WO2014051590A1

WO2014051590A1 - System and method for detecting a status of electrodes of a patient monitoring device

Info

Publication number: WO2014051590A1
Application number: PCT/US2012/057543
Authority: WO
Inventors: Daniel K. Freeman; Clifford Mark Risher-Kelly; David C. MAURER
Original assignee: Draeger Medical Systems, Inc.
Priority date: 2012-09-27
Filing date: 2012-09-27
Publication date: 2014-04-03

Abstract

An apparatus and method for detecting a condition of an electrode on a patient being used to monitor patient parameter data is provided. The apparatus detects, in a first mode of operation, a condition associated with respective ones of a plurality of electrodes selectively coupled to a patient. A respective one of the plurality of electrodes is selected and configured as a neutral drive electrode based on the detected condition. Monitoring of at least one patient parameter in response to the selection and configuration of the neutral drive electrode is enabled and, in a second mode of operation, detecting the condition of the neutral drive electrode and others of the plurality of electrodes to determine if reconfiguration of the neutral drive electrode is required based on the detected condition in the second mode of operation.

Description

System and Method for Detecting a Status of

Electrodes of a Patient Monitoring Device

Field of the Invention

This invention concerns a system and method for patient monitoring devices and, more specifically, for measuring contact impedance of at least one electrode connected to a patient monitoring device to determine the connection quality of the at least one electrode.

Background of the Invention

In the course of providing healthcare to patients, it is necessary to monitor vital statistics and other patient parameters. Different types of patient monitoring devices are able to monitor the physiological state of the patient via at least one electrode that is coupled to the skin of the patient at various locations on the body. For example, the electrical activity of the heart is routinely monitored in clinical environments using an electrocardiogram (ECG) monitor. The ECG monitor is connected to the patient by a plurality of electrodes that monitor the electrical impulses of the patient's heart. In order for the ECG monitor to effectively record the electrical impulses of the patient, electrodes extending therefrom conventionally include a conductive gel that is embedded in an adhesive pad used to secure the electrode to the body of a patient. Wires from the monitor are selectively connected to the electrode in order to communicate voltages detected to the ECG monitoring device to provide a healthcare practitioner with data regarding the patient's heart function.

It is well known that the quality of the recorded signal depends on the electrical connection and the impedance between the electrode and the patient's body, referred to as contact impedance. Therefore, it is desirable to measure the contact impedance to insure that the signal being monitored is of a sufficient quality. Conventional ECG monitoring systems alert clinicians when the contact impedance of one of the electrodes approaches or exceeds a threshold impedance value indicating that the connection of this electrode is poor. When the contact impedance exceeds an impedance threshold, the electrode is indicated as being in a "lead-off state which means the quality of the signal being sensed by the electrode is poor and insufficient for use in patient monitoring. In contrast, if the contact impedance is at least one of below the threshold value and within a predetermined acceptable impedance range, the electrode is indicated as being in a "lead-on" state.

Conventionally, there are two approaches to detecting the lead off status of an electrode. One approach is to detect the status of the electrode by measuring impedance values during monitoring of a patient parameter. The second approach is to interrupt patient parameter monitoring and measuring the impedance of the electrodes to detect the status of the respective electrodes. In clinical environments, the first approach to detecting lead off status is preferred to the second because there is no disruption in patient monitoring. However, the first approach whereby lead off detection occurs simultaneously with patient monitoring can be unreliable, resulting in a significant number of false positive lead off detections. In contrast, the second approach provides a more precise determination of contact impedance but requires patient monitoring to be temporarily suspended to allow for the determination to be made. It is therefore desirable to provide a contact impedance measurement system that can measure contact impedance simultaneously with patient parameter monitoring while minimizing the number of false positive detections of a lead off condition. Further, when at least one of the leads is determined to be lead off, it is desirable to interrupt monitoring so that the contact impedance of all of the electrodes can be precisely measured to allow for reconfiguration of the electrodes being used to monitor the patient parameter. A system according to invention principles addresses deficiencies of known systems.

Summary of the Invention

In one embodiment, a method of detecting a condition of an electrode on a patient being used to monitor patient parameter data is provided. The apparatus detects, in a first mode of operation, a condition associated with respective ones of a plurality of electrodes selectively coupled to a patient. A respective one of the plurality of electrodes is selected and configured as a neutral drive electrode based on the detected condition. Monitoring of at least one patient parameter in response to the selection and configuration of the neutral drive electrode is enabled and, in a second mode of operation, detecting the condition of the neutral drive electrode and others of the plurality of electrodes to determine if reconfiguration of the neutral drive electrode is required based on the detected condition in the second mode of operation.

In another embodiment, an apparatus that detects a condition of an electrode on a patient being used to monitor patient parameter data is provided. The apparatus includes a first circuit having a first electrode, a first amplifier and a first multiplexer and a second circuit including a second electrode, a second amplifier and a second multiplexer. A current source is selectively connected to at least one of the first electrode and the second electrode. A neutral drive circuit is connected to the first multiplexer and the second multiplexer; and a controller is connected to the current source, the first multiplexer and second multiplexer. In a first mode, the controller controls the first and second multiplexer to connect the first and second electrodes with respective first and second amplifiers and the current source to detect a condition of the first and second electrodes to determine which one of the first and second electrodes is to be connected to the neutral drive circuit. In a second mode of operation, the controller controls the one of the first and second multiplexers associated with the one of the first and second electrodes determined to be connected to the neutral drive circuit to connect the one of the first and second electrodes with the neutral drive circuit and controls the other of the first and second multiplexer to connect the other of the first and second electrodes with a respective amplifier thereby enabling monitoring of at least patient parameter, the controller controls the current source to apply a current to the other of the first and second electrodes to detect a condition of the other of the first and second electrode while the at least one patient parameter is being monitored.

Brief Description of the Drawings

Figure 1 is an exemplary block diagram of the system for detecting electrode status according to invention principles;

Figure 2 is a flow diagram detailing an exemplary algorithm executed by the system for detecting electrode status according to invention principles;

Figure 3 is a flow diagram detailing an exemplary algorithm executed by the system for detecting electrode status according to invention principles;

Figure 4 is an exemplary block diagram of another embodiment of the system for detecting electrode status according to invention principles; Figure 5 is an exemplary circuit diagram of the system for detecting electrode status according to invention principles;

Figure 6 is an exemplary circuit diagram of the system for detecting electrode status according to invention principles;

Figure 7 is an exemplary circuit diagram of the system for detecting electrode status according to invention principles; and

Figure 8 is an exemplary circuit diagram of the system for detecting electrode status according to invention principles.

Detailed Description

The present system advantageously detects a condition of at least one electrode that is coupled to and senses electrophysiological data from a patient which may be used to for determining data representing at least one patient parameter. The condition may represent a connection quality of the electrode and may be determined by measuring impedance at the interface between an electrode connected to the patient and the skin of the patient. This is known as the contact impedance and the system advantageously measures and determines the contact impedance for each electrode at least one of (a) prior to patient monitoring; (b) during the course of patient monitoring; and (c) during an interruption in patient monitoring. By measuring the contact impedance, healthcare practitioners may be notified in real time of a condition representative of a degrading connection of one or more electrodes connected to the patient. This enables a healthcare practitioner to remedy a situation that would otherwise lead to a signal sensed by the electrode having a less than desirable signal quality causing data that does not accurate represent a current condition of the patient to be generated. Additionally, determining the connection quality of electrodes enables the monitoring device to automatically reconfigure itself to choose electrodes having the lowest levels of impedance for use in sensing electrophysiological signals from the patient to determine patient parameter data therefrom.

In one embodiment, the system may be implemented in a patient monitoring device including but not limited to an electrocardiogram (ECG) monitoring device or an electroencephalogram (EEG) monitoring device. In another embodiment, the system may be implemented in front end circuitry for a patient monitoring device. The system advantageously measures contact impedance of the at least one electrode to identify whether the contact impedance exceeds a threshold impedance value resulting in the electrode being identified as being in a first condition indicating that the signal being sensed by the at least one electrode is of insufficient quality for use in determining a patient parameter. In one embodiment, the first condition may indicate that the electrode is in a "lead- off status. If the contact impedance measured by the system is below the threshold impedance value, the system identifies the at least one electrode as being in a second condition indicating that the signal sensed thereby is acceptable for use in determining patient parameter data for the particular patient. In one embodiment, the second condition may indicate that the electrode is in a "lead-on" status.

The system advantageously employs a dual mode condition detection algorithm that determines a condition of each electrode connected to the patient. The dual mode condition detection algorithm advantageously uses different condition detection algorithms at different times to generate condition data associated with each electrode. The system may use the condition data for configuring which electrodes the patient monitoring device are used to determine the at least one patient parameter.

A first condition detection algorithm advantageously provides a precise measurement of the contact impedance for each electrode. This is particularly useful when determining which electrode for a set of electrodes connected to a patient should be used as the neutral driver for the patient. The first condition detection algorithm may be executed when the patient monitoring device is not sensing electro physiological signals from the patient to be used in determining patient parameter data for the patient. In one embodiment, the first condition detection algorithm is executed upon initializing the patient monitoring device and prior to monitoring the patient in any manner. The first condition detection algorithm may be executed when the system interrupts the continuous monitoring of a patient parameter for the particular patient when the system determines that the contact impedance for a particular electrode has exceeded the impedance threshold.

In response to execution of the first contact impedance detection algorithm, condition data representing an impedance for the respective electrode may be stored. The system may use the condition data to provide an indicator to a clinician identifying an electrode having the highest connection quality to allow the clinician to select which electrode is to be designated the neutral driver. In another embodiment, the system may automatically configure the electrodes in a predetermined electrode configuration pattern based on the stored condition data for each electrode. The configuration pattern may include identifying an electrode to be the neutral drive, a first group of electrodes as primary electrodes and a second group of electrodes as non-primary electrodes.

Once the electrodes have been configured and the system is set to sense electrophysiological signals from a patient for use in determining the patient parameter, the system employs a second condition detection algorithm to detect the status of the electrodes simultaneously with monitoring the patient. The use of the second condition detection algorithm advantageously provides clinicians with an indication as to whether or not the contact impedances of any of the electrodes have reached a threshold impedance indicating that the connection quality of the electrode at a given time is poor resulting in the particular electrode status being "lead-off. In response to detecting that an electrode is in lead off condition, the system automatically stores condition data representing the status in a memory and may provide an indication to a clinician that one or more electrodes is in lead off. In one embodiment, if an electrode is determined to be lead off, the system automatically identifies which electrode is in this condition and uses electrode configuration rules to determine how to respond. Electrode configuration rules enable the system to automatically take predefined actions when determining a electrode is in lead off status. In one embodiment, configuration rules may cause the system to alert a clinician regarding the lead off status of an electrode indicating that an intervention by the clinician is needed at that time (e.g. replacement of the electrode because it has dried out). In another embodiment, the configuration rules may cause the system to automatically interrupt monitoring of the patient when it is determined that the electrode indentified as being lead-off has met a certain criteria for that type of electrode.

One exemplary configuration rule may require interruption of patient monitoring if the electrode identified as being lead off is determined to be the electrode on which the neutral driver is operating. This configuration rule may also re-initiate the first condition detection algorithm to determine the precise impedance values for the other electrodes not indicated as being lead off in order to reconfigure the electrodes for monitoring and select a different electrode to be the neutral driver and re-initiate patient monitoring and execution of the second condition detection algorithm. The second condition detection algorithm further advantageously checks the condition of all electrodes including any electrode that is determined to be lead off to identify if the condition has changed and the electrode has gone from lead off to lead on indicating that it may again be used for patient monitoring and automatically reconfigures the electrodes accordingly.

The first condition detection algorithm advantageously precisely and accurately determines contact impedance for each electrode. The first condition detection algorithm may use voltage/current sources that are connected to each electrode and are turned on in a sequential manner. The resulting current/voltage responses are measured and these measurements are then used to calculate the contact impedance of each electrode. One exemplary first condition detection algorithm may include applying a current source to each electrode while simultaneously shorting all of the other electrodes together. This advantageously enables the system to identify which electrodes have the highest connection quality for use in configuring the patient monitoring device.

The second condition detection algorithm may include any impedance measurement algorithm that runs continuously during patient monitoring. One exemplary second condition detection algorithm using pull-up/pull-down resistors where each electrode is connected to a resistor (generally tens of megaohms) in series with a voltage source and the impedance is calculated therefrom. Another exemplary second condition detection algorithm includes injecting a high- frequency AC current to all electrodes during patient parameter monitoring and measuring the impedance of each electrode. However, while these algorithms are able to detect the condition of electrodes during patient monitoring, there are certain drawbacks associated with these exemplary algorithms. With respect to the pull up/pull down resistors, the number of false positive detections of lead off conditions is unacceptably high. The use of high frequency AC signals may reduce the number of false positive lead off detections but this may also result in interfering with some of the ancillary functions performed by the patient monitoring devices regarding the monitoring and detection of other patient parameters. For example, if the patient monitoring device is an ECG monitoring device able to monitor ECG data of the patient, using the second condition detection algorithm that employs the high frequency AC signal may interfere with measuring data representing patient respiration or detecting pacemaker activity associated with the patient.

Thus, the system advantageously employs a second condition detection algorithm that controls a current source connected to each electrode that is not identified as the neutral drive to inject a current of a predetermined amount thereon. The total current injected by all current sources at a given time is equal to the current used to drive the neutral drive circuit which maintains a voltage of the patient's body at substantially zero volts. This advantageously enables the neutral drive to remain active and allow for patient parameter monitoring while still being able to determine the condition status for each electrode.

In order to calculate the contact impedance for a given electrode, it is necessary to know the current through the electrode, as well as the voltage across the electrode. In this case, the current through each electrode is known and is defined by the magnitude of the current source used to inject current through each electrode. Also, the voltage on the side of the electrode inside of the body is forced by the neutral drive circuit to be equal to the ground of the system. The voltage on the side of the electrode outside the skin is measured, and therefore it is possible to compute the contact impedance simply by dividing the measured voltage by the magnitude of the current. However, the calculation will differ slightly for the neutral electrode because for this case, the magnitude of the current is determined by the total number of electrodes in the system. For example, if there are three electrodes attached to the patient in addition to the neutral electrode, and if a current of magnitude I is injected into all 3 electrodes, then a total current of magnitude 31 will return through the neutral electrode. In this case, the contact impedance is calculated as the voltage on the neutral electrode divided by 31.

In some patient monitoring systems, the voltage on each electrode is not measured, but instead the differential voltage between electrodes is measured. In this case, it will be necessary to record the electrode voltage of at least one of the primary electrodes. Then, using this voltage in conjunction with the differential voltages of the three primary leads, the electrode voltages of all primary electrodes can be calculated. If the calculated impedance exceeds the threshold impedance, the condition of the electrode is indicated as being lead-off and data representing the condition may be stored in memory.

Condition data may be used to at least one of (a) generate an alert to a clinician indicating that one or more electrodes have entered a lead-off condition and (b) automatically change an operating mode of the patient monitoring device to interrupt monitoring and re-initiate the first condition detection algorithm to reconfigure the electrodes by selecting a new electrode to be the neutral drive.

Thus, the system advantageously employs a dual condition detection algorithm that uses two levels of monitoring each of which compensate for any deficiency associated with using the detection algorithm individually. The first detection algorithm is initiated prior to monitoring to calculate precise impedances for each electrode and to configure which electrode will be the neutral drive electrode. The impedance values for each electrode calculated prior to monitoring may be stored and used as base impedance values for comparisons at a later time when, for example, the monitoring device may need to be reconfigured. Once the system has determined the base impedance values for the electrodes, patient monitoring is initiated and the system executes the second condition detection algorithm at predefined intervals to detect the condition of each electrode as monitoring progresses. The second condition detection algorithm advantageously detects lead off status and will interrupt monitoring if the detected lead off status of an electrode meets a predetermine criteria and re-initiate the first detection algorithm to reconfigure the patient monitoring device. The dual mode detection algorithm compensates for any unreliability associated with lead off detection during patient monitoring by temporarily interrupting patient monitoring to initiate a more precise mechanism to measure contact impedance resulting in an improved ability to reconfigure patient monitoring devices in real time when a contact impedance of one or more electrodes has reached an unacceptable level. By providing an algorithm that only interrupts monitoring when a lead off condition on an electrode has been detected and only when the system identifies that the electrode on which the lead off condition has been detected meets a certain criteria (e.g. the neutral drive), the number of interruptions to patient parameter monitoring is minimized.

In one embodiment, the patient monitoring device may be an ECG monitor and the dual mode detection algorithm advantageously improves the ability of the ECG monitor to determine which electrode will be used for the neutral driving circuit. By default, the right leg (RL) electrode is usually used as the neutral driver. Thus, when the first condition detection algorithm is executed and measures the impedance of each electrode, presuming the impedance is below a threshold level, the system will automatically select the RL electrode to be used as the neutral driver. Once monitoring has begun, the system automatically determines if the voltage on the neutral drive is approaching or exceeds a clamp voltage and, if so, the system temporarily suspends patient monitoring to reinitiate the first condition detection algorithm enabling reconfiguration of the neutral drive electrode. If contact impedance of this electrode reaches or exceeds a threshold impedance, it is determined to be lead-off and the system advantageously reconfigures the electrodes to select a different electrode to be the neutral driver. ECG monitoring is temporarily disrupted to obtain precise measurements of impedances for all electrodes to intelligently determine which electrode should be reconfigured as the new neutral driver electrode. This is an improvement over arbitrarily selecting a different electrode as the neutral driver because arbitrary selection does not take into account the current condition of the electrode being selected. This may lead to additional switching after and within a few seconds of reconfiguration of the new neutral driver. By temporarily suspending ECG monitoring to obtain precise measurements of the other electrodes, the system advantageously is able to select an electrode having a sufficiently low impedance for use as the new neutral driver such that a stable ECG signal can be provided. This is advantageous because the additional time required for the suspension and measurement of the other electrodes is small as compared to a settling time of the reconfiguration of the neutral electrode. This avoids the need to continuously and arbitrarily hunt when reconfiguring electrodes as the neutral which requires the system to settle after each selection to determine if the newly configured electrode is acceptable. By suspending monitoring, reconfiguring and checking the impedance on the newly configured and all other electrodes, there is no need to wait for settling after each check and the overall time patient monitoring is suspended is reduced.

Figure 1 is a block diagram of an exemplary monitoring device 100 that selectively monitors electrical impulses from a patient via at least one electrode 102 that is connected to a predetermined location on the patient. While the patient monitoring device 100 shown in Figure 1 only shows a single electrode, one skilled in the art will understand that additional configurations having more than one electrode exist and the respective elements and components described herein can readily be extended to these configurations. Thus, the description of the monitoring device 100 with one electrode is for purpose of ease of understanding and simplicity regarding the operation thereof. Thus, while the description of the electrode 102 is shown herein, it should be appreciated that any number of electrodes may be used to monitor the electrical impulses of the patient and the number of electrodes employed depends on the type of data being monitored by the monitoring device 100. In one embodiment, the patient monitoring device 100 is an ECG monitor including a plurality of electrodes that are attached to the patient's limbs and chest. One skilled in the art understands that the electrodes are commonly positioned on the right arm (RA), left arm (LA), right leg (RL), left leg (LL), and in some cases there are several electrodes placed on the chest. Of these electrodes, RA, LA and LL are generally referred to as "primary electrodes", the electrodes on the chest are referred to as "V-leads", and RL is usually referred to as the "neutral electrode", although in practice, any electrode can be designated as the neutral electrode. We use the term "secondary electrodes" to refer collectively to the V-leads and neutral electrode. The primary electrodes RA, LA and RL are coupled to an averager (not shown) which automatically averages the voltages of the primary electrodes in order to generate a reference voltage known as the Wilson Point.

The reference voltage generated by the averager may be used as discussed below to determine the impedances for any of the V-leads. Significant noise is introduced to the body through capacitive coupling to various sources of electric fields (e.g. power line noise). This noise is referred to as common mode noise and it can obscure the ECG signal. In order to reduce such noise, most ECG monitors will take the average voltage of the three primary electrodes and input this signal to an inverting amplifier. The output of this amplifier is connected to the neutral electrode. By inverting the signal that is common to all electrodes and injecting it back into the body, the noise levels experience by the ECG monitor is dramatically reduced. Additionally, the V-leads are known to include electrodes Vi - V₆ positioned at predetermined locations on the chest of the patient in a known manner. The manner in which the monitoring device monitors the electrical impulses to generate and output ECG waveforms is known, is not germane to the present invention and will not be discussed further.

Referring back to Figure 1 , a condition representing the connection quality of the electrode 102 may be determined by calculating the contact impedance for the electrode 102. The contact impedance for the electrode is represented in Figure 1 generally by "ZN" where N represents the number of electrodes connected to the monitoring device 100 at a given time. The electrode 102 is coupled to the monitoring device 100 in the following manner.

The monitoring device 100 includes a multiplexer (MUX) 104, a differential amplifier 106 and neutral drive circuitry 108. The electrode 102, the differential amplifier 106 and the neutral drive circuitry 108 are coupled together via the multiplexer 104. The multiplexer 104 enables bidirectional transmission of signals between elements connected thereto. In one mode of operation, the multiplexer 104 may couple the electrode 102 with a first input 105 of the differential amplifier 106 enabling the differential amplifier 106 to receive an electrical signal sensed by the electrode 102. The differential amplifier 106 may also receive, at a second input 107, an input signal derived from a second source 109. The amplifier 106 uses the first and second input signals to calculate a voltage differential between a voltage on electrode 102 and the voltage associated with the second source 109. The second source 109 includes one of (a) an input sensed by a second different electrode; (b) a reference voltage (e.g. Wilson Point) that is derived by averaging the voltages of a predetermined number of other electrodes; or (c) ground. The differential amplifier 106 is further coupled to a parameter processor 110 via a switch 112 that, when in the closed position, enables the parameter processor 110 to use the voltage output by the amplifier 106 (along with the voltages of any other amplifiers in the device 100) to determine at least one patient parameter.

In another mode of operation the multiplexer 104 may be configured to couple the neutral drive circuit 108 to the electrode 102 resulting in the electrode 102 being designated the neutral electrode and applying current to the patient as described above. Thus, for each electrode 102 connected to the monitoring device 100, a respective multiplexer 104 couples the respective electrode 102 to a respective one of the differential amplifiers 106. Additionally, each multiplexer 104 may selectively couple the neutral drive circuitry 108 to the respective electrode 102. Exemplary configurations of the patient monitoring device will be discussed hereinafter with respect to Figures 5 - 7 which will show the manner in which all the individual elements described in Figure 1 may be connected to one another.

The monitoring device 100 also includes a current source 114 that is coupled to the electrode 102 via a switch 116. The switch 116, when in the closed position, results in current of a predetermined amount to be applied to electrode 102 and enables the monitoring device 100 to determine contact impedance for the electrode 102 as discussed below. The current being injected on the electrode 102 is returned to the monitoring device 100 via the electrode that is configured as the neutral drive electrode.

The monitoring device 100 further includes a controller 118 able to selectively control various components of the monitoring device 100 to measure and detect a condition representing the contact impedance of the electrode 102 and a memory 120 coupled thereto for storing data representing a condition of an electrode 102. The controller 118 selectively configures the monitoring device 100 to operate in one of a first operating mode whereby the monitoring device is not monitoring patient parameter data and a second mode whereby the monitoring device 100 is selectively using the signals sensed by the electrode 102 to monitor and determine patient parameter data. The controller 118 is coupled to switch 112 and, in the first operating mode (e.g. prior to monitoring beginning or during a controlled interruption in patient monitoring), the controller 118 controls the switch 112 to disconnect the output of amplifier 106 from the parameter processor 110. In the second mode of operation, the controller 118 causes the switch 112 to connect the output of the differential amplifier 106 to the parameter processor 110 thereby allowing data output by the amplifier 108 to be used by the parameter processor 110 to generate patient parameter data.

The controller 118 further may advantageously control the monitoring device 100 to execute a dual condition detection algorithm that determines a condition of the electrode 102 in both the first operating mode and the second operating mode. The controller 118 may execute a first condition detection algorithm to determine condition data of an electrode when the electrode is not monitoring patient parameter data in the first mode of operation and a second condition detection algorithm to detect electrode condition data when the monitoring device is monitoring patient parameter data in the second mode of operation. Electrode condition data may include one of (a) lead off status indicating that the signal being sensed thereby is insufficient for use in generating patient parameter data by a parameter processor 110; (b) lead on status indicating that the signal being sensed thereby is sufficient for use in generating a patient parameter data by the parameter processor 110 and (c) an actual impedance value of a particular electrode at a given time. The controller 118 may detect a condition of the electrode at least one of (a) during patient monitoring; (b) prior to beginning patient monitoring; (c) during an interruption in patient monitoring; and (d) at an end of patient monitoring. The condition data detected by the controller 118 may be stored in memory 120 which is coupled thereto.

In the first operating mode, the controller 118 initiates a first condition detection algorithm that detects the condition of the electrode when no patient monitoring is occurring. An exemplary first condition detection algorithm may include measuring contact impedance using pull-up/pull-down resistors where each electrode is connected to a resistor (generally tens of megaohms) in series with a voltage source. This will cause the electrode voltage to be drawn near the applied voltage level when the contact impedance increases to the tens of megaohms range. This indicates the presence of a poor connection and suggests the signal being sensed is of sub-optimal quality and thus be determined to be Lead Off. Another exemplary first condition detection algorithm measures contact impedance by applying a current to a given electrode which returns to ground through the other connected electrode. This results in a voltage drop across the electrode to which the current was applied and a corresponding voltage drop across a parallel combination of all other electrodes. By repeating this measurement for each of N total electrodes, a set of N nonlinear equations and N- unknowns can be derived where N is equal to the number of electrodes connected to the system. A further exemplary first condition detection algorithm may include selectively applying the first current source having a predetermined magnitude to one of the three electrodes and simultaneously applying the second current source having the predetermined magnitude to another one of the three electrodes. The first and second current source may be sequentially applied to each combination of electrode pairs for the three electrodes. A voltage differential between each of the electrodes may be measured and, because the current level is of a predetermined value, a linear equation wherein the voltage differential between two electrodes is equal to the sum of the contact impedances of a first and second electrode times the current level may be generated for each electrode pair. Thereafter, the three generated linear equations representing each electrode pair (e.g. Electrode 1 and 2, Electrode 2 and 3, and Electrode 1 and 3) may be resolved and the respective contact impedances for each electrode determined. The contact impedance values are compared to a threshold contact impedance to determine if the signal being sensed by the respective electrode is of a sufficient quality and thus is Lead On. These first condition detection algorithms are describe for purposes of example only and any manner of measuring the contact impedance of electrodes connected to the patient may be used.

In one embodiment, in the first condition detection algorithm, the controller causes switch 112 to disconnect the amplifier 106 from the parameter processor 110 at least one of preventing patient monitoring from beginning or interrupting on-going patient monitoring. The controller 118 controls the switch 116 to connect the current source 114 to the electrode 102 and controls the multiplexer 104 to couple the electrode to the first input 105 of the amplifier 106. The controller 118 also determines which second source 109 will be connected to the second input 107 of the amplifier 106 to allow the voltage of the electrode 102 to be measured by the amplifier. In one embodiment, the controller causes the second input 107 of the amplifier to be connected to ground to obtain the voltage on electrode 102. In another embodiment, the controller 118 may cause a second electrode to be connected to the second input 107 of the amplifier 106 and a differential voltage between electrode 102 and the second electrode is determined by the amplifier 106. The voltage determined by the amplifier 106 is provided to the controller 118 and data representing a condition of the electrode 102 is determined. These steps are repeated to connect a second channel in the same manner in order to obtain the differential voltage between the electrodes to measure condition data. The condition data associated with the electrode may be a contact impedance value ZN. The condition data representing the contact impedance value as determined by the controller 118 may be stored in memory 120. In one embodiment, when the condition detection algorithm is executed prior to any patient monitoring (e.g. upon initialization and configuration of the monitoring device 100 for a particular patient) the contact impedance value may be stored, and representing a base condition associated with electrode 102.

In one embodiment, the controller 118 may confirm an electrode configuration input by a clinician to identify the number of electrodes that will be used.. In this embodiment, the configuration of electrodes 102 connected to the patient may be identified in advance by the clinician and, upon detecting the condition of each electrode, the condition data is compared to acceptable condition data values associated with the particular type of electrode 102 being configured and, if within an acceptable range, the controller 118 selects, as the neutral drive electrode, an electrode having condition data in the acceptable range and being highest on a priority list of neutral drive electrodes based on the type of electrode configuration selected by the clinician. For example, if the clinician indicates the electrodes connected to the patient are in a 10 Lead electrode configuration, electrode 102 in Figure 1 is identified as being the RL electrode and set by the monitoring device to be the neutral driver. So long as the the condition data (e.g. contract impedance) of the electrode is determined to be below a threshold value (e.g. below a threshold contact impedance value), the controller 118 confirms that the RL electrode is able to be the neutral drive electrode and causes the multiplexer 104 to couple the neutral drive circuit 108 to the electrode 102.

In another embodiment, the controller 118 may determine an electrode configuration for the monitoring device 100 accordingly. In this embodiment, the controller 118 may detect condition data for electrode 102 and compare it to a threshold to determine if the condition is acceptable for patient monitoring and store the condition data in memory 120. The controller 118 repeats this detection for all electrodes connected to the monitoring device and stores condition data in the memory 120. The controller may compare the detected condition data associated with each electrode to a set of electrode configuration information and determine which electrode is Lead ON or Lead Off. In one embodiment, the condition data may represent the impedances for all electrodes which may be compared to electrode specific impedance thresholds to determine if the respective electrode is Lead On or Lead Off. Data representing the condition of each electrode is stored in memory 120. Once the condition status of each electrode is determined, the controller 118 uses a neutral drive priority list in conjunction with the determined electrode status to select which electrode will be designated as the neutral drive electrode. In one embodiment, the neutral drive electrode priority list is RL, V2, RA, LA, LL, VI, V3, V4, V5 and V6 in descending order where RL is the first choice and has the highest priority and V6 is the lowest. Thus, the electrode highest on the priority list that is also indicated as being Lead On (or NOT Lead Off) will be selected and designated as the neutral drive electrode. Upon configuring an electrode as a neutral drive electrode, the controller signals the multiplexer 104 to connect the electrode with the neutral drive circuitry.

While only the configuration of the neutral electrode is described in this first operating mode prior to any patient monitoring, the controller 118 is able to detect condition data for any type of electrode and store this data for later use in determining whether or not the connection quality of the particular electrode has diminished.

Upon configuring which electrode 102 will be used as the neutral drive, the controller 118 signals the switch 112 to connect the amplifier 106 with parameter processor 110 to allow patient monitoring to begin (or continue if the above discussed first mode of operation was initiated during an interruption of patient monitoring). At predetermined intervals, the controller 118 controls switch 116 to couple the current source 114 to the electrode 102 to apply a predetermined current to the electrode 102. The current source 114 may be able to provide current / ranging between 6 nA and 24 nA. As shown herein, the current source 114 is providing a current of 12nA when connected to electrode 102. By applying the predetermined current /, a voltage of the electrode 102 can be measured and used to determine the condition of the electrode by calculating an impedance ZN associated with the electrode 102 and comparing the calculated impedance with a threshold impedance to determine if the electrode is lead off or lead on.

If electrode 102 is identified and operating as the neutral drive electrode, the controller 118 may exit the patient monitoring mode and cause the multiplexer 104 to decouple the neutral drive circuit 108 from the electrode 102, couple the electrode 102 with the first input 105 of the amplifier 106 and use a voltage from the second source 109 received at input 107 to measure the voltage on electrode 102. Upon disconnecting the neutral drive circuit 108 from the electrode 102, the controller 118 will cause a multiplexer 104 associated with a second different electrode (not shown) to couple the second different electrode with the neutral drive circuit 108 as a operational neutral drive circuit and reinitiate patient monitoring if needed to detect the condition of the electrodes while patient monitoring is occurring. If the electrode 102 is not the neutral drive, the controller 118 need only selectively control switch 116 to connect the current source 114 to the electrode 102.

The controller 118 causes each current sources connected to each electrode 102 to be connected simultaneously thereby injecting predetermined current / on each electrode. Thus, the total current injected into the system is represented in Equation 1 as

T=I(N-1) (1) where T is the total current injected into the system, / is the predetermined current injected on each electrode and N is the total number of electrodes connected to the patient. The total current T cumulatively injected into the electrodes 102 by all current sources 114 is equal to the current injected by the neutral drive circuitry 108. In this manner, the neutral drive circuitry 108 is able to overcome the total current T cumulatively injected by all current sources. By being able to remove (e.g. balance all other current sources depending on the direction of the current) the total current T, the neutral driver is able to maintain the voltage of a patient at substantially zero volts which allows for the monitoring device 100 to monitor patient parameter data while simultaneously checking a condition of the respective electrode 102. A further advantage provided by T being equal to the current injected by the neutral drive relates to the maximum impedance of the neutral electrode able to be measured at a given time without saturating the neutral drive. As the total current T increases, the maximum impedance able to be used on the neutral drive decreases until saturation occurs. Thus, by reducing the amount of current needed to detect the electrode status, the system advantageously provides a greater range of neutral drive impedance that is usable by the system. The current T though the neutral drive causes a voltage to appear on the amplifier coupled to the neutral drive electrode. If this voltage becomes greater than the clamp voltage, the impedance on the neutral drive electrode can no longer be measured and the neutral drive would be indicated as Lead Off causing the system to exit patient monitoring to reconfigure which electrode will be the neutral drive electrode.

Upon applying the current from current source 114 to electrode 102, the voltage is measured by the amplifier 106 relative to the second source 109. In one embodiment, the second source is ground which results in a true voltage measurement associated with electrode 102. In another embodiment, the voltage measured may be a differential voltage between the voltage on electrode 102 and a voltage derived from one of (a) a different electrode or (b) a reference voltage, depending on the type of electrode 102. For example, if electrode 102 is a primary electrode, the voltage differential can be obtained between it and another primary electrode. Alternatively, if electrode 102 is a V-Lead, then the voltage differential will be between it and the reference voltage. Once the voltage is determined by amplifier 106, the controller 118 compares the voltage with a threshold voltage value that indicates a lead off status. If the voltage determined by amplifier 106 is below the threshold, the controller 118 determines that the condition of the electrode is lead on and updates data representing the electrode condition in memory 120. The controller 118 may then disconnect the current source 114 from the electrode 102 using the switch 116.

If the voltage detected by the amplifier equals or exceeds the threshold voltage, the controller 118 determines that the condition of the electrode is lead- off and updates the status of the particular electrode in memory 120. The controller 118 then determines what type of electrode 102 is now being indicated as lead off. If the electrode 102 determined to be lead off is the neutral driver electrode, the controller 118 interrupts patient parameter monitoring by disconnecting the amplifier 106 from the parameter processor 110 using switch 112 and re-initiates the first condition detection algorithm to obtain a precise measurement of the contact impedance for electrode 102 to check if the lead off status determined using the second condition detection algorithm is a false positive result. If the precise contact impedance measured using the first condition detection algorithm results in the electrode impedance being below the impedance threshold, the controller 118 reconnects the amplifier 106 with the parameter processor 110 and displays patient parameter data for a predetermined amount of time. If the displayed patient parameter data is acceptable, the controller 118 updates the status of the electrode in memory 120 as being lead on and patient parameter monitoring continues. However, if the contact impedance for electrode 102 meets or exceeds the contact impedance threshold, the controller 118 automatically reconfigures the neutral electrode configuration. The controller 118 selects a different electrode to be used as a neutral drive electrode by comparing the contact impedances measured using the first condition detection algorithm run in response to interrupting patient monitoring. In one embodiment, the controller 118 may select a different primary electrode or one of the V-Lead electrodes as a new neutral driver. The controller 118, in selecting a new electrode to be used as neutral driver may also reconfigure which electrodes are being used to derive the reference voltage that is used as part of the second condition detection algorithm. Another advantage of measuring the actual impedance of the electrodes is that the impedances can be displayed on the monitor with high accuracy to help the clinician evaluate the quality of the electrodes being used.

In response to reconfiguring the neutral driver electrode, the controller 118 re-initiates the second condition detection algorithm to further monitor and detect any changes in electrode status as monitoring continues. This re-initiation of the second condition detection algorithm continues to detect the condition of each electrode, even the electrode that was originally designated the neutral driver and which has been disconnected. This advantageously enables the controller 118 to see if the lead off status has changed for that electrode and allow the system to further reconfigure that electrode as the neutral driver when the contact impedance has been reduced below the threshold by, for example, changing the electrode pad or due to movement of the patient.

The system 100 may also include a set of diode clamps 122 positioned between the electrode 102 and the multiplexer 104 and current source 114. The diode clamps 122 provide defibrillation protection by preventing any voltages from a defibrillator from entering the monitoring device 100 and damaging the circuitry. The voltage of the diode clamps 122 should be above the highest acceptable input voltage of the amplifier 106. If this condition is not met, the monitoring device 100 will always determine that the particular electrode 102 is in lead-off status.

Figure 2 is a flow diagram detailing the operation of the system according to invention principles. The following description of the activities in Figure 2 will be made with reference to certain components described in Figure 1 that may implement the activities. Additionally, the operation of the system will be described with respect to a register 250 that may be stored in memory 120. The register 250 may include a table having a predetermined number of columns collectively referred to with reference numeral 254 and a predetermined number of rows collectively referred to with reference numeral 252. The columns 254 of register 250 represent a number of individual electrodes present in a particular electrode configuration. The rows 252 of the register may include at least three rows and the data stored therein may indicate a status of a corresponding electrode in the particular column for a given purpose and/or operation. Additionally, the columns are organized according to a priority level for the electrodes wherein the priority level is associated with a respective purpose and/or operation of the system.

In one embodiment, the system is a monitoring device for monitoring ECG data from a particular patient. It is beneficial to measure and obtain certain information about the electrodes connected to the patient that sense the electrophysiological data. The register 250 has a number of columns 254 equal to a number of electrodes being used in the current electrode configuration. As shown here, the monitoring device is configured as a 12-Lead ECG monitor having 10 electrodes coupled to the patient. The 10 electrodes include the primary leads (RA, LA, RL, LL) and secondary leads (VI- V6). This is shown for purposes of example only and the monitoring device may be configured to operate in any known ECG electrode configuration such as a 3 -Lead or 5 -Lead electrode configuration. The rows 252 of the register 250 enable the system to monitor and track information associated with respective electrodes in the columns 254. As shown herein, a first row 1 represents Lead-Off Status data whereby upon detection of a lead off status or a particular electrode, the controller 118 may insert a flag in the corresponding column of row 252a when a particular lead is determined to be in Lead-Off status. A second row 2 indicates which electrode has been selected to be the neutral drive electrode. As shown herein, the monitor is configured to allow a first subset of the set of electrodes to be selected as the neutral drive and provides data fields that correspond to the subset of electrodes. Additionally, as selection of the neutral drive electrode is an aspect of system operation, the columns are organized priority level associated with neutral drive selection. For example, the first column 254a represents the RL electrode which is the ideal electrode to be designated the neutral drive. The second column 254b, immediately to the right thereof, represents the V2 electrode which is the second best electrode to be designated as the neutral electrode. The column order from left to right represent a decreasing desirability to have the respective electrode be designated the neutral electrode. Thus, the priority of electrodes for a 12-lead, 10 electrode ECG monitoring configuration is the RL 254a, V2 254b, RA 254c, LA 254d, LL 254e, VI 254f, V3 254g, V4 254h, V5 254i, V6 254j. A third row 3 indicates a second subset of the electrodes that are being used to generate a reference voltage (e.g. Wilson Point) that is used during ECG monitoring and which is derived from at least two of the second subset of electrodes. As shown herein, the second subset of electrodes include the RA, LA and LL electrodes. The manner in which the particular fields of register 250 are to be updated will be discussed below with respect to the particular aspects of the algorithm of Figure 2.

In step 202, the controller 118 initiates the first condition detection algorithm which is a discontinuous monitoring algorithm that measures the contact impedance value for each electrode and allows for the selection and configuration of the neutral drive electrode. The discontinuous monitoring algorithm also enables selection of a subset of the primary electrodes from which a reference voltage to be used by the secondary electrodes that is used by the secondary electrodes. Step 202 also enables identification of a Lead Off status for each electrode coupled thereto by comparing calculated impedances for each electrode to impedance thresholds. In response to detecting any electrode being Lead Off, the controller 118 updates a fields in Row 1 of register 250 to indicate that an electrode is Lead Off by inserting a flag or other identifier in the column corresponding to the electrode determined to be Lead Off. If no electrode is determined to be Lead Off, the data field is left blank. Alternatively, the controller may update the data fields using a first type of indicator to indicate that the electrode is Lead Off and a second different indicator to indicate that the electrode is Lead On. Once the neutral electrode is configured, the controller 118 updates the data field in Row 2 corresponding to the electrode selected as the neutral drive electrode. Additionally, in response to determining the lead-off status of the electrodes, the controller 118 may use the lead off determination to select which of the primary electrodes will be used to generate the reference voltage. In response to this selection, the data fields in Row 3 corresponding to the selected electrodes are updated with a flag to indicate they are being used. As is known in the art, to generate the reference voltage at least two of the three primary electrodes are selected for this purpose. For example, if there is no flag in columns 254c - 254e of Row 1, the controller 118 may select all of the primary leads to generate the reference voltage and update columns 245c - 254e in Row 3 to indicate such.

In step 204, the controller 118 initiates monitoring and runs the second condition detection algorithm to detect the condition of the electrodes while patient monitoring is occurring. The controller 118 queries whether a condition (e.g. lead off status) of any of the electrodes has changed in step 206. An exemplary second condition algorithm is described hereinafter with respect to Figure 3. As the second condition algorithm determines of the condition is present (e.g. that any of the electrodes are lead off), the controller 118 automatically updates the data fields in Row 1 accordingly. The controller 118 queries whether or not the condition of the electrodes have changed in step 206. If the result of the query in step 206 is negative, the method reverts back to step 204 and the second condition detection algorithm is repeated to continually determine if the condition on the electrodes is present. If the result of the query in step 206 is positive (e.g. the condition of the electrodes has changed and is now lead off), the controller 118 further queries whether the changed electrode condition is associated with the neutral drive electrode in step 208.

If the query in step 208 is positive, the controller 118 interrupts patient monitoring and runs the first condition detection algorithm to determine the precise impedance measurement for the neutral electrode in step 210. The controller 118 updates the data field in Row 1 to indicate that the presently configured neutral electrode which is indicated by a flag in Row 2 is Lead Off. This results in flags being placed in the same column of Rows 1 and 2. For example, if the originally configured neutral electrode was RL, column 254a of Row 2 would have a flag placed therein. Upon detecting in step 210 that the neutral electrode is Lead Off, the controller 118 would insert a flag in column 254a in Row 1. Further, the controller 118 may perform a check to determine if there is also a flag in the same column of Row 3. If so, the controller 118 automatically removes that electrode from being used to calculate the reference voltage. It should be noted that even though the workflow described herein refers to the neutral electrode, the controller 118, in step 210, may determine the precise impedance measurements for all electrodes and store those values in memory 120 for use in reconfiguring the electrodes accordingly. In doing so, the controller 118 may selectively update any field in Row 1 to indicate whether or not the respective electrode is, at the current time, is Lead Off. In step 212, the controller 118 compares the determined impedance of the neutral electrode with a threshold impedance value and determines if the measured impedance exceeds the threshold. If the determined impedance in step 212 exceeds the threshold impedance for the neutral drive, the controller 118 automatically reconfigures a different electrode to be the neutral electrode in step 215. By doing so, the controller 118 queries the indicators in Row 2 of register 250 to find a new electrode for the neutral drive. At first, the controller 118 locates the indicator in Row 2 indicating the currently set neutral drive electrode that has exceeded the impedance threshold and then determines if there are any higher priority electrodes (e.g. electrodes in columns to the left of the currently configured neutral electrode) that are not indicated in Row 1 as being Lead Off. If this condition is not met, the controller 118 goes down the priority list (e.g. looks to electrodes in columns to the right of the currently configured neutral) to find the next highest priority electrode that is not indicated as being lead off in Row 1 of register 250. Upon locating the electrode to be configured as the neutral, the controller 118 updates Row 2 of register 250 to reflect which electrode has been configured to be the neutral. Additionally, the controller may also have to update Row 3 of register 250 in the event that the newly configured neutral electrode is one of the primary electrodes that was previously being used to calculate the reference voltage. Should this occur, the controller 118 will deselect the electrode in Row 3 resulting in the reference voltage being derived from the other primary electrodes. Once a new neutral electrode has been selected, the controller 118 automatically updates the Lead Off status of all other electrodes in step 220. The algorithm then reverts back to step 204 whereby patient parameter monitoring (ECG monitoring) is re-initiated.

Referring back to step 212, if the determined impedance does not exceed the threshold, the controller 118 queries whether or not there is a higher priority neutral electrode available in step 214. The controller 118 performs this check by looking to the identifier in Row 2 that is in a column to the left of the currently identified neutral that also does not have an indicator in Row 1 of being Lead Off. If the query in step 214 is positive, a new electrode is configured to be the neutral drive electrode in step 217. The reconfiguration of a new neutral, including all updates and modifications to the register occur in substantially the same manner as those described above in step 215 and need not be repeated. Once a new neutral has been configured in step 217, the controller 118 reinitiates patient parameter monitoring and displays the patient parameter data for a predetermined amount of time to insure that the displayed patient parameter data is acceptable. This advantageously provides a false positive check whereby the patient parameter data will be displayed for lOsec without running the first condition detection algorithm in order to give time for stabilization of the circuit since changing the neutral drive causes current to flow between electrodes that must charge or discharge capacitance in the system due to the cable wires. This advantageously prevents the system from locking up in a state where no patient monitoring is occurring and the first condition detection algorithm is continually repeated resulting in no display of patient parameter data. Thereafter, once step 218 runs without any errors, data representing the condition of the electrode is updated in the user interface to reflect that the electrode is lead on in step 220.

Referring back to step 214, if the determination therein is negative and there is no higher priority neutral, the controller 118 reinitiates patient parameter monitoring and displays the patient parameter data for a predetermined amount of time to insure that the displayed patient parameter data is acceptable. This advantageously provides a false positive check whereby the patient parameter data will be displayed for lOsec without running the first condition detection algorithm in order to give time for stabilization of the circuit since changing the neutral drive causes current to flow between electrodes that must charge or discharge capacitance in the system due to the cable wires. Thereafter, once step 218 runs without any errors, data representing the condition of the electrode is updated in the user interface to reflect that the electrode is lead on in step 220.

Referring back to step 208, if the query therein results in the neutral drive not being off as determined using the second condition detection algorithm, the flow continues in step 209. In step 209, the controller 118 queries whether any of the primary electrodes (RA, LA, LL) are currently configured as the neutral drive. If the query in step 209 is negative, the user interface data representing the condition of the electrode is updated to reflect that the electrode is lead on in step 220.

If the query in step 209 is positive, the controller 118 interrupts patient monitoring and runs the first condition detection algorithm to determine the precise impedance measurement for all electrodes in step 211. The operation of step 211 is substantially similar to the operation of step 210 and need not be repeated here. Because the currently configured neutral drive electrode is determined to be a primary electrode, the controller 118 recognizes that there may be electrodes that are more desirable to be configured as the neutral. These other electrodes (e.g. RL or V2) are higher in the neutral drive electrode priority list used by the controller 118 in selecting and reconfiguring the neutral drive electrode. Thus, in step 213 the controller 118 uses the measured impedances of all the electrodes to determine whether or not the impedance on a first or second choice electrode (e.g. RL or V2)is less than the threshold impedance indicating that the first and second choice electrodes are Lead On. If the query in step 213 is negative, user interface data representing the condition of the electrode is updated to reflect that the electrode is lead on in step 220. If the query in step 213 is positive, the controller 118 changes the neutral drive in step 215 to be one of the first or second choice electrode being higher on the priority list and being indicated as Lead On. The manner in which the neutral drive electrode is reconfigured in step 215 is described in greater detail above and need not be repeated herein. Upon changing the neutral drive in step 215, user interface data representing the condition of the electrode is updated to reflect that the electrode is lead on in step 220.

Figure 3 is a flow diagram detailing the steps of the algorithm shown in step 204 of Figure 2. Similarly to Figure 2, the algorithm in Figure 3 is described with respect to the components shown in Figure 1. Once the discontinuous monitoring algorithm has concluded resulting in the neutral drive being selected and identifying at least two leads of the primary leads for use in generating the reference voltage, patient monitoring may begin. In one embodiment where the patient monitoring device is an ECG monitor, the discontinuous monitoring algorithm may configure the RL electrode to be the neutral drive electrode and determine that the reference voltage (e.g. Wilson point) will be calculated using the RA, LA and LL leads. In step 302, the controller 118 causes respective switches 116 to connect respective current sources 114 to respective electrodes 102. The voltages on each electrode are measured in step 304. The voltages may be measured in any manner. Thereafter, in step 306, an impedance for each electrode is calculated. In one embodiment, step 306 may include calculating an impedance on the primary leads (RA, LA, LL) by directly measuring a voltage of one of the primary leads. For purposes of example and ease of understanding, the voltage of the RA lead will be directly measured. Thereafter, a differential voltage of the directly measured electrode RA and each of a second primary electrode LA and a third one of the primary electrodes LL is obtained. Additionally a differential voltage between the second and third primary electrode LA and LL is also obtained. The controller 118 may calculate the actual voltages on the second and third primary electrodes using the Equations 2 - 4 below

VI = RA-LA (2) V2 = LA - LL (3) V3 = LL - RA (4) Here VI, V2 and V3 are the differential voltages measured by respective amplifiers 106. Because we have measured the actual voltage of a first primary electrode RA, and we know the voltage differentials, the controller 118 is able to quickly calculate the respective voltages for LA and LL using linear equations 2 - 4. Once the voltages are known, the controller can calculate the contact impedance for each electrode by dividing the voltage of the electrode by the current applied by the respective current source and compare the contact impedance values with a threshold to determine whether or not the condition of the respective electrode is lead off.

In another embodiment, step 306 may also include calculating an impedance for the secondary electrodes (V-Leads) to determine the condition of the secondary electrodes. The controller 118 subtracts the voltage on respective secondary leads from the reference voltage and divides the resulting differential voltage by the current injected on the respective secondary leads. This results in the contact impedance for the respective secondary leads. The controller 118 may then compare the determined contact impedances with the threshold contact impedance to determine whether or not the condition of the respective secondary electrode is lead off.

In step 308, the calculated impedances for each electrode are compared to thresholds impedances that correspond to the particular type of electrode to determine if the respective electrode is Lead On or Lead Off in order to update a Status table stored in the memory 120. For example the impedance threshold indicative of a lead off condition for the neutral electrode is lower than the impedance threshold indicative of a lead off condition for any of the primary or secondary leads. The threshold for the neutral drive may also be dependant on the total numbers of electrodes being used.

In one embodiment, if it is determined that the electrode configured as the neutral drive is lead off. Monitoring ceases and the algorithm reverts back to a discontinuous monitoring algorithm whereby reconfiguration of the neutral electrode occurs. For example, the controller 118 may interrupt patient monitoring to reconfigure the neutral drive reverts the system back step 208 in Figure 2. In one embodiment, the neutral drive may be reconfigured by the controller 118 causing the multiplexer 104 to disconnect the neutral drive circuit 108 from the electrode 102 and instead connect the electrode 102 to an input 105 of the amplifier 106. The controller 118 causes the switch 116 to move from an open position to a closed position connecting the current source 114 with the electrode 102. The controller 118 selects a new electrode to be used as the neutral drive electrode based on (a) a predetermined order indicating the next best electrode to be the neutral drive, and (b) the actual impedance values of each of the electrodes calculated using the first condition detection algorithm. The controller 118 includes a predetermined priority order of leads for use as the neutral drive electrode and selects the electrode based on this priority and the actual impedances for each electrode. For example, if the originally configured neutral was the RL lead and it has been determined to be in a lead off status, the controller looks to the next lead in the priority list which is the V2. The controller 118 further determines if the impedance on the next lead V2 is below the threshold impedance and, if so, the controller 118 reconfigures the V2 lead to be the neutral drive electrode and patient monitoring can recommence. Once an electrode is determined to be the new neutral, in this example V2, the controller 118 disconnects the current source 114 from the V2 electrode and signals the multiplexer to couple the V2 electrode with the neutral drive circuit 108. In another embodiment, if the first and second choice electrodes RL and V2, respectively, was determined to be lead off, the controller would select the RA electrode. However, as this is a primary electrode, use of this or any other primary electrodes (RA, LA, LL), requires the controller 118 to remove this electrode from being an input used in calculating the reference voltage. At any point in the future, should neutral drive electrode be reconfigured to be either the RL or V2 or any of the other V-leads (VI or V3 - V6), the controller 118 may reconfigure the RA (or an other of the primary leads) to be once again used to calculate the reference voltage.

Figure 4 is a block diagram of an alternate embodiment of a monitoring device 400 according to invention principles. The monitoring device 400 includes many of the components discussed above with respect to Figure 1 and are labeled with equivalent reference numerals indicating that these components operate in a similar manner. For purposes of ease and clarity, only the additional components in Figure 4 and their features will be discussed. The additional components shown herein are associated with detecting the condition of the electrode 102 when the controller 118 is executing the second condition detection algorithm. Instead of the controller 118 comparing the differential voltages of electrodes, the embodiment in Figure 4 employs a comparator 402 and a digital to analog converter (DAC) 404 which compares the voltage on electrode 102 with different voltage thresholds programmed on the DAC to determine if the impedance on the electrode 102 has reached the threshold impedance corresponding to a lead off condition. The voltage level in the DAC 404 that is indicative of a lead off condition may be determined in accordance with Equation 5 which states:

Zxhres = (Dv)/I (5) where Zxhres is the threshold impedance indicative of a lead off condition, Dy is the voltage level of the DAC 404 that is used by comparator 402 when comparing the voltage on electrode 102 and I is the current applied to electrode 102. For example, as shown herein V = IV and 1= 12nA resulting in Zxhres being substantially 70 mega ohms. While Zxhres indicative of lead off detection could be reduced by increasing the current applied, there are advantages to maintaining the current applied at a lower level. For example, the lower current applied minimizes saturation of the neutral drive electrode. Additionally, to ensure that the comparator 402 is able to detect lead off, the clamp voltage of the diode 122 must be above the lowest comparator threshold. Thus, the comparator 402 and DAC 404 may be used to determine if the condition of the electrode is lead off but cannot determine if the condition of the electrode is lead on. Therefore, this embodiment may be used as a first indicator for lead off indicating that the controller 118 may need to interrupt monitoring to execute the first detection algorithm and measure an impedance for all of the leads that may be on. In this embodiment, the controller 118 causes the switch 116 to move from a first open position to a second closed positioned causing a predetermined current to be applied to electrode 102 to create a voltage on the electrode 102. Instead of using that voltage to calculate an impedance, the voltage may be received at an input of the comparator 402 to determine a predefined threshold for an impedance which is too great for useful patient monitoring. The DAC 404 is used to determine the voltage threshold whereby a series of levels may be pre-stored in the DAC 404 that may correspond with an impedance indicative of a lead off condition. In the configuration described herein, the DAC 404 sets a threshold voltage that corresponds with the threshold impedance value discussed above in Figure 1. Thus, this circuit advantageously provides a course way to determine the lead impedance. For example, with a 6 bit DAC, 64 levels can be programmed as thresholds. By sequencing the DAC through each of the 64 bit levels, the voltage of the lead can be calculated to a granularity of 64 levels. Once this voltage is known, the impedance can be calculated as described in the previous methods.

Figures 5 - 8 depict one type of patient monitoring device described above in Figure 1 that may implement the present system. As shown herein, the patient monitoring device is an ECG monitor that senses electrophysiological signals from the heart of a patient to generate ECG data. The manner in which these signals sensed by the electrodes are used to generate ECG data are known and are not germane to the invention and therefore will not be discussed further. The present system advantageously detects the conditions of the electrodes connected to the patient to ensure that the signals sensed thereby are of a sufficient quality to generate ECG data for the patient.

Figure 5 is an exemplary circuit diagram that depicts the electrical connections of the ECG monitoring device after selection of a neutral drive electrode has been made. The ECG monitor includes a 6 lead electrode configuration for sensing signals from the patient' s heart. A plurality of electrodes 502a - f are designated as the RA, LA, LL, RL, VI and V2 electrodes, respectively. The electrodes 502a - c may be identified as primary electrodes RA, LA, and LL.. Electrode 502d may be identified as a neutral electrode RL, and electrodes 502e - f may be identified as secondary electrodes VI and V2. Electrodes 502a- f are shown coupled to the monitoring device 500. Although not shown, a person skilled in the art understands that each of electrodes 502a-f comprising a 6-lead ECG configuration are also connected to the patient in a known manner.

The electrodes 502a - 502f are coupled respective in pairs thereof to inputs of amplifiers 506a - 506f. A respective current source 514a - 514f is coupled to each electrode 502a -502f.. Each amplifier 506a - 506f outputs data representing a voltage differential between a respective pair of electrodes coupled there to on a particular channel CH 1 -CH 6 when a current from the respective current source 514a - 514f is applied to an electrode of the respective pair. Additionally one of the primary electrodes 502a - 502f is connected to an input of amplifier 505. A second input of amplifier 505 is connected so the amplifier may measure the actual voltage on the electrode connected thereto. The direct measurement of the voltage of the electrode connected to the amplifier 505 allows the controller to generate linear equations enabling determination of the voltages for each of the other primary electrodes which may be used to calculate contact impedances for the primary electrodes. As shown herein, electrode 502a is connected to the first input of amplifier 505 which measures the voltage on electrode 502a relative to ground upon application of current from current source 514a. Electrode 502a is also connected to amplifier 506a and amplifier 506c. Electrode 502b is connected to amplifier 506a and 506b and electrode 502c is connected to amplifier 502b and 502c. Thus, in addition to directly measuring the voltage on electrode 502a, the system can measure the voltage differentials between electrodes 502a and 502b, 502a and 502c, 502b and 502c. The connection of electrode 502a to amplifier 505 is shown for purposes of example only and any individual primary electrode may be directly connected to amplifier 505 to measure the actual voltage theron. One skilled in the art would readily understand how to reconfigure the circuit should a different electrode be connected to amplifier 505 in order to obtain the voltage differentials for each electrode pair.

Also connected to each of the primary electrodes 502a - 502c is reference voltage circuitry 520. The reference voltage circuitry that uses the average of the voltages sensed by electrodes 502a - 502c to generate the reference voltage provided to amplifier 522. The reference voltage is used to determine the voltage on the secondary electrodes 502e and 502f. Secondary electrodes 502e and 502f are coupled to respective first inputs of amplifiers 506e and 506f and have current sources 514e and 514f, respectively connected thereto. The reference voltage output by amplifier 522 is provided at respective second inputs of amplifiers 506e and 506f enabling a voltage differential to be calculated between the reference voltage and the voltage on electrodes 502e and 502f caused by the application of current from current sources 514e and 514f, respectively.

Electrode 502d is designated the neutral electrode and is shown coupled to a multiplexer 504d. The multiplexer connects electrode 502d to either the neutral drive circuitry 508 or to an amplifier 506d. When the multiplexer 504d connects electrode 502d with amplifier 506d the amplifier 506d can measure a voltage on electrode 502d when a current is applied thereto. When the multiplexer 504d connects the neutral drive circuit to the electrode, a current flows from the neutral drive circuit 508 through the electrode 502d and into the body thereby eliminating common mode noise and maintains the body at zero volts to allow for patient monitoring to occur. The multiplexer 504d switches the connection of electrode 502d from the neutral drive circuit 508 to the amplifier 506d in response to a control signal from a controller (not shown). While this figure depicts only a multiplexer 504d, it should be understood that this is for purpose of example only and in practice, each electrode is connected to a respective multiplexer 504 that can selectively couple the electrode to one of a respective amplifier or the neutral drive circuitry. This advantageously enables the monitoring device to reconfigure the system to connect any electrode as the neutral drive in response to determining that an impedance on the originally configured neutral drive electrode has reached or exceeded a threshold value indicating that the condition of the original neutral electrode is lead off.

While not shown, each of the current sources 514a - 514f and multiplexers on each electrode are coupled to a controller similar in structure and operation as discussed above in Figures 1 and 4. Thus, in operation, the system is able to determine condition data for each electrode when the controller causes currents to be applied from current sources to the electrodes in the manner described above in Figures 1 - 4. Upon determining the contact impedances associated with each electrode using the voltages measured by amplifiers 506a-f and comparing those voltages to threshold impedance values indicative of a lead off status, the system may automatically reconfigure the electrodes to select a new neutral drive electrode and modify the configuration of electrodes used for generating the reference voltage in order to maximize the signal quality. To ensure that the system is operating correctly, the reference voltage when applied to amplifier 507 and compared relative to ground on Channel 7 is converted and checked by the controller to see that the voltage is substantially zero volts (within +/- 200 mV) as long as the neutral drive electrode is not saturated. If the voltage is not substantially zero volts, it means that there is not enough current being applied to the body through the other electrodes connected to the patient. To remedy this, the controller 118 may control the current source to increase the current being applied to the body through the other electrodes. By increasing the current on the other leads, the current being removed from the body by the neutral drive is also increased to equal the total current being applied through the other leads. The impact of increasing the current applied to the body to drive the body to substantially zero volts is a decrease in the maximum impedance able to be measured on the respective electrodes. Alternatively, if the voltage on Channel 8 is substantially saturated, the controller will reconfigure the system to connect a different electrode as the neutral electrode. This enables the present system to determine the impedance of the neutral drive without having to saturate the neutral drive circuit and advantageously enables impedance calculation of the neutral electrode while that electrode is configured as the neutral electrode. In one embodiment, the difference in voltages between Channel 8 and Channel 7 provides the voltage of the neutral drive electrode. The determined voltage of the neutral drive electrode may then be divided by the current applied on all other electrodes to yield the impedance of the neutral drive electrode. The current used to calculate the neutral drive impedance is the total of the individual currents applied by the current sources on each electrode other than the neutral drive electrode. In one embodiment that includes a 6-lead ECG configuration as shown in Figure 5 five (5) electrodes other than the neutral electrode are provided with and each electrode has 12nA current applied by respective current sources. Thus, the total current would be 60nA. In another embodiment, that includes a three lead configuration, 2 electrodes other than the neutral drive electrode are present resulting in the total current being 24 nA. In a further embodiment that includes a 12 lead ECG configuration with 10 electrodes, the total current of 108 nA when summing the currents of each of the 9 electrodes present other than the neutral drive electrode. These currents determine the maximum impedance of the neutral drive able to be detected. There are certain other embodiments where voltages of only a select group of other electrodes are used in neutral drive impedance calculation. For example, if the system determines that a voltage on one or more of the electrodes is substantially equal to the clamp voltage as defined by the diode clamps (122 in Figs 1 and 4), that electrode is removed from the calculation and the total current would be derived from electrodes other than the neutral electrode and the electrode equaling or exceeding the clamp voltage.

The controller determines the condition of the secondary leads 502e and 502f by converting the voltage on Ch5 and Ch6, respectively. The reference voltage on Ch 7 is subtracted from each of the voltages on Ch 5 and Ch6, respectively, and the resulting voltages are divided by the current applied to each electrode to yield the actual impedance of VI and V2. This same method of calculating impedance for secondary leads 502e and 502f can also be used to determine the impedances for any additional secondary V leads when the system is in a 12 lead (10 electrode) configuration. Thereafter, the controller calculates the impedance on the primary electrodes. The voltage of the RA electrode is determined relative to ground by amplifier 505. Once the RA voltage is measured, the resulting voltage may be divided by the current to yield the RA electrode impedance. The controller uses the measured voltage of RA to generate and solve linear equations to yield the voltages of the other primary electrodes. In order to measure the other primary lead impedances the RA voltage is used in Equations 2 - 4 discussed above after the reference voltage is subtracted from the measured RA voltage. This occurs because RA is recorded relative to ground, we do not have to subtract a reference signal from RA. The measured voltage on Channel 0 is simply RA (no subtraction needed).

Upon determining the impedances of each electrode 502a - 502f, the system uses these impedances to detect the condition of the electrode to identify whether the electrode is in lead off status. The following discussion illustrates exemplary reconfiguration that may occur when certain electrodes are determined to be in lead off condition. In one embodiment, if the neutral drive electrode 502d is not substantially zero volts but the reference voltage is substantially zero volts, the controller determines that the impedance on the originally configured neutral drive is too high thus requiring reconfiguration of the neutral drive electrode. In this instance, the controller interrupts patient monitoring to initiate the first condition detection algorithm to obtain precise impedance measurements for each of the other electrodes and determine which other electrode should be configured as the new neutral drive electrode. For example, the controller, after determining actual impedance values for the electrodes when patient monitoring is not ongoing, may select a new neutral according to a predetermined order of other electrodes and use the highest electrode in the predetermined order having the lowest actual impedance. One exemplary predetermined order may be, in descending order of priority, the RL, V2, VI, RA, LA, LL, where RL is the highest priority and thus most desirable electrode to be designated the neutral drive electrode.

An exemplary circuit diagram showing a reconfigured system is shown in Figure 6 which shows that the electrode 502e (VI) has been determined to have an impedance below the threshold and RL and V2 were determined to have an impedance that exceeds the threshold and is classified as Lead Off. Thus, electrode 502e was the electrode with the next highest priority and is selected as the new neutral electrode. To accomplish this, the controller signals the multiplexer 504d (in Fig. 5) of the originally configured neutral drive to decouple electrode 502d from the neutral drive circuitry 505. The controller further signals multiplexer 504e connected to electrode 502e to decouple electrode 502e from amplifier 506e and couple electrode 502e to the neutral drive circuit 505.

Another exemplary circuit diagram showing a reconfigured system is shown in Figure 7. Figure 7 shows that electrode 502d and 502e have impedances that exceed the threshold impedance level resulting in electrode 502a being selected as the new neutral drive electrode as it has an impedance below the threshold and is the next highest priority electrode. The controller signals the multiplexer 504d (in Fig. 5) which connected the originally configured neutral drive to decouple electrode 502d from the neutral drive circuitry 508. The controller further signals multiplexer 504a connected to electrode 502a to decouple electrode 502a from amplifier 506a and couple electrode 502a to the neutral drive circuit 508. In this instance when a primary lead is selected as the new neutral lead, the controller also reconfigures the circuitry used to determine the reference voltage 522. Because electrode 502a is the new neutral drive electrode, the voltage thereon cannot be used into the reference voltage calculation. Thus, the controller disconnects electrode 502a from the reference voltage calculation by causing switch 521a to move from a first closed position to a second open position. In this embodiment, the reference voltage would be derived from electrodes 502b and 502c which, as shown herein, are connected respectively by switches 521b and 521c which are shown in the closed position (dotted lines) to the reference voltage calculator 522.

The reconfigurations shown in Figures 6 and 7 are described for purposes of example only and reconfiguration of the neutral drive to any electrode 502a - 502f is possible.

Upon reconfiguration and selection of the new neutral drive electrode, the controller can reinitiate ECG monitoring and reinitiate the second condition detection algorithm to continually check and update electrode condition data. In the event that condition of electrode 502d (RL) changes from lead-off, the controller may interrupt monitoring again to re-measure actual impedances to see if the neutral drive should be reconfigured back to be on electrode 502d.

Figure 8 is another embodiment of the system that operates in accordance with the same principles discussed above with respect to Figures 1 - 7. In this embodiment, a second multiplexer 702a - 702f is positioned on each of channels 1 - 6. The second multiplexer 702a - 702f is connected between respective first multiplexers 504a - 504f and amplifiers 506a - 506f. The second multiplexers 702a - 702f connect respective electrodes 502a - 502f via respective first multiplexers 504a - 504f to amplifiers 506a - 506f. Second multiplexers 702a - 702f may also connect each respective amplifier 506a - 506f to one of ground and to the first electrode 502a. This is advantageous because it simplifies the circuit resulting in one less amplifier channel and its associated analog-to-digital converter.

Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention. This disclosure is intended to cover any adaptations or variations of the embodiments discussed herein.

Claims

CLAIMS What is claimed is:

1. An apparatus that detects a condition of an electrode on a patient being used to monitor patient parameter data, the apparatus comprising:

a first circuit including a first electrode, a first amplifier and a first multiplexer;

a second circuit including a second electrode, a second amplifier and a second multiplexer;

a current source selectively connected to at least one of the first electrode and the second electrode;

a neutral drive circuit connected to the first multiplexer and the second multiplexer; and

a controller connected to the current source, the first multiplexer and second multiplexer, wherein

in a first mode, the controller controls the first and second multiplexer to connect the first and second electrodes with respective first and second amplifiers and the current source to detect a condition of the first and second electrodes to determine which one of the first and second electrodes is to be connected to the neutral drive circuit, and

in a second mode of operation, the controller controls the one of the first and second multiplexers associated with the one of the first and second electrodes determined to be connected to the neutral drive circuit to connect the one of the first and second electrodes with the neutral drive circuit and controls the other of the first and second multiplexer to connect the other of the first and second electrodes with a respective amplifier thereby enabling monitoring of at least patient parameter, the controller controls the current source to apply a current to the other of the first and second electrodes to detect a condition of the other of the first and second electrode while the at least one patient parameter is being monitored.

2. The apparatus as recited in claim 1 , wherein in the first mode of operation, the controller detects the condition of the first and second electrode by causing the current source to apply a current to the first and second electrodes enabling the amplifier to measure a voltage on the first and second electrodes for determining an impedance of the first and second electrode and compares the impedances of the first and second electrodes to a threshold to identify the condition is present on the first and second electrode in response to determining that the impedances one of equal the threshold or exceeds the threshold.

3. The apparatus as recited in claim 1, wherein

in the second mode of operation, the controller detects the condition of the other of the first and second electrode by measuring, at the respective amplifier of the other of the first and second electrode, a voltage on the other of the first and second electrodes to identify the condition is present on the other of the first and second electrodes in response to determining if an impedance is at least one of equal to or exceeds a threshold .

4. The apparatus as recited in claim 1 , wherein

the controller interrupts patient monitoring and automatically reinitiates the first mode of operation in response to determining that the other of the first and second electrode is connected to the patient.

5. The apparatus as recited in claim 1, wherein

the controller detects the condition of the one of the first and second electrodes connected to the neutral drive circuit by measuring a difference between a voltage associated with the neutral drive circuit and a reference voltage and comparing the determined difference to a threshold to determine if the neutral drive electrode is connected to the patient.

6. The apparatus as recited in claim 5, wherein

the controller interrupts patient parameter monitoring and reinitiates the first mode of operation in response to detecting the one of the first and second electrodes is not connected to the patient.

7. The apparatus as recited in claim 6, wherein the controller, in the first mode of operation, measures an actual impedance of the one of the first and second electrode to determine if the actual impedance exceeds a maximum impedance associated with the electrode and automatically selects another electrode to be connected to the neutral drive circuit.

8. The apparatus as recited in claim 7, wherein

the controller reinitiates patient parameter monitoring for a predetermined period of time in response to determining that the actual impedance of the one of the first and second electrodes has not exceeded the maximum impedance threshold.

9. The apparatus as recited in claim 1, further comprising

a plurality of circuits, each circuit including an electrode, a multiplexer and an amplifier and said current source is selectively connectable to each electrode.

10. The apparatus as recited in claim 9, wherein

in the second mode of operation, the controller connects the current source to all electrodes not determined to be connected to the neutral drive circuit enabling detection of the condition of all electrodes not determined to be connected to the neutral drive circuit.

11. The apparatus as recited in claim 10, wherein

the controller updates a status of each electrode based on the determined condition of the electrodes.

12. A method of detecting a condition of an electrode on a patient being used to monitor patient parameter data comprising the activities of:

detecting, in a first mode of operation, a condition associated with respective ones of a plurality of electrodes selectively coupled to a patient;

selecting and configuring a respective one of the plurality of electrodes as a neutral drive electrode based on the detected condition; enabling monitoring of at least one patient parameter in response to the selection and configuration of the neutral drive electrode;

detecting, in a second mode of operation, the condition of the neutral drive electrode and others of the plurality of electrodes to determine if reconfiguration of the neutral drive electrode is required based on the detected condition in the second mode of operation.

13. The method as recited in claim 12, wherein the activity of detecting in the first mode includes

connecting respective one of the plurality of electrodes with respective amplifiers and a current source to detect a condition of the first and second electrodes;

determining which one of the respective electrodes is to be connected to a neutral drive circuit;

configuring the determined electrode as the neutral drive circuit.

14. The method as recited in claim 12, wherein the activity of detecting in the second mode includes

connecting the determined electrode to the neutral drive circuit; connecting others of the plurality of electrodes to a respective amplifier thereby enabling monitoring of at least patient parameter,

applying current from a current source to the neutral drive electrode and the others of the plurality of electrodes to detect a condition of the other of the first and second electrode while the at least one patient parameter is being monitored.

15. The method as recited in claim 13, wherein

in the first mode of operation, further comprising the activities of detecting the condition of the respective electrodes by applying a current thereto;

measuring at respective amplifiers, a voltage on the plurality of electrodes;

determining an impedance of the plurality of electrodes; and the comparing the determined impedances of the plurality of electrodes to a threshold to identify the condition is present on any of the plurality of electrodes in response to determining that the impedances one of equal the threshold or exceeds the threshold.

16. The method as recited in claim 14, wherein

in the second mode of operation, further comprising the activities of

detecting the condition of the others of the plurality of electrodes by measuring a voltage, at respective amplifiers of the others of the plurality of electrodes;

determining that the condition is present on the others of the plurality of electrodes in response to determining if an impedance is at least one of equal to or exceeds a threshold.

17. The method as recited in claim 12, further comprising the activity of

interrupting patient monitoring and automatically reinitiating the first mode of operation in response to determining that one of the plurality of electrodes is not connected to the patient.

18. The method as recited in claim 12, further including

detecting the condition of the electrode connected to the neutral drive circuit by measuring a difference between a voltage associated with the neutral drive circuit and a reference voltage;

comparing the determined difference to a threshold to determine if the neutral drive electrode is connected to the patient.

19. The method as recited in claim 18, wherein

interrupting patient parameter monitoring and reinitiating the first mode of operation in response to detecting the one of the plurality of electrodes is not connected to the patient.

20. The method as recited in claim 19, wherein in the first mode of operation, further comprising the activity of measuring an actual impedance of the one of the first and second electrode to determine if the actual impedance exceeds a maximum impedance associated with the electrode and automatically selects another electrode to be connected to the neutral drive circuit.

21. The method as recited in claim 20, further comprising

reinitiating patient parameter monitoring for a predetermined period of time in response to determining that the actual impedance of the one of the plurality electrodes has not exceeded the maximum impedance threshold.

22. The method as recited in claim 12, wherein