WO2005117699A1

WO2005117699A1 - Method and apparatus for monitoring a sedated patient

Info

Publication number: WO2005117699A1
Application number: PCT/NO2005/000183
Authority: WO
Inventors: Hanne Storm
Original assignee: Hanne Storm
Priority date: 2004-06-04
Filing date: 2005-06-02
Publication date: 2005-12-15
Also published as: NO20042324D0

Abstract

The invention relates to a method and an apparatus for monitoring a sedated patient, in particular in order to establish an output signal which indicates the state of awakening in the patient. The method comprises the steps of providing a skin conductance signal measured at an area of the patient's skin, calculating an average value of the amplitude of fluctuation peaks in the skin conductance signal through a time interval, comparing the average value with a limit value, and establishing an output signal which indicates the state of awakening in the patient if the average value exceeds the limit value.

Description

Method and apparatus for monitoring a sedated patient

Technical field

The invention relates in general to medical technology, and in particular to a method and an apparatus for monitoring patients during surgery and general anaesthesia.

Background of the invention

During surgery it is very important to observe the patient's level of consciousness and awareness. Few reliable methods of observation exist today. In the field of medical technology there is a problem in producing physical measurements representing the activity in an individual's autonomous nervous system, i.e. in the part of the nervous system, which is beyond the control of the will.

Particularly, there is a special need to monitor the autonomous nervous system of a sedated, non-verbal patient, e.g. a patient in anaesthesia or an artificially ventilated patient, in order to detect if the patient needs more hypnotics because of awakening stimuli. Tests have shown that the skin's conductance changes as a time variable signal which, in addition to a basal, slowly varying value (the so-called basal level or the average conductance level through a certain interval), also has a component consisting of spontaneous waves or fluctuations. When an outgoing sympathetic nervous burst occurs to the skin, the palmar and plantar sweat glands are filled up, and the skin conductance increases before the sweat is removed and the skin conductance decreases. This creates a skin conductance fluctuation, and the number of such rapid fluctuations per time unit may be expressed as a score: number of skin conductance fluctuations. An increase in the number of skin conductance can be interpreted as increased activity in this part of the sympathetic nervous system. Unlike heart rate and blood pressure changes, emotional sweating is not influenced by circulatory changes. Number of skin conductance fluctuation changes may therefore be more specifically linked to pain responses than blood pressure and heart rate, at least in patients that are circulatory unstable such as in patients with changes in blood volume, cardiac contractility and with changes in drug effects on the cardiovascular system. Acid-base disturbances may also attenuate haemodynamic responses to noxious stimuli. Moreover, the amplitude of the fluctuations correlates to the strength of the sympathetic nerve burst. If the patient wakes up during surgery, this is followed by a much stronger sympathetic nervous system activity than if the sedated patient is exposed to discomfort stimuli such as intubation for artificial ventilation. Therefore, during awakening the amplitudes of the fluctuations will increase. Related background art

WO-03/94726 discloses a method and an apparatus for monitoring the autonomous nervous system of a sedated patient. In the method, a skin conductance signal is measured at an area of the patient's skin. Certain characteristics, including the average value of the skin conductance signal through a time interval and the number of fluctuation peaks through the interval, is calculated. Based on these characteristics, two output signals are established, indicating pain discomfort and awakening in the patient, respectively. The awakening signal is established based on the number of fluctuations and the average value through an interval.

Summary of the invention

An object of the present invention is to provide a method and an apparatus for monitoring a sedated patient, in particular a method and an apparatus for establishing a signal indicating the state of awakening in the patient, based on skin conductance measurements.

Another object of the present invention is to provide such a method and apparatus which provides reliable output indications.

Another object of the present invention is to provide such a method and apparatus which do not rely on the calculating of the number of fluctuation peaks or average value of the skin conductance signal through any measurement interval.

The above and additional objects are obtained by a method as set forth in the appended independent claim 1 and by the apparatus as set forth in the appended independent claim 6. A corresponding method for establishing an output signal in an apparatus for monitoring a sedated patient is set forth in the independent claim 1 1.

Further advantages are achieved by the preferred embodiments set forth in the dependent claims.

Brief description of the drawings

The invention will now be described in more detail with reference to the attached drawings, in which

Figure 1 is a block diagram for an apparatus according to the invention, and

Figure 2 is a flow chart illustrating a method according to the invention.

Detailed description of the invention

Figure 1 illustrates a block diagram for a preferred embodiment of an apparatus according to the invention. Substantial parts of the apparatus' hardware structure is previously described in the Applicant's related international patent application WO- 03/94726, with particular reference to the block diagram in fig. 1 and the corresponding, detailed description. The disclosure of this publication, and the hardware structure and hardware components in particular, is hereby expressly incorporated by reference.

Figure 1 illustrates a block diagram for a preferred embodiment of an apparatus according to the invention. On an area 2 of the skin on a body part 1 of the patient, sensor means 3 are placed for measuring the skin's conductance. The body part 1 is preferably a hand or a foot, and the area 2 of the skin on the body part 1 is preferably the palmar side of the hand (in the palm of the hand) or the plantar side of the foot (under the sole of the foot). The sensor means 3 comprise contact electrodes where at least two electrodes are placed on the skin area 2. In a preferred embodiment the sensor means 3 consist of three electrodes: a signal electrode, a measuring electrode and a reference voltage electrode, which ensures a constant application of voltage over the stratum corneum (the surface layer of the skin) under the measuring electrode. The measuring electrode and the signal electrode are preferably placed on the skin area 2. The reference voltage electrode may also be placed on the skin area 2, but it is preferably placed in a nearby location, suitable for the measuring arrangement concerned.

In a preferred embodiment an alternating current is used for measuring the skin's conductance. The alternating current advantageously has a frequency in the range of up to 1000 Hz, corresponding to the area where the skin's conductance is approximately linear. A frequency should be selected which ensures that the measuring signal is influenced to the least possible extent by interference from, e.g., the mains frequency. In a preferred embodiment the frequency is 88 Hz. A signal generator, operating at the specified frequency, applies a signal current to the signal electrode.

In the case of alternating current the conductance is identical to the real part of the complex admittance, and therefore not necessarily identical with the inverse value of the resistance. An advantage of using alternating current instead of direct current in conductance measurement is that by this means one avoids the invidious effect on the measurements of the skin's electrical polarizing properties.

The resulting current through the measuring electrode is conveyed to a measurement converter 4. This comprises a current to voltage converter, which in a preferred embodiment is a transresistance amplifier, but in its simplest form may be a resistance, which converts the current from the measuring electrode to a voltage.

The measurement converter further comprises a decomposition circuit, preferably in the form of a synchronous rectifier, which decomposes the complex admittance in a real part (the conductance) and an imaginary part (the susceptance). However, it is sufficient if the decomposition circuit only comprises means for deriving the conductance. The synchronous rectifier multiplies the measured voltage with the voltage from the signal generator. The two signals are in-phase. After multiplication, the result is according to the cosine (2u) equation, where the result is a DC component and one component at 2u frequency. In the preferred embodiment, this becomes 176 Hz. In the preferred embodiment, this synchronous rectifier is realized as an analog circuit with the required accuracy.

The measurement converter 4 may also comprise amplifier and filter circuits. In the preferred embodiment the measurement converter contains low-pass filters, both at the input and at the output. The object of the input low-pass filter is to attenuate high-frequency noise, for instance coming from other medical equipments, and also to serve as anti-aliasing filter to prevent high frequency components from being received by subsequent circuits for time discretization. The output low-pass filter shall attenuate the 2u components that result from the multiplication operation in the synchronous rectifier so that only the signal near DC is used for further processing.

By means of the choice of components and design details, moreover, the measurement converter is designed with a view to obtaining high sensitivity and a low noise level.

The control unit 5 comprises a time discretization unit 51 for time discretization of the signal from the measurement converter. The time discretization takes place at a sampling rate, which may advantageously be in the order of 20 to 200 samplings per second. The control unit further comprises an analog-digital converter 52, which converts measurement data to digital form. The choice of circuits for time discretization and analog-digital conversion implies technical decisions suitable for a person skilled in the art. In the preferred embodiment, time discretization is done in an integrated circuit, which combines oversampling, filtering and discretization.

The control unit may advantageously comprise additional analog and possibly also digital inputs (not illustrated), in addition to the input from the measurement converter 4. In this case the control unit 5 can either be equipped with a plurality of analog-digital converters 52, or it can employ various multiplexing techniques well- known to those skilled in the art in order to increase the number of analog inputs. These additional analog inputs may, for example, be arranged for additional electrodermal measurements, or for other physiological measurements which may advantageously be performed simultaneously or parallel with the electrodermal measurement, such as temperature, pulse, ECG, respiratory measurements, oxygen saturation measurements in the blood, or EEG (bispectral index).

The control unit 5 also comprises a processing unit 53 for processing the digitized measurement data, storage means in the form of at least one store for storing data and programs, illustrated as a non-volatile memory 54 and a random access memory 55. The control unit 5 further comprises an interface circuit 61 , which provides output signals 71 , 72. Preferably, the control unit 5 further comprises a further interface circuit 81 , which is further connected to display unit 8. The control unit 5 may also advantageously comprise a communication port 56 for digital communication with an external unit, such as a personal computer 10. Such communication is well-suited for loading or altering the program which is kept stored in the memory 54, 55 in the control unit, or for adding or altering other data which are kept stored in the memory 54, 55 in the control unit. Such communication is also well suited for read-out of data from the memory 54, 55 in the apparatus, thus enabling them to be transferred to the external computer 10 for further, subsequent analysis or storage. A communication port 56 in the control unit will be advantageously designed in accordance with requirements for equipment safety for patients, as described in more detail below.

In a preferred embodiment the non-volatile memory 54 comprises a read-only storage in the form of programmable ROM circuits, containing at least a program code and permanent data, and the random access memory 55 comprises a read and write storage in the form of RAM circuits, for storage of measurement data and other provisional data.

The control unit 5 also comprises an oscillator (not shown), which delivers a clock signal for controlling the processing unit 53. The processing unit 53 also contains timing means (not shown) in order to provide an expression of the current time, for use in the analysis of the measurements. Such timing means are well-known to those skilled in the art, and are often included in micro controllers or processor systems which the skilled person will find suitable for use with the present invention.

The control unit 5 may be realized as a microprocessor-based unit with connected input, output, memory and other peripheral circuits, or it may be realized as a micro controller unit where some or all of the connected circuits are integrated. The time discretization unit 51 and/or analog-digital converter 52 may also be included in such a unit. The choice of a suitable form of control unit 5 involves decisions, which are suitable for a person skilled in the art.

An alternative solution is to realize the control unit as a digital signal processor (DSP).

Thus, structural hardware components that are corresponding in the present invention and in WO-03/94726 include the measurement converter 4; which in a preferred embodiment may include a synchronous rectifier and a low pass filter; which converts the measured signal into a voltage. This voltage is further sent to control unit 5; which includes time discretization module 51 and analog-digital converter 52, which converts measurement data to digital form. The choice of circuits for time discretization and analog-digital conversion implies technical decisions suitable for a person skilled in the art. In the preferred embodiment, time discretization is done in an integrated circuit, which combines oversampling, filtering and discretization.

The data processing unit 53 is arranged for analysing the measured and digitized signal provided by the A/D converter 52. The signal is analysed in order to extract different types of information.

The control unit 5 is arranged to read time-discrete and quantized measurements for the skin conductance from the measurement converter 4, preferably by means of an executable program code, which is stored in the non-volatile memory 54 and which is executed by the processing unit 53. It is further arranged to enable measurements to be stored in the read and write memory 55. By means of the program code, the control unit 5 is further arranged to analyze the measurements in real time, i.e. simultaneously or parallel with the performance of the measurements. The method or process performed by the control unit 5, in order to analyze the skin conductance signal, is distinctive and substantially different from the method/process disclosed in WO-03/94726.

In this context, simultaneously or parallel should be understood to mean simultaneously or parallel for practical purposes, viewed in connection with the time constants which are in the nature of the measurements. This means that input, storage and analysis can be undertaken in separate time intervals, but in this case these time intervals, and the time between them, are so short that the individual actions appear to occur concurrently.

The control unit 5 is further arranged to identify the fluctuations in the time- discrete, quantized measuring signal, by means of a program code portion which is stored in the non-volatile memory 54 and which is executed by the processing unit

53.

The control unit 5 is further arranged to calculate the amplitude of the fluctuation peaks in the time-discrete, quantized measuring signal during a time interval, by means of a program code portion which is stored in the non-volatile memory 54 and which is executed by the processing unit 53.

The processing unit 53, the memories 54, 55, the analog/digital converter 52, the communication port 56, the interface circuit 81 and the interface circuit 61 are all connected to a bus unit 59. The detailed construction of such bus architecture for the design of a microprocessor-based instrument is regarded as well-known for a person skilled in the art. The interface circuit 61 is a digital port circuit, which derives output signals 71 , 72 from the processing unit 53 via the bus unit 59 when the interface circuit 61 is addressed by the program code executed by the processing unit 53.

The first output signal 71 indicates that the analysis of the skin conductance measurement has detected that the patient is receiving awakening stimuli and may need more hypnotics. The second output signal 72 may be used for future enhancement of the system, explained later.

In a special application of the invention the warning signals 71 , 72 or another signal derived from the processing means in the analysis of the skin conductance measurements may be used to control an automatic administration of a medication to the patient. Particularly, the administration of a sleep-inducing medication may be controlled by the first signal 71 indicating awakening. Each of the signals may be used, for example, to control a device for intravenous supply of medication. In this case the invention will form part of a feedback loop for control of the activity in the patient's autonomous nervous system.

In a preferred embodiment the display means 8 consist of a screen for graphic visualization of the conductance signal, and a digital display for displaying the frequency and amplitude of the measured signal fluctuations. The display units are preferably of a type whose power consumption is low, such as an LCD screen and LCD display. The display means may be separate or integrated in one and the same unit.

The apparatus further comprises a power supply unit 9 for supplying operating power to the various parts of the apparatus. The power supply may be a battery or a mains supply of a known type.

The apparatus may advantageously be adapted to suit the requirements regarding hospital equipment, which ensures patient safety. Such safety requirements are relatively easy to fulfill if the apparatus is battery-operated. If, on the other hand, the apparatus is mains operated, the power supply shall meet special requirements, or requirements are made regarding a galvanic partition between parts of the apparatus (for example, battery operated), which are safe for the patient and parts of the apparatus, which are unsafe for the patient. If the apparatus has to be connected to external equipment, which is mains operated and unsafe for the patient, the connection between the apparatus, which is safe for the patient and the unsafe external equipment requires to be galvanically separated. Galvanic separation of this kind can advantageously be achieved by means of an optical partition. Safety requirements for equipment close to the patient and solutions for fulfilling such requirements in an apparatus like that in the present invention are well-known to those skilled in the art. Figure 2 illustrates a flow chart for a method for controlling a warning signal in an apparatus for monitoring the autonomous nervous system of a sedated patient, and especially for detecting stress or discomfort and awakening.

The method starts at reference 31.

The first process step 32 is an initial step, establishing initial values for use in the remaining, repeated process steps.

In the first step 32, a skin conductance signal or EDR (electrodermal response) signal is measured, time-quantized and converted to digital form using the equipment described with reference to fig. 1. An initial time-series of a certain duration, typically a period of 20 seconds, containing skin conductance data, is acquired during this step. With a sampling rate of 20 - 200 samples per second, the time-series may contain 400 - 4000 samples.

In step 33, the skin conductance signal is measured, time-quantized and converted to digital form in the same way as in step 32. The amplitude of the fluctuation peaks in the conductance signal through the current time-series is then calculated. This is performed by detecting local peaks or local maximum values and by detecting local valleys or local minimum values and then calculating the amplitude as the difference in skin conductance between a peak and a valley. The mean amplitude for the time interval is then calculated based on the average of the single amplitudes that are measured in the time interval.

The existence of a peak is established if the derivative of the signal changes sign through a small period in the interval. The derivative of the signal is calculated as the difference between two subsequent sample values. In addition, it is possible to use a simple digital filter that needs to see two or more subsequent sign changes before the sign change is accepted.

In the calculation step 33 it may be necessary to establish additional criteria for when a signal amplitude should be considered as valid. In their simplest form such criteria may be based on the fact that the signal amplitude has to exceed an absolute limit in order to be able to be considered a valid fluctuation. A recommended, such limit value for the conductance is 0.02 μS.

Alternatively or in addition, it is an advantage to base the criteria on the fact that the signal actually has formed a peak that has lasted a certain time. The criteria may also be based on the fact that the increase in the skin conductance signal value as a function of time must remain below a certain limit, typically 20 μS/s, if the maximum value is to be considered valid. Another possible condition for establishing a valid peak is that the absolute value of the change in the conductance signal from a local peak to the following local valley exceeds a predetermined value, such as 0.02μS.

Also, a maximum value appearing at the border of the interval, i.e. the starting point or ending point of the interval should preferably not be regarded as a valid peak.

The object is thereby achieved that artifacts, which can occur in error situations such as, e.g., electrodes working loose from the skin, or other sources of noise or disturbances, does not lead to the erroneously detection of peaks.

The mean amplitude of the fluctuations calculated in step 33 is stored and used as the amplitude during the execution of the comparison step 35 below.

The step 34 is optional and will be explained later in the description.

The purpose of the following steps 35 - 37 is to realize the following functions:

In step 35, the calculated mean amplitude is compared against a preset limit value. The Applicant's tests have shown that a suitable limit value is 0,05 uS. Other values could possible be determined from clinical tests, in order to further optimize the performance and reliability of the output indications. If the mean amplitude, calculated in step 33, is above the reference value (output denoted Y), this indicates that the patient is receiving awakening stimuli and may need more hypnotics. The process continues to step 36, where the output signal 71 is activated.

If the calculated mean amplitude is equal to or lower than the preset limit value, (output denoted N), the process is continued at step 37, where the output signal 71, if previously activated, is reset.

The process is then repeated from step 33.

The process may be interrupted or terminated by an operating device (not shown) or by a command input from the communication port 56.

A first improvement to the method illustrated in figure 2 will be described in the following:

In the comparison step 35 in fig. 2, the current mean amplitude value is compared with a preset limit value. Even more reliable results may be achieved for the awakening indications if this comparison is modified. In the modified comparison step 35, the current mean amplitude is compared with the preset limit value and with the previous mean amplitude. If the current mean amplitude is larger than both the limit value and the previous mean amplitude, the process continues to step 36. If on the other hand the mean amplitude is equal to or less than the limit value or the previous mean amplitude, or both, the process continues to step 37. In order to perform this extended comparison, an additional step 34 should be performed subsequent to step 33, wherein the mean amplitude in the conductance signal, which is calculated in step 34, is stored and used as the "preset limit value" in the comparison step 35.

A second improvement to the embodiment illustrated in fig 2 will be described in the following:

In the embodiment in figure 2, a time-series is first acquired and subsequently analyzed. As an advantageous alternative, data acquisition and analysis are performed as separate, independent processes, concurrently executed by the processing unit 53.

A data acquisition process is then performed, which virtually continuously updates a portion of the memory 55 with the latest e.g. 20 seconds of skin conductance signal values.

An analysis process is initiated e.g. every 1 second. This process will analyze the latest e.g. 20 seconds of skin conductance data, acquired by the concurrently executed data acquisition process. All the process steps 33-37 are performed by the analysis process, while the initial process step 32 is performed in advance, as initial step.

This solution leads to an even faster and more reliable indication of awakening, compared to the simpler method described with reference to figure 2.

Another improvement to the embodiment illustrated in fig 2 will be described in the following:

In addition to the calculation in step 33, where the mean amplitude of the fluctuation peaks in the conductance signal is calculated, it is also possible to calculate the average conductance level. As explained in WO-03/094726, this level indicates that a state of awakening has occurred in the patient. This information may be used to activate output signal 72 and it may also be used as part of the comparison step 35 in order to verify the output signal 71.

The above description and drawings present a specific embodiment of the invention. It will be obvious to the skilled person that alternative or equivalent embodiments exist within the scope of the present invention. For instance, the use of skin impedance instead of skin conductance will of course lead to equivalent results, if the inverse nature of these variables is taken into account.

When the term "patient" is used throughout the specification and claims, is should be appreciated that although the present invention is primarily directed towards the monitoring of human beings, the invention has also been proven to be applicable for monitoring animals, in particular mammals. Consequently, the term "patient" should be interpreted as covering both human and animal patients.

Claims

1. Method for monitoring a sedated patient, comprising providing a skin conductance signal measured at an area of the patient's skin, calculating an average value of the amplitude of fluctuation peaks in the skin conductance signal through a time interval, comparing the average value with a limit value, and establishing an output signal which indicates the state of awakening in the patient if the comparing step indicates that the average value exceeds the limit value.

2. Method according to claim 1, wherein said limit value is a predetermined value in the area 0.01 to 0.2 uS, preferably about 0.05 uS.

3. Method according to claim 1 or 2, further comprising the step of selecting the limit value as the larger of

- a predetermined value in the area 0.01 to 0.2 uS, preferably about 0.05 uS, and

- a recently calculated average value of the amplitude of fluctuation peaks.

4. Method according to one of the preceding claims, wherein said steps of providing a skin conductance signal and said step of calculating the average value of the amplitude of fluctuation peaks in the skin conductance signal are performed concurrently in the time interval.

5. Method according to one of the claims 1 -4, wherein said calculating step, said comparing step and said establishing step are performed by a data processing unit included in an apparatus for monitoring a sedated patient.

6. Apparatus for monitoring a sedated patient, comprising measurement equipment for providing a skin conductance signal measured at an area of the patient's skin, and a control unit, arranged for calculating an average value of the amplitude of fluctuation peaks in the skin conductance signal through a time interval, comparing the average value with a limit value, and establishing an output signal which indicates the state of awakening in the patient if the comparing step indicates that the average value exceeds the limit value.

7. Apparatus according to claim 6, wherein said limit value is a predetermined value in the area 0.01 to 0.2 uS, preferably about 0.05 uS.

8. Apparatus according to claim 6 or 7, wherein said control unit is further arranged for selecting the limit value as the larger of a predetermined value in the area 0.01 to 0.2 uS, preferably about 0.05 uS, and a recently calculated average value of the amplitude of fluctuation peaks.

9. > Apparatus according to one of the claims 6 to 8, wherein said control unit is further arranged for providing the skin conductance signal and calculating the average value of the amplitude of fluctuation peaks in the skin conductance signal as concurrent subprocesses, executed in the time interval.

10.. Apparatus according to one of the claims 6-9, wherein said measurement equipment comprises skin electrodes and a measurement converter, and wherein said control unit comprises a time discretization unit, an A/D converter, a processing unit, and a memory.

1 1. Method for establishing an output signal in an apparatus for monitoring a sedated patient, said method comprising providing a skin conductance signal measured at an area of the patient's skin, calculating an average value of the amplitude of fluctuation peaks in the skin conductance signal through a time interval, comparing the average value with a limit value, and establishing an output signal which indicates the state of awakening in the patient if the comparing step indicates that the average value exceeds the limit value.

12. Method according to claim 1 1, wherein said limit value is a predetermined value in the area 0.01 to 0.2 uS, preferably about 0.05 uS.

13. Method according to claim 1 1 or 12, further comprising the step of selecting the limit value as the larger of

- a recently calculated average value of the amplitude of fluctuation peaks.

14. Method according to one of the preceding claims 1 1 -13, wherein said steps of providing a skin conductance signal and said step of calculating the average value of the amplitude of fluctuation peaks in the skin conductance signal are performed concurrently in the time interval.

15. Method according to one of the claims 1 1-14, wherein said calculating step, said comparing step and said establishing step are performed by a data processing unit included in said apparatus.