CN101484068A - Wearable Blood Pressure Monitoring System - Google Patents

Abstract

A system for measuring/monitoring a patient's vital signs, particularly blood pressure, includes: at least a plurality of electrodes disposed in an underwear waistband, and means for deriving the measurement value from the electrodes using pulse conduction time.

Description

Wearable Blood Pressure Monitoring System

The present invention relates to a monitoring system adapted to continuously or at least periodically monitor vital signs of a subject, and in particular to a system for measuring blood pressure.

Cardiovascular diseases such as myocardial infarction, congestive heart failure or hypertension have an increasing impact on mortality and morbidity in all developed countries. Consequently, there has been an increasing need for long-term continuous monitoring of patient vital signs, which provides the opportunity to evaluate the performance of the cardiovascular system. In the past, various blood pressure measurement systems were commonly used that required a compression cuff or similar device that had to be specifically attached to the patient when the measurement was to be taken, and that required suitable and skilled clinicians to operate them. Thus, such devices are generally limited to use at the doctor's premises or in hospitals, eg they are not suitable for continuous or periodic monitoring purposes.

Therefore, the present invention aims to provide a blood pressure measurement system. The system employs the pulse transit time method for deriving measurements from detected signals such as an electrocardiogram, but the system is also used to monitor other vital signs. It is particularly suitable for implementation in continuously wearable underwear integrated with measurement sensors or electrodes for direct contact with the subject's skin. Preferably, the garment comprises at least 4 electrodes in order to allow PTT measurements without additional connections to the patient's body.

Preferably, the sensor is one that does not require a specific attachment system, gel or paste to make a proper electrical contact, for example, they could be a recently developed dry electrode made of conductive rubber that only Relying on the natural secretion of sweat can form a conductive bridge between the skin and the electrodes. Preferably, the underwear comprises panties having built-in electrodes at least in the waistband region.

Preferably, the electrodes are arranged to measure the transmission of the pulse through the central artery, and the left and right femoral arteries, as well as the ECG. The system can also be arranged to monitor the body temperature, posture and activity level of the subject.

Preferably, the pulse detection is realized by using a bioimpedance method by injecting a little alternating current with the first electrode pair, and detecting the voltage change caused by the injected current with the second electrode pair, thereby generating an impedance plethysmogram. The preferred placement of the electrodes is such that it enables measurement of the plethysmogram of the central aorta as well as the left and right femoral arteries. Also, it is possible to measure ECG with dry electrodes in the belt.

With reference to the accompanying drawings, some embodiments of the present invention will be described as follows by way of example, wherein:

Figure 1 is a simplified diagram showing the major arteries in the human body;

Figure 2 shows a typical electrode arrangement for impedance plethysmography;

Figure 3 shows a conductive rubber dry electrode;

Figure 4 shows underwear with integrated dry electrodes;

Figure 5 shows an ECG signal measured at the waist;

Figure 6 diagrammatically shows the relative positions and associated measurement areas of integrated dry electrodes in underwear;

Fig. 7 is a schematic diagram of a signal processing unit;

Figure 8 shows ECG, IPG1 and IPG2 signals measured with the system of the present invention; and

FIG. 9 is a close-up of the signal in FIG. 8 .

The present invention contemplates the use of the pulse wave velocity method as a means of measuring vital parameters.

The pulse wave velocity (PWV) method is an adequate method for monitoring mechanical parameters, but it requires a set (at least two) of sensors distributed around the body. For example, recent studies have demonstrated a good correlation between blood pressure BP and pulse wave velocity (PWV). For example, this technique allows the beat-to-beat determination of BP via calibration with reference measurements of blood pressure obtained with a cuff. Typically, the relationship between blood pressure in an artery and PWV is expressed by the Moens-Korteweg relationship, which can be derived from fluid mechanics theory:

$\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="E\; t"\; filenames="A200780025297E00111.png,A200780025297E00151.png"\; state="\{\{state\}\}">E</figure-callout></span><figure-callout\; id="2"\; label="E\; t"\; filenames="A200780025297E00111.png,A200780025297E00151.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&rho;R</span>}}}$

Equation 1: The Moens-Korteweg equation is often used to describe the relationship between pulse wave velocity and blood pressure

Where: c = pulse wave velocity, E _t = tangential elastic modulus, ρ = density

R = arterial radius, h = arterial wall thickness.

The experimentally verified relationship is:

E＝E ₀ e ^αp , α≈0.017mmHg ^-1

The above relationship provides a link between pulse wave velocity and changes in blood pressure (P). This calibration step is necessary to measure the conversion of PWV to BP, the other parameters (α, E ₀ , h, r) are obviously subject dependent and difficult to measure directly.

This PWV can be determined by measuring the time it takes for a pressure wave to travel a certain distance in different ways in the arterial system (this time will be called the pulse transit time PTT), for example:

1. The time difference between two points where the pulse passes through the distance d.

2. The time difference between the R peak in the ECG signal and the transmitted pulse in the artery in a certain body position.

Typical setups in the literature are:

1. ECG and photoplethysmogram PPG; PTT is given by the time difference between the R peak and the characteristic points in the PPG. PPG can be measured at various locations on the body such as the ear or fingers.

2. ECG and bioimpedance measurements of the arm (impedance plethysmography IPG); PTT is given by the time difference between the R peak and the characteristic points in the IPG.

3. Impedance cardiography (ICG) of the chest and bioimpedance measurement (IPG) of the arm; the PTT is given by the time difference between the characteristic points in the ICG and the characteristic points in the IPG.

4. Impedance plethysmography (IPG1) at the first position on the arm and bioimpedance measurement (IPG2) at the second position on the arm; PTT is given by the time difference between the characteristic points in IPG1 and the characteristic points in IPG2 out.

All of these approaches have some drawbacks, especially in personal healthcare applications, if employed with clinical standard sensors or methods. State-of-the-art sensors, such as finger or ear sensors that measure photoplethysmography or bioimpedance methods, are rather inconvenient in daily life where finger and ear PPG sensors or specific medical electrodes are required, which must be stuck on the skin. Therefore, this state-of-the-art sensor is not suitable for long-term continuous monitoring in personal healthcare applications.

The general principle of bioimpedance measurement is illustrated in the diagram in FIG. 2 , which shows the technique applied to a patient's leg 2 , where a small alternating current is passed through the leg 2 via the first electrode pair 4 . The excitation current is a constant high frequency alternating current with very low amplitude (approximately 1 milliampere), so the current is not felt by the patient and does not have any significant physiological effects.

Another electrode pair 6 is then used to detect the voltage change caused by this excitation current, which is a measure of the impedance change caused by blood volume and velocity changes. This enables measurement of arterial volume pulsations via the control/measurement circuit 8 .

It should be noted that the same principle can also be applied to measurements on other areas of the body. Therefore, the present invention intends to perform measurements on the subject's lumbar region indicated by 10 in the schematic diagram of Fig. 1 and the left and right femoral arteries indicated by 12 and 14, since these locations represent the main arteries of the subject's arterial system. branch point. At the same time, the belt position also has obvious advantages, namely: since the belt position corresponds to the normal position of the belt of the clothes, the subject will more naturally accept electrodes properly installed in this area. This location is also significantly closer to the patient's heart for ECG measurements, and produces less static pressure effects and motion artifacts than other possible monitoring points such as extremities.

Figure 3 shows a dry electrode made of conductive rubber, which has a flexible body and is therefore ideally suited for integration into a certain part of clothing. In a preferred arrangement of the invention, such electrodes are integrated into an undergarment, such as underpants 16 as shown in FIG. In this way, these electrodes are adapted to make good electrical contact with the wearer's skin, without the need for any special pastes or gels, solely through the conductivity of naturally secreted sweat. Also as shown in this figure, behind the zipper pocket shown at 20, the signal processing circuit 20 corresponding to the circuit 8 shown in FIG. 2 can also be integrated into the waistband of the clothes.

Figure 5 shows a typical ECG of a subject at rest, which was taken using the electrode arrangement of Figure 4, where the electrodes are positioned close to the patient's buttocks so that the electrodes can be relatively well spaced, And thus, these electrodes surround a suitably large volume of the subject's body. It can be seen that all important regions, namely the P wave, QRS complex and T wave, are clearly delineated in this signal.

Figure 6 shows in diagram form the different areas where a suitable undergarment may have integrated electrodes to enable measurements at the left and right femoral artery locations as well as in the waistband area as shown in Figure 4 . It can be seen in this figure that the first electrode pair 22 in the waist belt is arranged at the buttocks, and the current I1 is injected into the first electrode pair, which generally corresponds to the electrode 4 shown in Figure 2. Meanwhile, the other electrodes in the waist belt A pair of electrodes 24 is used for the corresponding measurement of the voltage change V1 , which generally corresponds to the electrodes 6 in FIG. 2 .

Further electrodes 26 and 28 are respectively arranged at the left and right leg positions and, in this way, for example, a current I2 can be injected between the corresponding waistband electrode 22 and the right leg electrode 26 so that the voltage V2 is at the electrode 30 with respect to the waistband position. Measured here. Similarly, by injecting a current I3 between the left leg electrode 28 and the corresponding waistband electrode 22, a voltage V3 can be measured at the electrode 32 with respect to the waistband.

Figure 7 shows a signal processing circuit suitable for integration into a garment as shown at 20 in Figure 4, where multiple functions are combined in the same unit. Front-end circuitry 34 for impedance and front-end circuitry 36 for ECG measurements are connected to the central processing unit 38 and to allow compensation for patient motion and detection of patient posture and activity, a 3-axis accelerometer 40 and temperature sensing device 42 .

A power source 44 is used to power the unit, preferably a long life or rechargeable battery, and a radio frequency transceiver 46 allows the device to communicate data with external systems such as a user interface. A storage device 48 may also be incorporated to allow storage or caching of data as necessary. In this way, the device can also be used as a "Holter monitor" (holter device), in order to record heart activity over an extended period of time.

Figure 8 shows an example of signal measurements made by the system, where Figure 8(a) shows an ECG measurement taken at the belt. Likewise, Figure 8(b) shows IPG1 (impedance plethysmography) measured at the waistband and Figure 8(c) shows the corresponding plethysmogram IPG2 taken at the right femoral artery.

Figure 9 is an enlarged close-up view of the signal in Figure 8, showing how features are extracted from the signal. By comparing Figure 9(a) with Figure 9(b), i.e., from ECG to IPG1, the pulse transit time 1 can be derived, and, by comparing Figure 9(a) with Figure 9(c), i.e., from ECG to IPG2, also The pulse transit time PTT2 can be derived.

Thus, it should be noted that the system of the present invention allows the following different measurements to be taken:

Measured signal:

●The ECG of the subject, the electrical activity of the heart (no standard leads)

• Blood volume pulses into the left and right legs during a heartbeat (measured by impedance plethysmography (IPG) of the left and right legs), which indicate the mechanical properties of cardiac activity.

3-axis acceleration (static and dynamic)

●Temperature

●time

Thus, the system is able to provide information on a large number of cardiovascular parameters derived from the measured signals, such as:

●heart rate

●Arrhythmia detection

●4 several pulse conduction times (ECG->IPG1, ECG->IPG2, ECG->IPG3, IPG1->IPG2)

●Arterial stiffness/enhancement index

● Relative blood pressure change

●Absolute blood pressure

●Blood perfusion to left and right legs

●stroke volume

● Activity measures (resting, posture, movement)

● Background information (time, temperature, acceleration) derived from these signals

Thus, various useful applications are possible, such as:

- Continuous blood pressure monitoring

-Sleep quality detection

- Hypertension management

-Blood pressure Holter monitoring

- Elderly care

- Continuous monitoring, eg during cardiac rehabilitation.

Claims (14)

1. A system for measuring/monitoring a patient's vital signs, especially blood pressure, comprising: a plurality of electrodes disposed at least in the waistband of the underwear, and Means for deriving measurements from said electrodes using pulse transit time. 2. The system of claim 1, wherein the electrodes comprise conductive rubber dry electrodes. 3. A system according to claim 1 or 2, wherein there are four electrodes in the waistband, the four electrodes comprising a first electrode pair for injecting current and a second electrode pair for measuring the induced voltage change. Two electrode pairs, so that the bio-impedance technique can be used for measurement. 4. A measurement system according to any preceding claim, wherein the electrodes are arranged to measure a pulse transmitted in the central aorta of the patient. 5. A measurement system according to any preceding claim, wherein the underwear comprises panties. 6. The measurement system according to claim 5, wherein additional electrodes are provided on the legs of the panty for measuring the pulse transmitted in the left and right muscular arteries. 7. A measurement system according to any preceding claim, wherein pulse wave velocity measurements are made by detecting pulse transit time between spaced apart sensors. 8. The measurement system according to any one of claims 1 to 6, wherein the pulse wave velocity measurement is performed by detecting the time difference between the R-peak in the ECG and the pulse transmitted in the artery at a given sensor location on the body . 9. A measurement system according to any preceding claim, further comprising a 3-axis accelerometer by means of which the system can detect the patient's posture or activity level and can compensate for signal artifacts caused by motion film. 10. Measuring system according to any one of the preceding claims, comprising a signal processing unit having a front-end circuit for receiving sensor signals, means for storing data, and for communicating with external data processing and/or Or a radio frequency transceiver device that communicates with the control device. 11. The measurement system of claim 10, wherein the unit is further adapted to receive manual input from the patient and to send information relating to the condition of the system and the patient to the external device. 12. A measurement system according to claim 10 or 11, further comprising an alarm for warning the patient of a critical condition. 13. A measurement system according to any one of claims 10-12, further comprising means for sending a warning to an external source of professional assistance. 14. A measurement system according to any one of claims 10-13, wherein the system is arranged to operate as a Holter monitor or as an actigraphy device to record cardiac activity over an extended period of time.

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