WO2005065042A2

WO2005065042A2 - Uncompression noninvasive method of blood pressure measurement

Info

Publication number: WO2005065042A2
Application number: PCT/IL2005/000006
Authority: WO
Inventors: Oleg Zudov
Original assignee: Ferder, Alexander
Priority date: 2004-01-06
Filing date: 2005-01-03
Publication date: 2005-07-21
Also published as: WO2005065042A3

Abstract

A non-invasive technique to measure a subject's blood pressure without compression. A sensor is applied to the limb of the subject to measure a pulse signal. The pulse signal is measured when the subject's limb is placed in two different positions. The mean blood pressure of the subject is calculated based on the magnitudes ratio of the pulse signal when the subject's limb is in a first and a second position. The pulse blood pressure is obtained by the change in the mean value of the pulse signal when the subject's limb is moved from the first position to the second position. The systolic and diastolic blood pressure values that are obtained by scaling the pulse signal in pressure terms (using the hydrostatic pressure addition that is due to the height difference) and taking the maximum and minimum points of the scaled pulse signal.

Description

UNCOMPRESSION NONINVASIVE METHOD OF BLOOD PRESSURE MEASUREMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

[000 II ] This patent application claims the benefit of the filing date of United States Provisional Application for Patent having Serial Number 60/534,552 and having been filed on January 6, 2004.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

100021 Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDLX

[00031 Not applicable.

BACKGROUND OF THE INVENTION

[0004] This invention relates to the measurement of blood pressure (BP) and methods for monitoring blood pressure and, more specifically, to a system and method for providing blood pressure measurements without compression using scaling of the pulse signal but without calibration.

[0005] Everyone is quite familiar with the standard, state of the art techniques used for measuring and obtaining your blood pressure level. Sitting on the somewhat spongy elevated table on a fresh, crisp sheet of butcher paper, feet dangling down to a pull-out foot stool that is always just a little out of reach, and a fresh linen gown with a gaping hole in the back, the nurse walks in with arm wrap. The arm wrap is stretched around your arm and secured with Nelcro. A little rubber tube extends from the arm wrap to a small red rubber bulb with a pressure release valve. The nurse then pumps it up to the point that you feel as though your arm is going to collapse, and then she slaps the stethoscope to your arm and listens to your pulse as the arm wrap slowly deflates. This is the most common non-invasive technique for measuring blood pressure. Blood pressure can be measured using invasive and non-invasive methods. A common invasive method comprises insertion a pressure sensor (catheter) directly into an artery. This is the most accurate and reliable method but it is painful in use and can provide a pathway for infection. Therefore it can't be used for routine measurement and monitoring.

[0006] Νon-invasive methods such as compression methods utilize the occlusion of patient's arm by compression cuff. Detecting of Korotkoff sounds or oscillations in the cuff allows determining systolic and diastolic blood pressure. Such techniques are described in PCT Application Number WO03082100 and U.S. Pat. No.6,517,495. These methods and devices are widely used in hospitals and clinics for making routine blood pressure measurements but don't provide continuous blood pressure monitoring. In addition, using cuff measurements for a long time to monitor blood pressure may impose significant discomfort to patients. [0007] So-called vascular unloading systems attempt to cause the external applied pressure to be equal to the arterial blood pressure. A description of such systems can be found in U.S. Patent Number 4,869,261]. These systems provide continuous measurements but have the same disadvantages that all occlusion methods have. [0008] There are other blood measurement and monitoring approaches that exploit the correlation between blood pressure and the pulse wave transit time (PWTT) or pulse wave velocity (PWV). A description of these approaches can be found in U.S. Patent. Number5,921,936 and European Patent application NumberEPOl 81067]. The approaches described in these patents and patent applications are o f low accuracy and need to be individually calibrated. [0009] Another method that has been used in the industry is the pulse signal scaling methods (so called tonometry). A description of the tonometry methods can be found in U.S. Pat. Number 4,873,987. Tonometry methods are used for continuous measurements but, this technique often requires individual calibration.

[0010] A more recent measurement and monitoring approach is presented by MEDWANE IΝC, a U.S. company, in U.S. Patent Number 6,589,185. This method includes applying a varying pressure to the radial artery by the sensor. This is similar to the occlusion method but much has proven to be much more comfortable for patients. However, this method still requires a difficult procedure of s etting a s ensor above the artery accurately.

[0011] Despite progress that has been made in the field of measuring and monitoring blood pressure, there remains a need in the art for methods to measure and monitor blood pressure that have acceptable accuracy, do not require compression and do not require the complexities associated with individual calibration.

SUMMARY OF THE INVENTION

[0012] The present invention overcomes the above-described needs in the prior art by providing a new method and system for measuring blood pressure that avoids the need for occlusion and other external pressures on the artery. The scaling of the plethysmogram in pressure term is provided by the measurement procedure automatically without calibration. This procedure includes the raising of the patient's arm with pulse sensors on the definite height that changes the pressure in the artery on definite hydrostatic value. [0013] An exemplary embodiment of the present invention may register the plethysmogram two times: at or approximately near to the level of the heart and at another level that is substantially higher or lower than the heart, and the scaling is carried out using the definite difference of height between these levels. It should be appreciated that the measurements can be taken in a variety of positions and the described embodiment is simply one example. For instance, if the pulse measurement is being taken from the arm, the measurements can be taken when the arm is in a lowered position and then again when the arm is in a raised position. Alternatively, the measurements can be taken when the arm is in a raised position and then again when the ami is in a lowered position. In addition, the lowered position may be hanging by the subject's side, at or near the level of the subjects heart, or at a variety of positions between these points. Similarly, the raised position may be directly in the air, as if the subject were raising their arm to get attention, at or near the level of the heart, or at a variety of positions between these p oints. T he p lethysmogram may be registered by two channels: the pulse signal and the mean value. The first signal changing between two levels is used for mean blood pressure calculation and the second signal changing is used for pulse blood pressure calculation. The systolic and diastolic blood pressure are calculated using mean and pulse blood pressure, as well as the pulse waveform.

[001 ] Thus, the present invention enables measuring blood pressure repeatedly during a long time without imposing significant discomfort to patient. The procedure of measurement is simple and lasts only for a few tens of seconds. Exemplary embodiments of the invention are suitable for non-invasive blood pressure monitoring without occlusion. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be more readily understood from reading the following description and by reference to the accompanying drawings (forming a part of the description), wherein an example of various embodiments and aspects of the invention are shown.

[0016] FIG. 1 is a block diagram with the relevant elements of a measurement system that is built according to an exemplary embodiment of the present invention; [0017] FIG. 2 is illustrates signals of the first and the second channels that are illustrated in FIG. 1; [00Ϊ8] FIG. 3 is a diagram illustrating non-linear dependence between artery cross section area and the pressure inside it.

DESCRIPTION OF THE VARIOUS EMBODIMENTS OF THE INVENTION [0019] Referring now to the drawings, in which like numerals refer to like parts throughout the several views, exemplary embodiments and aspects of the present invention are described. [0020] Fig. 1 illustrates a block diagram of an exemplary measurement system. The exemplary embodiment comprises a pulse sensor 110, two registration channels: channel 120 for the mean level and channel 130 for the pulse signal and a computer 140. [0021] An exemplary pulse sensor 110 may be an impedance plethysmography sensor (a rheogram sensor) that measures the electric impedance at the body surface. However, the present invention is not limited to impedance plethysmography sensors and a variety of other types of sensors may be used in order to measure the pulse signal. In some embodiments, two or more sensors may be used in parallel. The sensor may be placed around the arm or other limb of a subject. [0022] The signal from sensor 110 is registered by the two channels. Channel 120 registers the mean level of the pulse. The output of channel 120 is illustrated by FIG. 2 chl. Channel 130 registers the amplitude (magnitude) of the pulse. The output of channel 130 is illustrated by FIG. 2 clι2. The signal from both channels feed the computer 140. Computer 140 may be any computing device, such as but not limited to, a special purpose device, a PC, a desktop computer, lap top computer, a PDA etc. Computer 140 may operate to control the measuring and may also operate to process the signals.

[0023] hi another exemplary embodiment (not shown in the drawings) the pulse sensor may be connected via an analog-to-digital converter to a computer. In addition to other tasks the computer may run a software program that reads the digital values on the input and then calculates the mean level of the pulse and the magnitude of the pulse.

[0024] The following paragraphs disclose an exemplary method that may be used for measuring the blood pressure of a subject. The subject is instructed to place the arm with the sensor in a lower position and to keep it stable. The lower position may be a position in which the sensor is in the same level as the heart. When the computer determines that the measurement is stable, then the pulse amplitude and the mean value are sampled and stored for the first position. Exemplary stable signals, in the lower position, are illustrated in FIG. 2 as period 210 which extends to the point at which T= 8 seconds. [0025] After storing the measurements for the lower position, an instruction is given to the subject to raise his or her arm to an upper position. The upper position may be in a definite difference of height from the lower position. Dming the transit period 220, which is illustrated in FIG. 2 in the time interval starting at T= 9 seconds to T= 20 seconds, the system idles.

[0026] When the computer determines that the measurement is stable (i.e., the arm is in the upper position) then new values of the pulse amplitude and the mean value are sampled and stored. E xemplary s table signals, in the upper position, are illustrated in FIG. 2 during the period 230 starting at T=20 seconds to T= 26 seconds. Next, the difference in the height of the arm between the upper position and the lower position 'Ah' is entered into the computer. [0027] An exemplary embodiment of the present invention may determine the measuring periods by running a task that identifies a steady state. Other examples may determine that a transit period may be a fix period of time, for example 30 seconds, and may perform the reading at the end of the 30 seconds from instructing the subject to change positions. Another example may use a manual instruction to trigger the reading. [0028] After measuring the signals in both locations of the arm, two values are calculated. The first value is: ΔP=pgΔh, where ' P'is hydrostatic pressure addition that is created when the limb is moved from one position to another position, '/?' is blood density, g=9.8m/s² and Ah is the distance that the limb is moved. The value of p typically changes over a very narrow range - 1.060 to 1.064 kg/1. This is lower than 0.5%, so for all practical purposes, this can be assumed to be constant.

[0029] Based on the above calculations of ' lP' the value of the blood pressure is calculated based on the following formula. AP/P K-As/s Where 'P' is the magnitude of the blood pressure - so called pulse pressure or systolic blood pressure minus diastolic blood pressure, is the magnitude (amplitude) of the pulse signal that was measured in the lower position of the arm, 'As' is the mean level changing value of the pulse signal and 'K' is a proportionality coefficient depending only on the channel's gain parity. 'K' is the instrumental constant of device. This parameter is known and does not change from one measurement (or patient) to other. So this value is not an individual constant and does not invoke the need for instrument calibration. As the dependence between the pulse signal and the blood pressure is nonlinear, the pulse blood pressure should be obtained from a more complicated equation according to selected physical model and the type of sensor utilized to perform the measurement.

[0030] The changes that occur in the pulse signal are shown in Fig. 2 below. The magnitude in change after arm is raised to the upper position can be seen by examining the height of the signal during period 2 10 in comparison with the height of the signal during period 230. To gain a better understanding of this phenomenon, the nonlinear dependence between the radius of the artery radius Ar and the pressure P inside the artery is illustrated in Fig. 3. This dependence can be described as Δr =E-f(P), where E is the elasticity of arterial wall and f(P) is some nonlinear function of blood pressure. As this dependence changes linearly with the artery wall elasticity variation, it is not difficult to show that pulse signal magnitude ratio is definitely connected with hydrostatic pressure difference IP and the mean blood pressure.

[00311 Thus having the pulse wave form and the values of mean and pulse blood pressure, the pulse signal can be scaled in pressure terms. Then systolic and diastolic blood pressure values can be obtained as maximum and minimum points of the scaled pulse signal.

[00321 A clinical test of an embodiment of the present invention has been conducted. To conduct the test, a noninvasive blood pressure meter, which provides the measurements without compression, was developed. The clinical examination was carried out to estimate the accuracy of the device and to validate the various aspects of the present invention.

[0033] The method uses the scaling of pulse signal. The Rheogram pulse signal is the most suitable pulse signal to use in this method due to the fact that it can be described by a simple physical model.

[0034] Due to specific measurement procedures, the rheogram signal is scaled in pressure terms. The scaling of the rheogram signal is provided automatically without the need to perform any calibration. The procedure includes a patient raising his or her arm from one position to another position while pulse sensors are attached to the arm. The height of raising the arm is of sufficient distance to cause a change of pressure in the artery resulting in a change in the hydrostatic value.

[0035] The device registers the rheogram two times: when the arm is at or near the level of the heart and at the alternate (either raised or lowered) position. The scaling is carried out using the definite difference between the pressures at these levels. The rheogram is registered by two channels: the pulse signal and the mean value. The first signal changing between two levels is used for mean blood pressure calculation and the second signal changing is used for pulse blood pressure calculation. Then systolic and diastolic blood pressure are calculated using mean and pulse blood pressure, as well as the pulse waveform. [0036] Thus, embodiments of the present invention enable a subject's blood pressure to be measured repeatedly over an extended period of time without imposing significant discomfort to the patient. The procedure of measurement is simple and lasts only 10 to 30 seconds. Preferred embodiments of the method are suitable for non-invasive blood pressure monitoring and measurement without occlusion. [0037] For the clinical accuracy estimation, two standard protocols were used: American AAMI/ANSI (1992) - American national standard: electronic or automated sphygmomanometers. Arlington, VA: Association for the Advancement of Medical Instrumentation, 1993; and British BHS (1993) - O'Brien E., Petrie J., Littler W.A. et al. The British Hypertension Society Protocol for the evaluation of blood pressure measm-ing devices // J. Hypertens. 1993. V. 11. Suppl. 2. S43-S62. [0038] Both of these protocols use as a gold standard, the result of measurement with Korotkoff sounds method carried out by two doctors-experts. The second protocol also is recommended by the European Society of Hypertension.

[0039] The test was conducted in a policlinic where patients had a routine inspection and their ages ranged up to 45 years old. 87 patient were tested (49 women and 38 men). Dr Ovsienko (cardiologist) and Dr Kudlay (internist) measured the patient's blood pressure manually as experts.

[0040] According to the protocols of the test, the measurements were done using one compression cuff of the testing device and Korotkoff sounds were registered under it. However, embodiments of the present invention do not utilize a cuff because the present invention is directed towards a method that does not use compression. The BHS protocol permits serial measurements in cases when simultaneous measurements are impossible. So, some measurements were made simultaneously using our system on one arm and doctors using a compression cuff and stethoscope on the another arm. However, most of the tests were performed on the same arm consistently.

[004 ϊ] In both scenarios, there was some level of error that could be introduced. In the first scenario, the error could be introduced due to a difference in the blood pressure between the two arms. In the second scenario, the error could be introduced due to a change in the blood pressure between the performances of the two measurements. The BHS protocol separates devices into three classes (class A, B and C) depending on their accuracy. For consistent testing procedures, more "soft" requirements are permitted than for simultaneous testing scenarios. These requirements to the accuracy for consistent measurements are given in Table 1. Table 1

[0042] This information is available in the publication O'Brien E., Pickering Th. et al. Working Group on Blood Pressure Monitoring of the European Society of Hypertension Intemational Protocol for validation of blood pressure measuring devices in adults // Blood Press. Monit. 2002. V. 7. P. 3-17.

[0043] Table 2 provides the results of the test. The results are listed separately for systolic and diastolic blood pressure. Table 2

[0044] These results illustrate that device used in the test and the embodiment of the invention corresponds to C class for systolic and almost B class for diastolic blood pressure measurements.

[0045] In the description and claims of the present application, each of the verbs, "comprise," "include," and "have," and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements, or parts of their respective subjects or verb. [0046] The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention.

[0047] The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons skilled in the art. The scope of the invention is limited only by the following claims.

Claims

CLAIMS What is claimed is: 1. A method for measuring blood pressure , the method comprising the steps of: using a sensor coupled to a patient's limb to register a first pulse wave signal (plethysmogram) while the limb is in a first position; using the sensor to register a second pulse wave signal (plethysmogram) while the limb is in a second position; scaling the first and second registrations in pressure terms based on the difference of height between these levels; and calculating the blood pressure as a function of the scaled pulse signals and the differences in the characteristics of the registered pulse signals.

2. The method of claim 1 wherein the plethysmogram is registered by two channels: a pulse signal value and a mean value; and the first signal changing between two levels is used for mean blood pressure calculation and the second signal changing is used for pulse blood pressure calculation; and then systolic and diastolic blood pressure are calculated using mean arid pulse blood pressure and the pulse wave form.

3. A method for measuring a subject's blood pressure, the method comprising the steps of: applying a sensor to a limb of the subject to sense a pulse signal; placing the subject's limb into a first position; measuring the mean value and the amplitude value of the pulse signal while the limb is in the first position; moving the subject's limb to a second position measuring the mean value and the amplitude value of the pulse signal while the limb is in the second position; calculating the blood pressure based on the amplitude value of the pulse signal while the limb is in the first position, the change in the mean value between the first limb position and the second limb position and the distance between the first limb position and the second limb position.

4. The method of claim 3, wherein the step of calculating the blood pressure further comprises: scaling the pulse signal in pressure terms based on the mean values and the amplitude values; determining the systolic blood pressure value and the diastolic blood pressure values based on the maximum and minimmri points of the scaled pulse signal; and further calculating the blood pressure based on the determined systolic blood pressure value and diastolic blood pressure values.

5. The method of claim 3, wherein the step of applying a sensor to the limb of a subject further comprises applying the sensor in such a manner to sense the rheogram pulse signal.

6. The method of claim 3, wherein the subject's limb is the subject's arm and the step of placing the subject's limb in a first position further comprises placing the subject's arm at approximately the level of the subject's heart.

7. The method of claim 3, wherein the subject's limb is the subject's arm and the step of placing the subject's limb in a second position further comprises placing the subject's arm at a position substantially above the subjects heart.

8. The method of claim 3, wherein the subject's limb is the subject's arm and the step of placing the subject's limb in a second position further comprises placing the subject's arm at a position substantially below the subjects heart.

9. The method of claim 3, wherein the subject's limb is the subject's arm and the step of placing the subject's limb in a first position further comprises placing the subject's arm at approximately the level of the subjects heart and placing the subjects arm into the second position further comprises moving the subjects arm substantially above the subject's heart.

10. The method of claim 3, wherein the subject's limb is the subject's arm and the step of placing the subject's limb in a first position further comprises placing the subject's arm at one position relative to the subjects heart and placing the subjects arm into the second position further comprises moving the subjects arm to another position relative to the subjects heart.

11. A system for measuring blood pressure using a non-invasive and uncompressed technique, the system comprising: a sensor that can be applied to the limb of a subject for sensing a pulse signal; a first channel coupled to the sensor for register the mean level of the pulse signal; a second channel coupled to the sensor for registering the amplitude of the pulse signal; a processing device for receiving the registered mean level and the registered amplitude from the first channel and the second channel, and for receiving the pulse signal from the sensor, the processor being operative to: determine the amplitudes of the pulse signal when the limb of the subject is in a first position and in a second position; determine the change in the mean value of the pulse signal between when the limb is in the first position and the second position; scale the pulse signal in pressure terms using hydrostatic pressure difference between the two positions; and calculate the blood pressure of the subject based at least in part on these values.

12. The system of claim 11, wherein the pulse signal is the rheogram pulse signal.