WO2008135892A2

WO2008135892A2 - Apparatus for performing pulse wave velocity measurements

Info

Publication number: WO2008135892A2
Application number: PCT/IB2008/051621
Authority: WO
Inventors: Thomas Vollmer
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-05-02
Filing date: 2008-04-28
Publication date: 2008-11-13
Also published as: WO2008135892A3

Abstract

The invention relates to an apparatus (100, 200) for performing pulse wave velocity measurements of blood flowing through a blood vessel, comprising at least a first and a second pulse detector, first selecting means (106) for selecting one of the first and the second detectors (104) and an amplifier (108) for amplifying a signal provided by the selected one of the first (102) and second (104) detectors.

Description

Apparatus for performing pulse wave velocity measurements

TECHNICAL FIELD

The invention relates to an apparatus for performing non-invasive pulse wave velocity measurements of blood flowing through a blood vessel, a method for performing pulse wave velocity measurements of blood flowing through a blood vessel and a computer program product comprising computer executable instructions for performing the method for performing pulse wave velocity measurements of blood flow flowing through a blood vessel.

BACKGROUND AND RELATED ART

High blood pressure is a big problem occurring nowadays in western societies. The problem with high blood pressure is that for high blood pressure the risks of a stroke, heart attack and heart failure are increasing. Since high blood pressure rarely has any symptoms, the only way to know if a patient has high blood pressure is to measure it. For control of blood pressure, accurate and, if possible continuous measurements of blood pressure is necessary for monitoring blood pressure behavior of a patient over time.

Besides the usage of an arm cuff with a connected electric monitor and display, which allows blood pressure measuring in an easy way, new blood pressure measurement techniques have been developed in the past years. For example, US 6,331,162 describes a device, which allows measuring of pulse wave velocity for analyzing blood flow in the human body. Thereby, the blood pressure of a person can be estimated by using the measured pulse wave velocity. A general overview on pulse wave velocity measurements in the human body can be found in US 4,425,920. Besides high blood pressure, the problem of dehydration of elderly people is another increasing problem in our aging society. Dehydration is frequent etiology of morbidity and mortality in elderly people. It causes the hospitalization of many patients and its outcome may be fatal. Older people have been shown to have a higher risk of developing dehydration than young adults. Dehydration is often linked to infection, stroke or heart attack, and if it is overlooked, mortality may be over 50%. Therefore, the prevention of dehydration is essential. Early detection of dehydration or hydration monitoring followed by prompt and adequate fluid supplement can substantially reduce the risk of severe dehydration, and consequently, are of great clinical need. Conventionally, the clinical assessment of dehydration severity is mainly based on a physical examination. Common symptoms include dry mouth and mucous membrane, sunken eyes, orthostatic hypertension, delayed capillary fill, and poor skin turgor. However, physical tests are subjective and have a low sensitivity and specificity in general. Laboratory tests on blood and urine samples have also been used to determine dehydration status. The main advantage of these tests over physical examination is that they provide objective and quantitative measurements. Nevertheless, they require special lab equipment and are usually time consuming and costly.

Because pulse wave velocity at known blood pressure is a function of blood viscosity and changes in blood viscosity itself are indicative for the hematocrit and thus the body hydration status, pulse wave velocity measurement is suitable for monitoring and detecting fluid deficit (dehydration). By means of two plethysmography transducers mounted in a defined distance, the pulse wave velocity can be derived from a pulse transit time correlation measurement.

One of the major problems occurring in practical application of transit time correlation measurements is to be found in stochastic measuring errors. Their occurrence probability depends upon the sample properties of the object as well as upon the dynamic systems behavior of the applied sensors, and is difficult to be determined. Therefore, transit time correlation measurements are rarely being used in practice so far. SUMMARY OF THE INVENTION

The present invention provides an apparatus for performing pulse wave velocity measurements of blood flowing through a blood vessel. The apparatus comprises at least a first and a second pulse detector, first selecting means for selecting one of the first and the second detectors and an amplifier for amplifying a signal provided by one of the first and second detectors. Thereby, each detector comprises a first and second transducer, wherein the first selecting means is adapted for selecting one of the first transducers of the first and second detectors. The apparatus according to the invention allows robust pulse wave velocity measurements derived from pulse transit time correlation measurements by averaging several pulse wave velocity measurements from a single transducer pair over time, i.e. over several heart beats. A robust pulse wave velocity measurement can thereby for example be a motion tolerant measurement. By switching between said detectors and averaging the pulse wave velocity derived from the pulse transit time correlation readings from the respective transducer pairs, a robust measurement of the pulse wave velocity can be achieved in real time.

In accordance with an embodiment of the invention, the apparatus for performing pulse wave velocity measurements further comprises a second selecting means, wherein the first selecting means is adapted for switching between each first transducer of the first and second detectors and wherein the second selecting means is adapted for switching between each second transducer of the first and second detectors. This allows the usage of any arbitrary combination of transducers which allows to perform highly flexible pulse wave velocity measurements. This becomes important for example, if there is a requirement for an extremely highly accurate pulse wave velocity measurement, which requires nearly perfect contact of used transducers with the skin above an artery. Therewith, suitable transducers can be individually selected with the first and second selecting means and combined in any arbitrary reasonable combinations. In accordance with an embodiment of the invention, the amplifier is connected to the first and second selecting means. The apparatus further comprises a controller, the controller being adapted for controlling a predefined switching sequence of the first and/or the second selecting means, wherein the switching from the first to the second detector is triggered by for example detection of a pulse wave at a first detector. By switching between transducer pairs and averaging the pulse wave velocity derived from the pulse transit time correlation readings from the respective transducer pairs, a robust measurement of the pulse wave velocity can be achieved.

Thereby, in accordance with an embodiment of the invention, the switching is for example performed continuously. The first and second transducers are adapted as plethysmography electrodes, wherein the plethysmography electrodes are bio- impedance and/or optical based electrodes, wherein for bio-impedance based electrodes said electrodes are adapted for capacitive and/or galvanic tissue electrode coupling.

In accordance with an embodiment of the invention, said electrodes are adapted for impedance measurements at various measurement frequencies. By tuning the measurement frequency to different values, a spectroscopic measurement can be achieved.

In accordance with an embodiment of the invention, the apparatus for performing pulse wave velocity measurements further comprises phase sensitive detection means. The usage of phase sensitive detection means like lock-in amplifiers enables to achieve high sensitivities.

In accordance with an embodiment of the invention, the apparatus further comprises blood pressure measurement means. Such blood pressure measurement means can for example be adapted as a blood pressure cuff. According to a preferred embodiment of the invention, the detectors for performing pulse wave velocity measurements are thereby integrated in such a blood pressure cuff. The combination of pulse wave velocity measurements with blood pressure measurements allows to monitor changes in blood viscosity and therefore determination of a body hydration status. In accordance with an embodiment of the invention, the apparatus for performing pulse wave velocity measurements further comprises a data processing system, whereby the data processing system is adapted for performing pulse transit time analysis. In accordance with an embodiment of the invention, the data processing system is adapted for receiving input signals representative of a blood pressure and the data processing system is further adapted for performing blood viscosity analysis.

In accordance with an embodiment of the invention, multiple detectors are adapted on a detector array. Thereby, said detector array is adapted as a self- adhesive detector array patch. Such a pulse wave velocity detector array patch with for example 10 to 100 detectors arranged at predefined distances on said detector array allows effective averaging of pulse wave velocity measurements derived from pulse transit time correlation readings from said detectors. Therefore this allows for accurate, reliable and fast measurements of either the blood pressure and/or a body hydration status.

In another patch embodiment, said detector array is combined with ECG (electrocardiogram) measurement means on the same patch, building a combined pulse wave velocity and ECG detecting patch. This embodiment allows the measurement of an additional pulse transit time, depicted pulse arrival time (PAT), which is the transit time from the ECG (e.g. R peak) to the pulse detected with a pulse detector on said patch.

In another aspect, the invention relates to a method for performing pulse wave velocity measurements of blood flowing through a blood vessel, the method comprising selecting of a first detector, the first detector being positioned at a first location for detecting pulse waves. The method further comprises detecting the first pulse waveform of the blood flow at the first location and amplification of said first pulse waveform with means of an amplifier. This is followed by selecting a second detector, the second detector being positioned at a second location along the arterial tree for detecting pulse waves, wherein starting from the first location, the second location is located at a defined distance from the first location in the direction of blood flow. By detecting a second pulse waveform of the blood flow at a second location and amplification of said second pulse waveform with means of an amplifier, a pulse transit time analysis is performed based on the detected first and second pulse. In accordance with an embodiment of the invention, the method for performing pulse wave velocity measurements further comprises a blood pressure measurement and further comprises performing a blood viscosity analysis based on analysis of the pulse transit time and the blood pressure measurement. Thereby the pulse transit time analysis is performed using a cross correlation of the first and second waveform signals. Since the cross correlation is a well known function of the relative time between two signals, performing a cross correlation of the first and second pulse waveform signals enables an easy and accurate measurement of pulse transit times. In accordance with an embodiment of the invention, the pulse wave velocity measurements are performed using multiple detectors. In accordance with a further embodiment of the invention, at least two sets of multiple detectors, whereby multiple detectors correspond to a detector array, are used for performing pulse wave velocity measurements, wherein the at least two sets of multiple detectors are located in a defined distance apart from each other, wherein the pulse transit time analysis is performed at the pulse transit time between said two sets of multiple detectors. By using such kind of sensor superposition, an even more robust estimation of the pulse wave velocity can be achieved compared to a measurement based on one set of multiple detectors only.

In accordance with an embodiment of the invention, the pulse transit time analysis further comprises averaging of pulse transit times determined from multiple detectors. Only pulse waveform signals from detectors with a predetermined signal quality are for example used for the averaging of pulse transit times. Thereby said signal quality is determined based on the signal to noise ratio of the detected pulse waveform signals. This allows a further reduction of measuring errors for example due to motion artifacts or bad contacts. In accordance with an embodiment of the invention, the pulse wave velocity measurements are performed beat-to-beat. This allows a continuous measurement of said pulse wave velocity.

In another aspect, the invention relates to a computer program product comprising computer executable instructions for performing the method of pulse wave velocity measurements of blood flowing through a blood vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention are described in greater detail by way of example only making reference to the drawings in which:

Figure 1 is a block diagram of an embodiment of an apparatus for performing pulse wave velocity measurements of blood flowing through a blood vessel,

Figure 2 shows a further block diagram of an embodiment of an apparatus for performing pulse wave velocity measurements of blood flowing through a blood vessel, Figure 3 shows a block diagram of an embodiment according to the invention utilizing a blood pressure cuff for combined detection of blood pressure and pulse wave velocity, Figure 4 shows a flowchart illustrating a method for performing pulse wave velocity measurements of blood flowing through a blood vessel.

DETAILED DESCRIPTION

Fig. 1 is a block diagram of an embodiment of an apparatus 100 for performing pulse wave velocity measurements of blood flowing through a blood vessel. The apparatus 100 comprises a first pulse detector 102 and a second pulse detector 104.

Thereby the second detector 104 is positioned at a second location for detecting pulse waves, wherein starting from the first location the first detector 102 being positioned at, the second location is located at a distance d from the first location in the direction of blood flow 110.

A switch 106 is adapted for selecting one of the first and the second detectors 102, 104. Thereby, selecting could be either realized by controlling the on-off state of each detector or, in case all detectors permanently provide signals to the switch, selecting individual signals provided by said detectors. An amplifier 108 which is connected to the first switch 106 is adapted for amplifying a signal provided by the one of the selected first and second detectors 102 and 104.

Each detector 102 and 104 comprises a first transducer 118 and a second transducer 120. Thereby, in the embodiment shown in fig. 1, all first transducers 118 are connected to the switch 106. Further, all second transducers 120 are connected to each other, as well as to voltage source 114.

The apparatus for performing pulse wave velocity measurements 100 further comprises a controller 122, whereby the controller 122 is adapted for controlling a predefined switching sequence of the first switch 106. Thereby, said switching is preferably performed continuously. It has to be ensured, that the switching rate for switching between multiple detectors is high enough in order to reliably detect pulse waves at any arbitrary pulse rate.

The apparatus 100 for performing pulse wave velocity measurements further comprises a data processing system 132, whereby the data processing system 132 is adapted for performing pulse transit time analysis. The controller 122 is connected to the data processing system with means of an interface 124. In a similar manner, the amplifier 108 and the voltage source 114 are connected to the data processing system

100 with means of interfaces 126 and 128, respectively. An interface 130 serves to connect the data processing system 132 with a computer network or other medical devices.

The interface 130 can for example be used, to further make use of a blood pressure cuff, which is connected to interface 130 to provide blood pressure measurement values to the data processing system 132. This is essential in case pulse wave velocity measurements at known blood pressure have to be performed in order to measure for example blood viscosity in order to determine a body hydration status.

The data processing system 132 further comprises input means like a keyboard and/or a mouse 134, a display 136 and a processor 138. The processor 138 is adapted to execute computer executable instructions 142 comprised in a memory 140. The computer executable instructions 142 are for example adapted for controlling the controller 122, the amplifier 108, the voltage source 114, as well as performing any kind of analysis like blood viscosity analysis, calculating cross correlations between first and second pulse waveform signals originating from the first detector 102 and the second detector 104 and performing pulse transit time analysis.

For bio-impedance based measurements, the detectors 102 and 104 are applied on the skin above an artery, such that the pulse wave in the artery can be sampled by switching between the first transducers 118 and the second transducers 120. The transducers might be isolated (capacitive tissue electrode coupling) or non-isolated (galvanic tissue electrode coupling). In analogy, for an optical measurement principle, the bio-impedance detectors are replaced by co-located reflective or transmissive photo plethysmography light source and detector elements, respectively.

It has to be noted that the controller 122 is not necessarily required in order to perform pulse wave velocity measurements, controlling of the switch 106 can also be performed by the data processing system 132. However, the controller 122 could also be additionally adapted to perform some signal processing in interconnection with the data processing system 132.

Fig. 2 shows a further block diagram of an embodiment of an apparatus 200 for performing pulse wave velocity measurements of blood flowing through a blood vessel. Compared to the apparatus 100 in fig. 1, the apparatus 200 in fig. 2 further comprises a switch 202. Any other components which correspond to components in fig. 1 are denoted with the same reference numerals as in fig. 1.

The additional switch 202 is connected to each of the second transducers 120 of the first detector 102 and the second detector 104. The switches 106 and 202 are controlled by the controller 122. The switch 106 is further connected to the voltage measurement amplifier 108, the output of which is connected to a demodulator 212. The demodulator 212 itself is connected to an analogue-digital converter 208, the output of which is communicating with the interface 126. The purpose of the demodulator 212 is to enable performing a phase sensitive signal detection. The demodulator 212 is further connected to current measurement means 206 and a signal generator 204, whereby the signal generator 204 is controlled by the interface 128.

The signal generator 204 is further connected to a current source 210, which first (positive) output is connected to the switch 202. The second (negative) output of the current source 210 is also connected to the switch 202 with means of the current measurement means 206. The current source 210 is controlled by the interface 130 of the data processing system 132.

It is to be understandable that the principle of using a first detector 102 and a second detector 104 is expandable to a plurality of detectors, typically in between 10 and 100 detectors. Therewith, expanding the principle denoted in apparatus 200 for two detectors to m=l, ..., M first transducers and n=l, ..., N second transducers, the additional switch 202 which can be separately switched from the first switch 106, allows to scan arbitrary transducer combinations (m,n) leading to measurement voltages V_M,_N (k) at sampling time kT (T being the sampling time, and V_T the sampling rate of the switch 106 and/or 202, controlled by the controller 122). Thereby the switching sequence is for example alternating (m=l, 8, 2, 7, ...). Thereby each switching between the transducers 102 and 104 can for example be performed triggered by detection of a pulse wave at any of the detectors or the switching can be performed continuously. The usage of the additional switch 202 allows also a highly flexible switching between various transducers. This enables for example exclusion of transducers which show for example a bad signal to noise ratio or this even allows to eliminate false signals from defect transducers. Another advantage to use a second switch 202 is that this allows a flexible utilization of different capacitances between respective transducers. For example, switching can be performed between transducer pairs (n,m)=(l,l), (8,8), (1,2), (8,7), ... This also allows the combined usage of various capacitive and/or galvanic bio-impedance detectors and/or optical based detectors.

For signal processing in case of multiple detectors, computer executable instructions 142 can be used to calculate the cross-correlation C(_m,_n),(_m',n')(l) of the measurement voltages V_(m, _n)(k) and V_(m' _n')(k) Between two transducer pairings (m,n) and (m',n'): C_(m,_n),(m',n')(l) = A{V_(m,_n)(k) * V_(m'_n')(k+1)}. From the maximum of C(m,n),(m',n')(l), the PTT(_{m n})_;(_m'_n') (pulse transit time) can be derived. With the defined distance d(_m,_n),(m',n') between said transducers and PTT(_m;n);(_m'_;n')_; the pulse wave velocity v_(m,n),(m',n') between _(m,_n),(m',n') can be derived:

V(m,n),(m',n') ⁼ d(_m;n),(m',n') / PTT(_m;n)_;(_m'_;n').

By averaging all V(_m,_n),(_m',n'), a robust estimation of the pulse wave velocity v in an artery segment can be achieved: v = A{ V(_m,_n),(_m',n') } • By further using the well known Moens-Korteweg equation, the blood density p and therewith the blood viscosity can be calculated:

Thereby, r is the vessel radius, h the vessel wall thickness and E the Young elasticity modulus of the wall, which is directly related to the blood pressure P: E=E_oe^aP. This relation therewith provides a direct link between the pulse wave velocity and the blood pressure.

By further using a suitably adapted current source 210 in terms of for example a special signal generator, galvanic coupled bio-impedance spectroscopy with current excitation can be performed. In this embodiment a phase sensitive detector (PSP, demodulator or lock-in amplifier) can be used in order to achieve high sensitivity. By tuning the measurement frequencies for each detector, spectroscopic measurements can be achieved.

Fig. 3 shows a block diagram of an embodiment according to the invention utilizing a blood pressure cuff 300 for combined detection of blood pressure and pulse wave velocity. The blood pressure cuff 300 is thereby connected to a data processing system 302 adapted to perform blood pressure measurements. The data processing system 302 is further adapted for performing pulse transit time analysis by using pulse wave velocity measurements performed with a detector array 304 integrated within the blood pressure cuff 300. By a combined analysis of said pulse wave velocity measurements and blood pressure measurements, the data processing system 302 can perform blood viscosity analysis and therewith dehydration or hydration monitoring.

It has to be noted that preferably the detector array 304 extends over the whole inner circumference of the blood pressure cuff 300. Fig. 4 shows a flow chart illustrating a method for performing pulse wave velocity measurements of blood flowing through a blood vessel. In step 400 a first detector is selected, whereby the first detector is positioned at a first location for detecting pulse waves. In step 402, a first pulse waveform of the blood flow at the first location is detected. In step 404, said first pulse waveform is amplified with means of an amplifier. This is followed by step 406, where a second detector is selected, whereby the second detector is positioned at a second location for detecting pulse waves, wherein starting from the first location the second location is located at a distance from the first location in the direction of blood flow. In step 408 a second pulse waveform of the blood flow at the second location is detected. In step 410 said second pulse waveform is amplified with means of the amplifier. In step 412 a pulse transit time analysis is performed based on the detected first and second pulse, whereby the analysis is performed with the data processing system. LIST OF REFERENCE NUMERALS

Claims

CLAIMS:

1. An apparatus (100, 200) for performing pulse wave velocity measurements of blood flowing through a blood vessel comprising:

- at least a first (102) and a second (104) pulse detector, - first selecting means (106) for selecting one of the first (102) and the second (104) detectors,

- an amplifier (108) for amplifying a signal provided by the selected one of the first (102) and second (104) detectors.

2. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1, wherein each detector (102, 104) comprises a first (118) and a second (120) transducer, wherein the first selecting means (106) is adapted for selecting one of the first transducers (118) of the first (102) and second (104) detectors.

3. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 2, further comprising a second selecting means (202), wherein the first selecting means (106) is adapted for switching between each first transducer (118) of the first (102) and second detectors (104) and wherein the second selecting means (202) is adapted for switching between each second transducer (120) of the first (102) and second (104) detectors, wherein the amplifier (108) is connected to the first (106) and the second (202) selecting means .

4. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1, further comprising a controller (122), the controller (122) being adapted for controlling a predefined switching sequence of the first (106= and/or the second (202) selecting means, wherein the switching from the first (102) to the second (104) detector is triggered by detection of a pulse wave at the first detector (102).

5. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 4, wherein the switching is performed continuously.

6. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1, wherein the first (118) and second (120) transducers are adapted as plethysmography electrodes, wherein the plethysmography electrodes are bio-impedance and/or optical based electrodes, wherein for bio- impedance based electrodes said electrodes are adapted for capacitive and/or galvanic tissue-electrode coupling.

7. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 6, wherein the electrodes are adapted for impedance measurements at various measurement frequencies.

8. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 7, further comprising phase sensitive detection means.

9. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1 , further comprising blood pressure measurement means (300).

10. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1, further comprising a data processing system (132, 302), the data processing system (132, 302) being adapted for performing pulse transit time analysis.

11. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 10, wherein the data processing system (132, 302) is adapted for receiving input signals representative of a blood pressure and wherein the data processing system (132, 302) is adapted for performing blood viscosity analysis.

12. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1 , wherein the detectors are integrated in a blood pressure cuff (300).

13. The apparatus (100, 200) for performing pulse wave velocity measurements of claim 1, wherein the detectors are adapted on a detector array (304).

14. A method for performing pulse wave velocity measurements of blood flowing through a blood vessel, the method comprising:

- selecting a first detector (102), the first detector (102) being positioned at a first location for detecting pulse waves, - detecting a first pulse waveform of the blood flow at the first location,

- amplifying said first pulse waveform with means of an amplifier,

- selecting a second detector (104), the second detector (104) being positioned at a second location for detecting pulse waves, wherein starting from the first location the second location is located at a distance (</)from the first location in the direction (110) of the blood flow,

- detecting a second pulse waveform of the blood flow at the second location,

- amplifying said second pulse waveform with means of the amplifier (108), - performing a pulse transit time analysis based on the detected first and second pulse.

15. The method for performing pulse wave velocity measurements of claim 14, further comprising a blood pressure measurement and further comprising performing a blood viscosity analysis based on analysis of the pulse transit time and the blood pressure measurement.

16. The method for performing pulse wave velocity measurements of claim 14, wherein the pulse transit time analysis is performed using a cross correlation of the first and second pulse waveform signals.

17. The method for performing pulse wave velocity measurements of claim 14, wherein the pulse wave velocity measurements are performed using multiple detectors (102, 104).

18. The method for performing pulse wave velocity measurements of claim 17, wherein at least two sets of multiple detectors (102, 104) are used for performing pulse wave velocity measurements, wherein the at least two sets of multiple detectors (102, 104) are located in a defined distance apart from each other, wherein the pulse transit time analysis is performed on the pulse transit time between said two sets of multiple detectors (102, 104).

19. The method for performing pulse wave velocity measurements of claim 17, wherein the pulse transit time analysis further comprises averaging of pulse transit times determined for multiple detectors (102, 104).

20. A computer program product comprising computer executable instructions for performing any of the method claims 14 to 19.