Intelligent multichannel sensors for pulse wave analysis

Abstract

Aortic pulse wave velocity is an independent predictive indicator for all cause mortality and cardiovascular morbidity. Unfortunately it is only invasively accessible and thus the A. carotis–A. femoralis pulse wave velocity (cfPWV) is recommended as a non-invasive substitute. This work presents a model based analysis method for the beat-to-beat online determination of an arbitrary, peripheral pulse transit time (PTT). The method is based on the recording of a three lead electrocardiography (ECG) and of pulse waves (PW) at a peripheral site such as the A. carotis by means of a multiple sensor array. The two modules for the signal acquisition work autonomously but time-wise simultaneously and transmit the data via a radio unit to the central processing unit. There the algorithms for the pulse transit time determination exploit these signals. In doing so the main focus is on an efficient implementation to assure real-time usability. The evaluation of the developed modules and algorithms was done in two separate trials. First the algorithms were tested offline against manual signal annotation using invasive data previously recorded to proof their accuracy. The resulting mean differences in PTT for pulse waves assessed at the aortic root and the aortic bifurcation are 2.86 ms (4.72 ms SD) and 2.00 ms (2.28 ms SD). To evaluate the whole system integrity in a second step online measurements on probands were carried out and compared to data from literature. The trials resulted in a mean PTT of 174.6 ms (17.7 ms SD) for the A. radialis and of 81.9 ms (13.2 ms SD) for the A. carotis. The results suggest that the method may be useful and deployable at general practitioners (GP) and in ambulatory care of (chronic) cardiovascular diseases.

Introduction

The medical research regarding hypertension has changed considerably during the last two decades. Around the year 1990 the diastolic blood pressure has been the most important value to look at [18] and about ten years later the focus has been on the systolic blood pressure. With the beginning of the new millennium the topic of arterial stiffness of major vessels related to hypertension has slowly arisen in clinical practice. Arterial stiffness and its indicators have been mentioned for the first time in the ESH-ESC guidelines for hypertension treatment in the year 2003 [8]. As parameters to measure arterial stiffness primarily the methods of pulse wave analysis (PWA) and pulse wave velocity (PWV) have been proposed.

Ageing and pathological changes (e.g. arteriosclerosis or subclinical organ damage) are the main factors for the stiffening of vessels and as a result an increase of PWV may occur. As a consequence increased and premature pressure reflections emerge, which superimpose the generic pulse wave ejected by the heart earlier and more intensely. The superposition may cause a pathological increase of the aortic systolic blood pressure and subsequently an increase of the cardiovascular risk [22], [25], [32].

Meanwhile aortic PWV has been shown to have an independent predictive value for all cause mortality and cardiovascular morbidity, coronary events and strokes in patients with uncomplicated essential hypertension [4], with impaired glucose tolerance [5] and with renal failure [3], in elderly individuals [20] and in the general population [34]. In the update of the ESH-ESC guidelines for hypertension treatment in the year 2007 [19] the consequences of arterial stiffness on cardiovascular mortality play a major role. These guidelines additionally claim widely suitable measuring devices for the measurement of arterial wall stiffness and describe the influence of arterial stiffness on the aortic blood pressure. The authors have recently introduced a simple method for PWA based on an inverted simulation model to be used in oscillometric devices [29]. The aim of this work is the presentation of a model based approach for simple PWV measurements.