Pulse wave analysis

Abstract

‘ Since the information which the pulse affords is of so great importance, and so often consulted, surely it must be to our advantage to appreciate fully all it tells us, and to draw from it every detail that it is capable of imparting’ F.A. Mahomed 1872 [1] It is now possible to generate the ascending aortic pressure wave from the arterial pressure pulse, recorded noninvasively by applanation tonometry in the radial or carotid artery. This represents a blend of nineteenth century sphygmography with cuff sphygmomanometry, and is made possible by introduction of high fidelity tonometers, by characterization of arterial hydraulic properties in the upper limb and neck, and through application of mathematical engineering techniques in modern computer systems. This review will consider historical development, theoretic background, present status and future potential, as well as comparing this technique with radial and carotid tonometry alone, and with analysis of flow pulse and volume pulse waveforms as determined by Doppler or photoplethysmographic techniques. The arterial pulse is the most fundamental sign in clinical medicine, and has since antiquity been identified with the physician and the art of medicine. Palpation of the pulse forms the crest of the Royal College of Physicians of London, which was established to improve the scientific basis and practice of medicine and in an era when William Harvey, as anatomist to that college, wrote his classic text ‘de Motu Cordis…’[2]. William Bright [3] based his diagnosis of high blood pressure on ‘hardness’ of the pulse, and on the pressure required to extinguish the pulse. A scientific basis only arose after Marey [4], and then Mahomed [1] developed graphic methods to record the arterial pulse. By the beginning of the twentieth century, sphygmography was well established in medical journals and in medical textbooks and had been used to describe heart block and effects of antianginal medication as well as hypertension and other conditions [5–9]. In life insurance examinations sphygmocardiography was widely used for detecting persons with ‘arterial senility’ and increased risk of premature death [10]. Unfortunately sphygmography lapsed with introduction of the cuff sphygmomanometer, which provided numbers for the extremes of the pulse, and a veneer of scientific accuracy. Frederick Akbar Mohamed established the foundation of pulse wave analysis in a short medical lifetime from 1872 to 1884. He described the normal radial pressure waveform and showed the difference between this and the carotid wave [1]. He showed the effect of high blood pressure on the radial waveform, and used the waveform to describe the natural history of essential hypertension, and the difference between this and chronic nephritis [7, 8]. He also described the effects of arterial degeneration with ageing on the arterial pulse [7]. These features were identified and utilized in the life insurance studies of the late nineteenth century [10]. Mahomed's sphygmogram, and the popular Dudgeon sphygmogram which followed, and which was used by Sir James MacKenzie [9] were mechanical devices, awkward to use and prone to artifact. Modern tonometer systems are piezo-electric and are far more accurate, reliable, and easy to use. While originally introduced clinically to measure intraocular pressure, they have been adapted for vascular use by Drzwiecki [11], Millar and others [5, 6]. While Mahomed was the first to recognize the difference between pressure waves in central and peripheral arteries, McDonald [12] was responsible for explaining this phenomenon on the basis of wave reflection, and for introducing transfer functions to characterize properties of vascular beds in the frequency domain, and (with his colleague J.R. Womersley) for establishing the validity of assuming linearity in the arterial tree [13]. The work of McDonald, Womersley, Taylor and others, originally from Harvey's own hospital (St Bartholomew's, London) has led on to the techniques described here for pulse wave analysis. The technique of noninvasive aortic pulse wave analysis, as described here, depends on accurate recording of the radial pressure wave, its calibration against brachial pressure, then generation of the ascending aortic pressure waveform through use of a generalized transfer function in a computerized process. Ascending aortic waveforms are ensemble averaged into a single calibrated wave whose different features can be identified automatically with clinically important measures of pressure and time intervals measured and printed out in an interpretive report (Figure 1). Steps in the process are described below. The Sphygmocardiograph: computerized report on analysis of radial artery and synthesized aortic pressure waves. A series of radial artery pressure waves, recorded over an 8 s period (upper continuous tracing) are used to synthesize a series of ascending aortic pressure waves (lower continuous trace) using a convol