Carotid Artery Tonometry: Pros and Cons

Palpation of the arterial pulse is one of the oldest and most basic parts of the physical examination, and is classically undertaken at the radial site. Paintings of medical practice over centuries have come to typify the physician conversing with the patient and in physical contact with the finger(s) over the wrist in a gesture that combines humanity and trust, greeting, comfort and reassurance. To adequately feel the pulse, the physician needs exert enough pressure to flatten the arterial wall so as to gauge the pressure within. 1 The physician will not detect the pulse if he/she presses so lightly that the artery is not deformed, or presses so heavily that the artery is occluded. The physics of this process involves applanation (flattening), and so, sensing the pressure within the artery by flattening a small part of the arterial wall under the sensor 2 —which in this case comprises the Paccinian corpuscles in the physician’s fingers over the flattened segment of artery. The pulse feels stronger if arterial pressure within is high or if the radial artery is large so that more Paccinian corpuscles are activated. Instruments to measure the arterial pulse at the wrist were introduced first by Marey in Paris then extended by Mahomed in England and taken up for clinical use by Mackenzie, Lewis and others 3–5 for clinical practice. At the turn of the 19th century the Dudgeon sphygmogram ( Figure 1 ) was widely used in specialist clinical and even general clinical practice. Sensors for recording pulses in the neck were introduced by Mackenzie, Lewis, Wiggers, 3–5 and others to record pulsations attributable to venous as well as arterial pulsations. These combined a tambour (like a stethoscope head) and air-filled tubing connected to a stylus which moved up and down with the carotid and jugular pulsations, tracing a pulse waveform with the stylus moving on smoked paper over a drum which was rotated by a clockwork motor. Improvements on this purely mechanical technique were used to obtain pressure wave tracings from the carotid arteries for measurement of systolic time intervals as indices of left ventricular function in more recent times, 6 and for distinguishing the different effects of predominantly large arterial vasodilators (such as nitrates and calcium-channel blockers), and arteriolar vasodilators (such as hydralazine and alpha-adrenergic blockers). 7 Dudgeon sphygmogram as used by Sir James Mackenzie circa 1900. Applanation tonometry has long been used by ophthalmologists on the eyeball to measure intra-ocular pressure in patients with suspected glaucoma. This technique flattens (applanates) a small part of the eyeball under a tiny sensor. It was first applied to measurement of the arterial pulse by Drzweicki et al . 2 and then popularized with introduction of the very precise applanation tonometer by Huntley Millar as a derivative of his catheter tip intra-arterial sensor. 8 Utility and accuracy of this for noninvasive determination of the arterial pulse was established by Kelly et al . 8 , and the instrument is now widely used in clinical practice and research to measure arterial waveforms, and arterial stiffness from determination of pulse wave velocity (PWV) over segments of artery. A scientific statement on arterial stiffness from the American Heart Association has just been released. 9 This relies heavily on use of applanation tonometry for measurement of aortic stiffness, and its direct and indirect effects from change in timing and amplitude of wave reflection. Use of applanation tonometry in the neck over the carotid artery remains central to measurement of arterial stiffness as “aortic” carotid/femoral PWV (cf-PWV) since it depends on measurement of wave speed along the wall of the arterial segment through identification of a feature (usually the wave foot) which is not affected by wave reflection. Through identification of the same feature on the femoral artery and from the time delay and distance between carotid and femoral sites, one can measure cf-PWV. The most recent 9 and earlier 10 consensus documents identify this as the most useful clinical measure of arterial stiffening with age, and an independent predictor of cardiovascular events. Use of carotid tonometry for measurement of cf-PWV requires gentle pressure on the carotid artery in the neck, just sufficient to identify its most obvious feature—a sudden upstroke which identifies the beginning of left ventricular ejection into the aorta. Carotid tonometry is now less favored for pulse wave analysis than in the past. It is difficult to be certain of obtaining adequate applanation since the artery can move freely under the sensor, and needs to be stabilized by pressure on other neck structures by the operator. Artifact is common—as from respiration and the procedure is uncomfortable to many. 11 Further it carries the possibility of activating baroreceptors, so leading to reflex changes in heart rate and arterial pressure. It also carries the risk of dislodging carotid arterial plaque or thrombus. Other problems with carotid tonometry arise from uncertainty in its calibration from conventional brachial cuff and upper limb tonometry. Anomalous results 12 are attributable to failure of applanation at the brachial as at the carotid sites. 13–15 The original European 10 and the recent AHA 9 consensus statements do not recommend tonometry for waveform analysis at either the brachial or carotid sites. Waveforms derived therefrom have not been shown to predict risk of cardiovascular events. The favored site for applanation tonometry remains the same (radial) site which has been used over eons past. 1 , 3 , 5 Carotid tonometry and potential problems with this are reviewed in the present issue of American Journal of Hypertension16 on the basis of a study by an experienced nurse in 26 patients, and with attention to time delays and changes in arterial pressure and heart rate. Findings confirm that carotid applanation tonometry as used for determining cf-PWV can be used with little change in heart rate or arterial pressure, whether used sequentially or simultaneously at the carotid or femoral site. A study such as this cannot however address the more serious issues of transient asystole and fitting (which the writer has seen) or of transient contra lateral hemiplegia (of which the writer has been informed). These problems are likely to be seen only in a large cohort and may not be reported. They are possible and need be considered as a reason for approaching carotid tonometry with caution, for using radial artery tonometry where possible for generation of central aortic pressure, and never applying pressure simultaneously on both carotid arteries as mentioned in this article. I would advise use of carotid tonometry with gentle pressure only for measurement of carotid femoral PWV, and for checking unusual central pressure waves generated from the radial waves, and not for routine measurement of central pressure waveforms. Michael O’Rourke is a founding director of AtCor Medical Pty Limited, manufacturer of systems for analyzing the arterial pulse and Aortic Wrap Pty Limited, developer of devices to improve aortic distensibility, and a consultant to Novartis and to Merck.