Calibration of Central Pressures: A Flaw in Brachial Applanation Tonometry?: 3c.03

Amplification of the pressure pulse in the upper limb causes systolic and pulse pressures (PP) to be higher at the wrist and elbow than in the ascending aorta. While one method for measuring central (ascending aorta) pressure from the radial artery pressure wave has shown superiority of aortic pressure for predicting outcomes, another, based on carotid and brachial tonometry, has not. We considered that a cause for this may be in accuracy of brachial tonometry to applanate the artery and register the arterial waveform. Introduction of Form Factor (FF) to characterize the waveforms at different sites provides a method to study this. FF is = (Mean Pressure - Diastolic Pressure) / PP. For calculation of central PP (and systolic pressure), mean and diastolic pressure are considered identical at both sites so that PP at each sites is inversely proportional to FF at each site; i.e. for Carotid PP ∼ 1/FF(carotid) and for Brachial PP ∼ 1/FF(brachial). We measured carotid, brachial and radial pressure waves by a Millar applanation tonometer at the 3 sites in 101 patiens and volunteers aged 21–93 years at an outpatients clinic and assigned to them the same diastolic and mean pressure, then calculated systolic pressure and FF at the 3 sites. FF was almost identical in carotid and brachial sites (40.2 c.f. 39.1%; p = NS) and so PP was almost identical (65.1 c.f. 66.6 mmHg; p = NS), whereas FF was quite different between carotid and radial sites (40.2 c.f. 34.5; p = .000) and amplification was substantial (65.1 to 75.1 mmHg; p = .000), as seen in invasive studies. Similarity of carotid and brachial waveform can be attributed to failure of applanation at the brachial site, since the artery is unsupported and needs be sensed through the bicipital aponeurosis. Identity of invasive and tonometric waves has been confirmed at radial and carotid sites, but never at the brachial artery.