We thank Drs Cameron and Dart for drawing our attention to their studies of carotid applanation tonometry . In our article (their reference 1) we showed that inappropriate use of applanation tonometry at the brachial site overestimates brachial mean pressure, and form factor [(mean pressure − diastolic pressure)/pulse pressure], such that the method of scaling the central pressure waveform to brachial diastolic and mean pressure leads to overestimation of central systolic and pulse pressure. It results in identity of central and upper limb systolic and pulse pressure values. This method was introduced by Kelly and Fitchett in 1992 , and was used by Cameron, Dart and colleagues in the three articles they cite – “Waveforms were … scaled to brachial diastolic and mean pressure” (authors references 2 to 4). The Kelly and Fitchett method was confirmed by us (correspondents’ reference 1, Figure 4, right panels,) to generate identical central from radial pressure waveforms compared with those generated with a validated transfer function technique (US FDA K012487 and K002742). The use of brachial waveforms however, generated central waves with higher mean, systolic, and pulse pressures (same Figure 4). Cameron and Dart had used sphygmomanometric values of mean, as well as systolic and diastolic pressures, and we misinterpreted this to indicate that they had used brachial tonometry. They now emphasise that they did not derive mean pressure from brachial waveforms, but from an oscillometric cuff system.
Cameron and Dart have been studying carotid applanation tonometry for many years. In earlier studies on exercise [3,4], they measured carotid pressure waveforms, considered these identical to aortic pressure waveforms (from limited data in three patients), and took carotid systolic and diastolic pressures to be identical to brachial. Apparent improvement in aortic compliance could reasonably be attributed to improved endothelial function and decrease in the secondary peak of the carotid pressure wave. In the subsequent studies (correspondents’reference 2–4), brachial pressure was measured by an early model Dynamap instrument, which had been calibrated against invasive aortic rather than brachial cuff pressure; this overestimated systolic, diastolic and mean pressures [5,6]. Cameron and Dart introduced the method as an improvement on the previous approach which overlooked upper limb amplification, but its use also led to identical systolic and pulse pressure at the central and brachial sites, that is, pulse pressure 54 vs. 53, 79 vs. 79, 84.vs. 85 mmHg (P-value not significant in any) in their references 2, 3 and 4, respectively. Systolic pressures showed no amplification when available in the three articles. With central systolic and pulse pressures identical at central and brachial sites, it is surprising that they found one superior to the other in any of these articles. We agree with Cameron and Dart to “misclassification”, and with the need to be clear in describing methods, but we cannot accept that any of their methods generate accurate values of central pressure or improve on the standard cuff sphygmomanometer. None have been accepted by regulatory bodies such as the US FDA.
But other problems arise from consideration of these and other publications by the Cameron/ Dart group. In their reference 2, Table 1 shows brachial mean pressure to be just 5 mmHg greater than diastolic pressure for placebo and rampril groups, so that form factor was less than 10% instead of the usual 25–45% published elsewhere. They dismissed use of a generalized transfer function for generating central from radial pressure waveforms  on the basis of a mathematically flawed calculation of phase that had peripheral pressure preceding central pressure at higher frequencies .
We stand by our analyses, but agree with our colleagues that we have misclassified their problem; inappropriate brachial tonometry is just one source of error in estimation of mean pressure, and so in generating accurate central blood pressure.
In an accompanying commentary to our article (reference 1 of Cameron and Dart letter page 1551), referee 1 did not appreciate identity of the aortic waves calculated from radial waveforms using the transfer function and Kelly/ Fitchett methods . We thank our colleagues and editors for enabling us to clarify these issues for journal readers.
Conflicts of interest
M.F.O. is the founding director of AtCor Medical P/L, manufacturer of pulse wave analysis system and of Aortic Wrap P/L, developer of methods to reduce arterial stiffness, and consultant to Novartis. A.A. has no disclosure.
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