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Electrical activity of the heart 119

(GTF) to estimate CAP out of brachial or radial pressure waveform (Yao et al., 2018).
Moreover, the need for a rigid structure in the vicinity of the artery, which may promote
uniform compression and occlusion of the vessel, recommends the selection of an artery
close to the body surface, for example, the radial or the brachial arteries (Stergiou et al.,
2006; Lee and Nam, 2009; OMRON, 2020). The GTF implements a low-pass filter
that cuts off the high harmonics of the pressure waveform traveling from central aorta to
the periphery, and it can provide not only quantitative CAP but also CAP waveform
(Chen et al., 1997; Lee and Nam, 2009). GTF is implemented in the first device accepted
by US Food and Drug Administration for the estimation of CAP (Pauca et al., 2001),
and it is the most widely used method so far (Yao et al., 2018). On the other hand, GTF
was questioned in chronic kidney disease or arterial stiffness, and not all algorithms that
implementGTFshavethe same accuracy (Hope et al., 2003; Yao et al., 2018).
For adults from midlife onward, systolic CAP can be calculated via a regression
equation using the second systolic peak as an independent variable because the RPW
peak in the periphery approximates the central SBP—the pressure gradients in the
arterial system are relatively small during late systole, and the late systolic shoulder
represents the major peak for them (Pauca et al., 2001; OMRON, 2020). Other GTF
specialized methods for CAP estimation are available (Yao et al., 2018): N-point mov-
ing average, NPMA (a first-order low-pass filter that removes the pressure wave reflec-
tions, providing only the central aortic SBP); adaptive transfer function (for tuning the
GTF), individualized transfer function (ITF; uses an individualized physical transmis-
sion line for the aorta-brachial and aorta-radial model), blind system identification, BSI
(reconstructs the input out of two or more outputs). Whatever method is used, the
tonometry waveforms in carotid artery are calibrated to brachial SBP and DBP.
Applanation tonometry (AT), an oscillometric method, was introduced in ophthal-
mology to assess the pressure exerted by intraocular fluids on the cornea (Applanation
Tonometry, 2020). Eventually the tonometric estimation was used to measure the pulse
wave of a superficial artery noninvasively too (Kelly et al., 1989) and it evolved into the
arterial applanation tonometry (AAT) (Pressman and Newgard, 1963). While the sphyg-
momanometer measures only AP and DP, the AAT provides continuous pulse wave-
form with pressure sensor placed over a superficial artery. AAT is use to diagnose
atherosclerosis and the factors that can cause myocardial infarction, and it is aimed to
estimate BCP (Kips et al., 2011; Cheng et al., 2013; Zayat et al., 2017) and solve for the
disagreement between CAP and PBP that was evidenced to augment with the posology
of vasoactive agents (Mackenzie et al., 2009).

The augmentation index
The AAT pressure readout is not identical to the invasively measured one, and the
pressure applied to flatten the arterial wall and compress overlying tissues must be