Page 131 - Computational Modeling in Biomedical Engineering and Medical Physics

120 Computational Modeling in Biomedical Engineering and Medical Physics

Figure 4.20 The pulse wave—a superposition of the ejected and reflected pulse waves (left)—and
the augmentation index (right).

accounted for. The AAT master pressure wavy profile (Fig. 4.20) shows off an incisura
(a deep indentation) associated with the RPW. The weights of the DPW and RPW
define the augmentation index (AI), which is a measure of arterial stiffness: the larger
the RPW the higher the AI, hence the stiffer the arteries are.
In fact, arterial stiffness progresses with aging and due to disorders such as hyper-
tension, hypercholesterolemia, and diabetes. The left ventricular ejection pressure
wave propagates faster through stiffer arteries, which leads to faster return of the RPW
to the left ventricle. The RPW arriving during systole augments the late SBP (after-
load) on the left ventricle. Moreover, the peak of the RPW approaches that of the
EPW. Consequently, the heart has to enhance the myocardial contractility to increase
the blood pressure, which poses a higher “load on the heart.” If this action lasts longer
the heart eventually gets strained. The reduction of coronary artery perfusion pressure
leads to greater risk of angina, heart attack, stroke, and heart failure.
The pulse waveform obtained using AAT and GTF may satisfactorily estimate the
arterial compliance. However, some concern is noted regarding AI recovery because the
postprocessing relies on rendering the wave profile with higher fidelity. AI is calculated
as the difference between the second and first systolic peak pressure (P2 P1) divided
through the pulse pressure, expressed as percentage of the BCP (Sievi et al., 2015). Vital
hemodynamic parameters may be thus reliably obtained through tonometry, which may
be designed specifically to measure the cardiac output, the stroke volume (Zayatetal.,
2017), the arterial blood pressure (Kemmotsu et al., 1991a,b), and others.

The generalized transfer function

The aortic pressure waveform for AI calculation can be estimated either from the
radial artery waveform, using a transfer function, or from the common carotid wave-
form. The AT method (including the tonometer) may be seen as a metrological device
that convolves some input (arterial pressure here) to be presented as a readout signal.
Its functioning is actually a transfer function (TF) that maps the output signal