Page 129 - Computational Modeling in Biomedical Engineering and Medical Physics

118 Computational Modeling in Biomedical Engineering and Medical Physics

(Cox, 1971; Arts et al., 1979; Paquerot and Remoissenet, 1994). For example, at the
wrist, the pulse is felt when touching the radial artery with a 100 ms delay from the
moment of blood ejection in the aorta, due to different pressure wave velocity values
along the arterial network (lower speeds in large arteries which increase as the blood
vessel diameter shortens). Ageing hardens the smaller arterial walls and the pulse waves
may travel faster for older subjects. Meaningful information on the cardiovascular con-
dition may be obtained using pressure measurements (Benetos et al., 2012) performed
at the ascending aorta level, or the central aortic pressure (CAP) and cardiovascular
composite events and surrogate markers of cardiovascular disorders are linked signifi-
cantly to CAP (Agabiti-Rosei et al., 2007; Ghiadoni et al., 2009; Yao et al., 2018),
while peripheral vasculature and target organs [brain, heart, kidney (Mitchell, 2008)]
are directly exposed to it. Unfortunately, the “gold standard” CAP measurements are
invasive, only applicable during catheterization, and thus cannot be performed rou-
tinely therefore there is growing concern in the noninvasive measurement of CAP.
On the other hand, peripheral blood pressure (PBP) information can be obtained non-
invasively, and many noninvasive methods for reliable measuring BP waves based on
oscillometry, auscultation and tonometry are currently accessible (Bronzino, 2004; Lee and
Nam, 2009). The auscultation method uses the stethoscope to analyze the noises that occur
during the slow decompression of the cuff sited on the elbow. Oscillometry detects the
maximum pulsations matching the phenomena in the systolic diastolic sequence, sent out
by the arteries compressed by a pneumatic sleeve (Stergiou et al., 2006; Sorvoja, 2006).
Systolic blood pressure (SBP) is measured during the period of ventricular contraction.
Diastolic blood pressure (DBP) is measured between periods of ventricular contraction.
However, PBP information (e.g., brachial blood pressure) per se, conveys less accu-
rately CAP information, and shows off different response to certain drugs (O’Rourke,
2006; O’Rourke et al., 2001). To circumvent these difficulties and shortcomings and
use PBP, recently, there is growing concern in the development of methods and
devices for the noninvasive extract CAP out of PBP (Yao, et al., 2018), and the pulse
wave reflection is the underlying signal that may be used to this aim.
The contraction of the heart increases the pressure in the aorta eliciting a direct,
“ejected” pressure wave (EPW) that drives the blood flow throughout the body. The
EPW may then be partially reflected from an artery bifurcation or from a peripheral
artery: the structure and properties of the downstream vascular network, the peripheral
vascular resistance, the elasticity and compliance of blood vessels may cause a reverse,
reflected pressure wave (RPW), which interacts with the EPW and patterns the mor-
phology of the pulse wave (Wang and Parker, 2004; Wang et al., 2004, 2006; van der
Vosse and Stergiopulos, 2011).
The radial, ulnar, and brachial arteries stiffness does not change significantly with
aging, hypertension, and exercise among subjects with similar physiological and patholog-
ical characteristics, which sets the theoretical grounds for the generalized transfer function