ABC | Volume 113, Nº6, December 2019

Original Article Elias Neto et al. Great arteries behavior during orthostasis Arq Bras Cardiol. 2019; 113(6):1072-1081 Because no significant increase in SBP was observed in orthostatic position, the increase in PWV may have resulted from circulatory dynamics disorders resulting from gravitational force, in association with structural and geometric characteristics of the aorta. This hypothesis is based on the Moens-Korteweg formula and on the knowledge that PWV depends on vascular radius and thickness, as well as on the vascular elastic module. Measurement of PWV during the tilt test exposes the arterial segments to gravitational stress in a distinguished form, imitating, to a certain extent, what occurs during active orthostatic position. In fact, the immediate consequence of orthostasis is that gravity favors a progressive increase in blood pressure in the segments below the cardiac level in orthostatic position. 8,16–18 The hydrostatic pressure generated by the gravitational force changes the indifferent hydrostatic point, defined as the axial reference in which the venous blood column pressure is not altered by postural reorientation. Such point is located at the right atrial level in supine position and in the infradiaphragmatic aortic territory in orthostatic position. Because of this, an increase in blood flow to the arterial segments with greater elastic module and smaller radius occurs, evidencing the increase in themeasured carotid-femoral PWV. 11,16 This increase in PWV accounts for the early return of the reflected waves from the peripheral sites to the ascending aorta. This wave, reflected earlier (during the ventricular ejection period), adds to the incident wave generated by left ventricular ejection Figure 3 – Sketch of the mechanism proposed for pulse wave velocity behavior in a healthy young individual. A – In the supine position, the reflected component occurs during the diastole due to a smaller PWV. B – In orthostatic position, due to the gravitational force, the indifferent hydrostatic point moves to the subdiaphragmatic aorta, which has a smaller radius and a greater elastic module; therefore, PWV increases, leading to an earlier return of the reflected component of the pulse wave, which then occurs with a systolic “pulse summation”. Thus, pulse wave morphology is altered. 321x263 mm (72 x 72 DPI) and influences the contour of the pressure and flow waves. 10 In other words, the earlier return of the reflected component, occurring during the systolic component of the pulse wave, leads to an increase in pulse pressure (pulse summation). 19–20 This increase, provided by the reflected wave in the initial portion of the arterial pulse wave, may result from a complex, evolutionary, functional, anatomical-humoral adaptation of the vascular system, to maintain an effective cerebral blood flow in response to bipedalism (Figure 3). 21 The observed increase in PWVp can also be attributed to the emergence of new sites of reflection of the pulse in the peripheral circulation due to a possible increase in peripheral vascular resistance in response to standing. A phasic response of the central hemodynamics to gravitational stress has been reported in experimental studies with baboons. A later systolic peak resulting from the reflected component of the pulse wave does not occur immediately after taking orthostatic position in these animals. Therefore, the reflected wave appears later in diastole, suggesting a decrease in PWV. Then, the so-called compensatory phase occurs during response to baroreflex. 12,22 The role played by wave reflection in the circulatory homeostasis in orthostatic position is reinforced by the observation that nitroglycerin, used sublingually for tilt test sensitization, causes peripheral vasodilation, leading to a delay in the reflected component of the pulse wave and consequent reduction in the proximal systolic pressure, culminating in symptoms of low cerebral blood flow in patients with neuromediated syncope. 23-24 1077