Influences on the Functional Behavior of Great Arteries during Orthostasis

Abstract

Background: Arterial compliance reduction has been associated with aging and hypertension in supine position. However, the dynamic effects of orthostatism on aortic distensibility has not been defined.

Objective: We sought to determine the orthostatic influence and the interference of age, blood pressure (BP) and heart rate (HR) on the great arteries during gravitational stress.

Methods: Ninety-three healthy volunteers (age 42 ± 16 years). Carotid-femoral pulse wave velocity (PWV) assumed as aortic stiffness was assessed in supine position (basal phase), during tilt test (TT) (orthostatic phase) and after return to supine position (recovery phase). Simultaneously with PWV acquisition, measures of BP and HR rate were recorded.

Results: PWV during TT increased significantly compared to the basal and recovery phases (11.7 ± 2.5 m/s vs. 10.1 ± 2.3 m/s and 9.5 ± 2.0 m/s). Systolic BP (r = 0.55, r = 0.46 and r = 0.39) and age (r = 0.59, r = 0.63 and r = 0.39) correlated with PWV in all phases. The significance level for all tests was established as α = 0.05.

Conclusion: We conclude that there is a permanent increase in PWV during orthostatic position that was returned to basal level at the recovery phase. This dynamic pattern of PWV response, during postural changes, can be explained by an increase in hydrostatic pressure at the level of abdominal aorta which with smaller radius and an increased elastic modulus, propagates the pulse in a faster way. Considering that it could increase central pulse reflection during the orthostatic position, we speculate that this mechanism may play a role in the overall adaptation of humans to gravitational stress.

Figure 1

Chart showing the dynamic behavior of hemodynamic parameters monitored during the protocol. 151x155 mm (96 x 96 DPI)

Figure 2

Chart of dispersion between carotid-femoral pulse wave velocity in the standing position (PWVp) and: A – basal carotid-femoral pulse wave velocity (basal PWV), p < 0.01; B – systolic blood pressure (SBP), p < 0.001; C – age, in 93 participants, p < 0.001; D – heart rate (HR) in 93 participants, p < 0.001.

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)

Figure 4

Hypothesis proposed for the role played by the increase in carotid-femoral pulse wave velocity in the maintenance of circulatory homeostasis in response to orthostatic stress. 309x165 mm (72 x 72 DPI). PP: pulse pressure; PWV: pulse wave velocity; MBP: medium blood pressure; HR: heart rate.