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 In reality, although a decrease in the coefficient of reflection is observed using nitroglycerin, an increase in aortic stiffness is paradoxically observed. This secondary aortic stiffness was attributed to a possible reflex activation of the sympathetic nervous system. 25 Analysis of PWV behavior in this series of patients also showed that the increase in PWV was positively correlated with age, showing that, even in the elderly, who have higher baseline PWV levels, an additional increase occurred by taking orthostatic position (r 2  = 0.357, p < 0.001 in the supine position and r 2  = 0.406, p < 0.001 in orthostatic position). In reality, this additional increase in PWV in the elderly results from the addition of structural findings with the dynamic postural component. Another intriguing aspect of this study was the discovery of a negative correlation betweenHR and PWV in orthostatic position (r = –0.36, p < 0.001). Age was the major variable responsible for HR behavior in orthostatic position (r 2  = 0.27, p < 0.001). Aiming at better understanding that finding , the correlation between HR measured during orthostatic stress and baseline PWV was also assessed. A negative correlation between HR and baseline PWV (r = –0.29, p < 0.01) was observed. This result shows a clear association of the baseline pattern of arterial compliance with the level of HR response to orthostatic position. Initial experimental studies and studies in human beings in the supine position reported a positive correlation between increased HR and increased aortic stiffness. 1,7,26–27 However, Wilkinson et al., 14 in a study assessing PWV behavior and the invasive augmentation index in healthy individuals undergoing atrial stimulation, reported no significant alterations in aortic distensibility due to an increase in HR. 14 Further to the lack of consensus on the effects of heart rate on PWV, the possible mechanisms contributing to PWV changes with heart rate have yet to be fully elucidated, although many investigators have attributed heart-rate related changes in arterial stiffness to the viscoelasticity of the arterial wall. With high heart rate being an independent prognostic factor of cardiovascular disease and its association with hypertension, the interaction between heart rate and PWV continues to be relevant in assessing cardiovascular risk. 28 Although a first analysis of these data point to a potential disagreement with our findings, they should be considered as complementary to each other and analyzed within a dynamic context, because they were obtained in very distinct physiological conditions. Cross-sectional studies have reported that baseline HR does not differ between young and elderly individuals in the supine position. 29–30 However, HR assessment of healthy individuals in the sitting position has shown that HR decreases with age in both sexes. On the other hand, studies using the tilt test to assess cardiovascular adaptation to orthostatic stress have also reported a significantly lower response of HR in elderly individuals. 16 The decrease in HR variability due to postural change observed in elderly patients compared with that in young individuals has been attributed to a decrease in the recruitment of the activity of baroreceptors in orthostatic position. 16,29–30 Considering the dynamic behavior of aortic compliance due to postural change, a greater decrease in systolic volume (SV) in young individuals could lead to a decrease in pulsatile aortic strain, with a subsequent decrease in baroreceptor stimulation and an increase in HR. 31 Maintenance of systolic volume in elderly individuals has been attributed to lower venous compliance in this group, allowing preservation of cardiac filling volume, and, consequently, of systolic volume. 11 However, a lower HR associated with age, such as that observed in this study, could also mean an adaptive mechanism of man to bipedalism. Heart rate is known to affect preload via its effect on diastolic filling time and to modulate the myocardial contractility status, altering the myocardial concentration of Ca 2+ and Na + . As a consequence of increase in myocardial contractility, HR also modulates end-systolic volume, systolic volume and ejection fraction. 11 Thus, a lower HR could initially allow better cardiac performance in the presence of a stiffer arterial system. Considering all that has been presented so far, the following hypothesis was formulated: the return of the reflected component of the pulse wave is fundamental to the immediate adaptation to orthostatic position, as well as to the adequate adaptation of baroreceptors. Based on data from this study, it is not possible to determine whether this increase is not only due to the dynamic circulatory changes secondary to gravity, but also to sympathetic activation in response to decreased stroke volume and pulse pressure. If the above hypothesis is correct, one may consider the role played by our findings in some very common clinical conditions associated with orthostatic position (Figure 4). 9,32–34 Study limitations This study has some limitations. Assessment of sympathetic response was not performed during the tilt test through venous measurement of catecholamines or recording of neural sympathetic activity through electroneuromyography. 35 However, although the present study did not directly assess vasomotor activity, an increase in mean arterial pressure (supine: 099 ± 15 mmHg vs. orthostasis: 103 ± 11 mmHg, p < 0.01) allowed us to infer that there has been an increase in peripheral vascular resistance due to the reflex sympathetic activation induced by the drop of pulse pressure in orthostasis. Another limiting factor was the lack of study of the baroreflex response to orthostatic stress caused by the tilt test, which resulted in lack of data referring to its dysfunction and to the specific location of alterations in its reflex arch. Change in HR was the only response observed, with no other baroreflex measurements, particularly those of systolic volume, although they have been shown to be closely related. 36 Another possible source of error in PWV measurement lies in determining the arterial segment. Its superficial and noninvasive measurement allows only an estimation of the distance traveled by the pulse wave. Obtainment of carotid-femoral PWV comprises the analysis of a relatively long arterial segment, which may be extremely tortuous from the three-dimensional point of view. Another factor is that the vessel may be distorted by the direct application of 1078