Cardiac Autonomic Response to Hypovolemia --- Effect of Age




Wang, Hong-Wei


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Objective: The beat-to-beat variation in heart rate reflects the dynamic response of the cardiovascular control systems to physiological perturbations such as respiration and postural change. The heart rate variation (HRV) is a part of the rapidly reacting component of cardiovascular homeostasis largely influenced by parasympathetic and sympathetic input. Thus, beat-to-beat variation in heart rate can be used as a measure of cardiac autonomic responses. The standard deviation of R-R interval is a measure of the overall variability in heart rate and has been shown to decrease with aging (9,23). This measure, however, can not identify individual sources of the variation. Spectral analysis reduces a signal to its constituent frequency components and the relative power of these components has been indicated related to parasympathetic mediation, combined sympathetic and parasympathetic mediation, and sympathetic mediation (1, 14, 17). Limited data are available regarding power spectral analysis of heart rate variation to study aging changes under orthostatic stress. An attenuated cardiac sympathetic nerve activity was found in older group vs younger individuals (7, 11, 19) during posture change. However, these studies were carried out using either posture change from to upright or passive head-up tilt. During these posture changes, both cardiopulmonary baroreceptors (i.e., intrathoracic hypovolemia) and arterial baroreceptors (postural hypotension) were unloaded. Therefore, it is unclear whether there is any difference in the HR variability between the young and the elderly during unloading of cardiopulmonary baroreceptors (or low-pressure baroreceptors) alone. In this study, we investigated the age difference in cardiac autonomic modulation of heart rate during unloading of cardiopulmonary baroreceptors with or without systemic arterial hypotension. For the purpose, we examined beat-to-beat heart rate variability in both the time and frequency domain using power spectral analyses in healthy individuals from ages 18 to 68 under basal conditions and in response to graded lower body negative pressure induced central hypovolemia. Not only individual low and high frequency spectral content were analyzed and those parameters were compared in order to find a quantitative evaluation of sympathetic and parasympathetic modulation and under the graded lower body negative pressure.