EFFECTS OF HYPOVOLEMIA AND POSTURE ON RESPONSES TO THE VALSALVA MANEUVER

Introduction: We tested the hypotheses that hypovolemia would result i n attenuated elevation in blood pressure, greater baroreflex-mediated tachycardia, and reduced capacity for vasoconstriction during a Valsal va maneuver (VM). Methods: Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat before strain, during a 15-s VM strai n at 30 mmHg expiratory pressure, and post-strain. Eight subjects perf ormed three VM trials in each of three postures (supine, sitting, and standing) under two experimental conditions (normovolemic and hypovole mic). Hypovolemia was acutely induced by a bolus injection of 30 mg fu rosemide. Each experimental condition was conducted on a different day , separated by one week. Delta MAP was used in analyses of phase I, la te phase II (an indicator of vasoconstriction) and phase III of VM. Th e ratio Delta HR/Delta MAP, an index of nonspecific baroreflex control of HR, was used in analysis of early phase II and phase IV of the VM. Results: Compared to normovolemia, hypovolemia resulted in 12% lower plasma volume (p = 0.0001). Delta MAP for both phase I and phase III o f the VM differed between postures (p = 0.0132 and p = 0.0003, respect ively) and was lower in the hypovolemic condition than in the normovol emic condition for phase I in the standing posture (-5 mmHg, p = 0.038 5). Conclusions: HR and blood pressure responses to alterations in int rathoracic pressure are affected by fluid redistribution (posture chan ge), but not by circulating blood volume. Therefore, out results did n ot support our hypothesis that hypovolemia would result in attenuated elevation in blood pressure, greater baroreflex-mediated tachycardia, and reduced capacity for vasoconstriction during a Valsalva maneuver. However, moderate hypovolemia can be specifically predicted by the pha se I response to a VM performed in the standing posture.