265 HIGHER RESTING SUPINE BLOOD PRESSURE IS ASSOCIATED WITH AN INITIAL RISE IN CARDIAC OUTPUT ON STANDING

Abstract

Background: Active standing is normally associated with a large transient reduction in systolic blood pressure (SBP) within the first 30 sec. This can be associated with symptoms of pre-syncope and occasionally loss of consciousness. The initial hemodynamic response to active standing is complex and a transient reduction in total peripheral resistance (TPR) is a characteristic feature, dependent on cardiovascular autonomic reflexes related to redistribution of blood and skeletal muscle contraction. Methods: SBP, heart rate (HR), cardiac output (CO), stroke volume (SV) and TPR were measured continuously (Finometer Midi) before and on standing in 309 medical students of median age 22 (54% female). Results: Although SBP fell [median: -44 (interquartile range: 23) mmHg] and heart rate rose [+34 (12) min−1], there were both qualitative rises (n = 175, +1.3 L/min) and falls (n = 134, -0.8 L/min) in CO during the first 30 seconds on standing, without significant differences in age, sex, weight or height. Those in whom CO rose on standing had a lesser fall in SBP (-36 v -55 mmHg, P < 0.0001). Compared with people who show an early fall in CO on standing, those who show an early increase in CO have a slightly higher supine SBP (115 v 111 mmHg, P < 0.0001) and higher TPR (1051 v 894 units, P < 0.0001). Conclusions: A rise in CO upon standing might define a phenotypic subgroup in which cardiovascular reflexes program higher supine SBP and TPR. Such a quantitative phenotype might be useful for genetic studies.