Effects of moderate exercise training on plasma volume, baroreceptor sensitivity and orthostatic tolerance in healthy subjects

The effect of physical training on an individual's ability to withstand an orthostatic stress is unclear. This study was undertaken to determine the e ffects on orthostatic tolerance in healthy volunteers of training at a leve l appropriate for unfit subjects and cardiorespiratory patients. In 11 asym ptomatic, untrained subjects the following assessments were made: plasma vo lume by Evans Blue dye dilution and blood volume derived from haematocrit; carotid baroreceptor sensitivity from the slope of the regression of change in cardiac interval against pressure applied to a neck chamber; orthostati c tolerance as time to presyncope in a test of head-up tilting combined wit h lower body suction; exercise test relating heart rate to oxygen consumpti on. Subjects were then given a training schedule (5 BX/XBX, Royal Canadian Air Force) involving 11-12 min of mixed exercises per day until an age/sex related 'target' was reached. Following training all subjects showed eviden ce of improved fitness, seen as decreases in heart rate at an oxygen uptake ((V) over dot (O2)) of 1.5 l min(-1) and in the elevation of the regressio n line between heart rate and (V) over dot (O2). All also had increases in plasma and blood volumes and decreases in baroreceptor sensitivity: Seven o f the subjects showed increases in orthostatic tolerance. Improvement in or thostatic tolerance was related to a low initial tolerance, and was associa ted with increases in plasma volume and decreases in baroreceptor sensitivi ty. These results show that moderate exercise training increases orthostati c tolerance in subjects who do not already have a high initial tolerance an d suggest that training may be of value in the management of untrained pati ents with attacks of syncope due to orthostatic intolerance.