EFFECTS OF ACUTE EXPANSION OF PLASMA-VOLUME ON CARDIOVASCULAR AND THERMAL FUNCTION DURING PROLONGED EXERCISE

To investigate the hypothesis that an increase in plasma volume (PV) i s obligatory in reducing the cardiovascular drift that is associated w ith prolonged exercise following training, a plasma expander (Macrodex ) was used to acutely elevate PV. Eight untrained volunteers [maximal oxygen consumption; VO2max 45.2 (2.2) ml.kg(-1).min(-1), mean (SE)] cy cled for 2 h [at 46 (4)% VO2max] in ambient conditions either with no PV expansion (CON! or following PV expansions of either 14% (LOW) or 2 1% (HIGH). During CON? heart rate (HR) increased (P < 0.05) from 147 ( 2.4) beats.min(-1) to 173 (3.6) beats.min(-1) from 15 to 120 min of ex ercise. Both LOW and HIGH conditions depressed (P < 0.05) HR. an effec t that was manifested following 15 min of exercise. In contrast. strok e volume (SV) was elevated following PV expansion, with values (mi) of 89.6 (6.8), 97.8 (5.9) and 104 (4.6) noted by 15 min of exercise for CON, LOW and HIGH conditions, respectively. Acute PV expansion, regard less of magnitude, also resulted in elevations in cardiac output (Q(c) ). These differences between conditions persisted throughout the exerc ise? as did the elevation in Q(c) that was noted with LOW and HIGH con ditions. No difference between Q(c), HR or SV was found between LOW an d HIGH. In addition. neither LOW nor HIGH conditions altered the chang e in rectal temperature that was observed during exercise. These resul ts demonstrate that. at least for moderate exercise performed in ambie nt conditions, PV expansion serves only to alter cardiac function (Q(c ), HR, SV) early in exercise, and not to attenuate the drift that occu rs as the exercise is prolonged.