Effect of saline infusion during exercise on thermal and circulatory regulations

To quantify the effect of an acute increase in plasma volume (PV) on forearm blood flow (FBF), heart rate (HR), and esophageal temperature (T(es)) during exercise, we studied six male volunteers who exercised on a cycle ergometer at 60% of maximal aerobic power for 50 min in a warm [(W), 30°C, < 30% relative humidity (rh)] or cool environment [(C), 22°C, < 30% rh] with isotonic saline infusion [Inf(+)] or without infusion [Inf(-)]. The infusion was performed at a constant rate of 0.29 ml·kg body wt^-1·min^-1 for 20-50 min of exercise to mimic fluid intake during exercise. PV decreased by ~5 ml/kg body wt within the first 10 min of exercise in all protocols. Therefore, PV in Inf(-) was maintained at the same reduced level by 50 min of exercise in both ambient temperatures, whereas PV in Inf(+) increased toward the preexercise level and recovered ~4.5 ml/kg body wt by 50 min in both temperatures. The restoration of PV during exercise suppressed the HR increase by 6 beats/min at 50 min of exercise in W; however, infusion had no effect on HR in C. In W, FBF in Inf(+) continued to increase linearly as T(es) rose to 38.1°C by the end of exercise, whereas FBF in Inf(-) plateaued when T(es) reached ~37.7°C. The infusion in C had only a minor effect on FBF. These results suggest that even a partial restoration of the lost PV during exercise is useful to suppress the gradual drift in HR and to promote better heat transfer from the body core to the skin in a warm environment.