Distribution of cardiac output during exercise/heat stress: influence of age and fitness level

During dynamic exercise in warm environments, the increase in skin blood flow (SkBF) is supported by an increase in cardiac output (Qc) and a decrease in splanchnic (SBF) and renal blood flow (RBF). To examine interactions between age and fitness on these regional blood flows, men (6 per group I were recruited in 4 categories: young-fit (YF; 24±2 yr, VO2max 62±2 ml/kg/min, 99th %ile), young-sedentary (YS; 26±2, 43±1. 45th), old-fit (OF: 64±2, 42+}, 90th), and old-sedentary (OS; 65±l, 28+1. 30th). Subjects exercised at 36°C at 35% (20 min) and 60% (30 min) of cycle VO2peak. Qc was measured by a CO2-rebreathing technique, SBF by ICG clearance, and RBF by PAH clearance under steady state conditions at each intensity. Forearm blood flow (FBF, venous occlusion plethysmography) was used to. examine changes in SkBF. At 60% VO2peak, YF had a significantly higher Qc and FBF than any other group. In the O group, fitness level had no significant effect on any measured variable. Regardless of fitness level. Y subjects had greater (p<.02) decreases in SBF and RBF during exercise. Total flow redirected from these 2 vascular beds (ASBF + ARBF) was as follows: YF: 1128±69 mL/min, YS: 779±110, OF: 568±68. OS: 427±8I. It was concluded that during moderate exercise at a given relative exercise intensity, regional blood flow is affected by age, with O men showing smaller increases in SkBF and smaller decreases in SBF and RBF. This age effect may not be preventable by regular aerobic exercise. Conversely, the higher SkBF observed in very fit Y men (compared to their sedentary counterparts) is a function of both a higher Qc and a greater redistribution of flow from splanchnic and renal circulations.