Six subjects, aged 25.8 ± 1 years, performed steady-state dynamic exercise to investigate the cardiorespiratory responses to reductions in blood flow during exercise. Each subject exercised at 40% and 70% of their maximal oxygen uptake (˙VO2max) for 20 minutes each. At the tenth minute of each exercise bout, a cardioselective β1-antagonist (Esmolol, 0.5 ml/min/kg) was administered in order to decrease heart rate (HR) and cardiac output. HR (beats·min-1), mean arterial pressure (MAP; mmHg), cardiac output (L·min-1), oxygen uptake(ml·min-1·kg-1), arteriovenous oxygen (O2) difference (A-V O2diff; ml·100ml-1), and percent oxygen extraction (% O2 ext.) were measured, see mean data intable (*P<0.05).
Oxygen uptake and stroke volume (SV) were not significantly altered byβ1-blockade at either condition. Thus, as flow (cardiac output) was reduced, O2 extraction increased in order to compensate for a lack of O2 delivery to active muscle without an increase in SV or ˙VO2. We conclude that O2 extraction is the primary compensation for reductions in flow during dynamic exercise and must be tightly coupled with O2 diffusion gradients within the muscle.
E-22 POSTER CARDIOVASCULAR