Physical fitness and cardiovascular response to lower body negative pressure

Fourteen young male volunteers (mean age 28.1 yr) underwent maximal exercise performance testing and lower body negative pressure (LBNP) challenge to -50 Torr. Two distinct groups, fit (F, n = 8), mean maximal aerobic capacity (V̇O(2 max)) = 70.2 ± 2.6 (SE) ml O₂ kg^-1.min^-1, and average fit (AF, n = 6), mean V̇O(2 max)) = 41.3 ± 2.9 ml O₂ kg^-1.min^-1, P<0.001, were evaluated. Rebreathing CO₂ cardiac outputs, heart rate (HR), blood pressure (BP), and leg circumference changes were monitored at each stage of progressive increases in LBNP to -50 Torr. The overall hemodynamic responses of both groups of subjects to LBNP were qualitatively similar to previous findings. There were no differences between F and AF in peripheral venous pooling as shown by a leg compliance (Δ leg volume/Δ LBNP) for the F of 12.6 ± 1.1 and for the AF 11.6 ± 2.0 P<0.05. The F subjects had significantly less tachycardic response [ΔHR/Δ systolic BP of F = 0.7 beats/Torr] to LBNP to -50 Torr than the AF subjects [ΔHR/Δ systolic BP of unfit (UF) = 1.36 beats/Torr], P<0.05. In addition, overall calculated peripheral vascular resistance was significantly higher in the AF subjects (P<0.001), and there was a more marked decrease in systolic BP of the F subjects between the LBN pressures of -32 to -50 Torr. We concluded that the reflex response to central hypovolemia was altered by endurance exercise training.