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 (VO2max) = 70.2 +/- 2.6 (SE) ml O2 kg-1 X min-1, and average fit (AF, n = 6), mean VO2 max V 41.3 +/- 2.9 ml O2 kg-1 X min-1, P less than 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 (delta leg volume/delta LBNP) for the F of 12.6 +/- 1.1 and for the AF 11.6 +/- 2.0, P greater than 0.05. The F subjects had significantly less tachycardic response [delta HR/delta systolic BP of F = 0.7 beats/Torr] to LBNP to -50 Torr than the AF subjects [delta HR/delta systolic BP of unfit (UF) = 1.36 beats/Torr], P less than 0.05. In addition, overall calculated peripheral vascular resistance was significantly higher in the AF subjects (P less than 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.