Effects of blood volume distribution on ventilatory variables at rest and during exercise.

Ventilatory variables and heart rate (HR) were investigated in eight supine subjects during dynamic leg exercise at 0 and 100 W with and without the lower portion of the body exposed to a pressure of -50 mmHg (lower body negative pressure, LBNP). Resting values of inspired minute volume (V1), and respiratory drive in terms of mouth occlusion pressure (P0.1) were unchanged, whereas HR was higher during LBNP than in the control condition. Exercise values for HR in steady state were not affected by LBNP, whereas V1 was 15 and 11% lower and P0.1 was 20 and 11% lower in this condition at the 0 and 100 W workload levels, respectively. Time courses for V1 at the onset of 100 W exercise were similar with and without exposure to LBNP, indicating that the sudden increase in venous return occurring upon onset of pedalling during LBNP affords no significant stimulus in the initial development of exercise hyperpnoea. That exercise ventilation and P0.1 in steady state were reduced by LBNP suggests diminished humoral and/or locally induced chemical drive due to improved blood flow in exercising muscles resulting in reduced production of muscle metabolites or facilitation of their removal.