Effects of lower body pressure changes on pulmonary function

Purpose: During and following exercise there are a number of changes in pulmonary function, among which is a decrease in forced vital capacity (FVC). Several potential mechanisms may explain this decreased FVC, including an exercise-induced increase in thoracic blood volume. Methods: We tested the hypothesis that altered thoracic blood volume alone, as produced by the application of 30 mm Hg lower body negative (LBNP) or positive pressure (LBPP) for 5 min, would change FVC and forced expiratory volume in 1 s (FEV_1.0). Further, we tested whether the changes in pulmonary function were related to initial lung volume and whether the lower body pressure changes led to an altered lung compliance as measured by static pressure- volume curves. Results: Results indicated that with LBNP, FVC, and FEV_1.0 were significantly increased by approximately 0.15 L and 0.18 L, respectively. When LBPP was applied, FVC and FEV_1.0 were decreased by approximately 0.18 and 0.14 L, respectively. The increase in FVC with LBNP was significantly related to the original FVC (r = 0.66, P < 0.05). There was no significant correlation between the increase in FEV_1.0 and the original FEV_1.0 (r = 0.48, P > 0.05). Pulmonary compliance was not changed significantly by the application of LBPP. Conclusions: These results suggest that part of the change in pulmonary function following heavy exercise is related to an increased thoracic blood volume. The lack of change in lung compliance suggests that the effect of altered thoracic blood volume is to displace air and not to change the mechanical properties of the lungs.