The Impact of the Vasodilator, Sildenafil, on the Pulmonary Circulation During Exercise in Healthy Trained and Untrained Humans

Background

There is emerging evidence suggesting that the pulmonary vasculature and right heart may play a role in the limitation of exercise capacity in healthy individuals. It is well established that aerobic training improves cardiovascular function. While the pulmonary system is integral to the function of the cardiopulmonary system, it has been traditionally accepted that lung function does not respond to exercise training. However, recent research suggests pulmonary vascular function adaptations may occur with aerobic training, and this may contribute to enhanced exercise tolerance.

Research has highlighted that increased capillary blood volume (Vc) and diffusion capacity for carbon monoxide (DLCO) are correlated with higher cardiorespiratory fitness at rest. Additionally, endurance trained participants have increased exercise DLCO concomitant to higher resting Vc when compared to untrained participants, and during exercise this difference seems to be driven by higher membrane diffusing capacity (Dm), independent of Vc or VA (alveolar volume). Of importance is also the evidence that highlights endurance trained participants having reduced pulmonary arterial pressures at rest and during exercise. Reduced pulmonary arterial pressure in endurance trained participants despite endurance trained participants consistently displaying increased diffusion capacity/pulmonary perfusion at rest and during exercise suggests a lower threshold pressure for pulmonary capillary recruitment. Together, this cross-sectional evidence suggests improvements in the pulmonary circulation due to exercise training in order to facilitate gas exchange. Whether this apparent improvement in pulmonary circulation is due to enhanced pulmonary vascular function via NO mediated vasodilation must be determined experimentally. If sildenafil administration improves DLCO, Vc, and Dm, this would provide evidence that the NO mediated vasodilatory pathway plays a role in the regulation of vascular tone, function, and perfusion across the pulmonary vasculature. Should a larger response to sildenafil be observed in untrained persons, this would suggest better baseline vascular function in trained participants compared to untrained. This would provide strong evidence that aerobic training improves pulmonary vasculature function which is contrary to the conventional understanding of aerobic training on the cardiopulmonary system.

Some investigations on the pulmonary vasculature have been completed with the known pulmonary vasodilator, sildenafil. Sildenafil administration has been shown to reduce pulmonary artery pressure and improve exercise tolerance in normobaric hypoxic conditions in young healthy individuals. Further, one of these investigations determined sildenafil did not alter maximal oxygen consumption (V̇O2peak) in normoxia in moderately fit participants; however, this study was likely underpowered to detect a response. While some sildenafil interventional work has been conducted, no study to date has determined the effect of sildenafil on DLCO, Vc, and Dm during exercise in untrained vs. trained participants. If sildenafil administration improves DLCO, Vc, and Dm, this would provide evidence that the NO mediated vasodilatory pathway plays a role in the regulation of vascular tone, function, and perfusion across the pulmonary vasculature. Should a larger response to sildenafil be observed in untrained persons, this would suggest better baseline vascular function in trained participants compared to untrained. This would provide strong evidence that aerobic training improves pulmonary vasculature function which is contrary to the conventional understanding of aerobic training on the cardiopulmonary system.

Purpose