Exercise physiology and pulmonary arterial hypertension.

The lungs are the only organ that receives the entire cardiac output with every stroke. The pulmonary circulation is normally a high-flow, low-resistance, low-pressure system that carries blood into the pulmonary microcirculation. In pulmonary artery hypertension (PAH)vascular remodeling contributes to a sustained elevation of pulmonary vascular resistance (PVR) and pulmonary artery pressure (PAP) as a result of vascular remodeling characterized largely by vascular smooth muscle cell proliferation and medial hypertrophy, and endothelial cell proliferation resulting in lumen obliteration. The loss of pulmonary arterial compliance and development of elevated PVR puts an excessive burden on the right ventricle due to the increased workload necessary to overcome the downstream pressure, ultimately leading to right-sided heart failure. The functional status of the pulmonary circulation and the levels of PVR and PAP ultimately determine the outcome of patients with PAH. Study of the pressure-flow relationships in the pulmonary vascular bed will provide an improved appreciation of the pathophysiology of pulmonary hypertension.