Decreased exercise-induced pulmonary capillary recruitment in pulmonary arterial hypertension.

The human lung normally accommodates exercise-induced cardiac output increases mainly via recruitment of non-concomitantly perfused pulmonary capillaries. Recruitment is detectable by measuring the first-pass transpulmonary metabolism of 3H-benzoyl-Phe-Ala-Pro, providing an estimate of functional capillary surface area (FCSA). Pulmonary arterial hypertension (PAH) results from luminal narrowing of small precapillary arterioles, reducing downstream perfused FCSA. We hypothesized that exercising PAH patients would not be able to recruit FCSA normally. We studied two patients with severe PAH. Despite exercising to maximal dyspnea, neither could recruit FCSA. These limited data are the first direct measurements of FCSA in exercising PAH patients.