These limited data are the first direct measurements of functional capillary surface area in exercising PAH patients, demonstrating that despite exercising to maximal dyspnea, neither patient with severe PAH could recruit functional capillary surface area normally.
Key Findings
Results
PAH patients were unable to recruit functional capillary surface area (FCSA) during exercise despite exercising to maximal dyspnea.
Two patients with severe pulmonary arterial hypertension were studied.
Neither patient could recruit FCSA during exercise, in contrast to the normal physiological response of pulmonary capillary recruitment during increased cardiac output.
Patients exercised to maximal dyspnea, indicating the limitation was not due to submaximal effort.
These are described as 'the first direct measurements of FCSA in exercising PAH patients.'
Methods
Functional capillary surface area (FCSA) can be estimated by measuring first-pass transpulmonary metabolism of 3H-benzoyl-Phe-Ala-Pro.
The method measures first-pass transpulmonary metabolism of 3H-benzoyl-Phe-Ala-Pro to provide an estimate of FCSA.
This approach detects pulmonary capillary recruitment by quantifying functional capillary surface area.
The technique allows direct measurement of FCSA, which reflects the surface area of concomitantly perfused pulmonary capillaries.
Background
The normal human lung accommodates exercise-induced cardiac output increases mainly via recruitment of non-concomitantly perfused pulmonary capillaries.
Under normal physiological conditions, increased cardiac output during exercise is accommodated predominantly through pulmonary capillary recruitment rather than distension alone.
Non-concomitantly perfused capillaries are recruited during exercise to expand the functional capillary surface area.
This recruitment mechanism is the basis for the hypothesis that PAH patients would show impaired FCSA recruitment.
Background
Pulmonary arterial hypertension results from luminal narrowing of small precapillary arterioles, which reduces downstream perfused FCSA.
The pathophysiology of PAH involves luminal narrowing of small precapillary arterioles.
This narrowing reduces the downstream perfused functional capillary surface area at baseline.
The authors hypothesized that this structural limitation would prevent normal exercise-induced FCSA recruitment in PAH patients.
What This Means
This research suggests that patients with severe pulmonary arterial hypertension (PAH) — a serious condition involving high blood pressure in the lungs — are unable to expand their lung's blood vessel network during exercise the way healthy people can. Normally, when a person exercises and their heart pumps more blood, the lungs respond by opening up additional tiny blood vessels (capillaries) that weren't in use at rest, increasing the surface area available for gas exchange. The researchers measured this 'functional capillary surface area' using a specialized radioactive tracer technique, and found that two PAH patients could not recruit these additional capillaries even when they exercised as hard as they possibly could until limited by severe breathlessness.
This finding makes physiological sense because PAH causes the small arteries feeding the lung capillaries to become narrowed, which would physically prevent blood from reaching and opening those reserve capillaries during exercise. The inability to recruit additional capillary surface area during exercise likely contributes to the severe exercise intolerance that PAH patients experience.
While the study only involved two patients — making it preliminary and exploratory in nature — the authors note these are the first direct measurements of functional capillary surface area in exercising PAH patients. This research suggests a new way to understand and potentially measure the functional consequences of PAH on the lung circulation during exercise, which could have implications for assessing disease severity or treatment responses in the future.
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Langleben D, Orfanos S, Fox B, Giovinazzo M, Kaddis M, Catravas J. (2026). Decreased exercise-induced pulmonary capillary recruitment in pulmonary arterial hypertension.. Vascular pharmacology. https://doi.org/10.1016/j.vph.2026.107615