Is Pulmonary Microvascular Recruitment Possible during Supine Bicycle Exercise? - The 63rd Annual Conference of the Israel Heart Society in association with the Israel Society of Cardiothoracic Surgery 12-13 April 2016, David Intercontinental Hotel, Tel-Aviv, Israel

David Langleben ¹ Stylianos Orfanos ² Michele Giovinazzo ¹ Robert Schlesinger ¹ Fay Blenkhorn ¹ John Catravas ³

¹Center for Pulmonary Vascular Disease, Div. of Cardiology, Jewish General Hospital, McGill University, Montreal, Quebec
²Pulmonary Hypertension Clinic, Dept. of Critical Care, Attikon Hospital, University of Athens, Athens
³Frank Reidy Research Center for Bioelectrics, Old Dominion University, Norfolk, VA

Introduction: Because it normally operates at low blood pressure and a low resistance, the human pulmonary circulation demonstrates gravity-induced gradients in perfusion from the apex to base, in the upright body position. Thus, there are normally “unrecruited” unperfused capillaries in the vertical upper lung at rest. These capillaries can normally be recruited during high pulmonary blood flow states, such as during exercise. It has been proposed that, in the supine position, gravitational gradients are minimized and the supine lung at rest is “maximally or nearly maximally recruited”. Using a technique that sensitively measures the perfused functional capillary surface area (FCSA), we are testing whether FCSA recruitment is possible during supine exercise in humans. There have previously been no studies in normal subjects and, in the course of studies to evaluate patients with presumed pulmonary hypertension, we have been fortunate to identify 2 rare normals that underwent exercise-heart catheterization.

Methods: We assessed the amount of FCSA by measuring the first-pass transpulmonary metabolism of ³H-benzoyl-Phe-Ala-Pro, by the endothelial ectoenzyme angiotensin converting-enzyme, at rest and at peak exercise using a bicycle ergometer during cardiac catheterization.
Results: The subjects exercised for a mean of 792 seconds to a mean workload of 52 Watts. They had normal levels of FCSA at rest. Cardiac output doubled from mean 5.8 L/min to 12.1 L/min. FCSA increased from mean 3729 mL/min to 5800 mL/min, a 56% increase. The increases in FCSA in relation to flow were parallel and identical in both subjects.

Conclusion: Although the increase in FCSA was not fully proportionate (i.e. 1:1) to increased pulmonary blood flow (cardiac output), these studies provide the first evidence that in exercising normal supine humans, FCSA recruitment occurs. This validated technique can therefore be used to study abnormal patterns of recruitment in pulmonary hypertension.