The transition point from predominantly pulmonary capillary recruitment to pulmonary capillary distention in exercising humans. - The 67th Annual Conference of the Israel Heart Society

Benjamin Fox ¹ David Langleben ² Stylianos Orfanos ³ Michele Giovinazzo ² Robert Schlesinger ² Ali Abualsaud ² Lawrence Rudski ² John Catravas ⁴

¹Respirology, Yitzhak Shamir Medical Center, Tel Aviv University, Israel
²Center for Pulmonary Vascular Disease, Azrieli Heart Center, Jewish General Hospital, McGill University, Canada
³Pulmonary Hypertension Center, Attikon Hospital, University of Athens, Greece
⁴Frank Reidy Research Center for Bioelectrics, Old Dominion University, USA

Introduction: The human lung accommodates large increases in cardiac output (CO) during exercise, with small increases in pulmonary arterial pressure (PAP). At moderate resting CO, increasing blood flow causes principally recruitment of non-concomitantly perfused pulmonary capillaries, while at higher CO further increases appear to cause distention of already perfused capillaries (Am J Physiol Lung Cell Mol Physiol 2019;317:L361–L368). We now identify the transition range from principally recruitment to increasing distention in humans.
Materials and methods: Research Ethics Committee approved, and informed written consent was obtained. Ten subjects (9 female) performed symptom-limited supine exercise during cardiac catheterization. Six had resting mean PAP (PAPm) ≤ 20 mmHg, and four had PAPm between 21 and 24 mmHg. The first-pass transpulmonary circulatory metabolism of injected [³H]benzoyl-Phe-Ala-Pro (BPAP) was measured at rest and peak exercise, and functional capillary surface area (FCSA) was calculated. A well-fit (r=0.89, p<0.001) polynomial curve with 95% confidence intervals, for FCSA versus CO, was compared to known theoretical curves for pure capillary recruitment, capillary distention without increased FCSA, and distention with some increase in FCSA.
Results: Resting CO (6.3±1.6 L/min) increased up to 3.3 x baseline during exercise. At moderate increases of flow, based on the polynomial curve and 95% confidence intervals, the pattern of increased FCSA was that of nearly pure recruitment. However, at approximately a CO of 12.5 L/min, the curve progressively diverged from pure recruitment, indicating the increase of capillary distention.
Conclusions: Our findings clarify an important physiologic question. Humans undergo both pulmonary capillary recruitment and distention during supine exercise, depending on the amount of pulmonary blood flow. The transition point is between 2 and 3 fold resting flow. This pattern is similar to that described in other mammalian lungs.