Pulmonary capillary blood volume and diffusing membrane capacity during exercise in humans: role of pulmonary artery pressure

During exercise, lung diffusing capacity for carbon monoxide (DL_CO), pulmonary capillary blood volume (V_c), and diffusing membrane capacity (D_M) increase secondary to a rise in pulmonary artery pressure (PAP) and central blood volume mobilization. Although the role of central blood volume on DL_CO is well established, the impact of PAP on DL_CO, V_c, and D_M during exercise is less clear. Based on previous work, we tested the hypothesis that acute increases in PAP will potentiate exercise DL_CO via increases in D_M. Fifteen healthy young adults (7 females; age: 24 ± 4 yr) completed two bouts of cycling exercise at 60 W, with (CUFF) or without (CON) bilateral thigh cuff inflation pressurized to 90 mmHg. The multiple fractions of the inspired O₂-DL_CO method were used to determine DL_CO, V_c, and D_M at baseline and during both exercise conditions alongside estimates of cardiac output (Q̇_c; impedance cardiography) and right ventricular systolic pressure (RVSP; echocardiography). CUFF exercise resulted in a larger increase in RVSP (CUFF: 44.7 ± 6.1 vs. CON: 38.9 ± 5.5 mmHg; <i>P</i> = 0.036) but not Q̇_c (<i>P</i> = 0.644) or V̇o₂ (<i>P</i> = 0.976) compared with CON. DL_CO was higher during the CUFF exercise (CUFF: 41 ± 6 vs. CON: 38 ± 6 mL/min/mmHg; <i>P</i> = 0.001) and was mediated by increases in D_M (CUFF: 138 ± 55 vs. CON: 90 ± 39 mL/min/mmHg; <i>P</i> = 0.032), not V_c (CUFF: 85 ± 18 vs. CON: 98 ± 27 mL/min/mmHg; <i>P</i> = 0.820). Increases in RVSP were positively related to D_M (<i>r</i>_rm = 0.82; <i>P</i> = 0.024) but inversely related to V_c (<i>r</i>_rm = -0.80, <i>P</i> = 0.029). Collectively, these data indicate that PAP primarily contributes to DL_CO during low-intensity exercise via increases in capillary recruitment (i.e., D_M).<b>NEW & NOTEWORTHY</b> Pulmonary artery pressure contributes to DL_CO recruitment during exercise. However, it is unclear how pulmonary artery pressure impacts the pulmonary microcirculatory adjustments that comprise DL_CO, namely V_c and D_M. Using subsystolic occlusion of the locomotor muscles during exercise to induce increases in pulmonary artery pressure, without changes in cardiac output or metabolic demand, we demonstrate that pulmonary artery pressure contributes to DL_CO during exercise via heightened capillary recruitment (D_M).