Alveolar–capillary reserve in COPD assessed by the pulmonary diffusing capacity response to an upright‐to‐supine postural change

Abstract In patients with chronic obstructive pulmonary disease (COPD) who exhibit reduced alveolar–capillary reserve, the combined assessment of pulmonary diffusing capacity for carbon monoxide and nitric oxide ( D L,CO,NO ) during exercise may pose difficulties, and the transition from upright to supine posture may offer a useful alternative. A total of 50 participants (35 with COPD and 15 healthy controls) underwent measurements of D L,CO,NO in the upright and supine postures. A subset (COPD: n = 12, controls: n = 12) also completed a 12‐week supervised high‐intensity interval training (HIIT) intervention. The reported D L,CO,NO metrics were diffusing capacity for nitric oxide and carbon monoxide ( D L,NO and D L,CO,5s , respectively), alveolar–capillary membrane diffusing capacity ( D M,CO ), pulmonary capillary blood volume ( V C ), and alveolar volume ( V A ). The upright‐to‐supine change in neither D L,NO ( P = 0.271), nor D M,CO ( P = 0.068) nor V A ( P = 0.934) differed between groups. In contrast, the upright‐to‐supine change in D L,CO5s was reduced in moderate and severe COPD compared with controls (control vs. moderate: median IQR 0.6 0.3, 0.9 mmol/min/kPa, P < 0.001; control vs. severe: 0.9, 0.2, 1.5 mmol/min/kPa, P = 0.006), whereas it did not differ between controls and mild COPD (0.3 −0.1, 0.7 mmol/min/kPa, P = 0.13). Similarly, the upright‐to‐supine V C change was reduced in moderate and severe COPD compared with healthy controls (control vs. moderate: 8.3 3.9, 12.8 mL, P < 0.001; control vs. severe: 10.7 1.2, 20.2 mL, P = 0.021), but not in mild COPD (5.3 −0.2, 10.8 mL, P = 0.063). The HIIT intervention had no effect on these metrics. The blunted V C response to an upright‐to‐supine postural change in moderate‐to‐severe COPD is consistent with reduced alveolar–capillary reserve and may be useful when measurements during exercise are not possible.