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

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.