Effect of varying alveolar oxygen partial pressure on diffusing capacity for nitric oxide and carbon monoxide, membrane diffusing capacity and lung capillary blood volume

1. To examine the effect of varying oxygen partial pressure (Pao₂) on nitric oxide (DLNO) and carbon monoxide (DLCO) diffusing capacity (transfer factor), 10 subjects performed combined DLCO/DLNO measurements with the inspired mixture made up with three different oxygen concentrations (25%, 18% and 15%) to give Pao₂ values of 12–20 kPa.

2. A novel method is described for calculating membrane diffusing capacity (DM) and pulmonary capillary volume (Qc) from DLNO and DLCO.

3. The mean DMCO was 52.89 mmol min⁻¹ kPa⁻¹ and Qc was 0.056 litre. Reducing Pao₂ from 20 to 12 kPa resulted in an increase in DLCO = −0.124 (O₂%) + 11.67 (P < 0.001) and a fall in DLNO = 0.538 (O₂%) + 32.01 (P < 0.001) and a fall in DLNO/DLCO = 0.107 (O₂%) + 2.52 (P < 0.001). DM (P = 0.59) and Qc (P = 0.64) also tended to fall with falling Pao₂.

4. It appears more likely that the minor reduction in DLNO that we have observed with falling Pao₂ is due to diffusion rather than reaction limitation.