Lung diffusing capacity for nitric oxide at lowered and raised ambient pressures

- The lung diffusing capacity for nitric oxide (DLNO) was determined in healthy humans at ambient pressures ranging 0.5-4.0 atmospheres absolute (ATA).
- The diffusivity for NO in the background gas was 8 times lower at the highest pressure than at the lowest.
- Data fitted a model with a decreasing conductance through the gas phase in the peripheral lung with higher pressure.
- DLNO was estimated to be 5% lower than DmNO at 1.0 ATA and 17% lower than DmNO at 4.0 ATA.

Lung diffusing capacity for NO (DLNO) was determined in eight subjects at ambient pressures of 505, 1015, and 4053 hPa (379, 761 and 3040 mmHg) as they breathed normoxic gases. Mean values were 116.9 ± 11.1 (SEM), 113.4 ± 11.1 and 99.3 ± 10.1 ml min−1 hPa−1 at 505, 1015, and 4053 hPa, with a 13% difference between the two higher pressures (P = 0.017). The data were applied to a model with two serially coupled conductances; the gas phase (DgNO, variable with pressure), and the alveolo-capillary membrane (DmNO, constant). The data fitted the model well and we conclude that diffusive transport of NO in the peripheral lung is inversely related to gas density. At normal pressure DmNO was approximately 5% larger than DLNO, suggesting that the Dg factor then is not negligible. We also conclude that the density of the breathing gas is likely to impact the backdiffusion of naturally formed NO from conducting airways to the alveoli.