Single-breath diffusing capacities for NO, CO and C18O2 in rabbits

Nitric oxide (NO) has been introduced recently for studying alveolar-capillary gas transfer. Due to extremely fast reaction kinetics for the association of NO with haemoglobin, pulmonary NO uptake is expected to depend only on diffusion, whereas in the case of carbon monoxide (CO) or oxygen-labelled carbon dioxide (C18O2) the alveolar-capillary transfer is, in addition, known to depend on a blood uptake component. To provide further data for NO, CO and C18O2, we determined the pulmonary diffusing capacities (DL) for the indicator gases mentioned, performing single-breath manoeuvres on ten rabbits. The inspired gas mixtures contained 0.05% NO and/or 0. 2% CO or 1% C18O2 in nitrogen. Applying respiratory mass spectrometry to the expirates we obtained the following mean +/- SD values: DL,NO/DL,CO = 3.55 +/- 0.4, DL,C18O2/DL,NO = 6.0 +/- 0.6, DL, C18O2/DL,CO = 21.4 +/- 2.5. Graham's law predicts DL ratios of 1.9 for NO/CO, 12 for C18O2/NO, and 23 for C18O2/CO. Thus we equally underestimated the predicted DL ratios for C18O2/NO and CO/NO by a factor of approximately 0.5. From this, and by excluding significant interactions between the indicator gases and lung tissues, we conclude that the closest approximation of the diffusive component of DL is indeed obtained by using NO.