SINGLE-BREATH DIFFUSING CAPACITIES FOR NO, CO AND (CO2)-O-18 IN RABBITS

Nitric oxide (NO) has been introduced recently for studying alveolar-c apillary gas transfer. Due to extremely fast reaction kinetics for the association of NO with haemoglobin, pulmonary NO uptake is expected t o depend only on diffusion, whereas in the case of carbon monoxide (CO ) or oxygen-labelled carbon dioxide ((CO2)-O-18) the alveolar-capillar y transfer is, in addition, known to depend on a blood uptake componen t. To provide further data for NO, CO and (CO2)-O-18, we determined th e pulmonary diffusing capacities (D-L) for the indicator gases mention ed, performing single-breath manoeuvres on ten rabbits. The inspired g as mixtures contained 0.05% NO and/or 0.2% CO or 1% (CO2)-O-18 in nitr ogen. Applying respiratory mass spectrometry to the expirates we obtai ned the following mean +/- SD values: D-L,D-NO/D-L,D-CO = 3.55 +/- 0.4 , D-L,D-C18O2/D-L,D-NO = 6.0 +/- 0.6, D-L,D-C18O2/D-L,D-CO = 21.4 +/- 2.5. Graham's law predicts D-L ratios of 1.9 for NO/CO, 12 for (CO2)-O -18/NO, and 23 for (CO2)-O-18/CO. Thus we equally underestimated the p redicted D-L ratios for (CO2)-O-18/NO and CO/NO by a factor of approxi mately 0.5. From this, and by excluding significant interactions betwe en the indicator gases and lung tissues, we conclude that the closest approximation of the diffusive component of D-L is indeed obtained by using NO.