Nitric oxide used to test pulmonary gas exchange in rabbits

To evaluate whether nitric oxide (NO) is an appropriate test gas for assess ing pulmonary gas exchange, we determined the rates of disappearance from t he alveolar space (lambda) of NO and singly and doubly O-18-labelled carbon dioxide ((COO)-O-16-O-18, (CO2)-O-18) by performing single-breath manoeuvr es on seven artificially ventilated rabbits. By exploiting unique features of both isotopic species and by analysing pulmonary gas transport and lambd a values with a commonly used model, we found that diffusion forms 98+/-6% (mean +/- SD) of the overall resistance to alveolar-capillary NO transfer. This means that measurements of pulmonary NO uptake reveal the entire diffu sive properties of the alveolar-capillary membrane, because the extremely f ast binding of NO to haemoglobin negates the "reactive" component within re d blood cells of pulmonary capillaries.