How to calculate the single-breath nitric oxide diffusing capacity in rabbits

Nitric oxide (NO) is a novel indicator gas for investigating alveolar capillary gas exchange conditions. In clinical practice, pulmonary gas uptake is determined by measuring the single-breath diffusing capacity (DL,NO). Different algorithms are employed to calculate DL,NO. To compare the accuracy of those most commonly used, we performed single-breath experiments on 12 artificially ventilated rabbits. In each animal four manoeuvres, executing breath-holds of 2, 4, 6 and 8 s, were carried out. In each case we administered 55 ml of an indicator gas mixture containing 0.05% NO. Alveolar gas was analysed by respiratory mass spectrometry. The two algorithms for calculating DL,NO based on the conventional solution of the breath-holding equation [Ogilvie et al. (1957) J Clin Invest 36:1-17 and Jones and Meade (1961) Q J Exp Physiol 46:131-143], were compared with the three-equation technique [Graham et al. (1980) IEEE Trans Biomed Eng 27:221-227] as the reference. The deviation between DL,NO calculated from the conventional methods and the reference decreased linearly with increasing duration of NO uptake (deltat). The mean deviations declined from 16.6% (Jones and Meade) or 7.7% (Ogilvie) at deltat=4 s to 5.7% (Jones and Meade) or 2.4% (Ogilvie) at deltat=10 s. The larger mean values are due to the conventional solution where three-tenths of the inflation time is subtracted from deltat. These findings qualify the common prediction that the latter method yields DL,NO values of the highest accuracy. We therefore recommend Ogilvie's procedure if the three-equation technique cannot be employed.