Nitrogen monoxide and carbon monoxide transfer interpretation: state of the art.

Just a few clinicians routinely measure the subcomponents of the lung diffusing capacity for Carbone monoxide (DL ). This is because the measurement of membrane and blood conductances for CO (Dm and Db  = θ  × V , respectively) by the classic Roughton and Forster method is complicated and time consuming. In addition, it mistakenly assumes a close relationship between alveolar oxygen partial pressure (PAO ) and mean intracapillary oxygen partial pressure (PcapO ) which is the true determinant of specific conductance of haemoglobin for CO (θ ). Besides that, the critical multistep oxygenation method along with different linear equations relating 1/θ to PcapO gave highly scattered Dm and V values. The Dm and V can also be derived from a simultaneous measurement of DL and DL with the blood resistance for NO assumed to be negligible. However, recent in vitro and in vivo experiments point towards a finite value of θ (about 4·5 ml  × ml  × min  × mmHg ). Putting together the arguments and our clinical data allows us to report here the state of the art in partitioning the CO diffusing capacity into its constitutive components, with the goal to encourage further studies examining the sensitivity of Dm and V to alterations observed in parenchymal diseases.