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_CO). This is because the measurement of membrane and blood conductances for CO (Dm_CO and Db_CO = θ_CO × V_c, 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 (PAO₂) and mean intracapillary oxygen partial pressure (PcapO₂) which is the true determinant of specific conductance of haemoglobin for CO (θ_CO). Besides that, the critical multistep oxygenation method along with different linear equations relating 1/θ_CO to PcapO₂ gave highly scattered Dm_CO and V_c values. The Dm and V_c can also be derived from a simultaneous measurement of DL_NO and DL_CO 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_NO (about 4·5 ml_NO × ml_blood^?1 × min^?1 × mmHg^?1). 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_CO and V_c to alterations observed in parenchymal diseases.