Reference equations for pulmonary diffusing capacity of carbon monoxide and nitric oxide in adult Caucasians

The aim of this study was to determine reference equations for the combined measurement of diffusing capacity of the lung for carbon monoxide (CO) and nitric oxide (NO) (D_LCONO). In addition, we wanted to appeal for consensus regarding methodology of the measurement including calculation of diffusing capacity of the alveolo-capillary membrane (D_m) and pulmonary capillary volume (V_c).D_LCONO was measured in 282 healthy individuals aged 18-97 years using the single-breath technique and a breath-hold time of 5 s (true apnoea period). The following values were used: 1) specific conductance of nitric oxide (θ_NO)=4.5 mL_NO·mL_blood^-1·min^-1·mmHg^-1; 2) ratio of diffusing capacity of the membrane for NO and CO (D_mNO/D_mCO)=1.97; and 3) 1/red cell CO conductance (1/θ_CO)=(1.30+0.0041·mean capillary oxygen pressure)·(14.6/Hb concentration in g·dL^-1).Reference equations were established for the outcomes of D_LCONO, including D_LCO and D_LNO and the calculated values D_m and V_c Independent variables were age, sex, height and age squared.By providing new reference equations and by appealing for consensus regarding the methodology, we hope to provide a basis for future studies and clinical use of this novel and interesting method.