Impact of the use of a finite value of NO conductance on Hb in the interpretation of TLNO and TLCO transfer in homogenous groups of patients

Rationale.The single breath NO/CO method in its original design makes the assumption of infinity for the NO conductance on Hb (ΘNOinf). Yet this assumption of infinity seems uncorrect. The ratio ΘNO/ΘCO would be 7.7.Compared to the original method the use of this ratio decrease Vc by 24% and increase DmCO sharply in a proportion depending on both TLCO and TLNO values. The aim of the study was to analyse the impact of the use of a fixed ΘNO on DmCO in 3 groups of patients and 1 group of healthy. DmCO7.7 (computed with ΘNO/ΘCO=7.7) and DmCOinf (with ΘNO/ΘCO =infinity) were compared. The specific question raised was: can value in DmCO be safely inferred from the calculation of DmCOinf or should we use DmCO7.7 ? Methods. 1 group of 307 healthy people, 3 groups of patients : 62 patients with diabetes and near normal lung function, 64 patients with pulmonary hypertension, 59 patients before lung surgery with mild obstruction. All patients and subjects performed 2 maneuvers. Results. Linear correlations between Dm7.7, and Dm inf (Eq 1), Dm7.7 and Vc 7.7 (Eq2) were calculated . R² values were taken as indexes of variability. DmCO and Vc were calculated taking either θNO as infinite (Dm inf, Vc inf) or with θNO/θCO=7.7 (Dm 7.7, Vc 7.7). In patients the correlations between Dm 7.7 and Dm inf were tight in diabetes group (R²=0.79) but scattered in the 2 other groups (R² 0.35 and 0.11 for PAH and preop groups respectively) even more for Eq 2(R² 0.03 and 0.11 respectively). Conclusion In patients with mild or severe alterations in lung function the calculation of Dm 7.7 is recommended as Dm value cannot be accurately inferred from Dm inf.