Accuracy for DLco impairment through ventilatory efficiency indices in smokers without COPD

Background: Smokers without COPD could manifest reduced lung diffusion capacity (D_Lco), uncovering an “excess” V´_E-V´CO₂ during exercise. Thus, the new index for ventilatory efficiency (ηV´_E %, Muller, P.T., Clin Physiol Func Imag, 2021 Jan, 41(1):103-109) has the additional advantage of “normalization” of the V´CO₂-log₁₀^V´_E slope for maximal voluntary ventilation (MVV). Objective: To test the accuracy of the V´_E/V´CO₂ slope, V´_E/V´CO₂ nadir, and ηV´_E to predict resting D_Lco below the lower limit of normality (LLN). Methods: We prospectively enrolled smokers and performed spirometry, D_Lco, and cardiopulmonary exercise testing (CPET). Receiver operating characteristics with the area under the curve (AUC) were applied, intended to determine sensitivity/specificity and cut-off values for groups with DLco ≥ LLN or DLco Results: Thirty smokers were included, comprising fifteen with DLco ≥ LLN and fifteen with DLco<LLN, aged 54±8 vs 54±7 yrs, body mass index 27±4 vs 25±4 kg*m^-2, FEV₁ (%Pred) 95±14 vs 93±12, peak V´O₂ (%Pred) 101±17 vs 94±18, and well-paired sex and pack-yrs, respectively (p>0.05 for all). Only reduced DLco (%Pred, p<0.001), increased V´_E/V´CO₂ nadir (p=0.036), and reduced ηV´_E (p=0.004) were found in the D_Lco<LLN group in comparison with DLco ≥ LLN group. The sensitivity/specificity (AUC) and best cut-off values were 80%/72% (0.730, p=0.030), 27, for V´_E/V´CO₂ slope, 80%/67% (0.730, p=0.030), 28, for V´_E/V´CO₂ nadir, and 87%/73% (0.820, p=0.002), 13%, for ηV´_E. Conclusions: The slightly better performance for ηV´_E warrants additional studies, but a mild degree of ventilatory constraints and an effect of “normalization” for MVV emerge as possible mechanisms.