Intercept of minute ventilation vs. carbon dioxide output relationship in chronic obstructive pulmonary disease: utility as an index of ventilatory inefficiency

Abstract

Background: Ventilatory inefficiency is known to be a contributor to exercise intolerance in chronic obstructive pulmonary disease (COPD). The intercept of the minute ventilation (V̇E) vs. carbon dioxide output (V̇CO2) plot is a key ventilator inefficiency parameter. However, its relationships with lung hyperinflation (LH) and airflow limitation are not known. This study aimed to evaluate the correlations between the V̇E/V̇CO2 intercept and LH in COPD to determine its utility as an index of functional impairment.Methods: We conducted a retrospective analysis of data from 53 COPD patients and 14 healthy controls performed incremental cardiopulmonary exercise tests and resting pulmonary function. Ventilatory inefficiency was represented by parameters reflecting the V̇E/V̇CO2 nadir and slope (linear region), and intercept of the V̇E/V̇CO2 plot. Their correlations with measures of LH and airflow limitation were evaluated.Results: Compared to the control, the slope (30.58±3.62) and intercept (4.85±1.11) higher in COPDstages1-2, leading to a higher nadir (31.47±4.47) (p<0.05). Despite an even higher intercept in COPDstages3-4 (7.16±1.41), the slope diminished with disease progression (from 30.58±3.62 in COPDstages1-2 to 28.36±4.58 in COPDstages3-4). Compared to the V̇E/V̇CO2 nadir and V̇E/V̇CO2 slope, the intercept was better correlated with peak V̇E/maximal voluntary ventilation (MVV) (r=0.489, p<0.001) and peak V̇O2/watt (r=0.354, p=0.003). The intercept was also significantly correlated with RV/TLC (r=0.588, p<0.001), IC/TLC (r=-0.574, p<0.001), peak VT/TLC (r=-0.585, p<0.001); and airflow limitation forced expiratory volume in 1s (FEV1) % predicted (r=-0.606, p<0.001) and FEV1/forced vital capacity (FVC) (r=-0.629, p<0.001).Conclusion: V̇E/V̇CO2intercept was consistently better correlated with worsening static and dynamic lung hyperinflation and airflow limitation in COPD. V̇E/V̇CO2 intercept emerged as a useful index of ventilatory inefficiency across the severity spectrum of COPD patients.