Ventilatory inefficiency during graded exercise in COPD: A pragmatic approach

The current approach to measuring ventilatory (in)efficiency (V'_E -V'CO₂ slope, nadir and intercept) presents critical drawbacks in the evaluation of COPD subjects, owing mainly to mechanical ventilatory constraints. Thus, we aimed to compare the current approach with a new method we have developed for ventilatory efficiency calculation.

The new procedure was based on measuring the amount of CO₂ cleared by the lungs (V'CO₂ , L/min) plotted against a predefined range of increase in minute ventilation (V'_E ) (ten-fold increase based on semilog scale) during incremental exercise to symptom-limited maximum tolerance. This value was compared to a hypothetical predicted maximum CO₂ output at the predicted maximal voluntary ventilation, defining ventilatory efficiency (ηV'_E , %). The results were used to compare 30 subjects with COPD (II-IV Global Initiative for Chronic Obstructive Lung Disease, GOLD) and 10 non-COPD smokers, to establish the best discriminative physiological variable for disease severity through logistic multinomial regression.

The new approach was more sensitive to progressive deterioration of airway obstruction, resulting in worse ηV'_E as lung function worsens throughout the GOLD panel (ηV'_E (%), p < .001), when compared with V'_E -V'CO₂ slope (p = .715) or V'_E -V'CO₂ nadir (p = .070), besides showing the best model based on the logistic regression approach.

Although requiring more complex calculations compared to the current procedure, the new approach is highly sensitive to true ventilatory/gas-exchange deterioration, even throughout more severe pulmonary lung function in COPD subjects.