Ventilatory inefficiency as a limiting factor for exercise in patients with COPD.

Background: Ventilatory inefficiency increases ventilatory demand; corresponds to an abnormal increase in the ratio of minute ventilation (V_dot(E)) to CO(2) production (V_dot(CO(2))); represents increased dead space, deregulation of respiratory control, and early lactic threshold; and is associated with expiratory flow limitation that enhances dynamic hyperinflation and may limit exercise capacity.

Objective: To evaluate the influence of ventilatory inefficiency over exercise capacity in COPD patients.

Methods: Prospective study of 35 COPD subjects with different levels of severity, in whom cardiopulmonary stress test was performed. Ventilatory inefficiency was represented by the V_dot(E) /V_dot(CO(2)) relation. Its influence over maximal oxygen consumption (V_dot(O(2))max), power (W), and ventilatory threshold was evaluated. Surrogate parameters of cardiac function, like oxygen pulse (V_dot(O(2))/heart rate) and circulatory power (%V_dot(O(2))max × peak systolic pressure), were also evaluated.

Results: Cardiopulmonary stress test was stopped due to dyspnea with elevated V_dot(E) and marked reduction of breathing reserve. A severe increase in V_dot(CO(2)) (mean ± SD 35.9 ± 5.6), a decrease of V_dot(O(2)) (mean ± SD 75.2 ± 20%), and a decrease of W (mean ± SD 68.6 ± 23.3%) were demonstrated. Twenty-eight patients presented dynamic hyperinflation. Linear regression showed a reduction of 2.04% on V_dot(O(2>))max (P < .001), 2.6% on W (P < .001), 1% on V_dot(O(2))/heart rate (P = .049), and 322.7 units on circulatory power (P = .02) per each unit of increment in V_dot(E)/V_dot(CO(2)), respectively.

Conclusions: Ventilatory inefficiency correlates with a reduction in exercise capacity in COPD patients. Including this parameter in the evaluation of exercise limitation in this patient population may mean a contribution toward the understanding of its pathophysiology.