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

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.

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

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.

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.

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.