Potassium and ventilation during positive and negative work inpatients with chronic obstructive pulmonary disease

In patients with chronic obstructive pulmonary disease (COPD),reduced ventilatory reserves limit exercise tolerance. In these patients, the ventilatoryrequirements of eccentric exercise (negative work, W_neg) are lower thanthose of concentric exercise (positive work, W_pos) at similar workloads.In this study, we investigated the relationship between plasma potassium levels and ventilationduring W_pos and W_neg in these patients. Twelvepatients with stable COPD mean (SD) FEV₁ 46% (16) of predicted]performed W_pos and W_neg on a cycle ergometer(6 min of exercise; interval ≥1 h) in a randomized order at a constant workload of50% of the individual maximum (positive) work capacity. Minute ventilation (V_E) and arterial plasma potassium concentration (K⁺]_a) were measured at rest, and at 1-min intervals during exercise and during 3 min ofrecovery. V_E increased less during W_neg thanduring W_pos 6 (range 3–26) vs. 18 (range 8–28) l min^?1; P<0·01]. V_E during W_neg was reduced in proportion to VCO ₂.The increase in K⁺]_a during W_pos and W_neg 0·45 (range 0·26–0·75) and0·34 (range 0·1–0·97) mM ] did not differsignificantly. V_E was closely correlated with VCO ₂ during both types of exercise. V_E was also closelycorrelated with K⁺]_a, but the slope of the relationship betweenK⁺]_a and V_E was steeper during W_pos than during W_neg 39·1 (range15·2–88·6) vs. 18·3 (range7·2–37·3) l min^?1 mM ^?1; P=0·012]. In contrast, the slope of the relationship betweenVCO ₂ and V_E was similar during bothtypes of exercise 27?8 (range 19·2–37·1) vs. 32·1 (range19·8–48·4)]. Thus, for a given increase in K⁺]_a, the increase in V_E was significantly less during W_neg. In patients with COPD, potassium did not explain the difference inexercise ventilation between W_neg and W_pos, andmay not play a significant role in the control of breathing during low-intensity exercise.