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

In patients with chronic obstructive pulmonary disease (COPD), reduced ventilatory reserves limit exercise tolerance. In these patients, the ventilatory requirements of eccentric exercise (negative work, Wneg) are lower than those of concentric exercise (positive work, Wpos) at similar workloads. In this study, we investigated the relationship between plasma potassium levels and ventilation during Wpos and Wneg in these patients. Twelve patients with stable COPD [mean (SD) FEV1 46% (16) of predicted] performed Wpos and Wneg on a cycle ergometer (6 min of exercise; interval > or = 1 h) in a randomized order at a constant workload of 50% of the individual maximum (positive) work capacity. Minute ventilation (VE) and arterial plasma potassium concentration ([K+]a) were measured at rest, and at 1-min intervals during exercise and during 3 min of recovery. VE increased less during Wneg than during Wpos [6 (range 3-26) vs. 18 (range 8-28) 1 min-1; P < 0.01]. VE during Wneg was reduced in proportion to VCO2. The increase in [K+]a during Wpos and Wneg [0.45 (range 0.26-0.75) and 0.34 (range 0.1-0.97) mM] did not differ significantly. VE was closely correlated with VCO2 during both types of exercise. VE was also closely correlated with [K+]a, but the slope of the relationship between [K+]a and VE was steeper during Wpos than during Wneg [39.1 (range 15.2-88.6) vs. 18.3 (range 7.2-37.3) 1 min-1 mM-1; P = 0.012]. In contrast, the slope of the relationship between VCO2 and VE was similar during both types of exercise [27.8 (range 19.2-37.1) vs. 32.1 (range 19.8-48.4)]. Thus, for a given increase in [K+]a, the increase in VE was significantly less during Wneg. In patients with COPD, potassium did not explain the difference in exercise ventilation between Wneg and Wpos, and may not play a significant role in the control of breathing during low-intensity exercise.