Exercise response after rapid intravenous infusion of saline in healthy humans

Patients with chronic heart failure have an abnormal pattern of exercise ventilation ((V)over dot E), characterized by small tidal volumes (VT), increased alveolar ventilation, and elevated physiological dead space (VD/VT). To investigate whether increased lung water in isolation could reproduce this pattern of exercise ventilation, 30 ml/kg of saline were rapidly infused into nine normal subjects, immediately before a symptom-limited incremental exercise test. Saline infusion significantly reduced forced vital capacity, 1-s forced expiratory volume, and alveolar volume (P < 0.01 for all). After saline, exercise ventilation assessed by the (V)over dot E/(V)over dot CO2 slope increased from 24.9 +/- 2.4 to 28.0 +/- 2.9 l/l, (P < 0.0002), associated with a small decrease in arterial PCO2, but without changes in VT, VD/VT, or alveolar-arterial O-2 difference. A reduction in maximal O-2 uptake of 175 +/- 184 ml/min (P < 0.02) was observed in the postsaline infusion exercise studies, associated with a consistent reduction in maximal exercise heart rate (8.1 +/- 5.9 beats/min, P < 0.01), but without a change in the O-2 pulse. Therefore, infusion of saline to normal subjects before exercise failed to reproduce either the increase in VD/VT or the smaller exercise VT described in heart failure patients. The observed increase in (V)over dot E can be attributed to dilution acidosis from infusion of the bicarbonate-free fluid and/or to afferent signals from lung and exercising muscles. The reduction in maximal power output, maximal O-2 uptake, and heart rate after saline infusion may be linked to accumulation of edema fluid in exercising muscle, impairing the diffusion of O-2 to muscle mitochondria.