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Exercise response after rapid intravenous infusion of saline in healthy humans

¹Department of Medicine, University of Washington, Seattle, Washington 98195-6522; ²Centro di Fisiopatologia Respiratoria e di Studio della Dispnea, Azienda Ospedaliera S. Croce e Carle, 12100 Cuneo; ³Centro Cardiologico Monzino IRCCS, Istituto di Cardiologia, Università di Milano, 20138 Milano; and ⁴Servizio di Fisiopatologia Respiratoria, Dipartimento di Medicina Interna, Università di Genova, 16132 Genova, Italy

Submitted 2 February 2004 ; accepted in final form 15 April 2004

Patients with chronic heart failure have an abnormal pattern of exercise ventilation (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 E/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 O₂ difference. A reduction in maximal O₂ 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 O₂ 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 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 O₂ uptake, and heart rate after saline infusion may be linked to accumulation of edema fluid in exercising muscle, impairing the diffusion of O₂ to muscle mitochondria.

edema; tidal volume; physiological dead space; exercise ventilation; maximal O₂ uptake

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