Failure of Aminophylline and Salbutamol to Improve Respiratory Muscle Function and Exercise Tolerance in Healthy Humans

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Abstract
We investigated the effects of intravenously administered aminophylline (A; 6 mg/kg over 20 min, followed by 0.9 mg/kg/h), salbutamol (S; 4 µg/kg over 20 min, followed by 3 µg/kg/h) and placebo (P; saline solution) on the strength of the respiratory muscles, the ventilatory endurance and the exercise tolerance in 7 healthy humans. Neither A nor S caused bronchodilatation, as shown by the lack of change in FEV1. The strength of respiratory muscles, as measured by maximal inspiratory pressure (MIP), and the ventilatory endurance, as measured by sustainable inspiratory pressure (SIP), were not statistically different after A (MIP: 136.5 ± SE 11.6 cm H2O; SIP: 104.2 ± 8.4 cm H2O or after S (MIP: 135.7 ± 11.5; SIP: 107.1 ± 10.4) compared to after P (MIP: 127.1 ± 10.1; SIP: 105.0 ± 5.9). Significant changes in 12-min walking distance, perceived exertion rate, anaerobic threshold, maximal work output, maximal O2 uptake were observed neither after A nor after S. The exchange ratio and heart rate were higher after A and S than after P at some steps of a progressive, symptom-limited, treadmill-based exercise test. This might be the result of metabolic or cardiovascular adaptations elicited by these drugs. We conclude that A or S at therapeutic concentrations have no clinically relevant beneficial effects on ventilatory muscle function and exercise tolerance in healthy subjects.

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