Ineffectiveness of intravenous β₂-agonists on improving exercise tolerance in patients with reversible chronic airway obstruction

The effects on exercise tolerance after acute administration of β₂-agonists were investigated in 11 patients with partly reversible chronic airway obstruction after 400 μg of salbutamol (S) given intravenously (i.v.) and after 400 μg i.v. of a new selective β₂-agonist, broxaterol (B), by a cardiopulmonary incremental exercise test. At rest, while VE increased in respect to basal conditions (C) after S (from 13.3 ± 2.2 to 14.4 ± 2.8 l/min;p < 0.05) and after B (from 13.6 ± 3.1 to 15.5 ± 3.61/min; p < 0.05), VO₂, VCO₂ and VO₂/HR showed no substantial variations. A small, not significant reduction of PaO₂ was observed both after S (from 82.7 ± 11.7 to 79.1 ± 16.7 mm Hg) and B (from 81.6 ± 10.5 to 78.0 ± 11.0 mm Hg). The maximum workload increased neither after S (from 67.5 ± 39.1 to 66.6 ± 37.0 W) nor after B (from 65.7 ± 39.3 to 60.0 ± 35.8 W). At peak of exercise, VO₂, VCO₂ and VO₂/HR did not change after S and B as compared with C, whereas VE remained higher after both β₂-agonists throughout the effort. VO₂ at ventilatory anaerobic threshold (AT) was significantly greater either after S (from 744 ± 378 to 815 ± 302 ml/min; p < 0.05) and after B (from 756 ± 290 to 842 ± 292 ml/min; p < 0.05). The PaO₂ increase shown by these patients during effort was greater after β₂-agonists administration, ΔPaO₂ from rest to peak of exercise amounting to 14.9 ± 14.3 vs. 7.8 ± 8.2 mm Hg after Sand to 17.8 ± 15.1 vs. 8.8 ± 10.9 mm Hg after B, in respect to relative baseline (p < 0.05). We conclude that β₂-agonists, when given acutely, do not improve exercise tolerance in patients with reversible chronic airflow obstruction, although these drugs can induce a small increment of ventilatory AT. In addition, arterial blood gases do not deteriorate at rest and are better preserved during exercise after β₂-agonists.