Summary
We evaluated the effects of clenbuterol HCl (0.8 μg/kg, of body weight, iv), a β2, agonist, on ventilation-perfusion matching and hemodynamic variables in anesthetized (by iv route), laterally recumbent horses. The multiple inert gas elimination technique was used to assess pulmonary gas exchange. Clenbuterol HCl induced a decrease in arterial oxygen tension (from 57.0 ± 1.8 to 49.3 ± 1.2 mm of Hg; mean ± sem) as a result of increased shunt fraction (from 6.6 ± 2.1 to 14.4 ± 3.1%) and ventilation to regions with high ventilation-perfusion ratios. In contrast, no changes in these variables were found in horses given sterile water. In horses given clenbuterol HCl, O2 consumption increased from 2.23 ± 0.18 to 2.70 ± 0.14 ml · min-1 · kg-1, and respiratory exchange ratio decreased from 0.80 ± 0.02 to 0.72 ± 0.01. Respiratory exchange ratio and O, consumption were not significantly modified in sterile water-treated (control) horses. Clenbuterol HCl administration was associated with increased cardiac index (from 57.4 ± 4.0 to 84.2 ± 6.3 ml. min-1 · kg-1), decreased total peripheral vascular resistance (from 108.3 ± 9.3 to 47.6 ± 2.8 mm of Hg · s · kg · ml-1), and decreased pulmonary vascular resistance (from 31.3 ± 3.8 to 13.6 ± 0.7 mm of Hg · s · kg · ml-1). Our findings indicated that clenbuterol HCl may potentiate hypoxemia as a result of increased shunt fraction in horses anesthetized by the iv route, and caused changes in hemodynamic variables that were consistent with its ability to stimulate β2-adrenergic receptors.
