EFFECTS OF CLENBUTEROL HYDROCHLORIDE ON PULMONARY GAS-EXCHANGE AND HEMODYNAMICS IN ANESTHETIZED HORSES

We evaluated the effects of clenbuterol HCl (0.8 mug/kg, of body weight, iv), a beta2 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 HCI 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 O2 consumption were not significantly modified in sterile water-treated (control) horses. Clenbuterol HCI 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 HCI 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 beta2-adrenergic receptors.