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 weigh t, iv), a beta2 agonist, on ventilation-perfusion matching and hemodyn amic variables in anesthetized (by IV route), laterally recumbent hors es. The multiple inert gas elimination technique was used to assess pu lmonary gas exchange. Clenbuterol HCI induced a decrease in arterial o xygen tension (from 57.0 +/- 1.8 to 49.3 +/- 1.2 mm of Hg; mean +/- SE M) 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 ra tios. In contrast, no changes in these variables were found in horses given sterile water. In horses given clenbuterol HCl, O2 consumption i ncreased from 2.23 +/ 0. 18 to 2.70 +/- 0. 14 ml . min-1 . kg-1, and r espiratory exchange ratio decreased from 0.80 +/- 0.02 to 0.72 +/- 0.0 1. Respiratory exchange ratio and O2 consumption were not significantl y 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 v ascular 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 increase d shunt fraction in horses anesthetized by the IV route, and caused ch anges in hemodynamic variables that were consistent with its ability t o stimulate beta2-adrenergic receptors.