Effects of dopamine, dobutamine, dopexamine, phenylephrine, and saline solution on intramuscular blood flow and other cardiopulmonary variables in halothane-anesthetized ponies.

OBJECTIVE: To evaluate the effect on intramuscular blood flow (IMBF) and hemodynamic variables of 4 antihypotensive agents given during anesthesia. ANIMALS: 8 ponies. PROCEDURE: Halothane-anesthetized ponies (n = 6) positioned in lateral recumbency received, on separate occasions, infusions of each of the following 4 agents in serially increasing dosages or saline solution: phenylephrine hydrochloride (0.25, 0.5, 1, and 2 microg/kg of body weight), dopamine (2.5, 5, 10, and 20 microg/kg), dobutamine (1, 2.5, 5, and 10 microg/kg), and dopexamine (0.5, 1, 5, and 10 microg/kg). Changes in IMBF (by laser-Doppler flowmetry) in nondependent and dependent triceps brachii muscles and cardiopulmonary variables were measured.
RESULTS: Phenylephrine at all dosages failed to improve IMBF or cardiac index (CI), but increased mean arterial pressure (MAP) and systemic vascular resistance (SVR); 2 ponies had forelimb lameness on recovery. Dopamine (10 microg/kg/min) increased CI, MAP, and IMBF in the dependent muscle. A higher dose (20 microg/kg/min) caused cardiac arrhythmias and muscular tremor. Dobutamine increased Cl, MAP, and IMBF of both forelimbs, effects being significant for 2.5 microg/kg/min, with further improvement as the dosage increased. In 2 ponies, 10 microg of dobutamine/kg/min caused cardiac arrhythmias. Dopexamine (1 and 5 microg/kg/min) increased CI, MAP, and IMBF in the nondependent muscle, and 10 microg/kg/min caused muscular tremor, sweating, and arrhythmias. SVR was reduced after infusion of dopamine, dobutamine, or dopexamine.
CONCLUSION: During anesthesia of equids, an increase in Cl and MAP is necessary to improve IMBF in the dependent forelimb. CLINICAL RELEVANCE: Of the agents investigated, dobutamine proved the most consistent in improving IMBF.