The effects of dobutamine and ephedrine on packed cell volume, total protein, heart rate, and blood pressure in anaesthetized horses

Anaesthetized horses frequently require treatment for arterial hypotension (Donaldson, 1988). The administration of inotropic drugs, such as dobutamine or ephedrine, is one of the primary methods used to restore acceptable blood pressure in anaesthetized horses. The haemodynamic effects of dobutamine, a catecholamine with direct acting b1-, b2-, and a-adrenoceptor agonist activity (Ruffolo & Messick, 1985; Vernon et al., 1992), have been well characterized in the anaesthetized horse (Swanson et al., 1985). Dobutamine (2.5±5.0 mg/kg/min) increased oxygen delivery in ponies (Gasthuys et al., 1991) and horses (Dyson & Pascoe, 1990; Wertz et al., 1992) partly by increasing cardiac output and partly by increasing haemoglobin concentration. Anaesthetized horses administered dobutamine (4 1.5 mg/kg/min) may develop reflex decreases in heart rate or a number of atrial and ventricular arrhythmias (Swanson et al., 1985; Donaldson, 1988; Light & Hellyer, 1993). Ephedrine is a nonselective synthetic catecholamine that acts directly and indirectly (through the release of noradrenaline) on a-and b-adrenergic receptors (Stoelting, 1987). The haemodynamic effects of ephedrine has been investigated in anaesthetized dogs (Wagner et al., 1993) and horses (Grandy et al., 1989). Ephedrine increased cardiac output with no changes in heart rate or rhythm (Grandy et al., 1989). Whereas a high dose of ephedrine (0.25 mg/kg, intravenously (i.v.)) increased ha emoglobin concentration in dogs (Wagner et al., 1993), the effects of ephedrine on packed cell volume (PCV) has not been reported in clinically anaesthetized horses. The objectives of this study were to determine the effects of dobutamine and ephedrine on heart rate, blood pressure, PCV and total plasma protein (TP) in clinically anesthetized, hypotensive horses. Data were collected from 41 client-owned horses which were anaesthetized at the Colorado State University Veterinary Teaching Hospital for a variety of surgical procedures. The anaesthetic protocol was tailored to the individual animal and was not standardized. Horses were sedated with xylazine and anaesthesia induced with guaifenesin or diazepam followed by ketamine or thiopental. Anaesthesia was maintained with either halothane or isoflurane in oxygen administered to effect. A balanced electrolyte solution (Normosol-R, Abbott Laboratories, North Chicago, IL) was administered at a rate of & 10 mL/kg/h, intravenously. Arterial blood pressure was measured with a catheter percutaneously placed in the facial, transverse facial, or greater metatarsal artery and connected to a calibrated strain gauge transducer (Code Transducer, Aragon Medicals, Athens, TX). Zero pressure was considered to be the point of the shoulder with the horse in dorsal recumbency, and the sternum in lateral recumbency. Horses with arterial hypotension (mean arterial pressure 5 65 mmHg) were administered either ephedrine (Ephedrine sulfate, Abbott Laboratories, North Chicago, IL) (0.06 mg/kg, i.v., 1±2 boluses) or dobutamine (Abbott Laboratories, North Chicago, IL) (1±4 mg/kg/min, i.v.) after obtaining baseline measurements of heart rate, mean arterial pressure, PCV and TP. Blood samples for the determination of PCV and TP were obtained percutaneously from a peripheral vein or the indwelling arterial catheter (n=1). The selection of inotrope to treat hypotension was left up to the discretion of the individual anaesthetist managing the case. Heart rate, mean arterial pressure, PCV and TP were obtained at 5 and 15 min after beginning inotrope administration. The effect of treatment (dobutamine, ephedrine) over time on measured parameters was analysed using a two-way ANOVA for repeated measures. If differences were significant, a Bonferroni/Dunn test was used for specific time comparisons. P 5 0.05 was considered statistically significant. The effects of ephedrine and dobutamine on measured parameters were not compared to each other as there was no attempt made to administer equivalent inotropic doses of the two drugs. Data are presented as mean+ standard deviation. Dobutamine was administered to 28 horses which were 6.0+4.8 years of age and weighing & 443+99 kg. Dobutamine was administered at a rate of & 1±2 mg/kg/min (n=24) and 2±4 mg/kg/min (n=4). Ephedrine was administered to 13 horses which were 3.3+3.9 years of age and weighing & 399+128 kg. Table 1 shows that dobutamine and ephedrine significantly increased mean arterial pressure at 5 min as compared to baseline. Dobutamine further increased mean arterial pressure at 15 min as compared to baseline and 5 min.