The haemodynamic effects of milrinone HCl in halothane anaesthetised horses

The haemodynamic effects of milrinone hydrochloride were determined in halothane-anaesthetised horses. Six healthy adult horses were anaesthetised with guaifenesin and thiamylal and maintained with halothane in oxygen (end-tidal halothane concentration of 1.15%). Baseline haemodynamic data were recorded after a 45 min stabilisation period. All 6 horses received a single loading dose of milrinone HCl, 0.2 microgram/kg i.v., followed by progressively increasing infusions of 2.5, 5, 10 and 20 micrograms/kg bwt/min. Each infusion lasted for 15 min and produced dose related increases in heart rate, mean arterial blood pressure, cardiac output, maximum rate of increase and decrease of left ventricular pressure (+/- dP/dtmax) and ejection fraction in halothane anesthetised horses. Median artery blood flow increased following milrinone administration. Right atrial and pulmonary artery pressures, systemic vascular resistance and left ventricular end-diastolic and end-systolic volumes decreased. Most haemodynamic changes were sustained throughout the infusion period and for 30 min following the termination of milrinone infusion. Systemic vascular resistance was increased above baseline values at 30 min following the termination of milrinone infusion. No adverse side effects were observed during this study although a milrinone infusion rate of 20 micrograms/kg bwt/min increased heart rate to values greater than 50 beats/min. The results of this study suggest that milrinone produces beneficial haemodynamic effects in halothane anaesthetised horses and is potentially useful in the treatment of patients with a reduced cardiac output.