Is milrinone equivalent to amrinone during enflurane anesthesia in the dog?

Dose-response curves for milrinone during 2.1-2.3% end-tidal enflurane anesthesia were studied in six dogs given three successive boluses and 30-minute infusions of milrinone: 1) 40 microgram/kg plus 3 microgram.kg-1.min-1 (plasma level at 5 and 30 minutes after beginning of infusion: 122 +/- 14 and 136 +/- 14 ng/ml); 2) 60 microgram/kg plus 6 microgram.kg-1.min-1 (285 +/- 31 and 304 +/- 19 ng/ml); 3) 80 microgram/kg plus 12 microgram.kg-1.min-1 (498 +/- 32 and 581 +/- 28 ng/ml), demonstrating a progressive improvement of cardiac performance. Differences between milrinone and amrinone were also studied during enflurane anesthesia in six other dogs given milrinone or amrinone at 3- to 4-week intervals using both a low dose that did not decrease mean arterial pressure significantly and a dose that decreased mean arterial pressure 20-25% below baseline values. There was a dose-related effect with both drugs on the measured hemodynamic variables. Plasma catecholamine levels did not change significantly in either group. The results of our studies show that during enflurane anesthesia 1) there is a correlation between milrinone plasma levels and improvement of cardiac performance and, 2) milrinone, at low and high doses studied without or with a significant decrease in mean arterial pressure, respectively, is similar to amrinone in its activity to improve cardiac performance by similar positive inotropic, chronotropic, and vasodilating effects.