Haemodynamic effects of intravenous methadone anaesthesia in dogs

Abstract

The cardiovascular effects of intravenous anaesthetic doses of methadone were evaluated in 30 mongrel dogs breathing oxygen. One half of the dogs were unpremedicated (group B) and the other half (group A) received atropine 1.5 mg intramuscularly 20 minutes before infusion of methadone. After collection of control cardiovascular data dogs in both groups were given 0.3 mg · kg-1 of methadone intravenously over a ten-minute period and cardiovascular dynamics were again recorded immediately following infusion and 20 minutes later. Following this 0.5 mg · kg-1 of methadone was infused over ten minutes and measurements were obtained as with the 0.3 mg · kg-1 dose. Data were recorded in a similar fashion before and following intravenous infusion of methadone 1.0, 1.5 and 2.0 mg · kg-1 in the same animals, so that after the final infusion each animal had received a total accumulated dose of methadone 5.3 mg · kg-1. Group A dogs had significantly higher heart rates, mean aortic pressures and cardiac output than group B dogs during control conditions and throughout the study. Group A dogs showed no change in any cardiovascular variable measured during or after infusion of methadone 0.3 or 0.5 mg · kg-1. Methadone 1.0, 1.5 and 2.0 mg · kg-1 produced slight (but similar) decreases in mean aortic blood pressure but did not change heart rate, cardiac output, pulmonary capillary wedge pressure, right atrial pressure or pulmonary systemic vascular resistance. In group B dogs methadone 0.5 mg · kg-1 produced modest decreases in heart rate, cardiac output and aortic blood pressure. Infusion of methadone 1.0,1.5 and 2.0 mg · kg-1 (total doses of 1.8, 3.3 and 5.3 mg · kg-1) produced changes similar to those with 0.5 mg · kg-1 of the compound in group B animals with the exception of heart rate, which was further decreased, and pulmonary vascular resistance, which was increased with 1.5 and 2.0 mg · kg-1 infusions. These data demonstrate that intravenous anaesthetic doses of methadone produce only minimal changes in cardiovascular dynamics, most of which can be avoided by prior administration of atropine. © 1980 Canadian Anesthesiologists.