Effects of Methadone on the Minimum Anesthetic Concentration of Isoflurane, and Its Effects on Heart Rate, Blood Pressure and Ventilation during Isoflurane Anesthesia in Hens (Gallus gallus domesticus)

Abstract

The aim of this study was to measure the temporal effects of intramuscular methadone administration on the minimum anesthetic concentration (MAC) of isoflurane in hens, and to evaluate the effects of the isoflurane-methadone combination on heart rate and rhythm, blood pressure and ventilation. Thirteen healthy adult hens weighing 1.7 ± 0.2 kg were used. The MAC of isoflurane was determined in each individual using the bracketing method. Subsequently, the reduction in isoflurane MAC produced by methadone (3 or 6 mg kg-1, IM) was determined by the up-and-down method. Stimulation was applied at 15 and 30 minutes, and at 45 minutes if the bird had not moved at 30 minutes. Isoflurane MAC reduction was calculated at each time point using logistic regression. After a washout period, birds were anesthetized with isoflurane and methadone, 6 mg kg-1 IM was administered. Heart rate and rhythm, respiratory rate, blood gas values and invasive blood pressure were measured at 1.0 and 0.7 isoflurane MAC, and during 45 minutes after administration of methadone once birds were anesthetized with 0.7 isoflurane MAC. Fifteen minutes after administration of 3 mg kg-1 of methadone, isoflurane MAC was reduced by 2 (-9 to 13)% [logistic regression estimate (95% Wald confidence interval)]. Administration of 6 mg kg-1 of methadone decreased isoflurane MAC by 29 (11 to 46)%, 27 (-3 to 56)% and 10 (-8 to 28)% after 15, 30 and 45 minutes, respectively. Methadone (6 mg kg-1) induced atrioventricular block in three animals and ventricular premature contractions in two. Methadone caused an increase in arterial blood pressure and arterial partial pressure of carbon dioxide, while heart rate and pH decreased. Methadone, 6 mg kg-1 IM significantly reduced isoflurane MAC by 30% in hens 15 minutes after administration. At this dose, methadone caused mild respiratory acidosis and increase in systemic blood pressure.

Effects of a single intravenous bolus of fentanyl on the minimum anesthetic concentration of isoflurane in chickens (Gallus gallus domesticus)

“…A combination of sevoflurane with butorphanol resulted in cardiopulmonary arrest in two guineafowl (Escobar et al 2014). Administration of isoflurane combined with methadone to chickens decreased the HR and increased sAP when compared with anesthesia using an equipotent concentration of isoflurane (Escobar et al 2016). However, equipotent concentrations of isoflurane with and without fentanyl were not maintained in this study as the MAC reduction at 5 minutes after fentanyl (>40%) was much larger than the MAC reduction used in the second phase (20%).…”

“…Time to standing was longer after the administration of 10 mg kg À1 , presumably because birds tested at this dose were anesthetized with a higher FE 0 Iso than those tested at the dose of 30 mg kg À1 . Thus, fentanyl did not appear to influence extubation and standing times, contrary to methadone in chickens anesthetized with isoflurane, which induced prolonged recovery times (Escobar et al 2016).…”

“…In the second phase of the study, there were no significant changes in cardiopulmonary variables before and after administration of fentanyl (30 mg kg À1 ) IV over 1 minute. Other studies in birds have reported the cardiopulmonary effects of equipotent concentrations of inhalation anesthetics combined or not with opioids (Escobar et al 2014(Escobar et al , 2016. A combination of sevoflurane with butorphanol resulted in cardiopulmonary arrest in two guineafowl (Escobar et al 2014).…”

“…The individual isoflurane MAC of 13 chickens had been previously determined in triplicate using the bracketing method and electrical stimulation (Escobar et al 2016). The MAC of isoflurane was determined in four additional chickens.…”

“…Intramuscular (IM) administration of morphine (3 or 6 mg kg À1 ) in chickens reduced the isoflurane MAC at 15 minutes by 15% and 22%, respectively (Vela et al 2014). Methadone (6 mg kg À1 ) IM decreased isoflurane MAC by 30% at 15 minutes after administration in chickens; however, the effect was gone by 30 minutes (Escobar et al 2016).…”