The effects of methadone on respiration and ventilation in Pygmy goats

The respiratory effects of dl-methadone administered i.v. were examined in awake of Pygmy goats. Three castrated males and three anestrus females received i.v. injections of methadone in increasing sequential saline treatments 10 days before the methadone injections. Serum dl-methadone was measured by radioimmunoassay. The PACO2 and PaCO2 decreased significantly (P < .001) following administration of methadone, while arterial blood pH, respiratory frequency, VE, VA, VO2, VE/VO2, and VA/VO2 significantly increased (P < .05). These results are consistent with relative hyperventilation. The changes were sex-related except for the increases in pH and VO2. There was a strong correlation between increasing serum methadone concentration and increasing ratio of VA/VO2. Naloxone did not antagonize the methadone effects. Since methadone induces hypoventilation in many species including man, the pygmy goat provides an interesting example of a sharply contrasting species difference in narcotic effects. The mechanism of the drug-induced hyperventilation and the possible role of female sex hormones in the potentiation of the ventilatory effects should be explored.