[Competitive study of the effects of naloxone and of almitrine on fentanyl analgesia in the anesthetized dog: effects on the muzzle opening reflex and blood gases].

UNLABELLED: The search for a technique making it possible to dissociate the analgesia and ventilatory depression of central analgesics led to a comparison of the effects of naloxone, a specific morphinomimetic antagonist, with almitrine, a ventilatory stimulant with a peripheral action, on muzzle opening reflex and blood gases. Five male dogs (Beagles, aged one year), anaesthetised with Alfetesine were treated separately with the two drugs used alone and after fentanyl analgesia (injection of fractionnated doses up to the threshold of apnoea). The association of the two drugs was also tested in tyhe dog after analgesia. The parameters studied were muzzle opening reflex, as an indication of analgesia, and blood gases, and were observed for 45 minutes, including 15 minutes control.

RESULTS: 1 - The intravenous injection of 1,2 mg of naloxone had the effect of increasing the surface area of muscle potentials with a maximum of 7 per cent (p 0.001) at the 15 th minute. By contrast, no significant change in blood gases was seen. In the same dogs given fentanyl analgesia, naloxone not only reversed respiratory depression but had a stimulatory effect on MOR reaching 7 per cent (p 0.001) at the 30 th minute. 2 - The effects of 1 mg.kg-1 of almitrine were characterised by a fall in MOR for a period equal to that of the study and a minimum of 7.8 per cent (p 0.001) at the 20 th minute. At the same time, marked ventilatory stimulation was seen. PO2 rose by 22.7 per cent (p 0.02) at the 5 th minute. PCO2 fell during the 30 minutes studied with a minimum of 39.6 per cent (p 0.01) at the 20 th minute. Almitrine did not antagonise the depression of MOR caused by fentanyl but reversed the respiratory depression of the analgesic, increasing PO2 by 26 per cent (p 0.01) and decreasing PCO2 by 25.7 per cent (p 0.01). 3 - The combination of both drugs cancelled out the abolition of the reflex by fentanyl then facilitated it up to 24.7 per cent (p 0.001) in comparison with the animal not receiving any analgesic. By contrast, the ventilatory action of almitrine was not potentialised by naloxone. In view of these data, and in the absence of any emergency, the choice of naloxone as an antagonist of ventilatory depression of central analgesics should not be preferential in order to avoid the rebound effect.