Prevention of late fentanyl-induced respiratory depression after the injection of opiate antagonists naltrexone and S-20682: comparison with naloxone.

Compared with naloxone, two opiate antagonists (naltrexone and S-20682) were tested in the intact dog in order to reverse respiratory depression, induced by fentanyl 50 micrograms kg-1 i.v. Respiratory rate and arterial blood-gases were measured at 5, 30, 60, 120, 180 and 240 min after the additional i.v. injection of the antagonist (1 microgram kg-1). Only S-20682, but not naloxone or naltrexone, prevented late (240 min) fentanyl-induced depression of respiratory drive. A late respiratory depression coincided with an increase in amplitudes and a reduction of frequency in electrical cortical activity (EEG). Naltrexone exhibited an antagonistic effect of duration twice (60 min) that of naloxone.