Nalorphine and naloxone were compared as to their effectiveness as pethidine antagonists. 85 infants were divided into a control group containing 19 newborn babies whose mothers did not receive pethidine and the babies received no antagonist, and three groups in which the mothers all received pethidine and the babies had either no antagonist (24), nalorphine IV (16), or naloxone IV (26). All the babies were assessed by measuring their neurobehavioural states and respiratory functions. A further 12 newborn babies had naloxone plasma levels measured by radioimmunoassay. Although standard doses of nalorphine effectively antagonised the depressive effect on respiration induced by pethidine, there was a pronounced and undesirable excitatory agonist action. Naloxone was not observed to have any agonist activity, but the recommended IV dose (0.01 mg/kg) had only a slight and delayed antagonist action as measured by respiratory function tests. A more rapid and improved antagonism was noted after this dose was doubled (0.02 mg/kg). The plasma elimination-phase half-life of naloxone after intravenous cord injection was about 3 hours.20
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