Plasma morphine concentrations and analgesic effects of lumbar extradural morphine and heroin

Patients undergoing lumbar laminectomy were given extradural narcotic, either 5 mg morphine sulphate or 5.5 mg heroin (diamorphine hydrochloride); the extradural catheter had been positioned adjacent to the dura under direct vision. Plasma morphine concentrations measured by specific radioimmunoassay showed that peak concentrations occurred significantly earlier with heroin (4.7 +/- 0.6 min, mean +/- SEM) than with morphine (7.6 +/- 0.9 min) and that peak concentrations were significantly higher after heroin 5-10 min after extradural injection. The fraction of extradural heroin crossing the dura was estimated to be 55% of the fraction of morphine crossing the dura. Postoperative fentanyl requirements using demand analgesia were the same with extradural morphine as with extradural heroin (mean, 6.6 micrograms/hr). Clinically significant slowing of respiratory rate occurred only after extradural heroin (three patients).