Epidural narcotics in volunteers: Sensitivity to pain and to carbon dioxide

Abstract

Tolerance to pain and sensitivity to rising concentrations of inhaled CO2 were measured before and after administration of methadone, 5 mg, or hydromorphine, 0.5 mg, by the i.v. route and by epidural injection in the lumbar or upper thoracic region in 5 subjects. Tolerance to periosteal pressure, cutaneous electrical stimulation and the cold pressor response to ice-water immersion were measured in both upper and lower limbs. Tolerance to all 3 pain modalities was greater in the epidural blocked limbs than in the unblocked limbs or after i.v. administration and this difference was sustained after a 2nd injection of narcotic. Sensitivity to CO2 was less depressed by epidural narcotic than by i.v. administration; but after a 2nd dose of narcotic, depression of CO2 sensitivity by epidural injection was comparable to that produced by i.v. injection. Apparently epidural narcotics have a segmental action and a systemic effect and both actions are dose-dependent.