Epidural narcotics in volunteers: sensitivity to pain and to carbon dioxide.

Tolerance to pain and sensitivity to rising concentrations of inhaled carbon dioxide were measured before and after administration of methadone, 5 mg, or hydromorphone, 0.5 mg, by the intravenous 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 three pain modalities was greater in the epidural "blocked" limbs than in the "unblocked" limbs or after intravenous administration, and this difference was sustained after a second injection of narcotic. Sensitivity to carbon dioxide was less depressed by epidural narcotic than by intravenous administration; however, after a second dose of narcotic, depression of CO2 sensitivity by epidural injection was comparable to that produced by intravenous injection. These observations support the hypothesis that epidural narcotics have a segmental action as well as a systemic effect, and that both actions are dose-dependent.