Epidural Morphine Following Epidural Local Anesthesia

Abstract

The minute inspired ventilation (.ovrhdot.VI) and airway occlusion pressure (P100) responses to CO2 were measured during rebreathing in 10 patients who were given epidural morphine for analgesia following lower extremity or lower abdominal surgery. All patients were studied and blood samples for morphine analysis were obtained at 4 different times: preoperatively, postoperatively premorphine and 1 and 6 h after a single 10-mg epidural dose of preservative-free morphine in 10 ml of saline. All patients reported effective analgesia with a duration ranging from 8-25.5 h. There were no differences between the pre- and postoperative .ovrhdot.VI vs. PCO2 and P100 vs. PCO2 response slopes, indicating that the epidural local anesthetic alone had no effect on respiratory drive. Administration of 10 mg morphine epidurally caused a significant 22% decrease in the average .ovrhdot.VI vs. PCO2 slope and a 33% decrease in the average P100 vs. PCO2 slope 1 h postmorphine when compared to the postoperative slopes. The average decrease in .ovrhdot.VI vs. PCO2 at 6 h postmorphine was not significant. The average P100 vs. PCO2 response slope was decreased significantly at 6 h postmorphine by 27%. There was no significant correlation between serum morphine concentration and the ventilatory responses. Morphine administered by the epidural route produces decreased respiratory drive. There is a high degree of individual variability in the magnitude and time course of this effect.