Respiratory depression following only 0.4 mg of intrathecal morphine

Abstract

Intrathecal and epidural opiates have been used successfully to produce postoperative analgesia. However, respiratory depression is a problem with this technique with doses of 1 mg or more. However, we observed a case of respiratory depression with a much smaller dose, 0.4 mg, of intrathecal morphine. A 74-year-old patient scheduled for a peripheral orthopedic procedure was premedicated with 5 mg of diazepam orally, 2 h preoperatively. Analgesia was obtained using 0.8 ml heavy lidocaine (i.e., 40 mg lidocaine) inserted via a spinal needle at L2-3 with the patient in the left lateral position and maintained 15 degrees head up. This was followed by 0.4 mg of preservative-free morphine in 5% dextrose water. Flunitrazepam, a total of 1 mg, was given iv intermittently during the procedure. Postoperatively, inspired oxygen was increased with the help of a 28% ventimask. It was instructed that she be kept slightly head up. Accidently the patient was put head down for half an hour. Two-and-a-half hours later, the patient was cyanotic, with a respiratory rate of 6 breaths · min-1, an arterial blood pressure of 70/50 mmHg, and a heart rate of 46 beats/min. Naloxone 0.4 mg was given iv and 0.4 mg im. This resulted in immediate improvement in respiratory rate to 10 breaths · min-1, arterial blood pressure to 100/50 mmHg, heart rate to 50 beats/min, and her color returned to normal. No further naloxone was required. The intrathecal morphine was the only narcotic given during the perioperative period. Pain relief lasted for 36 h. The cause of the respiratory depression is most likeky due to rostral spread of the morphine following the normal flow of cerebrospinal fluid from the lumbar region to the basal cisterns and which may then enter the ventricular system.