The Optimal Dosage of Intrathecal Morphine for Peripartum Analgesia

Regional anesthesia techniques such as combined spinal epidural (CSE) analgesia are very effective for the management of intrapartum pain. The advantages of these techniques are that they are safe when properly conducted and that they provide excellent analgesia while allowing the patient to remain awake and participate in the labor and delivery. The risks of maternal aspiration and fetal drug depression associated with general anesthesia are minimized. Finally, the effective analgesia associated with regional techniques blunt the hemodynamic effects caused by painful contractions and reduce maternal catecholamines, resulting in increased placental perfusion.1

Opioids in combination with local anesthetics in the spinal space provide effective pain relief during labor with minimal side effects. The advantages of spinal opioid administration include lack of motor blockade and faster onset of analgesia.2 In addition, since the opiate receptors are in the spinal space, a smaller amount of opioid can be used to provide excellent pain relief while minimizing the side effects. At Beth Israel Deaconess Medical Center (BIDMC), the obstetric anesthesiology group uses a standard spinal dosing for CSE during labor which includes: 1 ml of 0.25% bupivicaine with 12.5 mcg of fentanyl.

Yeh and colleagues have found that morphine 150 mcg added to the fentanyl-bupivicaine spinal injection can prolong the duration of spinal analgesia but was associated with increased side effects. 3 The side effect profile of spinal narcotics include: nausea, vomiting, pruritus, and urinary retention. Although these side effects for the most part can be easily treated, they can be bothersome to the post partum patient. In a previous study performed from our institution, the addition of 100 mcg of morphine to spinal bupivicaine and fentanyl reduced the rate of breakthrough pain during labor analgesia and prolonged the time to first request for supplementation. Overall, it was found that the incidence of side effects was low but the group that received the spinal morphine did have more nausea and vomiting compared with the placebo group. 4

In this current investigation, we would like to assess whether an even smaller dose of spinal morphine would provide an effective, pain free recovery from vaginal delivery while decreasing the incidence of side effects, specifically nausea and vomiting. We would like to perform a formal dose response study to identify the ideal dose of intrathecal morphine that would not compromise the pain relief during labor while minimizing the side effects.