Continuous Intravenous Morphine Infusion
40. Three months later, A.J. is again hospitalized because of a compression fracture of his spine. He is cachectic and weighs only 87 pounds. He has been receiving 20 mg of morphine IM every 3 hours since his admission. His pain is bothersome and his loss of muscle mass is not conducive to IM injections. What would be a reasonable IV analgesic program for A.J.?
Continuous IV infusions of morphine are superior to IM injections in maintaining a pain-free condition in cancer and surgical patients.144,145 In one study, six of eight children with cancer experienced complete pain control with 0.025 to 2.6 mg/kg/hour of morphine for 1 to 16 days.
A.J. has been receiving 160 mg of morphine daily, which is approximately 7 mg/hour. His initial IV infusion dose of morphine should be increased to 8 mg/hour because his pain is still bothersome. When 500 mg of morphine sulfate is added to 500 mL of 5% dextrose in water, the resulting 1-mg/mL solution can be infused IV at 8 mL/hour to deliver the 8-mg/hour dose. In addition, an order should be written for a 4-mg IV bolus of morphine every hour as needed for signs of pain or discomfort.
The supplemental bolus doses of IV morphine will facilitate subsequent adjustments of the continuous infusion rate. At the end of a predetermined period of time, the number of milligrams of drug that were administered by constant infusion is added to 1.5 times the number of milligrams of drug used in supplemental bolus doses. This amount of drug is then divided by the elapsed interval of time, thereby calculating the new infusion rate. For example, if at the end of 6 hours A.J. has received five supplemental doses of 4 mg each, then (6 hours at 8 mg/hour) plus (1.5 × 5 doses of 4 mg) = 78 mg per 6 hours. The new infusion rate should then be 13 mg/hour, and the new supplemental doses should be 6 mg (approximately equal to the amount of drug normally infused in 30 minutes) every hour as needed. Continual assessment of the patient and hourly adjustments of dose are probably superior to the method outlined previously, but the aforementioned method may be more practical for the busy clinician.
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