Ragavendra R. Baliga, M.D
Assistant Professor
Division of Cardiology
University Of Michigan

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Question 12 of 39:

As nitroglycerin is quite effective in relieving ischemic-type chest discomfort due to acute coronary syndromes, it is preferred to narcotic analgesics to manage the chest pain in this patient.

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Effective analgesia (e.g. intravenous morphine) should be administered promptly at the time of diagnosis and should not be delayed on the premise that to do so will obscure the ability to evaluate the results of anti-ischemic therapy.

Because the sympathetic drive that accompanies myocardial infarction arises from a combination of ischemic-type chest discomfort and anxiety, a primary objective of therapy is administration of sufficient doses of an analgesic such as morphine sulfate to relieve what many patients have described as impending doom. Patients should be instructed to notify the nurse immediately when discomfort occurs and describe its severity using a numeric scale (e.g. 1 to 10).

Morphine sulfate can be administered intravenously at a rate of 2 to 4 mg every 5 minutes, with some patients requiring as much as 25 to 30 mg before pain relief is adequate. The current practice of administering morphine in small increments to avoid paradoxic augmentation of sympathetic nervous system tone and respiratory depression may have a tendency to result in too low a cumulative dose being administered. Fear of inducing hypotension also tends to restrict the amount of morphine sulfate administered. It is important to realize that morphine-induced hypotension typically occurs in volume-depleted, orthostatic patients and is not a particular threat to supine patients. It may be more prudent to avoid concomitant use of other vasodilators such as intravenous nitroglycerin in patients with severe unremitting pain.

The depressant effect of morphine on ventilation is centrally mediated. In the setting of acute MI, respiratory depression is usually not a significant clinical problem because the sympathetic discharge associated with severe ischemic-type chest discomfort or pulmonary edema. Administration of 0.4 mg naloxone IV at up to 3-minute intervals to a maximum of 3 doses may be used to relieve morphine-induced respiratory depression should it occur.