Morphine increases myocardial infarction size in rats.

Morphine anesthesia is often recommended in patients with reduced cardiac reserve. As the effect of morphine on the balance between myocardial oxygen supply and demand is not well understood, the effect of large doses of morphine (3 mg/kg, subcutaneously) on experimental myocardial infarction size in the rat was evaluated. Morphine was administered 10 minutes before thoracotomy and coronary artery ligation. Myocardial infarction size was assessed by histologic techniques 48 hours later. Rats given morphine developed significantly larger infarctions than did rats receiving an injection of saline (45.8% of left ventricular area versus 35.3%, p less than 0.05). The data indicate that morphine increase the area of myocardial ischemia when administered before coronary artery occlusion in rats.