Autoradiographic method for measuring the ischemic myocardium at risk: Effects of verapamil on infarct size after experimental coronary artery occlusion

Investigation of the efficacy of pharmacologic agents affecting myocardial infarct size after coronary artery occlusion is complicated by the variability of collateral flow among experimental animals which results in variability of infarct size. To overcome this difficulty, we developed an autoradiographic method to delineate the ischemic area at risk of necrosis after coronary artery occlusion and we investigated the potential protective effect of a calcium antagonist verapamil. The left anterior descending coronary arteries of 25 barbiturate-anesthetized dogs were occluded. Thirty minutes later, highly radioactive human albumin microspheres labeled with ⁹⁹mTc were injected into the left atrium. One hour after coronary artery occlusion, dogs were randomized to control or treated groups; the latter received a 0.2 mg/kg loading dose and 0.6 mg/kg per hr maintenance dose of verapamil intravenously. Eight hours after coronary artery occlusion, the dogs were killed, the hearts were excised, and the left ventricle was sectioned parallel to the atrioventricular groove; infarct size was determined planimetrically after incubation in triphenyl tetrazolium chloride. The slices were then exposed to high-speed X-ray film with image-enhancing screens. The percentage of left ventricle that was ischemic, as determined by planimetry of autoradiographs, was similar in treated and control animals (36.6 ± 2.0% compared to 37.3 ± 2.8%, respectively). Of the ischemic area, 92.0 ± 4.3% was infarcted in control animals and 70.5 ± 5.1% was infarcted in treated animals (P < 0.01). Thus, this autoradiographic method using ⁹⁹mTc-labeled human albumin microspheres is useful in delineating the area of ischemia after coronary artery occlusion and in evaluating the efficacy of pharmacologic agents designed to protect ischemic myocardium. Verapamil, administered 1 hr after coronary artery occlusion, is effective in limiting infarct size.