The effects of nitroglycerin-methoxamine combination on infarct size in conscious dogs

The effect of nitroglycerin combined with methoxamine in reducing infarct weight was studied in conscious dogs. Ten minutes after permanent left anterior descending (LAD) coronary artery occlusion, 10 dogs received nitroglycerin (450 μg bolus IV, then 300 μg/min for 4 hours) and methoxamine as needed to maintain blood pressure and heart rate. No dogs were in heart failure. Ten control dogs received saline solution. Dogs were sacrificed 3 days later. The region at risk of infarction was delineated by simultaneously perfusing the aortic root with Evans blue and the distal LAD artery with saline solution under equal pressures. Slices of stained hearts were incubated with tetrazolium to identify infarct. Total weight of left ventricle (LV), risk region, and infarct was measured. Nitroglycerin-treated dogs showed no difference from control dogs in infarct weight (26.2 ± 5.9 gm ± SE vs 27.7 ± 5.6 gm), percent risk region/LV (36.0 ± 1.4% vs 37.9 ± 3.1%), or percent infarct/LV (23.5 ± 5.2% vs 24.8 ± 4.9%). In a subgroup with risk region/LV ≤ 35%, nitroglycerin reduced infarct weight by 45% (8.8 ± 8.5% vs 15.9 ± 7.9%), percent infarct/LV by 49% (7.1 ± 6.8% vs 13.8 ± 6.6%), and percent infarct/risk region by 41% (23.0 ± 22.0% vs 38.9 ± 15.9%). Because of the small number of dogs in the study, differences were not significant. In dogs with risk region/LV > 35%, nitroglycerin had no effect. Thus, in dogs without overt heart failure, nitroglycerin may salvage ischemic tissue within small areas at risk of infarction, but the results are not definitive. However, our results clearly demonstrate that in the absence of failure, nitroglycerin does not reduce the size of large infarcts.