Nitroglycerin in experimental myocardial infarction effects on regional left ventricular length and tension

Abstract

Controversy has existed concerning the value of nitroglycerin therapy in acute myocardial infarction. With use of Walton-Brodie strain gauge arches and mercury-in-Silastic segment length gauges, the effects of nitroglycerin (30 μg/kg intravenously) were studied on local tension and length characteristics of the ischemic, border and nonischemic zones of the left ventricle in 10 open chest dogs. In the nonischemic zone total tension increased to 144.8 ± 13.6 (mean ± standard error of the mean) percent ( P <0.005) of control levels, and segment length decreased to 79.4 ± 4.4 percent ( P <0.01) with infusion of nitroglycerin 15 to 30 minutes after coronary ligation. Changes were similar in the groups given nitroglycerin 45 to 60 minutes and 2 to 3 hours after ligation. The border zone exhibited an increase in total tension to 132.3 ± 8.4 percent ( P <0.005) of control level and a decrease in segment length to 79.0 ± 2.4 percent ( P <0.001) in the 15 to 30 minute group. In the 45 to 60 minute group, tension increased to 117.9 ± 4.8 percent ( P <0.005), whereas length decreased to 86.8 ± 1.4 percent ( P <0.001); and in the 2 to 3 hour group tension increased to 124.9 ± 6.0 percent ( P <0.005), and length decreased to 91.9 ± 3.5 percent ( P <0.001). The decrease in magnitude of responsiveness in the 45 to 60 minute and 2 to 3 hour postligation groups compared with the 15 to 30 minute postligation group was highly significant ( P <0.005). The central ischemic zone showed no significant change in tension or length in any group. Maximal response to nitroglycerin administration in both tension and length parameters was observed within 30 to 60 seconds and was concomitant with a decrease in systolic pressure to 73.3 ± 1.9 percent ( P <0.001) and an increase in heart rate to 107.2 ± 1.0 percent. Nitroglycerin improves the contractile performance of the nonischemic and border zones after coronary occlusion without affecting the central ischemic zone. However, the border zone exhibits a progressive decrease in contractile response as a function of time. These data suggest that nitroglycerin has the overall effect of functionally reducing the extent of the ischemic area and strongly supports its potential clinical usefulness in acute myocardial infarction.