Speed and sensitivity of mechanical versus electrographic indicators to mild or moderate myocardial ischemia in the pig.

Abstract

Intraoperative myocardial ischemia may be detected and quantified by indexes of myocardial contraction or by electrography. The relative reliability of these two approaches is controversial. Two issues are relevant: the timing of events after the onset of mild to moderate ischemia, and the sensitivity of measures to ischemia at steady state. In eight pigs, a carotid-to-left anterior descending coronary artery shunt with a flow meter was installed. Flow to the left anterior descending coronary artery was reduced in steps of 10% from baseline values to 50% of baseline. Wall thickness, myocardial QRS amplitude, and ST-segment deviation were measured every 1 min for 6 min at each step. Regional myocardial lactate extraction was measured at 6 min. Linear relations were found between the percentage of baseline coronary flow and all four dependent variables at steady state, indicating equal sensitivity (defined as rate of change with respect to flow reduction) to myocardial ischemia. After flow reduction, decreases in systolic wall thickening occurred first and were followed by a QRS amplitude decrease and then ST-segment elevation. The onset of ischemia was earlier with more severe reductions of coronary flow. Mechanical and electric measures of myocardial ischemia show equal sensitivity at steady state even though regional contraction changed more quickly than did QRS amplitude or the ST-segment after an abrupt reduction in coronary flow.