Mechanisms of ischemic ST-segment displacement. Evaluation by direct current recordings.

Abstract

The electrophysiologic basis of ischemic ST-segment displacement was investigated in 40 open chest dogs. Epicardial and subendocardial electrograms were recorded with direct current coupled amplifiers during partial and complete coronary artery occlusion. The time course and magnitude of DC potential changes, and the effects on the DC potentials of heart rate and subendocardial ischemia were investigated. TQ segment depression, representing loss of resting membrane potential, was the consistent and most specific mechanism of ST displacement due to ischemia. True ST-segment displacement, due to alterations of phase 2 of the transmembrane action potential, occurred less frequently, and was not specific for ischemia. The DC potential changes were similar in subendocardial and epicardial tissue. Increased heart rate by atrial pacing increased the magnitude of TQ segment depression and produced subendocardial ischemia during partial coronary flow reduction. The findings have importance in the clinical interpretation of ST-segment displacement. Potential limitations in the use of precordial ST-segment mapping to estimate infarct size or severity are described. The results help explain the variable and conflicting findings of previous studies investigating the mechanisms of ST displacement, and may help explain the poor correlation between rapid atrial pacing-induced ST displacement and coronary artery disease defined by angiography. ECG interpretation based on physiologic data such as those presented in this study should improve the usefulness and accuracy of the ECG examination.