Cellular Basis for ST-Segment Changes Observed During Ischemia Jose ´ M. Di Diego, MD, and Charles Antzelevitch, PhD Abstract: This study probes the cellular basis for ischemia-induced ST-segment elevation with the isolated arterially perfused canine ventricular wedge prepara- tion. Transmembrane action potentials (AP) from epicardial (Epi) and endocardial (Endo) regions, a pseudo-electrocardiogram (ECG), and 5 intramural unipolar electrograms were simultaneously recorded at a basic cycle length of 800 or 2,000 ms. Global ischemia was induced by an abrupt interruption of coronary flow for 30 minutes. Under control conditions, the ST segment was isoelectric because of the absence of voltage gradients at the level of AP plateau among the cells spanning the ventricular wall. Global ischemia could cause an all-or-none repolarization at the end of phase 1 of the AP in Epi but not Endo leading to ST-segment elevation and extrasystolic activity secondary to phase 2 re-entry. In the majority of preparations, global ischemia resulted in a progressive increase in transmural conduction time after 25 to 30 minutes of interruption of flow caused by a step delay of impulse transmission in the midmyocardium. The ECG assumed a “tombstone” configuration. Correlation of the APs and ECG activity revealed that the apparent severe ST-segment elevation encountered under these condi- tions is actually a markedly prolonged R wave. In control, Endo repolarized after Epi yielding upright T waves in the ECG. After 30 minutes of ischemia Epi repolarized after Endo causing reversal of repolarization gradients and T-wave inversion. The ischemia-induced electrophysiologic changes returned to nearly control values within 5 minutes of reperfusion. Our results indicate that 2 distinctly different mechanisms involving 1) loss of the epicardial action potential dome and 2) markedly delayed transmural conduction underlie the apparent ST-segment elevation encountered during acute ischemia. Key words: Isch- emia, ST-segment elevation, T-wave inversion. The electrocardiogram (ECG) has long been rec- ognized as an important tool for the diagnosis and localization of acute myocardial ischemia and in- farction. Under these conditions, changes in the surface ECG are thought to be related to changes in the resting potential, action potential morphology, action potential duration, and conduction charac- teristics of the injured myocardial region (1–3). Current concepts regarding the pathophysiological mechanisms underlying myocardial ischemia-in- duced changes in the ECG derive principally from theoretical models since attempts to record action potentials form discrete transmural sites of the ischemic myocardium in vivo have generally been limited to the epicardial surface (4 – 6). The present study attempts to overcome this From the Masonic Medical Research Laboratory, Utica, NY. This work supported by grants HL37396 and HL47678 from the National Institutes of Health, a Grant-in-Aid from the American Heart Association, New York State Affiliate, and New York and Florida Masons. Reprint requests: Charles Antzelevitch, Masonic Medical Re- search Laboratory, 2150 Bleecker St, Utica, NY 13504; e-mail: ca@mmrl.edu. © 2003 Elsevier Inc. All rights reserved. 0022-0736/03/360S-0001$30.00/0 doi:10.1016/j.jelectrocard.2003.09.001 Journal of Electrocardiology Vol. 36 Supplement 2003 1