Changes in Myocardial Electrical Impedance Induced by Coronary Artery Occlusion in Pigs With and Without Preconditioning

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Abstract
Background Myocardial ischemia increases tissue electrical resistivity leading to cell-to-cell uncoupling, and this effect is delayed by ischemic preconditioning in isolated myocardium. Alterations in myocardial resistivity elicited by ischemia in vivo may influence arrhythmogenesis and local ST-segment changes, but this is not well known. Methods and Results Myocardial impedance (resistivity [Ω · cm] and phase angle [°]), epicardial ST segment, and ventricular arrhythmias were analyzed during 4 hours of coronary artery occlusion in 11 anesthetized open-chest pigs; these were compared with 13 other pigs submitted to a similar coronary occlusion preceded by ischemic preconditioning. Myocardial resistivity rose slowly during the first 34±7 minutes of occlusion (237±41 to 359±59 Ω · cm), increased rapidly to 488±100 Ω · cm at 60 minutes, and reached a plateau value (718±266 Ω · cm, ANOVA; P<.01) at 150±69 minutes. By contrast, phase-angle changes began after 17 minutes of ischemia (−3.0±1.6° to −4.2±1.2° at 29±8 minutes) and evolved faster thereafter (−12.5±5.3° at 144±56 minutes). Marked changes in myocardial impedance were observed during the reversion of ST-segment elevation that occurred 1 to 4 hours after occlusion, but impedance changes were less apparent during the early ST-segment recovery seen at 15 to 35 minutes of ischemia. The second arrhythmia peak (30±5 minutes) coincided with the fast change in tissue impedance, and both were delayed (P<.05) by ischemic preconditioning. Conclusions A rapid impairment of myocardial impedance occurs after 30 minutes of coronary occlusion, and its onset is better defined by shift in phase angle than by rise in tissue resistivity. Phase 1b arrhythmias are associated with marked impedance changes, and both are delayed by preconditioning. Reversion of ST-segment elevation is partially associated with impairment of myocardial impedance, but other factors play a role as well.

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