Electrograms from the coronary sinus in acute myocardial infarction: implications for electrocardiographic guidance of pacing catheters.

Intracardiac electrocardiography has been reported to be the most reliable of the techniques used for guiding placement of transvenous pacing catheters. When a catheter electrode is in contact with the right ventricular apical endocardium, the intracardiac electrogram demonstrates marked ST segment elevation and, usually, a very large S wave. The case presented indicates that both of these features of the intracardiac electrogram should be sought since ST segment elevation alone may represent epicardial recording of an acute myocardial infarction when the catheter electrode is in the coronary venous system.