Detection of myocardial infarction in paced rhythm is often difficult due to the baseline abnormality in depolarization and secondary abnormality in repolarization. But if an ECG prior to myocardial infarction is available, careful lead by lead comparison will make detection of a fresh myocardial infarction rather easy. This should of course be complemented by clinical evaluation and estimation of biomarkers of myocardial necrosis.
ECG showing paced rhythm with anterior wall infarction
Large amplitude pacing spikes are seen before each QRS complex indicating unipolar ventricular pacing. Lead I and aVL shows left bundle branch block pattern and inferior leads show negative QRS complexes. Together these patterns suggest pacing from the right ventricular apex. Dissociated P waves suggest complete heart block as the reason for pacemaker implantation. T waves are inverted in leads V2-V6, in the presence of negative QRS complexes (concordant T waves). This would suggest a primary T wave abnormality.
Right ventricular apical pacing prior to infarction
Review of previous ECG, though partially faded, showed upright T waves in anterior leads. This in addition to the clinical history of anginal pain, wall motion abnormality on echo and elevated troponins confirmed the possibility of an anterior myocardial infarction.