This ECG is taken from an elderly man who has a history of complete heart block and AV sequential pacemaker.  On the day of this ECG, he presented to the Emergency Department with chest pain and shortness of breath. His vital signs were stable and within normal limits.  We do not have information about his treatment or outcome. 

I don’t see spikes.  How do we know this is a paced rhythm?  The ECG clearly shows the presence of an AV pacemaker.  There are very tiny pacer “spikes”, probably best seen in Leads III, aVF, aVL, and most of the precordial leads.  Other ECG signs that this is a paced rhythm are:  wide QRS at about .16 seconds (160 ms); abnormal left frontal plane axis; regular rhythm with AV dissociation (there are P waves seen occasionally that have no fixed relationship to the QRS complexes).  Also, V6 is negative.  That rules out left bundle branch block unless the electrodes are misplaced.  There are no capture beats in this strip.  The patient appears to be, at least right now, 100% dependent on the paced rhythm. 

Why does the presence of a pacemaker make it harder to diagnose an M.I. from the ECG?  Wide-QRS rhythms, such as right-ventricular paced rhythms, left bundle branch block, and ventricular ectopic rhythms, usually have “discordant ST and T wave changes”.  That is, when the QRS is positive (upright), the ST and T wave are negative.  The reverse is also true:  when the QRS is negative and wide, the ST and T wave changes are positive (ST elevation).  This is not true for right bundle branch block because the conduction delay that causes the widening of the QRS is in the right ventricle, and the ST segment is reflecting the LEFT ventricle’s repolarization.  Discordant ST changes can make it difficult to determine from the ECG alone that there is an ST elevation M.I. (STEMI).  Diagnosis usually must be made from patient presentation, ECG changes over time, and cardiac enzymes – or more definitively from cardiac angiogram. Pacemakers that produce narrow QRS complexes do not cause discordant ST changes. 

 Can we see an M.I. on this ECG?   Remember that this patient was complaining of chest pain.  Fortunately, his STEMI is pretty easy to see on the ECG.  He has ST ELEVATION in leads where there should be ST depression.  That is, the wide-QRS complex paced rhythm has POSITIVE QRS complexes in Leads I and aVL – he should have ST DEPRESSION from the paced rhythm.  Instead, he has ST ELEVATION.  This is anterior-lateral STEMI. Lead III shows ST depression where we would expect to see elevation.  This is a reciprocal change caused by the M.I.   Also, Leads V2 through V6 have ST ELEVATION that is more pronounced that one would expect from a paced rhythm alone.