This ECG was taken from a 78-year-old man who was experiencing chest pressure in the morning, after having left shoulder pain since the night before. He has a history of hypertension and hypercholesterolemia, and has an implanted pacemaker.

What does the ECG show?  The ECG shows an atrial paced rhythm, with two premature beats, beats number 5 and 12.  These are probably PVCs.  The patient has a functioning AV conduction system, so the paced atrial beats are conducting through the AV node and producing QRS complexes.  In the interventricular conduction system, the impulse encounters right bundle branch block. This causes each QRS to have an “extra” wave attached at the end, representing slightly delayed depolarization of the right ventricle.  Instead of an “rS” pattern in V1, for example, we see “rSR’ “.  The slight delay causes the QRS to be widened, as we are measuring the two ventricles separately, rather than synchronously.

There is definite ST segment elevation in V₂ and V₃, and the shape of the ST segment is straight, having lost it’s normal “concave upward” appearance.  In an ECG taken three minutes later, the STE extends to V₄.

Do the pacemaker or the right bundle branch block prevent us from diagnosing an ST-elevation M.I.?  The answer to that is a resounding “NO!” Pacemakers can sometimes make it difficult to assess ST elevation because ventricular pacing causes ST segment changes.  Pacing the right ventricle causes a depolarization delay in the left ventricle as the impulse travels “cell to cell” across the LV.  This means an RV-paced beat will resemble a PVC from the RV.  When LV depolarization is altered, repolarization will also be altered, causing ST elevation in leads with negative QRS complexes, and ST depression is leads with upright QRSs. These are called discordant ST changes. These changes are proportionate to the height or depth of the QRS, with very minimal or no ST changes in leads with short or biphasic QRS complexes.  We don’t have to worry about that in this situation – the pacemaker is not pacing the ventricles.

You might have heard that bundle branch block makes it hard or even impossible to diagnose STEMI.  Left bundle branch block can cause some difficulty because, in LBBB, the left ventricle is depolarized via the right ventricle, instead of via the left bundle branch.  Like in RV pacing, the slow, cell-to-cell depolarization that results from LBBB also causes ST changes that are discordant to the QRS complexes, and proportionate to the size of the QRSs. 

But this patient has right bundle branch block. The left ventricle is depolarizing (and repolarizing) normally, while the conduction delay is in the right ventricle.  Since the M.I. is in the left ventricle, these obvious ST elevations indicate acute STEMI.

Patient outcome.  The EMS crew called the hospital with a “STEMI Alert”.  A second ECG showed increasing STE.  The patient’s troponin level was 0.33 ng/mL in the Emergency Dept. (Normal troponin is less than 0.01 ng/mL). He was rushed to the cath lab, where an occlusion was found in his left anterior descending artery.  The artery was opened and stented successfully.

Thanks to Lew Steinberg, Ryan Winkelman, and Palm Beach Gardens Fire Rescue for this interesting tracing.