This ECG is from a 66-year-old woman who called 911 for a complaint of chest pain for the past four hours. She also complained of nausea, vomiting, and diarrhea for that time. She was pale and diaphoretic, and her BP was 77/43 sitting up, improving to 90/54 reclining. She denied “cardiac” history.  Her medications included:  aspirin, an SSRI, cilostazol, amlodipine, umeclidinium and vilanterol inhaler, atorvastatin, levothyroid, and metoprolol. We don’t have a previous ECG.  The EMS crew followed their chest pain protocol and delivered the patient to a facility with an interventional cath lab, but they did not designate a “STEMI Alert” because of the wide QRS.  It is their protocol to use the term “STEMI Alert” only when no M.I. mimics, such as left bundle branch block, are present. 

What does this ECG show?     There is an underlying sinus rhythm at 75 bpm.  There is AV dissociation, with regular, wide QRS complexes at a rate of 44 bpm.   This meets the criteria for complete heart block (third-degree AV block).  The morphology of the QRS complexes meets the criteria for left bundle branch block (wide, upright in Leads I and V6, negative in V1).  At a rate of 44 bpm, several options for this escape rhythm are possible:  1)  junctional escape with LBBB, 2) junctional escape with intraventricular conduction delay due to AMI,  and 3) idioventricular escape rhythm.   Also, in the presence of IWMI, AV node ischemia is very likely, resulting in AV blocks at the level of the AV node.  CHB at the AV node would result in junctional escape rhythm, and CHB below that, in the fascicles of the bundle branches, would result in idioventricular escape. The issue for this patient, and ANY patient, is cardiac output, and we see several reasons for cardiac output to be lower:

·         Wide QRS

This ECG is taken from a woman who had suffered for several years with intractable intermittent atrial fibrillation. She had tolerated medications poorly, and several attempts at electric cardioversion had resulted in only temporary relief. Ultimately, she chose to undergo AV node ablation.  In the electrophysiology lab, her AV node was destroyed, preventing the atrial fib impulses from penetrating into the ventricles.  This resulted in a “man-made” complete AV block.  A pacemaker was implanted in the EP lab.  When she is in atrial fibrillation, the fibrillatory waves of the atria INHIBIT the atrial pacing electrode from firing, so she has no paced P waves at that time.  The right ventricular pacing electrode functions without inhibition, and makes a wide QRS complex with a leftward axis deviation (normal for RV pacing). 

In this ECG, we see the patient WITHOUT atrial fib, and the pacemaker is pacing the atria AND the ventricles, in a sequential fashion.  The spikes are very hard to see, as this is a “bipolar” pacemaker, which makes much smaller spikes than a “unipolar” pacemaker.  Some ECG machines will automatically enhance the spikes, but this one did not.  We have marked a “sample” atrial spike in blue for you and one of the ventricular spikes in red.  Each beat on this ECG actually has appropriately-timed atrial and ventricular stimuli (spikes), and the patient has optimized cardiac output provided by the “atrial kick”. A P wave occurring just before a QRS indicates that the ventricles are filling from the forceful contraction of the atria.  This provides much better filling than when the atria are not beating or are fibrillating. 

This is a good ECG to use to show your students how we can recognize a paced rhythm without being sure of the spikes.  Of course, without other evidence (patient history and exam), we can’t know for sure that this is a paced rhythm, but the steady, normal rate, wide complexes, and left axis deviation are signs of RV pacing.  Look for negatively-deflected QRS complexes in II, III, and aVF and positive QRSs in aVL and aVR. 

When pacing only one ventricle, the impulse travels relatively slowly through the other ventricle, resulting in wide QRS complexes.  This then results in SECONDARY ST-T WAVE CHANGES.  Wherever the QRS is positive, you will normally see some ST depression and T wave inversion.  In leads with negative QRS complexes, the opposite is true, and you will see ST elevation and upright T waves.  This can make evaluation of ST segments for acute M.I. difficult.

This ECG was taken from a 90-year-old woman.  We have no other history, unfortunately.  It is a good example of a sinus rhythm with complete AV block, also called third-degree AV block.

The defining characteristics of this rhythm include:   1) an underlying rhythm that is regular and with a physiological rate.  In other words, the P waves are not so fast that they would not be expected to conduct one-to-one.  2)  a second rhythm of regular QRS complexes that is unrelated to the P waves.

Occasionally, a P wave may occur before a QRS and appear to have a PR interval.  This is just a chance meeting, as both rhythms (P waves and QRS complexes) are regular AT DIFFERENT RATES, so we would expect them to occur near each other from time to time.  NONE of the P waves are being conducted to the ventricles to produce QRS complexes. This is a good ECG to demonstrate "marching out" the P waves to see that they are very regular, even though some are hidden in the QRS, ST segment, or T waves.

In this case, the "escape rhythm" occurs from the AV junction.  The AV junctional pacemakers are "set" at a rate of about 40 - 60 beats per minute.  Normally, the sinus rhythm arrives in the AV junction faster than that, depolarizing the junctional pacemakers and preventing them from firing spontaneously.  In complete AV block, the atrial impulse never arrives, so the junctional pacemaker is free to "escape" and become the primary pacemaker of the heart.  We recognize this rhythm as junctional because the QRS complexes are narrow, and the rate is around 40 bpm.  Knowing that the escape rhythm is from the junction tells us that the AV block is in the AV node.  The AV junction is the first available pacemaker below the block.  Had the complete AV block been lower, in the bundle branches, the QRS would have come from the ventricles and would have been wide and slower.

In very general terms, this "supra-Hisian" type of AV block is preferable to a "sub-Hisian" block.  The rate is faster, and the QRS complexes narrow, both conditions causing a better cardiac output than wide QRS complexes and extremely slow rates.  However, the effect of the block on the patient has a lot to do with the cause of the block and the symptoms the slow rate cause.  Emergency treatment of the rate may be necessary if it causes a drop in blood pressure and perfusion.  Some patients with this type of block will need a permanent implanted pacemaker, but not all.