What Do You Think 52

Why three QRS complexes?

I was asked why this tracing has three different QRS configurations almost all with similar cycle lengths.

What do you think?

When interpreting ECGs, first determine the rhythm and if P waves are present is  AV conduction intact.

Regular sinus P waves are present.

Some P waves are conducted(solid arrows) and some non-conducted (red stippled arrows).

On the top row, the rhythm is 2:1 AV block with conducted broad QRS complexes (red highlight) suggesting distal conduction tissue disease.

Although we do not know which bundle branch is blocked, let us for educational purposes, call it a right bundle branch block.

Other beats are clearly non-sinus and are regular escape ventricular beats with a narrow QRS complex (yellow highlight).

Why does an escape ventricular complex have a narrow QRS, yet the conducted QRS has a bundle branch block?

Because there is a right bundle branch block, the ventricular escape focus lies within or next to the left bundle branch fascicles and conducts to the ventricle. However, it may also conduct rapidly to the right bundle branch either by interconnecting Purkinje bridges or retrograde via the bundle of His. This can occur if the right bundle branch block lies in the proximal bundle of His. The differentiation of the bundle branches occurs high in the Bundle of His before it splits into the two discrete bundles and is referred to as longitudinal dissociation. Also, although anterograde conduction is blocked in the right bundle branch, retrograde conduction may not be blocked and allows conduction.  

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There is a third QRS complex (blue highlight), which is narrow and preceded by a sinus P wave and a normal PR interval.

How do we explain this QRS?

These are ventricular fusion beats between the sinus generated and the ventricular escape focus generated QRS complexes.  

Why does fusion occur?

By chance, the sinus generated QRS complexes with 2:1 block (2200 ms) and the ventricular escape focus (2,200 ms) have exactly the same timing. Following a run of complete AV block, there is a conducted beat which sets up the synchronization of the two foci (red and blue arrows) for two fusion beats until complete AV block recurs.

I cannot explain why AV conduction (red highlight) results in the only premature beat in the tracing, but such premature AV conduction can occur during the supernormal phase of conductivity.  

The supernormal phase of conductivity is a narrow zone of enhanced excitability occurring at the end of the relative refractory period of the action potential, where a normally sub threshold stimulus from a conducted beat or pacing stimulus can elicit a response. During the relative refractory period, a stronger-than-usual stimulus is usually required to initiate an action potential. It therefore seems ironic that within phase 3 of the action potential, two zones exist with opposite responses to cardiac propagating stimuli or pacing. On the ECG, the supernormal phase of conduction occurs just beyond the T wave of the previous beat.  

Supernormal excitability and conduction is best seen and most frequently recognised with complete AV block where a P wave falling in the supernormal phase, may allow AV conduction. The pattern of AV conduction is dependent on the underlying pathology and the conducted QRS maybe narrow or broad.

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The narrow conducted QRS (≤120 ms).

Two examples of sinus rhythm(red arrows) with complete AV block (stippled red arrows) and a very slow narrow QRS escape rhythm, suggesting that the focus is in the AV junction.Identical premature QRS complexes (red highlight, solid red arrows) occur when the P waves fall in the supernormal phase and conduct to the ventricle along a conduction pathway resulting in QRS complexes near identical to the escape QRS complexes.

The broad conducted QRS (≥120ms).  

The escape QRS originates below the AV junction. The broad QRS of AV conduction will bear no resemblance to the escape QRS, which has a bundle branch block configuration. In this situation, supernormal phase of conduction is often mistaken for ventricular ectopics.

Sinus rhythm (red arrows) with complete heart block. The conducted QRS has a right bundle branch block, where as supernormal conduction results in a left bundle branch block configuration (red highlight). There is gradual sinus slowing during the tracing.

With a narrow QRS complete AV block, the conducted QRS may be wide (red highlight) because of prematurity and hence aberrant ventricular conduction.  

Harry Mond