What Do You Think 60

 I would like to present an ECG and go through the steps of preparation and analysis in order to get the answer.

The ECGs were sent to me by Pantea at CardioScan.

Thank you Pantea.

Here are the raw ECGs.

What do you think?

Bunched up together, they look messy. I have chosen the top tracing.

Now shortened (easier to see) and cleaned up, removing the annotation.

Are P waves present and if so, is it sinus rhythm?

The rhythm is sinus with regular P waves at a rate of 75 bpm.

What about the ventricular rate?

The ventricular rate is much slower and irregular with repetitive sequences of two long R-R intervals at1400 ms (43 bpm) followed by a short R-R interval at 960 ms (63 bpm). There are three sequences on the tracing (red highlight).

Seeing the sinus and R-R cycles are very different in rate, is there a relationship between them?

Most P waves do not conduct(red stippled arrows). Some do (red solid arrows) with a normal PR interval.Because the R-R interval prior to the conducted beat is shorter, this complex must be conducted.

The conducted and non-conducted QRS complexes have different configurations?

Although both QRS complexes are narrow, they are different in configuration with a marked T wave abnormality in the non-conducted QRS (blue highlight) suggesting the pacemaker focus lies high in the ventricle within the fascicles and thus this is a fascicular escape rhythm.

Why is there intermittent AV conduction?

The P waves that conduct always fall just beyond the T wave of the previous non-conducted QRS complex (green highlight).  

Why is this important?

There is a critical zone after the T wave called the supernormal phase of excitability or conductivity. When the propagating wave of depolarization arrives at the block during this zone, there is enhanced conduction, despite the underlying complete AV block. This narrow zone lies at the end of the relative refractory period, where phase 3 merges into phase 4 (red stippled arrow).

The concept of supernormal phase of conductivity creates a conundrum. The definition of complete AV block clearly states that unlike high degree AV block, there is no AV conduction. Recognising this anomaly, when discussing supernormal phase of conductivity, it is best to allow the term complete AV block to be retained.

To summarize:

When reviewing long rhythm strips of complete AV block, isolated single episodes of AV conduction can occasionally be seen. This is due to enhanced AV conduction during the supernormal phase of conductivity. The supernormal phase is a very narrow zone within the cardiac action potential at the junction of phases 3 and 4. With complete AV block, a propagating wave of depolarization upon reaching the area of block may now be conducted to the ventricle. The pattern of AV conduction is dependent on the underlying site of pathology and the conducted QRS may be narrow or broad.

Narrow conducted QRS (≤ 120ms).

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 pacemaker escape focus is in theAV 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, near identical to the escape QRS complexes.

Broad conducted QRS (≥120 ms).

Broad escape ventricular QRS complexes may originate anywhere below the His bundle. The QRS complexes of the escape rhythm and conducted beats bear no resemblance to each other. In this situation, supernormal phase of conduction may be diagnosed as ventricular ectopics. Look for the conducted P wave (red solid arrows).

The QRS has a right bundle branch block configuration. Supernormal conduction results in a left bundle branch block (red highlight) and would be interpreted as ventricular ectopics were it not for the preceding P waves in the zone of the supernormal phase of conductivity.

It is not always possible to be sure that the premature beat has been conducted.

Narrow QRS complete AV block.A P wave falls in the T wave (red highlight) and appears to conduct with aberrant ventricular conduction. Electrophysiologically this P wave may be too early and this is actually a ventricular ectopic.  

Supernormal phase of conductivity can also be demonstrated with ventricular pacing and high threshold exit block.

When the voltage requirements for ventricular pacing exceeds the output of the pulse generator, high threshold exit block occurs. On the ECG, the stimulus artefact (blue stippled arrows) is not followed by a QRS unless the artefact occurs at the time of the supernormal phase of conductivity (blue solid arrows).

Unipolar VVI pacing 60 bpm with native rhythm 85 bpm. There is both high threshold exit block and intermittent failure of sensing. Some stimulus artefacts occur in the refractory period of the heart.

Another example may occur with pre-excitation when there is competition between the accessory pathway and AV conduction.

Alternating conduction between the accessory pathway and AV conduction in a patient with pre-excitation. The alternating block lies in the accessory pathway with every second beat being conducted prematurely with a short PR interval. One can envisage, alternating conduction as a result of supernormal phase of conductivity within the accessory pathway.This is speculation on my behalf. If this can occur does this mean that dual AV nodal slow pathway conduction may also be triggered by supernormal phase of conductivity. Would love comments.

Harry Mond