What Do You Think 22

101-year old, one-handed butcher’s apprentice.

Not too difficult! 

What do you think?

The rhythm is sinus with ventricular trigeminy.

The PR interval varies, with a normal PR interval (red highlight) prior to the next ventricular ectopic and a short one (yellow highlight) following the ventricular ectopic.

How do you explain this?

The ventricular ectopics interrupt the sinus P wave timing. This is because the ventricular ectopics result in retrograde conduction and a pause or resetting of the P wave timing cycle. This is referred to as a partial compensatory pause.

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To summarize:

Full compensatory pause

There is a concealed non-conducted sinus P wave (red stippled arrow) within the ectopic QRS. We wait for the next sinus cycle and hence the full pause.

Partial compensatory pause

There is a retrograde P wave(green vertical arrow) from the ventricular ectopic. This  precedes the sinus P wave and inhibits the next sinus cycle (blue stippled arrow). The retrograde P wave restarts a sinus cycle. The two sinus cycles with the ventricular ectopic are therefore shorter than with a full compensatory pause.

So how does this alter the PR intervals?

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Prior to the ventricular ectopic, the PR  interval is normal (red highlight). This is because there is a preceding “normal” sinus cycle (red horizontal arrow). The cycle after the ventricular ectopic commences with a retrograde P wave (red vertical arrow) and although this is equivalent to a normal sinus cycle (blue arrow), the R-R interval is longer (green arrow). This longer R-R interval allows a junctional escape beat (yellow highlight).                                                                                                                  

Here is another example of a junctional escape beat in the next PR interval following the partial compensatory pause of a ventricular ectopic.

This rhythm mimics isorhythmic AV dissociation.

To summarize:

Iso-rhythmic AV dissociation is a common nocturnal rhythm, particularly in young or fit athletes, where increased vagal tone results in sinus bradycardia. When the sinus rate becomes slower than the AV node pacemaker, junctional rhythm emerges. This is frequently a rhythmic pattern related to sinus arrhythmia.  

Iso-rhythmic AV dissociation with conducted sinus rhythm only in the last complex (red highlight). There isa sinus P wave before most QRS complexes, but the PR intervals are very short and the QRS complexes are junctional escape beats which terminate normal AV conduction.

With vagal hypertonia, junctional rhythm may occur without a preceding sinus P wave and retrograde conduction may then occur, resetting the sinus node. By definition this is not iso-rhythmic AV dissociation.

Sinus rhythm (red highlight) with a junctional escape couplet (yellow highlight) and retrograde conduction(red arrows). 

On rare occasions, a junctional rhythm may predominate with an escape sinus node “ectopic” (red highlight).

Of course this may be an atrial ectopic adjacent to the sinus node called a crista-terminalis ectopic.   

idioventricular rhythm is often incorrectly referred to as iso-rhythmic AV dissociation.

The first complex is a sinus beat conducted with a left bundle branch block (red highlight). The last three complexes are an idioventricular rhythm with a right bundle branch block configuration and P waves being consumed by the QRS complexes (red arrows). Between these lies a fusion beat with the narrow QRS because of near simultaneous biventricular activation (blue highlight). The relationship of the sinus P waves to the QRS complexes caused the confusion.

Idioventricular rhythm may commence or terminate with a fusion beat.

Sinus rhythm (red highlight) and idioventricular rhythm (yellow highlight) with the sinus P waves being consumed by the ventricular rhythm. Fusion beats (blue highlight) are present. 

Some of you will ask whether the ventricular trigeminy is actually parasystole.

There are very minor differences in the sinus-ectopic coupling intervals (red highlight), but the ectopic coupling intervals probably exclude parasystole.      

Longer strips are required.