Sinus Rhythm Faster Than Itself

Report:

Nonparoxysmal junctional tachycardia (NPJT) 98-100/min

Sinus rhythm 94-96/min

Left atrial abnormality

Sinus capture beats

AV dissociation with interference

Left ventricular hypertrophy with ST/T changes (incomplete LBBB)

Comment:

Paradoxically, the overall tachycardia averaging 110/min is caused by normal sinus rhythm as much as by the borderline (in rate) junctional tachycardia. Pairs of sinus capture beats following a junctional beat form two groups of triplets, each with a rate of over 130/min.

How can sinus rhythm produce a ventricular rate faster than itself? This happens by crowding of the two conducted sinus QRS complexes in each triplet by drastic shortening of the second PR interval.

The second PR interval is shorter because its P wave is further apart from the preceding QRS complex and the AV node is more receptive to its conduction. There is a reciprocal P-R and R-P relationship, as described (more elegantly than here) by Marriott in his Pearls and Pitfalls76.

The sinus P waves following the triplets have even shorter P-R intervals, but are not necessarily conducted; if there is any conduction, the resultant QRS is a fusion beat between the sinus and the junctional impulses. Other P waves, merging into the ensuing QRS complexes, are obviously dissociated. They are – to borrow from Marriott once more – “antecedent but unattached”77.

The criteria for AV dissociation consist of (1) independent beating of the atria and the ventricles, (2) ventricular rate exceeding the atrial rate and (3) absence of retrograde conduction of the ventricular beats78. These are relatively narrow criteria, separating the dissociation from AV block and, one would like to hope, lessening the confusion and ambiguity surrounding it.

This example is one of incomplete AV dissociation, since there are at least four sinus capture beats. It is, traditionally (from Möbitz and Scherf, no less) called AV dissociation with interference, a term deprecated by Marriott79 but less so by his unfortunate successor, who found an example of “AV dissociation due to interference” in a series of retrogradely conducted junctional beats80!

While recognising Marriott’s objections, I still use the traditional term AV dissociation with interference81.