Illusion of Sinus Tachycardia

Report:

(Lead 2 sequential rhythm strips)

Sinus rhythm approximately 96/min

Frequent VEBs, some in triplets or short runs of VT

Concealed retrograde conduction

Comment:

This is a good example of the reciprocal R-P – P-R relationship. Much of the lower strips is allorhythmic alternation of VT triplets with a pair of conducted sinus beats. The first sinus beat has its P wave quite close to the preceding ectopic QRS and distorts its T wave. The shortness of this R-P interval is in contrast to the quite long (0.36”) P-R interval of the first sinus beat. Next P wave keeps more distance from the preceding (in this case, normal) QRS complex; its P-R interval is only modestly prolonged at 0.26”. The difference between the two P-R intervals causes approximation of the two normal QRS complexes, giving an illusion of much faster sinus rate, about 112/min. This is one of the mechanisms of sinus rate being faster than itself; the other involves Wenckebach conduction, as in the Case 12094.

The first conducted P wave in each pair has a long P-R interval because of the partial penetration of the last VEB into the AV node. This kind of retrograde conduction is only visible by its effect on the subsequent beat – hence ‘concealed’.