Tracing

This tracing shows inappropriate switching to DDD mode in the absence of atrioventricular conduction disorder, the BAV 1 criterion being fulfilled on an active junctional rhythm; initially, rate responsive atrial pacing and intrinsic atrioventricular conduction; slight acceleration of the ventricular rhythm corresponding to a probable active junctional rhythm; a complex marked Vr (third cycle of the tracing) corresponds to sensing in the safety window (absence of forced pacing in this context during operation in SafeR mode); ventricular activation being slightly faster than the atrial pacing rate, the ventricular complexes will progressively precede atrial pacing and occur immediately before marker Ap yielding a long Ap-Vs pseudo-pattern; the AR pseudo-intervals gradually decrease but remain higher than the programmed value; after 6 consecutive cycles, the pacemaker switches to DDD mode (vertical line); note that in the first cycle, ventricular sensing Vr occurs in a safety window triggering forced ventricular pacing (Vn) after a short AV delay of 95 ms at the end of the safety window (normal operation of the DDD mode); on the second cycle, ventricular sensing occurs in the post-atrial ventricular blanking and is therefore not detected (no inhibition of ventricular pacing, no forced pacing at the end of the safety window, and ventricular pacing at the end of programmed AV delay);

Comments

In SafeR mode, interrogation of the episodes of mode switching to DDD highlights a certain number of occasionally surprising patterns. One of the relatively common causes of inappropriate switching (absence of atrioventricular conduction disorder) is the presence of an active idioventricular or junctional rhythm and isorhythmic dissociation. The SafeR mode is predominantly programmed in patients with sinus dysfunction and preserved atrioventricular conduction. In some patients, there may be an increased automaticity at an ectopic focus (most often the atrioventricular node) which generates a so-called active rhythm that competes with activation stemming from the atrium. When the nodal « escape » is clearly faster than the programmed atrial rate, retrograde atrial conduction is observed if ventricular-atrial conduction is preserved with 1:1 ventricular-atrial activation. In SafeR mode, the pacemaker does not switch according to an AVB II, III or pause criterion (absence of blocked « P wave », no pause). On the other hand, it can switch according to an AVB 1 criterion if, over several consecutive cycles, the RP (Vs-Ap) intervals are relatively short and consequently the PR (Ap-Vs) intervals are long.

When the rate of the active junctional rhythm is relatively similar to that of the P wave or atrial pacing, there may be a competition between the two rhythms and one can thus observe capture or fusion complexes. There can also be a so-called isorhythmic dissociation, the atria and ventricles having an independent regular rhythm in the absence of atrioventricular conduction disorders. When the junctional rhythm is slightly faster than the pacing rate as observed in this example, intrinsic ventricular activity may precede atrial pacing and yield a pseudo-pattern of long PR leading to mode switching.

Interrogation of the device memory is relatively frequent in this type of episodes in patients with sinus dysfunction. Their number per patient, however, is most often limited, leading to a very modest increase in percentage of ventricular pacing. An increase in the minimum rate and the programming of a rate response allows eliminating or considerably reducing the occurrence of this type of switching, which is generally perfectly asymptomatic and can therefore, insofar as they are rare, be preserved and not induce changes in programming. Indeed, while the switching can, strictly-speaking, be deemed inappropriate (absence of atrioventricular conduction disorder), they nonetheless appear relatively adapted to the extent that they allow restoring a physiological atrioventricular synchrony.

Patients with sick sinus syndrome programmed in SafeR mode may exhibit asymptomatic junctional episodes revealed by inappropriate switching according to a AVB I criterion.