A-V dissociation: A reappraisal∗

Atrioventricular dissociation

Atrioventricular (AV) dissociation is an electrocardiographic syndrome; a descriptive term for a variety of conditions of abnormal cardiac conduction which all feature independent function of the atria and ventricles. AV dissociation can be subclassified as AV dissociation by default (an independent ventricular pacemaker responds to slowing of the dominant atrial pacemaker) versus AV dissociation by usurpation (acceleration of a latent pacemaker takes control of cardiac conduction by exceeding the intrinsic atrial rate). Inclusion of third degree AV block (complete heart block) as a manifestation of AV dissociation is controversial, yet is functionally appealing in that this disorder also features independent activity of the atria and ventricles.

The mechanism of synchronization in isorhythmic A-V dissociation. II. Clinical studies

The electrocardiographic patterns recorded from seven patients with isorhythmic A-V dissociation fall into two distinct groups. In pattern I, the P wave fluctuates cyclically back and forth across the QRS complex. The mechanism responsible for this type of A-V synchronization represents a typical biologic feedback control system. The P-R interval is a determinant of stroke volume, which in turn influences the arterial blood pressure. The blood pressure has an inverse effect on the discharge frequency of the S-A node through the baroreceptor reflex. The S-A nodal frequency then affects the P-R interval, to close the feedback loop. In pattern II, the P wave is in a fairly constant position relative to the QRS complex. It is usually coincident with the QRS complex or appears on the ST segment or first half of the T wave. The mechanism producing synchronization in pattern II type of isorhythmic dissociation has not been established conclusively.

Principles governing 2:1 AV block with interference dissociation

Images

The mechanism of synchronization in isorhythmic A-V dissociation. Some observations on the morphology and polarity of the P wave during retrograde capture of the atria

We studied 11 patients with spontaneous isorhythmic atrioventricular (A-V) dissociation occurring during cardiac surgery. The period of synchronization or accrochage was shown to result from retrograde capture of the atria by an A-V junctional rhythm. The P wave in leads II, III, and aV F was biphasic (–, +) during the period of synchronization, with the initial negative portion of the P wave or the entire P wave often buried within the QRS complex. It was concluded that isorhythmic A-V dissociation occurs only during the period when the rhythm alternates between an atrial rhythm and an A-V junctional rhythm and results when either the dominant pacemaker slows or the latent pacemaker accelerates.

Analogy of Electronic Pacemaker and Ventricular Parasystole with Observations on Refractory Period, Supernormal Phase, and Synchronization

The artificial pacemaker has been compared to a physiologic ventricular parasystolic focus. In hearts being driven by a pacemaker, in the absence of atrioventricular (AV) block, capture (interference) phenomena are constantly seen. While variations in QRS complexes have been observed when the external pacemaker stimulus occurs early in the T-wave period of a sinus conducted beat, no sequence of aberrant beats has been observed. One patient showed occasional sinus beats interpolated between ventricular beats arising from the artificial pacemaker, and these occurred at a very narrow time band suggesting that AV conduction was permitted by phenomenon of a supernormal phase. At a later date this patient showed also retrograde conduction believed related to a supernormal phase during recovery of junctional tissue penetrated by an impulse entering from above, though the impulse itself was blocked.

Records of two patients are used to illustrate grossly aberrant QRS complexes produced by the stimulus of the electric pacemaker when this fell in the semirefractory period. Noteworthy was the presence of a latent period before potentials of the propagated impulse were recorded, there being evident stimulus-to-QRS delays. The situation wherein the rhythm is most chaotic is that where AV block is absent, the sinus rate is fast, and the electric pacemaker rate is relatively slow. In two patients, retrograde activation of the atria occurred as a stable mechanism. In one patient, nodal rhythm of the RP type occurred, and sequences of the records suggested that the AV node might have become synchronized with the extrinsic pacemaker cycle. In this last instance three independent centers of effective impulse formation coexisted, the sinus node, AV node, and stimulating electrodes of the electronic pacemaker.

A marked increase in the refractory period of the ventricle, as measured by its response to the set stimulus of the electric pacemaker delivered at varying instants with respect to the sinus mechanism, occurred in one case within a few days. The possibility that reserpine therapy contributed to this change is possible but has not been established, for the effective refractory period, as measured, later remained constant over a period of many weeks.