Second-degree atrioventricular block with Wenckebach periodicity

ECG: Atrial sinus activity (positive P waves in leads I, II, V5, V6, negative in aVR); left atrial enlargement (increased P wave duration, bifid pattern in lead II, significant negativity in V1), Mobitz I second-degree AV block pattern with Wenckebach periods: prolongation of the PR interval followed by a blocked P wave (3 conducted P waves, one blocked P wave, then 5 conducted P waves, one blocked P wave.

Comments: Second-degree atrioventricular block (AV block) type 1 (Mobitz 1) is defined by the intermittent presence of a blocked P wave with a variable PR interval. The second-degree AV block with Wenckebach periodicity is a special form of second-degree AV block type 1; in this setting, we can observe a progressive prolongation of the PR interval until the occurrence of a blocked P wave; after the pause, the next P wave is conducted with a PR interval returning to the baseline value after which the same sequence is reproduced (Wenckebach periods). These periods may be of variable length, comprising between 2 to 10 conducted P waves for one blocked P wave. The increment in PR interval is often more significant at the onset of the Wenckebach cycle and tends to decrease from cycle to cycle. For example, a PR interval of 150 ms can be measured on the first cycle and then prolonged to 250 ms on the second (+100 ms), 300 ms on the next (+50 ms), and finally to 330 ms (+30 ms) before the P wave is blocked. The PP cycles being of stable length, a shortening of the RR intervals is thus observed before occurrence of the pause.

Atrioventricular node conduction is said to be decremental with a change in conduction velocities as a function of the impulse rate (involvement of calcium channels), which is not the case for the His-Purkinje distal network (involvement of potassium channels) where conduction behaves in an all-or-nothing manner.

In this asymptomatic patient, a former athlete, an exercise test showed a resumption of normal atrioventricular synchronism (absence of blocked P waves, prolonged PR then shortening upon exertion). The narrow QRS complexes are evocative of an intra-nodal disorder with a low risk of development toward a third-degree atrioventricular block. This patient remained asymptomatic in the absence of pacemaker implantation.

Take-home message: A second-degree AV block type 1 with Wenckebach periodicity corresponds to a progressive prolongation of the PR interval until the occurrence of a blocked P wave. The phenomenon recurs in periods of varying lengths.