SafeR 3: switch for second-degree atrioventricular block
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At the beginning of the tracing, atrial pacing with spontaneous atrioventricular conduction; second-degree atrioventricular block with alternating conducted atrial pacing and blocked atrial pacing (2:1 block pattern); after 3 non-consecutive blocked paced atrial complexes, the BAV II criterion is fulfilled (non-programmable parameter, 3 in 12 blocked atrial activities); switching to DDD mode (vertical line);
The second degree atrioventricular block type 1 (Mobitz 1) is defined by the intermittent presence of a blocked P wave with a variable PR interval. The Luciani-Wenckebach second-degree AV block corresponds to a gradual 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 (Luciani-Wenckebach periods). These periods can be more or less prolonged, consisting of between 2 and 10 conducted P waves for one blocked P wave. The second-degree AV block type 1 primarily corresponds to a conduction disorder located at the level of the atrioventricular node and to an exaggeration of the decremental conduction. This location explains the predominantly normal morphology of the QRS complexes. During an electrophysiological study, a nodal slowdown results in a progressive prolongation of the AH interval, with the HV interval being normal and the blocked P wave not followed by a Hisian potential. Indications for implantation in the setting of a second-degree AV block type 1 are controversial. The causal relationship between conduction disorder and the onset of symptoms is difficult to establish, especially when symptoms are modest and non-specific. There is a Class IIA indication in European recommendations when a second-degree AV block type I is responsible for symptoms or when the location of the conduction disorder is intra- or infra-Hisian (major risk of progression to complete atrioventricular block) upon electrophysiological study.
The second-degree atrioventricular block type 2 (Mobitz 2) is much rarer than type 1; blocked P waves not followed by QRS occur unexpectedly, without prior prolongation of the PR space. The electrocardiogram thus shows an intermittent blocking of the P wave; the recurrence of blocked P waves can be regular, although most often are irregular; the number of blocked P waves is generally increased by maneuvers that increase the heart rate (exercise, atropine) and vice versa for the maneuvers that decrease the heart rate. The PR interval of the conducted complexes remains constant. Ventricular pauses are twice the normal duration of the RR interval. Second-degree AV block type 2 is mostly the result of an irreversible degenerative anatomical lesion. The recording of the potentials of the bundle of His reveals that the origin of the intermittent conductance blockage is usually located in the distal part of the bundle of His or in one of the three branches, the other two already being blocked (intra- or infra-Hisian block). This explains the constancy of the PR interval before and after the blocked P wave, with conduction occurring according to an all-or-nothing principle. This type of block is an indisputable indication for pacemaker implantation. Indeed, the progression towards third-degree AV block is common; furthermore, because of the location of the block, potential escape (routes) are located inferiorly and are often slow and unstable, thereby increasing the risk of syncope or sudden death.
In the SafeR algorithm, the AVB II criterion is not programmable. The pacemaker switches to DDD mode when 3 in 12 atrial events (sensed outside of the relative refractory period or paced) are blocked. The ratio between blocked P waves and conducted P waves leading to mode switching cannot be changed.
The second-degree AV block type 1 with Luciani-Wenckebach sequences corresponds to a progressive prolongation of the PR interval until the occurrence of a blocked P wave. The second-degree AV block type 2 (Mobitz 2) corresponds to the occurrence of a blocked P wave without prior modification of the PR interval (PR constant before and after the pause). When the SafeR mode is programmed, the pacemaker switches to DDD mode when 3 in 12 atrial activities are blocked (non-programmable ratio).
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