Vagally Mediated Atrioventricular Block
Vagally mediated AV block is characteristically paroxysmal and can be recurrent. Some reports dealing with the ECG findings of this form of block have been published; it has been recorded as an asymptomatic event during Holter monitoring or as a symptomatic episode (syncope/presyncope) during Holter or implantable loop-recorder monitoring. These reports allow us to draw up some ECG criteria. The diagnosis of vagally mediated AV block can be made when there is a simultaneous depression of sinus node function and AV conduction; the ECG criteria can be summarised as follows (figures 1–3):
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Figure 1.
Vagally mediated second-degree atrioventricular block. Mobitz type I block associated with simultaneous slowing of sinus rate.
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Figure 2.
Vagally mediated second-degree atrioventricular block. The arrow shows a marked prolongation of the PP interval simultaneously with the blocked P wave.
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Figure 3.
Vagally mediated atrioventricular block during a syncopal episode detected by an implantable loop recorder. The triangle indicates the time of automatic activation of the device. The stored electrocardiogram shows progressively severe sinus bradycardia followed by complete atrioventricular block with long ventricular pauses of 14 s and 6 s.
At the baseline, PR and QRS intervals are normal in most cases; sinus bradycardia is present in some patients.
Slowing of the sinus rate does not usually interfere with the diagnosis of Mobitz I block, whereas it can interfere with the diagnosis of Mobitz II block. In 1978, Massie et al reported that a vagal surge can cause simultaneous sinus slowing and AV block that can simulate Mobitz II block, and termed this phenomenon 'apparent type II block'. Other authors have used the term 'pseudo-Mobitz II block'; we prefer this term, which appears to be clearer. In vagally mediated AV block, the blocked P wave is generally preceded by prolongation of the PR interval, but in some patients this prolongation is not present and the AV block appears abruptly. Moreover, the first PR interval after the blocked beat may be equal to the PR intervals before the vagal effect, either because of a fortuitous relationship of a non-conducted P wave with an escape QRS complex or as a result of an actual conduction of the P wave with an unchanged PR interval (Figure 4). The latter occurs when a slower sinus rate facilitates AV conduction and overcomes the depressant effect of a residual vagal effect on AV nodal conduction. Thus, when all the PR intervals remain constant, Mobitz type II block may be diagnosed erroneously if sinus slowing is ignored. In the study by Lange et al, the pseudo-Mobitz II block was present in 14% of patients with vagally mediated second-degree AV block.
(Enlarge Image)
Figure 4.
'Pseudo-Mobitz type II block'. Second-degree atrioventricular block with constant PR intervals and simultaneous slowing of the sinus rate.
Electrocardiographic Findings of Vagally Mediated AV Block
Vagally mediated AV block is characteristically paroxysmal and can be recurrent. Some reports dealing with the ECG findings of this form of block have been published; it has been recorded as an asymptomatic event during Holter monitoring or as a symptomatic episode (syncope/presyncope) during Holter or implantable loop-recorder monitoring. These reports allow us to draw up some ECG criteria. The diagnosis of vagally mediated AV block can be made when there is a simultaneous depression of sinus node function and AV conduction; the ECG criteria can be summarised as follows (figures 1–3):
(Enlarge Image)
Figure 1.
Vagally mediated second-degree atrioventricular block. Mobitz type I block associated with simultaneous slowing of sinus rate.
(Enlarge Image)
Figure 2.
Vagally mediated second-degree atrioventricular block. The arrow shows a marked prolongation of the PP interval simultaneously with the blocked P wave.
(Enlarge Image)
Figure 3.
Vagally mediated atrioventricular block during a syncopal episode detected by an implantable loop recorder. The triangle indicates the time of automatic activation of the device. The stored electrocardiogram shows progressively severe sinus bradycardia followed by complete atrioventricular block with long ventricular pauses of 14 s and 6 s.
Heterogeneous presentation of AV block: Mobitz type I, pseudo-Mobitz type II (see below), 2:1, advanced-degree, complete AV block or a combination of different types of AV block; ventricular asystole may be present;—simultaneous slowing of the sinus rate with longer and irregular PP intervals;—blocked P wave generally preceded by a prolongation of PR interval in one or more beats;—significant PR prolongation or Mobitz I block before initiation of complete AV block;—AV block sometimes preceded and/or followed by sinus arrest;—resumption of AV conduction, generally on sinus acceleration.
At the baseline, PR and QRS intervals are normal in most cases; sinus bradycardia is present in some patients.
Pseudo-Mobitz Type II Block
Slowing of the sinus rate does not usually interfere with the diagnosis of Mobitz I block, whereas it can interfere with the diagnosis of Mobitz II block. In 1978, Massie et al reported that a vagal surge can cause simultaneous sinus slowing and AV block that can simulate Mobitz II block, and termed this phenomenon 'apparent type II block'. Other authors have used the term 'pseudo-Mobitz II block'; we prefer this term, which appears to be clearer. In vagally mediated AV block, the blocked P wave is generally preceded by prolongation of the PR interval, but in some patients this prolongation is not present and the AV block appears abruptly. Moreover, the first PR interval after the blocked beat may be equal to the PR intervals before the vagal effect, either because of a fortuitous relationship of a non-conducted P wave with an escape QRS complex or as a result of an actual conduction of the P wave with an unchanged PR interval (Figure 4). The latter occurs when a slower sinus rate facilitates AV conduction and overcomes the depressant effect of a residual vagal effect on AV nodal conduction. Thus, when all the PR intervals remain constant, Mobitz type II block may be diagnosed erroneously if sinus slowing is ignored. In the study by Lange et al, the pseudo-Mobitz II block was present in 14% of patients with vagally mediated second-degree AV block.
(Enlarge Image)
Figure 4.
'Pseudo-Mobitz type II block'. Second-degree atrioventricular block with constant PR intervals and simultaneous slowing of the sinus rate.