Vagally Mediated Atrioventricular Block
Vagally Mediated Atrioventricular Block
Vagally mediated AV block has been fortuitously encountered as asymptomatic or paucisymptomatic episodes during Holter monitoring, or seen during typical situations characterised by paroxysmal vagal overactivity, such as carotid sinus massage (CSM), tilt-induced syncope, spontaneous neurally mediated syncope and emotional distress in animals.
Vagally mediated second- or third-degree AV block has been induced during CSM in patients with normal AV conduction, mainly during left-sided stimulation, though sinus arrest is seen much more often than AV block. Spontaneous vagally mediated AV block has been reproduced in a few patients during CSM, but we do not yet know the sensitivity and specificity of CSM -induced AV block.
There are few reports of AV block during tilt-induced-syncope. Zysko et al reported on 31 patients, 80% of whom had a normal PR interval at baseline. In all patients, AV block was associated with sinus slowing. Mobitz I, 2:1, advanced-degree and complete AV block were seen. In 67% of the patients, at least two types of AV block were recorded. Ventricular asystole was present in 58% of patients; in about half of these, sinus arrest occurred, either before or after the AV block. AV block during a positive tilt test is a rare phenomenon; indeed, in the study by Zysko et al it was seen in only 5% of patients with positive response. The same percentage was reported by Brignole et al, who observed that isolated depression of sinus node function was four times more common. The rarity of AV block during positive tilt testing may be due to the fact that the sympathetic and parasympathetic systems exert a fairly balanced effect on the normal AV node, whereas the normal sinus node is under the predominant effect of the parasympathetic system and, is probably more sensitive to vagal output. Moreover, vagal stimulation on the sinus node can mask the effect on the AV node—that is, the sinus arrest does not allow us to recognise AV conduction disturbances because of the lack of any atrial electrical activity.
There are some ECG reports of vagally mediated AV block (advanced-degree or complete) in patients with situational syncope, such as swallowing or coughing syncope or with vasovagal syncope; most of these patients had normal AV conduction. Mendoza et al reported on four patients with ECG-documented syncope during Holter recording, which showed a ventricular asystole with marked slowing of the sinus rate. As these patients had normal heart, normal AV conduction and positive tilt testing (without AV block), these syncopal episodes were diagnosed as neurally mediated. In the ISSUE-2 study, vagally mediated AV block (ventricular asystole and sinus slowing) was present in 8% of spontaneous implantable loop-recorder-documented syncopal episodes; in this case, too—that is, during spontaneous neurally mediated syncope, isolated sinus node dysfunction was much more common than AV block.
Transient second- or third-degree AV block associated with sinus slowing has been seen in rodents during emotional distress, such as during tonic immobility when the animals are suddenly attacked by a predator.
Typical Forms of Vagally Mediated AV Block
Vagally mediated AV block has been fortuitously encountered as asymptomatic or paucisymptomatic episodes during Holter monitoring, or seen during typical situations characterised by paroxysmal vagal overactivity, such as carotid sinus massage (CSM), tilt-induced syncope, spontaneous neurally mediated syncope and emotional distress in animals.
Carotid Sinus Massage
Vagally mediated second- or third-degree AV block has been induced during CSM in patients with normal AV conduction, mainly during left-sided stimulation, though sinus arrest is seen much more often than AV block. Spontaneous vagally mediated AV block has been reproduced in a few patients during CSM, but we do not yet know the sensitivity and specificity of CSM -induced AV block.
Tilt-induced Syncope
There are few reports of AV block during tilt-induced-syncope. Zysko et al reported on 31 patients, 80% of whom had a normal PR interval at baseline. In all patients, AV block was associated with sinus slowing. Mobitz I, 2:1, advanced-degree and complete AV block were seen. In 67% of the patients, at least two types of AV block were recorded. Ventricular asystole was present in 58% of patients; in about half of these, sinus arrest occurred, either before or after the AV block. AV block during a positive tilt test is a rare phenomenon; indeed, in the study by Zysko et al it was seen in only 5% of patients with positive response. The same percentage was reported by Brignole et al, who observed that isolated depression of sinus node function was four times more common. The rarity of AV block during positive tilt testing may be due to the fact that the sympathetic and parasympathetic systems exert a fairly balanced effect on the normal AV node, whereas the normal sinus node is under the predominant effect of the parasympathetic system and, is probably more sensitive to vagal output. Moreover, vagal stimulation on the sinus node can mask the effect on the AV node—that is, the sinus arrest does not allow us to recognise AV conduction disturbances because of the lack of any atrial electrical activity.
Spontaneous Neurally Mediated Syncope
There are some ECG reports of vagally mediated AV block (advanced-degree or complete) in patients with situational syncope, such as swallowing or coughing syncope or with vasovagal syncope; most of these patients had normal AV conduction. Mendoza et al reported on four patients with ECG-documented syncope during Holter recording, which showed a ventricular asystole with marked slowing of the sinus rate. As these patients had normal heart, normal AV conduction and positive tilt testing (without AV block), these syncopal episodes were diagnosed as neurally mediated. In the ISSUE-2 study, vagally mediated AV block (ventricular asystole and sinus slowing) was present in 8% of spontaneous implantable loop-recorder-documented syncopal episodes; in this case, too—that is, during spontaneous neurally mediated syncope, isolated sinus node dysfunction was much more common than AV block.
Emotional Distress in Animals
Transient second- or third-degree AV block associated with sinus slowing has been seen in rodents during emotional distress, such as during tonic immobility when the animals are suddenly attacked by a predator.
