Paroxysmal AV block pathophysiology

Overview

Intrinsic paroxysmal AV block (I-AVB) is an AV block secondary to an innate anatomical defect. Given the presence of such a defect it's prognosis, compared to extrinsic paroxysmal vagal AV block and extrinsic paroxysmal idiopathic AV block is poor. It may have a bradycardia or tachycardia component associated with it and is characterized by atrial/ventricular premature beats prior to the period of asystole. Extrinsic vagal paroxysmal AV Block occurs secondary to an increase in vagal tone. ECG findings reflecting this include sinus rate slowing and increasing PP interval/ PR interval prior to the period of asystole. Individuals with low levels of adenosine are susceptible to sudden surges in adenosine levels which act on the AV node and cause episodes of presyncope or syncope. This would be seen on an ECG as a sudden increase in sinus rate with narrow QRS complexes just prior to the period of asystole.

Intrinsic Paroxysmal AV Block

• Intrinsic AV block (I-AVB) is an AV block secondary to an innate anatomical defect.
• It is hugely recognized on an ECG as an atrial premature beat (APB) or ventricular premature beat (VPB) before and after a variable period of complete AV block/asystole.
  • Sinus rate increase/ decrease prior to the VPB/APB or during the period of asystole further divides it into Tachycardia Dependent AV block (TD-AVB) and Pause/Bradycardia dependent AV block (PD- AVB).

• Normal cardiac myocytes are associated with a more negative resting membrane potential, an increased amplitude of action potential and a fast depolarizing sodium current.
• An exact opposite is seen in diseased myocytes responsible for TD- PAVB. An imbalance between inward depolarizing sodium and calcium currents and outward repolarizing potassium currents causes an increase in recovery time and leads to a phenomenon called ‘post-repolarization refractoriness’.
• Despite repolarization being complete, a stimulus would not be able to induce an action potential.
• A hypothetical line of thinking that could be attributed to both PD-AVB and TD-AVB is a ‘concealed conduction’ in the intra His Bundle which serves as a source of a delayed escape rhythm, thereby disrupting the refractoriness and recovery time of the surrounding myocytes. This predisposes the patient to fatal complications such as syncope, presyncope, sudden cardiac death and atrial fibrillation with a rapid ventricular response rate.
• Certain studies hypothesize that ventricular or supraventricular impulses reach this ‘concealed conduction’ at a time when there is a local phase 4 block (when sodium channels are inactive.) This subsequent long pause is reflected by the increased H-H interval in EPS studies and confirms an intra His Bundle block (an entity commonly missed and mislabeled as an infra-His Bundle block or AV block on electrophysiological studies)
• Much debate surrounds this as it has also been documented that TD- AV/ PD-AV blocks are not related to phase 3 or phase 4 conduction defects, as previously hypothesized. It is related to myocardial ischemia, Mobitz type II block, RBBB and Intra His bundle conduction defects, retrograde ventricular premature beats and anterograde atrial premature beats; all factors that are independent of local phase 4 blocks.

Extrinsic Vagal Paroxysmal AV Block

• An extrinsic vagally mediated AV block (EV-AVB) may occur due to a vagal surge or a condition causing an increase in vagal tone such as during tilt table testing, carotid sinus massage, coughing, micturition, defecation, swallowing, myocardial infarction, injection of dypramidole and cardiac transplant rejection.
• It causes SA and AV node slowing and is therefore reflected on the ECG as sinus rate slowing, increasing/irregular PP and PR intervals prior to a period of compete AV block. A heterogenous presentation in terms of Mobitz type I or II and complete heart block may also be noted. This is followed by a period of sinus acceleration.
• Electrophysiological studies indicate a normal H-H interval and therefore it can be assumed that it does not have any effect on conduction in the bundle of His and is not associated with any anatomic involvement, as seen in intrinsic AV Block.
• The pathophysiology of EV-AVB may even be related to the autonomic control of the sinus and AV nodes. A parasympathetic predominance over the SA node and sympathetic predominance over the AV node is exerted in a normal autonomic nervous system.
  • A disruption in this regulation may cause parasympathetic bursts and therefore, an AV block.
• The effect of vagal stimulation depends on the method and intensity of stimulation and the resting sympathetic activity.
• Vasalva maneuver, carotid sinus massage, water face immersion, tilt table testing may or may not induce an EV- AVB and in some cases a reversal may be seen on atropine administration. "Paroxysmal vagally mediated av block with recurrent syncope - Talwar - 1985 - Clinical Cardiology - Wiley Online Library".

Extrinsic Idiopathic Paroxysmal AV Block

• The pathogenesis of extrinsic idiopathic paroxysmal AV block (EI-AVB) can be correlated to adenosine plasma levels (APL) and increased affinity of adenosine A1 receptors.
• There is a recurrent history of unexplained syncope, absence of ECG and cardiac abnormalities and a good prognosis.
• Due to innately low APL values seen in these patients, there is an upregulation of A1 receptors, such that even during a mild transient surge in endogenous adenosine levels, AV block occurs.
• A1 receptors, which are present more in the AV node than the SA node, impose an antiadrenergic action by antagonizing β1 receptors, the sympathetic nervous system, hyperpolarizing the SA and AV nodes through potassium channels and lowering intracellular cAMP levels.
• Therefore, in such patients an injection of adenosine or adenosine triphosphate (ATP) may reproduce the attack and adenosine antagonists such as theophylline may be an efficacious treatment option.
• On an ECG, there is an absence of signs of vagal stimulation, atrial/ventricular premature beats and there may be a presence of narrow QRS complexes prior to the period of complete AV Block/ asystole
• Certain studies have also noticed genetic polymorphisms in A2A receptors in a population of people experiencing recurrent unexplained syncope. .

References

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