Paroxysmal AV Block Extrinsic Idiopathic AV Block

Overview

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.

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.