Atrioventricular junctional tachycardia during heart block

Area of Science:

• Cardiology
• Electrophysiology
• Pharmacology

Background:

• Cholinergic activity profoundly impacts cardiac conduction, particularly within the atrioventricular (AV) junction.
• Understanding the interplay between cholinergic and adrenergic systems is crucial for interpreting cardiac arrhythmias and conduction abnormalities.

Purpose of the Study:

• To investigate the response of the AV junction to adrenergic stimulation during pharmacologically induced cholinergic depression.
• To determine the precise location and extent of acetylcholine's effect on AV conduction.

Main Methods:

• Six dogs were studied under conditions of second-degree heart block induced by acetylcholinesterase paralysis.
• Brief, intense local adrenergic stimulation was applied to the AV junction.
• Electrophysiological parameters, including transatrial and His bundle-ventricular conduction times, and atrial activation sequences were recorded.

Main Results:

• Despite profound AV conduction depression by cholinergic activity, adrenergic stimulation readily induced AV junctional tachycardia.
• Acetylcholine did not prolong transatrial or His bundle-ventricular conduction times.
• Retrograde atrial activation sequences and electrogram configurations remained unaltered during AV junctional rhythm with atrial capture.
• In some cases, retrograde atrial activation preceded the His bundle spike, indicating differential effects of acetylcholine on anterograde versus retrograde conduction.

Conclusions:

• Acetylcholine-induced depression of AV conduction is primarily confined to the AV node region.
• Retrograde atrial activation can precede His bundle activation, challenging assumptions based on P wave morphology or A-H intervals for pacemaker origin.
• The findings highlight the complexity of AV junctional control and the potential for misinterpretation of pacemaker origin under specific conditions.