Maintained prolongation of AV conduction time by acetylcholine.

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Pharmacology

Background:

• Cholinergic activation can cause heart rate responses to fade during sustained vagal stimulation.
• Previous research on atrioventricular (AV) conduction during cholinergic activation focused mainly on low-grade AV block.

Purpose of the Study:

• To investigate the dromotropic (AV conduction) responses to varying degrees of sustained cholinergic activation.
• To compare the characteristics of AV conduction changes with known chronotropic (heart rate) responses to acetylcholine.

Main Methods:

• Anesthetized mongrel dogs were used, with direct perfusion of the AV node artery using acetylcholine solutions.
• Electrograms were recorded from multiple cardiac sites (SA node, atrium, ventricle, His bundle).
• Different concentrations of acetylcholine were infused to induce varying levels of cholinergic activation.

Main Results:

• High acetylcholine concentrations consistently produced sustained complete heart block.
• Moderate concentrations led to maintained second-degree AV block or oscillations between first and second-degree block.
• Lower concentrations caused sustained AH interval prolongation, dependent on continuous perfusion.

Conclusions:

• Fading is not a characteristic of AV conduction during maintained cholinergic activation, unlike chronotropic responses.
• Acetylcholine exhibits differential effects on cardiac chronotropy and dromotropy.
• Sustained cholinergic stimulation does not lead to fading of AV conduction abnormalities.