Structure and function of specific regions in the canine atrioventricular node

Area of Science:

• Cardiovascular Physiology
• Cardiac Electrophysiology
• Comparative Anatomy

Background:

• Detailed functional subregions of the rabbit atrioventricular (AV) node are known.
• The normal canine AV node has not been as rigorously studied.
• Understanding canine AV node function is crucial for comparative cardiac research.

Purpose of the Study:

• To anatomically and functionally characterize the isolated canine AV node.
• To investigate the electrophysiological properties of different AV node regions.
• To correlate structural findings with functional observations in the canine AV node.

Main Methods:

• Excised atria and AV junctional tissues from 20 young dogs.
• Selective arterial perfusion of sinus node, AV node, and His bundle.
• Recording of AV node action potentials using beveled microelectrodes.
• Anatomic studies including electron microscopy.

Main Results:

• Canine AV node action potentials resemble those of the rabbit AV node.
• Slow conduction observed in the AV node (56 +/- 7 mm/s), slowest in the distal region (16 +/- 14 mm/s).
• Distal AV node showed diastolic depolarization and slowest action potential upstroke velocity.
• Proximal AV node composed of transitional cells; distal AV node contained transitional cells and P-cells.
• Numerous long gap junctions found between proximal AV node cells.

Conclusions:

• The canine AV node exhibits distinct functional subregions.
• The proximal AV node appears organized for atrial input triage.
• Large gap junctions in the proximal AV node may facilitate electrotonic interaction.
• The distal AV node is the site of slowest conduction and origin for most AV junctional rhythms.