Revisiting heart activation-conduction physiology, part I: atria.

Area of Science:

• Cardiology
• Electrophysiology
• Cardiac Physiology

Background:

• The atria function as a complex organ, evolving from a simple peristaltic tube.
• Atrial activation relies on a "two-layer" physiology utilizing the subendocardial layer for centripetal transmural activation.
• This physiological mechanism ensures robust signal transmission from the sinus node to the AV node.

Purpose of the Study:

• To re-examine atrial conduction-activation, considering its evolutionary complexity.
• To gain new insights by analyzing atrial physiology during sinus rhythm, atrial flutter, and atrial fibrillation (AF).
• To explore the relationship between normal atrial activation patterns and the pathophysiology of atrial tachycardias.

Main Methods:

• Observational analysis of atrial conduction and activation patterns.
• Review of existing literature on atrial physiology, including studies on atrial flutter and AF.
• Comparative analysis of activation during sinus rhythm, common flutter, and different stages of AF.

Main Results:

• The atria utilize a subendocardial layer for centripetal transmural activation, obviating the need for a specialized conduction system.
• Atrial flutter involves re-entry mechanisms supported by slow conduction areas, particularly in the subendocardial layer.
• Acute AF exhibits rotors consistent with normal 2D activation, suggesting reversibility by suppressing physiological transmural activation.
• Permanent AF shows abolished 2D activation, replaced by chaotic transmural patterns, indicating an irreversible shift in atrial conduction.

Conclusions:

• The "two-layer" atrial activation physiology is fundamental and robust.
• Understanding physiological activation is key to deciphering the mechanisms of atrial flutter and AF.
• Acute AF may be reversible by restoring normal physiological transmural activation, whereas permanent AF represents a new, irreversible physiology.