Atrioventricular NodeEdit
The atrioventricular node (AV node) is a compact, central component of the heart’s electrical wiring. It sits at the crossroads between the atrial and ventricular chambers, acting as a gatekeeper that coordinates the timing of the heart’s two lower chambers. By moderating how quickly impulses pass from the atria to the ventricles, the AV node ensures that the atria have a moment to contract and fill the ventricles before they themselves contract. This delay is vital for efficient pumping and overall cardiovascular performance. The AV node is one piece in the broader Conduction system of the heart that includes the SA node (the natural pacemaker of the heart) and the His-Purkinje system that carries impulses to the ventricles.
In humans, the AV node is typically located in the lower part of the right atrium, near the septum that divides the atria. Its precise position is classically described within an anatomical region known as Koch’s triangle, a small area bounded by landmarks such as the tricuspid valve annulus, the tendon of Todaro, and the ostium of the coronary sinus. The node’s location and structure can vary somewhat among individuals, but its role in coordinating atrioventricular conduction is a constant feature of mammalian cardiac physiology. The AV node receives its blood supply primarily from branches of the right coronary artery in most people, which helps explain why certain heart conditions that affect coronary circulation can influence AV nodal function. For readers exploring anatomy, related terms include Koch's triangle and AV nodal artery.
Structure and location
- Anatomy in the right atrium: The AV node is a compact cluster of nodal tissue that sits at or near the boundary between atrial tissue and the specialized conduction pathways leading toward the ventricles. It forms part of the posterior interatrial septum and is intimately connected with the surrounding conduction tissue.
- Boundaries and landmarks: Its neighborhood is defined by nearby structures including the tricuspid valve annulus and the coronary sinus ostium, with the tendon of Todaro serving as a guide in textbook descriptions of its location.
- Blood supply: The nodal artery (often a branch of the right coronary artery) penetrates the node to sustain its cellular activity and responsiveness to autonomic input.
- Variability: While the general position is consistent, minor anatomic variations exist, and imaging or electrophysiology studies may refine the exact course of conduction through an individual’s AV node.
Physiology and conduction
- Delayed conduction: A core function of the AV node is to delay the transmission of impulses from the atria to the ventricles. This delay, typically on the order of tens to a hundred milliseconds, allows the atria to complete contraction and optimally fill the ventricles with blood before the ventricles contract.
- Route of passage: Impulses originate in the SA node and travel through atrial myocardium to reach the AV node, where the electrical signal is momentarily slowed before advancing to the His bundle and onward to the Purkinje fibers and ventricles.
- Refractory properties: The AV node has distinctive refractory characteristics that help protect the ventricles from excessively rapid atrial rates and support orderly rhythm generation. These properties influence how the heart responds to arrhythmias and pharmacologic interventions.
- Autonomic modulation: Sympathetic stimulation tends to accelerate AV nodal conduction and reduce refractoriness, while parasympathetic (vagal) activity slows conduction. These autonomic influences help the heart adapt to changing demands during stress, rest, or disease.
Clinical significance
- Conduction blocks: Abnormal AV nodal conduction can produce varying degrees of atrioventricular block.
- First-degree AV block: Prolongation of the PR interval without dropped beats. This is often benign but may signal underlying conduction system disease or be influenced by medications.
- Second-degree AV block: Intermittent failure of conduction, with Mobitz type I (Wenckebach) typically reflecting nodal or proximal conduction disease, and Mobitz type II more often indicating block below the AV node. The distinction matters for prognosis and treatment.
- Third-degree (complete) heart block: No atrial impulses reach the ventricles, requiring prompt evaluation and often pacemaker therapy.
- AV nodal reentrant tachycardia (AVNRT): A common form of paroxysmal supraventricular tachycardia arising from a micro-reentrant circuit within or near the AV node. Management ranges from acute pharmacologic strategies (e.g., adenosine for rapid termination) to longer-term strategies such as catheter ablation that targets the slow pathway involved in the reentry circuit. See Atrioventricular nodal reentrant tachycardia for details.
- Therapeutic approaches: In patients with symptomatic conduction disease or AVNRT, therapies include medical management with rate-controlling drugs or antiarrhythmics, pacemaker implantation in cases of advanced block, and catheter-based ablation to modify the nodal tissue pathways responsible for arrhythmias. See Cardiac pacemaker and Catheter ablation for related concepts.
- Distinction from accessory pathways: Some arrhythmias involve extra conduction pathways outside the AV node (e.g., accessory AV pathways) that can create pre-excitation or reentrant circuits. Management and prognosis differ depending on whether the issue is nodal (AV node-centered) or accessory-pathway mediated.
History and perspective
- Discovery and naming: The conduction system of the heart was elucidated over the late 19th and early 20th centuries by researchers such as Wilhelm His, who described the His bundle, and Sunao Tawara, who clarified the atrioventricular conduction axis and its components in the awakening era of cardiac electrophysiology. These discoveries shaped modern understanding of how the AV node fits into the broader pacing and conduction framework. The AV node is commonly discussed alongside the His-Purkinje system and surrounding nodal tissue in medical literature and education.
- Evolving treatment paradigms: Advances in catheter-based mapping and ablation techniques have transformed the management of AVNRT and other nodal conditions. The balance between medication, device therapy, and ablation reflects ongoing debates in medicine about best practices, patient autonomy, cost-effectiveness, and access to specialized care. Proponents of targeted ablation emphasize durable rhythm control and reduced medication burden, while critics highlight the need to preserve conduction reserve and consider patient preference.