Junctional Rhythm

Excerpt

The heart's electrical conduction system is mediated by structures composed of specialized pacemaker cells. The primary pacemaker, the sinoatrial node, is located subepicardially in the right atrial wall near the junction with the superior vena cava. The atrioventricular node lies subendocardially within the triangle of Koch, bordered posteriorly by the coronary sinus ostium, superiorly by the tendon of Todaro, and anteriorly by the septal leaflet of the tricuspid valve. The atrioventricular node tapers into the bundle of His, which bifurcates into right and left bundle branches coursing along the interventricular septum and giving rise to Purkinje fibers in the subendocardial regions of the ventricles.

The sinoatrial node receives blood supply from the sinoatrial nodal artery, a branch of the right coronary artery in approximately 60% of individuals and the left circumflex coronary artery in the remaining 40%. The atrioventricular node is supplied by the atrioventricular nodal branch, most commonly from the right coronary artery (90%) or, less frequently, from the left circumflex artery (10%), depending on coronary dominance. Under normal conditions, electrical impulses originate in the sinoatrial node, initiating atrial contraction. The impulse then travels to the atrioventricular node, where conduction slows briefly before continuing through the bundle of His and Purkinje fibers to trigger ventricular contraction.

Each region containing specialized pacemaker cells can independently sustain the heart rate. Under physiologic conditions, secondary pacemaker sites remain suppressed by overdrive inhibition from the faster depolarization rate of the sinoatrial node, maintaining normal sinus rhythm. If the sinoatrial node slows below the intrinsic rate of another pacemaking structure, the faster region assumes control as the dominant pacemaker. Each region possesses a distinct intrinsic depolarization rate. Therefore, the resulting heart rate and rhythm depend on the predominant site. This mechanism underlies the various forms of junctional rhythm (see Image. Junctional Rhythm).

The terminology used to classify junctional rhythms is based on ventricular rate and includes the following categories:

1. Junctional bradycardia: Heart rate below 40 bpm

2. Junctional escape rhythm: Heart rate between 40 and 60 bpm

3. Accelerated junctional rhythm: Heart rate between 60 and 100 bpm

4. Junctional tachycardia: Heart rate exceeding 100 bpm

Classification of junctional rhythms by rate allows precise identification of underlying electrophysiologic disturbances. Accurate categorization is essential for guiding appropriate management and optimizing patient outcomes.