Junctional Rhythm

Excerpt

The heart's electrical conduction system operates through structures composed of specialized pacemaker cells. The primary pacemaker is the sinoatrial node, located subepicardially in the right atrium wall near the junction with the superior vena cava. The atrioventricular node lies subendocardially within the triangle of Koch, which is 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 that course along the interventricular septum and give 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 of 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%) and less frequently from the left circumflex artery (10%), depending on whether the coronary circulation is right- or left-dominant. 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 due to overdrive inhibition by the faster depolarization rate of the sinoatrial node, maintaining normal sinus rhythm. If the sinoatrial node slows to a rate below that of another pacemaking structure, the faster region assumes control as the dominant pacemaker. Since each region has a distinct intrinsic depolarization rate, the resulting heart rate and rhythm depend on which site becomes predominant. This mechanism underlies the various forms of junctional rhythm (see Image. Junctional Rhythm).

The terminology used to classify junctional rhythms is based on the 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

Classifying junctional rhythms by rate enables precise identification of underlying electrophysiologic disturbances. Such distinctions are critical for optimizing patient care.