Junctional & Ventricular Rhythms
Junctional Rhythms
Overview
Junctional rhythms involve pacing that originates from the AV junction, which is a critical segment of the heart's electrical system.
Anatomy of Junction
AV Node:
A cluster of specialized cells located in the lower part of the right atrium, just above the base of the tricuspid valve.
Function: The primary role of the AV node is to delay the electrical impulse. This delay facilitates the contraction of the atria, allowing for complete ventricular filling with blood prior to the next contraction.
Bundle of His:
Located in the upper region of the interventricular septum, it serves as a connection between the AV node and the two bundle branches.
Contains pacemaker cells that fire at a rate of 40-60 beats per minute (bpm).
AV Junction
Refers to the AV node and the nonbranching portion of the Bundle of His.
Although the SA node serves as the heart's primary pacemaker, the AV junction can take over heart pacing under certain conditions:
SA node fails to discharge: Example - sinus arrest.
Impulse from SA node blocked: Example - SA block.
SA node discharges slowly: Example - sinus bradycardia or slowing from sinus arrhythmia.
Impulse generated by SA node is not conducted: Example - AV block.
Electrical Impulse and Atrial Activation
When the AV junction paces the heart:
The electrical impulse travels retrograde (backward) to activate the atria.
If a P wave is present, it will be inverted in Leads II, III, and aVF as the impulse travels away from the positive electrode.
Presence of Inverted P Wave:
Occurs before the QRS complex if atria depolarize prior to the ventricles. Sometimes the PR interval is less than 0.12 seconds.
May not be visible if both atria and ventricles depolarize simultaneously, as it becomes obscured in the QRS complex.
Appears after the QRS if the atria depolarize following ventricular depolarization.
Premature Junctional Complex (PJC)
A PJC occurs when an irritable area within the AV junction fires prior to the next expected SA node impulse, thereby interrupting the sinus rhythm.
The electrical impulse travels through the ventricles as usual, with the QRS complex typically measuring 0.10 seconds or less.
An incomplete pause generally follows a PJC, indicating the time taken for the SA node to reset for its next impulse.
Timing of Junctional Complexes:
Early: Occurs before the next expected sinus beat (termed PJC).
Late: Follows the next expected sinus beat (termed Junctional Escape Beat).
To classify whether a complex is early or late, two sinus beats in a row must be observed to establish the regularity of the underlying rhythm.
Characteristics of PJC:
Rate: Usually remains within the normal range, depending on the underlying rhythm.
Rhythm: Regular, but may include premature beats.
P Waves: May occur before, during, or after the QRS complex; if visible, they are usually inverted in leads I, II, and aVF.
PR Interval: If a P wave appears prior to the QRS, the PR interval is typically 0.12 seconds or less; if absent, there is no PR interval.
QRS Duration: Usually 0.10 seconds or less, unless an abnormal conduction or an intraventricular conduction delay exists.
Junctional Escape Rhythm
Initiated in the AV junction, a junctional escape beat appears late after the expected sinus beat.
Common during sinus arrest episodes or following premature atrial contractions (PACs).
Characteristics of Junctional Escape Beats:
Rate: Generally normal, reliant on the underlying rhythm.
Rhythm: Regular, with late beats that may create an irregular appearance.
P Wave: May be before, during, or after the QRS; when visible, it is inverted in II, III, and aVF.
PR Interval: If a P wave occurs before the QRS, it is typically 0.12 seconds or less; otherwise, no PR interval.
QRS Duration: Usually 0.10 seconds or less unless a conduction delay exists.
A junctional rhythm consists of a series of junctional escape beats.
Normal Pacing: The intrinsic rate of the AV junction is 40-60 bpm.
A junctional rhythm typically produces a narrow QRS complex and a very regular rhythm.
Junctional Bradycardia: If pacing by the AV junction is at a rate slower than 40 bpm, the rhythm is termed junctional bradycardia. It should be noted that the AV junction’s normal pacing (40-60 bpm) is already regarded as bradycardic.
Accelerated Junctional Rhythm
An accelerated junctional rhythm is similar to the junctional escape rhythm but occurs at a rate of 60-100 bpm due to enhanced automaticity of the bundle of His.
It is generally not associated with signs of decreased cardiac output.
Junctional Tachycardia Rhythm
Junctional tachycardia rhythm represents a similar conduction pattern to both escape and accelerated rhythms, occurring at a heart rate between 100-150 bpm.
The source of this rhythm originates from the bundle of His.
With a significantly increased rate, patients are more likely to report symptoms such as palpitations or fluttering.
Supraventricular Tachycardia (SVT)
Defined as a tachycardic state arising from any area above the ventricles.
Not strictly a junctional dysrhythmia, but relevant due to the impulses emerging from areas akin to those controlled by the junction.
Heart rate exceeding 150 bpm is common, and due to the rapid pacing, P waves are often absent from the ECG.
Summary of Junctional Rhythms
Rhythm | Heart Rate (bpm) | P Waves | PR Interval | QRS Duration |
|---|---|---|---|---|
Junctional Rhythm | 40-60 | May be inverted | 0.12 sec or less | 0.10 sec or less |
Accelerated Junctional Rhythm | 60-100 | Often inverted | 0.12 sec or less | 0.10 sec or less |
Junctional Tachycardia | 100-150 | Not visible | None | 0.12 sec or greater |
Defibrillation
Refers to the application of an electrical current across the heart to disrupt an abnormal rhythm.
The shock aims to synchronize myocardial cells to enable the heart's natural pacemakers to regain control.
This method does not adhere to any particular phase of the cardiac cycle but aims to start the heart's recoverable rhythm when the myocardial cells repolarize.