6.1 Junctional escape, accelerated junctional & junctional tachycardia

The AV Junction as a Backup Pacemaker

The atrioventricular (AV) junction, the region encompassing the AV node and the proximal bundle of His, contains pacemaker cells that can assume control of the heart when the sinoatrial (SA) node fails or slows. Under normal circumstances the SA node's faster intrinsic rate (60-100 bpm) overdrive-suppresses the junction, so junctional cells never get the chance to fire. When the sinus rate drops below the junction's inherent rate, or when a sinus impulse is blocked before reaching the ventricles, the junction escapes and takes over. The inherent firing rate of the AV junction is 40-60 bpm, a number the CRAT exam expects you to recall instantly and to distinguish from the sinus node (60-100) and the ventricular Purkinje system (20-40).

The Signature P Wave

Because a junctional impulse originates below the atria, atrial depolarization travels backward (retrograde) toward the atria at the same time the impulse travels forward to the ventricles. This retrograde conduction produces the hallmark junctional P wave, which in leads II, III, and aVF is inverted (negative) because the atria depolarize in the opposite-to-normal direction. Where that inverted P wave lands depends on the relative timing of atrial and ventricular activation:

• Before the QRS with a short PR interval (less than 0.12 s): the atria depolarize slightly ahead of the ventricles.
• Hidden within the QRS (no visible P wave): atria and ventricles depolarize simultaneously, and the larger QRS buries the P wave.
• After the QRS: the ventricles depolarize first, so the retrograde P wave appears just after the QRS complex.

The QRS in pure junctional rhythms is narrow (0.10-0.12 s or less) because, once the impulse reaches the His-Purkinje system, the ventricles are activated by the normal pathway. A narrow QRS is your clue that the focus sits at or above the bundle branches.

Naming the Junctional Rhythms by Rate

All junctional rhythms share the same P-wave morphology rules; they differ only by rate. Memorize this ladder:

Junctional Escape Rhythm (40-60 bpm)

This is a protective rhythm. When the SA node stops, sinus arrest occurs, or a high-grade block prevents sinus impulses from reaching the ventricles, the junction rescues the heart at 40-60 bpm. The rhythm is regular, the QRS is narrow, and P waves are inverted, absent, or retrograde. Do not treat the escape beat itself; it is the safety net, and suppressing it with medication can be dangerous.

Accelerated Junctional Rhythm (60-100 bpm)

Here the junctional focus fires faster than its inherent rate but has not yet crossed into tachycardia. It is termed accelerated precisely because 60-100 exceeds the normal 40-60 junctional rate yet falls short of 100. Common causes tested on the exam include digoxin toxicity, myocardial ischemia (especially inferior wall MI), and electrolyte disturbance. The rhythm is regular with the characteristic junctional P waves.

Junctional Tachycardia (over 100 bpm)

When an irritable junctional focus fires above 100 bpm, the rhythm is junctional tachycardia, with rates typically running 100-140 bpm. Because the rate is fast and P waves are frequently buried, junctional tachycardia can resemble other supraventricular tachycardias; the narrow QRS with absent or inverted retrograde P waves points to the junction. Digoxin toxicity is again a classic cause.

Distinguishing Junctional Rhythms From Look-Alikes

Two rhythms are frequently mistaken for a junctional focus, and the exam rewards you for separating them. Sinus bradycardia also produces a slow, regular, narrow-complex rhythm at 40-60 bpm, but its P waves are upright in lead II with a normal PR interval, because the impulse still starts in the SA node. If the P wave is upright and precedes the QRS by a normal interval, the rhythm is sinus, not junctional. A wandering atrial pacemaker shows at least three different P-wave shapes as the pacing site drifts among the atria and junction, with a varying PR interval, whereas a pure junctional rhythm keeps one consistent inverted or absent P wave. When a retrograde P wave follows the QRS, the short gap between the QRS and that P wave is the RP interval; recognizing it confirms retrograde atrial activation and locks in the junctional diagnosis.

Premature Junctional Contractions (PJCs)

A premature junctional contraction is a single early beat arising from the junction that interrupts the underlying rhythm. Its features mirror junctional rhythm on a single-beat scale: a narrow QRS appearing earlier than expected, preceded or followed by (or lacking) an inverted P wave. A PJC is usually followed by a non-compensatory pause because the premature impulse conducts retrograde into the SA node and resets its timing. PJCs are generally benign but may signal digoxin excess, caffeine, or ischemia when frequent. On a strip, count them as ectopic beats and describe the underlying rhythm separately (for example, "sinus rhythm with frequent PJCs").

Exam tip: The single most reliable discriminator among the junctional rhythms is rate, and the single most reliable clue that a rhythm is junctional at all is the inverted or absent P wave paired with a narrow QRS. Keep those two anchors in mind and you can classify any junctional strip quickly.