Junctional Rhythms and AV Blocks

Junctional Rhythms and AV Blocks

Where the Rhythm Comes From

The heart's electrical system has a chain of command. The sinoatrial (SA) node fires first, depolarization sweeps through the atria, and the impulse arrives at the atrioventricular (AV) node. The AV node slows conduction briefly, giving the atria time to squeeze blood into the ventricles before the ventricles contract. When part of this chain fails or slows, the heart uses backup pacemakers, and the result appears on the EKG as a junctional rhythm or an AV block.

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Junctional Rhythms

The AV junction (the AV node and the bundle of His) can take over pacing when the SA node fails to lead. Because the impulse now originates in the junction rather than the SA node, atrial activation is abnormal. The P wave may appear:

• Before the QRS (but with a short PR interval, less than 0.12 s)
• Hidden inside the QRS
• After the QRS (retrograde, often inverted in leads II, III, and aVF)

The QRS complex itself is usually narrow (less than 0.12 s) because ventricular conduction through the bundle branches is normal.

Junctional escape rhythm fires at the junction's inherent rate of 40–60 beats per minute. It is a safety net: it keeps the patient alive when the SA node fails. The rate is slow, and the patient may feel lightheaded or hypotensive.

Accelerated junctional rhythm fires at 60–100 beats per minute. The junction is faster than normal for that pacemaker site, often caused by digitalis toxicity, inferior myocardial infarction, or increased vagal tone. Despite the name, the rate falls within the normal heart-rate range, which can make recognition tricky.

Junctional tachycardia exceeds 100 beats per minute and is covered in a separate lesson on supraventricular tachycardias.

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AV Blocks: The Core Concept

AV blocks describe a delay or failure of impulse conduction through the AV node or the bundle of His. They are graded by severity. The PR interval—measured from the start of the P wave to the start of the QRS complex—is your primary measurement tool. Normal PR interval is 0.12–0.20 seconds (three to five small boxes on standard paper at 25 mm/s).

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First-Degree AV Block

Every P wave conducts to the ventricles, but conduction is slower than normal. The PR interval is greater than 0.20 seconds and is constant beat to beat. The P:QRS ratio is 1:1. The QRS is typically narrow. This is not a true block—no beat is dropped—but it signals delayed conduction. First-degree AV block is often benign but may indicate medication effect (such as beta-blockers or calcium-channel blockers), electrolyte disturbance, or early AV node disease. Notify the supervising clinician per your facility protocol; it is not a rhythm emergency on its own.

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Second-Degree AV Block — Mobitz Type I (Wenckebach)

The PR interval progressively lengthens with each successive beat until one P wave fails to conduct and the QRS is dropped. After the dropped beat, the cycle resets and repeats. This pattern is called the Wenckebach phenomenon. Key features:

• PR interval: progressively lengthening
• Pattern: grouped beating (clusters of beats separated by a pause)
• P:QRS ratio: more P waves than QRS complexes (for example, 4:3 or 3:2)
• QRS: usually narrow

Mobitz I is usually at the level of the AV node and is often transient, seen with inferior MI or medication effect. It is less dangerous than Mobitz II, but report it promptly.

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Second-Degree AV Block — Mobitz Type II

The PR interval is constant from beat to beat, then suddenly a P wave fails to conduct with no warning lengthening. The dropped beat is unexpected. Key features:

• PR interval: constant (may be normal or prolonged, but does not change)
• Pattern: sudden dropped QRS without prior PR lengthening
• P:QRS ratio: more P waves than QRS complexes
• QRS: often wide, because the block is usually below the AV node in the bundle of His or bundle branches

Mobitz II is clinically serious. It can deteriorate rapidly into complete heart block and may require pacing. Report it immediately and be prepared to support the patient.

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Third-Degree (Complete) AV Block

No atrial impulses conduct to the ventricles. The atria and ventricles beat independently—atrial depolarization driven by the SA node and ventricular depolarization driven by an escape pacemaker in the junction or ventricles. On the EKG:

• P waves are present and regular (marching out at their own rate)
• QRS complexes are present and regular (but at a different, slower rate)
• No fixed relationship between P waves and QRS complexes (PR interval varies continuously)
• QRS may be narrow (junctional escape, 40–60 bpm) or wide (ventricular escape, 20–40 bpm)

Complete heart block is a medical emergency. Ventricular rates this slow cause severely reduced cardiac output. The patient may be unconscious, hypotensive, or in cardiac arrest. As an EKG technician, your scope-of-practice response is: do not leave the patient alone, immediately notify the nurse or supervising clinician, call for help using your facility's protocol, document what you observed and when, and be ready to assist as directed. You do not interpret or treat—you communicate findings accurately and urgently.

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Using Ladder Diagrams

A ladder diagram is a visual tool with rows representing the atria (A), AV node (AV), and ventricles (V). Lines drawn through those rows show where an impulse travels, where it slows, and where it blocks. Practicing with ladder diagrams helps you trace why each block looks the way it does on a strip, reinforcing the logic behind the pattern rather than relying on memorization alone.

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Safety Reminder for EKG Technicians

Identifying an abnormal rhythm accurately and reporting it without delay is your most important safety function. You do not diagnose or order treatment. When you see a rhythm that concerns you—especially complete heart block, Mobitz II, or any rhythm causing the patient to be unresponsive or symptomatic—report immediately, stay with the patient, and follow your facility protocol.