4.5 Atrioventricular (AV) Blocks (1st Degree, 2nd Degree Type I/II, 3rd Degree Complete)

Atrioventricular (AV) Blocks

Atrioventricular (AV) blocks occur when the electrical impulse from the atria is delayed or completely blocked at the AV node or Bundle of His before reaching the ventricles. Recognizing these blocks is a critical skill for the Certified Cardiographic Technician, as the severity of the block dictates patient management. Some AV blocks are benign and require no treatment, while others represent life-threatening emergencies requiring immediate pacing. The key to diagnosing AV blocks lies in analyzing the relationship between the P waves (atrial activity) and the QRS complexes (ventricular activity).

Comprehensive Comparison Table: AV Blocks

First-Degree AV Block

In a first-degree AV block, all impulses from the SA node successfully conduct to the ventricles, but they are delayed at the AV node longer than normal.

• Rhythm: Regular.
• Rate: Usually depends on the underlying rhythm (e.g., normal sinus rhythm).
• P Waves: Upright, uniform, one before every QRS.
• PR Interval: Consistently prolonged > 0.20 seconds (more than 5 small boxes). This is the hallmark criteria. The delay is identical for every single beat.
• QRS Complex: Usually narrow (< 0.12 seconds), unless a bundle branch block is also present.

First-degree AV block is generally benign and may be seen in healthy athletes due to high vagal tone, or in patients taking AV node-blocking medications like beta-blockers or calcium channel blockers. No specific treatment is required other than monitoring.

Second-Degree AV Block, Type I (Wenckebach / Mobitz I)

Second-degree blocks involve an intermittent failure of AV conduction. In Type I, the delay at the AV node progressively increases with each beat until an impulse is completely blocked.

• Rhythm: Irregular. The R-R intervals progressively shorten until a beat is dropped, creating a grouped beating pattern. The characteristic finding is "longer, longer, longer, drop."
• Rate: Atrial rate is regular; ventricular rate is slower than the atrial rate.
• P Waves: Normal in size and shape. There are more P waves than QRS complexes.
• PR Interval: Progressively lengthens with each consecutive beat until a P wave appears without a following QRS complex.
• QRS Complex: Usually narrow (< 0.12 seconds).

Wenckebach is often transient and associated with acute inferior myocardial infarction (due to right coronary artery ischemia affecting the AV node) or medication toxicity. It rarely progresses to complete heart block and is typically only treated if the patient becomes symptomatic from the resulting bradycardia.

Second-Degree AV Block, Type II (Mobitz II)

Type II is a more serious pathology, usually indicating structural damage below the AV node (in the Bundle of His or bundle branches). Conduction fails suddenly without warning.

• Rhythm: Atrial rhythm is regular; ventricular rhythm may be regular (if block is fixed, like 2:1 or 3:1) or irregular.
• Rate: Atrial rate is normal; ventricular rate is slower and often bradycardic.
• P Waves: Normal. More P waves than QRS complexes.
• PR Interval: Constant for the conducted beats. There is no progressive prolongation before the dropped QRS. This is the critical distinction from Type I.
• QRS Complex: Often wide (≥ 0.12 seconds) because the block is typically located lower in the conduction system, frequently associated with a bundle branch block.

Mobitz II is a dangerous rhythm because it can rapidly progress to complete heart block. Temporary or permanent pacing is usually indicated, even if the patient is relatively asymptomatic, due to the unpredictability of the block.

Third-Degree (Complete) AV Block

In third-degree AV block, there is a total failure of conduction between the atria and ventricles. The atria and ventricles beat independently of each other, controlled by different pacemakers. This is called AV dissociation. This creates a highly precarious hemodynamic state.

• Rhythm: Both the atrial rhythm (P-P interval) and the ventricular rhythm (R-R interval) are regular, but they are completely independent of one another. You will see P waves marching through the QRS complexes.
• Rate: Atrial rate is driven by the SA node (usually 60-100 bpm). The ventricular rate is driven by an escape pacemaker. If the escape focus is junctional, the rate is 40-60 bpm with a narrow QRS. If the escape focus is ventricular, the rate is 20-40 bpm with a wide QRS.
• P Waves: Normal in appearance, but they have no relationship to the QRS complexes. They may be hidden within QRS complexes or T waves.
• PR Interval: Totally variable. There is no true PR interval because the P waves do not conduct to the ventricles.
• QRS Complex: Narrow if the block is high (AV node) and escape is junctional; wide if the block is low (bundle branches) and escape is ventricular.

Complete heart block severely compromises cardiac output and is a medical emergency requiring immediate pacemaker intervention. Drugs like Atropine are frequently ineffective (especially if the block is low) and may even worsen the block, making transcutaneous or transvenous pacing the primary therapy.