6.2 First-degree & second-degree AV blocks

What "AV Block" Means

An AV block is a delay or interruption of impulse conduction between the atria and the ventricles. Every AV block is classified by measuring the PR interval (normal 0.12-0.20 s) and by determining the relationship between P waves and QRS complexes, specifically whether every P wave is followed by a QRS. For the CRAT exam, the discipline of counting P waves and QRS complexes and measuring every PR interval across the strip is what separates a correct answer from a guess.

First-Degree AV Block

First-degree AV block is not truly a block but a consistent delay. Every sinus impulse still reaches the ventricles, so every P wave is followed by a QRS, and the atrial and ventricular rates are equal. The defining criterion is a PR interval greater than 0.20 s (one large box) that stays constant beat to beat. The rhythm is otherwise regular with normal upright P waves and a narrow QRS.

Criteria checklist:

• PR interval greater than 0.20 s
• PR interval constant (does not change)
• Every P wave conducts (1:1 P-to-QRS ratio)
• Regular rhythm

First-degree block is usually benign and asymptomatic. It may result from increased vagal tone, AV-nodal medications (beta blockers, calcium channel blockers, digoxin), or ischemia.

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

In second-degree AV block, some P waves conduct and some do not, so there are more P waves than QRS complexes. Type I, also called Mobitz I or Wenckebach, has a distinctive, progressive pattern.

The Wenckebach Footprint

The PR interval lengthens progressively with each successive beat until finally a P wave is not conducted and a QRS is dropped. After the dropped beat the cycle resets: the PR shortens again and the pattern repeats. Because a beat is periodically dropped, the QRS complexes cluster into grouped beating, clusters of complexes separated by a pause. Additional clues:

• The R-R interval progressively shortens before the dropped beat, a classic finding because the increment of PR lengthening gets smaller with each beat.
• The pause containing the dropped QRS is less than twice the shortest R-R interval.
• The QRS is usually narrow, because the block is at the AV node.

The block site is typically within the AV node itself, and Wenckebach is often transient and benign, caused by high vagal tone, inferior wall MI, or AV-nodal drugs.

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

Type II, or Mobitz II, is the dangerous cousin. Here the PR interval of conducted beats stays constant, and then, without any warning lengthening, a P wave is suddenly not conducted and a QRS is dropped. There is no progressive PR prolongation; the conducted PR intervals are identical before and after the dropped beat.

Key features:

• Constant PR on all conducted beats
• Sudden, intermittent dropped QRS with no PR warning
• QRS is often wide, because the block is below the AV node in the His-Purkinje system
• May occur in fixed ratios (2:1, 3:1), so count the P-to-QRS ratio

Because the block sits low in the conduction system, Type II is more likely to progress to complete heart block and often requires a pacemaker. It is associated with anterior wall MI and structural conduction-system disease.

A Word on Ratios and Measurement

Second-degree blocks are often described by their conduction ratio, the number of P waves to conducted QRS complexes. A strip with four P waves for every three QRS complexes is a 4:3 block; two P waves for every QRS is a 2:1 block. Use calipers to walk out the P-P interval first and confirm it is regular, then mark each QRS; the extra, non-conducted P waves will stand out, sometimes distorting a T wave where a hidden P wave falls on it. Counting this ratio deliberately keeps you from mistaking a dropped beat for ordinary sinus irregularity, and it forces you to prove that the atrial rhythm itself is regular before you blame the AV node.

Telling Type I from Type II

This is the single most commonly confused distinction on the exam. Focus on the PR intervals of the conducted beats leading into the dropped beat:

The decisive question: Does the PR interval change before the dropped beat? If PR marches out longer and longer, it is Type I. If PR is rock-steady and a beat simply vanishes, it is Type II. A special case, 2:1 AV block, is ambiguous because you never see two conducted beats in a row to judge PR progression; classify it by QRS width and accompanying strips, where a narrow QRS suggests Type I and a wide QRS suggests Type II.

Exam tip: Always count atrial (P) and ventricular (QRS) rates separately in any second-degree block. More P waves than QRS complexes confirms a second-degree block; the PR behavior tells you which type.