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AV Heart Blocks - ECG Guide

Source: Braunwald's Heart Disease - A Textbook of Cardiovascular Medicine

What Is AV Block?

AV block is a disturbance of impulse conduction where the atrial impulse is conducted with delay or not at all to the ventricle when the AV junction is not physiologically refractory. The block site can be at the AV node, His bundle, or bundle branches. It must be distinguished from interference (normal refractoriness after a preceding impulse).

Classification by Degree

1st Degree AV Block

Definition: Every P wave conducts to the ventricle, but PR interval is prolonged > 200 ms (0.20 s).

ECG features:

PR interval > 200 ms - can extend as long as 1000 ms in severe cases
Regular rhythm - every P wave is followed by a QRS
No dropped beats

Site of block:

If QRS is narrow - delay is almost always in the AV node (prolonged A-H interval)
If QRS shows BBB pattern - delay may be in AV node OR His-Purkinje system; requires His bundle electrogram to differentiate

Clinical note: Usually benign. Vagal tone (e.g., carotid massage) or rate acceleration can cause 1st degree to progress to 2nd degree Wenckebach.

First-degree AV block with intracardiac recordings showing AV nodal delay (A-H = 310ms, left) vs His-Purkinje delay (H-V = 95ms, right)

Fig 68.6 from Braunwald's: Left panel - RBBB with AV nodal delay (A-H 310 ms). Right panel - LBBB with His-Purkinje delay (H-V 95 ms)

2nd Degree AV Block

Some (not all) P waves fail to conduct to the ventricle. Two subtypes:

Mobitz Type I (Wenckebach) - 2nd Degree

ECG features:

PR interval progressively lengthens from beat to beat
The RR interval shortens progressively (because each PR increment is smaller than the last)
A P wave is eventually dropped (not followed by QRS)
The pause after the dropped beat is less than twice the shortest PP interval
After the dropped beat, the cycle resets and PR shortens again

Ladder diagram - 4:3 Wenckebach cycle:

Wenckebach ladder diagram showing PR progression from 200ms to 350ms with dropped beat, RR intervals 1100ms then 1050ms then pause 1850ms

P-P = 1000 ms, PR goes 200 → 300 → 350 ms → blocked. Note: RR shortens (1100 → 1050) then long pause (1850 ms)

Memory aid for Wenckebach (PRWP mnemonic):

PR gets longer, longer, longer...
RR gets shorter, shorter, shorter...
Will drop a beat
Pause, then repeats

Site: Usually the AV node. Benign in most settings (especially inferior MI or athletes).

Mobitz Type II - 2nd Degree

ECG features:

PR interval is constant (no progressive lengthening)
A P wave is suddenly not conducted without warning
The pause containing the dropped beat is exactly twice the PP interval
QRS is often wide (BBB pattern), reflecting infranodal disease

Key distinction from Type I:

Feature	Mobitz I (Wenckebach)	Mobitz II
PR before dropped beat	Progressively longer	Constant
QRS width	Usually narrow	Usually wide
Site of block	AV node	His bundle / bundle branches
Clinical risk	Lower	Higher - prone to complete block
Response to atropine	Usually responds	Unpredictable / poor
Pacing needed?	Rarely (unless symptomatic)	Yes - permanent pacing often needed

Site: Infranodal - His bundle or bundle branches. Clinically more serious because it can progress unpredictably to complete (3rd degree) block.

High-Grade (Advanced) AV Block

When 2 or more consecutive P waves are blocked (e.g., 3:1 or 4:1 block). This is an intermediate category between 2nd and 3rd degree. The conducted beats may have a constant PR, and the ventricular rate can be very slow.

3rd Degree (Complete) AV Block

Definition: No atrial impulses conduct to the ventricle. Atria and ventricles are completely dissociated.

ECG features (from Tintinalli's Emergency Medicine):

Feature	Finding
P wave - QRS association	None - completely dissociated
Atrial rate vs. ventricular rate	Atrial rate greater than ventricular rate
Ventricular rhythm	Regular (escape rhythm is regular)
QRS morphology	Usually widened (junctional escape = narrow; ventricular escape = wide)

Escape rhythms:

AV nodal / junctional escape: Rate 40-60 bpm, narrow QRS - block is at or above His bundle
Ventricular escape: Rate 20-40 bpm, wide QRS - block is infranodal (bundle branch level); less reliable, hemodynamically worse

AV dissociation vs Complete Heart Block:

Complete heart block is one cause of AV dissociation, but they are not synonymous
In AV dissociation from enhanced automaticity, the ventricular rate may be faster than the atrial rate (isorhythmic dissociation); in complete heart block, the atrial rate is always faster

Common Causes Summary

Block Type	Common Causes
1st degree	Vagal tone, athletes, inferior MI, digoxin, beta-blockers, calcium channel blockers, myocarditis
2nd degree Mobitz I	Inferior MI (RCA lesion), increased vagal tone, drug effects (digoxin, opioids), myocarditis
2nd degree Mobitz II	Anterior MI, degenerative conduction system disease (Lev's/Lenegre's disease), cardiomyopathy, surgical trauma
3rd degree (complete)	Inferior MI (often temporary), anterior MI (often permanent), Lyme carditis, congenital, degenerative, surgical

Management Overview

Block	Management
1st degree	Usually no treatment; monitor; address reversible causes
2nd degree Mobitz I	Often no treatment if asymptomatic; atropine if symptomatic; pacing rarely needed
2nd degree Mobitz II	Permanent pacemaker in most cases; transcutaneous pacing if unstable
3rd degree (complete)	Immediate pacing if symptomatic; atropine for nodal blocks; transcutaneous pacing while arranging transvenous; permanent pacemaker eventually

Key clinical point: Infranodal blocks (Mobitz II, complete block with wide QRS escape) are unlikely to respond to atropine or vagolytic agents. Pacing is the definitive treatment. - Tintinalli's Emergency Medicine

Quick ECG Recognition Trick

What you see	Likely block
Every P has a QRS, PR > 200 ms	1st degree
PR getting longer → dropped beat → reset	2nd degree Mobitz I (Wenckebach)
Constant PR → sudden dropped beat, wide QRS	2nd degree Mobitz II
P waves and QRS completely unrelated, regular slow escape	3rd degree (complete)

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