Rhythm Interpretation: Third-degree AV Block
Wednesday, December 31, 2025
Rhythm Interpretation: Third-degree AV Block
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Third-degree AV block is a complete failure of electrical conduction. No sinus or atrial impulses reach the ventricles, and complete AV dissociation occurs. This results in a slow ventricular rate with activation from a backup pacemaker.
The ventricles may be activated by an impulse close to the AV node (narrow complex QRS) or from the purkinje fibers (wide complex QRS).
When the QRS is narrow in third-degree AV block, it indicates the impulse activating the ventricles is close to the AV node, before the bifurcation of the Bundle of His. This is considered a junctional pacemaker and may be seen with inferior wall ischemia or MI.
When the QRS is wide in third-degree AV block, it indicates the impulse activating the ventricles is below the Bundle of His and is coming from the purkinje fibers. This is considered a ventricular pacemaker and may be seen with anterior wall ischemia or MI. This type of block carries a higher mortality rate due to extensive damage to the myocardium and the conduction system.
In the clinical environment, third-degree AV block is most often seen in conjunction with sinus rhythm. However, third-degree AV block can also occur with atrial fibrillation and atrial flutter. Fibrillatory or flutter waves will be seen and the QRS complexes will be regular because of the complete AV dissociation.
When third-degree block develops gradually, the patient may be symptomatic, but stable. If it occurs suddenly, the patient will most likely be unstable. The course is unpredictable and can progress to ventricular standstill. Treatment must be quickly implemented, especially in the setting of ischemia or an acute MI. This includes application of an external pacemaker while a transvenous one is inserted. Vasoactive drugs such as epinephrine or dopamine infusions may be needed to augment pacing.
Rhythm Analysis in Sinus Rhythm with Third-degree AV block
Third-degree AV block with a ventricular pacemaker.
Sinus rate=88 BPM. Ventricular rate=38 BPM. QRS=0.14.
Third-degree AV block with a junctional pacemaker
Sinus rate=75 BPM. Ventricular rate=42 BPM. QRS=0.08.
Third-degree AV block with a ventricular pacemaker and progression to ventricular standstill
Sinus rate=100 BPM. Ventricular rate=40 BPM. QRS=0.16.
Rhythm Analysis in Atrial Fibrillation with Third-degree AV block
Atrial fibrillation with extreme bradycardia. HR=30 BPM. QRS complexes are regular because of the complete AV dissociation. Wide complex QRS indicating a pacemaker below the Bundle of His in the purkinje fibers which has an intrinsic rate of 30-40 BPM.
Rhythm Analysis in Atrial Flutter
Atrial flutter with variable AV conduction. The AV interval is constant indicating the flutter waves are causing the QRS complexes.
Rhythm Analysis in Atrial Flutter with Third-degree AV block
Atrial flutter with extreme bradycardia. HR=30 BPM. The AV interval is variable because the flutter waves are not causing the QRS complexes. The wide and regular QRS complexes indicates a pacemaker below the Bundle of His in the purkinje fibers which has an intrinsic rate of 30-40 BPM.
Summary
Most of the time third-degree AV block is seen with sinus rhythm. When the P waves are present, the rhythm is designated as sinus rhythm with third-degree AV block, or sinus bradycardia with third-degree AV block depending on the discharge rate of the SA node. It may also occur in the setting of sinus dysrhythmia and the P waves will vary by more than 0.12 seconds.
Third-degree AV block in atrial fibrillation or atrial flutter is infrequently seen and may be due to failure of the conduction system in disease or drug toxicity such as with digoxin.
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