Third-degree (complete) AV block
A complete failure of AV conduction: no atrial impulses reach the ventricles at all.
Key ECG features
- ✓No P wave conducts to the ventricles
- ✓P waves and QRS independent (AV dissociation)
- •Atrial rate faster than the ventricular rate
- •No consistent PR interval
- ±Escape rhythm: narrow if junctional, wide if ventricular
✓ required · • typical · ± may be present
ECG characteristics
P waves and QRS complexes march independently at their own rates (AV dissociation), with no consistent PR interval and an atrial rate faster than the ventricular one. A slow junctional (narrow QRS) or ventricular (wide QRS) escape rhythm keeps the ventricles going.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Degenerative or inflammatory disease of the AV node and His-Purkinje system, most often in older dogs. Also infiltrative myocardial disease and, less commonly, severe electrolyte disturbance.
Why it matters
The resulting bradycardia is frequently symptomatic in dogs - weakness, syncope - and complete block is a common indication for pacemaker implantation. Cats usually hold a faster escape rate and tolerate the same block far better, so in cats it is more often picked up incidentally and the decision to pace rests on clinical signs rather than on the ECG alone. The escape rhythm is the only thing maintaining output and must never be suppressed.
Differentiate from
- Second-degree AV blockIn second-degree block some P waves still conduct with a consistent PR interval; here none conduct and the PR varies constantly.
- Ventricular escape rhythmThe wide, slow rhythm keeping the ventricles going below the block is itself a ventricular escape rhythm.
- Junctional escape rhythmIf the escape focus sits in the AV junction the QRS is narrow instead, and the rate is a little faster.
- Sinus bradycardiaBoth are slow. Complete block has more P waves than QRS complexes, marching at their own independent rate.
- Second-degree AV block (Mobitz II)Mobitz II is the block most likely to progress to this. The difference is whether any P wave still conducts.