Pearls of Knowledge: Third degree AV Block

Dysrhythmias: Third-degree AV Block

Pearls of Knowledge ©

Third-degree AV block is a complete failure of electrical conduction. No sinus or atrial impulses reach the ventricles, and complete AV dissociation occurs. 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 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 treatment must be quickly implemented, especially in the setting of an acute MI. Treatment is a pacemaker. 

Rhythm analysis

The SA node will fire regular P waves, but they are completely dissociated, and none are “married” to the QRS complexes. The QRS will occur at regular intervals and the rate and width of the QRS complexes will be determined by the origin of the impulse.

Third-degree AV block with a ventricular pacemaker

Sinus rate=88 BPM. Ventricular rate=38 BPM. QRS=0.14. Ventricular pacemaker.

Third-degree AV block with a junctional pacemaker

 Sinus rate=75 BPM. Ventricular rate=42 BPM. QRS=0.08. Junctional pacemaker. 

 Ventricular standstill 

  • Sinus rate=100 BPM. Ventricular rate=40 BPM. QRS=0.16. Ventricular pacemaker.
  • Progression to ventricular standstill and pulselessness. 

Third-degree AV block in atrial fibrillation 

Rhythm analysis

 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. (Intrinsic rate 30-40 BPM).


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