Third-degree (complete) heart block – Example 1

Table of Contents

Table of Contents

Yilin Zhang, MD


  1. Identify complete (3rd degree) heart block

Teaching Instructions

Plan to spend ~ 5-10 min reviewing the teaching materials.  Have your readers develop a systematic approach to reading ECGs. Refer to How to Read an ECG for additional details. 

Rate: Ventricular rate is ~ 60 bpm. 

Axis: Right axis deviation (downwards QRS in lead I, upright in lead II)

Rhythm: The ventricular rhythm is regular but you may notice that the PR intervals seem to vary from beat to beat. This should raise suspicion for AV dissociation. The rhythm strip is typically the easiest lead to march out the P's and QRS's. Some P's may be hidden under a QRS complex or T wave and may be difficult to identify. However, your P-P interval should remain constant. Calipers are helpful in marching out P waves. If there is no association between the P waves and QRS, this is complete AV dissociation or complete heart block. 

Bonus: The morphology of the QRS depends on the location of the escape rhythm. All cells of the heart (sinus node, atria, AV node, bundle of His, ventricles) have pacemaker capability and an intrinsic rhythm. Typically the intrinsic pacemaker capability is suppressed by the sinus node. However, in complete AV block, there is no signal getting through from the atria. Thus, the intrinsic pacemaker ability of the AV node or ventricles take over. 

  • Sinus node – rate of 60-100 bpm (this will often be the rate of the P waves)
  • AV node (junctional escape) – narrow QRS, rate of 40-60 bpm
  • Ventricles (ventricular escape – wide QRS, rate of 20-40 bpm

This patient has a history of RBBB so the escape rhythm is likely at the level of the AV node or bundle of His to give a ventricular rate of ~ 60 bpm. 

Capture beats, which are a hallmark of AV dissociation, are QRS complexes that occur prematurely (relative to escape rhythm). These premature beats can have features of both the atrial impulse and the ectopic impulse, creating a fusion beat. Complete heart block is a form of AV dissociation but AV dissociation can occur in the absence of complete heart block. The difference is in the fact that with complete heart block, no atrial impulse can be conducted through the AV node. 

Final diagnosis: 3rd degree heart block with a junctional escape rhythm and pre-existing RBBB6

Presentation Board

Take Home Points

  1. Beat to beat variation of the PR interval should raise suspicion for AV dissociation and/or complete heart block.
  2. March out the P waves and QRS complexes to see if there is evidence of AV dissociation. The rate of the P waves if they originate from the sinus node will typically be 60-100 bpm. The ventricular rate depends on the location of the escape rhythm. 
  3. Complete heart block is a cardiac emergency and requires urgent/emergent pacing.


Goldberger AL, Goldberger ZD, and Shvilkin A. Atrioventricular Conduction Abnormalities, Part 1: Delays, Blocks, and Dissociation Syndromes. In: Goldberger AL, ed. Goldberger's Clinical Electrocardiography A Simplified Approach. 9th edition. Philadelphia, PA: Elsevier; 2018.

Yilin Zhang


Comment on this article