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Third Degree Heart Block - Heart Blocks.

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Description Part 1

  • Third Degree Heart Block is also known as “Complete Heart Block”.
  • This name more accurately describes the electrical event or problem occurring within the heart.
  • As a result of disease or tissue death, there is a blockage preventing electrical impulses within the atria from entering the ventricular conduction system.
  • The outcome of this impediment are two independently functioning pacemakers within the heart (typically one is supraventricular, the other is ventricular).
  • Essentially, the atria and ventricles are electrically separated (dissociated) from one another.

Description Part 2

  • What will be seen are regularly occurring P waves and QRS complexes, but at two distinctly different rates.
  • The QRS complexes may occur as a result of impulses coming from the His bundle or the Purkinje network.
  • Morphology and rate will often provide clues regarding the locus of ventricular impulse stimulation. A more narrow appearing QRS with a rate greater than 40 generally would indicate the impulse formation is coming from the His bundle. A wide, bizarre QRS with a rate of 40 or less indicates the impulses are originating in the Purkinje network.
  • Complete heart block presents with Regular P to P and R to R intervals and a variable P-R interval.
heart block ecg image 110

Practice Strip

heart block ecg image 111

Analyze this tracing using the five steps of rhythm analysis.

Show Answer
  • Rhythm: Atria and Ventricles - Regular
  • Rate: Atria – 94, Ventricles - 34
  • P Wave: Upright (some buried)
  • PR interval: Variable
  • QRS: 0.14 sec
  • Interpretation: Third Degree (Complete) Heart Block

Authors and Reviewers

Authored by Thomas O'Brien
Medically reviewed by Dr. Jonathan Keroes, MD, Cardiology
Last Update: 11/8/2020

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