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Second Degree Heart Block Type I - Heart Blocks.

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  • Also known as Wenckebach Phenomenon; this dysrhythmia is typically stable and often temporary with the patient remaining asymptomatic as long as the ventricular response remains within the “normal” range.
  • The unique feature (hallmark) of this dysrhythmia is the presence of a prolonging P-R interval from one cardiac complex to the next, until it reaches a point where the QRS complex is non-conducted ( blocked or more simply missing). Then the pattern starts over again.


  • In this dysrhythmia, if the third QRS complex is dropped/blocked, then it will always be the third complex that is blocked before re-setting in a repetitious pattern.
  • It is important to note the following:
    • The P – P intervals are regular and the R to R intervals are irregular.
    • here are more P waves than QRS complexes. Report the rate of each separately.

heart block ecg image 104

Practice Strip

heart block ecg image 105

Analyze this tracing using the five steps of rhythm analysis.

  • Show Answer
    • Rhythm: Atria – Regular, Ventricles - Irregular
    • Rate: Atria – 56, Ventricles - 40
    • P Wave: Upright
    • PR interval: Variable, progressive
    • QRS: 0.10 sec
    • Interpretation: Second Degree Heart Block Type I

  • 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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