Second Degree Heart Block Type I - Course # 316-5


  • 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

  • Analyze this tracing using the five steps of rhythm analysis.
  • Compare your answers with the answers on the next slide.

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  • 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
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316 Second Degree Heart Block Type I - Course # 316-5