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Identifying second-degree atrioventricular block (type I)

doi: 10.1097/01.NURSE.0000294287.10800.93
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SECOND-DEGREE ATRIOVENTRICULAR (AV) block occurs when one or more electrical impulses from the sinoatrial (SA) node aren't conducted to the ventricles. Second-degree AV block is subdivided into type I and type II.

Let's take a look at type I. Also called Wenckebach or Mobitz type I block, type I second-degree AV block occurs when each successive impulse from the SA node is delayed slightly longer than the previous one. This pattern of progressive prolongation of the PR interval continues until an impulse fails to be conducted to the ventricles.

Usually only a single impulse is blocked from reaching the ventricles and, following this nonconducted P wave or dropped beat, the pattern is repeated. This repetitive sequence of two or more consecutive beats followed by a dropped beat results in “group beating.” Type I second-degree AV block generally occurs at the level of the AV node.

Take a look at the defining characteristics of second-degree AV block, type I.



  • Atrial: Regular
  • Ventricular: Irregular


  • Atrial rate exceeds ventricular rate because of nonconducted beats
  • Both rates usually within normal limits

P wave

  • Normal size
  • Normal configuration
  • Each followed by a QRS complex, except blocked P wave

PR interval

  • Progressively longer with each cycle until a P wave appears without a QRS complex
  • Commonly described as “long, longer, dropped”
  • Slight variation in delay from cycle to cycle
  • After the nonconducted beat, shorter than the interval preceding it

QRS complex

  • Usually within normal limits
  • Periodically absent

T wave

  • Normal size
  • Normal configuration
  • Deflection may be opposite that of the QRS complex

QT interval

  • Usually within normal limits


  • Wenckebach pattern of grouped beats (footprints of Wenckebach)
  • R-R interval shortens until a P wave appears without a QRS complex; cycle then repeats

Practice strip

Use the 8-step method to interpret the following rhythm strip. Put your answers on the blank lines. Answers appear below.



Rhythm interpretation:Atrial—regular; ventricular—irregular; Rate:Atrial—that of underlying rhythm; ventricular—depends on number of impulses conducted through AV node; less than atrial rate; P waves: sinus origin; PR interval: varies; progressively lengthens until a P wave isn't conducted (P wave occurs without a QRS complex); a pause follows the dropped QRS complex; QRS complex: Normal (0.10 seconds or less).

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© 2007 Lippincott Williams & Wilkins, Inc.