Heart Blocks

First degree AV block

• mechanism: disorder of impulse conduction (block or delay)

• characterized by delay > 200 ms from onset of P wave to onset of QRS complex (i.e., PR > 200 ms)

• in this example, the delay is 260 ms

• if the delay exceeds 300 ms, they can get AV dissociation, leading to suboptimal filling. this manifests as shortness of breath, for example

Second degree AV block

• this means there is an occasional dropped beat—every so often, a P wave won’t go to the ventricle

• Mobitz type I (Wenckebach) block

  • gradually prolonging PR interval

  • then, the heart drops the beat and nothing gets to the ventricles

  • location of block is typically within AV node

  • does not usually progress to complete heart block, so generally don’t have to treat

• Mobitz type II

  • mechanism: disorder of impulse conduction: conduction block

  • fixed PR interval and intermittent dropped beats

  • the location of the block is below AV node

  • presents greater risk of progressing suddenly to complete heart block

  • evaluate pt for placement of permanent pacemaker, else sudden death may occur

Complete (third degree) heart block

• complete disruption of conduction between atria and ventricles, which are activated independently at different, fixed rates

• symptoms: decreased CO

  • fatigue

  • dyspnea

  • SOB

  • syncope

• risk for sudden death; require rx with pacemaker

Category: Pathology Notes