🫀 Mirza's Cardiology Notes

Third degree (3˚) AV Block

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Rule of Thumb: Atrial and Ventricular Rates

When the atrial rate is faster than the ventricular rate (and no atrial rates are conducted), CHB is present. If the ventricular rate is faster than the atrial rate, AV dissociation without CHB should be suspected.

  • Involves a complete block of the conduction of atrial stimuli to the ventricles → so atrial depolarizations are not conducted to the ventricles. Therefore, an automaticity focus below the complete block escapes to pace the ventricles at its inherent rate.
  • Perfusing rhythm (if present) is maintained by junctional or ventricular escape rhythm.
  • Distal/infranodal 3˚ AVB is much more likely to produce Sx than 3˚ AVB located in the AV node.1
  • ECG:
    • AV dissociation w/ independent atrial and ventricular rates
    • A going at one rate and V going at another rate (both at stable rates, i.e. regular P-P interval and regular R-R interval)
    • If CHB localized to the AV node:
      • usually accompanied by relatively reliable escape pacemaker, localized just distally to the AV node (“high junctional” escape rhythm). Often yields a baseline HR of 40-55 bpm and responds (at least moderately well) to ↑ sympathetic tone.1
      • You often see this pattern where the CHB is usually localized to the AV node with congenital CHB too.
    • If CHB localized distal to the AV node (infranodal):
      • wide-complex, slow (20-40 bpm) escape pacemaker is a strong indicator of distal/infranodal AVB
        • by contrast, a narrow-complex and more rapid (40-55 bpm) escape rhythm suggests AV nodal block
  • Evaluation:
    • BP - check for hypOtension, HD instability
    • QRS
      • if wide, suggests escape rhythm (bad!)
        • May need to get the dopamine going and begin coordinating for pacing
      • if narrow, good (relatively)
    • Syncope - bad if present
  • Treatment
    • Pacemaker
    • Consider temporary pacemaker until permanent pacemaker is implanted

Footnotes

  1. Fogoros, R. N., & Mandrola, J. M. (2017). Fogoros’ electrophysiologic testing. John Wiley & Sons. ↩ ↩2

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