Heart Block and Conduction Abnormalities

OVERVIEW

• abnormalities may occur at any part of the conduction system

TYPES

1st degree block

• prolongation of PR interval (>0.2s)

2nd degree Mobitz type I (Wenckebach) block

• progressive lengthening of PR interval with eventual dropped ventricular conduction

2nd degree Mobitz type II (Hay) block

• intermittent dropping of ventricular conduction

2nd degree (2:1 type) block

• alternate p-wave not conducted to ventricles

3rd degree block (complete heart block)

• complete dissociation between atria and ventricular

Left Anterior Fascicular Block (LAFB) (Left anterior hemiblock)

• LAD, Q waves in I and aVL, small R in III (and absence of LVH)

Left Posterior Fascicular Block (LPFB) (Left posterior hemiblock)

• RAD, small R in I, small Q in III (and absence of RVH)

Right bundle branch block (RBBB)

• RSR in V1 (‘M’), and ‘W’ in V6 (MARROW), normal axis

Left bundle branch block (LBBB)

• septal depolarisation reversed so there is a change in initial direction of QRS (WILLIAM), normal axis

Bifascicular block

• RBBB + block of either left anterior or posterior fascicle.
• RBBB + left anterior fascicle block -> LAD
• RBBB + left posterior fascicle block -> RAD

Trifascicular block – 3 types:

1. Prolonged PR interval + RBBB + LAD
2. LBBB + RAD
3. AF + RBBB + LAD

MANAGEMENT

By diagnosis

• 1st degree
  – nothing unless symptomatic and other causes of symptoms excluded
• 2nd degree (Mobitz type I)
  – nothing unless symptomatic and other causes of symptoms excluded
• 2nd degree (Mobitz type II)
  – pacemaker
• 3rd degree
  – pacemaker
• branch blocks
  – nothing unless progresses or symptomatic -> pacemaker
  — if sympatomatic -> pacemaker
  — trifascicular block -> pacemaker
• may need temporary pacing wire or external pacing
• also if rates too slow and unresponsive to drugs -> pace

Emergency management of bradycardia

• check blood pressure
• atropine 25microgram/kg or glycopyrolate 0.2mg
• isoprenaline 1-10mcg/min
• adrenaline 0.1-1.0mcg/kg/min
• temporary pacing – tranthoracic, transoesophageal, transvenous

References and Links

LITFL

• ECG Library — Conduction blocks
• ECG Library — ECG rhythm analysis
• ECG Library — A to Z by Diagnosis