see also see also cardiology, basics of the ECG, ECG Diagnosis of AMI

• a conduction block that affects one of the three major infranodal conduction pathways:
  • right bundle branch - see RBBB
  • left anterior superior fascicle (LASF) eg. left anterior hemiblock (LAHB)
    • the LASF is small and easily affected by focal pathology
    • ECG characteristics:
      • normal QRS duration
      • frontal QRS axis < -45deg
      • R wave in lead I > R wave in leads II or III
      • a qR complex in lead aVL
      • deep S wave in leads II, III & aVF
      • NB. exclude other causes of LAD such as inf AMI, hyperkalaemia, pre-excitation syndromes, or body habitus
  • left post inferior fascicle (LPIF) eg. left posterior hemiblock (LPHB)
    • LPIF is broad and blockage usually indicates widespread organic heart disease rather than a focal lesion
    • other causes of RAD are chronic cor pulmonale, RVH, and lateral infarct.
    • ECG characteristics:
      • normal QRS duration
      • frontal QRS axis > 110deg
      • small r and deep S in lead I
      • R wave in lead III > in lead II
      • a qR complex in lead III

• conduction block affecting 2 fascicles such as:
  • RBB and LASF
  • RBB and LPIF
  • LASF and LPIF - see LBBB
• management:
  • in general, new LBBB in a setting of ischaemic chest pain is managed as for ST elevation MI until proven otherwise
  • otherwise bifascicular block is generally managed as per trifascicular block below.

• conduction block affecting all 3 fascicles, such as:
  • RBB and LASF with 1st degree AV block
  • RBB and LPIF with 1st degree AV block
  • LBB with 1st degree AV block
  • alternating LBBB and RBBB
• management:
  • whilst these blocks indicate advanced organic heart disease, risk of sudden progression to complete heart block and sudden death is not high.
  • placement of a ventricular demand pacemaker is thus indicated if symptoms due to documented bradyarrhythmias occur of if this blockade occurs in a setting of acute MI when risks of complete heart block are much greater.