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1 Degree A-V block

• The depolarizing impulse is delayed (not blocked) between the atrial and Purkinje fibers.
• PR interval 0.21 or greater
• All P waves  followed by QRS complexes
• The "block" is located proximal to the bifurcation of the bundle branches in AV node
• Encountered in
  • Normal
  • Aging
  • Myocarditis
  • CAD
  • Digoxin

2 Degree A-V block Mobitz Type I (Wenckebach type)

• Progressive prolongation of PR interval in successive beats until a ventricular complex  (absent QRS) drops out
• Wenckebach period
• PR interval becomes normal again
• This pattern repeats
• Block is located proximal to the bifurcation of the bundle branches
• Most often seen with inferior wall MI and Dig excess
• Encountered in
  • Myocarditis
  • CAD
  • Digoxin

2 Degree A-V block Mobitz type II

• Ventricular beats dropped intermittently (random absence of QRS complexes) in presence of constant normal or prolonged PR interval
• PR interval >0.25 secs
• PR interval in the Ist or second beat following the ventricular pause may be of variable duration
• Atrial rhythm is regular
• Most commonly due to bilateral bundle branch block
• Common precursor of persistent complete AV block
• Encountered in
  • Anterior MI
  • Myocarditis
  • Digoxin

3 degree A-V block.

• Independent pacemakers control atria and ventricles
• PP intervals are equal
• RR intervals are equal
• P and R waves are disconnected and has no relationship
• Atrial rhythm may be sinus, faster than ventricular rate, or standstill , Atrial fibrillation etc
• Ventricular rate is bradycardic
• QRS complexes usually narrow(<0.12 secs ) if block above bifurcation of the bundle branches
• QRS complexes wide (0.12 sec or greater) if block below bifurcation of the bundle branches- bilateral bundle branch block

A-V dissociation

• Different pacemakers control atria and ventricles at different rates.
• there is slowing of Atrial pacemaker or acceleration of subsidiary pacemaker or both
• most common causes are dig excess and inferior wall MI
• Encountered in
  • MI
  • Myocarditis
  • Digoxin
  • Valve replacement Surgery

Complete bundle branch block

• The Ventricles depolarize one after another.
• Complete bundle branch block of either the left or right bundle results in a QRS duration of 0.12 second or more.
• Normal PR interval
• Complete right bundle branch block (CRBBB) always includes a QRS with a broad, slurred, terminal QRS component directed to the right and anteriorly, i.e. upright in V1, often a RSR pattern, and a deep S in V6.
• Complete left bundle branch block (CLBBB) alters the depolarization of the entire left ventricle.  The entire QRS is oriented to the left, i.e. upright in leads I, AVL and often, but not necessarily in V5 and V6.  A characteristic of CLBBB is the notch or slur just past midway in the QRS in I, AVL, and occasionally in V5 and V6.  Axis deviation is not part of the diagnosis in CLBBB or CRBBB.
• Deep S wave in V1 and V2
• The pattern of incomplete LBBB fulfills criteria for CLBBB except that the QRS duration is less than 0.12 seconds.

Complete LBBB

• QRS duration of 0.12 secs or greater in limb leads.
• Broad slurred R in V6 with absent q, depressed ST segment and inverted T wave.
• Usually a broad slurred R in I and avl.
• Deep S wave in V1-V2
• Normal PR interval
• Intrinsicoid deflection delayed 0.07 secs or more in V6.

Incomplete LBBB

• Criteria for LBBB are met but QRS duration is less than 0.12 secs.

Complete Right Bundle Branch Block

• QRS duration of 0.11 secs or more in limb leads
• RSR pattern in V1 with depressed ST segment and inverted T wave.
• Deep S wave in V5-V6
• Normal PR interval
• Intrinsicoid deflection delayed to 0.07 secs in V1.

Incomplete Right Bundle Branch Block   

• Criteria for right bundle branch block are met but the QRS duration is less than 0.11 secs.

Left Anterior Hemiblock (LAH)

Left Anterior Hemiblock (LAH).  Block of the Superior Division of the Left Bundle Branch.

• QRS axis of more than -30o
• q in lead I and avl of normal appearance
• QRS duration of less than 0.10 secs.
• Deep S wave in limb leads III and AVF

Pacemaker

Aberrant interventricular conduction

Aberrant interventricular conduction refers to a widened QRS due to block in any fascicle for one beat only - usually a premature supraventricular beat.  A run of these supraventricular beats can occur in a tachycardia all conducted abnormally.