Third degree AV block

  • May be isoarrhythmic- similar P and QRS rates.
  • Acute-
    • At AV node level- inferior wall MI, digoxin and rheumatic fever.
    • At bundle branch level- anterior wall MI.
  • Chronic-
    • Commonest cause- fibrosis (Lev, Lenegre)
    • Congenital (maternal anti-Ro antibodies)
  • Chronic third degree HB is usually due to damage to both right and left bundle branches.
  • When difficulty in distinguishing third degree HB from second degree HB-
    • QRS intervals are usually regular- if irregular usually suggests some amount of AV conduction.
    • Constant PR (or flutter-R) interval indicates second degree HB while varying PR (or flutter-R) interval indicates third degree HB.

Location of AV block

  • Sites-
    • AV node- most common
    • His bundle- rare (so differentiation is between the other two)
    • Bundle branches- second most common
  • With blocks lower than third degree heart blocks-
    • QRS duration-
      • Normal- AV node
      • Wide- can be either (AV node + aberrancy can also cause widening)
    • PR interval consistency-
      • Variable PR interval- AV node (only AV node has ability to vary conduction time)
      • Constant PR interval (when AV conduction is present)- bundle branch

Wenckebach sequence

    • Indicates AV nodal block
    • PR interval-
      • Progressive prolongation of PR interval – because successive atrial impulses arrive progressively earlier in the relative refractory period of the AV node.
      • The “increase in PR interval” (PR2-PR1, PR3-PR2 etc) progressively decreases and hence is maximum between the second beat and the first beat. This, in the setting of a constant PP interval, causes RR interval to shorten progressively.
    • RR interval-
      • RR interval progressively shortens.
      • Longest cycle (with the missed beat) is less than twice the shortest one. This is because the longest cycle has the shortest PR interval beat.
      • Smaller QRS groups are commoner than larger ones. Thus, 3:2 > 4:3 > 5:4 etc. Commonest is pairs.
    • PP interval- constant.

Purkinje block (in bundle branches)

    • Second degree heart block-
      • Purkinje cells have very short refractory period. So, either normal PR or no conduction. No increased PR interval.
    • Complete heart block-
      • Almost always, preceding bundle branch block is present.
      • Usually there is sudden transition from no heart block to complete heart block without an intervening first or second degree heart block.