CARDIOLOGIST IN YELAHANKA
SECOND DEGREE AV BLICK There are two basic types of second-degree AV block: AV nodal Möbitz type I (Wenckebach)
heart block, and the more distal and more sinister Möbitz type II heart block. Möbitz type I
heart block is much more common.
In Möbitz type I block the PR interval lengthens progressively with each cardiac cycle,
until an atrial wave is not conducted. There is recovery of conduction and the next a wave
is conducted with a shorter interval and the cycle begins again. The QRS complex is narrow
(Fig 3.10) (unless associated with pre-existing BBB). The increment is largest between the first
and second conducted P wave, and the PR interval continues to increase by less and less until
a P wave is dropped.
Möbitz type II heart block is almost always associated with a BBB (Fig 3.11), since its origin
is intraventricular (below the AV node), and it tends to lapse suddenly into extreme bradycardia
or asystole. It tends to be over-diagnosed, especially in the setting of 2:1 AV block (Fig 3.12).
There is no lengthening of the PR interval before an atrial wave is not conducted. At times,
atropine or exercise can demonstrate the site of the block, by increasing the block from 2:1 to a
higher grade when the underlying mechanism is Möbitz II. Conversely, Wenckebach conduction
may improve to 3:2 or better.
For a distinction to be made between Möbitz type I and Möbitz type II, at least two consecutively
conducted P waves have to be evaluated. This is impossible in 2:1 conduction (block)
and can only be reported as 2:1 AV block (Fig 3.12). Yet this is very commonly reported as