In this chapter, we address the basic notions of conduction abnormalities.
Impairment can occur in any part of the conduction system. Prolongation of the PR
interval over 200 ms is the characteristic feature of first degree A-V block. Second
degree A-V block includes Wenckebach, Mobitz II A-V block and 2:1 A-V block. The
characteristic ECG feature of Wenckebach block, also called Mobitz I, is progressive
lengthening of the PR interval until finally a beat is dropped. This is a more severe form
of second degree block. The characteristic ECG picture is that of a series of one or more
non-conducted P waves, 2:1, 3:1, 4:1 block. Two to one A-V block (2:1 block) can be
of the types Mobitz I or II. It can be nodal or infra-hisian. A P wave is conducted, the
following one is blocked and so on. Third degree A-V block, also known as complete
A-V block, is a complete disruption of A-V conduction. The atria and the ventricles are
paced independently. Block of conduction in the left bundle branch, prior to its
bifurcation, results primarily in delayed depolarisation of the left ventricle. In LBBB,
the septum depolarizes from right to left, since in this case depolarisation is initiated by
the right bundle branch. In rigth bundle branch block (RBBB) conduction along the
right branch of the His bundle no longer exists. The ventricles are activated only by the
left branch.
Sinus activity is not visible on the surface ECG. Thus first degree sino-atrial block is
only theoretical without electrophysiological sequelae. Only second and third degree
sino-atrial blocks are visible on the surface ECG and these do have some clinical
importance. Second degree block of the type Wenckebach occurs with a progressive
shortening of the PP interval and a slight increase of the heart rate followed by a pause
with a duration greater than the PP interval preceding it but less than the next PP
interval. Third degree block or complete block cannot be distinguished at all from true
atrial standstill: absence of atrial activity, His bundle escape rhythm with narrow QRS
complexes and sometimes retrograde P' waves with a polarity opposite to sinus P wave
polarity. The pacemaker rhythm can easily be recognized on the ECG. It shows
pacemaker spikes: vertical artifact signals that represent the electrical activity of the
pacemaker. Usually these spikes are more visible in unipolar than in bipolar pacing. The
morphology of the QRS complex helps to locate the site of ventricular pacing, typically
right bundle branch block morphology for left ventricular pacing and left bundle branch
block morphology for right ventricular pacing.
Keywords: Sino-atrial block, atrio-ventricular block, left bundle branch block,
rigth bundle branch block, left anterior hemiblock, left posterior hemiblock,
second degree atrioventricular block, first degree atrio-ventricular block, Mobitz I
block, Mobitz II block, complete atrio-ventricular block.
