Conduction Abnormalities

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.