4.4 Abnormalities of heart conduction function

The dedicated cardiac conducting system usually ensures synchronous conduction of each sinus impulse from the atria to the ventricles. Abnormalities of conduction of the sinus impulse to the ventricles may portend the development of heart block, which can eventually lead to syncope or cardiac arrest. In order to weigh up the clinical significance of conduction abnormalities, the physician must assess the site of conduction disturbance, the risk of progression to complete block, and the probability that a subsidiary escape rhythm arising distal to the site of block will be electrophysiologically and hemodynamically stable.

The specialized cardiac conducting system normally ensures synchronous conduction of each sinus impulse from the atria to the ventricles. Abnormalities of conduction of the sinus impulse to the ventricles may portend the development of heart block, which can ultimately lead to syncope or cardiac arrest. In order to evaluate the clinical significance of conduction abnormalities, the physician must assess the site of conduction disturbance, the risk of progression to complete block, and the probability that a subsidiary escape rhythm arising distal to the site of block will be electrophysiologically and hemodynamically stable. This latter point is perhaps the most important, since the rate and stability of the escape pacemaker determine what symptoms result from heart block. The escape pacemaker following AV nodal block is usually in the His bundle, which generally has a stable rate of 40 to 60 beats per minute and is associated with a QRS complex of normal duration (in the absence of a preexisting intraventricular conduction defect). This contrasts with escape rhythms arising in the distal His-Purkinje system, which have lower intrinsic rates (25 to 45 beats per minute), manifest wide QRS complexes with prolonged duration, and are unstable. Thus, the most important issue is to assess the risk of infra- or intra-His block (which always mandates a pacemaker) or AV nodal block in which the frequency of the escape pacemaker is not sufficient to meet hemodynamic requirements. Although prolonged QRS complexes are invariable when the distal His-Purkinje pacemakers form the escape mechanism, wide QRS complexes can also coexist with AV nodal block and a His bundle rhythm. Therefore, QRS morphology alone may not be adequate to identify the site of block.