Electrocardiogram: Some Practical Consequences

OKHAWA and his colleagues have provided histologic evidence that a lesion in the distal or branching portion of the His bundle, and thus actually within the ventricles, may produce complete atrioventricular (AV) block on the surface ECG associated with "AH" block on the His bundle electrogram (atrial impulses blocked proximal to the His deflection) and that this may be associated with an escape rhythm with narrow QRS complexes that are similar to normally conducted complexes (an "AV junctional escape rhythm"). They have also presented evidence that a lesion that blocks the two bundle branches near their origin may produce complete AV block on the surface ECG associated with an HV block on the His electrogram (an intraventricular block between the His depolarization and the ventricular depolarization), accompanied by an escape rhythm with wide aberrantly conducted QRS complexes (a "ventricular escape rhythm"). These observations are a further step to a better understanding of the origin of disorders of conduction and dysrhythmias and they remind us again that the surface ECG does not always provide a simple reflection of electrical events within the heart. When the ECG became available, it provided what appeared to be a new and beautifully simple representation of the process of cardiac depolarization: an impulse originating in the sinoatrial node depolarized the atria, producing the P wave; the delay that occurred as the impulse traversed the atria and the AV node was represented by the PR interval; the depolarization of the ventricles, initiated by the AV node, was represented by a QRS complex that lasted not more than 0.10 second and had a relatively invariant configuration in normal hearts. Several clinical corollaries arose: a delay between the onset of the P wave and the onset of the QRS represented an AV block, the site of which was somewhere in the atrium or AV node; a prolonged QRS of unusual configuration represented an intraventricular block originating somewhere in the ventricle; a QRS complex lasting no longer than 0.10 second, not preceded by a P wave, and having a configuration like that of a normally conducted complex must originate in the AV node; and a prolonged, aberrantly conducted QRS complex not preceded by a P wave must originate in the ventricles.