ESSENTIALS OF DIAGNOSIS
Conduction disturbance between the atrium and ventricle that can be physiologic (due to enhanced vagal tone) or pathologic.
Block occurs in the AV node (first-degree, second-degree Mobitz type I) or below the AV node (second-degree Mobitz type II, third-degree).
Symptomatic AV block or block below the AV node in the absence of a reversible cause usually warrants permanent pacemaker implantation.
AV block can be physiologic (due to increased vagal tone) or pathologic (due to underlying heart disease such as ischemia, myocarditis, fibrosis of the conduction system, or after cardiac surgery). AV block is categorized as first-degree (PR interval greater than 200 msec with all atrial impulses conducted), second-degree (intermittent blocked beats), or third-degree (complete heart block, in which no atrial impulses are conducted to the ventricles). Second-degree AV block is further subclassified into Mobitz type I (Wenckebach), in which the AV conduction time (PR interval) progressively lengthens before the blocked beat (see eFigure 10–59) and Mobitz type II, in which there are intermittently nonconducted atrial beats not preceded by lengthening AV conduction (see eFigure 10–60). When only 2:1 AV block is present on the ECG, the differentiation between Mobitz type I or Mobitz type II is more difficult. If the baseline PR interval is prolonged (greater than 200 msec) or the width of the QRS complex is narrow (less than 120 msec), the block is usually nodal (Mobitz type I); if the QRS complex is wide (greater than or equal to 120 msec), the block is more likely infranodal (Mobitz type II).
AV dissociation occurs when an intrinsic ventricular pacemaker is firing at a rate faster than or close to the sinus rate (accelerated idioventricular rhythm, ventricular premature beats, or ventricular tachycardia), such that atrial impulses arriving at the AV node when it is refractory may not be conducted. This phenomenon does not necessarily indicate AV block (eFigure 10–70). No treatment is required aside from management of the causative arrhythmia.
Atrioventricular (AV) dissociation with capture. Two rhythms are present. A regular atrial rhythm (arrows) is present at a rate of 60/min. The P waves are upright in aVR and inverted in lead aVF, indicating retrograde atrial activation, presumably from a focus low in the atrium. The QRS rhythm occurs at a rate of 85/min (determined from the regular RR intervals shown at A, B, and C). The rate of the QRS rhythm and the normal duration of the QRS complexes indicate a focus of origin in the AV junction or bundle of His. In aVF, the second, fifth, seventh, and tenth QRS complexes occur early relative to the prevailing QRS rate and are preceded by P waves at intervals of 0.12 s. These early QRS complexes represent capture beats—ie, they are stimulated by the P waves that precede them. Ventricular capture is the hallmark of AV dissociation. (Reproduced, with permission, from Goldschlager N, ...