Electrocardiography of Atrioventricular Block

Key points

Atrioventricular delayed conduction

Understanding various types of AV block requires knowledge of the normal flow of conduction through the cardiac electrical system as stated previously to understand better where delay or block may occur.

Delayed AV conduction will result in PR prolongation (PR interval >200 milliseconds). This is typically referred to as first degree AV block, but there is no actual block of impulses to the ventricle. The authors prefer first degree AV conduction delay, but realize that block is ingrained in ECG

Dual atrioventricular node physiology

Many patients have dual inputs to the AV node with evidence of fast and slow pathway conduction. The exact mechanism by which conduction passes into and through the AV node has not been completely elucidated despite the existence of multiple theories.^6,7 However, with dual AV node physiology, there may be a normal PR interval at baseline, but pacing maneuvers can unmask slow pathway conduction with an increase in the AH interval. Fig. 3 shows an uncommon occurrence where a baseline ECG shows

Second degree type 1 atrioventricular block (Wenckebach; Mobitz 1)

Grouped beating is the hallmark of type 1 second degree AV block but is not diagnostic. Second degree type 1 AV block typically begins as delay in the AV node, resulting in progressive PR prolongation followed by AV block with subsequent recovery of the PR interval (Fig. 4). Note that the first delay is more than subsequent delays (30 milliseconds vs 10 milliseconds), and this leads to the progressive shortening of the RR intervals before block. The RR interval containing the blocked P wave is

Second degree type 2 atrioventricular block (Mobitz II)

As one considers higher levels of AV block it is prudent to recognize the clinical importance of identifying these abnormalities and proceeding with appropriate intervention to avoid increased morbidity and mortality. Second degree type 2 AV block occurs without warning, that is, there is no prolongation of PR interval preceding the blocked P wave, but rather sudden AV block. With rare exception, block occurs within the HPS, and the QRS complex is wide. Typically, there is 1 nonconducted P wave

Two to one atrioventricular block

The phenomenon of 2:1 AV block is its own separate category, for every other beat is blocked, precluding diagnostic information gained from progressive or sudden changes in AV conduction. In a patient with normal QRS complex and substantial PR prolongation on the conducted beat, block is almost always in the AV node; if the PR interval is normal, one needs to consider block within the His bundle, for in the authors’ experience, it is rare to have 2:1 block in the AV node with a normal QRS and

Paroxysmal atrioventricular block

Paroxysmal AV block occurs suddenly following a premature atrial or ventricular complex (Fig. 8). In the few cases where there are intracardiac recordings during block, the block has been in the HPS. Block can be prolonged because of the lack of a stable escape rhythm and lead to syncope or even sudden death. The lack of a stable escape rhythm is concerning, and although this phenomenon is rare, it also warrants pacemaker implantation.

Complete heart block

In complete heart block (third degree AV block) there is complete AV dissociation, with an atrial rhythm totally independent of an escape rhythm. That escape rhythm can either be junctional (typical of congenital heart block), resulting in conduction with narrow QRS complex, or bundle branch block pattern if the patient had a bundle branch block at baseline, or from a subnodal focus with a wide QRS complex. Fig. 9 is an example from a patient with congenital heart block and shows a junctional

Clinics care points

Disclosure

None.