3rd degree av block with junctional escape

First-degree atrioventricular block

For 1st-degree block, conduction is slowed without skipped beats. All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.2 sec). For 3rd-degree block, there is no relationship between P waves and QRS complexes, and the P wave rate is greater than the QRS rate.

First-degree AV block may be physiologic in younger patients with high vagal tone and in well-trained athletes. First-degree AV block is rarely symptomatic, and no treatment is required. Further investigation may be indicated when 1st degree AV block accompanies another heart disorder or appears to be caused by drugs.

Some normal P waves are followed by QRS complexes, but some are not. Two types exist:

• Mobitz type I

• Mobitz type II

In Mobitz type I 2nd-degree AV block, the PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated (see figure Mobitz type I 2nd-degree AV block Mobitz type I 2nd-degree atrioventricular block

Mobitz type I 2nd-degree atrioventricular block

The PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated.

Mobitz type II 2nd-degree atrioventricular block

The PR interval remains constant. Beats are intermittently nonconducted, and QRS complexes dropped, usually in a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.

Second-degree atrioventricular block (high grade)

Every second P wave is blocked in this example of high-grade 2nd-degree AV block.

The distinction between Mobitz type I and Mobitz type II block is difficult to make because 2 P waves are never conducted in a row. Risk of complete AV block is difficult to predict, and a pacemaker is usually indicated.

Third-degree atrioventricular block

There is no relationship between P waves and QRS complexes (AV dissociation) in 3rd-degree AV block. The P wave rate is greater than the QRS rate. The underlying rhythm here is a junctional escape rhythm with narrow QRS complexes and a rate of approximately 65 beats/minute.

There is no electrical communication between the atria and ventricles and no relationship between P waves and QRS complexes (AV dissociation). Cardiac function is maintained by an escape junctional or ventricular pacemaker. Escape rhythms originating above the bifurcation of the His bundle produce narrow QRS complexes, relatively rapid (> 40 beats/minute) and reliable heart rates, and mild symptoms (eg, fatigue, postural light-headedness, effort intolerance). Escape rhythms originating below the bifurcation produce wider QRS complexes, slower and unreliable heart rates, and more severe symptoms (eg, presyncope, syncope, heart failure). Signs include those of AV dissociation, such as cannon a waves, blood pressure fluctuations, and changes in loudness of the 1st heart sound (S1). Risk of asystole-related syncope and sudden death is greater if low escape rhythms are present.

Most patients with congenital 3rd-degree AV block have a junctional escape rhythm that maintains a reasonable rate, but they require a permanent pacemaker before they reach middle age. Less commonly, patients with congenital AV block have a slow escape rhythm and require a permanent pacemaker at a young age, perhaps even during infancy.