Atrioventricular Block

All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.20 second—see figure First-degree atrioventricular block First-degree atrioventricular block ).

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 first-degree AV block accompanies another heart disorder or appears to be caused by medication.

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

In Mobitz type I second-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 second-degree AV block Mobitz type I second-degree atrioventricular block ).

Mobitz type I second-degree AV block may be physiologic in younger and more athletic patients. The block occurs at the AV node in about 75% of patients with a narrow QRS complex and at infranodal sites (His bundle, bundle branches, or fascicles) in the rest. If the block becomes complete, a reliable junctional escape rhythm typically develops. Treatment is therefore unnecessary unless the block causes symptomatic bradycardia Bradyarrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more and transient or reversible causes have been excluded. Treatment is pacemaker insertion Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more , which may also benefit asymptomatic patients with Mobitz type I second-degree AV block at infranodal sites detected by electrophysiologic studies done for other reasons.

In Mobitz type II second-degree AV 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 (see figure Mobitz type II 2nd-degree AV block Mobitz type II 2nd-degree atrioventricular block ).

Mobitz type II second-degree AV block is always pathologic; the block occurs at the His bundle in 20% of patients and in the bundle branches in the rest. Patients may be asymptomatic or experience light-headedness, presyncope, and syncope, depending on the ratio of conducted to blocked beats. Patients are at risk of developing symptomatic high-grade or complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow and unreliable to maintain systemic perfusion; therefore, a pacemaker Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more is indicated.

In high-grade second-degree AV block, every 2nd (or more) P wave is blocked (see figure Second-degree AV block (high grade) Second-degree atrioventricular block (high grade) ).

In patients with high-grade second-degree AV block, the distinction between Mobitz type I AV block and Mobitz type II AV 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.

Patients with any form of second-degree AV block and a structural heart disorder should be considered candidates for permanent pacing Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more unless there is a transient or reversible cause.

Heart block is complete in third-degree AV block (see figure Third-degree AV block Third-degree atrioventricular block ).

There is no electrical communication between the atria and ventricles and thus 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 variation 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 require a pacemaker Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more . If the block is caused by antiarrhythmic drugs Medications for Arrhythmias The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more , stopping the drug may be effective, although temporary pacing may be needed. A block caused by acute inferior myocardial infarction usually reflects atrioventricular nodal dysfunction and may respond to atropine or resolve spontaneously over several days. A block caused by anterior myocardial infarction usually reflects extensive myocardial necrosis involving the His-Purkinje system and requires immediate transvenous pacemaker insertion with interim external pacing as necessary. Spontaneous resolution may occur but warrants evaluation of AV nodal and infranodal conduction (eg, electrophysiologic study, exercise testing, 24-hour ECG).

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.