cardiovascular disorders
Atrioventricular Block
Second degree AV block
Some normal P waves are followed by QRS complexes, but some are not. Two types exist: Mobitz type I Mobitz type II 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 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 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 Bradyarrhythmias and transient or reversible causes have been excluded. Treatment is 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 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. 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 Cardiac Pacemakers is indicated. In high-grade second-degree AV block, every 2nd (or more) P wave is blocked (see figure Second-degree atrioventricular block (high grade) ). Every second P wave is blocked in this example of high-grade second-degree AV block. 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 Cardiac Pacemakers unless there is a transient or reversible cause. 