In second-degree type II AV nodal block (a.k.a. Mobitz Type II AV block), the AV node becomes completely refractory to conduction on an intermittent basis. For example, three consecutive P waves may be followed by a QRS complex, giving the ECG a normal appearance, then the fourth P wave may suddenly not be followed by a QRS complex since it does not conduct through the AV node to the ventricles.
The PR interval may be normal or prolonged, however it is constant in length unlike second-degree AV block Mobitz Type I (Wenckebach) in which the PR interval progressively lengthens until a P wave is not conducted. A second-degree type II AV block indicates significant conduction disease in this His-Purkinje system and is irreversible (not subject to autonomic tone or AV blocking medications). This is a very important distinguishing factor compared to second-degree type I AV block. Because of this, a permament pacemaker is indicated in every patient with second-degree type II AV block.
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