Curriculum · Cardiovascular and Hypertension
Second-degree AV block, Mobitz type II
What it is
AV conduction block comes in one of three degrees. First-degree block is a prolonged AV delay, greater than 0.2 sec, that is, a prolonged PR interval with a P:QRS ratio still 1:1. Second-degree block loses beats, and it has two types. Third-degree block, also called complete heart block, is complete AV conduction failure with no P:QRS relationship and a drop in cardiac output from the disordered contraction of atria and ventricles.
Mobitz type II is the second of the two second-degree types: intermittent loss of AV conduction with a fixed PR interval. The block may last for 2 or more beats.
How it is diagnosed
The diagnosis of Mobitz type II turns on one measurement: the PR interval of the conducted beats is fixed. Beats are lost, yet the PR interval before each conducted QRS is the same as the one before it.
That fixed PR interval is what separates it from Mobitz type I (Wenckebach), where the PR interval lengthens gradually until a QRS is lost. Mobitz type I is an intermittent blockade of electrical impulses from the atria to the ventricles at the level of the atrioventricular node, which prevents generation of a QRS complex; it shows progressive prolongation of the PR interval until a P wave is not followed by a QRS complex. P waves come at regular intervals, so PP intervals are normal, and following the missed QRS complex the PR interval returns to its baseline duration.