How to recognize heart blocks

Atrioventricular (AV) blocks (often referred to as heart blocks) are characterized by impaired conduction between the atria and the ventricles of the heart. They are classified into first, second or third degree heart block depending on the severity of the block – third being more severe than first.

The impairment causing heart blocks can be located on more levels of the hearts electrical conduction system: the AV node, the bundle of His or the bundle branches. In distal AV blocks the right bundle branch plus the left bundle branch or its two fascicles are impaired. In proximal AV blocks the impairment is localized at the AV node or the bundle of His.

First degree heart block is characterized by prolongation of the electrical conduction from the atria to the ventricles. The lesion is typically located in the AV node. On the EKG the PR interval is prolonged, i.e. more than 0,22 sec.

Second degree heart block is characterized by the dropping of a QRS interval after a P wave. In Mobitz type I (Wenckebach) second degree heart block the PR interval is progressively prolonged until a non-conducting P wave occurs. The lesion can be anywhere. In Mobitz type II second degree heart block the PR interval is constant followed by a non-conducting P wave. The lesion is never in the AV node. In advanced second degree heart block only the second (2:1) or third (3:1) P wave are conducting. This type cannot be classified into Mobitz I or II as it is impossible to observe whether prolongation of the PR interval occurs prior to the dropped QRS interval.

Complete heart block is characterized by complete AV dissociation, that is, no P waves are conducting.

Heart blocks can cause Adams Stokes syncope, occasionally with seizures, due to intermittent ventricular asystole. General speaking Mobitz II and third degree heart block require pacemaker implantation.

— Mathias

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