Second-degree AV block, Mobitz type I (Wenckebach)

What it is

Second degree Mobitz type I heart block, the Wenckebach block, is an intermittent blockade of electrical impulses from the atria to the ventricles at the level of the atrioventricular node. That blockade prevents generation of a QRS complex, so a beat is dropped.

It is the second of the three degrees of AV conduction block, and within second degree there are two types, Mobitz type I and Mobitz type II.

Causes and risk

Secondary causes to look for:

• Cardiac: ischemic heart disease.
• Metabolic: hypothyroidism, hyper/hypokalemia.
• Drug-induced: CCB, beta blockers.
• Rare but worth remembering: Lyme disease, sarcoidosis, hypothermia.

How it presents

Because the blockade is intermittent, the patient may present with intermittent syncopal episodes. A young male may present with intermittent syncopal episodes and no family history, and the electrophysiological diagnosis is made from the rhythm strip.

If the rate falls far enough the patient can develop shock with altered mental status, with a monitor showing a HR of 43 bpm with the same rhythm.