Second-degree atrioventricular block

Second-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart. It is a conduction block between the atria and ventricles. The presence of second-degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction. It is classified as a block of the AV node, falling between first-degree (slowed conduction) and third degree blocks (complete block). Most people with Wenckebach (Type I Mobitz) do not show symptoms. However, those that do usually display one or more of the following: Light-headedness Dizziness Syncope (fainting) There are two non-distinct types of second-degree AV block, called Type 1 and Type 2. In both types, a P wave is blocked from initiating a QRS complex; but, in Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern. Type 1 second-degree heart block is considered more benign than type 2 second-degree heart block. The type 1 does not have structural changes found on histology. Both types are named after Woldemar Mobitz. Type I is also named for Karel Frederik Wenckebach, and type II is also named for John Hay. Type 1 Second-degree AV block, also known as Mobitz I or Wenckebach periodicity, is almost always a disease of the AV node. Wenckebach published a paper in 1906 on progressively lengthening PR intervals that was later classified as Type I in Mobitz's 1924 paper. Thus, both "Mobitz type I" and "Wenckebach block" refer to the same pattern and pathophysiology. In Wenckebach's 1906 paper, his original observations were of increasing delay in contraction of the atria and ventricles that shortened after a brief pause. He later observed this in the electrocardiogram (ECG) after Einthoven's 1901 invention. In modern practice, Mobitz I heart block is characterized by progressive prolongation of the PR interval on consecutive beats followed by a blocked P wave (i.e., a dropped QRS complex).