Second degree AV block, mobitz I/wenckebach

See brief interpretation of 12 lead EKG above

- progressive prolongation of the PR interval in consecutive beats until a non-conducting P wave occurs

Second degree heart (or atrioventricular, AV) block Mobitz type I is also referred to as Wenckebach block. In this heart block the PR interval progressively lengthens until a blocked P wave occurs. After the blocked P wave the cycle continues. The lesion is often at the atrioventricular node.

The number of beats before the non-conducted P wave vary from a couple to several beats. Below a detail of the 12 lead EKG. The conduction ratio – that is the ratio of P waves to QRS complexes (P:QRS) – is 3:2.

The distinction between Mobitz type 1 heart block and complete heart block can sometimes be tricky. In complete heart block the PR interval also appears variable and you may have the impression of a Wenckebach relationship between the P waves and the QRS complexes while in fact there is no relationship at all (AV dissociation).

The key is to check the ventricular rhythm. In complete heart block the ventricular rhythm is regular, while in Wenckebach block there are periodic pauses with the QRS complexes clumping together in repeating cycles. This is sometimes referred to as “group beating”. The distinction is clinical relevant as complete heart block requires pacemaker, while Wenckebach heart block in general does not.

Take a second look at the 12 lead EKG above. Also look for clues to what may have caused the conduction defect.

– Mathias