See quick 12 lead ECG interpretation
Interpreting Wenckebach block
In Mobitz type I AV block – also called Wenckebach 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 ECG. The conduction ratio – that is the ratio of p waves to QRS complexes (P:QRS) – is 3:2.
Wenckebach phenomenon versus third degree AV block
The distinction between Mobitz type 1 AV block and third degree AV block can sometimes be tricky. In third degree AV 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 third degree AV 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 third degree AV block requires pacemaker, while Wenckebach 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
Group beating
