WPW: “The Great Imposter” | STEMI Mimic
(C) 2020 Vernon R Stanley, MD, PhD | Co-editor Courtney Stanley, MS, PA-C
The WPW is often referred to as the great imposter. A common mimic of the WPW is the old MI. You will frequently find significantly wide Q-waves in the inferior leads. This will often be interpreted by the computer as “old inferior MI”. However these Q-waves usually do not reflect an MI but are simply secondary to the WPW. It is a pseudoinfarction pattern.
Please note the WPW in the following tracing which demonstrates wide Qs in Leads III, avF (pseudoinfarction).
Although a fairly uncommon finding, it is important to discuss the WPW syndrome predicted wave shape. The general category of the preexcitation syndrome (of which WPW is an example) is the result of a congenital abnormality of the electrical conduction system of the heart. There are several variants on the theme, but for examples’ sake, let us consider the specific case of WPW and the Bundle of Kent. This is a special situation where an accessory pathway exists which short circuits the AV node. The AV node is the primary reason for the long duration of the PR interval (0.12 to 0.20 sec), since the conduction across the AV node is extremely slow. In the case of WPW, the Bundle of Kent has short-circuited the AV node and it follows that the PR interval will be short, i.e. PR < 0.12 sec. The electrical signal will travel via the Bundle of Kent and will depolarize the ventricle earlier than usual (the ventricle is pre-excited). The Bundle of Kent is composed of myocardial tissue and its electrical conduction is slow, nevertheless much more rapid than that traversing the AV node. Please see the diagrams below demonstrating the Bundle of Kent and it’s effect on the resultant waveform.
There are three important observations to make about the above illustration of WPW.
- The PR-interval is shortened by the Bundle of Kent. More specifically: the PR < 0.12 sec.
- The QRS duration is widened because of the early depolarization of the septum. More specifically: |QRS| > 0.10 sec
- When the electrical signal and depolarization wavefronts rejoin the old pathways, there is a sudden change in direction (instantaneous slope change) in the QRS waveform. This portion of the waveform is called a Delta wave. The provider should be aware that the Delta wave may not be apparent in all the leads and certainly may be negative in some leads.
In summary, the classic triad criteria for the WPW is as follows:
- |QRS| > 0.10 sec
- PR < 0.12 sec
- Presence of Delta wave in some of the leads
The important conclusion is as follows: If one measures a short PR-interval on the ECG, one should “think” WPW and proceed to look for the presence of the other two criteria. It is no trite statement to say that if you do not look for the WPW, you will never find it.