This is Part I of the discussion for 17 year old male CC: Chest pain and palpitations. You may wish to review the case.
If you recall from the case, our crew was performing an interfacility transport of a 17 year old male with a rapid, irregular tachycardia which required cardioversion.
At the receiving facility he was diagnosed with Wolff-Parkinson-White Syndrome (WPW). After undergoing ablation, he is expected to do just fine.
What exactly is WPW Syndrome?
In 1930, Wolff, Parkinson and White described a series of young patients who had ECG findings of a bundle branch block pattern, short PR interval, and paroxysms of tachycardia.
In WPW Syndrome, patient's have a congenital abnormality involving the presence of abnormal conduction tissue between the atria and ventricles associated with supraventricular tachycardias. It is thought to affect about 0.1 to 3 per 1000 in the general population.
WPW is a preexcitation syndrome, where impulses are transmitted from the atria to the ventricles by way of an accessory pathway–or AP–instead of the atrioventricular conduction system.
Usually, this AP is the "Bundle of Kent", which connects the atrium directly to the ventricle. Unike the AV node, this AP transmits the impulse to the ventricle without delay, and at least a portion of the ventricular myocardium is activated before the impulse can reach the ventricles through the normal AV pathway. This is what creates the "pre"-excitation pattern:
It is this preexcitation that causes the slurred start of the QRS, known as a Delta Wave, as part of the ventricle is activated early. As this slurring occurs earlier than normal in the cardiac cycle, the PR interval will be short. As the rest of the ventricular myocardium is activated via the normal AV conduction system, the rest of the QRS may be normal.
We can clearly see this WPW pattern on the prehospital 12 Lead of our patient:
Classic ECG findings that are associated with WPW Syndrome:
- Presence of a short PR interval, less than 120 ms
- A widened QRS complex with a delta wave, the slurred onset of the QRS waveform
- Secondary ST-T wave changes
The amount of ventricular myocardium activated early by the AP will determine the size of the delta wave and the amount of PRi shortening.
Another important consideration in WPW is that the secondary ST-T changes of pre-excitation can be confused with an MI. As with other mimics of AMI, these changes involve a widened QRS/ST angle, i.e. ST/T wave changes opposite the direction of the QRS complex. In fact, repolarization abnormalities are very common with WPW, and they often look like the changes associated with injury and ischemia commonly known as a pseudoinfarction pattern.
What does it mean for our patients?
While someone with WPW can remain asymptomatic for some time, the presence of an accessory pathway can result in arrhythmias. Patients may present with symptoms from mild chest discomfort, to palpitations with or without a syncopal episode, or even sudden cardiac death. As in our patient, symptoms are usually accompanied by a decreased tolerance for activity.
SVT, atrial fibrillation and atrial flutter are the most common arrhythmias associated with WPW. Because two different pathways are available a "loop" can be formed causing a reentrant tachycardia.
Depending on which direction the "loop" is traveling, there may be no part of the ventricular myocardium which is preexcited (orthodromic, 95% of cases). However, if the direction of the loop is one in which the impulse travels first down the AP, and then back up the AV node in a retrograde fashion (antidromic), the ventricles are almost fully activated by the AP, and the QRS will be abnormally wide (5%).
Regardless of which direction the loop travels, a regular reentry tachycardia will rely on the AV node to continue. Because of this, AV blocking maneuvers or medications can terminate the rhythm.
When a patient with preexcitation syndrome develops atrial fibrillation this is life threatening. Because of the potential for extremely fast conduction across the bypass tract, the rate may sometimes approach 300/min.
How do we recognize it?
Atrial fibrillation and WPW is an irregularly irregular tachycardia, with wide and bizarre complexes of differing morphologies, and an R-R interval that can be 250 ms or less.
As in our patient's ECG above, some impulses are transmitted through the the AP and some through the AV node, hence the varying morphogies. If we knock out the AV node with antiarrhythmics, all that will be left is the accessory pathway, which will gladly conduct the rapid impulses of the atria directly through to the ventricles.
Without the protective phsyiologic blocking of the AV node, the result may be ventricular fibrillation! Adenosine, beta blockers, and calcium channel blockers are all absolutely contraindicated.
The best treatment for an unstable patient with atrial fibrillation and WPW is electrical cardioversion.
Once identified and appropriately treated, WPW is associated with an excellent prognosis. For more information about WPW Syndrome, click here.
Stay tuned for Part II of this conclusion where we delve further into this important syndrome!