Wolff-parkinson-white pattern, or WPW, is a type of heart arrhythmia caused by an accessory
pathway, or an “extra” electrical conduction pathway connecting the atria and ventricles,
or upper and lower chambers of the heart.
Normally, an electrical signal starts at the sinoatrial or SA node in the right atrium,
it then propagates out through both atria, including bachmann’s bundle in the left
atrium, and contracts both atria, it’s then is delayed just slightly as it goes through
the atrioventricular or AV node, before it passes through the Bundle of His and to the
Purkinje fibers of the left and right ventricles, causing them to contract as well.
On an electrocardiogram, the P-wave corresponds to atrial contraction, the PR interval corresponds
to the slight delay through the AV node, and the QRS complex is ventricular contraction.
Now in a normal electrical conduction system, the AV node is the only place where the signal
can get through from the atria to the ventricles, it’s kind of like there’s this gatekeeper
that has to stop the signal to make sure everything’s good before letting it pass, so there’s
always a slight delay here.
People with WPW essentially have a secret, backdoor entrance.
This entrance—being secret and all—doesn’t have a gatekeeper, and therefore there’s
no delay as the signal moves through it.
This secret backdoor entrance is a tiny bundle of cardiac tissue that conducts electrical
signals really well and it’s called the Bundle of Kent.
Using the Bundle of Kent means the ventricles start to contract a little bit early, called
pre-excitation.
If the Bundle’s on the left side of the heart, it’s called “type A pre-excitation,”
if it’s on the right side, it’s called “type B pre-excitation,” although type
A, on the left side, is a lot more common..
Alright, even though the signal sneaks through early, the signal waiting at the AV node eventually
makes it’s way through and the two signals essentially combine to contract the ventricles.
So on an ECG, people with WPW have a short PR interval with a delta wave as well as QRS
prolongation, which makes sense because the signal’s taking the shortcut and contracting
the ventricles early, which means the PR’s shorter and overall QRS complex is longer.
People with WPW usually have a PR interval less than 120 ms, and a QRS complex greater
than 110 ms.
Also, the ST segment and T wave, which represent repolarization, will often be directed opposite
the QRS complex.
This WPW pattern doesn’t typically cause any symptoms and it’s relatively benign.
In some cases, however, this pattern can facilitate certain arrhythmias, or basically make certain
arrhythmias more severe and potentially even cause sudden cardiac death, in which case
it would be called Wolff-Parkinson-White syndrome.
For example, people with atrial arrhythmias might have atrial rates in the 200 to 300
beats per minute range.
Normally, the AV node doesn’t allow all of these signals through, and so the ventricles
will contract at a lower number, like for every 3 atrial beats, there’s only 1 ventricular
beat, so for 300 beats per minute in the atrium, the ventricle would be going 100 beats per
minute.
For people with a Bundle of Kent, though, those signals aren’t held up at the AV node,
and the ventricles contract at the same rate as the atrium, in this case at 300 beats per
minute, which is way too fast for the ventricles, and this can quickly lead to cardiogenic shock,
since the heart doesn’t even have time to fill with blood before each contraction.
And so it’s barely pumping any blood.
In other cases, the extra pathway can set up a reentry circuit.
In the case of a reentry circuit, the signal might move back up the accessory pathway,
since the majority of these Bundles of Kent are actually bidirectional, meaning the signal
can go from atrium to ventricle as well as from ventricle to atrium.
So an example of this would be if an electrical signal goes down the AV node, goes through
the ventricles, then goes up the bundle of Kent, causing the atria to contract, and then
it goes back down the AV node, contracting the ventricles and goes back up again, creating
a reentry circuit.
This type where it goes up from ventricle to atrium, is called Atrioventricular reentrant
tachycardia (or AVRT) with orthodromic conduction, and can lead to very high ventricular rates,
between 200 and 300 bpm.
Less commonly, the signal can move in the opposite direction, called AVRT with antidromic
conduction.
These circuits can be initiated by several mechanisms, like premature contractions in
the atria or ventricles.
It’s estimated that about 1 in 1000 of people have Wolff-Parkinson-White pattern, which
is congenital and present at birth.
A small proportion of this already small proportion go on to have the symptoms that we just discussed.
If they do develop dangerous tachyarrhythmias, however, they may be treated pharmacologically,
but a definitive treatment is radiofrequency catheter ablation of the accessory pathway,
or Bundle of Kent.
This treatment essentially uses cautery to cut and destroy the pathway, essentially boarding
off the secret back door, leaving behind only one connection between the atria and the ventricles—the
AV node.