DESCRIPTION

Technically this isn’t an arrhythmia (actually, even non-technically it isn’t!) but this condition provides the substrate for some crazy arrhythmias, so I thought I’d include it. Your attendings always like to throw this ECG at you, so if you can recognize preexcitation you will definitely impress them.

Preexcitation is to the electrophysiologist as foreplay is to married couples. Let’s face it, those studies are long and keep those electrophysiology guys in the lab all night!

Actually, this is a situation where there is an extra, or accessory, electrical connection between the atrium and ventricle, which can provide a quicker route than the atrioventricular (AV) node for electricity to travel between the two chambers. Creative physiologists named these accessory pathways accessory pathways, which are strands of muscle that are congenitally present and “bridge” the fibrous tissue that electrically insulates the atria from the ventricles. These pathways can conduct electricity and often manifest themselves on the surface ECG by producing a characteristic short PR (because the impulse skips the AV node) and a wide QRS (because the initial part of ventricular activation is via the accessory pathway which activates the ventricle separately from its normal conduction system). The latter part of the ventricular complex, however, is narrow, as the impulse that is working its way through the AV node finally flashes down the normal conduction system. The QRS has a hump at its onset, called a delta wave (see arrow), not to be confused with the faucet or the force.

In sinus rhythm, unless someone looked at the ECG, no one would know preexcitation was present. In sinus rhythm preexcitation is completely asymptomatic. However, these dual pathways can make for some funky arrhythmias: an impulse can travel down the accessory pathway and up the AV node (or the other way around), creating a small, rapidly conducting circuit which can cause an impressive tachycardia. These would look for all the world, depending on the direction the arrhythmia took, like a supraventricular tachycardia or a ventricular tachycardia. Worse yet, patients with preexcitation are prone to developing atrial fibrillation which can be particularly rapid since the accessory pathway may not slow the impulses the way the normal AV node does.

HABITAT

These can be found anywhere, but most are found accidentally when somebody has an ECG done for another reason.

CALL

“Is Wolff–Parkinson–White a sausage company or a law firm?”

RESEMBLANCE TO OTHER ARRHYTHMIAS

In sinus rhythm, preexcitation can be confused with sinus rhythm with a bundle branch block. With preexcitation, the key is the short PR and the delta wave, with the terminal part of the QRS being narrow. During a tachycardia all bets are off; in atrial fibrillation your only clue to the presence of preexcitation may be that it is particularly rapid, with a mix of narrow and wide beats, depending on which pathway conducts the impulse. During other tachycardias it may be very hard to tell if there is underlying preexcitation.

CARE AND FEEDING

If the patient is in sinus rhythm, you don’t have to do anything except maybe make an appointment for him or her to see an electrophysiologist, since some of these patients are actually at risk for sudden death from their rhythm disturbances, especially rapid atrial fibrillation. If the patient is known to have preexcitation and is actually in a tachycardia, especially atrial fibrillation, like Siberian tigers or polar bears it is best left to the experts to care for. Many of the responses to pharmacology are paradoxic; if there was an emergency and the patient was unstable, probably the safest thing to do is just to cardiovert the rhythm back to sinus rhythm without getting sucked into the murk of odd drug reactions.

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