Can you give adenosine to a patient with WPW?

Maybe you can answer the question in the title completely and confidently. If so, feel free to skip this post, and go on to some more entertaining corner of the web. However, it seems to me that many emergency providers are unsure about how to approach a confusing issue with adenosine:

Can (or should) you give adenosine to a patient with known or suspected WPW?

Spoiler: The answer to this questions is

More specifically:

1. Adenosine is safe and effective for terminating SVT (narrow or wide) caused by WPW, but...
2. Giving adenosine for WPW with atrial fibrillation, however, can be lethal.

One recent case that was treated in the field illustrates principle #1. Fortunately, no local medics have provided us with a demonstration of principle #2!

(As always - Follow your local protocols for WPW and arrhythmias!)

Can you give adenosine to a patient with WPW?
Case report:
This is best illustrated with a case! Paramedic Harvey had a patient with palpitations, tachycardia, and a good BP, and she obtained the following ECG.

A narrow-complex, regular tachycardia in a stable patient? Looks like an SVT!  Medic Harvey decided, appropriately, to bust out the adenosine.

A rapid bolus of 6 mg broke the tachycardia, and restored NSR on the monitor, but the follow-up 12-lead looked a little odd:

A few minutes later a second post-conversion ECG was obtained.

Huh. You know, that sort of looks like...

It was at this point that the patient told Heather 

"I probably should have told you that I have WPW."

Wait, what?!
Oooooo.... Isn't WPW supposed to be a dangerous rhythm, where you can send the the patient into VF just by thinking about adenosine? Holy cow, did the medic just get lucky, or what?

Brief Review of WPW
Let me go over the pathophysiology briefly. 
(For a fuller description, check out a great new e-book by Ken Grauer, entitled ACLS - 2013 - ePub, for more education on WPW and other topics in ECG interpretation and ACLS.)

The 1993 edition was pure gold when I was in paramedic school.
It's only gotten better - seriously, download this now!

There are 2 common causes of a regular narrow-complex tachycardia, or SVT. 

• The most common is AV nodal reentrant tachycardia (AVNRT), which involves a "loop" of electrical current that is confined within the AV node. These are narrow unless aberrant conduction occurs.
• By contrast, AV reentrant tachycardia (AVRT), involves a "short circuit," or accessory pathway, between the atria and the ventricles that bypasses the AV node. During normal sinus rhythm, the ECG will often demonstrate signs of "pre-excitation" of the ventricles; a short PR, a delta wave, and a wide QRS.

Grauer K: ACLS-2013-ePub, KG/EKG Press (available in kindle/ibooks/nook/kobo)

When an episode of AVRT (aka SVT) is triggered, however, the QRS will usually be narrow, and the delta wave will disappear (as seen in Panel A below). As noted in Panel B, however, in a rare minority of patients the circuit of conduction will be "backwards," and produce a wide QRS.

Grauer K: ACLS-2013-ePub, KG/EKG Press (available in kindle/ibooks/nook/kobo)

So, there are some important differences between the two main causes of SVT (AVNRT and AVRT). It's important to emphasize, though, that...

AVNRT and AVRT have 3 important things in common:

• Both rely on the AV node to complete the "loop" of electrical current that generates the tachycardia;
• In both, the sinus node is generating normal, controlled signals; i.e., 60 - 100 beats per minute.; and 
• Adenosine will terminate the reentrant thythm in both by shutting down the AV node.

In AVNRT, adenosine effectively "cuts" all electrical connections between the atria and ventricles (temporarily!). In AVRT/WPW, however, the bypass tract is unaffected by the drug, and is still capable of conducting electrical signals to the ventricles. This could potentially lead to sinus impulses being conducted anterograde down the AP, producing wide, "aberrant," QRS complexes, but at a controlled rate (i.e. 60-100 bpm).

WPW with AF can be lethal if treated with adenosine
This is entirely different when the patient has WPW and atrial fibrillation. Remember that, in AF, the atria are firing off at 300 - 400 times a minute, not the controlled rate of 60-100 that the SA node generates. 

So, if these all of these electrical impulses are transmitted to the ventricles through the bypass tract, the myocardium will freak out. Like this.

Source: Shah 2001

WPW without AF is safe to treat with adenosine
We probably don't need to worry*, however, about giving adenosine in patients with WPW, but without AF. There's a few reasons why.

1) First off all, about a third of the SVTs that you have given adenosine to in the past were actually caused by AVRT/WPW, just based on the epidemiology. Since practically none of these developed VF (right?), this is apparently a very safe practice.

2) Second, there was a concern in the past that a certain percentage of wide-complex tachycardia were actually WPW with antidromic conduction, and so the advice was to avoid adenosine. The rationale was that since the bypass tract was capable of retrograde conduction, shutting down the AV node could "expose" the ventricles to potentially unregulated pacing. However studies such as this one have convinced a number of people, including the AHA, that adenosine is pretty safe in anyone with a (regular!) wide-complex tachycardia. 

3) Third, many cardiology experts believe that adenosine is safe in regular WCTs. The authors of a 1991 study found it to be quite safe in this setting, and concluded that 

"Adenosine may cause acceleration of preexcited atrial arrhythmias, but these effects are transient and should not discourage the use of adenosine as a diagnostic agent in broad complex, regular tachycardias of uncertain origin."

Dr. Stephen Smith, of Dr Smith's ECG Blog fame, believes that we don't even need clinical evidence to prove that adenosine is safe in regular tachycardias, wide or narrow, in a person with a history of WPW. A fundamental understanding of cardiac physiology is enough to sh0w this:

"Anyone who knows what AVRT is, and what it is that electrical circuits do, knows that it is safe to give adenosine. If you ask an electrophysiologist for an article on this, they will say 'There are no articles, because this is so obvious that it needs no proof.'

[A]denosine is safe in VT. One need not prove that it is safe in AVRT ."

The Bottom Line
So it turns out that adenosine was quite safe with Harvey's patient, as well as effective. This probably wasn't a fluke!

Although many medics have been taught that Wolff-Parkinson-White (WPW) and adenosine are a dangerous combination, this isn't often the case. On the contrary - it seems likely that medics and physicians have frequently given adenosine to patients with undiagnosed WPW, without apparent ill effects. 

Furthermore, the recent emphasis on giving adenosine for regular wide-complex tachycardias of uncertain etiology makes it even more likely that YOU will give adenosine to someone with undiagnosed WPW. A small number of those WCTs represent WPW with antidromic conduction, but adenosine administration has not been documented to cause any problems to date.

So, follow your local protocols for arrhythmias and WPW, but keep in mind the available evidence, as well as expert opinion, suggests that adenosine is safe in most cases.
__________________________________
* Okay, there are a few, totally theoretic, reasons to worry about giving adenosine to patients with WPW. I mention these only out of a sense of completeness. 

First of all, adenosine is not totally benign, and it has been shown to induce atrial fibrillation in a number of case reports. Although the half-life is fairly short, it's conceivable that adenosine could first trigger AF in a patient with a history of WPW, and that the AF could then immediately degenerate into VF. One case report describes how a young woman with SVT developed AF after performing a Valsalva maneuver. Adenosine was then given, and the rhythm degenerated into a pulseless irregular WCT. Cardioversion restored a pulse and sinus conduction.

Second, although there are no case reports that I know of have shown an antidromic WPW tachycardia deteriorating after adenosine administration, this is a rare rhythm, and even a moderate relative risk of adverse effects from adenosine would produce a small absolute number of complications. For example, the study by Maril (that demonstrated the safety of using adenosine in WCTs) only enrolled 2 patients with a history of WPW. One of those patients was determined to have been having a ventricular (non-accessory pathway dependent-) rhythm upon enrollment. It's hard to determine the absolute safety of a drug for such a rare rhythm. 

Nonetheless, even a "worst-case scenario" suggests that an adverse event would be very rare.