You gave adensoine for THAT? (part 3)

To recap - the paramedic had a patient who was throwing off long segments of both narrow-complex tachycardias, and wide-complex tachycardias. An initial dose of adenosine managed to transiently establish a sinus rhythm.

Sinus rhythm for 3 whole beats! *Pumps fist in air*

So what was converted; SVT or VT?
It's tempting to think that, beacause it was converted by adenosine, it must have been a SVT with aberrant conduction. A few things suggest that this was, instead, true VT.

Playing the odds...
First, a wide-complex rhythm in an older patient is far more likely to be VT than SVT - if you're betting, that's where yo should put your money. 

Your instructor was right - axis is important!
Second, you could be tempted to call this a left bundle branch block, but the axis is unusual. A LBBB usually points off towards the left.

Note the large R waves in I and aVL (Source)

Our patient, on the other hand, shows an axis oriented a bit to the right.

Note the large R waves in II, III, and aVF

Fusion beats
Third, the patient keeps kicking out fusion beats, or QRS complexes that show features of both a supraventricular beat, and a simultaneous ventricular beat. They usually look like an "average" of a PAC and a PVC. Here's an example from Life in the Fast Lane:

Not wide, not narrow, just ... average.
(source)

 When we look at the initial rhythm strip we see a bunch of sorta wide/sorta narrow beats.

Another ECG, done in the ED, also shows multiple fusion beats, indicated by the arrows.

Such a finding strongly suggests that this patient had ventricular tachycardia that was converted by adenosine. Crazy, right?

Well, it turns out that not everything that converts with adenosine is SVT!  One infrequent kind of VT can convert with adenosine - right ventricular outflow tachycardia.

RVOT
A fairly uncommon kind of VT is know as RVOT, named for the location of the problem. It basically looks like a LBBB, but with an inferior, rather than a left-oriented axis. A great example comes courtesy of Dr. Ken Grauer. Compare it with the initial 12-lead we have.

RVOT - from Dr Gauer's collection.

I'm not going to go on further about this rhythm, since it's pretty uncommon, but if you want to know more go check out Dr Grauer's blog.

(I'll note again that my cardiology education started when I read the 1993 edition of his ACLS textbook. He has a 2014 update, available on a variety of e-readers. Buy it!)

Uh, so there's this rare RVOT thing. Why should I care?
As I mentioned in Part 2, there are two important things to know about giving adenosine for a WCT:

1. Adenosine is relatively safe in regular, monomorphic WCT.
2. However, it can convert certain types of VT.

Since the publication of the 2010 ACLS Guidelines, a large number of EMS agencies have adopted adenosine as the first agent to administer in the treatment of a stable, regular, and monomorphic wide-complex tachycardia. I don't have any hard numbers on how common this protocol is, but instead of a study, I can give you a collage! 

I makes it for u.

Despite the popularity of this protocol, many clinicians have reservations about the wisdom
of this approach, since there are a number of potential pitfalls. One of the big ones would be to assume that adenosine is a reliable test for supraventricular rhythms. Keep in mind that he ACLS guidelines only state that "adenosine is relatively safe for both treatment and diagnosis."  

Safe, not accurate!

The Bottom Line
The SHCGB protocols allow for the use adenosine when the etiology of a WCT isn't clear. Keep in mind that, even if adenosine is successful, the etiology may still be unclear!