Sunday, September 4, 2011

Anaphylactic reactions - 5 things.

I'm a little concerned that the treatment of anaphylaxis is not as well understood by EM folks as it should be. Now, in the acute management of ACS, you can be forgiven for not being familiar about the latest literature regarding the various antithrombotics, the merits of different DES, the proper initial dose of clopidogrel, etc. Too many trials, too much evidence.

But in anaphylaxis. there is hardly any evidence! It's possible to review the entirety of the best evidence in one pithy bog post, so let's get on with it.

First, get this paper. It's the best reference for current diagnosis and treatment of anaphylaxis out there.

Okay, 5 things:

1. Epinephrine is given IM (intramuscular), and in the lateral thigh. Not IM in the deltoid, not SQ in the buttocks. IM in the thigh, period.

The first time I ordered epi this way, the conversation with the nurse went something like this:
"Okay, let's give some epi. I want 0.5 mg IM in the thigh."
"No prob, I already drew it up." (Holds up syringe with 1/2 inch long needle on it)
"Uh, I don't think that'll make it to the muscle..."
"Well, it's only supposed to be SQ, right?"
"No, I want 0.5 mg epi, IM, in the thigh."
"Sure." (attaches longer needle, then rolls up patient's shirt sleeve, exposing the deltoid)
"Hold on a moment - I want that to go in the thigh, not the deltoid."
"You sure? We always do it this way."
"Yes, I want 0.5 mg epi IM in the thigh.
"Okay, you're the doc." (Turns around, eye rolling.)

Just in case you didn't catch it, epinephrine is given IM in the lateral thigh. Quiz later.

Okay, if this is the only thing you learned today, I'm happy. But let's look at the evidence for this, because it's beautiful. Some investigators decided to pay some healthy folks to get epi shots or saline injections. They were administered either IM or SQ, and in the area of either  the deltoid, or lateral thigh. They then measured the serum epi levels found in each of the possible combinations.

So elegant, I could cry. If they didn't give the epi IM in the thigh, the bump in serum epi was no different than placebo. Remember as well, these are healthy volunteers, without any cutaneous vasoconstriction.

2. The dose is 0.01 mg/kg, maxing out at 0.5 mg.  You can repeat this ever 5-15 minutes PRN.

3. The antihistamines and steroids are given only for the (putative) biphasic reaction, and have only a minor role in  acute management. A lot of folks think that this biphasic reaction doesn't even exist, and that "relapses" that occur in the following day or two just represent an incompletely treated MONOphasic reaction. In fact, if you look at this trial that involved giving bee stings to people who had had anaphylactic reactions to bee stings in the past, there is no mention of steroids or antihistamines.

I'm not even bothering to put in the Cochrane Collaboration analyses on the use of these adjunctive measures - both say "No evidence."

4. IV fluids.
They may not have as big a role in trauma these days, but when it comes to anaphylaxis, you want large-bore, proximal IVs, with 2 bags of saline hanging.  A patient in true anaphylactic shock not only has a distributive shock, they actually can lose up to 33% of their intravascular volume in the the initial few minutes. This is the time to break out the 14g catheters!

5.  Intravenous epinephrine
Here  we go - the patient's pressure is tanking, their thigh looks like a pincushion from all the IM epi you've given, and you have the 3rd and 4th liters of saline running in on pressure bags. Time for the IV epi! How do you do this without causing more problems than you already have? Try the 1-1-1 method, probably the best way to give enough, but not too much.

First, grab 1 mg of epinephrine - any concentration will do, the number work out with either the 1:10 or 1:1000 versions.

Next, put this in 1 liter NS.

You'll have a concentration of 1mcg/ml, and you want to start at 1 ml/minute, and titrate up quickly. In the protocol described in the bee sting study above, they would start off at 5 mcg/minute, but then it turns into the 1-1-5 rule. Just titrate up quickly.

Okay, a final word: Isn't epi dangerous? Well, yeah, if you give too much! There are a mess of case reports that show all manner of badness from epinephrine, but most of them appear to result from extraordinary doses.

For example, look at this case: a young adult was given what they describe as "low-dose" IV epi, and subsequently bumped her cardiac enzymes, despite a clean cath. However, what they call "low-dose" was actually 100µg IV, or 20 times the dose that the bee-sting protocol started with, and 100 times what we describe in this post!

Anyway, just remember that you want epinephrine, 0.01mg/kg, given IM, in the lateral thigh!

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