Anaphylaxis: I can't breathe!

On an EMS ride along, you respond to a dispatch for a patient having an allergic reaction to some food.  You arrive on scene and find the patient having difficulty speaking and having stridor, in clear respiratory distress.

Clinical Question:

What is the ideal treatment of anaphylaxis?

Literature:

The lifetime prevalence of anaphylaxis is estimated between 0.5-2% with mortality rates ranging between 0.65%-2%, resulting in approximately 1500 deaths annually.  When fatal, death usually occurs minutes after contact with the trigger.  Fatal food reactions usually caused respiratory arrest 30-35 minutes after initial contact.

The first line treatment is epinephrine since it counters many of the symptoms of anaphylaxis.  As an alpha-receptor agonist, it reverses peripheral vasodilation and reduces edema.  The beta-receptor activity dilates bronchial airways, increases myocardial contraction, suppresses histamine and leukotriene release, inhibits mast cell activation.

There are several methods for epinephrine injection.  In a study comparing IM vs subcutaneously route, it found that an IM injection of epinephrine in the anterolateral aspect of the thigh achieved a higher and faster plasma epinephrine concentration peak.  IM injection into the deltoid of the arm as well as subcutaneous injection into the deltoid had lower and longer time to peak concentrations.  The greater blood supply to the vastus lateralis muscle is theorized to account for this difference.  The IM injection site also offers a greater margin of safety as well as ease of administration compared to IV epinephrine.  The IM dose of epinephrine can be repeated in 5-15 minutes if symptoms are not improving.  IV epinephrine has been associated with fatal cardiac arrhythmias and myocardial infarction and should be reserved for those unresponsive to conventional treatment in a controlled setting.

The recommended dose of epinephrine IM is 0.3-0.5mg of 1:1000 epinephrine (0.3-0.5 ml).  For pediatric patients, the recommended dosage is 0.01mg/kg.  Initial resuscitation should also include a bolus of normal saline given the vasodilatory effects of anaphylaxis.

Antihistamines are considered second line treatment.  H1 blockers such as diphenhydramine can be given to alleviate cutaneous symptoms, however it does not acutely treat the life threatening aspects of anaphylaxis.  There is little evidence to support the routine use of H2 blockers.  Steroids can also be given, although there is also weak evidence for their use.  In theory, steroids are used to prevent a biphasic reaction (when symptoms resolve, but recur within 1-72h) but there is little evidence that it is effective in reducing biphasic reactions.  Many providers will give a one time dose of methylprednisolone 1-2mg/kg in the ED, which is generally considered sufficient.  Bronchodilators can also be given if the patient is wheezing, experiencing dyspnea, or coughing.

Take home points:

-First line treatment for anaphylaxis is epinephrine 0.3-0.5mg IM in anterolateral thigh
-IV fluids should be given to counteract vasodilation
-H1 blocker may provide symptomatic relief, however is not first line therapy
-Bronchodilators may provide symptomatic relief
-H2 blockers have not been proven to be effective
-Steroids have not been proven to be effective

References:

1)Pumphrey RS. Lessons for management of anaphylaxis from a study of fatal reactions. Clin Exp Allergy. 2000;30(8):1144-1150
2) Soar J, Pumphrey R, Cant A, Clarke S, Corbett A, Dawson P, Ewan P, Foex B, Gabbott D, Griffiths M, Hall J, Harper N, Jewkes F, Maconochie I, Mitchell S, Nasser S, Nolan J, Rylance G, Sheikh A, Unsworth DJ, Warrell D. Emergency treatment of anaphylactic reactions – guidelines for healthcare providers. Resuscitation. 2008 May;77(2):157-69
3) Simons FE, Ardusso LR, Bilò MB, Cardona V, Ebisawa M, El-Gamal YM, Lieberman P, Lockey RF, Muraro A, Roberts G, Sanchez-Borges M, Sheikh A, Shek LP, Wallace DV, Worm M. International consensus on (ICON) anaphylaxis. World Allergy Organ J. 2014 May;7(1):9
4) Simons FE, Gu X, Simons KJ. Epinephrine absorption in adults: intramuscular versus subcutaneous injection. J Allergy Clin Immunol. 2001 Nov;108(5):871-3.
5) Zilberstein J, McCurdy MT, Winters ME. Anaphylaxis. J Emerg Med. 2014 Aug;47(20):182-7

Contributed by Steven Hung, PGY-2