Local Anesthetic Allergy

Rarely, a patient presents with a history of being allergic to lidocaine or another kind of local anesthetic. Often, the "allergic" reaction described (racing heart, light-headed, dizziness, etc) is more attributable to the epinephrine which is often used in conjunction with the anesthetic; therefore not a true allergy, but a not unexpected possible side effect.

Also, if a true allergy is present, it may not be to the anesthetic itself, but the preservative contained within, typically metabisulfite and/or methylparaben. The exception are single-dose vials which are preservative-free, unless they contain epinephrine. Any local anesthetic containing epinephrine, whether a single-dose or multi-dose vial, has metabisulfite in it. The preservative in the multi-dose vial of lidocaine and other anesthetics typically contain methylparaben. As such, the muti-dose vial of lidocaine with epinephrine would contain both metabisulfite and methylparaben.

In any case, if a true allergy to the anesthetic itself is present, there is the possibility that another type of anesthetic can be used. Why?

There are two different classes of local anesthetics divided into amino esters and amino amides. Chemically, amino amides have an amide link between the intermediate chain and the aromatic end, whereas amino esters have an ester link between the intermediate chain and the aromatic end.

Amino Ester

Amino Amide

There are other differences. Pharmacologically, amino esters are metabolized in the plasma whereas amino amides are metabolized in the liver. Amino esters are unstable in solution, but amino amides are very stable in solution.

Amino esters are also much more likely than amino amides to cause allergic hypersensitivity reactions.

Amino Esters (only one "i" in the spelling of the name):

• Cocaine
• Procaine
• Tetracaine (in the form of cetacaine)*
• Chloroprocaine*
• Benzocaine (in the form of cetacaine)*
• Cetacaine (benzocaine and tetracaine)*
• Propoxycaine

Amino Amides (two "i"'s in the spelling of the name):

• Lidocaine*
• Mepivacaine*
• Prilocaine
• Bupivacaine*
• Ropivacaine*
• Levobupivacaine
• Articaine
• Etidocaine
• Trimecaine
• EMLA (lidocaine/prilocaine)

* is next to those commonly carried in hospitals.

Much appreciate the assistance of Margaret Rowe, PharmD who helped write this blog article.

Source:
Local Anesthetics. Emedicine 4/19/13

Local Anesthetic. Wikipedia

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FDA Approves New Epinephrine Injector That Talks

Sanofi US has introduced a new epinephrine injector (competitor to the more commonly known EpiPen made by Dey Pharma LP) that is equipped with audio and visual cues that helps a patient or care-giver administer epinephrine in cases of a severe allergic reaction (anaphylaxis).

Known as Auvi-Q, it was FDA approved Aug 13, 2012, and should soon become available for purchase by prescription.

Just like EpiPen, Auvi-Q comes in two different dosages of epinephrine... 0.3mg for patients who weigh more than 66 pounds and 0.15mg for those who weigh between 33 and 66 pounds. Each pack contains two devices with active drug plus a non-active training device.

During a life-threatening allergic reaction, Auvi-Q talks the patient or care-giver through each step of the injection process. If more time is needed, it can repeat the instructions step-by-step. The device also comes with written instructions printed on the side. In addition, there are visual cues with an alert light to signal when the injection is complete. Watch video.

Auvi-Q is about the size and shape of a credit card, the thickness of a cell phone and fits comfortably in a pocket or small purse. This size and shape is in contrast to the EpiPen which looks like a fat marker. Being compact and easy to carry should remove one obstacle that could be a reason why as many as two-thirds of patients do not carry their epinephrine injector device.

Like the EpiPen, the device also has an automatic retractable needle mechanism to prevent accidental needle sticks.

Source:
FDA Approves Auvi-Q Voice Activated Epinephrine Injector. EmaxHealth 8/13/12

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Why Does EpiPen Have to be Injected into the Thigh?

As any severely food allergic patients know, when suffering from a potentially life-threatening anaphylactic reaction, they need to jab an epinephrine needle into the side of their thigh...

But... why the thigh? Why not the arm like most other shots?

Per AAAAI, there have been 3 anatomic sites investigated to determine which location offers the fastest systemic absorption of epinephrine when injected intramuscularly and/or subcutaneously.

• Intramuscular injection in the upper arm (deltoid muscle) just like a flu shot.
• Subcutaneous injection in the upper arm just like an allergy shot.
• Intramuscular injection in the lateral thigh (vastus lateralis muscle).

Of these sites, the intramuscular thigh injection demonstrated the fastest rise in blood epinephrine.

When dealing with a life-threatening anaphylaxis, you need the speed because if the drug is too slow, the person may die before adequate epinephrine levels are achieved... and the thigh muscle provides the speed whereas the other sites do not.

Why might that be?

It's probably because the thigh muscle is large with lots of blood vessels allowing for multiple points of access for the injected epinephrine to enter into the blood circulation.


References:
Epinephrine absorption in children with a history of anaphylaxis. J Allergy Clin Immunology 1998; 101(1):33-37.

Epinephrine absorption in adults: intramuscular versus subcutaneous injection. J Allergy Clin Immunol 2001; 108(5):871-873.

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