Anaphylaxis

Anaphylaxis, often called anaphylactic shock, is an acute allergic reaction caused by exposure to an antigen to which the patient is hypersensitive. It is characteristic of the condition that significant distress can begin within short minutes of exposure. While the patient may be aware of the hypersensitivity, it may trigger on a first exposure, as to an insect sting.

Signs of the reaction can include a raised red rash ("hives", urticaria), respiratory distress, cardiovascular failure, shock and death. While anaphylaxis can progress at a slower rate, it can take effect with frightening speed. Patients aware of sensitivities that could trigger anaphylaxis should carry an epinephrine autoinjector, and other recommended drugs. Emergency responder doing triage must place these patients in the highest priority, because the condition is usually controllable with prompt treatment.

Pathophysiology
"Anaphylaxis" includes to mast cell and basophil degranulation, triggered either by an immunoglobulin E (IgE) mediated "anaphylactic reaction", or by degranulation mediated by non-IgE factors, or "anaphylactoid reaction".

In either case, the released cytokines include histamine, which increases blood vessel permeability, as well as contraction of smooth muscles, such as the bronchi. The increased vascular permeability can cause shock by shifting as much as 50% of the body's fluids to the extravascular compartment.

Evaluation
The patient, if conscious, will be apprehensive and appear seriously ill. If the patient or a companion can inform the treating clinician of known anaphylaxis, this is critical information. If there is no informant, search for an identifying bracelet, necklace, or identification card.

Assuming it is possible to get history, key questions included the existence of cardiac disease or hypertension, and if the patient is taking beta-adrenergic antagonists for any reason.

Bystander history of a possible trigger, sudden onset, and rapid progression is informative. The presence of urticaria or general flushing, hypotension, bronchospasm, laryngeal edema, back pain, dilation of the pupils, and convulsions, or combinations of them, are warning signs; the patient is at risk for immediate death. Cardiac arrest, in suspected anaphylaxis, calls for vigorous resuscitation since it is a potentially reversable condition.

Treatment
Patients in anaphylaxis should be transported by advanced life support capable ambulance when available. If the trigger point of entry can be identified (e.g., a bee sting), and it is on an extremity, a tourniquet, released every 5 minutes, can be applied. If a stinger or other source is present, remove it as quickly as possible.

Epinephrine is the critical component of therapy. While some physicians are reluctant to administer it due to possible side effects, only the most extreme and confirmed reasons can justify withholding it. In mild to moderate cases, it is given intramuscularly in 1:1,000 solution, 0.3 to 0.5 ml for adults. Repeat every 5-10 minutes until the signs disappear. Obvious care must be taken in patients with cardiac or hypertensive disease, but a patient is likelier to die quickly from anaphylaxis than the effects of epinephrine.

In all cases, intravenous access should be gained immediately, giving adults 500 ml to 1 L of normal saline over 30 minutes, with additional fluids given to support the clinical situation, which is likely to involve shock. Wnen anaphylaxis is severe, give epinephrine intravenously or by endotracheal tube, 1.0 ml. of 1:10,000 solution. A continuous infusion may be needed.

note well: 1:1,000 solution intramuscular, 1:10,000 intravenous or endotracheal

Antihistamines support epinephrine, but do not replace it. The combination of a histamine H1 antagonist (e.g., diphenhydramine) and H2 antagonist (e.g., ranitidine) has been demonstrated to be superior to a single type.

In the presence of laryngeal swelling, nebulized epinephrine may help, but evidence of such swelling will usually justify epinephrine. Bronchospasm, if present, should be treated with a nebulized short-acting β2 agonist such as albuterol

While they will not have a short-term effect, corticosteroids should be started early in the incident; anaphylaxis may have a late-phase component.

It has become understood that maintaining adequate blood pressure can be the most difficult part of treatment, in an intensive care unit if the episode is prolonged. Fluid replacement is the first part of therapy for hypotension, which may require very aggressive volumes of saline or other crystalloid. Fluids must be individualized, based on blood pressure and urine volume; in severe cases, invasive monitoring of cardiac output and central venous pressure are appropriate.

Pharmacologic support of blood pressure can include continuous epinephrine infusion, as well as dopamine. This is one of the few remaining indications for military antishock trousers. .

If the patient is taking a beta-blocker, its effects must be reversed. There is anecdotal and animal evidence that glucagon can be helpful. It also provides inotropic effects and chronotropic effects on the heart by increasing intracellular levels of cAMP.

Prevention
Obviously, the patient should avoid antigens to which they know they are sensitive. If, for example, there is hypersensitivity to latex, all medical personnel should be informed, and the patient should wear an alerting bracelet or necklace. When insect stings are a trigger, the patient must take precautions to avoid attracting insects while outside, such as wearing strong perfumes.

It is also advisable to carry an emergency drug kit, which minimally consists of an epinephrine autoinjector, and perhaps antihistamines.

Allergic desensitization should be considered seriously if feasible.