In medicine, anaphylaxis is a severe and rapid systemic allergic reaction. Anaphylaxis occurs when a person is exposed to a trigger substance, called an allergen, to which they have become sensitized. Minute amounts of allergens may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, inhalation, skin contact or injection of an allergen. The most severe type of anaphylaxis—anaphylactic shock—will usually lead to death in minutes if left untreated.
Anaphylactic shock is a life-threatening medical emergency because of rapid constriction of the airway, often within minutes of onset. Calling for help immediately is important, as brain damage occurs rapidly without oxygen. Anaphylactic shock requires advanced medical care immediately; but other first aid measures include rescue breathing (part of CPR) and administration of epinephrine. Rescue breathing may be hindered by the constricted airways but can help if the victim stops breathing on their own. If the patient has previously been diagnosed with anaphylaxis, they may be carrying an EpiPen (or similar device) for immediate administration of epinephrine (adrenaline) by a layperson to help keep open the airway. Repetitive administration can cause tachycardia (rapid heartbeat) and occasionally ventricular tachycardia with heart rates potentially reaching 240 beats per minute, which can also be fatal. This is why some protocols advise Intramuscular injection of only 0.3–0.5mL of a 1:1,000 dilution. The epinephrine will prevent worsening of the airway constriction, and may be life-saving.
Epinephrine will act on Beta-2 adrenergic receptors in the lung as a powerful bronchodilator relieving allergic or histamine-induced acute asthmatic attack or anaphylaxis. Tachycardia results from stimulation of Beta-1 adrenergic receptors of the heart increasing contractility (ionotropic effect) and frequency (chronotropic effect) and thus cardiac output.
Symptoms of anaphylaxis are related to the action of immunoglobulin E (IgE) and other anaphylatoxins, which act to release histamine and other mediator substances from mast cells (degranulation). In addition to other effects, histamine induces vasodilation and bronchospasm (constriction of the airways).
Symptoms can include the following:
- respiratory distress,
- hypotension (low blood pressure),
- urticaria (hives),
- flushed appearance,
- angioedema (swelling of the face, neck and throat),
- tears (due to angioedema and stress),
- itching, and
- anxiety, including a sense of impending doom
The time between ingestion of the allergen and anaphylaxis symptoms can vary for some patients depending on the amount of allergen ingested and sensitivity. Symptoms can appear immediately, or can be delayed by half an hour to several hours after ingestion. However, symptoms of anaphylaxis usually appear very quickly once they do begin.
Common causative agents in humans include:
- foods (e.g. milk, cheese, nuts, peanuts, soybeans and other legumes, fish and shellfish, wheat and eggs);
- drugs (e.g. penicillin and other cephalosporins, contrast media, ASA and other NSAIDs such as ibuprofen and diclofenac);
- Hymenoptera stings from insects such as bees, wasps, yellow jackets, hornets, and some stinging ants; and
- exercise (see exercise-induced anaphylaxis).
The anaphylactic reaction is mediated by antibodies. Anaphylactic sensitivity can be transferred to a normal (non-sensitive) recipient by means of serum containing such antibodies. This is then called passive transfer of the allergy or hypersensitivity.
The source of antibody may be from the same species, e.g. the human species, for which the predominant immunoglobulin involved is of the IgE class. Some known cases of deaths resulting from passive transfer involve organ transplants. It may also be transferred from animals of a different species. For example, a guinea pig may be sensitized by an intravenous injection of rabbit antibody to ovalbumin. When challenged (that is, brought in contact) with ovalbumin 48 hours later, the guinea pig will suffer fatal anaphylactic shock.
Paramedic treatment in the field may include injection with epinephrine, administration of oxygen therapy and, if necessary, intubation during transport to advanced medical care. In profuse angioedema, cricothyroidectomy or tracheotomy may be required to maintain oxygenation.
The clinical treatment of anaphylaxis by a doctor and in the hospital setting aims to treat the cellular hypersensitivity reaction as well as the symptoms. Antihistamine drugs (which inhibit the effects of histamine at histamine receptors) are given but are usually not sufficient in anaphylaxis, and high doses of intravenous corticosteroids are often required. Hypotension is treated with intravenous fluids and sometimes vasoconstrictor drugs. For bronchospasm, bronchodilator drugs (e.g. Salbutamol, known as Albuterol in the United States) are used. In severe cases, immediate treatment with epinephrine can be lifesaving. Supportive care with mechanical ventilation may be required.
Patients must be monitored for four hours after being transported to medical care for the possibility of biphasic reactions (recurrence of anaphylaxis) .
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