Anaphylactic (allergic) shock is also referred to as anaphylaxis. As the clinical features present within a few minutes of exposure to an antigen it is classified as a severe form of type 1 hypersensitivity reaction. It is an extreme abnormal allergic reaction to a drug or other substance introduced into the body. Reactions usually present suddenly, within seconds to a few minutes after exposure to the antigenic substance. However, reactions delayed by up to half an hour may occur. When a person is first exposed to a substance to which they are hypersensitive, the B lymphocytes will produce antibodies. These antibodies become attached to mast cells. On subsequent exposure to the antigenic substance the antigen will combine with these antibodies causing the mast cells to release large amounts of histamine and other inflammatory and vasodilatory substances into the blood and tissue fluids. Histamine is a powerful vasodilator and bronchoconstrictor. Arteriole vasodilation reduces the peripheral resistance and therefore blood pressure. In addition to this the capillaries become more permeable so fluid leaks from the blood into the tissues, leading to oedema and hypovolaemia. Pulmonary oedema may also develop. Heart rate will usually increase in an attempt to compensate for the hypotension
Clinical features
Often the first indication of a developing reaction is patient anxiety and unease. This has been described as a feeling of impending doom. The severity of reactions may vary considerably from skin irritation and a feeling of unease to complete collapse. Indeed in young children the collapse has been of such severity that the child becomes completely flaccid, so called ‘rag doll’ syndrome. In addition to the hypotension and bronchospasm already described, angioedema may develop. This may affect the face, tongue and larynx resulting in progressive occlusion of the upper airway, compounding the respiratory embarrassment caused by the bronchospasm. These respiratory problems will lead to wheezing, distress, stridor and cyanosis. Because pathological vasodilation is caused by histamine and other substances, anaphylaxis often causes patchy or global redness of the skin. In addition to redness, intensely itchy urticarial wheals may develop. Sneezing and other irritation of the respiratory tract may be a feature. Young children rarely, if ever, faint after a medical procedure such as a vaccination, so any case of collapse in children will be organic in nature.
Adults however frequently faint, and so this is the most likely cause of acute unconsciousness. In a faint the patient regains consciousness very quickly when lying flat and there is no redness or wheals on the skin. A central pulse is maintained during a faint or convulsion. Central pulses should be palpated for 5 to 10 seconds as there is often a bradycardia during a faint. Anaphylactic reactions are more common in people with a history of allergy or previous reactions; there may also be a history of asthma. There is often a history of previous localized allergic reactions to the offending antigen. Gaining information about an individual’s allergies and any previous abnormal reactions
is therefore a vital part of a patient assessment. However, almost any agent may cause anaphylaxis in idiosyncratically sensitive individuals and present without warning.
Management principles in anaphylaxis
The causative agent should be identified and if possible discontinued or removed. Airway patency must be established and high concentrations of oxygen should be given if available. The patient should lie flat to maximize cerebral circulation. Intravenous fluids may be needed to improve blood pressure. If there is nocardiac output the situation may present as a cardiac arrest and then should be treated as such. Epinephrine (adrenaline) should be given promptly via deep intramuscular injection, normally an initial dose of 0.5mg for an adult with a corresponding reduced dose for children. This dose may be repeated after 5 to 10 minutes if indicated. Epinephrine is a potent bronchodilator and vasoconstrictor; it is therefore capable of reversing the principle effects of
histamine. (Inadvertent intravenous injection of a bolus dose of epinephrine may well lead to ventricular fibrillation so is one to avoid.) Chlorpheniramine (piriton), hydrocortisone and salbutamol may also play a role in management.
All patients who have had an anaphylactic reaction should be automatically admitted to hospital for review by a physician. Once the causative agent is identified patients must be instructed to avoid it in future. Patients should also be issued with self-injectible epinephrine and they and their relatives should be taught how and when to administer this. MedicAlert bracelets should be worn. Alert stickers should be placed on patients’ notes and prescription charts.
6 авг 2024
