Define Anaphylactic Shock. Describe its clinical features and management.

The Mechanism

Anaphylactic shock is a severe form of distributive shock from an immediate (type I) hypersensitivity reaction. Prior sensitisation produces IgE that coats mast cells; on re-exposure, the allergen cross-links this IgE and triggers explosive release of histamine and other mediators, causing at once vasodilatation (hypotension), increased capillary permeability (fluid leaks out of vessels) and bronchospasm with mucosal oedema (airway compromise). An 'anaphylactoid' (non-IgE) reaction gives an identical picture through direct mast-cell activation (e.g. Contrast) without prior sensitisation — treatment is the same. Common triggers: drugs (antibiotics, muscle relaxants), latex, contrast, colloids, stings and foods.

Clinical Features

Onset is rapid:

• flushing, urticaria and angioedema of the lips/face
• stridor from laryngeal oedema and wheeze from bronchospasm with hypoxia
• hypotension/shock, often with a sense of impending doom. Severity ranges from skin-only to airway compromise and collapse — any respiratory or cardiovascular involvement mandates immediate adrenaline.

Recognition & Pitfalls

The diagnosis is clinical — rapid airway/breathing/circulation problems, usually with skin changes, after a likely trigger. Do not confuse it with a simple vasovagal faint (bradycardia and pallor but no urticaria, wheeze or angioedema). In the anaesthetised, draped patient, unexplained hypotension, high airway pressures or a rash may be the only clues.

Management — Adrenaline First

The life-saving action is immediate intramuscular adrenaline, which reverses the whole pathology: via α-action it vasoconstricts (raising BP, reducing oedema) and via β-action it bronchodilates and stabilises mast cells. Then remove the trigger, give high-flow oxygen, lie the patient flat with legs raised, and give a rapid IV fluid bolus. Antihistamines and corticosteroids are second-line adjuncts, not substitutes. Observe for a biphasic reaction (recurrence hours later); send mast-cell tryptase, and on discharge provide an adrenaline auto-injector and allergy referral.

The Resuscitation Sequence

Recognise anaphylaxis + remove the trigger; call for help → IM adrenaline into the anterolateral thigh (repeat every 5 min) → Airway:

• high-flow oxygen
• prepare for early intubation if laryngeal oedema → Positioning: lie flat, legs raised (sit up only if breathing is the priority) → Rapid IV crystalloid bolus for hypotension → Refractory case: IV adrenaline infusion + senior/ICU help → Adjuncts (antihistamine, hydrocortisone)
• observe for a biphasic reaction

Refractory Anaphylaxis

A minority do not respond to repeated IM adrenaline and fluids. These patients need an IV adrenaline infusion under continuous monitoring by an experienced clinician, further fluid, and additional agents; patients on beta-blockers may respond poorly to adrenaline and can benefit from glucagon. This is why help is called early and the patient managed in a resuscitation area.

The Mediators & Severity Grading

The clinical picture maps onto the released mediators:

• histamine causes vasodilatation, vascular leak and bronchospasm
• leukotrienes and prostaglandins intensify bronchoconstriction and leak
• tryptase is a useful diagnostic marker measured during and after the episode. Severity is graded from mild (skin/mucosa only), through moderate (respiratory or cardiovascular involvement — dyspnoea, wheeze, presyncope), to severe (hypoxia, hypotension, collapse). Prevention matters as much as treatment — take an allergy history before any prescription or anaesthetic, label allergies clearly, and refer previous reactors to an allergy service.

• Adrenaline (IM) — adult 0.5 mg (0.5 mL of 1:1000), anterolateral thigh; repeat every 5 min as needed.
• IV fluids — rapid crystalloid bolus for hypotension; high-flow oxygen; adjuncts (antihistamine, hydrocortisone).

Intramuscular adrenaline precedes all other treatment.

• Distributive shock from type I (IgE) hypersensitivity → mediator release → vasodilatation + capillary leak + bronchospasm; anaphylactoid = non-IgE, same picture.
• Triggers: drugs (antibiotics, muscle relaxants), latex, contrast, colloids, stings, foods.
• Rapid urticaria/angioedema + stridor/wheeze + hypotension; distinguish from vasovagal faint.
• IM adrenaline first-line and life-saving (α → vasoconstriction, β → bronchodilation): 0.5 mg 1:1000, repeat q5min.
• Add O2, positioning, IV fluids; antihistamine/steroid adjuncts; observe for biphasic reaction; tryptase + auto-injector + allergy referral.