Anaphylactic Shock

Anaphylactic shock is life threatening systemic hypersensitivity with circulatory failure (hypotension with or without airway and breathing compromise) caused by mediator release from mast cells and basophils. Diagnosis rests on sudden onset of skin or mucosal signs with airway, breathing or circulation compromise, or hypotension after a known allergen.

Epidemiology. Lifetime prevalence is about 0.3 to 5%. Common triggers are foods (peanut, tree nuts, milk, egg, shellfish), drugs (antibiotics, particularly beta lactams, neuromuscular blockers, NSAIDs, contrast media), insect venom and latex. Idiopathic anaphylaxis occurs in some patients.

Risk factors. Asthma, mastocytosis or high baseline tryptase, cardiovascular disease, beta blocker or angiotensin converting enzyme inhibitor use, alcohol, exercise, infection and older age.

Pathophysiology. Immunoglobulin E-mediated, immune complex or direct mast cell activation releases histamine, tryptase, leukotrienes and prostaglandins. Vasodilation and capillary leak can lose up to 35% of intravascular volume within minutes. Upper airway oedema, bronchospasm and myocardial depression add to the picture.

Presentation is urticaria, angioedema, flushing, itching, wheeze, stridor, voice change, abdominal pain, vomiting, diarrhoea, dizziness, tachycardia, hypotension, collapse and cardiac arrest. Skin signs are absent in about 10 to 20%. Under general anaesthesia, hypotension, bronchospasm and high airway pressures may be the only signs.

Red flags.

• Stridor, voice change or tongue swelling.
• Hypotension, collapse or altered consciousness.
• Beta blocker therapy (adrenaline resistance) and asthma.

Diagnosis is clinical. Do not delay adrenaline for tests. Measure serum tryptase as soon as feasible after resuscitation started, a second sample 1 to 2 hours after onset (within 6 hours), and a baseline value at least 24 hours later. A rise of more than 20% plus 2 microgram/L over baseline supports the diagnosis. Refer for allergy testing 4 to 6 weeks later.

Differential diagnosis. Vasovagal syncope, panic attack, acute asthma, septic shock, hereditary angioedema, scombroid poisoning, carcinoid, pulmonary embolism and myocardial infarction.

Immediate (within minutes).

• Remove the trigger when possible (stop infusion, remove sting).
• Lay the patient flat with legs raised (sitting up if breathless, and left lateral if pregnant). Do not stand them up suddenly.
• Adrenaline IM into the anterolateral mid thigh: adults and children over 12 years 0.5 mg (0.5 mL of 1 mg/mL), children 6 to 12 years 0.3 mg, 6 months to 6 years 0.15 mg, under 6 months 0.1 to 0.15 mg. Repeat every 5 minutes if no improvement.
• Oxygen 10 to 15 L/min by non-rebreather mask. Secure large bore IV access.
• IV fluid: 500 to 1000 mL rapidly, to a total of 20 mL/kg in children, then reassess. Large volumes may be required.

Refractory anaphylaxis (after 2 to 3 IM doses).

• Adrenaline infusion 0.05 to 0.1 microgram/kg/min IV (1 to 10 microgram/min in adults), titrated to effect, with continuous ECG and blood pressure monitoring. IV bolus 50 microgram (0.5 mL of 1 in 10,000) only by experienced clinicians in monitored settings.
• Noradrenaline or vasopressin 0.03 units/min if shock persists.
• Nebulised adrenaline 5 mg (5 mL of 1 mg/mL) for upper airway oedema, and salbutamol 5 mg nebulised for bronchospasm.
• Glucagon 1 to 5 mg IV over 5 minutes, then 5 to 15 microgram/min infusion, in patients taking beta blockers who do not respond to adrenaline. Expect vomiting, so protect the airway.
• Intubate early for stridor or airway oedema, by an expert with difficult airway equipment. Consider extracorporeal support in refractory arrest.

Adjuncts (do not replace adrenaline). Chlorphenamine 10 mg IV slowly (1 to 2 mg/kg is not used in children; 2.5 to 5 mg IV for 1 to 6 years), and hydrocortisone 200 mg IV. Neither prevents biphasic reactions with certainty.

Observation. Observe for at least 6 hours after symptoms resolve, and 12 to 24 hours for severe reactions, those needing more than one adrenaline dose, asthma, or poor access to care. Biphasic reactions occur in about 5% of patients.

Discharge. Prescribe two adrenaline auto injectors (0.3 mg for weight 30 kg or more, 0.15 mg for 15 to 30 kg), teach their use, give a written action plan, avoid the trigger, and refer to an allergist.

Monitoring. Continuous ECG, blood pressure, oximetry and airway observation.

Referral and escalation. Intensive care for refractory shock, stridor needing airway management, or cardiac arrest. Allergy specialist for all.

Special groups. In pregnancy, adrenaline is not withheld, and the patient is placed in left lateral position. Beta blocker users may need glucagon. In mastocytosis, lifelong adrenaline availability and venom immunotherapy.