Anaphylaxis

Question 1

What is anaphylaxis?

Answer 1

Sudden-onset systemic hypersensitivity reaction associated with life-threatening airway/breathing/circulation problems after exposure to an allergen.

Question 2

What is a non-immunological anaphylaxis? Aetiology?

Answer 2

Anaphylactoid reaction. Not associated with IgE antibodies but is due to direct mast cell degranulation. Causes include NSAIDs which may trigger mast cells by altering arachidonic acid metabolism.

Question 3

What is the aetiology of anaphylaxis?

Answer 3

Allergens such as food, drugs, insect bites. Sometimes, a co-factor is required such as NSAIDs, alcohol, exercise and other foods e.g., anaphylaxis after a patient exercised within 2-4 hours of ingestion of a specific food.

Question 4

What is the pathophysiology of anaphylaxis? (x3)

Answer 4

• IgE-mediated hypersensitivity reaction to allergen leading to massive degranulation of pro-inflammatory and vasoactive mediators and cytokines from mast cells and basophils (histamine, prostaglandin D2, leukotrienes).
• SHOCK: is caused by cytokines which depress myocardial function, cause vasodilation and oedema
• AIRWAY OBSTRUCTION: oedema in URT and altered smooth muscle tone resulting in bronchospasm

Question 5

What is the epidemiology of anaphylaxis: Where? Age?

Answer 5

Higher incidence in western countries. Highest in young children.

Question 6

What are the signs and symptoms of anaphylaxis?

Answer 6

• ABCDE APPROACH. Capitalised symptoms are the triad cardinal signs.
• A – airway swelling such as throat and tongue, hoarse voice and stridor
• B – SOB, wheeze, STRIDOR, patient becoming tired, confusion from hypoxia, cyanosis (late sign), respiratory arrest
• C – signs of shock (pale, clammy), tachycardia, HYPOTENSION, decreased levels of consciousness, cardiac arrest (if unmanaged)
• D&E – itching, diarrhoea, vomiting, erythema, URTICARIA, angio-oedema (swelling of dep tissues including eyelids, lips, mouth, throat)

Question 7

What are the investigations for anaphylaxis? (x3)

Answer 7

• Diagnosis is clinical
• MAST CELL TRYPTASE: measure after resuscitation in all adults, and children where allergy is not food related. Elevated
• ECG: non-specific ST changes are common post-adrenaline, including signs of MI with normal coronary arteries
• ABG: metabolic acidosis from elevated lactate

Question 8

How is anaphylaxis managed?

Answer 8

1. Lie flat and raise legs
2. Adrenaline given IM; repeat after 5 mins if no better
3. THEN Establish airway with high flow oxygen
4. IV fluid challenge
5. Chlorphenamine: antihistamine to prevent biphasic reaction
6. Hydrocortisone: to prevent biphasic reaction

Question 9

Dosing of adrenaline in anaphylaxis?

Answer 9

500 micrograms IM (0.5mL) (1:1000 adrenaline). Less for children under 12.

Question 10

What should you use in IV fluid challenge? Amounts?

Answer 10

500-1000mL colloid in adults. 20mL/kg in children.

Question 11

Dosing of Chlorphenamine in anaphylaxis?

Answer 11

IM or slow IV – 10mg; 5mg for 6-12yrs; 2.5mg for younger.

Question 12

Dosing of Hydrocortisone in anaphylaxis?

Answer 12

200mg for adult, 100mg for 6-12yrs, 50mg for younger.

Question 13

How does adrenaline treat airway obstruction and shock?

Answer 13

• SHOCK: agonist to alpha-1 receptors, causing peripheral vasoconstriction, which reduces hypotension and mucosal oedema. Therefore, blood pressure may be normal due to this compensatory mechanism increasing peripheral resistance. Adrenaline is also an agonist to Beta-1 receptors, which increases the rate and force of cardiac contractions and reduce hypotension.
• AIRWAY OBSTRUCTION: Beta-2 receptors, reducing inflammatory mediator release from mast cells and basophils and causing bronchodilation.

Question 14

What are the complications of anaphylaxis?

Answer 14

MI from cardiac ischaemia leading to arrest.