Anaphylaxis

Question 1

Give a brief definition of anaphylaxis

Answer 1

A systemic allergic reaction, caused by an allergen that is life-threatening.

Question 2

Describe the sensitisation process of anaphylaxis

Answer 2

An allergen enters the circulation for the first time - T-helper cells that are hypersensitive to the allergen stimulate B cells to turn into plasma cells and create specific IgE to the allergen. These IgE antibodies bind to the surface of mast cells which remain inactive until repeat exposure.

Question 3

Describe what happens with repeat exposure to the allergen

Answer 3

Antigen cross-links with IgE-mast cell complex - this causes degranulation of mast cell which releases inflammatory mediators such as histamine, Ach and bradykinins. This exocytosis also causes the release of arachidonic acid from the cell membrane, activating the synthesis of prostaglandins and leukotrienes.

Question 4

What are the main effects of the inflammatory mediators?

Answer 4

Histamine: activates H1-H3 receptors causing systemic effects.
-Respiratory: bronchospasm, mucus production, airway oedema
-Cardiovascular: widespread vasodilation, increased capillary permeability = loss of blood volume, increased HR
-GIT: increased GIT SMC activity and mucus secretion = nausea, diarrhoea, vomiting
-Integ: vasodilation and permeability = urticaria, hives, redness
Prostaglandins and leukotrienes: vasodilation, increased vascular permeability, bronchospasm, chemoattraction of basophils and eosinophils = further inflam mediators

Question 5

What are the signs and symptoms that can be seen in anaphylaxis?

Answer 5

• Respiratory: SOB, wheezing, stridor, increased WOB, dyspnoea, hoarse voice, cyanosis, hypoxia, tachypnoea
• CV: hypotension, tachycardia, chest tightness
• NEURO: anxiety, altered GCS, dizzyness/lightheadedness, syncope
• GIT: nausea, vomiting, diarrhoa, stomah cramping
• INTEG: urticaria, redness, itchyness, hives, angioedema of lips, tongue, eyes, diaphoresis

Question 6

How would you treat a mild allergic reaction?

Answer 6

May be a simple skin rash or local reaction

• Remove cause of reaction
• Pain relief
• Symptomatic care
• Fexofenadine 180mg (not for paeds)

Question 7

Outline your treatment for anaphylaxis

Answer 7

-Basic care and high flow 02
If hypotension, severe bronchospasm OR respiratory distress due to angioedema is present:
-Adrenaline 10mcg/kg to max of 500mcg IM, repeated every 5 mins as required
-Saline IV to maintain adequate BP
-If IPPV required allow slow ventilation rate
-Transport and notify

Question 8

Describe the pharmacology of fexofenadine

Answer 8

Drug type: antihistamine
Onset: 1-3 hours  Duration: 24 hours
MOA: Antagonises the H1 receptor, stabilising it and preventing the effect of histamine
Indications: local allergic reactions
Contraindication: allergy, paediatrics

Question 9

What is the rationale behind adrenaline use?

Answer 9

Adrenaline is used for it’s adrenergic effects. A1 vasoconstriction (prevents loss of blood volume from vascular permeability and vasodilation) = increase in BP.
B2: bronchodilates airways, counteracting the effect of inflammatory mediators

Question 10

Why give fluids in anaphylaxis?

Answer 10

Fluids are given until a normal blood pressure is acheived.
This would be considered after adrenaline however, as fluid may continue to leak out of permeable capillaries, meaning the saline is not having any effect other then causing further oedema.