1.3 Anaphylaxis & Acute Airway Emergencies
Key Takeaways
Anaphylaxis is an acute, life-threatening systemic hypersensitivity reaction characterized by rapidly progressive Airway, Breathing, or Circulation compromise, usually accompanied by mucosal or cutaneous features.
Intramuscular adrenaline 1:1000 (1 mg/mL) injected into the anterolateral mid-thigh is the immediate first-line drug: 500 micrograms for adults (>12 yr), 300 micrograms for children aged 6–12 yr, and 150 micrograms for children aged 6 mo–6 yr.
Antihistamines and corticosteroids are not first-line: the RCUK 2021 guideline no longer advises routine corticosteroids and prefers a non-sedating oral antihistamine once the patient is stable; neither may delay adrenaline or airway management.
Serial mast cell tryptase measurements should be obtained at presentation, at 1–2 hours post-reaction, and at least 24 hours later to establish the patient's true baseline.
Anaphylaxis & Acute Airway Emergencies
Anaphylaxis is a severe, life-threatening, generalised or systemic hypersensitivity reaction characterized by rapidly developing airway compromise, breathing difficulty, and/or circulatory collapse. Prompt administration of intramuscular adrenaline is the single most critical intervention to prevent fatal asphyxiation or cardiovascular collapse.
Diagnostic Criteria & Clinical Presentation
According to Resuscitation Council UK (RCUK) guidelines, anaphylaxis is likely when a patient presents with sudden onset and rapid progression of symptoms involving:
- A – Airway Problems: Pharyngeal or laryngeal oedema, hoarse voice, stridor, tongue and lip swelling, difficulty swallowing.
- B – Breathing Problems: Bronchospasm, expiratory wheeze, tachypnoea, intercostal recession, exhaustion, hypoxaemia (cyanosis).
- C – Circulation Problems: Profound hypotension (SBP <90 mmHg or faintness), tachycardia (or relative bradycardia), cold clammy skin, pallor, dizziness, cardiac arrest.
Warning
Skin and Mucosal Signs Are NOT Obligatory: While urticaria, generalized erythema, angioedema, and intense pruritus occur in over 80% of anaphylactic reactions, up to 20% of severe or rapidly fatal presentations present with isolated circulatory collapse or acute airway obstruction without visible cutaneous manifestations. Never withhold adrenaline because a rash is absent!
Immediate Resuscitation & Adrenaline Dosing
Patient Positioning
- Hypotension/Circulatory Collapse: Lie the patient flat and elevate the legs to enhance venous return to the heart.
- Airway/Breathing Distress: Patients struggling to breathe may prefer sitting upright, but monitor blood pressure closely.
- Late Pregnancy: Position in the left lateral tilt position to prevent aortocaval compression by the gravid uterus.
- Fatal Pitfall: Never allow the patient to sit up suddenly, stand, or walk. Sudden standing causes acute vena caval pooling, fatal loss of cardiac filling pressure (empty ventricle syndrome), and pulseless electrical activity arrest.
Intramuscular Adrenaline Dosing (1:1000 = 1 mg/mL)
Adrenaline acts rapidly to reverse peripheral vasodilation (alpha-1 agonism), relieve bronchospasm and laryngeal oedema (beta-2 agonism), and suppress mast cell degranulation.
| Patient Age / Category | IM Adrenaline (1:1000) Dose | Volume of 1:1000 Solution |
|---|---|---|
| Adult and Child > 12 years | 500 micrograms | 0.5 mL |
| Child 6 – 12 years | 300 micrograms | 0.3 mL |
| Child 6 months – 6 years | 150 micrograms | 0.15 mL |
| Infant < 6 months | 100 – 150 micrograms | 0.1 – 0.15 mL |
- Injection Site: The anterolateral aspect of the middle third of the thigh (vastus lateralis). The high vascularity of this muscle bed achieves significantly faster peak plasma concentrations than deltoid or subcutaneous routes.
- Repeat Dosing: If there is no clinical improvement or if the patient deteriorates, repeat the IM dose after 5 minutes.
Fluid Resuscitation & Refractory Anaphylaxis
Massive capillary endothelial permeability causes rapid loss of up to 35% of circulating intravascular volume into the interstitial space within minutes. Administer an immediate rapid IV fluid challenge of 500–1000 mL of balanced crystalloid in adults (10 mL/kg in children, per RCUK 2021).
Refractory Anaphylaxis: Defined as anaphylaxis that requires ongoing treatment despite two appropriate doses of intramuscular adrenaline. Management requires immediate critical care escalation, expert airway management, and initiation of a titrated intravenous adrenaline infusion (typically 0.1 micrograms/kg/min) by trained critical care personnel under continuous ECG and arterial line monitoring. Intravenous adrenaline boluses are strictly avoided outside cardiac arrest due to high risks of fatal ventricular tachyarrhythmias and myocardial infarction.
Second-Line Adjunctive Therapies & Clarifications
In older protocols, antihistamines and corticosteroids were routinely administered alongside adrenaline. The RCUK 2021 anaphylaxis guideline explicitly demotes these drugs to secondary, non-urgent roles:
- Antihistamines (third-line): Once the patient is stable, a non-sedating oral antihistamine (e.g. cetirizine) is preferred over chlorphenamine for skin symptoms (parenteral chlorphenamine can worsen hypotension). Antihistamines treat pruritus, urticaria and erythema. They have no effect on life-threatening laryngeal oedema or shock. Antihistamines must never delay adrenaline administration.
- Corticosteroids (e.g., Hydrocortisone 200 mg IV in adults): Historically administered to prevent late-phase biphasic anaphylactic reactions. However, systematic reviews demonstrate no robust evidence of clinical benefit. RCUK guidelines state corticosteroids are no longer routinely recommended for the acute management of anaphylaxis.
- Inhaled Bronchodilators (Salbutamol): Useful adjunctive therapy for persistent lower airway bronchospasm that persists after IM adrenaline has stabilized systemic hemodynamics.
Diagnostic Confirmation: Mast Cell Tryptase
Serum mast cell tryptase is a neutral protease stored inside secretory granules of mast cells. A marked elevation confirms systemic mast cell degranulation.
| Timing of Blood Sample | Clinical Purpose |
|---|---|
| Sample 1: Immediate | Drawn as soon as practical after starting resuscitation (do not delay treatment). |
| Sample 2: 1 to 2 Hours | Drawn 1–2 hours post-onset of symptoms (corresponds to peak circulating tryptase levels). |
| Sample 3: ≥ 24 Hours | Drawn at least 24 hours later (or in follow-up allergy clinic) to establish the true baseline level. |
Discharge Planning & Auto-Injectors
- Observation Period (RCUK 2021, timed from resolution of symptoms): fast-track discharge after 2 hours can be considered after a good response (within 5–10 minutes) to a single dose of adrenaline given within 30 minutes of onset, with complete resolution, unused adrenaline auto-injectors and training, and adequate supervision; observe for at least 6 hours if two doses of IM adrenaline were needed or there is a previous biphasic reaction; observe for at least 12 hours after a severe reaction needing more than two doses, with severe asthma or severe respiratory compromise, possible continuing absorption of the allergen (e.g. slow-release medicines), a late-night presentation, or poor access to emergency care.
- Discharge Essentials: Prescribe two adrenaline auto-injectors (AAIs) (e.g., 300 mcg or 500 mcg devices), provide face-to-face device training with a trainer kit, supply a written anaphylaxis action plan, and make an urgent referral to a specialist allergy clinic.
Acute Upper Airway Emergencies
| Emergency Condition | Pathophysiology & Presentation | Immediate Clinical Management |
|---|---|---|
| Foreign Body Airway Obstruction (FBAO) | Sudden choking while eating; severe: unable to speak/cough, clutching neck, silent stridor, cyanosis | Conscious: 5 sharp back blows between scapulae, followed by 5 abdominal thrusts (chest thrusts in infants/pregnant). Unconscious: Call 2222; commence CPR 30:2 immediately. |
| Acute Epiglottitis | Rapid supraglottic inflammation (Haemophilus influenzae, Streptococcus); sore throat, high fever, drooling, dysphagia, muffled voice, tripod position, inspiratory stridor | DO NOT examine the throat with a tongue depressor (precipitates fatal complete laryngospasm). Keep patient calm with parents/carers. Call senior anaesthetist and ENT surgeon immediately for emergency fiberoptic intubation in the operating theatre. IV ceftriaxone. |
| Ludwig's Angina | Rapidly spreading cellulitis of bilateral submandibular and sublingual spaces, typically from infected lower molars (2nd/3rd); "woody" brawny floor-of-mouth swelling, protruding tongue, neck crepitus, airway occlusion | Urgent senior airway call; awake fiberoptic intubation or awake tracheostomy before complete occlusion. IV broad-spectrum antibiotics (co-amoxiclav + metronidazole or piperacillin-tazobactam) and surgical incision and drainage. |
A 24-year-old woman collapses in a restaurant within 10 minutes of consuming a dessert containing walnuts. On arrival of the emergency medical team, she is cyanotic, gasping for breath, and unable to speak. Physical examination reveals audible inspiratory stridor, widespread expiratory wheeze, swollen lips, and a blood pressure of 72/38 mmHg with a heart rate of 142 bpm. What is the most appropriate first-line pharmacological treatment?
Chlorphenamine 10 mg intravenously
Adrenaline 100 micrograms intravenously as an immediate bolus
Adrenaline 500 micrograms intramuscularly into the anterolateral thigh
Hydrocortisone 200 mg intravenously
A 38-year-old man who developed anaphylactic shock following an amoxicillin-clavulanic acid injection has been successfully resuscitated in the emergency department with two doses of intramuscular adrenaline and IV fluids. He is now hemodynamically stable. The medical team plans to measure serial serum mast cell tryptase to confirm an anaphylactic mechanism. What is the recommended sampling schedule for mast cell tryptase according to Resuscitation Council UK guidelines?
Immediately, at 1–2 hours, and at least 24 hours after the reaction
A single baseline sample drawn immediately prior to hospital discharge
A single measurement taken exactly 30 minutes following initial recovery
Hourly serum samples taken for the first 6 consecutive hours following presentation
A 34-year-old previously healthy man presents to the emergency department with a 12-hour history of severe sore throat, fever, difficulty swallowing his own saliva, and a muffled 'hot potato' voice. He is sitting upright, leaning forward with his neck extended (tripod position). He has audible inspiratory stridor, a temperature of 39.2°C, respiratory rate of 28 breaths/min, and oxygen saturation of 94% on air. What is the most critical and safe next step in his clinical management?
Attempt urgent bedside blind orotracheal intubation in the department
Examine the throat with a tongue depressor to look for a peritonsillar abscess before referral
Prescribe oral co-amoxiclav and discharge with urgent outpatient clinic review
Senior anaesthetic and ENT help to secure the airway in theatre, without examining the throat
Sections you finish are checked off in the contents.