Anaphylaxis

Quick Overview

Anaphylaxis is a life-threatening systemic hypersensitivity reaction requiring immediate recognition and treatment. NICE CG134 emphasizes IM adrenaline 500 micrograms (1:1000) as first-line therapy, repeated every 5 minutes if needed. Key challenges include recognizing atypical presentations, managing biphasic reactions (occurring in 1-20% of cases 1-72 hours later), and ensuring appropriate discharge with two adrenaline auto-injectors.

Core Facts & Concepts

Diagnostic Criteria (NICE CG134)

• Anaphylaxis = sudden onset (<2 hours) + life-threatening airway/breathing/circulation problems ± skin/mucosal changes
• Skin/mucosal changes present in 80-90% but NOT required for diagnosis
• Absence of skin signs more common with iatrogenic triggers (drugs, IV contrast)

Critical Drug Dosing

• 💊 IM Adrenaline 1:1000: 500 micrograms (0.5ml) adults; 300 micrograms (0.3ml) ages 6-12; 150 micrograms (0.15ml) <6 years
• Route: anterolateral thigh (vastus lateralis) - NOT deltoid
• Repeat every 5 minutes if no improvement
• IV adrenaline ONLY for experienced specialists with cardiac monitoring

Mast Cell Tryptase Timing

• 📊 Three samples required:
  • Immediately after resuscitation started
  • 1-2 hours from symptom onset (peak)
  • Baseline (>24 hours or follow-up clinic)
• Elevated tryptase confirms mast cell degranulation

Problem-Solving Approach

Immediate Management (ABCDE approach)

1. Remove trigger (stop drug infusion, remove stinger)
2. IM adrenaline 500 micrograms - do NOT delay for IV access
3. Position: Lie flat + raise legs (unless vomiting/pregnant/breathing difficulty)
4. High-flow oxygen 15L via non-rebreather mask
5. IV fluid bolus 500-1000ml crystalloid if hypotensive
6. Monitor continuously: BP, pulse, respiratory rate, SpO₂, ECG

Refractory Anaphylaxis (after 2 IM adrenaline doses)

• Consider IV adrenaline infusion (specialist use only)
• 💊 IV hydrocortisone 200mg + chlorphenamine 10mg (adjuncts, NOT first-line)
• May need glucagon 1-2mg if on beta-blockers

🚩 Red Flags: Sudden deterioration after initial improvement suggests biphasic reaction; abrupt onset without skin signs; patient on beta-blockers (reduced adrenaline response)

Analysis Framework

Biphasic Reaction Risk Factors

• Delayed/inadequate initial adrenaline
• Severe initial presentation requiring >2 adrenaline doses
• Previous biphasic reactions
• Observation period: Minimum 6-12 hours post-reaction

Visual Aid

Discharge Checklist (NICE CG134)

• ✓ Two adrenaline auto-injectors prescribed (EpiPen/Jext/Emerade)
• ✓ Training on auto-injector use provided
• ✓ Written emergency management plan
• ✓ Mast cell tryptase samples taken
• ✓ Specialist allergy referral arranged
• ✓ Medical alert bracelet advised

Key Points Summary

✓ IM adrenaline 500 micrograms (1:1000) is first-line; repeat every 5 minutes if needed - anterolateral thigh route

✓ Diagnosis does NOT require skin signs - 10-20% present without urticaria/angioedema (especially iatrogenic causes)

✓ Biphasic reactions occur in 1-20% within 1-72 hours; observe minimum 6-12 hours post-reaction

✓ Mast cell tryptase timing: immediate, 1-2 hours (peak), and baseline (>24 hours) - confirms diagnosis retrospectively

✓ Discharge with TWO auto-injectors + specialist allergy referral + training (NICE CG134 mandatory)

✓ 🚩 Refractory anaphylaxis: Consider after 2 IM doses; check beta-blocker use (may need glucagon 1-2mg)

✓ Position matters: Lie flat + legs raised improves venous return (avoid in vomiting/pregnancy/severe dyspnea)

⚠️ Warning: Never delay IM adrenaline for IV access or antihistamines - adrenaline is the ONLY life-saving intervention