Allergic Reaction to a Vaccine and Shock

The Reaction: Anaphylaxis

Type of Shock: Distributive, Not True Hypovolemic

"Anaphylaxis is predominantly an IgE-mediated allergic reaction... in which there is a profound distributive type of shock possibly mediated through histamine release. In this setting, there is evidence of both venous and arterial vasodilation."

Why There IS a Hypovolemic Component

1. Massive vasodilation → blood pools in peripheral vasculature → reduced effective circulating volume
2. Increased vascular permeability (histamine, leukotrienes) → fluid shifts from intravascular to interstitial space (third-spacing) → loss of up to 50% of intravascular volume within minutes
3. Urticaria and angioedema further reflect this capillary leak

Pathophysiology Summary

Vaccine allergen → IgE cross-linking on mast cells/basophils
        ↓
Degranulation → Histamine, Tryptase, Leukotrienes, Prostaglandins
        ↓
Vasodilation + ↑ Capillary permeability
        ↓
Relative & Absolute ↓ Intravascular volume
        ↓
↓ Preload → ↓ Cardiac Output → Tissue hypoperfusion → SHOCK

Clinical Presentation

• Urticaria, flushing, angioedema
• Bronchospasm, stridor (airway compromise)
• Hypotension, tachycardia
• Altered consciousness
• Onset typically within minutes of vaccine administration

Management

1. Epinephrine IM (0.3–0.5 mg of 1:1000 into the lateral thigh) — first-line, life-saving
2. IV fluid resuscitation — addresses the relative/absolute hypovolemia
3. Supplemental oxygen / airway management
4. H1 + H2 antihistamines (adjuncts only, not first-line for shock)
5. Systemic corticosteroids (prevent biphasic reaction)
6. Vasopressors (norepinephrine/dopamine) if refractory to epinephrine

Bottom Line

Indications for Adrenaline (Epinephrine) in Anaphylaxis

When to Give Epinephrine

Key rule: If the patient has been exposed to a known/suspected trigger and develops symptoms involving two or more body systems, or has any cardiovascular or respiratory compromise — give epinephrine immediately.

• Mild, localized reactions (e.g., isolated urticaria at the injection site with no systemic features)
• Vasovagal reactions (bradycardia + hypotension without urticaria/angioedema/bronchospasm)

Dosing of IM Adrenaline

• Route: Intramuscular into the anterolateral thigh (vastus lateralis) — preferred over deltoid due to faster absorption
• Concentration: 1:1000 (1 mg/mL)
• Dose:
  • Adults: 0.3–0.5 mg (0.3–0.5 mL)
  • Children/teenagers: 0.01 mg/kg, up to 0.3 mg
  • Maximum single dose: 0.5 mg

How Many Times Can IM Epinephrine Be Repeated?

• Repeat every 5–15 minutes if signs and symptoms of anaphylaxis persist
• There is no strict maximum number of doses — repeat doses are given as long as the patient continues to show anaphylaxis features and IV epinephrine/vasopressor escalation has not yet been initiated

Harrison's also warns that failure to use epinephrine within the first 20 minutes of symptom onset is a significant risk factor for poor outcomes. Early use is far more important than concern about repeating doses.

Important Practical Notes

1. Posture matters: Lay the patient supine with legs elevated (or in recovery position if airway at risk). Sitting/standing position in a hypotensive patient can cause "empty ventricle syndrome" — Harrison's specifically warns that epinephrine's chronotropic effects can worsen this if the patient is upright.
2. After 2 IM doses without response → this is a signal to escalate care: IV access, IV fluids, IV epinephrine infusion, and ICU-level monitoring.
3. Biphasic anaphylaxis can occur 1–72 hours after the initial reaction — patients should be observed for a minimum of 4–6 hours after resolution.

Indications for IV Adrenaline in Anaphylaxis

Specific Indications

When NOT to Jump to IV Epinephrine

• Do not give IV epinephrine as a first step unless the patient is in cardiac arrest or has no palpable pulse
• Always attempt IM epinephrine first in all other scenarios — IV epinephrine in a non-arrested, non-monitored patient carries a high risk of fatal ventricular arrhythmia and hypertensive emergency

IV Epinephrine Dosing in Anaphylaxis

This must only be given in a monitored setting with cardiac monitoring and IV access established.

• Dilute 1 mg/mL (1:1000) → to 1:10,000 (0.1 mg/mL) by adding 1 mL epinephrine to 9 mL normal saline
• Give 0.1–0.2 mg (1–2 mL of 1:10,000) IV slowly over 5–10 minutes
• Monitor BP and cardiac rhythm continuously

• 0.05–0.1 mcg/kg/min, titrated to response
• Prepared as: 1 mg epinephrine in 250 mL NS → infuse at rate guided by hemodynamic response

Escalation Ladder Summary

Anaphylaxis diagnosed
        ↓
IM Epinephrine 0.3–0.5 mg (anterolateral thigh)
        ↓ No response in 5–15 min
Repeat IM Epinephrine (2nd dose)
        ↓ Still no response
IV Access + IV Fluid Bolus (500–1000 mL NS)
        ↓ Still refractory
IV Epinephrine infusion (0.05–0.1 mcg/kg/min)
        ↓ Cardiac arrest
Standard ALS/ACLS protocol + IV epinephrine per arrest protocol

Key Safety Points

1. Never give undiluted 1:1000 epinephrine IV — this concentration is for IM use only; IV use of 1:1000 can cause fatal hypertension and arrhythmia
2. Continuous cardiac monitoring is mandatory before and during IV epinephrine
3. If IV access is delayed and the patient is deteriorating, IO (intraosseous) route is an acceptable alternative
4. In patients on beta-blockers, anaphylaxis may be refractory to epinephrine — consider glucagon 1–2 mg IV as an adjunct in these cases