Why is norepinephrine used for septic shock but epinephrine (adrenaline) for anaphylactic shock?

Norepinephrine for Septic Shock vs. Epinephrine for Anaphylactic Shock: Pharmacological Rationale

Norepinephrine is recommended as first-line therapy for septic shock due to its predominant alpha-adrenergic effects that restore vascular tone, while epinephrine is preferred for anaphylactic shock because its combined alpha and beta effects address both hypotension and bronchospasm simultaneously. 1, 2

Pathophysiological Differences Between Shock Types

Septic Shock

• Characterized by vasodilation and decreased systemic vascular resistance
• Requires primarily vasoconstrictive support
• Often involves myocardial dysfunction but primarily needs vascular tone restoration

Anaphylactic Shock

• Features both vasodilation and bronchospasm
• Requires both vasoconstrictive and bronchodilatory effects
• Rapid onset requiring immediate intervention

Pharmacological Properties Explaining the Difference

Norepinephrine in Septic Shock

• Mechanism: Predominantly alpha-1 adrenergic effects with minimal beta effects
• Benefits:
  • Potent vasoconstrictor that increases systemic vascular resistance
  • Minimal tachycardia compared to other vasopressors
  • Less arrhythmogenic than dopamine
  • Better survival outcomes compared to dopamine 3
  • Recommended as first-line by Surviving Sepsis Campaign guidelines 1

Epinephrine in Anaphylactic Shock

• Mechanism: Combined alpha and beta adrenergic effects
• Benefits:
  • Alpha-1 effects restore vascular tone and blood pressure
  • Beta-1 effects increase cardiac output and heart rate
  • Critical beta-2 effects reverse bronchospasm and reduce mediator release from mast cells
  • Rapid onset of action suitable for the acute nature of anaphylaxis

Clinical Evidence Supporting These Choices

For Septic Shock

• Norepinephrine is associated with decreased all-cause mortality compared to dopamine (11% absolute risk reduction) 3
• Norepinephrine has fewer cardiac arrhythmias than dopamine 3
• While epinephrine can be used as a second-line agent in septic shock, it causes more metabolic disturbances (lactic acidosis) and cardiac adverse effects 4

For Anaphylactic Shock

• Epinephrine's bronchodilatory effects are essential for managing the respiratory symptoms of anaphylaxis
• The combined vasoconstrictive and inotropic effects address both hypotension and poor cardiac output

Common Pitfalls to Avoid

1. Delaying vasopressor initiation: In severe shock, early vasopressor use may be necessary even before completing fluid resuscitation 2

2. Using dopamine as first-line: Associated with higher mortality and arrhythmia rates in septic shock 3

3. Forgetting central access: Vasopressors should ideally be administered through central venous catheters to prevent extravasation 2

4. Inadequate monitoring: Continuous arterial pressure monitoring and assessment of tissue perfusion markers are essential 2

5. Overlooking fluid status: Adequate fluid resuscitation should ideally be achieved before or concurrent with vasopressor therapy 2

In summary, the choice between norepinephrine for septic shock and epinephrine for anaphylactic shock is based on matching the pharmacological properties of each agent to the specific pathophysiology of each shock state, with the ultimate goal of optimizing tissue perfusion and patient outcomes.