A 32-year-old woman with acute anaphylaxis receives intramuscular epinephrine, while a 58-year-old man in septic shock receives intravenous norepinephrine. Which hemodynamic feature best distinguishes the expected vascular response in these two patients?

Question

Accepted Answer

B. Epinephrine decreases peripheral vascular resistance at therapeutic doses; norepinephrine increases it markedly. ## Hemodynamic Distinction: Epinephrine vs Norepinephrine

### Peripheral Vascular Resistance (PVR) Response

**Key Point:** The most reliable hemodynamic discriminator is the effect on peripheral vascular resistance. Epinephrine's β₂ activity causes dose-dependent changes in PVR, while norepinephrine consistently increases PVR.

### Dose-Dependent Effects of Epinephrine

1. **Low dose (0.5–2 μg/min):** β₂ > α₁
   - Skeletal muscle vasodilation
   - **PVR decreases**
   - MAP may decrease (diastolic more than systolic)
   - Heart rate increases

2. **High dose (>4 μg/min):** α₁ > β₂
   - Systemic vasoconstriction
   - **PVR increases**
   - MAP increases markedly

### Norepinephrine Response

- **Predominantly α₁ activity** at all doses
- Consistent, dose-dependent **increase in PVR**
- Systolic and diastolic pressures increase proportionally
- Reflex bradycardia (due to baroreceptor activation from hypertension)

### Comparative Table

| Parameter | Epinephrine (low dose) | Epinephrine (high dose) | Norepinephrine |
|-----------|------------------------|-------------------------|----------------|
| **PVR** | ↓ (β₂ > α₁) | ↑ (α₁ > β₂) | ↑↑ (α₁ dominant) |
| **MAP** | ↓ or → | ↑↑ | ↑↑ |
| **HR** | ↑↑ | ↑ | ↓ (reflex) |
| **Cardiac output** | ↑↑ | ↑ | → or ↓ |
| **Skeletal muscle flow** | ↑ | → or ↓ | ↓ |

**High-Yield:** In anaphylaxis, low-dose epinephrine (0.3–0.5 mg IM) causes vasodilation in skeletal muscle (improving tissue perfusion) while maintaining coronary and cerebral flow. In septic shock, norepinephrine's consistent PVR increase restores perfusion pressure without the "steal" phenomenon.

**Clinical Pearl:** Epinephrine's biphasic vascular response is why dose titration is critical. At low doses in anaphylaxis, it reverses the pathological vasodilation and improves perfusion. Norepinephrine's uniform vasoconstriction makes it more predictable in distributive shock but less suitable when bronchodilation is needed.

**Warning:** Do not confuse epinephrine's low-dose PVR decrease with systemic vasodilation — the effect is selective to skeletal muscle and skin, not global.

[cite:KD Tripathi 8e Ch 12]

Answer

A. Epinephrine increases diastolic blood pressure more than systolic; norepinephrine increases both equally

Answer

C. Epinephrine causes systemic vasoconstriction; norepinephrine causes systemic vasodilation

Answer

D. Epinephrine dilates skeletal muscle vessels while maintaining coronary perfusion; norepinephrine constricts all vascular beds uniformly

A 32-year-old woman with acute anaphylaxis receives intramuscular epinephrine, while a 58-year-old man in septic shock receives intravenous norepinephrine. Which hemodynamic feature best distinguishes the expected vascular response in these two patients?

Explanation

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