## Cardiovascular Comparison: Phenylephrine vs Epinephrine ### Receptor Selectivity & Mechanism **Key Point:** Phenylephrine is a selective α₁-adrenergic agonist, while epinephrine is a non-selective agonist (α₁, α₂, β₁, β₂). | Feature | Phenylephrine | Epinephrine | |---------|---------------|-------------| | **Primary receptor** | α₁ only | α₁, α₂, β₁, β₂ | | **Systolic BP** | ↑↑ (marked) | ↑ (moderate) | | **Diastolic BP** | ↑↑ (marked) | ↑ (moderate) | | **Heart rate** | ↓ (reflex bradycardia) | ↑ (direct β₁ effect) | | **Cardiac output** | ↓ (due to bradycardia) | ↑ (β₁ stimulation) | | **Mechanism of HR change** | Baroreceptor reflex from ↑ BP | Direct myocardial stimulation | ### Why Reflex Bradycardia Occurs with Phenylephrine 1. Phenylephrine causes **pure vasoconstriction** (α₁ effect) → marked ↑ in BP 2. Baroreceptors sense the acute hypertension 3. Parasympathetic (vagal) reflex is triggered → **bradycardia** 4. This is **NOT** a direct drug effect but a **homeostatic response** ### Why Epinephrine Causes Tachycardia 1. β₁ receptors on the heart are directly stimulated → ↑ HR and contractility 2. The β₂-mediated vasodilation in skeletal muscle limits the BP rise 3. **No strong baroreceptor reflex** because the BP rise is modest 4. Net result: **tachycardia** despite some peripheral vasoconstriction **High-Yield:** This distinction is classic NEET PG material. Phenylephrine = α₁ only = reflex bradycardia. Epinephrine = mixed = direct tachycardia. **Clinical Pearl:** Phenylephrine is used in anesthesia to treat hypotension *without* increasing HR (useful in tachycardic patients). Epinephrine is the drug of choice for anaphylaxis because it increases both BP and cardiac output.
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