Phenylephrine

Phenylephrine is a pure α₁-adrenergic agonist used in ICU primarily as a vasopressor.

• Class: Direct-acting sympathomimetic
• Receptor activity: Selective α₁ agonist (no β activity)
• Main effect: Vasoconstriction → ↑ SVR → ↑ MAP

Hemodynamic Effects 

Parameter	Effect
SVR	↑↑ (marked increase)
MAP	↑
Heart rate	↓ (reflex bradycardia)
Cardiac output	↓ / ↔
Contractility	↔
Venous return	↑ (venoconstriction)

Key Concept:
Pure vasoconstrictor → may reduce cardiac output, especially in LV dysfunction.

Dosing in ICU

Bolus (peri-intubation / acute hypotension)

• 50–200 mcg IV bolus

Infusion

• 0.1–3 mcg/kg/min (titrate to MAP)

Common practice:

• Start low (e.g., 0.2–0.5 mcg/kg/min)
• Titrate based on MAP target (≥65 mmHg)

Indications

1. Vasodilatory shock with tachycardia

• e.g., septic shock with AF / SVT
• Advantage: no β₁ stimulation → avoids tachyarrhythmias

2. Neurogenic shock

• Restores vascular tone

3. Peri-intubation hypotension

• Preferred for rapid BP support

4. Anesthesia-induced hypotension

• Common intraoperative vasopressor

5. Dynamic LVOT obstruction

• e.g., Hypertrophic obstructive cardiomyopathy
• ↑ afterload → ↓ obstruction

 Contraindications / Avoid

Situation	Reason
Cardiogenic shock	↓ CO worsens perfusion
Severe LV dysfunction	↑ afterload harmful
Hypovolemia (untreated)	worsens tissue perfusion
Bradycardia	may worsen further

 Adverse Effects

Cardiovascular

• Reflex bradycardia
• ↓ Cardiac output
• Hypertension (overcorrection)

Peripheral

• Digital ischemia
• Skin necrosis (extravasation)

Organ perfusion

• ↓ renal / splanchnic perfusion (controversial but important exam point)

Extravasation Management

• Stop infusion immediately
• Infiltrate area with phentolamine
• Elevate limb

 Comparison with Other Vasopressors 

Drug	Receptors	HR	CO	SVR	Use
Phenylephrine	α₁ only	↓	↓/↔	↑↑	Tachyarrhythmia states
Noradrenaline	α₁ + β₁	↔/↑	↑	↑	First-line septic shock
Adrenaline	α + β	↑↑	↑↑	↑	Anaphylaxis
Vasopressin	V1	↔	↔	↑	Refractory shock