Phenylephrine

Summary

Phenylephrine is a sympathomimetic drug. At lower doses, phenylephrine activates alpha-1 receptors.  At higher doses, phenylephrine can activate alpha-2 receptors.

The physiologic effects of phenylephrine include vasoconstriction, which can cause an increase in blood pressure and compensatory decrease in heart rate. Phenylephrine may or may not decrease cardiac output.

Clinically, phenylephrine can be used to treat hypotension. It can also be used as a decongestant to treat rhinitis. It can also be given in droplet form to cause mydriasis for ocular procedures. Lastly, phenylephrine can be used to treat priapism.

Key Points

• Phenylephrine
  • Drug Class
    • Sympathomimetic
      • mimics the action of endogenous hormones on adrenergic receptors
  • Site of Action
    • Activates Alpha 1 (Alpha-1 Agonist) at low doses
      • Causes vasoconstriction
    • Activates Alpha 2 (Alpha-2 Agonist) at high doses
      • Alpha-2 receptors are mostly found centrally and work to inhibit sympathetic outflow; this effect is clinically negligible due to strong agonism of alpha-1 receptors
  • Physiologic Effects
    • Vasoconstriction
      • Due to strong alpha-1 agonism
    • Increases Blood Pressure
      • Due to vasoconstriction
    • ↓ HR
      • Due to reflex bradycardia caused by increasing blood pressure
    • -/↓ CO
      • Cardiac output may or may not decrease due to reflex bradycardia
  • Clinical Use
    • Treats Hypotension
      • Vasoconstriction increases systemic vascular resistance to increase blood pressure
      • Phenylephrine is a first-line vasopressor used to elevate BP in patients with hemodynamic instability in the hospital
    • Causes Mydriasis (Used in Ocular Procedures)
      • Activation of iris dilator muscle leads to mydriasis
    • Treats Rhinitis (Decongestant)
      • Decreased blood flow causes nasal tissue to shrink, thereby clearing the congestion
    • Treats ischemic priapism