Phenylephrine

General

• Type: Vasopressors
• Dosage Forms: Commonly 50 mg per 5 mL (10 mg/mL)
• Common Trade Names: Neosynephrine
• Onset of action: About 1 minute
• Duration of action: 10-20 minutes

Adult Dosing

Hypotension

• Start 100-200 mcg/min then taper down
  • 40-60 mcg/min works for most

• "Push dose"
  • Inject 1 mL (10 mg) into 100 mL NS Bag = 100 mcg/mL
  • Draw this into a syringe, with a concentration of 100mcg/mL
  • Administer 0.5-2 mL (50-200 mcg) every 2-5 minutes

Low Flow Priapism

• Dilute phenylephrine 1mg in 9mL NS for final concentration of 100mcg/mL
  • For 500 mcg/ml, take 0.5 ml of 10mg/ml phenylephrine, and dilute in 9.5 cc NS
• Inject base of penis with 29-Ga needle (after blood aspiration to confirm position)
  • 100-500 mcg every 3-5min (max 1000 mcg) until resolution or 1 hour

Pediatric Dosing

• Hypotension/Shock
  • IV Bolus: 5-20mcg/kg q10-15min prn, 1st dose should not exceed 500mcg with a max dose of 1000mcg
  • IV Infusion: 0.1-0.5mcg/kg/min, titrate to desired BP
• Nasal Congestion (OTC)
  • 4-<6yrs: 2.5mg q4hr prn for <7 days
  • 6-<12yrs: 5mg q4hr prn for <7 days

Special Populations

• Pregnancy Rating: C
• Lactation: infant risk cannot be ruled out
• Renal Dosing
  • Adult
  • Pediatric
• Hepatic Dosing
  • Adult
  • Pediatric

Contraindications

• Allergy to class/drug
• Severe hypertension
• Ventricular tachycardia
• Closed angle glaucoma
• Caution if:
  • Heart failure or cardiogenic shock (cardiac output may decrease with increased SVR)
  • Spinal cord injury or other autonomic dysfunction (may experience exaggerated increase in blood pressure)
  • Hyperthyroidism

Adverse Reactions

• Baroreceptor-mediated reflex bradycardia, particularly in patients with preexisting cardiac dysfunction or severe CAD
• If extravasates use phentolamine

Serious

• Bradycardia
• Pulmonary edema
• Metabolic acidosis
• Decreased renal perfusion
• Extravasation

Common

• Hypertension
• Anxiety
• Headache
• Rebound congestion, sneezing
• Nausea, gastric irritation

Pharmacology

• Half-life: 2.5 hours
• Metabolism: extensively in intestinal wall, moderately in liver
• Excretion: Urinary
• Mechanism of Action: selective α₁ agonist causes vasoconstriction

Primary Receptor

• α₁

Relative Effects

• ↑SVR
• ↓HR (reflex bradycardia)

Notes

• Use with caution in patients with spinal cord injury-related bradycardia
• Useful for treatment of vasodilatory shock when norepinephrine or dopamine have precipitated tachyarrhythmias
• In patients with ↓LV function, unopposed α₁ may lead to decreased CO or myocardial ischemia
  • However clinical trials do not support these effects when used in clinically appropriate dose range
• Commonly used as an alternative to norepinephrine in the setting of atrial fibrillation with rapid ventricular response (associated with lower heart rate after initiation of vasopressor)

Indication

• Neurogenic Shock
• Second line agent for septic shock

See Also

• http://www.atsjournals.org/doi/abs/10.1164/rccm.201006-0972CI?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed#readcube-epdf