Why is a woman in labor with hypotension given an intramuscular (IM) injection of ephedrine in the thigh?

Management of Hypotension in Laboring Women

Intramuscular ephedrine is administered in the thigh of a hypotensive woman in labor because the anterolateral thigh (vastus lateralis) injection site produces higher and more rapid peak plasma levels compared to arm injections, which is critical for quickly restoring maternal blood pressure and maintaining uteroplacental perfusion. 1

Rationale for Ephedrine Use in Obstetric Hypotension

• Ephedrine is indicated for the treatment of clinically important hypotension occurring in the setting of anesthesia, including during labor and delivery 2
• Ephedrine has been shown to raise systolic and mean blood pressure when administered as a bolus dose following the development of hypotension during anesthesia 2
• Both ephedrine and phenylephrine are acceptable drugs for treating hypotension during neuraxial anesthesia for obstetric patients 3

Advantages of Thigh Injection Site

• Intramuscular injections into the anterolateral thigh (vastus lateralis) produce higher and more rapid peak plasma levels compared with injections administered in the arm (deltoid) 3, 1
• The pharmacokinetic advantage of thigh injection is particularly important in emergency situations where rapid correction of hypotension is needed 1

Benefits of Ephedrine for Maternal-Fetal Outcomes

• Bolus administration of intravenous ephedrine reverses the decrease in diastolic uteroplacental blood flow velocity during uterine contractions 4
• Ephedrine increases uterine blood flow and fetal arterial oxygen tension during hypotension, which is critical for preventing fetal distress 5
• Prophylactic ephedrine prevents maternal hypotension and fetal late decelerations, though it may be associated with fetal tachycardia 6

Dosing Considerations

• For hypotension during labor or anesthesia, ephedrine is typically administered at doses of 5-10 mg intravenously or 10-25 mg intramuscularly 7
• Higher prophylactic doses (15-20 mg) of ephedrine are more effective than lower doses (10 mg) in preventing hypotension during spinal anesthesia for cesarean section 7

Clinical Algorithm for Hypotension Management in Labor

1. Identify hypotension (systolic BP <100 mmHg or >20% decrease from baseline)
2. Position patient in left lateral tilt to relieve aortocaval compression 3
3. Administer intravenous fluid bolus if IV access is available 3
4. If hypotension persists or IV access is not immediately available:
   • Administer ephedrine 10-25 mg intramuscularly in the anterolateral thigh 1, 7
   • Monitor maternal blood pressure every 2-5 minutes 7
   • Monitor fetal heart rate patterns for signs of distress 6
5. If hypotension persists after initial dose, additional doses may be administered as needed 7

Potential Pitfalls and Caveats

• While ephedrine is effective for treating hypotension, it may cause fetal tachycardia, which should be monitored 6
• In severe, refractory hypotension, additional interventions may be necessary, including volume expansion with crystalloid solutions and consideration of vasopressor infusions 3
• For patients with pre-existing hypertension or cardiac conditions, dosing should be adjusted to avoid excessive blood pressure elevation 8