## Drug of Choice for Labor Augmentation **Key Point:** Oxytocin is the first-line agent for augmentation of labor in the active phase when contractions are inadequate. ### Mechanism of Action Oxytocin is a posterior pituitary hormone that: - Increases frequency and amplitude of uterine contractions - Acts on oxytocin receptors on myometrial smooth muscle - Produces rhythmic contractions without tetany when dosed appropriately ### Dosing Protocol - **Starting dose:** 2 mIU/min IV infusion - **Increment:** Increase by 2 mIU/min every 15–20 minutes - **Maximum dose:** 20–40 mIU/min (rarely exceeded) - **Monitoring:** Continuous fetal heart rate monitoring and uterine contraction assessment mandatory ### Advantages Over Alternatives | Feature | Oxytocin | Ergot Alkaloids | Prostaglandins | |---------|----------|-----------------|----------------| | **Timing** | Active labor, adequate cervical dilation | 3rd stage (contraindicated in 1st/2nd stage) | Can be used earlier but more side effects | | **Uterine effect** | Rhythmic contractions | Sustained tetanus (risk of uterine rupture) | Tetanic contractions, risk of uterine rupture | | **Safety profile** | Safest in active labor | Contraindicated before full dilation | Higher incidence of side effects | | **Reversibility** | Short half-life (~3 min), easily reversible | Prolonged action | Prolonged action | **High-Yield:** Oxytocin is safe in active labor (cervix ≥6 cm) with reassuring fetal status because its short half-life allows rapid adjustment if hyperstimulation occurs. **Clinical Pearl:** Ergot alkaloids cause sustained tetanic contractions and are reserved for the 3rd stage of labor (after delivery of baby) to control postpartum hemorrhage. Using them in the 1st or 2nd stage risks uterine rupture and placental abruption. **Warning:** Prostaglandins (PGF2α, carboprost) cause more severe side effects (diarrhea, bronchospasm, hypertension) and are not preferred for routine augmentation; they are reserved for specific indications like retained placenta or postpartum hemorrhage refractory to oxytocin. ### Contraindications to Oxytocin Augmentation - Cephalopelvic disproportion - Placenta previa - Vasa previa - Transverse lie - Previous uterine scar (relative; VBAC may be attempted with caution) [cite:Williams Obstetrics 26e Ch 21]
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