Oxytocin
Oxytocin
Generic Name
Oxytocin
Mechanism
Oxytocin exerts its effects primarily by binding to the oxytocin receptors (OTRs)—a G‑protein coupled receptor expressed on uterine myometrium, myoepithelial breast cells, and placental vascular smooth muscle.
* Activation → Gq protein → phospholipase C → IP3/DAG → ↑ intracellular Ca²⁺
* Result → smooth‑muscle contraction, milk let‑down from mammary glands, increased placental blood flow
* Secondary actions include vasoconstriction of placental vessels and modulation of oxytocin‑modulated nerve activity.
Pharmacokinetics
| Parameter | Typical Data |
| Absorption | Intravenous: 100 %; Intramuscular (IM)/Subcutaneous: ~30‑50 %, peak at 5‑10 min |
| Distribution | Small Vd (~0.2 L/kg); crosses blood‑brain barrier minimally |
| Metabolism | Inactivated by enzymatic hydrolysis (ACE, NEP, oxidase) in liver and kidney |
| Elimination | Renal clearance; terminal half‑life ~3‑5 min (IV), 30‑90 min (IM) |
| Protein Binding | ~10 % |
Because of its short IV half‑life, continuous infusion is required for sustained therapeutic effect.
Indications
* Labor induction (term or post‑term)
* Augmentation of inadequate uterine contractions
* Post‑partum hemorrhage due to uterine atony
* Control of uterine bleeding in postpartum or ante‑partum hemorrhage
* Management of preterm labor (particularly after stabilization)
* Breast feeding support (milk let‑down)
* Non‑obstetric: used in neuro‑critically ill patients for coagulopathy correction (experimental) – not routine.
Contraindications
| Category | Note |
| Absolute contraindications | Active fetal demise, in utero death, placenta accreta or previa? (pre‑existing placental abruption) |
| Relative contraindications | Uterine scarring (C‑section, myomectomy), uterine oversensitivity (e.g., postpartum hemorrhage) |
| Warnings |
• Hypotension via vasodilatory effect • Water intoxication / hyponatremia in high doses • Uterine hyperstimulation leading to fetal distress • ACE inhibitor‑induced miscarriage (use with caution in pregnant women) |
Continuous monitoring of blood pressure, uterine activity, and fetal heart rate is mandatory.
Dosing
| Setting | Dose/Infusion |
| Induction of labor | 5 U IV bolus, then 10 U/h infusion (adjust 5‑10 U/h); titrate to cumulative 100 U (regional guidelines) |
| Augmentation | 10 U IV bolus, then 5‑10 U/h infusion (repeat bolus if inadequate response) |
| Post‑partum hemorrhage | 10 U IV bolus, then 5‑10 U/h; target total >40 U if bleeding persists |
| Immunoblot | If IM application for lactation – 10‑20 U/qd |
*Infusion rates should be adjusted in real time per uterine response and fetal status. Use syringe pumps with 3‑min stop‑clock intervals for stable readings.*
* Intramuscular: 10 U IM to promote milk let‑down; not for uterine actions.*
Adverse Effects
| Adverse Effect | Frequency | Notes |
| Uterine hyperstimulation (tachysystole) | Common | Monitor fetal heart rate; may necessitate drug taper/stop |
| Hypotension | Common | Associate with vasodilatory effect; give IV fluids if needed |
| Headache, flushing | Mild | Transient, often self‑limited |
| Water intoxication / hyponatremia | Rare | Higher risk with >400 U total or prolonged infusion |
| Bronchospasm (rare) | Very rare | Consider if asthmatic history |
| Positive inotropic effect on heart (rare) | Very rare | Occurs at high doses; ECG monitoring useful |
| Allergic reaction | Very rare | Anaphylaxis improbable due to synthetic nature but possible |
Serious adverse events are typically dose‑related; tight monitoring mitigates risk.
Monitoring
- Uterine contraction chart: frequency, amplitude, pattern
- Non‑stress test / fetal heart rate tracing (every 15‑30 min during induction/augmentation)
- Maternal vitals: BP, HR, O₂ sat, temperature
- Fluid balance: inputs/outputs, daily weights (to detect water intoxication)
- Electrolytes: serum Na⁺, K⁺, particularly if >200 U administered
- Blood gases (if indicated) for fetal acidosis evaluation
- Post‑partum bleeding volume: estimate via vaginal pads, clots, hemoglobin drop
Clinical Pearls
- Start low, go slow: A 5‑U bolus can trigger tachysystole; titrate infusions in 5‑U steps to achieve 10‑15 min post‑delivery uterine contraction pattern.
- Observe the fetal heart: Tachysystole >100 bpm × 1 min or >5 min may indicate uteroplacental insufficiency; stop drug before fetal distress escalates.
- Avoid additive catecholamines: When used with methyldopa or clonidine, vasodilatory effect may be amplified, increasing hypotension risk.
- Water‑intoxication surveillance: Adults receiving >400 U over 24 h often develop hyponatremia; monitor electrolytes every 12 h in high‑dose protocols.
- Route‑dependent effects: IM is useful for lactation support but ineffective for uterine contraction; ensure clear patient instruction.
- Patient variability: Women with prior cesarean scar or uterine atony respond unpredictably; baseline uterine tone assessment guides dose.
- Reporting: Include odour or appearance abnormality – indicates peptide degradation; replace vial if compromised.
Key takeaway: *Oxytocin is a short‑acting, powerful uterotonic; precise dosing, vigilant monitoring, and patient‑tailored titration are essential for safe, effective obstetric practice.*