Oxytocin - Complete Clinical Reference for Obstetric Anaesthesia

1. Drug Overview

• Tietz Textbook of Laboratory Medicine, 7th Edition, p. 2251

2. Mechanism of Action

Uterotonic Mechanism

1. Receptor binding - Oxytocin binds specific G-protein-coupled receptors (OXTR) on myometrial cells. The receptor is a 389-amino acid protein with 7 transmembrane domains, encoded on chromosome 3p25.
2. G-protein activation - Receptor activation stimulates Gαq/11 subfamily G-proteins, which activate phospholipase C (PLC).
3. Calcium mobilisation - PLC generates inositol triphosphate (IP3), causing:
   • Release of Ca²+ from intracellular stores (sarcoplasmic reticulum)
   • Influx of extracellular Ca²+ through voltage-sensitive calcium channels
4. Myosin activation - Elevated intracellular Ca²+ activates myosin light chain kinase (MLCK), leading to actin-myosin crossbridge formation and uterine contraction.
5. Dual indirect pathway - Oxytocin also stimulates prostaglandin (PGF2α) production in the decidua and fetal membranes, amplifying contractions.

Milk Ejection Mechanism

Receptor Regulation in Pregnancy

• Myometrial oxytocin receptors increase 50-100x in the 1st trimester vs non-pregnant state
• A further 200-300x increase occurs during pregnancy, peaking at the onset of labour
• Estrogen upregulates, progesterone downregulates OXTR gene expression
• This explains why the myometrium becomes dramatically more sensitive near term
• Creasy & Resnik's Maternal-Fetal Medicine, p. 556-561

3. Pharmacokinetics

• Creasy & Resnik's Maternal-Fetal Medicine, p. 556-557

4. Indications

Obstetric

1. Induction of labour - at or near term when vaginal delivery is indicated and the cervix is favourable (Bishop score ≥ 6)
2. Augmentation of labour - slow progress in active labour (uterine inertia/hypotonic uterine dysfunction)
3. Prevention of postpartum haemorrhage (PPH) - most important uterotonic in PPH prevention; given after delivery of the baby (and ideally placenta)
4. Treatment of PPH - along with other uterotonics (ergometrine, misoprostol, carboprost)
5. Uterine atony - after Caesarean section or vaginal delivery
6. Management of incomplete/inevitable abortion - to augment contractions
7. Management of missed abortion (used in conjunction with other agents)

Non-obstetric (less common)

• Milk let-down stimulation (intranasal spray)
• Research uses in social behaviour/stress modulation (CNS receptors)

5. Contraindications

Absolute

• Cephalopelvic disproportion - risk of uterine rupture
• Unfavourable foetal presentation (e.g. transverse lie, brow presentation without correction)
• Previous classical (vertical) uterine incision or extensive uterine surgery
• Placenta praevia / vasa praevia
• Active genital herpes (in some guidelines)
• Fetal distress where delivery is not imminent (oxytocin worsens hypoxia by increasing uterine tone)
• Cord prolapse
• Hypertonic uterine contractions (uterine tetany)

Relative

• Previous lower segment Caesarean section (LSCS) - increased risk of uterine rupture (use with close monitoring)
• Grand multiparity (≥5 pregnancies) - risk of uterine rupture
• Overdistended uterus (polyhydramnios, multiple gestation) - increased risk of atony and rupture
• History of uterine surgery
• Significant cardiovascular disease - risk of haemodynamic compromise
• Renal impairment - risk of water intoxication (antidiuretic effect)

6. Adverse Effects

7. Anaesthetic Considerations in Obstetrics

7a. Cardiovascular Effects - The Core Problem

• Hypotension - peripheral vasodilation via nitric oxide release
• Tachycardia - reflex and direct
• ST-segment depression and T-wave changes (ECG changes) - myocardial ischaemia
• Decreased cardiac output (despite tachycardia)
• Chest pain - due to coronary vasospasm or reduced coronary perfusion

7b. Bolus vs Infusion - Practical Protocol

7c. Receptor Tachyphylaxis (Receptor Downregulation)

• Patients who have received oxytocin for labour augmentation before proceeding to Caesarean section have downregulated myometrial oxytocin receptors
• These patients require higher doses of oxytocin post-delivery to achieve adequate uterine tone
• This is clinically important - do not assume the standard dose is sufficient in augmented labours
• If adequate tone is not achieved with oxytocin, escalate to ergometrine, carboprost (PGF2α analogue), or tranexamic acid as part of a PPH bundle

7d. High-Risk Patients - Special Considerations

• Oxytocin boluses are particularly dangerous - can precipitate acute haemodynamic decompensation
• Use smallest effective dose as slow infusion only
• Have vasopressors ready (phenylephrine/ephedrine)
• Consider alternatives: carbetocin (longer-acting synthetic oxytocin analogue with less CV instability) or misoprostol

• Already haemodynamically unstable and receiving antihypertensives/magnesium
• Slow infusion mandatory; avoid bolus entirely
• Risk of pulmonary oedema (especially with magnesium co-administration)

• Oxytocin-induced vasodilation is particularly dangerous; haemodynamic collapse can occur
• Restore intravascular volume before or simultaneously with oxytocin

7e. Interaction with Anaesthetic Agents

7f. Water Intoxication Risk

• Risk when oxytocin is infused at high rates (>20 mU/min) over prolonged periods
• Compounded by large volumes of hypotonic IV fluids (5% dextrose) used to dilute the drug
• Results in: hyponatraemia, cerebral oedema, seizures, coma
• Always use normal saline or Hartmann's as the diluent - never hypotonic solutions
• Monitor fluid balance and electrolytes in prolonged infusions

7g. Post-Caesarean Oxytocin Protocol (Summary)

1. After delivery of baby and cord clamping: 1-3 IU slow IV push over 30-60 seconds (some units use 5 IU but more slowly)
2. Start maintenance infusion: 10-20 IU in 500 mL NS at 100-125 mL/hr (approx 33-40 mIU/min) for 4 hours
3. Have vasopressor (phenylephrine) prepared and ready to treat hypotension
4. Monitor BP every 1-2 minutes after bolus dose
5. If inadequate uterine tone: add ergometrine 250-500 mcg IM (caution in hypertension), misoprostol 600-1000 mcg PR/SL, or carboprost 250 mcg IM (caution in asthma)

8. Summary Table

• Creasy & Resnik's Maternal-Fetal Medicine, Principles and Practice - Oxytocin, pp. 556-563
• Tietz Textbook of Laboratory Medicine, 7th Edition - Oxytocin biochemistry, p. 2251
• Thomas JS et al., Br J Anaesth 2007 - haemodynamic effects of oxytocin bolus vs infusion at Caesarean section
• APSF Obstetric Anaesthesia Safety Guidelines