Oxytocin and Other Drugs acting on Uterus SN 1. Oxytocics* - classify*, uses, adverse effects; enumerate, clinical indications of each agent; Oxytocin and Prostaglandin Analogue - compare and contrast as oxytocics 2. Ergometrine in PPH - pharamacological basis, Contraindications to use of Ergometrine 3. Oxytocin and Ergometrine - compare and contrast*** 4. Why is Oxytocin used for Induction of Labour and not Methylergometrine. I have exam explain imp in simple language with more content
Drugs Acting on the Uterus - Complete Exam Notes
SN 1: OXYTOCICS - Classification, Uses, Adverse Effects & Clinical Indications
What are Oxytocics?
CLASSIFICATION OF OXYTOCICS
| Class | Agents |
|---|---|
| A. Posterior pituitary hormone | Oxytocin (Syntocinon) |
| B. Ergot alkaloids | Ergometrine (Ergonovine), Methylergometrine (Methergine) |
| C. Prostaglandins | PGE2 - Dinoprostone; PGE1 analogue - Misoprostol; PGF2α analogue - Carboprost (15-methyl PGF2α) |
| D. Miscellaneous | Quinine (weak), Sparteine (rarely used) |
A. OXYTOCIN
Source & Nature
- Nonapeptide hormone secreted by the posterior pituitary (hypothalamus synthesizes, posterior pituitary releases)
- Available as synthetic oxytocin (identical to natural)
Mechanism of Action
- Binds to Gq-coupled oxytocin receptors (OTR) on myometrial cells
- Activates phospholipase C → IP3 → Ca²⁺ release from SR + increased Ca²⁺ entry
- Increased intracellular Ca²⁺ → activates myosin light chain kinase (MLCK) → myometrial contraction
- Sensitivity increases dramatically at term due to upregulation of OTRs (estrogen increases receptors; progesterone decreases them)
Pharmacokinetics
- Route: IV (for induction), IM (for PPH)
- Half-life: ~3-5 minutes (very short - inactivated by oxytocinase)
- NOT given orally (destroyed by gut peptidases)
- Onset IV: immediate; IM: 3-5 minutes
Uterine Effects
- Low doses (physiological): rhythmic, coordinated contractions - mimics normal labour
- High doses: sustained tonic contraction (spasm) - dangerous
- Effect is dose-dependent and estrogen-dependent (works well near term)
- At term - very sensitive; in early pregnancy - relatively insensitive (progesterone dominance)
Clinical Indications / Uses
- Induction of labour (at/near term, cervix ripe) - gold standard
- Augmentation of labour (sluggish uterine activity)
- Prevention of PPH (post-delivery IM or IV infusion)
- Treatment of PPH - IV infusion
- Management of incomplete abortion (helps expel products)
- Oxytocin challenge test (OCT) - fetal well-being assessment
Adverse Effects
- Water intoxication (ADH-like effect) - most important; hyponatremia, convulsions
- Uterine hyperstimulation / tetanic contraction - fetal distress, uterine rupture
- Fetal hypoxia (due to sustained contraction reducing placental blood flow)
- Hypotension (at high IV doses - vasodilation)
- Nausea, vomiting
Contraindications
- Cephalopelvic disproportion
- Fetal malpresentation
- Previous uterine scar (relative C/I)
- Placenta praevia
B. ERGOT ALKALOIDS
Ergometrine (Ergonovine) and Methylergometrine (Methergine)
- Act on α-adrenergic receptors and serotonin (5-HT) receptors on the uterus
- Produce sustained tonic contraction (not rhythmic) of the uterus
- The contraction is powerful and prolonged - entire uterus contracts like a clamp
| Property | Ergometrine | Methylergometrine |
|---|---|---|
| Route | IV/IM/Oral | IM/Oral |
| Onset (IM) | 2-5 min | 2-5 min |
| Onset (IV) | Immediate | Immediate |
| Duration | 45 min - 3 hours | Similar |
| Bioavailability | Good orally | Good orally |
- PPH (Postpartum Haemorrhage) - primary use (IM after delivery of placenta)
- Subinvolution of uterus - promotes uterine involution postpartum
- Management of incomplete abortion
- Causes TONIC (sustained, non-rhythmic) contraction → fetal hypoxia/asphyxia
- Dangerous for the fetus
- Causes hypertension (vasoconstriction)
- Hypertension (vasoconstriction via α-receptors and 5-HT receptors) - MAJOR concern
- Nausea, vomiting
- Headache, dizziness
- Ergotism (with prolonged use): peripheral vascular spasm, gangrene, hallucinations (rare with modern use)
- Coronary artery spasm (angina)
C. PROSTAGLANDINS
PGE2 - Dinoprostone (Prostin E2)
- Cervical ripening (gel/pessary/tablet) and induction of labour
- Mid-trimester termination of pregnancy
PGE1 analogue - Misoprostol (Cytotec, Oxaprost)
- Cheap, heat-stable (no refrigeration needed) - ideal for developing countries
- Routes: oral, sublingual, vaginal, rectal
- Uses: induction of labour, medical abortion (with mifepristone), PPH
- SE: fever, shivering, diarrhoea, uterine hyperstimulation
PGF2α analogue - Carboprost (15-methyl PGF2α; Hemabate)
- Intractable PPH unresponsive to oxytocin and ergometrine
- Dose: 250 mcg IM every 15-90 min (max 8 doses)
- SE: Bronchoconstriction (C/I in asthma), diarrhoea, fever, hypertension
SN 1 (continued): OXYTOCIN vs PROSTAGLANDIN ANALOGUE - Compare and Contrast as Oxytocics
| Feature | Oxytocin | Prostaglandins (e.g., Misoprostol) |
|---|---|---|
| Nature | Nonapeptide hormone | Modified arachidonic acid derivatives |
| Receptor | Oxytocin receptor (Gq-coupled) | EP/FP receptors (Gs and Gq) |
| Mechanism | IP3 → Ca²⁺ release → MLCK activation | cAMP and IP3 pathways → Ca²⁺ |
| Type of contraction | Rhythmic, coordinated (mimics labour) | Rhythmic but also promotes cervical ripening |
| Effect on cervix | Minimal cervical ripening | Excellent cervical ripening (softening) |
| Route | IV/IM only (destroyed orally) | Oral, sublingual, vaginal, rectal |
| Storage | Requires refrigeration | Misoprostol is heat-stable (no refrigeration) |
| Titration | Easily titratable (IV infusion) | Less easy to titrate (oral/vaginal doses) |
| Sensitivity | Only effective near term (estrogen-dependent) | Effective at any gestational age |
| Uses | Induction, augmentation, PPH | Induction, MTP, cervical ripening, PPH |
| Side effects | Water intoxication, hypotension | Fever, shivering, diarrhoea, bronchospasm (carboprost) |
| Use in early pregnancy | Relatively ineffective | Highly effective (MTP) |
| Use in asthma | Safe | Carboprost contraindicated; misoprostol relatively safe |
| Cost | Moderate | Misoprostol is cheap |
| Key advantage | Easy IV titration, physiological contraction | Cervical ripening, oral route, gestational age independence |
SN 2: ERGOMETRINE IN PPH - Pharmacological Basis & Contraindications
What is PPH?
Pharmacological Basis of Ergometrine in PPH
- Mechanism: Ergometrine acts on α-adrenergic and 5-HT receptors on uterine smooth muscle
- Effect: Produces powerful, sustained tonic contraction of the entire uterus (body + lower segment)
- This tonic contraction mechanically compresses and obliterates the blood vessels supplying the placental bed → stops bleeding
- Think of it as: the uterus acts like a "living ligature" clamping the blood vessels
- Onset: Rapid (2-5 min IM; immediate IV)
- Dose: Ergometrine 0.25 mg IM or slow IV; Methylergometrine 0.2 mg IM
- A combination of Oxytocin 5 IU + Ergometrine 0.5 mg (Syntometrine) is commonly given IM at delivery of anterior shoulder - combines the rapid onset of oxytocin with the prolonged effect of ergometrine.
Contraindications to Ergometrine
| Contraindication | Reason |
|---|---|
| Hypertension / Pre-eclampsia / Eclampsia | Ergometrine causes vasoconstriction → further raise in BP → stroke, pulmonary oedema |
| Heart disease / Cardiac patients | Vasoconstriction increases cardiac workload; can cause coronary vasospasm |
| Peripheral vascular disease | Worsens ischaemia |
| Induction/augmentation of labour | Causes tonic contraction → fetal asphyxia, uterine rupture |
| Second/third stage not completed | Risk of trapping placenta/fetus |
| Renal failure / Hepatic failure | Impaired metabolism/excretion |
| Migraine | Ergot-related vasoconstriction worsens |
| Sepsis | Risk of gangrene due to peripheral vasoconstriction |
| Bronchial asthma | Some bronchospasm risk |
Key exam point: The most important contraindication is HYPERTENSION (including PIH/pre-eclampsia). In hypertensive mothers with PPH, use oxytocin or carboprost instead.
SN 3: OXYTOCIN vs ERGOMETRINE - Compare and Contrast ★★★
| Feature | Oxytocin | Ergometrine / Methylergometrine |
|---|---|---|
| Source | Posterior pituitary (synthetic) | Ergot alkaloid (Claviceps purpurea fungus) |
| Nature | Nonapeptide | Ergot alkaloid (indole compound) |
| Receptor | Oxytocin receptor (Gq-GPCR) | α-adrenergic + 5-HT receptors |
| Type of uterine contraction | Rhythmic, coordinated contractions (mimics normal labour - contract + relax) | Sustained tonic contraction (entire uterus = one continuous clamp) |
| Sensitivity (gestational age) | Near term only (requires estrogen-primed uterus with OTRs) | Effective at any stage of pregnancy and postpartum |
| Effect on lower uterine segment | Acts mainly on upper segment | Acts on entire uterus including lower segment and cervix |
| Effect on blood vessels | Causes mild vasodilation (hypotension at high doses) | Causes vasoconstriction (raises BP - dangerous in hypertension) |
| Route | IV infusion (induction), IM (PPH) | IM, IV (slow), oral |
| Oral bioavailability | None (peptide - destroyed) | Good (can be given orally) |
| Onset (IM) | 3-5 min | 2-5 min |
| Duration | Short (half-life 3-5 min) | Longer (45 min to 3 hours) |
| Effect on BP | Mild hypotension (high doses) | Hypertension (vasoconstriction) |
| Effect on milk ejection | Yes (milk let-down reflex) | No |
| ADH-like effect | Yes (water retention) | No |
| Use in labour induction | ✅ YES - drug of choice | ❌ NO - causes fetal asphyxia |
| Use in PPH | ✅ YES - first line | ✅ YES - effective |
| Use in hypertensive mother | ✅ SAFE | ❌ CONTRAINDICATED |
| Use in cardiac patient | Caution (hypotension at high doses) | ❌ AVOID (coronary spasm) |
| Fetal safety during labour | ✅ Safe when properly dosed | ❌ Dangerous (tonic contraction → fetal hypoxia) |
| Key advantage | Physiological, titratable, safe for fetus | Longer-acting, good for PPH, oral route available |
| Key disadvantage | Short-acting, water intoxication, no oral use | Hypertension, tonic spasm, contraindicated in HTN |
SN 4: Why Oxytocin is Used for Induction of Labour and NOT Methylergometrine ★
Oxytocin produces RHYTHMIC contractions; Methylergometrine produces TONIC contractions
- Contractions that come and go (rhythmic)
- Between contractions: uterus relaxes → blood flows to placenta → fetus gets O₂
- Cervix dilates progressively
- Baby descends safely
- Mimics natural labour contractions - rhythmic, coordinated, with relaxation between
- Strength and frequency can be titrated (increased/decreased via IV infusion)
- Acts only on upper uterine segment - lower segment & cervix stay relaxed → baby can pass
- Effect disappears quickly if stopped (half-life 3-5 min) - easy to manage complications
- Works via physiological OT receptors
| Reason | Explanation |
|---|---|
| Tonic contraction | Uterus contracts continuously - no relaxation phase - placental blood flow stops → fetal hypoxia → asphyxia → stillbirth |
| Fetal danger | Continuous contraction compresses umbilical vessels - baby cannot get O₂ |
| Cannot titrate | No IV infusion possible; long duration; cannot stop quickly once given IM |
| Acts on entire uterus | Lower segment also contracts - cervix doesn't dilate - obstructed labour |
| Causes hypertension | Vasoconstriction - dangerous in labour |
| Uterine rupture risk | Tonic spasm in a scarred uterus = catastrophic |
| Not selective for upper segment | Cervix contracts too - no descent of presenting part |
Oxytocin = a bouncer who opens and closes the door rhythmically - people can pass through. Methylergometrine = a bouncer who LOCKS the door shut - nobody can get out, and the baby is trapped without oxygen.
QUICK SUMMARY TABLE - All Oxytocics at a Glance
| Oxytocin | Ergometrine | Misoprostol | Carboprost | |
|---|---|---|---|---|
| Induction of labour | ✅ 1st choice | ❌ | ✅ (if cervix unripe) | ❌ |
| PPH prevention | ✅ | ✅ | ✅ | ✅ |
| PPH treatment | ✅ 1st line | ✅ 2nd line | ✅ | ✅ refractory PPH |
| MTP / abortion | Limited | ❌ | ✅ (with mifepristone) | ✅ (2nd trimester) |
| Cervical ripening | ❌ | ❌ | ✅ | ❌ |
| Safe in hypertension | ✅ | ❌ | ✅ | ✅ |
| Safe in asthma | ✅ | ✅ | ✅ | ❌ |
| Oral use | ❌ | ✅ | ✅ | ❌ |
HIGH-YIELD EXAM POINTS TO MEMORIZE
- Oxytocin = rhythmic contractions + only near term + titrable IV + water intoxication
- Ergometrine = tonic contraction + any trimester + raises BP + C/I in hypertension
- Misoprostol = oral/SL/vaginal + cervical ripening + any gestation + heat stable + cheap
- Carboprost = refractory PPH + C/I in asthma (bronchoconstriction)
- Syntometrine = Oxytocin 5 IU + Ergometrine 0.5 mg - used in active management of 3rd stage
- Why not ergometrine for induction: tonic contraction → fetal asphyxia + uterus cannot relax + BP rises + cannot stop quickly
- Why not oxytocin for PPH alone in resistant cases: short-acting, no vasoconstriction, may need ergometrine's sustained effect
- Contraindication to ergometrine: Hypertension (most important), cardiac disease, PVD
- "Syntometrine" combines the best of both: fast onset of oxytocin + prolonged effect of ergometrine
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3D cutaway medical illustration of the uterus showing oxytocin mechanism of action. Cross-section of uterus with myometrial cells visible inside the wall. Show oxytocin molecules binding to receptors on myometrial cells, with signaling cascade: Gq receptor → phospholipase C → IP3 → calcium release from sarcoplasmic reticulum → MLCK activation → muscle contraction. Show rhythmic wave-like contractions with arrows. Label: oxytocin receptor, Ca2+ ions, IP3 pathway, myometrial fibers contracting rhythmically. Also show posterior pituitary at top releasing oxytocin into bloodstream. Rich anatomical detail, vibrant colors, educational medical poster style, dark background, glowing labels.
3D cutaway medical illustration comparing oxytocin vs ergometrine uterine contractions. Split view of uterus: LEFT HALF shows oxytocin effect - rhythmic wave contractions (sine wave pattern) with relaxation periods between, labeled "RHYTHMIC - safe for fetus", fetal oxygen supply intact, cervix open. RIGHT HALF shows ergometrine effect - sustained tonic clamp contraction, entire uterus squeezed tight like a fist, labeled "TONIC CONTRACTION - fetal asphyxia risk", cervix closed, no relaxation. Show fetus inside for scale. Blood vessels visible. Color coded: oxytocin side blue/green, ergometrine side orange/red. Bold labels, medical education style, dark background.
3D cutaway anatomical illustration showing ergometrine pharmacological basis in postpartum hemorrhage (PPH). Large cross-section of postpartum uterus showing: placental bed with open blood vessels bleeding (red), ergometrine molecules binding to alpha-adrenergic and serotonin 5-HT receptors on uterine muscle cells, resulting in powerful tonic contraction that mechanically compresses placental blood vessels like a living ligature, stopping hemorrhage. Show before/after: left side bleeding open vessels, right side contracted uterus with closed compressed vessels. Label all structures: uterine wall, myometrium, placental site, blood vessels, alpha receptors, 5-HT receptors. Rich 3D detail, vibrant medical poster style.
3D cutaway medical chart infographic showing classification of all oxytocic drugs acting on uterus. Central 3D uterus with 4 branches extending outward: Branch 1 (blue) - Oxytocin: posterior pituitary molecule, IV drip, rhythmic contractions wave; Branch 2 (red) - Ergot Alkaloids: Ergometrine/Methylergometrine, fungus source Claviceps purpurea, tonic contraction clamp icon; Branch 3 (green) - Prostaglandins: PGE2 Dinoprostone, PGE1 Misoprostol pill, PGF2alpha Carboprost injection; Branch 4 (purple) - Miscellaneous: Quinine. Each branch has mechanism icons. Clinical uses listed: induction of labour, PPH, MTP, cervical ripening. Color-coded, 3D depth, bold text labels, dark background, educational poster.
3D cutaway medical illustration explaining why oxytocin is used for induction of labour and NOT methylergometrine. Show pregnant uterus with full-term fetus inside. LEFT PANEL labeled "OXYTOCIN - SAFE FOR INDUCTION": uterus shows rhythmic contractions with relaxation, cervix open and dilating, fetus descending with umbilical cord carrying oxygen (bright red blood), IV drip shown with titration dial, contraction wave graph showing peaks and valleys. RIGHT PANEL labeled "METHYLERGOMETRINE - DANGEROUS": uterus in continuous tonic spasm (red/orange), cervix clamped shut, fetus trapped, umbilical cord compressed (blue = no oxygen), fetal distress symbol, blood pressure arrow going up, danger warning symbols. Very detailed, bold educational style.
3D cutaway medical illustration of prostaglandin analogue misoprostol mechanism on uterus and cervix. Cross-section showing: uterus and cervix in detail. Show misoprostol (PGE1 analogue) binding to EP receptors on myometrial cells and cervical stromal cells. Two effects shown: 1) UTERINE EFFECT - myometrial contractions with calcium signaling cascade, cAMP and IP3 pathways labeled; 2) CERVICAL RIPENING - cervical collagen breakdown, prostaglandin softening the cervix (show collagen fibers dissolving), cervix becoming soft and dilated. Oral pill shown with arrows to multiple routes: oral, sublingual, vaginal, rectal. Heat-stable molecule shown. Side effects bubble: fever thermometer, diarrhea icon. Rich 3D colors, dark background, medical education style.
3D medical comparison table poster: Oxytocin vs Ergometrine vs Misoprostol vs Carboprost - all oxytocics on uterus. Four 3D columns each topped with drug molecule icon. Rows comparing: Source (pituitary gland, fungus, synthetic prostaglandin), Receptor (OT receptor, alpha+5HT, EP receptor, FP receptor), Contraction type (rhythmic wave, tonic clamp, rhythmic+ripening, tonic), Effect on BP (down arrow mild, UP arrow danger red, neutral, up), Oral use (X cross, checkmark green, checkmark green, X), Use in HTN (safe green tick, DANGER red X, safe green tick, safe green tick), Use in Asthma (safe, safe, safe, DANGER red X carboprost bronchospasm), Key use labels. 3D depth shadows, vibrant color coding, bold educational medical poster, dark navy background.
3D cutaway anatomical medical poster showing contraindications of ergometrine. Central 3D uterus with ergometrine molecule. Surrounding it are 8 red danger zones showing contraindications: 1) Hypertension - blood pressure gauge going high with brain stroke icon; 2) Pre-eclampsia/Eclampsia - pregnant woman with swollen face; 3) Heart disease - 3D heart with coronary artery spasm; 4) Peripheral vascular disease - leg with blocked vessel; 5) Fetal presence (induction) - fetus with asphyxia warning; 6) Renal failure - kidney icon; 7) Hepatic failure - liver icon; 8) Migraine - head with pain. Each contraindication shows WHY: vasoconstriction arrows, BP spike graphs, coronary spasm. Bold red warning labels. Dark background, dramatic 3D medical illustration style.
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