Uterotonics and tocolytics
Stem
A 30-year-old woman, 64 kg, at 30 weeks is in preterm labor. She has chronic hypertension (blood pressure 152/98 mmHg) and asthma treated with an albuterol inhaler. A magnesium sulfate infusion has run for 20 hours, and she received two doses of subcutaneous terbutaline this morning. Her heart rate is 116 and potassium 3.1 mmol/L. The fetus is breech and labor is advancing, so the obstetrician books an urgent cesarean.
Question
What do her tocolytics mean for your anesthetic, and which uterotonics will you use after delivery?
Ideal answer
HeadlineHer tocolytics shape both the anesthetic and the bleeding risk: a spinal with a magnesium-aware backup, and uterotonics chosen around her hypertension and asthma.
- Glucose checked, fluids careful, lungs watched. Terbutaline explains the tachycardia and low potassium.1
- Continue magnesium; check reflexes and breathing. For neuroprotection; loss of the patellar reflex is the first sign of toxicity.2
- Spinal; if general, full intubating dose, reduced later doses. Magnesium makes a spinal the clear choice: it potentiates neuromuscular blockers, so quantitative monitoring confirms reversal.3
- Oxytocin 10–40 units in up to 1 liter. Titrated to uterine tone: magnesium and terbutaline both relax the uterus, so I expect atony.4
- Then misoprostol 600–1000 mcg; tranexamic acid 1 g early. Methylergonovine is contraindicated by her hypertension and carboprost avoided in asthma;5,6 TXA over 10 minutes if bleeding continues.6
Watch for: quantified blood loss, with blood products called early.
Breaking down the answer
- Read the drug list as physiologic problems — beta-agonists also cause hyperglycemia, arrhythmias, myocardial ischemia and pulmonary edema;1 magnesium running close to 24 hours before delivery can depress the newborn, so I warn the neonatal team.2
- Oxytocin is first line, with respect — it has an intrinsic antidiuretic effect, and water intoxication with seizures has occurred with prolonged high-dose infusions.4
- Match second-line drugs to contraindications — methylergonovine is contraindicated in hypertension and preeclampsia and is given IV only as a lifesaving measure over at least 60 seconds; it can cause coronary vasospasm.5 Carboprost carries a caution in asthma and cardiovascular disease.7
- Why the clock matters — in the WOMAN trial, TXA reduced deaths from bleeding when given within 3 hours of birth; its ampoules are kept away from the neuraxial tray to prevent a wrong-route intrathecal injection.6
- Tocolytics have time limits — terbutaline should not be used beyond 48–72 hours or for maintenance, and magnesium beyond 5–7 days can cause fetal hypocalcemia and bone abnormalities.1,2
Follow-up question
The uterus stays boggy after oxytocin, and before you can object the obstetrician injects carboprost 250 mcg into the myometrium. Five minutes later your awake patient is wheezing, her respiratory rate is 32, and SpO2 has fallen to 89%. What do you do?
Follow-up answer
HeadlineProstaglandin F2-alpha bronchospasm in an asthmatic, possibly worse supine; I tell the obstetrician no more carboprost and call for help.
100% oxygen by face mask, sitting her up as far as the surgery allows, nebulized albuterol in line with the oxygen. I check the block level to rule out a high spinal. If she keeps deteriorating, IV epinephrine in small doses and preparation to intubate: ketamine or propofol, a magnesium-adjusted relaxant plan, and a volatile kept low, because it bronchodilates but also relaxes the atonic uterus. Atony continues to be managed with misoprostol, tranexamic acid and mechanical methods.6 Then wheeze, saturation, blood loss and uterine tone again, and I document the reaction.
Pitfalls that cost points
- Pushing large undiluted oxytocin boluses; it is an infusion, not a rapid IV push.
- Never mentioning tranexamic acid or the risk of wrong-route injection.
- Stopping magnesium at incision without a plan, or ignoring neonatal depression.
Rapid-fire
Q: What is the carboprost dosing limit? A: 250 mcg IM, repeated every 15–90 minutes, no more than 2 mg (8 doses) in total.7
Q: Which drugs interact dangerously with methylergonovine? A: Strong CYP3A4 inhibitors such as protease inhibitors, macrolides and azole antifungals.5
Q: What serum magnesium controls seizures? A: 2.5–5 mEq/L; reflexes diminish above 4 mEq/L.2
Q: How fast does IM oxytocin work? A: Within 3–5 minutes, lasting 2–3 hours; IV works almost immediately and fades within an hour.4
Q: Does the rocuronium label address magnesium? A: Yes; it lists magnesium salts as enhancing neuromuscular block, so I reduce redosing, not the intubating dose.3
Q: What tranexamic acid regimen? A: 1 g IV over 10 minutes, with a second 1 g if bleeding continues after 30 minutes.6
Numbers to know
| Item | Value |
|---|---|
| Oxytocin for postpartum bleeding | 10–40 U in up to 1000 mL IV, or 10 U IM4 |
| Methylergonovine | 0.2 mg IM every 2–4 h; IV over ≥60 s only5 |
| Carboprost | 250 mcg IM every 15–90 min, max 2 mg7 |
| Misoprostol | 600–1000 mcg6 |
| Tranexamic acid | 1 g IV over 10 min, within 3 h of birth6 |
| Terbutaline limit | Not beyond 48–72 h1 |
| Magnesium limit | Fetal bone effects beyond 5–7 days2 |
Links
- Obstetric hemorrhage: previa, accreta, atony, Pre-eclampsia and eclampsia, General anesthesia for cesarean delivery, Neuromuscular blockade, monitoring and reversal, Intraoperative bronchospasm
- ACOG Practice Bulletin 183 (Postpartum Hemorrhage); FDA labeling for oxytocin, methylergonovine, carboprost, magnesium sulfate and terbutaline
Sources
- 1.Terbutaline sulfate injection prescribing information (boxed warning) · FDA / DailyMed (Areva) · dailymed.nlm.nih.gov
- 2.Magnesium Sulfate in Water for Injection prescribing information · FDA / DailyMed (Hospira) · dailymed.nlm.nih.gov
- 3.Rocuronium bromide injection prescribing information · FDA / DailyMed · dailymed.nlm.nih.govSection 7.6: magnesium salts given for toxemia may enhance neuromuscular blockade.
- 4.PITOCIN (oxytocin) injection prescribing information · FDA / DailyMed (Par Health) · dailymed.nlm.nih.gov
- 5.Methylergonovine maleate injection prescribing information · FDA / DailyMed (American Regent) · dailymed.nlm.nih.gov
- 6.Postpartum Hemorrhage: Medications to Treat Uterine Atony · The ObG Project, 2022 · obgproject.comSummary drawing on ACOG Practice Bulletin 183 and the WOMAN trial.
- 7.Hemabate (carboprost tromethamine) prescribing information · Pfizer (FDA labeling) · pfizermedical.com
Facts on this page were checked against these sources on Oct 15, 2026. Guidelines change; confirm doses against current institutional protocols.