Tocolytics
Key Points
- Tocolytics delay uterine contractions to prevent preterm birth — used between 20–34 weeks’ gestation to allow time for corticosteroid administration and fetal lung maturation
- Goal: Delay delivery 24–48 hours (up to 7 days) to administer antenatal corticosteroids (betamethasone) → ↑ fetal surfactant production
- Magnesium sulfate (MgSO4): Most commonly used tocolytic; CNS and muscular depressant; monitor for toxicity (loss of patellar reflex, respiratory depression)
- Antidote for Mg toxicity: Calcium gluconate (or calcium chloride) IV — must be available at bedside during MgSO4 infusion
- Terbutaline: Beta-2 agonist; used acutely for tachysystole management during oxytocin therapy; NOT for long-term preterm labor
- Gestational-age strategy matters: indomethacin is generally limited to pregnancies under 32 weeks and routine tocolysis is often not used beyond 34 weeks.
Drug Class Overview
Tocolytic drugs inhibit or suppress uterine contractions. The primary clinical indication is preterm labor — defined as regular uterine contractions with cervical change before 37 weeks’ gestation. Tocolytics are used between 20 and 34 weeks to allow time for:
- Administration of antenatal corticosteroids (betamethasone) to accelerate fetal lung maturation
- Transfer to a higher-level neonatal care facility if necessary
Tocolytics do not prevent preterm birth indefinitely — they typically delay delivery by 24–48 hours to 7 days. Because of their potential for serious adverse effects, these medications require continuous maternal and fetal monitoring in an acute care setting.
Magnesium Sulfate (MgSO4)
Mechanism: Acts as a CNS and muscular depressant by:
- Blocking neuromuscular transmission of acetylcholine
- Calcium antagonism — prevents calcium from entering cells and triggering muscle contractions
Primary Clinical Use: Seizure prevention in preeclampsia and eclampsia (definitive use); tocolysis is a secondary/controversial indication.
Dosing:
| Phase | Route | Dose |
|---|---|---|
| Loading dose | IV | 4–6 g over 20–30 minutes |
| Maintenance infusion | IV continuous | 1–3 g/hour; titrate to clinical response |
| IM alternative | Bilateral gluteal IM | 4–5 g each side simultaneously |
Duration: Do NOT use >5–7 days as tocolytic — risk of fetal hypocalcemia, skeletal demineralization, osteopenia, and bone abnormalities with prolonged fetal exposure.
Magnesium Toxicity Monitoring
Magnesium toxicity is life-threatening and requires close monitoring throughout infusion:
| Serum Mg Level (mEq/L) | Clinical Finding | Action |
|---|---|---|
| 4–8 mEq/L | Therapeutic range for preeclampsia/seizure prevention | Continue; monitor |
| 7–10 mEq/L | Loss of patellar (deep tendon) reflexes | Notify provider; hold or reduce rate |
| 10–13 mEq/L | Respiratory depression (<12 breaths/min) | Stop infusion; antidote |
| >15 mEq/L | Cardiac arrest | Emergency response |
Magnesium Toxicity — STOP Infusion Immediately If:
- Patellar reflex absent (DTR = 0) → earliest sign of toxicity
- Respiratory rate <12 breaths/min
- Urine output <30 mL/hour (decreased renal excretion → accumulation)
- Level of consciousness decreased (confusion, lethargy, obtundation)
Antidote: Calcium gluconate 1 g IV slowly — must be at bedside during all MgSO4 infusions
Adverse Effects (Birthing Parent):
- Flushing, sweating, hypotension
- Depressed reflexes, flaccid paralysis, hypothermia
- Circulatory collapse, cardiac and CNS depression progressing to respiratory paralysis
- Hypocalcemia (tetany risk)
Adverse Effects (Fetus/Neonate):
- Heart rate changes, hypotonia (poor muscle tone)
- Respiratory depression, possibly death with prolonged use
Contraindications: Hypersensitivity; toxemia within 2 hours before delivery; myocardial damage; heart block; hypermagnesemia; hypercalcemia; use cautiously in myasthenia gravis and renal impairment.
Terbutaline — Acute Tocolysis
Mechanism: Selective beta-2 adrenergic agonist → relaxes uterine smooth muscle (uterine relaxation).
Primary Use in Labor Management: Acute management of uterine tachysystole (>5 contractions in 10 minutes) during oxytocin administration — rapidly relaxes the uterus.
Dose: 0.25 mg subcutaneous (SQ) — may repeat once in 15–30 minutes if no response.
NOT for long-term use: FDA Black Box Warning — oral or parenteral terbutaline is not approved for prevention or prolonged treatment of preterm labor due to risk of serious maternal cardiac events and death.
Additional limits and contraindications:
- Do not use for prolonged tocolysis beyond 48 to 72 hours.
- Avoid in ischemic heart disease, uncontrolled hypertension, dysrhythmias, hyperthyroidism, poorly controlled diabetes, seizure disorders, or maternal heart rate greater than 120/min.
Adverse Effects: Tachycardia (maternal and fetal), palpitations, tremor, hypokalemia, hypoglycemia, pulmonary edema (with prolonged use).
Additional Short-Term Tocolytic Options
- Indomethacin: COX inhibitor that reduces prostaglandin-mediated cervical ripening and myometrial contraction. Avoid after 32 weeks because of fetal ductus arteriosus constriction risk; also avoid with maternal thrombocytopenia, renal impairment, or GI bleeding.
- Nifedipine: Calcium-channel blocker that relaxes uterine smooth muscle. Monitor for hypotension, dizziness, and headache; avoid in maternal hypotension, heart failure, or significant liver dysfunction.
- Common gestational-age protocol context: Under 32 weeks, indomethacin plus corticosteroids for about 48 hours is common; from 32 to 34 weeks, nifedipine plus corticosteroids is common with selective terbutaline add-on if progression persists.
- More than 34 weeks: Routine tocolysis is often avoided because expected neonatal benefit is lower and maternal-fetal medication risk may outweigh benefit.
Corticosteroids — Companion Therapy
When tocolytics delay preterm delivery, antenatal corticosteroids are administered to accelerate fetal lung maturation:
- Betamethasone 12 mg IM × 2 doses, 24 hours apart — most common regimen
- Promotes fetal surfactant synthesis → reduces risk of neonatal respiratory distress syndrome (RDS)
- Optimal benefit requires ≥24 hours after first dose; maximum benefit at 48 hours–7 days
Nursing Assessment and Monitoring
Before Administration (MgSO4):
- Obtain baseline vital signs, DTRs, respiratory rate, and urine output
- Confirm serum magnesium level and renal function (Mg is renally excreted)
- Ensure calcium gluconate 1 g IV at bedside (antidote for toxicity)
- Apply continuous EFM — monitor for fetal heart rate changes
During MgSO4 Infusion (Every 1–2 Hours):
- Patellar reflex — absent reflex = imminent toxicity; stop infusion
- Respiratory rate — <12/min = toxicity; stop infusion
- Level of consciousness — confusion or lethargy signals accumulation
- Urine output — <30 mL/hr indicates decreased excretion; notify provider
- Serum magnesium levels per institutional protocol
Patient Education:
- Explain goal of therapy: to delay delivery and allow corticosteroids to work
- Describe expected side effects: flushing, warmth, feeling “heavy” or “foggy”
- Instruct to immediately report: difficulty breathing, chest tightness, vision changes, severe weakness
Related Concepts
- uterotonics — Drug class with opposite effect; terbutaline used to counteract oxytocin tachysystole
- preeclampsia — Primary indication for MgSO4; seizure prevention
- nursing-care-during-the-first-stage-of-labor — Labor monitoring context for tocolytic use
- labor-analgesics — Concurrent pain management during preterm labor
- fhr-and-uc-intervention-framework — FHR patterns and uterine contraction assessment
- conditions-limited-to-pregnancy — Preeclampsia, eclampsia, preterm labor as indications
Self-Check
- A patient on MgSO4 infusion has absent patellar reflexes and a respiratory rate of 10. What is the immediate nursing action and what medication is administered?
- Why is MgSO4 used for no more than 5–7 days as a tocolytic even though preterm labor may recur?
- A nurse administers terbutaline 0.25 mg SQ to a patient experiencing tachysystole from oxytocin augmentation. After 20 minutes, the uterus has not relaxed. What additional actions should the nurse take?