Preterm Labor

Magnesium prolongs nondepolarizing blockade - Magnesium potentiates rocuronium, vecuronium, and cisatracurium. Give small doses and titrate to a twitch monitor. It does not potentiate succinylcholine, so RSI dosing is unchanged — just expect diminished or absent fasciculations.

Check for magnesium toxicity at the bedside - Therapeutic range is 5 to 9 mg/dL. PR prolongation and QRS widening appear at 6 to 12 mg/dL, loss of the patellar reflex around 12 mg/dL, respiratory arrest and SA/AV block at 15 to 20 mg/dL, cardiac arrest at 24 mg/dL or above. The patellar reflex is the cheapest monitor in the room — check it every time you walk in. Treatment is stopping the infusion and calcium gluconate 1 g IV over 10 minutes.

Magnesium raises hemorrhage risk - Magnesium relaxes the uterus and increases the risk of postpartum hemorrhage; it is also a listed risk factor for uterine atony. Have oxytocin running and a second uterotonic in the room at delivery.

Don't stack magnesium on a calcium channel blocker - Combining magnesium with nifedipine outside the neuroprotection indication risks maternal respiratory depression. Know which tocolytics she has actually received before you add sedation.

Nifedipine plus neuraxial sympathectomy - Nifedipine causes flushing, headache, dizziness, and hypotension on its own. Layer an epidural on top and the fall is bigger than you planned for. Pre-load and have a vasopressor drawn up before you dose the block.

Terbutaline gives you a tachycardic mother - Beta-2 agonists produce maternal tachycardia, arrhythmias, hypotension, and nausea, plus fetal tachycardia. Sort drug effect from hypovolemia before you treat the number. Terbutaline sulfate injection carries a boxed warning against prolonged tocolysis beyond 48 to 72 hours and against maintenance tocolysis at home: serious reactions including death have been reported in pregnant women, specifically increased heart rate, transient hyperglycemia, hypokalemia, cardiac arrhythmias, pulmonary edema, and myocardial ischemia, with fetal tachycardia and neonatal hypoglycemia. A tachycardic, dyspneic mother on day 3 of terbutaline is pulmonary edema until you have proven otherwise.

Indomethacin - A nonselective COX inhibitor, contraindicated after 32 weeks because of premature ductal closure, and contraindicated with bleeding disorders, gastritis, aspirin hypersensitivity, or hepatic impairment. It causes renal afferent vasoconstriction and GI mucosal injury.

Magnesium for fetal neuroprotection - When delivery is expected before 32 weeks, magnesium reduces cerebral palsy risk: typically a 4 g load over 30 minutes then 1 g/hour, given as close as possible to 4 hours before delivery. This one keeps running — it is a fetal indication, not a tocolytic one.

Tocolysis buys 48 hours, not a term pregnancy - The whole point is the antenatal steroid window, GBS status, and transport to a center with a NICU. Use that window to place a labor epidural electively and test it rather than being called for a crash induction.

Know why tocolysis was refused - Preeclampsia with severe features, chorioamnionitis, hemorrhage, intrauterine fetal demise, lethal fetal anomaly, and significant maternal cardiac disease all contraindicate tocolytics. Those are the patients who go to the OR, and each brings its own anesthetic problem.

Rescue cerclage - Cervical cerclage for advanced dilation before 24 weeks is typically done under regional anesthesia in dorsal lithotomy. Short case, but the uterus is irritable and the cervix is already open.

Preterm labor is parturition between 20 0/7 and 36 6/7 weeks — early preterm before 33 weeks, late preterm 34 to 36 weeks. Three things have to happen: cervical change, persistent uterine contractions, and activation of the decidua and membranes. At term that sequence is physiologic; preterm it is pathologic, and infection is the usual driver. A fetal inflammatory response with rising fetal interleukin-6 triggers fetal CRH, ACTH, and cortisol release, while inflammatory cells flooding the cervical stroma release cytokines and prostaglandins that degrade collagen and ripen the cervix. Estrogen accelerates that collagen breakdown, progesterone blocks it, which is why progesterone is used to hold the cervix closed.

What reaches you is a parturient already on tocolytics and steroids. Magnesium, nifedipine, indomethacin, and terbutaline each change her hemodynamics or her response to your drugs, and delivery can be hours away or minutes.