Magnesium sulfate: the most tested obstetric drug
Magnesium sulfate is used to prevent seizures in pre-eclampsia and eclampsia and, in some settings, for fetal neuroprotection in preterm labour. It is a central nervous system depressant, so every monitoring parameter on the exam relates to detecting excessive depression before it becomes dangerous.
The three parameters to memorize are deep tendon reflexes, respiratory rate and urine output. Reflexes disappear first as levels rise, making them an early warning. A respiratory rate below 12 breaths per minute signals respiratory depression. Urine output below 30 mL per hour matters because magnesium is renally excreted, so oliguria causes accumulation. Any of these findings means stopping the infusion and notifying the provider.
Calcium gluconate is the antidote and should be immediately available at the bedside. Expect SATA items asking what equipment and medications must be present during a magnesium infusion: calcium gluconate, an infusion pump, continuous fetal monitoring and frequent maternal assessment all belong in the correct set.
| Parameter | Expected | Action if abnormal |
|---|---|---|
| Deep tendon reflexes | Present, may be diminished | Absent reflexes: stop infusion, notify provider |
| Respiratory rate | 12 or more per minute | Below 12: stop infusion, give calcium gluconate as ordered |
| Urine output | 30 mL/hr or more | Below 30 mL/hr: stop or slow, notify provider |
| Level of consciousness | Alert, may be drowsy | Marked lethargy: reassess and notify |
| Fetal heart rate | Reassuring baseline | Non-reassuring: reposition, oxygen, notify |
Oxytocin and uterine stimulants
Oxytocin is used to induce or augment labour and to control postpartum bleeding, and the exam tests two very different contexts. During labour, oxytocin is titrated to achieve an adequate contraction pattern while protecting the fetus. Tachysystole — more than five contractions in ten minutes averaged over thirty minutes — reduces placental perfusion, and the nursing response is to stop the infusion, reposition the client to a side-lying position, give oxygen by non-rebreather mask, increase the intravenous fluid rate and notify the provider.
After birth, oxytocin is first line for uterine atony, the most common cause of early postpartum haemorrhage. Nursing care begins with fundal massage and emptying the bladder, because a distended bladder displaces the uterus and prevents contraction. If bleeding continues, additional uterotonics are used.
Methylergonovine is effective but contraindicated in hypertension and pre-eclampsia because it causes vasoconstriction; a stem describing a hypertensive postpartum client with atony is testing whether you withhold it and question the order. Carboprost is avoided in asthma because it can cause bronchospasm, and misoprostol commonly causes fever and shivering.
- •Tachysystole: stop oxytocin, side-lying position, oxygen, fluids, notify
- •Postpartum atony: massage the fundus and empty the bladder first
- •Methylergonovine: check blood pressure, withhold if hypertensive
- •Carboprost: avoid in asthma; expect diarrhoea and fever as side effects
Preterm labour and fetal lung maturity
Tocolytics buy time rather than stopping labour indefinitely, and the time bought is used to give corticosteroids. Terbutaline relaxes uterine smooth muscle and reliably causes maternal tachycardia, palpitations, tremor and hyperglycaemia; a maternal heart rate above roughly 120 beats per minute or chest pain is a reason to hold the dose and notify. Nifedipine and indomethacin are also used, with nifedipine causing hypotension and headache.
Betamethasone or dexamethasone is given intramuscularly to the mother to accelerate fetal lung surfactant production, ideally between 24 and 34 weeks and at least 24 to 48 hours before delivery. Maternal hyperglycaemia is an expected effect, which matters for clients with diabetes.
Magnesium sulfate reappears here for fetal neuroprotection in anticipated very preterm birth, with the same monitoring parameters as in pre-eclampsia. Recognizing that one drug has several obstetric indications, each with identical safety checks, simplifies a large amount of content.
Rh immune globulin and other essentials
Rho(D) immune globulin prevents maternal sensitization when an Rh-negative mother carries an Rh-positive fetus. The standard timing is at about 28 weeks of gestation and again within 72 hours of birth if the newborn is Rh positive, plus after any event that can cause fetomaternal bleeding such as amniocentesis, abdominal trauma, miscarriage or ectopic pregnancy. It is given to the mother, never to the newborn, and it is not given if the mother is already sensitized.
Other recurring items include folic acid supplementation to prevent neural tube defects, iron supplementation with vitamin C for absorption and a warning about constipation and dark stools, and the categorical avoidance of live vaccines, isotretinoin, warfarin and ACE inhibitors in pregnancy. For newborns, vitamin K prevents haemorrhagic disease and erythromycin eye ointment prevents ophthalmia neonatorum.
Practise these with full maternity sets so the drugs appear in clinical context rather than as a list. Each of the maternal and newborn practice sets mixes pharmacology with assessment, which is how they appear on the real exam.