Magnesium Sulfate in Obstetrics

SWARAJ HOSPITAL & RESEARCH INSTITUTE

BOLANGIR, ODISHA  •  OBSTETRICS & GYNAECOLOGY CLINICAL PATHWAY SERIES

Clinician Reference Infographic

Magnesium Sulfate in Obstetrics — Eclampsia Prevention, Treatment & Safe Use

Magnesium sulfate prevents and treats eclamptic fits. This sheet gives who needs it, the two standard regimens, the hourly safety checks, the signs of toxicity, and what to do about them.

🎯 AT A GLANCE — MAGNESIUM SULFATE IN OBSTETRICS

Magnesium sulfate (MgSO₄) is the drug of choice to prevent and treat eclamptic fits; it works on the brain and is not an antihypertensive

In the Magpie trial, women with severe pre-eclampsia given MgSO₄ had a 58% lower risk of eclampsia than those given placebo, and in the Eclampsia Trial recurrent fits fell by 52% against diazepam and 67% against phenytoin

Safe use depends on watching three things: patellar reflexes, breathing rate, and urine output, with calcium gluconate ready as the antidote

It is also given to women at risk of preterm birth under about 32 weeks to protect the baby’s brain (from memory, check your protocol)

The safe dose is close to the toxic dose. Never give a repeat dose without checking reflexes, respiration and urine output.

🚨 STOP THE MAGNESIUM AND CALL FOR SENIOR HELP IF…

Patellar reflexes are absent — the earliest sign of toxicity

Respiratory rate is below 12 a minute (some protocols use below 16), or oxygen saturation falls

Urine output is below 30 mL an hour — hold the next dose and check kidney function

She is drowsy, confused, has slurred speech, chest pain, or an abnormal heart rhythm

A fit recurs while on MgSO₄ — treat and look for another cause; do not just repeat the dose without senior advice

STEP 1 · WHO NEEDS IT — indications

🩺 INDICATIONS AND CAUTIONS

Situation	Use of magnesium sulfate
Eclampsia	Treat the fit and prevent further fits; this is the first-line anticonvulsant
Severe pre-eclampsia	Give for prevention of eclampsia, and in many protocols for pre-eclampsia with severe features, such as very high blood pressure, severe headache, visual disturbance, upper abdominal pain, or abnormal blood tests (from memory; follow local protocol)
Mild pre-eclampsia with no severe features	Usually not needed; watch closely and decide with the senior obstetrician (from memory)
Preterm birth expected under about 32 weeks	Considered for fetal neuroprotection (from memory; regimens differ)
Preterm labour, for stopping contractions	Not recommended as a tocolytic (from memory)
Also, remember	Blood pressure still needs its own treatment, and delivery is the definitive treatment for pre-eclampsia

STEP 2 · GIVE IT SAFELY — regimens, set-up and monitoring

💉 THE TWO STANDARD REGIMENS

Regimen	Loading dose	Maintenance dose
Zuspan (IV only)	4 g IV over 20 minutes	1 g an hour IV for 24 hours
Pritchard (IV and IM)	4 g IV, plus 10 g IM (5 g deep in each buttock)	5 g IM every 4 hours, given only if the checks are normal

Doses above are from a published pharmacokinetic study (Du et al., J Clin Pharmacol). The items below are from memory: check them against your hospital and national protocol.

Strength: the 50% solution has 5 g in 10 mL; dilute IV doses as per protocol, and give the IM dose deep with a little local anaesthetic in the syringe to reduce pain (from memory)

Duration: continue for 24 hours after delivery or after the last fit, whichever is later (from memory)

Recurrent fit: a further small IV bolus (about 2 g) is commonly used, with senior advice (from memory)

Kidney disease (raised creatinine or low urine output): maintenance usually needs reduction and blood levels, if available; dose can need adjusting for body weight and creatinine

Use an infusion pump or a clearly labelled, double-checked syringe; magnesium overdose from a dosing error is a recognised harm

📈 SERUM LEVELS AND SIGNS

🔍 MONITORING EVERY HOUR (AND BEFORE EACH IM DOSE)

Check	Safe to continue if…	If not
Patellar reflex	Present	Hold the dose and call a senior
Breathing rate	At least 12 a minute (or your protocol’s limit)	Stop; give oxygen; call a senior
Urine output	At least 30 mL an hour (catheter with hourly bag)	Hold the dose; check kidneys
Oxygen saturation, pulse, blood pressure	Normal and recorded	Escalate
Level of consciousness	Alert	Stop; look for toxicity or another cause
Fetal heart	Normal	Inform the obstetrician

🧪 IF TOXICITY IS SUSPECTED

1 · StopStop the infusion and hold further IM doses

2 · CallCall the senior obstetrician and anaesthetist

3 · AirwayGive oxygen, support breathing with a bag and mask, and be ready to ventilate

4 · AntidoteGive calcium gluconate 1 g (10 mL of 10%) IV slowly over about 10 minutes (from memory; check your protocol)

5 · ReviewCheck the level if available, the urine output and creatinine, and keep monitoring

⚠️ SPECIAL SITUATIONS

Situation	Care
Kidney impairment	Magnesium is cleared by the kidney, so it builds up; reduce the maintenance dose and monitor closely
Myasthenia gravis	Generally avoid (from memory)
Calcium channel blocker (nifedipine) given too	Can be used together, but watch more closely for low blood pressure and weakness (from memory)
Pulmonary oedema or heart disease	Watch fluid balance; avoid fluid overload from the infusion
After delivery	Continue the course; watch for bleeding, as magnesium can relax the uterus (from memory)
Newborn	If magnesium was given close to birth, check the baby for low tone and weak breathing (from memory)

🔬 WHAT THE EVIDENCE SHOWS

A review of 15 randomised trials found magnesium sulfate use was associated with lower maternal mortality (Cochrane, as cited in a published review)

Eclampsia is uncommon in pre-eclampsia given magnesium, so very large trials would be needed to compare different regimens; shorter or lower-dose regimens have not been shown to differ, but the data are limited

Standard regimens are the safest choice unless your protocol says otherwise.

⚠️ COMMON PITFALLS

Giving the next dose without checking the reflexes, breathing and urine

Forgetting that low urine output makes toxicity more likely

No calcium gluconate or bag and mask at the bedside

Treating the fit with magnesium but leaving the blood pressure uncontrolled

Stopping too early or too late: keep it going for 24 hours after delivery

Dose or concentration mix-ups: label, double-check and use a pump

Delaying delivery decisions: magnesium protects against fits, but is not a cure

Memory Aid

CHECK

Five reminders for safe magnesium sulfate.

C

Calcium gluconate and a bag and mask at the bedside

H

Hourly reflexes — absent reflex means stop

E

Examine breathing — at least 12 a minute

C

Count the urine — at least 30 mL an hour

K

Keep going for 24 hours after delivery or the last fit

Remember: magnesium sulfate prevents fits, but it needs a nurse who watches the reflexes, the breathing, and the urine.

Swaraj Hospital & Research Institute · Department of Obstetrics & Gynaecology, Bolangir, Odisha

Clinician-facing summary. Checked against published reviews: the two standard regimens and the pharmacokinetic study (Du et al., J Clin Pharmacol), a Medscape review (therapeutic and toxic levels, urine limit, Magpie and Eclampsia Trial results) and a Cochrane figure cited in it. Items marked “from memory” (indications beyond eclampsia, reflex and breathing limits, calcium gluconate dose, recurrence dose, neuroprotection, special situations) were not checked. Doses and limits differ between guidelines: follow your local protocol and involve a senior obstetrician.