Magnesium sulphate toxicity in severe pre-eclampsia when the obstetrician is in theatre
Mrs A, a 26-year-old primigravida at 36 weeks' gestation, was admitted 6 hours ago with severe pre-eclampsia: BP 166/110 mmHg, 3+ proteinuria, headache and brisk reflexes. She received a loading dose of magnesium sulphate 4 g IV over 20 minutes, followed by a maintenance infusion of 1 g/hour through a syringe pump. It is now 19:30; the infusion was started at 14:20. Her hourly check reveals: patellar reflexes absent, respiratory rate 10/min and shallow, SpO2 94% on room air, urine output 20 mL in the past hour, and she is drowsy but rousable. The fetal heart rate is 140 beats/min. The obstetrician is scrubbed in theatre for an emergency caesarean and the anaesthetist is with her. The ward stocks 10% calcium gluconate 10 mL ampoules and there is no serum magnesium assay available at night. The nurse is also responsible for five other antenatal patients.
1. Interpret the findings and give your priority order of actions, with the reason for each.
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Interpretation: magnesium sulphate toxicity. Absent patellar reflexes occur at roughly 7–10 mEq/L, respiratory depression at 10–13 mEq/L and cardiac arrest above about 15 mEq/L. She has two of the three classic warning signs (loss of deep tendon reflexes, respiratory depression) with drowsiness. Oliguria (20 mL/h) is the likely precipitant: magnesium is cleared by the kidney, so a fixed 1 g/hour accumulates when urine output falls.
Priority order:
- Stop the infusion. The commonest error is to slow it instead of stopping it; absorption already in the circulation will not fall while magnesium continues to enter.
- Call for help - obstetrician in theatre, anaesthetist, labour ward; bring the resuscitation trolley, oxygen and suction.
- Airway and breathing: oxygen, correct positioning, bag-mask ventilation ready, continuous SpO2 and respiratory monitoring. Respiratory arrest is the immediate threat to life.
- Give the antidote once available: 10% calcium gluconate 10 mL (1 g) IV slowly over 3–10 minutes with cardiac monitoring.
- Monitor hourly: reflexes, respiratory rate, SpO2, urine output, conscious level and fetal heart; send blood for magnesium, creatinine and electrolytes as soon as the laboratory opens.
- Do not restart magnesium until reflexes and respiration are normal; control BP with parenteral labetalol or hydralazine and plan delivery - delivery is the definitive treatment for severe pre-eclampsia.
2. Calculate the cumulative magnesium dose she has received. Explain why the urine output matters and write out exactly how the antidote should be prescribed and given.
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Cumulative dose: loading 4 g at 14:00, then 1 g/hour from about 14:20 to 19:30 - 5 hours 10 minutes, approximately 5.2 g. Total ≈ 9.2–9.5 g. The precise figure matters less than the trend: a large cumulative dose has been given while her urine output has fallen.
Why urine output matters: magnesium is excreted almost entirely unchanged by the kidney. At 20 mL/h she is oliguric, so even a "standard" 1 g/hour maintenance rate raises plasma magnesium. This is why the maintenance rate must be reduced or withheld in renal impairment and why hourly urine output is a mandatory observation during magnesium therapy, alongside reflexes and respiratory rate.
Antidote prescription to expect and check: calcium gluconate 10% 10 mL (equivalent to 1 g) by slow intravenous injection over 3–10 minutes, with continuous cardiac monitoring; a second dose may be given if respiratory depression persists. Calcium chloride 10% 10 mL is an alternative but delivers about three times the elemental calcium, is highly irritant and must never be given intramuscularly or as a rapid bolus.
Document time of stopping the magnesium, time and dose of calcium gluconate, reflex and respiratory response after each dose, and who was informed. Keep the antidote and the resuscitation trolley at the bedside until reflexes return.
3. The obstetric registrar telephones and says: "Just halve the infusion and continue; we will deliver in the morning." Critique this instruction and state your plan, including what you would document.
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Critique: the instruction is unsafe on two counts. First, she already has clinical toxicity with oliguria and no assay is available overnight; continuing any magnesium when reflexes are absent and the respiratory rate is 10/min risks apnoea and cardiac arrest, because the level cannot be assumed to fall while magnesium is still entering. Second, the definitive treatment of severe pre-eclampsia is delivery; deferring it to morning for theatre convenience exposes mother and fetus to eclampsia, abruption and HELLP syndrome.
Plan: keep the magnesium stopped. Monitor reflexes, respiratory rate, SpO2, urine output and conscious level at least hourly, with calcium gluconate 10 mL at the bedside and resuscitation equipment checked. Control BP with parenteral labetalol or hydralazine to keep systolic about 130–150 mmHg and diastolic 80–100 mmHg. Maintain continuous fetal monitoring and left lateral tilt. Escalate to the consultant obstetrician on call and to the nursing supervisor, and request senior review now rather than in the morning.
Documentation: record the registrar's instruction verbatim, your concern and its clinical basis, the actions you took, whom you informed and at what time, and the patient's condition at each review. A contemporaneous, factual note is both good care and the nurse's professional protection.