Magnesium Sulfate Infusion in Preeclampsia: Recognising Toxicity at 34 Weeks
Ms. C is 31 years old, G1P0 at 34 weeks and 2 days, admitted 6 hours ago with preeclampsia. She is receiving magnesium sulfate 2 g/h IV after a 4 g loading dose, plus labetalol 200 mg orally twice daily. Her blood pressure is now 158/102 mmHg. Over the last two hours: respiratory rate 11 breaths/min, urine output 25 mL/h via catheter, deep tendon reflexes 1+ and decreasing from 2+, and she says she feels 'flushed and a bit foggy'. The cardiotocograph shows a baseline fetal heart rate of 140 beats/min with normal variability. A serum magnesium level drawn an hour ago is 8.2 mEq/L (therapeutic range 4–7). The charge nurse is off the floor and the obstetrician is in theatre.
1. Interpret these findings. What is happening, and which sign is the earliest indicator that something is wrong?
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This is evolving magnesium toxicity, not worsening preeclampsia.
Evidence:
- Magnesium level 8.2 mEq/L against a therapeutic range of 4–7 - above target.
- Deep tendon reflexes 1+ and falling - the earliest clinical sign of magnesium toxicity. Reflexes are lost before the respiratory and cardiac effects appear, which is precisely why they are checked hourly.
- Respiratory rate 11 breaths/min - trending towards respiratory depression (loss of the reflex arc precedes loss of respiratory drive).
- Urine output 25 mL/h (below the usual 30 mL/h threshold) - magnesium is cleared renally, so oliguria causes accumulation even at an unchanged infusion rate. This is the likely mechanism.
- Flushing, warmth and a 'foggy' feeling - consistent with mild magnesium excess and reduced clearance.
Why it matters: untreated progression is loss of reflexes → respiratory depression/apnoea → cardiac arrest. This is a stop-the-infusion situation, not an observation situation.
Discriminator: worsening preeclampsia gives hyperreflexia or clonus, headache, visual disturbance, epigastric/right upper quadrant pain and rising blood pressure. Magnesium toxicity gives hyporeflexia, falling respiratory rate, oliguria, flushing and drowsiness. Her blood pressure of 158/102 needs treating, but it is not the emergency in front of you.
2. What are your actions in the next five minutes, and what must be at the bedside before the infusion is ever restarted?
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Next five minutes:
- Stop the magnesium infusion (do not flush the line with the infusion).
- Call for help - rapid response/obstetric emergency call, notify the obstetrician in theatre and the covering registrar, and inform the charge nurse/midwife-coordinator.
- Airway, breathing, circulation: position, apply oxygen, continuous SpO2 and cardiac monitoring, count respiratory rate yourself, check conscious state.
- Have calcium gluconate 10% 10 mL available at the bedside as the antidote - give as prescribed if there is respiratory depression, loss of reflexes with hypoventilation, or cardiac arrest; it reverses magnesium toxicity but does not lower the serum level.
- Send an urgent repeat magnesium level and check renal function (raised creatinine explains the accumulation).
- Assess the fetus - continue CTG, report any deceleration or loss of variability.
- Document the time the infusion was stopped, vitals, reflex findings, who was notified.
Before any restart:
- A prescribed order from the obstetrician.
- Normal deep tendon reflexes, respiratory rate ≥12–16/min, urine output ≥30 mL/h.
- Magnesium level back within therapeutic range.
- A reduced infusion rate or alternative agent if renal function is impaired.
Recheck reflexes, respiratory rate and urine output hourly throughout any magnesium infusion - they are the bedside early-warning system that blood tests lag behind.
3. The registrar says, 'She is probably just tired - keep the magnesium running, her pressure is still high.' How do you respond, and what do you do next?
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You escalate, clearly and with the data - this is a patient-safety conversation, not a disagreement about opinion.
What to say (SBAR format):
- Situation: 'Ms. C on magnesium sulfate 2 g/h - I have stopped the infusion.'
- Background: 34+2 weeks, preeclampsia, labetalol 200 mg BD, loading dose 4 g completed 6 hours ago.
- Assessment: 'Reflexes have dropped from 2+ to 1+, respiratory rate is 11, urine output 25 mL/h, magnesium 8.2 mEq/L, and she is drowsy and flushed. Those are the recognised early signs of magnesium toxicity, and I am not comfortable restarting at the current rate.'
- Recommendation: 'I need you to review her now or give me a written plan: repeat magnesium level, renal function, reduce or stop the infusion, and how you want the blood pressure treated with something other than more magnesium.'
Then:
- Ask for the plan in writing and document the conversation and the response times.
- Use the chain of command if the registrar declines and you remain concerned - the consultant obstetrician on call, the nurse/midwife in charge, or the rapid response team. Escalating is not insubordination; it is the expected response to a deteriorating patient.
- Separate the two problems: her hypertension does need treatment - but the answer is an antihypertensive (e.g. labetalol/hydralazine per protocol), not continuing a toxic magnesium infusion. Say this explicitly so the plan addresses both.
- Do not restart the infusion to 'keep the peace' while waiting. Holding a toxic infusion is the reversible, defensible act.