Nursing care
Absent reflexes during a magnesium sulfate infusion: what the nurse does first
Written and reviewed by Leilani Kealoha, ADN, RN, PALS · 4 min read · Updated October 2026
Short answer
Stop the magnesium sulfate infusion. Loss of the patellar reflex is an early clinical sign of magnesium toxicity, and respiratory depression can follow. Then assess breathing and level of consciousness, notify the provider, and have calcium gluconate, the antidote, ready as prescribed. Charting the finding and rechecking reflexes later lets magnesium keep rising.
First action: stop the infusion
The first action is to stop the magnesium sulfate infusion. Drug labelling describes disappearance of the patellar reflex as a useful sign of the onset of magnesium toxicity. Magnesium depresses nerve and muscle function, and reflexes fade before the more dangerous effects on breathing and the heart, so a lost reflex is a warning to act now.
This is a breathing and safety priority. As magnesium levels climb, labels describe drowsiness, loss of reflexes and low blood pressure, then respiratory depression, paralysis and cardiac arrest. Stopping the infusion halts further rise. Because magnesium is cleared by the kidneys, a client with poor urine output can accumulate it even on a steady infusion.
Check breathing, then the rest of the picture
Immediately after stopping the infusion, assess the respiratory rate, depth and oxygen saturation, and the client's level of consciousness. Respiratory depression is the most serious effect, and labels state that breathing support must be provided if needed until calcium can be given. Keep the airway clear and have suction and a bag valve mask available.
Then review the urine output, blood pressure and fetal heart rate if the client is pregnant. Low urine output is a recognised cause of accumulation. Notify the provider promptly with the findings, and expect a serum magnesium level to be ordered. The provider decides whether and when the infusion restarts, and at what rate.
Calcium gluconate: ready, not routine
Calcium antagonises the effects of magnesium, and labelling states that injectable calcium gluconate should be immediately available whenever magnesium sulfate is given. In a toxicity scenario the nurse prepares it and gives it as prescribed or under protocol. It is not a reason to delay stopping the infusion or assessing breathing.
Some tasks can wait or be shared. Another nurse or an assistant can fetch the emergency equipment, call the provider or rapid response team, and help position the client. Ongoing reflex checks, breathing assessment and deciding on escalation stay with the registered nurse. Documentation of the times, findings and actions follows once the client is stable.
Worked scenario: preeclampsia on a magnesium infusion
Hypothetical practice question: a client with severe preeclampsia receiving magnesium sulfate has absent patellar reflexes and is drowsy. The respiratory rate is lower than at the last check. Which action should the nurse take first? Options: document the finding and recheck in an hour; give calcium gluconate; stop the infusion; increase the maintenance IV fluid.
Stopping the infusion is correct because it ends further magnesium delivery. Calcium gluconate is the antidote and is very tempting, but it needs a prescription or protocol and the source of toxicity should be removed first. Documenting and rechecking lets the level keep rising. Extra fluid does not reverse toxicity and risks overload in preeclampsia.
Why the tempting distractors are wrong
Decreasing the rate rather than stopping is a common distractor. It feels moderate, but with reflexes gone and breathing slowing, any further magnesium adds to the risk. Checking reflexes on the other side or in the arms can be part of the assessment, but it should not delay stopping the infusion when the finding fits the wider picture.
Hyperactive reflexes are a different finding. In preeclampsia they suggest nervous system irritability and seizure risk, the concern magnesium is used to prevent. The exam contrast is direction: brisk reflexes are a reason for the drug, while absent or sharply reduced reflexes on the drug are a warning of toxicity that needs action.
Sources and further reading
Magnesium Sulfate 50% w/v Solution for Injection/Infusion, Summary of Product Characteristics, electronic medicines compendium. Checking the patellar reflex, respiratory rate and urine output; progression of toxicity signs; calcium gluconate kept immediately available; calcium antagonising magnesium.
Preeclampsia and Eclampsia, MSD Manual Professional Edition. Use of magnesium sulfate in preeclampsia and calcium gluconate for symptomatic toxicity, including a sudden decrease in reflexes or hypoventilation.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our maternity and newborn practice questions are the closest set to what this page covers.
Common questions
Which assessments are checked during a magnesium sulfate infusion?
Labelling and nursing protocols focus on the patellar reflex, respiratory rate and urine output, along with level of consciousness and blood pressure. In pregnancy the fetal heart rate is also monitored. The exact thresholds and how often to check come from the facility protocol and the prescriber.
Why does low urine output matter on magnesium sulfate?
Magnesium leaves the body through the kidneys. If urine output falls, magnesium can build up even when the infusion rate has not changed. That is why reduced output is reported promptly and reviewed alongside reflexes and breathing when toxicity is suspected.
Is flushing on magnesium sulfate a sign of toxicity?
Flushing, warmth and nausea are described as common effects, and labels list flushing among early signs as levels rise. On their own they do not prove toxicity. Loss of reflexes, drowsiness, slowed breathing or low blood pressure are more concerning and need prompt action.