Absent reflexes: stop the magnesium
This client is receiving IV magnesium sulfate for a serum magnesium of 1.0 mg/dL. At the 2-hour check, her patellar reflexes, which were 3+ at the start, are now absent. Loss of the deep tendon reflexes is the earliest clinical sign of magnesium toxicity, appearing before respiratory depression, so the nurse stops the infusion and reports the finding. Her respiratory rate of 14/min and urine output of 40 mL/h are reassuring for now, but they do not make it safe to keep giving magnesium.
Mechanism: magnesium and neuromuscular transmission
Magnesium blocks calcium entry at nerve terminals and reduces acetylcholine release at the neuromuscular junction. As levels rise, transmission is depressed in a predictable order: first the deep tendon reflexes disappear, then respiratory depression develops as the respiratory muscles weaken, and at the highest levels cardiac conduction is affected and cardiac arrest can occur. Reflexes therefore act as an early warning that allows the infusion to be stopped while breathing is still adequate. Magnesium is excreted by the kidneys, so urine output is monitored throughout.
| Sign | Meaning | Nursing response |
|---|
| Reflexes present, RR normal, urine 30 mL/h or more | Safe to continue as ordered | Continue monitoring |
| Reflexes absent | Early toxicity | Stop infusion and report |
| Respiratory depression | Advanced toxicity | Stop, support breathing, give calcium gluconate as ordered |
| Low urine output | Reduced magnesium excretion | Report; toxicity risk rises |
Why the other options are unsafe
Continuing because breathing and urine output are adequate ignores the warning sign that comes before respiratory depression. Slowing the infusion by half still adds magnesium while a toxicity sign is present. Calcium gluconate is the antidote and should be kept ready, but the first step is to stop the source; the antidote is given as ordered, mainly for respiratory or cardiac depression, not as a substitute for stopping the infusion.
Watch out! In magnesium questions, the order of signs is often the key. Reflexes go first, then breathing, then the heart. Absent reflexes with normal breathing still mean stop.
Exam takeaway
Key point! During IV magnesium, check deep tendon reflexes, respiratory rate, urine output, and level of consciousness regularly. If reflexes disappear, stop the infusion, notify the provider, and keep calcium gluconate at hand. The same rule applies whether magnesium is given for hypomagnesemia or for seizure prevention in preeclampsia.