It's 0130 and you're taking handoff on a 33-weeker admitted for preeclampsia with severe features. The magnesium sulfate has been infusing at 2 grams an hour since 1900. The off-going nurse tells you the patient is "finally resting comfortably." You go in to do your own assessment, and here's what you find: she's drowsy but arousable, her respiratory rate is 11, and you have to work to find a patellar reflex at all. The last charted assessment, an hour ago, reads reflexes 2+, respirations 16.

Resting comfortably — or accumulating magnesium? That question is the whole job when you have a patient on mag, and the answer never comes from the pump or the monitor first. It comes from you, at the bedside, every hour.

What magnesium sulfate is doing — and what it isn't

Magnesium sulfate is given in preeclampsia with severe features and eclampsia for one reason: seizure prophylaxis. It is not an antihypertensive. This trips up more new nurses than almost anything else on a hypertensive patient. If the blood pressure is 165/112 and the mag is running, the mag is not treating that number — an antihypertensive like IV labetalol, IV hydralazine, or oral nifedipine does that work, on its own timeline, with its own escalation path. If you're watching a severe-range pressure and thinking "well, she's on mag," read our guide to hypertensive emergency in pregnancy, because those are two separate emergencies running in parallel.

A typical regimen per ACOG guidance is an IV loading dose of 4 to 6 grams over 20 to 30 minutes, then a maintenance infusion of 1 to 2 grams per hour, often continued through roughly 24 hours after delivery — but your order set and your facility protocol govern, always. What matters for your practice is this: magnesium is a high-alert medication with a narrow margin between the therapeutic range and toxicity, and it is cleared almost entirely by the kidneys. A preeclamptic patient is exactly the patient whose kidneys may not be working well. That combination — narrow margin plus unreliable clearance in the very disease you're treating — is why the hourly assessment exists.

During the loading dose, expect flushing, warmth, and sometimes nausea. Tell the patient before it starts: "You're going to feel hot, like a sunburn from the inside. That's expected and it passes." A patient who's been warned tolerates the load; a patient who wasn't is frightened, and you spend the next twenty minutes reassuring instead of assessing. One more flag worth carrying in your head: magnesium is generally avoided in patients with myasthenia gravis — if that's in the history, speak up before the bag is hung.

Recognize: the hourly assessment that catches toxicity first

Magnesium toxicity announces itself in a predictable order, and the order is the gift. Deep tendon reflexes diminish and then disappear before respiratory depression develops, and respiratory depression develops before cardiac compromise. That sequence is why the patellar reflex check is not a box to click — it's your earliest warning system. If you can still elicit reflexes, the dangerous territory is still ahead of you. If the reflexes are gone, you are already standing in it.

So every hour, with your own hands and eyes:

  • Deep tendon reflexes. Check the patellar reflex and grade it the same way every time. If the patient has an epidural, patellar reflexes may be unreliable — move up and use the biceps or brachioradialis instead, and chart which site you used so the next nurse trends the same one.
  • Respiratory rate and effort. Count a full minute yourself; don't trust the monitor's number on a drowsy patient. Most protocols treat a rate below 12 as a stop-and-escalate finding. Watch depth and effort, not just the count — shallow and slow together is worse than either alone.
  • Urine output. Because magnesium leaves through the kidneys, falling urine output means rising magnesium even when the pump rate never changed. Many protocols flag output below about 30 mL per hour. An indwelling catheter with a urometer isn't overkill here; it's how you get a trustworthy hourly number.
  • Level of consciousness. "Drowsy on mag" is common and expected. The question is trajectory: is she the same amount of drowsy as last hour, or more? Arousable to voice, or only to touch? Chart the words, not a vibe.

Two situations change the math and deserve extra vigilance: any bolus or rate increase, and any downtrend in urine output. If output is falling, the same 2 grams an hour is no longer the same dose — clearance has changed underneath you. That's a provider conversation before it becomes a toxicity event, not after.

Know where your calcium gluconate is before you ever need it. It's the reversal agent for magnesium toxicity, and on most units it lives at the bedside or in the top drawer of the cart for exactly this patient. If you can't put your hand on it in under a minute, fix that at the start of your shift, not at 0300.

Escalate: what you say when the picture changes

Back to your 0130 patient: respirations 11, reflexes barely present, more drowsy than an hour ago, and when you check the urometer, 20 mL for the past hour. You do not watch and re-check in an hour. Most facility protocols direct you to stop the infusion for signs of toxicity and notify the provider immediately — know whether yours does, because seconds of hesitation over "am I allowed to stop this?" are seconds you don't owe anyone. Stay with the patient, put eyes on the airway, and make the call sound like this:

"Dr. Patel, this is Maloree on L&D calling about Jordan Reyes in Room 4 — 33 weeks, preeclampsia with severe features, on magnesium at 2 grams an hour since 1900. In the last hour her respiratory rate dropped from 16 to 11, her patellar reflexes went from 2+ to barely present, and she's harder to rouse. Urine output was 20 mL this past hour. I've stopped the magnesium per our protocol and calcium gluconate is at the bedside. I need you at the bedside now, and I need an order for a stat magnesium level. How soon can you be here?"

Notice the anatomy of that call: the trend, not just the snapshot ("dropped from 16 to 11," not "RR is 11"); what you've already done; and a specific ask with a timeline question that requires a commitment. A provider can wave off a number. It's much harder to wave off a direction of travel plus a nurse who has already acted. If the response doesn't match the urgency, that's a chain-of-command moment — and the same severe-features patient who earned the mag in the first place has usually given you other warning signs worth naming on that call; our piece on preeclampsia with severe features covers the ones nurses catch first.

Document: the pattern that protects your patient and your license

Magnesium charting fails in one predictable way: "WNL" and copied-forward assessments. If the 0030 entry says reflexes 2+ and respirations 16, and the truth at 0030 was already drifting, the 0130 nurse inherits a false baseline and the trend is invisible until it's dramatic. Chart the actual findings, every hour, even when they're boring — especially when they're boring, because the boring entries are what make the first abnormal entry stand out.

A charting pattern that holds up:

  • Every hour: infusion rate, reflex grade and site, respiratory rate you counted, urine output in mL, and level of consciousness in plain words ("arousable to voice, oriented, falls back asleep").
  • At any rate change or bolus: the order, the time, the second-nurse verification your policy requires for a high-alert infusion, and your next assessment moved up accordingly.
  • When you escalate: time of call, who you spoke with by name, exactly what you reported, the orders received, and the provider's stated arrival time. "Dr. Patel notified at 0134 of RR 11, absent patellar reflexes, UOP 20 mL/hr; mag stopped at 0132 per protocol; Dr. Patel to bedside, stat mag level ordered." Then chart that each of those things happened.

If you stopped the infusion, chart the time you stopped it and why. A stopped pump with no note looks like an error; a stopped pump with a documented clinical reason is a nurse doing her job.

This is education, not medical advice — your facility's policy, provider orders, and clinical judgment always come first.

Mag patients are where new L&D nurses learn that the most dangerous hour is the quiet one. Build the hourly ritual, trust your own count over the monitor, and know your first three moves before you need them. For the rest of the hypertensive playbook in one place, Download the free OB Emergency Command Manual.