Acute Hypomagnesaemia: The Evidence Behind the Protocol

A 64-year-old retired schoolteacher is admitted with weakness and palpitations. He has taken pantoprazole every morning for nine years, and furosemide for his heart failure since last winter. His potassium is 2.9 mEq/L. He has had potassium chloride for two days, and the potassium has hardly moved. On the third morning someone sends a magnesium. It comes back at 1.2 mg/dL, and his ECG shows a long QTc. This is an illustrative case, not a real patient.

By the end of this article you will know why his potassium would not rise, how much magnesium to give him and how fast, and what to do about the pantoprazole.

1. Why the Potassium Would Not Rise

“His problem is potassium. Why are we talking about magnesium?” Because magnesium sits underneath both. The tool used to open its Evidence tab with this sentence:

Magnesium is a cofactor for over 300 enzymatic reactions, including the Na+/K+-ATPase pump that holds the potassium gradient, the adenylate cyclase that carries PTH signalling, and ATP metabolism itself.

A cofactor is a helper that an enzyme needs before it can work. The Na+/K+-ATPase is the pump in every cell membrane that pushes sodium out and pulls potassium in. Adenylate cyclase carries the signal of parathyroid hormone (PTH) inside the cell. Take the magnesium away and all three falter at once.

That is why the tool says, and still says, that fixing magnesium first is often the key to fixing everything else. It names three consequences: a refractory hypokalaemia, because the kidney goes on wasting potassium through its ROMK channels; a refractory hypocalcaemia, because PTH is neither released nor obeyed; and cardiac arrhythmias, a prolonged QT and torsades de pointes. Our patient has the first and the third. When the potassium is low as well, the tool’s result says so plainly: it will not respond to KCl until the magnesium is corrected.

2. How Much, and How Fast

His magnesium of 1.2 mg/dL falls in the moderate band, 1.0 to 1.4 mg/dL. A prolonged QTc, or symptoms, also count as moderate in the tool. Here is the intravenous table the tool used to carry as one. Look at the rate column first, because that is where the danger lies:

For him, that is 2 to 4 g of magnesium sulphate over 4 to 6 hours, followed by oral maintenance. Only the severe band and the arrest get a fast first dose.

“Then why not give the 4 g in half an hour and be done?” Because magnesium in excess depresses the nerves and the heart, and the signs come in a fixed order. The tool keeps that order in red: loss of the deep tendon reflexes first, then respiratory depression, then hypotension, then cardiac arrest. So check the knee jerk before each dose. A reflex that has gone is your warning, and it comes well before the breathing slows.

If toxicity does appear, the antidote is intravenous calcium gluconate 10%, 10 to 20 mL (1 to 2 g), over 5 to 10 minutes. It antagonises magnesium rather than removing it, so the effect wears off and the dose may need repeating. The kidney is what normally clears magnesium. In anuria or established renal failure, haemodialysis is what removes it.

3. The Pantoprazole

“He has taken it for nine years without trouble. Can it really be the pantoprazole?” It can, and the nine years are part of the story. Chronic use of a proton pump inhibitor (PPI) reduces magnesium absorption from the gut by inhibiting the TRPM6/7 channels. The effect is not dose-dependent, and it appears only after months to years of use. That is exactly why it is missed in a patient who has taken the drug uneventfully for a decade.

Here is the part that matters at the bedside. The low magnesium is not corrected by intravenous or oral magnesium while the PPI continues, because the absorptive defect is still there. The tool’s advice is to switch to an H2-blocker if possible. If the PPI is essential, add high-dose oral magnesium and monitor the level regularly.

4. Which Tablet?

Once the intravenous course is done, he goes home on tablets. Not every magnesium tablet carries the same amount of magnesium, and not every gut accepts it. The tool used to set out the choice in one table:

Read the first row against the third column. Magnesium oxide is 60% magnesium by weight, but very little of it is absorbed. Even so, it is the oral magnesium most commonly available in Indian government hospitals: Mag-SR 400 mg carries about 240 mg of elemental magnesium, and it works at higher doses.

“Then why not glycinate for everyone?” It is the best tolerated, but it is harder to find, and the tool lists only imported brands for it. The real limit with the oxide is diarrhoea, and diarrhoea wastes magnesium of its own. So the right tablet is the one the patient can buy and keep taking. If the oxide gives him loose stools, the dose is working against itself.

5. Torsades: the One Time the Level Does Not Matter

Intravenous magnesium sulphate 2 g is the first-line treatment for torsades de pointes, regardless of the serum magnesium. Magnesium suppresses the early afterdepolarisations, the small extra electrical waves after each beat that trigger the rhythm. So it is given emergently even when the level is normal: 2 g over 2 to 5 minutes in active torsades, then an infusion of 1 to 2 g/hour for 4 to 6 hours. In cardiac arrest an undiluted push is acceptable.

6. The Common Mistake

The common mistake is to keep chasing the potassium with more KCl. It is easy to see why. The potassium is the number on the report that is flagged, and KCl is the drug we know. But the kidney will pour it out again for as long as the magnesium stays low. The second mistake is to be reassured by a normal level. Most of the body’s magnesium sits inside the cells and the bone, and the tool reminds you that a level inside the reference range does not exclude depletion. Read the number against the patient.

Back to Our Patient

He gets 2 to 4 g of magnesium sulphate over 4 to 6 hours, with his knee jerks checked before each dose. His potassium and calcium are rechecked every 6 to 12 hours, and his magnesium in 24 to 48 hours, as the tool’s monitoring plan asks. The pantoprazole is changed to an H2-blocker. He goes home on oral magnesium, and because he stays on a diuretic, his magnesium is checked again every 3 to 6 months. This time, the potassium rises.

If he had been a heavy drinker, one more step applies. The thiamine-dependent enzymes also need magnesium, so thiamine repletion can fail while the magnesium is low. Correct both together; the alcohol withdrawal article has the thiamine cover.

Try This

A 58-year-old woman on day 3 after a bowel resection develops torsades de pointes on the monitor. She has a pulse. Her magnesium this morning was 2.0 mg/dL. Does she need magnesium? Illustrative case.

Answer. Yes. Torsades is the one situation where the level does not decide. Give magnesium sulphate 2 g IV over 2 to 5 minutes, then 1 to 2 g/hour for 4 to 6 hours. Keep a defibrillator at the bedside. If she becomes pulseless, follow the ACLS protocol.

Take-Home Points

• A potassium that will not rise on KCl is a reason to check the magnesium, and to fix the magnesium first.
• Moderate hypomagnesaemia (1.0 to 1.4 mg/dL) gets 2 to 4 g of magnesium sulphate over 4 to 6 hours, not a fast push.
• Check the deep tendon reflexes before each dose; calcium gluconate 10%, 10 to 20 mL, is the antidote.
• On a proton pump inhibitor, replacement alone will not hold: switch to an H2-blocker if possible.
• Torsades de pointes gets magnesium sulphate 2 g IV whatever the serum level.

The same magnesium lies behind two other tools: Acute Hypokalaemia and Acute Hypocalcaemia. For the long QT, see The QT Interval and the Electrolytes.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 5 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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