Hypomagnesemia
at the bedside
- check vitals · confirm Mg < 0.7 mmol/L
- look for tremor, tetany, seizures, weakness, delirium
- ECG → wide QRS, peaked T → long PR, flat T, arrhythmias as it falls
- spurious? EDTA-tube contamination (unexplained high K + low Ca) or low albumin → repeat
- low Mg blocks K and Ca correction: replace Mg first
- identify + treat cause
escalate now if
- torsades, arrhythmia, or unstable → magnesium sulfate 2 g IV over 10–15 min → tachycardia
- seizures or tetany → IV replacement
- Mg < 0.4 with K⁺ or Ca²⁺ not correcting → IV magnesium
focused check
- GI → diarrhea, malabsorption, pancreatitis, PPIs
- urinary → loop / thiazide diuretics, aminoglycosides, amphotericin, cisplatin, calcineurin inhibitors, cetuximab, digoxin
- other → alcohol, uncontrolled diabetes, hypercalcemia, recovery from ATN, post-obstructive diuresis
send now
- K⁺ + Ca²⁺ with the Mg
- ECG if symptomatic or arrhythmia
- cause unclear → urine Mg (GI vs renal loss)
treat
- MgSO₄ 50%: 1 g = 2 mL = 4 mmol; dilute in 100 mL NaCl 0.9% (≤ 20%)
- myasthenia gravis → avoid IV magnesium unless essential: it can precipitate a myasthenic crisis; if it must be given, neurology aware and monitored
torsades, arrhythmia or unstable
- MgSO₄ 2 g IV over 10–15 min; repeat if still unstable
- then MgSO₄ 4–8 g IV over 12–24 h
symptomatic (tetany, seizures), stable
- MgSO₄ 2 g IV over 30–60 min, then 4–8 g IV over 12–24 h
no symptoms, IV needed (NPO, GI intolerance, big deficit)
- Mg < 0.4 → MgSO₄ 4–8 g IV over 12–24 h, repeat as needed
- Mg 0.4–0.6 → MgSO₄ 2–4 g IV over 4–12 h
- Mg 0.6–0.7 → MgSO₄ 1–2 g IV over 1–2 h
no symptoms, can take oral
- oral Mg 10–40 mmol/day (240–1000 mg elemental) divided; sustained-release salts first, oxide causes diarrhea
- slow outside emergencies → up to 50% of a fast infusion is lost in urine
- keep Mg > 0.4 mmol/L; Mg 6–12 h after each IV dose before repeating
- hand over → last Mg + time, next check, escalation trigger if refractory
- CKD / AKI → 25–50% of the dose; severe renal impairment → senior first
available magnesium
| product | content | usual adult dose | bedside note |
|---|---|---|---|
| IV magnesium sulfate 50% | 2 mmol/mL: 10 mL ampoule = 5 g = 20 mmol Mg (1 g = 4 mmol) | 2 g IV over 10–15 min (unstable) · 2 g over 30–60 min (symptomatic) · 1–8 g over 1–24 h by level (treat) | dilute to ≤ 20% (e.g. 2 g in ≥ 50 mL NaCl 0.9% or glucose 5%); max 8 mmol/h (2 g/h) outside emergencies; renal impairment: 25–50% of the dose, monitor levels |
| oral magnesium salts | citrate, chloride or lactate preferred; oxide poorly absorbed | 10–40 mmol/day divided (treat) | diarrhea is dose-limiting; mmol per tablet varies by product: local formulation verification needed |
hypermagnesemia
- causes → renal failure, Mg load (infusions, antacids, laxatives, enemas)
- features (usually > 2 mmol/L): lethargy, lost reflexes, weakness, hypotension, bradycardia; ECG long PR / wide QRS / long QT at 2.5–5; heart block or arrest above ~7.5
- stop every magnesium source
symptomatic
- calcium gluconate 10% 10–20 mL (1–2 g) IV over 5–10 min, monitored; repeat every 5 min until symptoms settle
- arrest → ALS + calcium chloride 0.5–1 g IV over 2–5 min
- kidneys normal or moderately impaired + symptoms → furosemide 20–40 mg IV + NaCl 0.9% unless overloaded; no symptoms → trend Mg only
- eGFR < 15, on dialysis, or moderate–severe AKI + symptoms → nephrology for dialysis (clears in 2–4 h); saline + furosemide while arranging
evidence
| source | type |
|---|---|
| Standard electrolyte-replacement practice, renal dose-adjustment | common practice |
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
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reviewed Jun 2026updated Oct 2026file lab/rft-magnesium