Prolonged QT, torsades de pointes, digoxin sensitivity
Fluid, Electrolyte & Acid-Base · Topic 4 of 23
Hypomagnesemia
A patient's potassium keeps dropping despite aggressive replacement — the hidden culprit might be a magnesium level no one checked.
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Hypomagnesemia
Na⁺SodiumWNL
K⁺PotassiumWNL
Cl⁻ChlorideWNL
Ca²⁺CalciumWNL
Mg²⁺MagnesiumPriority
PO₄³⁻PhosphateWNL
ECFVolume (ECF)WNL
pHpHWNL
Bedside monitor · pattern
Hypomagnesemia
Low magnesium removes a membrane brake and unblocks renal potassium wasting, so potassium and calcium fall with it.
Direct physiologyThreshold falls→Hyperexcitable→QT lengthens→Torsades
ECG responseNormal sinus rhythm
Neuronal excitabilityHyperexcitable
SuppressedBalancedHyper
PerfusionEuvolemic
100%
Extracellular volume at baseline. The dashed line marks euvolemia.
What the nurse would notice
Hypomagnesemia across body systems
Tremor, cramps, tetany, hyperreflexia, Chvostek and Trousseau
Agitation, confusion; seizures when severe
Nausea, anorexia, decreased motility
Teaching model, not a clinical calculator. Patterns show high-yield directional relationships under the stated assumption. Real symptoms, ranges and treatment depend on cause, acuity, comorbidities and local protocols.
Signs & symptoms
Present on this panel. Tap any sign to see which values produce it.
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Fluids & Electrolytes · Mg²⁺
Hypomagnesemia
Too little magnesium. Nerves, muscles and the heart get twitchy — and low potassium and calcium refuse to come up.
Lead II · example patient2.0Mg²⁺ mEq/L
Low Mg → long QT → can turn into torsades. Keep them on a monitor.
Illustration · normal Mg²⁺ is 1.5 – 2.5
01 · The numberMg²⁺ below 1.5
mEq/L · normal 1.5 – 2.5 · severe about 1.0 or lower
02 · The dangerTorsades de pointes
Prolonged QT → a twisting V-tach · keep on a continuous cardiac monitor
03 · The fixReplace Mg to fix K⁺ and Ca²⁺
Low K⁺ or Ca²⁺ won't correct until magnesium is replaced · during IV Mg, check deep tendon reflexes
01
Why won't the K⁺ come up?
Side by sideLow Mg makes the kidneys keep dumping K⁺ — replace the Mg or the K⁺ won’t stick.
Mg still lowK⁺2.9Low
K⁺ given — but it leaks out in the urine. The number stays low.
Mg replacedK⁺2.9Low
The same K⁺ given — now the kidneys keep it. The number rises.
Normal K⁺ is 3.5 – 5.0 mEq/L
Nurse does
K⁺ won’t come up? Check the magnesium. Replace the Mg — then the K⁺ replacement stays in. Keep the cardiac monitor on; recheck K⁺ and Mg.
Illustration — the same K⁺ dose in both; the numbers show the idea, not real timing or doses. Why: low Mg opens a kidney K⁺ channel (ROMK). Low calcium behaves the same way: Mg is needed to release and respond to parathyroid hormone, so Ca²⁺ won’t come up until Mg is replaced either.
02
What causes it
Watch forClassic cause: alcohol use disorder
Too little taken in
Alcohol use disorderthe classic causeMalnutrition
Lost from the gut or not absorbed
DiarrheaLong-term PPIsproton pump inhibitors
Lost through the kidneys
Loop diureticsThiazide diuretics
03
What you'll see
SignsLow Mg makes nerves and muscles hyperexcitable
Tremors · muscle twitching early
Hyperreflexia (brisk, overactive reflexes) early
Positive Chvostek and Trousseau signs earlythe face twitches when tapped · the hand spasms under a BP cuff
Tetany (sustained muscle spasms) late
Seizures late
Prolonged QT → torsades de pointes late
Low K⁺ and low Ca²⁺ often come with it — check all three.
04
Labs & diagnostics
CheckSerum Mg²⁺ < 1.5 mEq/L
Serum Mg²⁺ below 1.5 mEq/Lnormal 1.5 – 2.5 (labs vary; many US labs report about 1.7 – 2.2 mg/dL) · severe about 1.0 or lower
Low K⁺ that won't come uplow Mg opens a kidney K⁺ channel (ROMK), so K⁺ is wasted in the urine
Low Ca²⁺ that won't come upMg is needed to release and respond to parathyroid hormone
12-lead ECG + continuous monitoringprolonged QT · watch for torsades
On digoxin?low Mg raises the risk of digoxin toxicity
Kidney function & urine outputMg leaves only through the kidneys — lower IV doses if they're impaired
05
Red flags
Act nowTorsades de pointes or a long QT report nowIV magnesium sulfate treats torsades
Seizure report nowprotect the airway · seizure precautions
Lost patellar reflex during IV magnesium holdfirst sign of toxicity — stop the infusion, notify
Slow breathing, low BP or slow HR during IV magnesium holdgiven too fast — stop it and notify
Low urine output during IV magnesiumthe kidneys can't clear it — it builds up
06
Treatment: replace magnesium
Oral or IVReplace Mg — K⁺ and Ca²⁺ won't correct without it
Do right now
1
→Cardiac monitor oncontinuous
2
→Seizure precautionsif severe or symptomatic
3
→Replace Mg as orderedby mouth if mild · IV if severe
4
Check reflexes during IV Mglost knee-jerk → stop, notify
OralMild · no symptoms
Oral magnesium
Take it with meals.
Diarrhea limits the doseIV infusionSevere (≤ 1.0) or symptomatic
Magnesium sulfate IV
Run it slowly on a pump. Too fast → flushing, low BP, slow HR or heart block, slow breathing.
Check reflexes during the infusionAntidoteAt the bedside
Calcium gluconate IV
Keep IV calcium gluconate at the bedside — it reverses magnesium toxicity.
Toxicity: reflexes fade firstDoses & special cases
Seizures or severe symptoms: 2 – 4 g magnesium sulfate IV over 5 – 10 minless urgent repletion runs about 1 g per hour
Kidney impairment: lower dosesMg leaves only through the kidneys — watch urine output and serum Mg
Reflexes start to fade above about 4 mEq/Llost above about 7 · respiratory depression above about 10
IV magnesium sulfate alone can briefly worsen a low calciumsulfate binds ionized calcium
07
Nursing priorities
In orderContinuous cardiac monitorprolonged QT, torsades
Seizure precautionsif severe or symptomatic
Check deep tendon reflexes during IV magnesiumlost patellar reflex → stop the infusion, notify
Keep IV calcium gluconate at the bedsidethe antidote for magnesium toxicity
Replace Mg along with K⁺ and Ca²⁺they won't correct without it
Watch urine output, breathing and BP · recheck Mg
08
Hypo or hyper magnesium?
| Level | Mg²⁺ below 1.5 mEq/L |
|---|---|
| Nerves & muscles | Hyperexcitable — tremors, tetany, leg cramps, seizures |
| Reflexes | Hyperactive (brisk) |
| Heart | Prolonged QT · torsades de pointes |
| Danger | Dysrhythmias · seizures |
| Usual cause | Alcohol use disorder · loop and thiazide diuretics · diarrhea |
| Fix | Magnesium — oral, or IV slowly on a pump |
09
Teach your patient
DischargeEat magnesium-rich foodsgreen leafy vegetables, almonds, peanut butter, legumes, citrus, chocolate
Limit alcoholthe classic cause
Take oral magnesium with mealsdiarrhea is the common side effect
On a PPI or diuretic long-term? Ask about your Mg levelthey can drain magnesium
Report tremors, twitching, cramps or palpitations
Low magnesium = hyperexcitable (tremors, brisk reflexes, torsades). If K⁺ or Ca²⁺ won't come up, replace the magnesium.
Sources · Levels, doses and toxicity checks from Merck Manual Professional: Hypomagnesemia, the magnesium sulfate injection label and Negru et al., hypomagnesemia and cardiac arrhythmias (2022); calcium link from Merck Manual Professional: Hypocalcemia; nursing care from OpenStax Medical-Surgical Nursing 10.3 and OpenStax Pharmacology 5.2. Typical adult values — follow your protocol.
Hypomagnesemia
A patient's potassium keeps dropping despite aggressive replacement — the hidden culprit might be a magnesium level no one checked. You can't fix one without the other.
Magnesium (normal 1.5–2.5 mEq/L) is a gatekeeper electrolyte: it stabilizes cardiac cell membranes, facilitates potassium uptake into cells, and is required for PTH secretion and calcium regulation. When magnesium falls below 1.5 mEq/L, three downstream problems emerge. First, refractory hypokalemia — the kidneys waste potassium through ROMK channels that magnesium normally blocks, so potassium replacement fails until magnesium is corrected. Second, hypocalcemia — low magnesium suppresses PTH release and causes end-organ resistance to PTH, dropping calcium simultaneously. Third, neuromuscular hyperexcitability — tremors, hyperactive deep tendon reflexes, positive Trousseau's and Chvostek's signs (shared with hypocalcemia because magnesium deficiency causes it). Cardiac effects include prolonged QT interval, torsades de pointes, and increased digoxin sensitivity. The most common causes are chronic alcohol use, loop and thiazide diuretics, proton pump inhibitors with prolonged use, and GI losses (diarrhea, NG suction). IV magnesium sulfate is the replacement route for severe deficiency (< 1.0 mEq/L) or symptomatic patients — administered slowly with continuous cardiac monitoring because rapid infusion causes hypotension and respiratory depression. Oral magnesium causes diarrhea, which paradoxically worsens losses.
Key Distinctions
Don't confuse hypomagnesemia's hyperexcitable presentation (tremors, seizures, hyperreflexia) with hypermagnesemia's sedated presentation (hyporeflexia, respiratory depression) — they are mirror opposites. Students often treat refractory hypokalemia by giving more potassium; the correct move is checking and replacing magnesium first. Trousseau's and Chvostek's signs appear in both hypomagnesemia and hypocalcemia — low magnesium is frequently the upstream cause of the low calcium.
Clinical Pearl
Mag is the mother electrolyte: if potassium won't come up or calcium won't come up, check magnesium first — it controls them both.
Knowledge Check
3 quick questions on the must-knows for this topic.
Question 1 of 3
Refractory hypokalemia often requires correcting which electrolyte first?
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Go further
- SimulatorFluid & Electrolyte SimulatorChange one lab. Watch the whole patient respond. Sodium, potassium, calcium, magnesium, pH and volume, all linked — with a live ECG and an IV pole where every bag runs.
- GuideThe 6 Electrolyte Relationships Every Nurse Gets Tested On (Mg–K, Ca–PO₄, pH–K and More)Why potassium will not correct until magnesium does, why calcium and phosphate move in opposite directions, how pH moves potassium and ionized calcium, why sodium is a water problem, and why low potassium makes digoxin dangerous — with the mechanism and the nursing move for each, and a simulator to break them on purpose.
- GuideFluid and Electrolyte NCLEX Questions: The 7 Patterns the Exam Keeps TestingThe priority is the organ, not the number. Anything changing the ECG comes first. Magnesium before potassium and calcium. Volume before the electrolyte. The lab that is lying. Slow is the rule for correction. The assessment that gates the drug. Learn the seven patterns and the individual questions stop being surprising.
- SimulatorAcid–Base & ABG SimulatorBuild a gas. Watch compensation happen. Move CO₂ or bicarbonate, then read the gas in three calls: pH, cause, compensation.
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