HYPOMAGNESEMIA

HYPOMAGNESEMIA 

Serum magnesium < 1.7 mg/dL/< 0.7 mmol/L/< 1.4 mEq/L

Serum magnesium represents <1% of total body magnesium → normal serum levels do not exclude intracellular depletion.

Severity Classification

Severity	Serum Mg Level
Mild	1.6–1.8 mg/dL (0.66–0.74 mmol/L)
Moderate	1.2–1.5 mg/dL (0.49–0.62 mmol/L)
Severe	<1.2 mg/dL (<0.49 mmol/L)

Normal serum magnesium: 1.8–2.4 mg/dL (0.74–1.0 mmol/L)

Physiology

Intestinal Absorption (30–50%)

• Mainly in ileum & colon

Renal Handling (Key Regulator)

Nephron Segment	% Reabsorbed	Mechanism
Proximal tubule	15–20%	Passive
Thick ascending limb (TAL)	60–70%	Paracellular (claudin-16, ROMK)
Distal convoluted tubule (DCT)	5–10%	Active (TRPM6)

DCT is the final checkpoint → no reabsorption beyond this

Key Cellular Roles of Magnesium

• Cofactor for >300 enzymatic reactions
• Regulates:
• Na⁺-K⁺-ATPase
• Calcium channels
• Potassium transport
• ATP metabolism
• Neuromuscular stability

Electrolyte Interactions 

Hypomagnesemia → Refractory Hypokalemia

• Mg deficiency → ↑ ROMK activity
• ↑ renal K⁺ wasting
• Potassium will not correct unless Mg is replaced

Hypomagnesemia → Hypocalcemia

• ↓ PTH secretion
• End-organ resistance to PTH

Etiology 

1. Reduced Intake / Absorption

• Poor nutrition—Starvation
• Chronic alcoholism
• Malabsorption syndromes—Celiac disease,Crohn disease,Short bowel syndrome,Chronic diarrhea
• Proton pump inhibitors (PPIs) (↓ TRPM6)

2. Gastrointestinal Losses

• Chronic diarrhea,Laxative abuse
• High-output stomas
• Nasogastric suction
• Biliary or pancreatic fistulae

3. Renal Magnesium Wasting (MOST COMMON in ICU)

Drugs

Drug Class	Examples
Diuretics	Loop > thiazides
Aminoglycosides	Gentamicin, amikacin
Amphotericin B	Tubular toxicity
Cisplatin	DCT injury
Calcineurin inhibitors	Tacrolimus, cyclosporine
Digoxin	↑ urinary Mg loss
PPIs	↓ intestinal absorption

Renal Tubular Disorders

• Gitelman syndrome
• Bartter syndrome
• Post-AKI diuretic phase
• Post-transplant tubular dysfunction
• Recovery phase of ATN

4. Redistribution (Shift into Cells)

• Refeeding syndrome
• Insulin therapy (DKA)
• Acute pancreatitis(due to binding by free fatty acids)
• Hungry bone syndrome
• Catecholamine surge
• Respiratory alkalosis
• Sepsis 

5. Endocrine / Metabolic Causes

• Uncontrolled diabetes mellitus (osmotic diuresis)
• Hyperaldosteronism
• Hyperparathyroidism
• Hyperthyroidism
• SIADH (dilutional)
• Hypercalcemia
• Hypophosphatemia

Clinical Manifestations

Features of hyperexitability(cardiac,neuromuscular )

Neuromuscular(Hypomagnesemia cause hypocalcemia by inhibition of PTH release or activity.)

• Tremors
• Muscle weakness
• Fasciculations
• Tetany
• Carpopedal spasm
• Seizures
• Hyperreflexia

Cardiovascular

• Ventricular arrhythmias
• Torsades de pointes
• Atrial fibrillation
• Prolonged QT
• Digitalis toxicity (↑ sensitivity)

Neuropsychiatric

• Apathy
• Delirium
• Depression
• Confusion
• Agitation

Metabolic

• Refractory hypokalemia
• Hypocalcemia
• Insulin resistance

ECG Changes

• Prolonged QT interval
• Flattened T waves
• ST depression
• Ventricular ectopy
• Polymorphic VT (torsades)

Diagnosis

Laboratory

• Serum Mg <1.7 mg/dL
• Check:Potassium—Calcium—Phosphate—Albumin

Advance Investigations(Routinely not required)

• 24-Hour Urinary Magnesium-30 mg/day → renal loss
• Fractional Excretion of Magnesium (FeMg)

       FeMg=(0.7 x PMg ×UCr )(UMg ×PCr ) ×100

FeMg	Interpretation
<2%	GI loss
>4%	Renal wasting

Management

General Principles

• Treat symptoms, not just numbers
• Treat hypokalemia,hypocalcemia
• Monitor ECG in severe cases

Oral Magnesium 

• side effect—Diarrhea
• Renal failure (relative contraindication)
• Indicated in mild hypomagnesemia,Asymptomatic Patient

Preparation	Elemental Mg	Dose
Mg oxide	High Mg, poor absorption	400 mg(= 20 mEq magnesium.) BD
Mg citrate	Better absorption	30 ml(1,745 mg MgCitrate = 282 mg Mg = 23.2 mEq) B.D
magnesium sulfate	1 gm = 8 mEq of magnesium.	2.5 gm  BD
magnesium hydroxide (Milk of Magnesia	5 ml = 400 mg MgOH = 13.7 mEq magnesium.	Indigestion: 5-15 ml q4hr Hypomagnesemia: 7.5 ml BD Constipation: 30-60 ml in divided doses (82-164 mEq Mg).

IV Magnesium 

Indication moderate to severe hypomagnesemia

Drug

• Magnesium sulfate (MgSO₄) 1 g = 8 mEq Mg²⁺~98 mg elemental Mg
• Ampoule : Magnesium sulfate 50% 1 g in 2 mL

Contraindications

Condition	Recommendation	Reason
Myasthenia Gravis (MG)	Relative contraindication. Avoid IV magnesium if possible. Prefer oral replacement when feasible. If severe or life-threatening hypomagnesemia exists, give IV magnesium slowly with continuous monitoring.	Magnesium inhibits acetylcholine release at the neuromuscular junction and can precipitate severe muscle weakness or respiratory failure.
High-Grade Heart Block (2nd-degree Mobitz II or 3rd-degree AV block)	Relative contraindication. Use cautiously and with ECG monitoring.	Magnesium slows AV nodal conduction and may worsen existing heart block.
Severe Hypocalcemia	Correct calcium along with magnesium. Avoid rapid large magnesium doses until calcium status is addressed.	Magnesium can transiently suppress PTH release and may aggravate symptomatic hypocalcemia, increasing risk of tetany or seizures.
Advanced Renal Failure (eGFR <30 mL/min/1.73 m²)	Not an absolute contraindication, but dose reduction (usually ~50%) and frequent monitoring are required.	Reduced renal excretion leads to magnesium accumulation and hypermagnesemia.
Continuous Magnesium Infusion with GFR <30 mL/min	Generally avoid prolonged continuous infusions unless under intensive monitoring.	High risk of magnesium toxicity due to impaired clearance.
Severe Hypotension or Shock	Infuse slowly and monitor blood pressure.	Rapid magnesium administration can cause vasodilation and worsen hypotension.

1. Asymptomatic Hypomagnesemia 

• Give 2–4 g magnesium sulfate IV intermittently
• Infuse slowly rather than rapidly
• Slower infusion improves intracellular uptake and reduces urinary magnesium wasting
• Rapid administration causes high serum peaks, resulting in increased renal excretion

Suggested Dosing (GFR >50 mL/min)

Serum Mg	Severity	Suggested Dose Over 24 Hours
1.6–1.8 mg/dL	Mild	Weight (kg) ÷ 16 = grams MgSO₄
1.2–1.5 mg/dL	Moderate	Weight (kg) ÷ 8 = grams MgSO₄

Practical ICU Regimen

Mild Hypomagnesemia (1.6–1.8 mg/dL)

• 2–4 g MgSO₄ IV over 2–4 hours
• Recheck magnesium the following day

Moderate Hypomagnesemia (1.2–1.5 mg/dL)

• 4–8 g MgSO₄ IV over 12–24 hours
• Repeat levels in 6–12 hours if critically ill

After Correction

Transition to oral magnesium when possible:

• Magnesium oxide 400–800 mg PO BID

This helps prevent recurrence, especially in patients with:Diuretic use,Chronic diarrhea,Gitelman syndrome,PPI-induced hypomagnesemia

2. Severe Asymptomatic Hypomagnesemia 

Option A: Repeated Intermittent IV Doses(Common ICU approach)

Magnesium Sulfate

• 2–4 g IV every 6–8 hours

Examples:

• 4 g IV q8h
• 2 g IV q6h

Continue until:

• Magnesium >1.6–1.8 mg/dL
• Underlying cause addressed

Option B: Continuous Magnesium Infusion

Useful when:

• Large magnesium deficits exist
• Ongoing magnesium losses continue
• Frequent re-dosing is impractical

Recommended Dose (GFR >50 mL/min)

Total magnesium sulfate dose over 24 hours:

Weight (kg) ÷ 4 = grams MgSO₄

3. Symptomatic hypomagnesemia

Loading Dose

• Step 1–2 g MgSO₄ IV over 5–15 minutes
• Step 2—Additional 2 g MgSO₄ IV over 30–60 minutes
• Total Loading Dose—4 g magnesium sulfate

This rapidly replenishes extracellular magnesium and stabilizes myocardial membranes.

Maintenance Therapy After Loading

GFR >30 mL/min

Continuous magnesium infusion may be used—1gm/hour

GFR <30 mL/min

Continuous infusion is generally avoided because of the risk of magnesium accumulation.

Preferred Strategy

• Check magnesium levels frequently
• Give intermittent doses as needed
• Re-dose according to serum levels and clinical response

Monitoring During Aggressive Magnesium Therapy

Clinical Monitoring

Watch for:

• Improvement in arrhythmia
• Resolution of seizures
• Normalization of QT interval
• Improvement in neuromuscular symptoms

Situation	Monitoring Frequency
Continuous infusion	Every 4–6 hr
Severe hypomagnesemia	Every 6 hr
Stable replacement	Daily

Complications of Overcorrection

• Hypotension
• Bradycardia
• Loss of DTRs
• Respiratory depression
• Cardiac arrest (very high levels)