How should severe hypomagnesemia, severe hypocalcemia, and hypokalemia be corrected in this patient?

Management of Severe Electrolyte Derangements: Hypomagnesemia, Hypocalcemia, and Hypokalemia

Correct magnesium first with intravenous magnesium sulfate 1-2 g over 15 minutes, followed by continuous infusion, because both hypocalcemia and hypokalemia will remain refractory to supplementation until magnesium is normalized. 1, 2, 3

Critical First Step: Volume Assessment and Repletion

Before initiating any electrolyte replacement, assess for volume depletion by checking orthostatic vital signs and measuring urinary sodium (< 10 mEq/L suggests volume depletion with secondary hyperaldosteronism). 1, 2 If volume depleted, administer intravenous normal saline 2-4 L/day initially to eliminate secondary hyperaldosteronism, which drives renal magnesium and potassium wasting and will prevent effective repletion despite supplementation. 1, 2

Immediate Magnesium Replacement Protocol

Intravenous Magnesium Administration

• Administer 1-2 g magnesium sulfate IV over 15 minutes as the initial bolus for severe symptomatic hypomagnesemia (Mg 1.2 mg/dL = 0.49 mmol/L). 4, 2
• Follow with continuous infusion of 1-4 mg/min magnesium sulfate or repeated doses of 1-2 g every 6-8 hours until serum magnesium reaches > 1.3 mEq/L. 2
• For life-threatening presentations (cardiac arrhythmias, seizures, tetany), give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline level. 4, 2

Monitoring During IV Replacement

• Check serum magnesium, potassium, calcium, and creatinine every 6-12 hours during intravenous replacement. 2
• Monitor for magnesium toxicity: loss of patellar reflexes, respiratory depression, hypotension, and bradycardia. 2
• Obtain ECG immediately given the severe electrolyte abnormalities and risk of ventricular arrhythmias or QTc prolongation. 4, 2

Renal Function Assessment

Check creatinine clearance before any magnesium administration—if < 20 mL/min, magnesium supplementation is absolutely contraindicated due to life-threatening hypermagnesemia risk. 1, 2 Between 20-30 mL/min, use extreme caution with reduced doses and close monitoring. 2

Calcium Replacement Strategy

Do not attempt calcium correction until magnesium is normalized—hypocalcemia is refractory to calcium supplementation when hypomagnesemia persists because magnesium deficiency impairs PTH secretion and activity. 2, 5, 3, 6

• Once magnesium is repleting (typically after 12-24 hours of IV magnesium), calcium will often normalize spontaneously within 24-72 hours without additional calcium supplementation. 2, 5
• If symptomatic hypocalcemia persists after magnesium correction, give calcium gluconate 1-2 g IV over 10 minutes, followed by continuous infusion. 2
• Monitor serum calcium every 6-12 hours during replacement. 2

Potassium Replacement Strategy

Hypokalemia will remain resistant to potassium treatment until magnesium is corrected because hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion. 1, 2, 3

• Begin potassium replacement simultaneously with magnesium, but expect potassium to normalize only after magnesium levels improve. 1, 2
• Administer potassium chloride 10-20 mEq/hour IV (maximum 40 mEq/hour via central line for severe symptomatic hypokalemia). 2
• Target serum potassium > 4.0 mEq/L to reduce arrhythmia risk. 2
• Recheck potassium every 2-4 hours during active replacement. 2

Transition to Oral Therapy

Once the patient is asymptomatic and serum magnesium reaches > 1.3 mEq/L:

• Start oral magnesium oxide 12 mmol (≈480 mg elemental magnesium) at night when intestinal transit is slowest for maximal absorption. 1, 2
• If levels remain low after 1-2 weeks, increase to 24 mmol daily (single or divided doses). 1, 2
• Continue oral potassium chloride 20-40 mEq three times daily until serum potassium stabilizes > 4.0 mEq/L. 2
• Add calcium carbonate 1,000-1,500 mg elemental calcium daily with vitamin D3 if calcium remains low after magnesium normalization. 2, 6

Follow-Up Monitoring

• Recheck magnesium, potassium, and calcium 2-3 weeks after starting oral supplementation. 1, 2
• Once stable, monitor electrolytes every 3 months. 2
• Assess for symptom resolution: muscle cramps, tetany, fatigue, paresthesias, and cardiac arrhythmias. 2, 6

Critical Pitfalls to Avoid

• Never attempt to correct hypokalemia or hypocalcemia before normalizing magnesium—these electrolyte abnormalities are refractory to supplementation until magnesium is corrected. 1, 2, 3
• Never start magnesium supplementation in volume-depleted patients without first correcting sodium and water depletion with IV saline—secondary hyperaldosteronism will cause continued renal magnesium wasting despite supplementation. 1, 2
• Do not assume normal serum magnesium excludes deficiency in future assessments—less than 1% of total body magnesium is in blood, so normal levels can coexist with significant intracellular depletion. 1, 2
• Avoid giving bolus potassium for suspected hypokalemia-related cardiac arrest without concurrent magnesium replacement. 2

Underlying Cause Investigation

While initiating replacement therapy, investigate the cause of these combined electrolyte abnormalities:

• Review medications: diuretics (loop or thiazide), proton pump inhibitors, aminoglycosides, cisplatin, calcineurin inhibitors. 2, 7, 8
• Assess for gastrointestinal losses: chronic diarrhea, short bowel syndrome, high-output stoma, malabsorption. 1, 2, 3
• Consider genetic disorders if young patient with recurrent episodes: Gitelman syndrome, Bartter syndrome, familial hypomagnesemia. 2, 8
• Evaluate for alcohol use disorder, diabetes, or refeeding syndrome risk. 2, 8