Management of Hypocalcemia and Hypomagnesemia with QT Prolongation
Immediately correct both electrolyte abnormalities with intravenous calcium gluconate and magnesium sulfate while initiating continuous cardiac monitoring, as this patient's severe hypocalcemia (7.0 mg/dL) and hypomagnesemia (1.5 mg/dL) are directly causing the QT prolongation and life-threatening arrhythmia risk.
Immediate Management Algorithm
Step 1: Initiate Continuous Cardiac Monitoring
- Place the patient on continuous ECG telemetry immediately, as QT prolongation from electrolyte disturbances carries significant risk for torsades de pointes and sudden cardiac death 1
- The combination of hand spasms (tetany) and orthopnea with severe electrolyte derangements indicates symptomatic hypocalcemia requiring urgent intervention 2
Step 2: Correct Magnesium FIRST
- Administer intravenous magnesium sulfate as the initial priority, as hypomagnesemia must be corrected before calcium replacement will be effective 3, 2
- Magnesium deficiency impairs parathyroid hormone secretion and creates functional hypoparathyroidism, preventing effective calcium correction 2
- The patient's magnesium level of 1.5 mg/dL is at the lower limit of normal but in the context of severe hypocalcemia suggests true magnesium depletion 3
Step 3: Administer Intravenous Calcium
- After initiating magnesium replacement, administer calcium gluconate intravenously at a rate not exceeding 200 mg/minute with continuous ECG monitoring 4
- Hypocalcemia below 7.5 mg/dL characteristically produces QT prolongation through lengthening of the ST segment 1
- Rapid correction is essential as this patient has symptomatic hypocalcemia with tetany (hand spasms) 4
Step 4: Monitor for Arrhythmias
- The QT prolongation from hypocalcemia occurs through impaired calcium-dependent inactivation of L-type calcium channels, prolonging ventricular repolarization and creating risk for early afterdepolarizations and torsades de pointes 5
- If torsades de pointes develops, administer 2g IV magnesium sulfate immediately regardless of serum magnesium level 6, 7
Critical Electrolyte Targets
- Maintain calcium >8.5 mg/dL to normalize QT interval 1
- Maintain magnesium >2.0 mg/dL to prevent recurrent hypocalcemia and arrhythmias 1, 6
- Recheck electrolytes every 4-6 hours during acute correction phase 8
Management of Orthopnea
- The orthopnea may represent heart failure exacerbated by electrolyte abnormalities, as hypomagnesemia and hypocalcemia can impair cardiac contractility 2
- Avoid aggressive diuresis until electrolytes are corrected, as this will worsen hypomagnesemia 2
- If diuretics are necessary, replace magnesium and calcium losses aggressively 1
Common Pitfalls to Avoid
- Do not attempt calcium correction without first addressing magnesium deficiency, as calcium replacement will be ineffective and potentially dangerous 2
- Do not administer calcium rapidly (>200 mg/minute), as this can cause vasodilation, hypotension, bradycardia, and cardiac arrest 4
- Do not use calcium in patients on digoxin without extreme caution and continuous ECG monitoring, as hypercalcemia increases digoxin toxicity and synergistic arrhythmias can occur 4
- Do not ignore the QT prolongation as "just electrolytes"—this patient is at immediate risk for torsades de pointes and sudden death 9, 8
Monitoring During Treatment
- Obtain 12-lead ECG every 2-4 hours until QT interval normalizes 8
- Continue telemetry monitoring until QTc <500 ms 6, 7
- If QTc exceeds 500 ms or increases >60 ms from baseline despite electrolyte correction, obtain urgent cardiology consultation 6, 7
Investigation of Underlying Cause
- Once stabilized, investigate the cause of combined hypocalcemia and hypomagnesemia: chronic PPI use, malabsorption, chronic diarrhea, alcoholism, or renal losses 2
- Check parathyroid hormone level, as functional hypoparathyroidism commonly accompanies severe hypomagnesemia 2
- Review all medications for drugs that promote electrolyte wasting or QT prolongation 1