What are the monitoring and management strategies for magnesium (MgSO4) toxicity in a pregnant woman with preeclampsia?

Monitoring and Management of Magnesium Sulfate Toxicity

Empirical calcium administration (calcium chloride 10% 5-10 mL or calcium gluconate 10% 15-30 mL IV over 2-5 minutes) is lifesaving and should be given immediately when magnesium toxicity is suspected—do not delay for laboratory confirmation. 1, 2

Clinical Monitoring Protocol

Clinical examination is the primary monitoring strategy; routine serum magnesium levels are not necessary in patients with normal renal function. 3 The following parameters must be assessed before each dose:

Essential Clinical Parameters

• Patellar (deep tendon) reflexes: Loss of reflexes occurs at 3.5-5 mmol/L and is the first warning sign of impending toxicity 4, 5
• Respiratory rate: Must be ≥12-16 breaths/minute; respiratory paralysis occurs at 5-6.5 mmol/L 3, 4, 5
• Urine output: Must maintain ≥30 mL/hour (or ≥100 mL over 4 hours); oliguria dramatically increases toxicity risk as magnesium is renally excreted 3, 4
• Oxygen saturation: Maintain >90% 3

When to Check Serum Magnesium Levels

Laboratory monitoring is indicated only in specific high-risk situations 3:

• Renal impairment (elevated creatinine)
• Urine output <30 mL/hour
• Loss of patellar reflexes
• Respiratory rate <12 breaths/minute

Clinical Manifestations by Severity

Mild-Moderate Toxicity (2.5-5 mmol/L)

• ECG changes: prolonged PR, QRS, and QT intervals 1, 2
• Loss of deep tendon reflexes 1, 2
• Flushing and sweating 4
• Nausea and vomiting 1, 2

Severe Toxicity (4-10 mmol/L)

• Sedation and severe muscular weakness 1, 2
• Respiratory depression (4-5 mmol/L) 1, 2
• AV nodal conduction block and bradycardia (6-10 mmol/L) 1, 2
• Hypotension 1, 2

Life-Threatening Toxicity (>7.5 mmol/L)

• Altered cardiac conduction (>7.5 mmol/L) 5
• Cardiac arrest (>12.5 mmol/L) 5

Immediate Management of Toxicity

First-Line Treatment

Administer IV calcium immediately as a physiological antagonist to magnesium 2:

• Calcium chloride 10%: 5-10 mL IV over 2-5 minutes 2
• Calcium gluconate 10%: 15-30 mL IV over 2-5 minutes 2

Supportive Measures

• Stop magnesium sulfate infusion immediately 6
• Continuous cardiac monitoring for arrhythmias 2
• Correct electrolyte abnormalities, particularly potassium 2
• Blood pressure support with vasopressors if hypotension persists despite calcium and fluid resuscitation 2
• Prepare for respiratory support: intubation may be necessary if respiratory depression occurs 1, 2

Critical Safety Considerations

High-Risk Populations

Patients with renal impairment develop toxicity at relatively lower doses 1, 2:

• Maximum dose: 20 grams/48 hours in severe renal insufficiency 4
• Check serum creatinine before initiating therapy 3
• Frequent serum magnesium monitoring required 4

Dangerous Drug Interactions

Never combine magnesium sulfate with calcium channel blockers (especially IV or sublingual nifedipine)—this causes severe myocardial depression and precipitous hypotension 3, 7

Pregnancy-Specific Concerns

• Iatrogenic overdose is particularly common in oliguric pregnant women receiving magnesium for preeclampsia 1
• Continuous administration beyond 5-7 days causes fetal abnormalities including hypocalcemia, skeletal demineralization, and osteopenia 4
• Continue for 24 hours postpartum as eclamptic seizures may develop for the first time in early postpartum period 3

Dosing and Administration Guidelines

Therapeutic Dosing for Preeclampsia/Eclampsia

• Loading dose: 4-5 g IV over 3-4 minutes or diluted in 250 mL fluid 4
• Maintenance: 1-2 g/hour by continuous IV infusion (preferred) or 4-5 g IM every 4 hours 4, 8
• Maximum rate: Do not exceed 150 mg/minute IV except in severe eclampsia with active seizures 4
• Maximum total dose: 30-40 g per 24 hours 4

Fluid Management

Limit total fluid intake to 60-80 mL/hour to avoid pulmonary edema, as preeclamptic women are at risk for both pulmonary edema and acute kidney injury 3

Common Pitfalls to Avoid

• Do not delay calcium administration while waiting for laboratory confirmation when clinical signs strongly suggest toxicity 2
• Do not continue magnesium when oliguria develops without dose adjustment or discontinuation 1, 2
• Do not rely solely on patellar reflex assessment—agreement between deep tendon reflex and serum magnesium is slight and not significant 9
• Do not use magnesium for blood pressure control—it is for seizure prevention only; use hydralazine, labetalol, or nifedipine for acute hypertension 3

Monitoring Frequency

• Before each dose: Check patellar reflexes, respiratory rate, and urine output 4, 8
• During infusion: Continuous monitoring if IV route used 2
• Postpartum: Monitor blood pressure and clinical condition at least every 4 hours while awake for at least 3 days 3