Pritchard Protocol for Magnesium Sulfate in Preeclampsia
The Pritchard protocol consists of a combined loading dose of 4 grams IV over 10-20 minutes plus 10 grams IM (5 grams in each buttock), followed by 5 grams IM every 4 hours in alternate buttocks for 24 hours postpartum. 1, 2
Loading Dose Administration
Combined IV/IM approach:
- Administer 4 grams IV as a 20% solution over 10-15 minutes (or up to 5 grams over 3-4 minutes if using 10-20% concentration) 2, 3
- Simultaneously give 10 grams IM total: 5 grams (10 mL of undiluted 50% solution) deep into each buttock 1, 2
- This combined loading achieves therapeutic magnesium levels (1.8-3.0 mmol/L) within 60 minutes via the IM route, while the IV component provides immediate effect 4, 5
Maintenance Dosing
Standard Pritchard maintenance:
- Give 5 grams IM (10 mL of 50% solution) every 4 hours in alternate buttocks 1, 2
- Continue for 24 hours postpartum in most cases 1, 6
- Maximum total daily dose should not exceed 30-40 grams 2
When to repeat or adjust:
- If seizures recur despite loading dose, give additional 5 grams IM 3
- In severe renal insufficiency, maximum dosage is 20 grams per 48 hours with frequent serum level monitoring 2
Clinical Monitoring Requirements
Essential safety parameters before each IM dose:
- Patellar reflexes must be present (loss occurs at 3.5-5 mmol/L) 1, 4
- Respiratory rate ≥12 breaths/minute (paralysis occurs at 5-6.5 mmol/L) 6, 4
- Urine output ≥30 mL/hour (oliguria increases toxicity risk as magnesium is renally excreted) 6, 7
Do not routinely check serum magnesium levels unless high-risk situations exist: renal impairment, oliguria <30 mL/hour, absent reflexes, or respiratory rate <12/minute 7
Critical Safety Considerations
Absolute contraindications and warnings:
- Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) as this causes severe myocardial depression and precipitous hypotension 1, 6, 7
- Limit total IV fluids to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 1, 6
- Do not continue magnesium sulfate beyond 5-7 days as prolonged use causes fetal abnormalities 2
Toxicity management:
- Keep calcium gluconate 1 gram IV available as antidote for magnesium toxicity 4
- Cardiac arrest occurs at magnesium levels >12.5 mmol/L 4
Advantages of Pritchard Protocol
When this regimen is preferred:
- Resource-limited settings where continuous IV infusion pumps are unavailable 1
- Limited IV access or difficulty maintaining IV lines 1
- Can be administered by trained midwives or nursing staff without requiring intensive monitoring equipment 7
- Validated in the landmark MAGPIE trial involving over 4,000 women 1
Alternative IV-Only Regimen
If continuous IV access is reliable, the alternative is 4-6 grams IV loading over 20-30 minutes followed by 1-2 grams/hour continuous infusion (use 2 grams/hour for BMI ≥25 kg/m²) 1, 6. However, the Pritchard protocol remains the gold standard in settings without infusion pumps 1.
Common Pitfalls to Avoid
- Do not use NSAIDs for postpartum pain in preeclamptic patients as they worsen hypertension and increase acute kidney injury risk 1, 6
- In community settings where full protocol cannot be administered, give at least the loading dose (10 grams IM total: 5 grams each buttock) before referral—this is better than no treatment 1
- Do not rely on proteinuria levels to guide therapy; clinical parameters (reflexes, respiratory rate, urine output) are sufficient 7
- The 1 gram/hour IV maintenance dose is inadequate and produces subtherapeutic levels 5