Nephrology Book

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Magnesium SulfateAka: Magnesium Replacement

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  1. Mechanism
    1. Magnesium is cofactor in enzymatic reactions
      1. Sodium-Potassium ATPase pump
      2. Blocks Neuromuscular transmission
    2. Magnesium Deficiency effects
      1. Cardiac arrhythmia
      2. Refractory Ventricular Fibrillation
      3. Cardiac Insufficiency
      4. Sudden Cardiac Death
  2. Indications
    1. Hypomagnesemia
    2. Arrhythmia Treatment and Prevention
      1. Torsades de pointes
    3. Acute Myocardial Infarction
      1. Arrhythmia prevention (variable efficacy)
    4. Preeclampsia (Pregnancy Induced Hypertension)
    5. Preterm Labor (not effective)
  3. Contraindications
    1. Myasthenia Gravis
    2. Impaired Renal Function
  4. Preparations
    1. Oral
      1. Milk of Magnesia (1 ml = 0.3 mEq)
    2. Parenteral (IV or IM): Magnesium Sulfate (MgSO4)
      1. One gram MgSO4 contains 8.12 meq Magnesium
      2. One ml MgSO4 50% Solution = 4 meq Magnesium
      3. One ml MgSO4 10% Solution = 0.8 meq Magnesium
  5. General Pointers
    1. Do NOT exceed 100 meq/day
    2. Adjust replacement for decreased Renal Function
  6. IV Dosing of Magnesium Sulfate
    1. Replacement of documented Magnesium deficiency
      1. MgSO4 0.5-1.0 grams/hour (4-8 meq/hour) for 24 hours
    2. Preeclampsia or Pregnancy Induced Hypertension
      1. Preparation
        1. Magnesium sulfate 5 grams in 250 ml D5W (20 mg/ml)
        2. Final concentration: 2 grams/hour = 100 ml/hour
      2. Load: 4-6 grams MgSO4 in 100 ml IV over 20-30 minutes
      3. Maintenance: 2-3 grams MgSO4 per hour
      4. Continue until diuresis or about 24 hours postpartum
      5. See monitoring in obstetrics below
    3. Preterm Labor (not effective)
      1. Same doses as in Preeclampsia
      2. Does not prevent preterm birth
        1. Gyetvai (1999) Obstet Gynecol 94:869
    4. Maintenance in prolonged IV fluid
      1. Add 1-2 grams (2-4 ml 50% MgSO4) to total IVF per day
      2. Delivers 0.3 to 0.7 meq per hour
    5. Arrhythmia
      1. Prepare 1-2 grams (2-4 ml 50% MgSO4) in 10 ml D5W
      2. Ventricular Tachycardia: 1-2 g MgSO4 IV over 1-2 min
      3. Ventricular Fibrillation: 1-2 g MgSO4 IV Push
      4. Torsades de pointes: 1-2 g (up to 10 g) MgSO4 IV
  7. Adverse Effects (see level related effects below)
    1. Too rapid Magnesium Sulfate administration
      1. Flushing
      2. Sweating
      3. Mild Bradycardia
      4. Hypotension
    2. Hypermagnesemia
      1. Muscle Weakness
      2. Hyporeflexia
      3. Respiratory Depression
      4. Pulmonary edema
      5. Headache
  8. Monitoring (particularly in Obstetrics)
    1. Vital Signs and Reflexes
      1. First hour: Check every 15 minutes
      2. Later: Monitor Vital signs every hour
    2. Intake and Urine output
      1. Consider Foley Catheter
      2. Fluid restrict to 2400 cc per 24 hours
    3. Examination
      1. Mental status
      2. Respiratory status
      3. Lung exam
      4. Deep Tendon Reflexes
    4. Check Serum Magnesium Level
      1. Obtain 6 hours and 12 hours after MgSO4 load
      2. See also Serum Magnesium and Hypermagnesemia
      3. Interpretation (See also Hypermagnesemia)
        1. Normal in pregnancy: 1.3 to 2.6 mg/dl
        2. Therapeutic: 5.5-7.5 mg/dl
        3. Loss of patellar reflex: 10-12 mg/dl
        4. Respiratory depression: 15-17 mg/dl
        5. Paralysis: 15-17 mg/dl
        6. Cardiac Arrest: 30-35 mg/dl
    5. Indications to stop magnesium, check level immediately
      1. Urine output <25 ml/hour
      2. Respiratory Rate <12 per minute
      3. Loss of Deep Tendon Reflexes
  9. Antidote for Magnesium Toxicity
    1. Calcium Gluconate 1 gram IV slowly over 3 minutes

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