Hypo Mg


Background

  • Often presents as lab detected value
    • Complicates care of ill pts
       
  • Symptomatic Mg depletion
    • Often assoc w/ other electrolyte abnormalities
      • Hypokalemia, hypocalcemia, met acidosis, etc.
    • Sx secondary to electrolyte disturbance
      • D/t Mg & another electrolyte abnormality
         
  • Critically low serum Mg levels are <0.5 mmol/l; manifest clinically as:
    • Seizures, tetany, life-threatening arrhythmias
    • When acute problem superimposed on chronic depletion
      • Ie, Pts w/ DM during correction of ketoacidosis OR
      • Alcoholics: vomiting, diarrhea, pancreatitis
         
  • Mg levels in moderate depletion range (0.5-0.7 mmol/l) may be life-threatening in
    • Acute MI, CHF
    • Marked hypomagnesemia & other electrolyte disturbances
      • Potentiate risk of life-threatening arrhythmias
      • Incr risk for digoxin toxicity
      • Esp w/ acute MI, cardiomyopathy

Risk Factors

  • Two main causes
    • Decreased GI absorption
    • Increased renal loss
       
  • Physiologic Mg levels
    • Intracellular: 94-99%
    • Protein-bound: 33%
    • Stored in bones: 50-60%
       
  • Risk Factors
    • Pharmacological
      • Laxative abuse
      • Diuretics
      • Digoxin
      • Cisplatin
      • Aminoglycosides
        • Gentamicin
      • Hydrofluoric acid
      • TPN
      • Prolonged NG suction
         
    • Medical
      • Chronic alcoholism
      • Vomiting, malnutrition
      • Diarrhea (acute, chronic)
      • Malabsorption syndromes
        • Chronic pancreatitis (decr Ca in pancreatitis), cirrhosis
        • Sprue, Steatorrhea
        • Surgical resection of bowel
      • Hungry bones syndrome
      • Nephropathy
        • Renal tubular defects: primary Mg wasting, renal tubular acidosis
        • ATN, interstitial nephritis, glomerulonephritis, post-obstructive diuresis
      • Hypocalcemia, hypophosphatemia
      • Polyuric states
      • Pregnancy, eclampsia
      • Acute MI
      • GI fistulas, IBS
      • DM, Familial hypomagnesemia (autosomal recessive)

History/Symptoms

  • Alcoholism
    • High prevalence assoc w/ EtOH withdrawal seizures
  • Weakness, numbness, vertigo
  • Palpitations
  • Nausea, vomiting, abdominal pain
     

Physical Exam

  • General
    • Altered mental status, altered personality
  • HEENT
    • Possible nystagmus
  • Cardiovascular
    • Dysrhythmias
  • GI/GU
    • Abd tenderness possible (vague)
  • Musculoskeletal/neurological
    • Muscle fasciculations, tremors, incr DTRs
    • Paresthesias, seizures, tetany (w/ superimposed decr Ca)
    • Dizziness, ataxia, myoclonus
    • Encephalopathy

Diagnosis

  • Labs/Tests
    • CMP
      • Hypo-Mg
      • Hypo-Ca & hypo-K (occassionally concurrently present)
    • UA/drug screen
      • Urine Mg usually <1 mEq/l [0.50 mmol/l]
      • Fractional excretion of Mg >2% w/ normal renal fxn
        • Indicates renal Mg wasting
        • D/t drugs (diuretics, aminoglycosides, cisplatin)
           
  • Imaging
    • CT/MRI head/spine: r/o possible tumors, encephalopathy, spine injuries
       
  • EKG
    • PR & QT prolongation, QRS widening
    • T-wave abnormalities
      • Acute: peaked
      • Chronic: flattened
      • Possible U-waves
    • Arrhythmias, Torsades possible

Treatment

  1. Monitor vitals, Tx underlying cause(s)
  2. Mild, asymptomatic
    • Oral supplement
      • Slow Mag™ or Mag-Tab SR™ 2 tabs QD
      • PO Mg Gluconate 1g PO TID
      • Dietary supplementation (cereals, green vegetables, meat, fish)
         
      • Magnesium hydroxide 80 mg/mL suspension (Milk of Magnesia) 7.5 - 15 mL daily or bid
      • Magnesium oxide 400 - 800 mg (1 - 2 tabs) daily or bid
        • Both these options are limited by their main side effect: diarrhea
    • IV (for patients NPO otherwise PO route should be used): Magnesium sulfate 2 g IV x 1
       
  3. Moderate, Mild symptomatic (<1.6 mEq)
    • Slow Mag™ or Mag-Tab SR™ 2 tabs BID
    • MgSO4 5g IV over 5hr
    • PO Mg Oxide 400 mg PO QD-BID
       
  4. Severe & symptomatic (tetany, seizures, shock, arrhythmia)
    • Mg dosing repeated until concentration >0.8 mEq/L
    • MgSO4, 1-2 g IV over 15 min, may repeat
      • Maintenance: 1-2 g IV/hr until target concentration & Sx abate
         
    • Torsade: 2 g IV over 2 min, repeat until arrhythmia resolved
       
    • Slow Mag™ or Mag-Tab SR™
      • 2 tabs QID for severe Mg depletion w/ mild Sx
      • Gradual Tx just as efficacious as rapid in this setting
      • Avoids excessive Mg excretion by kidneys
    •  
    • Mg levels can be verified 6 - 8h post-infusion.
    • Add Mg to daily BW

Disposition

  1. Admit if :
    • Serum Mg <1.5 mEq/l [0.75 mmol/l], or symptomatic
    • Severe Sx or level <1.2 mEq/l [0.60 mmol/l] requires monitored bed
       
  2. D/c w/ outpt f/u if :
    • asymptomatic hypo-Mg
 

Admission Order Set

1. Admit to:
2. Diagnosis: Hypomagnesemia
3. Condition:
4. Vital Signs: q6h
5. Activity: Up ad lib
6. Diet: Regular
7. Special Medications:
-Magnesium sulfate 4-6 gm in 500 mL D5W IV at 1 gm/hr. (Estimation of Mgdeficit = 0.2 x kg weight x desired increase in Mg concentration; give deficit over 2-3d) OR
-Magnesium sulfate (severe hypomagnesemia <1.0) 1-2 gm (2-4 mL of 50% solution) IV over 15 min, OR
-Magnesium chloride (Slow-Mag) 65-130 mg (1-2 tabs) PO tid-qid (64 mg or 5.3 mEq/tab) OR
8. Extras: ECG
9. Labs: Magnesium, calcium, chem 7&12. Urine Mg, electrolytes.