Hypophosphatemia

Background

• PO₄^-3< 2.3mg/dL
• Common Causes
  • • Internal redistribution, reduced intestinal absorption
    • Malnourished pts: refeeding syndrome and intracellular movement/use of phos
    • Insulin moves phos intracellular as does respiratory alkalosis
    • Phos binders on purpose or inadvertently (calcium, aluminum, magnesium antacids)
    • Excessive loss (diarrhea, CRRT, increased urinary excretion)
    • Proximal tubular dysfunction such as in Fanconi Syndrome
    • Hyperparathyroidism causes renal phos wasting
    • Post-parathyroidectomy leading to hungry bone syndrome
    • Vitamin D deficiency or resistance
• PO₄^-3is primarily found in bone and intracellular space
• Required for metabolic pathways (ATP production!)
• Most renal reabsorption occurs in proximal tubule via sodium-phosphate cotransporter

Presentation

• Mild Hypophosphatemia (serum >2.0) rarely symptomatic
• PO₄^-3< 2.0: Muscle weakness
• PO₄^-3< 1.0: Heart failure, respiratory failure, rhabdomyolysis, seizures
• Failure to wean from ventilator

Evaluation

• Urine PO₄^-3 level if cause not readily apparent
• Calculate Fe PO₄^-3 ([U PO₄^-3 x PCr x 100]/[P PO₄^-3x UCr]).
  • • Fe PO₄^-3 < 5% = normal renal response to hypophos: redistribution or ↓ absorption
    • Fe PO₄^-3 > 5% = renal phos wasting

Management

• Caution repleting pts with impaired renal function: start with half suggested dose
• If K⁺ > 4 and patient requires IV repletion, may need to use sodium PO₄^-3 in place of K⁺ PO₄^-3 IV; po preferred unless severe or symptomatic, or patient cannot take po
• K-Phos neutral: oral, each 250mg tablet has 8 mmol of PO₄^-3 and 1.1mEq of K⁺
• K⁺ PO₄^-3: IV, each mL has 3mmol PO₄^-3, 4.4 meq K⁺
• Na⁺ PO₄^-3: IV, each mL has 3mmol PO₄^-3
• IV repletion is diluted in 250mL NS or D5W and infused over 4-6 hrs
• PO₄^-3>1.5: PO: 40 – 80 mmol K⁺Phos neutral (aim for 1 mmol/kg) divided into 3-4 doses/day
• PO₄^-31.25 - 1.5: oral 100 mmol K⁺ PO₄^-3neutral in 3-4 divided doses if asymptomatic
• IV: 30 mmol K⁺ PO₄^-3over 6 hours (aim for 0.4mmol/kg) if symptomatic 
• PO₄^-3<1.25: IV: 80mmol K⁺Phos over approximately 12 hours (aim for 0.5mmol/kg)
  • • Check serum PO₄^-3 2-12 hrs after last dose of PO₄^-3 to determine if additional needs