Hypophosphatemia

Hypophosphatemia – Peter Thorne

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