Hypophosphataemia

OVERVIEW

• important intracellular anion
• 85% is stored in bone as hydroxyapapitie crystals, 14% in soft tissues, 1% in blood
• intracellular concentration = 100mmol/L
• plasma concentration = 0.8-1.3mmol/L

-> thus hypophosphataemia describes total body depletion

Hypophosphataemia = < 0.8

MILD – 0.65-0.8
MODERATE – 0.32-0.65
SEVERE – actions on intestine, kidneys and bone

• PTH -> increase in phosphate and Ca2+ release from bone, but increases excretion in kidney by inhibiting reabsorption in the proximal tubule
• vitamin D from kidneys acts on jejunum to increase absorption of Ca2+ and phosphate

FUNCTIONS – many!

• ATP
• nucleic acids
• phospholipids
• enzymatic co-factors
• cGMP
• cAMP
• 2, 3 DPG
• enzymes in glycolytic pathway
• buffer in maintenance of plasma pH
• immune system integrity
• coagulation cascade

CAUSES

Hypophosphataemia

• intake: malnutrition, phosphate binders, vitamin D, malabsorption, TPN
• redistribution: refeeding syndrome, insulin in DKA
• output:
  urinary – diuretics, osmotic diuresis, hyperparathyroidism, proximal tubular dysfunction (Fanconi’s syndrome)
  non-urinary –
  -> upper GI
  -> mid GI
  -> lower GI – diarrhoea
  -> other – sweat, burns, sepsis, bleeding

HISTORY

• look for symptoms of hypophosphataemia + cause (renal loss, intake, redistribution into cells, catabolic state)
• SOB
• ventilator dependence
• weakness
• altered mental state
• heart failure symptoms
• shock

EXAMINATION

• CVS: reversible dilated cardiomyopathy, inotrope requirement
• RESP: respiratory failure, ventilator dependence, left shift of oxy-Hb dissociation curve
• NEURO: altered mental state, weakness, gait disturbance, paraesthesiaes
• HAEM: haemolysis, disorders of WCC function,
• ENDO: bone demineralisation
• MUSKULO: rhabdomyolysis

INVESTIGATIONS

• aimed at quantifying severity and finding cause
• phosphate, Ca2+, K+, Mg2+
• ECG

MANAGEMENT

• feed adequately (caution in refeeding syndrome)
• if phosphate 0.65-0.89 give oral phosphate
• IV phosphate:

-> KH₂PO₄ – 10mmol of phosphate and 10mmol of K in 10mL
-> NaKH₂PO₄ – 13.4mmol of phosphate, 21.4mmol Na+, 2.6mmol K in 20mL

• administer 1 ampoule over 1 hour
• bewared of phosphate administration in renal failure
• monitor for hyperphosphataemia, hypocalcaemia, hypotension, tetany and ECG changes

References and Links

• CCC – Hyperphosphataemia
• CCC – Hyperphosphataemia Mind Map (PDF)
• CCC – Hypophosphataemia
• CCC – Hypophosphataemia Mind Map (PDF)