Lab Case 125 – Interpretation

A 70 year old man presents feeling unwell. His blood tests show:Severe hypernatraemia with renal failure

Osmolality = 332.9+Glucose ie very high

Predominant increase in Urea, likely indicating severe dehydration requiring fluid replacement orally(water) or iv (5% dextrose) to correct fluid/ Na /osmolality over 1-2 days to prevent complications.

Free water deficit Calculation:

TBW % X (Na-140)/140

TBW 50% males and 40% females

Approach to hypernatraemia:

Water loss in excess of salt deficit – hypernatremia is usually due to insufficient water (primarily in patients who either do not experience thirst normally, or cannot act on it) or less commonly excess salt which causes a rise in serum osmolality.

Decreased water intake (with normal fluid loss)

  • Disordered thirst perception  e.g. hypothalamic lesion
  • Lack of environmental water
  • Inability to communicate water needs e.g. Coma, CVA, Intubated patients and Kids

Hypotonic fluid loss (Water loss in excess of salt loss)

  • Skin:
    • Sweat in hot climate or exercise (Heat stroke, Heat exhaustion)
    • Burns
  • GI disturbances (especially with salt replacement)
    • Vomit, diarrhoea, fistula
  • Renal disease Impaired renal concentrating ability
      • Diabetes Insipidus – Central, Nephrogenic or Drugs (alcohol, phenytoin, lithium, colchicine, Amphotericin, gentamicin)
      • Osmotic Diuresis
        • CRF, mannitol, Hyperglycaemia, hypokalaemia
      • Renal disease
        • Nephropathy, myeloma, TIN, obstructive uropathy, PKD

Increased salt

  • Acute salt poisoning
    • Ingestion of seawater or salt tablets, IV NaHCO3, hypertonic saline
  • Increased mineralocorticoid
    • Primary hyperaldosteronism (Increased BP, decreased K, and alkalosis)
  • Increased glucocorticoid (Cushings)
  • Ectopic ACTH

Aim of treatment:

  • Stop ongoing losses
  • Correct water deficit
  • Correct sodium deficit if hypovolaemic
  • Treat the underlying cause

Complications of treatment

  • Intracerebral haemorrhage (especially in neonates: complication of NaHCO3 administration)
  • Coma and seizures
  • Cerebral oedema (if correction too rapid)
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