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Hypovolaemic hyponatraemia

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

  • presence of clinically detectable decreased extracellular fluid (ECF) volume generally reflects hypovolaemia from some degree of body solute depletion
  • hyponatraemia with volume depletion can arise in a variety of settings
    • renal loss with water retention include:
      • diuretic therapy
      • mineralocorticoid deficiency
      • arenal hemorrhage
      • cerebral salt wasting
      • adrenal enzyme deficiencies (congenital adrenal hyperplasia)
      • bicarbonaturia, glucosuria, ketonuria
    • extra renal loss with water retention include:
      • gastrointestinal losses - vomiting, diarrhoea
      • third space losses - bowel obstruction;pancreatitis; muscle trauma;burns
      • sweat losses e.g. endurance exercise
  • volume depletion is generally diagnosed clinically from the history, physical examination, and laboratory results
  • clinical signs of volume depletion include
    • orthostatic decreases in blood pressure and increases in pulse rate, dry mucus membranes, decreased skin turgor
    • if signs of volume depletion and hyponatraemia
      • then should be considered hypovolaemic hyponatraemia unless there are alternative explanations for these findings (1)
    • elevations of urea, creatinine, urea–creatinine ratio, and uric acid level indicate possible volume depletion
      • however these findings are neither sensitive nor specific, and they can be affected by other factors (eg, dietary protein intake, use of glucocorticoids).
    • urine sodium excretion is generally more helpful
      • spot urine [Na+] should be <30 mmol/L in patients with hypovolaemic hyponatraemia unless the kidney is the site of sodium loss
  • if cllinical assessment is equivocal
    • a trial of volume expansion can be a useful diagnostic tool (also will be therapeutic if volume depletion is the cause of the hyponatraemia)
      • a 0.5 to 1 L infusion of isotonic (0.9%) NaCl, patients with hypovolaemic hyponatraemia will begin to correct their hyponatraemia without developing signs of volume overload
      • in contrast, if SIADH
        • urine [Na+] will increase but the serum [Na+] will remain unchanged or decrease as the administered water is retained and the sodium load excreted in a smaller volume of concentrated urine (1)

Reference:

  • (1) hyponatraemia Treatment Guidelines 2007: Expert Panel Recommendations The American Journal of Medicine 2007; 120 (11);S1:S1-S21.

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