Tag Archives: urea

Urinary Osmolality, Elderly Patients, Alcoholics and Hyponatremia

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This discussion came about following a discussion with my colleague, Dr Bairbre McNicholas. It focuses principally on the problem of hyponatremia in elderly patients and undernourished alcoholics. I explain why the lack of dietary salt and protein intake massively inhibits water excretion resulting in hypotonic hyponatremia, often with fluid overload. The traditional approach to managing hyponatremia – fluid restriction – frequently fails because it is a problem of solute “underload” rather than water overload. Commencing iv fluids may precipitate a rapid and potentially dangerous diuresis – hence the most effective therapy for these patients is the FEED them.

I guarantee you’ll learn something.

The Syndrome of Antidiuresis (SIADH)

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This tutorial is about the Syndrome of Inappropriate Diuresis. SIAD also known as SIADH is a form of hypotonic hyponatremia associated with iso- or hypervolemia, high urinary osmolality and high urinary sodium. Traditionally this is associated with high levels of circulating vasopressin (antidiuretic hormone – ADH), that may be associated with sepsis, acute critical illness, pneumonia or mechanical ventilation. However, SIAD is also associated with a variety of brain injuries, drugs (SSRIs and anticonvulsants) and a variety of cancers.

Treatment of symptomatic SIAD is with hypertonic saline (150ml of 3% over 20 minutes). Chronic or asymptomatic SIAD is treated with fluid restriction (determined by the Furst equation uNa + uK/pNa – if the result is less than1 the patient is suitable for fluid restriction).

Alternative inexpensive therapies include Urea (30 to 60mg per day), salt tablets plus frusemide or demeclocycline.

Vaptan agents, the block the V2 receptors, appear to be effective for long term therapy. Tolvaptan is available commercially but quite expensive for the majority of patients.

Cerebral salt wasting is associated with subarachnoid hemorrhage. It shares the same blood and urinary profile as SIAD(H) but is associated with hypovolemia. The disorder is self limiting and is treated with isotonic fluids.