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A free water clearance of zero means the kidney is producing urine isosmotic with respect to the plasma. Values greater than zero imply that the kidney is producing dilute urine through the excretion of solute-free water. Values less than zero imply that the kidney is conserving water (likely under the influence of antidiuretic hormone, ADH ...
[3] [27] Once the patient is stable, it is important to identify the underlying cause of hypernatremia as that may affect the treatment plan. [3] [27] The final step in treatment is to calculate the patients free water deficit, and to replace it at a steady rate using a combination of oral or IV fluids.
The underlying mechanism typically involves too little free water in the body. [6] If the onset of hypernatremia was over a few hours, then it can be corrected relatively quickly using intravenous normal saline and 5% dextrose in water. [1] Otherwise, correction should occur slowly with, for those unable to drink water, half-normal saline. [1]
aldosterone, which stimulates active sodium re-absorption (and water as a result) anti-diuretic hormone, which stimulates passive water re-absorption; Both hormones exert their effects principally on the collecting ducts. Tubular secretion occurs simultaneously during re-absorption of filtrate.
The fractional excretion of sodium (FE Na) is the percentage of the sodium filtered by the kidney which is excreted in the urine.It is measured in terms of plasma and urine sodium, rather than by the interpretation of urinary sodium concentration alone, as urinary sodium concentrations can vary with water reabsorption.
The resultant value is the approximate volume of free water required to correct a hypernatremic state. In practice, the value rarely approximates the actual amount of free water required to correct a deficit due to insensible losses, urinary output, and differences in water distribution among patients. [13]
Example: a child with diarrhea who has been given salty soup to drink, or insufficiently diluted infant formula. Overall there is more sodium than water. The water will move out of the cell toward the intravascular compartment down the osmotic gradient. This can cause tissue breakage (in case of muscle breakage it is called rhabdomyolysis).
Excessive ADH causes an inappropriate increase in the reabsorption in the kidneys of solute-free water ("free water"): excess water moves from the distal convoluted tubules (DCTs) and collecting tubules of the nephrons – via activation of aquaporins, the site of the ADH receptors – back into the circulation. This has two consequences.