How is volume depletion managed in prerenal azotemia?

Updated: Sep 19, 2018
  • Author: Moro O Salifu, MD, MPH, FACP; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

If volume depletion is due to free water loss, the serum sodium is often elevated by 10 mEq/L from baseline. The amount of fluid replacement in liters—that is, the free water deficit—can be estimated from serum sodium (mg/dL) and patient weight (kg) as follows:

[(Na/140) – 1] × 0.5 × weight

The volume of fluid to be administered is equal to the sum of the free-water deficit and daily maintenance fluids. Fifty percent of this total volume should be administered in the first 24 hours, and a new calculation should be performed at 24 hours based on new laboratory results.

Maintenance fluid can be roughly estimated at 1.5-2 L/day; however, it can also be estimated from caloric intake since 1 kCal requires 1 mL of water in the metabolic process. Normal caloric intake is about 30 Kcal/kg (low catabolic state requires < 30 kCal/kg and high catabolic state requires >40 kCal/kg). A 70-Kg person at normal caloric intake requires 2100 Kcal/day or 2.1 L of fluid intake. This volume should be added to the free-water deficit and administered as noted above.

Alternatively, the total free water deficit is usually quite close to the sum of 50% free-water deficit and daily maintenance fluids. Therefore, for all practical purposes, the total free-water deficit can be administered intravenously in 24 hours.

The fluid to be administered should consist of hypotonic solutions such as 0.5% saline or 5% dextrose in water (D5W). Alert patients should be encouraged to drink as much free water as they can tolerate; otherwise, free water can be administered via a nasogastric tube.

Serum sodium should be measured every 6-8 hours, and fluid replacement should be adjusted to avoid a precipitous decline in the serum sodium. To prevent brain edema, the rate of decrease in serum sodium should be no more than 0.7 mEq/h (17 mEq/24 h). Volume depletion due to blood loss requires IV saline and transfusion to maintain pressure (as well as interventions to halt further loss).


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