Fluids, Electrolytes, Acid-Base Disorders, and Nutrition Support
Body water compartments
Total body water (TBW): About 60% of body weight for men; about 50% of body weight for
women; lower percentage for those with obesity and for older adults (0.5 L/kg for men; 0.45 L/kg
for women)
About 60% of TBW is intracellular.
About 40% of TBW is extracellular water (about 75% is interstitial fluid; about 25% plasma volume).
| d. | Fluid compartments are separated by membranes that are freely permeable to water. The movement |
|---|
of fluids through these compartments is due to hydrostatic pressure or osmotic pressure.
Simple, weight-based estimation: 30–35 mL/kg (overestimates needs for patients with higher BMI;
underestimates needs for patients with lower BMI)
Holliday-Segar formula: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, and 20 mL/
kg thereafter
Increased insensible losses occur with diarrhea and fever (around 10%–15% for every degree
Celsius greater than 37°C).
| d. | Fluids received from other sources should be considered when estimating a patient’s fluid needs to |
|---|
avoid fluid overload.
Rectal Temperature (°C)
No. of Patients
Mean Fluid Loss (mL/m2/day)
36.7–37.7
37.8–38.2
38.3–38.8
38.9–40
Estimating electrolyte requirements
Approximate electrolyte concentrations in the extracellular and intracellular fluids (ECFs and ICFs)
(Fluid, Electrolyte, and Acid-Base Disorders, Vol 1. New York: Churchill Livingstone, 1985:1-38)
Electrolyte
Extracellular Fluid
(mEq/L)
Intracellular Fluid
(mEq/L)
(plasma)
(muscle)
Sodium
Potassium
4.5
Chloride
Bicarbonate
Calcium
5.0 (10 mg/dL)
—
Magnesium
1.5 (1.8 mg/dL)
Phosphate
2 (3.5 mg/dL)