FLUID MANAGEMENT OF SPECIFIC CLINICAL CONDITIONS

The following guidelines are intended to facilitate initiating therapy, but the choice of fluid and rate of administration must be adjusted to achieve physiologic goals. These guidelines are only a starting point for patients without other major comorbidities of vital organs. Careful observation of the patient's response forms the basis for ongoing modification in a continuous feedback loop.

Routine Maintenance Fluids

Routine maintenance fluids are described for a 70-kg postoperative patient. The patient requires 110 mL H₂ O and 110 kcal/hour, or 2640 mL and 2640 kcal/day. This example is based on the 4-2-1 rule ( Table 46-21 ), which provides a close approximation of water requirements. The sodium requirement (1.5 mEq/kg/day) is dissolved in the daily fluid requirement of 2.64 L; the 100 mEq/kg/day requirement for potassium is placed in the 2.64 L/day water requirement: 100 mEq K/2.64 L = 42 mEq/L. However, the potassium concentration needs to be limited if the fluid is to be infused into a peripheral vein because of the chemical irritation induced by high concentrations of potassium. The obligatory glucose needs of the brain and RBCs are roughly 2 mg/kg/min. Because dextrose contains 3.41 kcal/g instead of 4 kcal/g of glucose, about 17% more dextrose than glucose is required. If carbohydrate is not provided, glycogenolysis and gluconeogenesis from amino acid pools provide the necessary glucose but accelerate protein catabolism.

Carbohydrate is said to prevent catabolism (i.e., protein sparing), but the benefit of this dose of dextrose is not clear. Total starvation may be preferable because insulin concentrations decrease to very low levels, facilitating lipolysis as a caloric source. The osmolarity of 7.5% dextrose is 417 mOsm/L, to which is added 156 mOsm/kg H₂ O, resulting in a highly hyperosmolar solution. A compromise between the need for glucose and hyperosmolality has been 5% dextrose.

If there are other losses (e.g., gastric drainage), additional sodium and water are required. Gastric drainage of 0.5 L/day loses 30 to 50 mEq of sodium and 50 to 65 mEq of chloride ( Table 46-22 ). When these are added to the maintenance fluid, the concentration approximates 0.45% NaCl. This solution is commonly used as a maintenance intravenous fluid postoperatively in patients with nasogastric drainage.