Maintenance Requirements for Water, Sodium, and Potassium. In healthy adults, sufficient water is required to balance gastrointestinal losses (100–200 mL/day), insensible losses (500–1,000 mL/day representing respiratory and cutaneous losses), and urinary losses (1,000 mL/day)
Water maintenance requirements are often calculated on the basis of body weight. For a 70-kg adult, the daily water maintenance requirement is about 2,500 mL (Table 14-13: Maintenance Water Requirements).
Renal sodium conservation is highly efficient, such that the average daily maintenance requirement in an adult is about 75 mEq.
The average daily maintenance requirement of potassium is about 40 mEq. Physiologic diuresis induces an obligate potassium loss of at least 10 mEq for every 1,000 mL of urine.
Electrolytes such as chloride, calcium, and magnesium do not require short-term replacement, although they must be supplied during chronic IV fluid maintenance.
Dextrose. Addition of glucose to maintenance fluid solutions is indicated only in patients considered to be at risk for developing hypoglycemia (infants, patients on insulin therapy). Otherwise, the normal hyperglycemic response to surgical stress is sufficient to prevent hypoglycemia.
Iatrogenic hyperglycemia can limit the effectiveness of fluid resuscitation by inducing an osmotic diuresis.
In critically ill patients, some evidence suggests that tight control of plasma glucose (80–110 mg/dL) is associated with better outcomes, but other evidence suggests the opposite.
Surgical Fluid Requirements
Water and Electrolyte Composition of Fluid Losses. Surgical patients require replacement of plasma volume and ECF secondary to hemorrhage and tissue manipulation (third-space loss). Lactated Ringer's solution is often selected for replacement of third-space losses as well as for gastrointestinal secretions.
Influence of Perioperative Fluid Infusion Rates on Clinical Outcomes. Conventionally, intraoperative fluid management included replacement of fluids assumed to accumulate extravascularly in surgically manipulated tissues. Until recently, perioperative clinical practice included, in addition to maintenance fluids and blood loss, 4 to 6 mL/kg/hr for procedures involving minimal tissue trauma, 6 to 8 mL/kg/hr for those involving moderate trauma, and 8 to 12 mL/kg/hr for those involving extreme trauma. Yet perioperative fluid management may be linked to minor and major morbidity.
Fluid restriction appears to be less well tolerated than liberal fluid administration in patients undergoing surgery of a limited scope (e.g., knee arthroscopy).
In patients undergoing major intraabdominal surgery, restrictive fluid administration is associated with combinations of positive and negative effects.
Critically ill patients with acute lung injury may benefit from conservative fluid replacement without an increased incidence of renal failure.