When altered, fluids and electrolytes should be corrected in the following order: (a) volume; (b) pH; (c) potassium, calcium, and magnesium; and (d) sodium and chloride. Reestablishment of tissue perfusion often equilibrates the fluid-electrolyte and acid-base balances. Because the osmolarity of normal saline (NS) matches that of serum, it is an excellent fluid for volume replacement. Hypotonic fluids such as 5% dextrose in water (D5W) should never be used to replace volume. Lactated Ringer solution is commonly used for surgical patients or trauma patients; however, only NS can be given in the same line with blood components. D5½NS, with or without potassium, is given as a maintenance fluid. The more concentrated dextrose solutions, D10W or D20W, are used for patients with compromised ability to mobilize glucose stores, such as patients with hepatic failure, or as part of total parental nutrition solutions.
Volume loss and dehydration can be inferred by the patient history. Historical features include: vomiting, diarrhea, fever, adverse working conditions, decreased fluid intake, chronic disease, altered level of consciousness, and reduced urine output. Tachycardia and hypotension are late signs of dehydration. On physical examination, one may find dry mucosa, shrunken tongue (excellent indicator), and decreased skin turgor. In infants and children, sunken fontanelles, decreased capillary refill, lack of tears, and decreased wet diapers are typical signs and symptoms of dehydration. Lethargy and coma are more ominous signs and may indicate a significant comorbid condition. Laboratory values are not reliable indicators of fluid status. Plasma and urine osmolarity are perhaps the most reliable measures of dehydration. Blood urea nitrogen (BUN), creatinine, hematocrit, and other chemistries are insensitive.
Volume overload is a purely clinical diagnosis and presents with edema (central or peripheral), respiratory distress (pulmonary edema), and jugular venous distention (in congestive heart failure). The significant risk factors for volume overload are renal, cardiovascular, and liver diseases. Blood pressure does not necessarily correlate with volume status alone; patients with volume overload can present with hypotension or hypertension.
- Adult: D5½NS at 75 to 125 mL/h + 20 mEq/L potassium chloride for an average adult (approximately 70 kilograms).
- Children: D5½NS or D10½NS, 100 mL/kilogram/d for the first 10 kilograms of body weight, 50 mL/kilogram/d for the second 10 kilograms, and 20 mL/kilogram/d for every kilograms thereafter. (See Chapter 81 for further discussion of pediatric fluid management.)
If the clinical picture and the laboratory data conflict, repeat the lab test prior to initiating therapy. Correcting a single abnormality may not be the only intervention needed because most electrolytes exist in equilibrium with others. Abnormalities should be corrected at the same rate they develop; however, slower correction is usually safe unless the condition warrants rapid or early intervention (eg, hypoglycemia or hyperkalemia). Evaluation of electrolyte disorders frequently requires a comparison of the measured and calculated osmolarities (number of particles per liter of solution). To calculate osmolarity, measured serum values in mEq/L are used: