Lactated Ringer's solution isn't always the safest choice for fluid therapy. In fact, administering the right fluid, whether a crystalloid, a colloid, or both, is essential to ensuring a correct fluid balance without causing adverse effects that can worsen a patient's condition. This article will help you assess each patient's status and choose the best fluid therapy option.

NORMAL BODY FLUID RETENTION

Total body water normally constitutes about 60% of most mammals' body weight, although this percentage can vary slightly with age, gender, and body condition. The two main fluid compartments of the body are intracellular and extracellular. About 67% of total body water is intracellular.^1-3 The remaining 33% is extracellular, in the intravascular and interstitial extravascular spaces. In addition, a small amount of extracellular fluid, known as transcellular fluid, is located in specialized compartments (i.e gastrointestinal tract fluids, synovial fluid, and cerebrospinal fluid).

The correct balance of fluids and electrolytes is necessary for normal body functioning and cellular processes. In addition to normal fluid intake from eating and drinking, water is also produced during the oxidation of food materials. Fluid is lost with panting, vomiting, diarrhea, and urination. Sensible fluid losses can be measured and constitute about two-thirds of the body's daily maintenance fluid requirements. Sensible fluids include urine, feces, and, abnormally, vomitus. Insensible fluid loss, such as evaporation from the respiratory tract, is estimated. Insensible fluid losses can be excessive because of evaporation from open body cavities during prolonged surgeries and in animals with hyperthermia, hemorrhage, or severe panting or salivation.^1,2

In healthy animals, fluid intake and excretion are kept in balance by sodium and chloride ion activity as well as serum osmolality.^2,3 Osmoreceptors in the hypothalamus sense the sodium and chloride concentrations in the vascular space. As the serum sodium concentration rises because of increased sodium intake or an increase in free water loss, serum osmolality rises. An increase in serum osmolality stimulates the release of arginine vasopressin, or antidiuretic hormone, from the hypothalamus. Antidiuretic hormone stimulates the opening of water channels in the collecting ducts of the renal tubules, which stimulates water reabsorption. Once water is retained in the vascular space, sodium, urea, and glucose—the major contributors to serum osmolality—are diluted, and serum osmolality decreases. Hypothalamic secretion of antidiuretic hormone ceases once serum osmolality returns to normal.

CALCULATING DAILY FLUID REQUIREMENTS

Daily fluid requirements are based on the metabolic water requirements of a patient in a state of equilibrium. During a state of equilibrium, a patient's daily water intake equals water loss, creating no net loss or gain of fluid. For each kilocalorie of energy metabolized, 1 ml water is consumed. Metabolic energy requirements are calculated based on the following linear formula¹ :

kcal/day = (30 × body weight_kg) + 70

By substituting 1 ml water for 1 kcal, the following formula can be used to estimate a patient's daily metabolic water requirements:

ml/day = (30 × body weight_kg) + 70

Recent studies indicate that metabolic energy requirements rarely increase during states of critical illness except in cases of sepsis.^4,5 Because our patients frequently pant and may have excessive evaporative losses or sensible fluid losses from vomiting, diarrhea, wound exudates, or body cavity effusions, daily fluid requirements may be greater than that calculated above. Thus, the above formula should be used as a guideline, and careful assessment and measurement of ongoing losses should be added to a patient's daily fluid therapy as needed to prevent further dehydration.