Readily quantifiable

The volume of fluid required to correct TBW depletion equals the basal fluid requirement plus ongoing exceptional losses plus the fluid deficit. Basal daily fluid requirements are calculated using the formulas in Table 27-2. For an adult, this represents 1,500 mL/day for the first 20 kg of body weight plus 20 mL/day for each additional kilogram. The volume of replacement fluids required for a given patient (the fluid deficit) can be estimated by the acute weight change in the patient (1 kg = 1 L of fluid). Because the precise weight change is not typically known, it is often calculated as follows: fluid deficit = normal TBW - present TBW. Normal TBW is estimated based on the patient's height using the formulas in Table 27-2, and the present TBW is estimated based on the patient's current body weight. The choice of fluids used for replacement is guided by the presence of concurrent electrolyte abnormalities. The adequacy of replacement is guided by each patient's objective response to fluid replacement (improved skin turgor, adequate urine output, normalization of heart rate, BP, etc.).

Once TBW has been restored, the volume of "maintenance" fluid equals the basal fluid requirement plus ongoing exceptional losses. If the pathophysiologic process leading to TBW depletion has not been identified and corrected (or accounted for in the calculation of maintenance fluid requirements), TBW depletion will quickly recur. To review the concepts involved in the calculation of replacement fluids for a representative patient see Patient Encounter 2.

Compared to TBW depletion, ECF depletion tends to occur acutely. In this setting, rapid and aggressive fluid replacement is required to maintain adequate organ perfusion. Because ECF depletion is generally due to the loss of isotonic fluid (proportional losses of sodium and water), major disturbances of plasma osmolality are not common. ECF depletion manifests clinically as signs and symptoms associated with decreased tissue perfusion: dizziness, orthostasis, tachycardia, decreased urine output, increased hematocrit, decreased central venous pressure, and/or hypovolemic shock. Common causes of ECF depletion include external fluid losses (burns, hemorrhage, diuresis, GI losses, and adrenal insufficiency) and third spacing of fluids (septic shock, anaphylactic shock, or abdominal ascites).

Table 27-2 Useful Calculations for the Estimation of Patient Maintenance Fluid Requirements

Child (10-20 kg) = 1,000 mL + 50 mL for each kilogram greater than 10

Adult (greater than 20 kg) = 1,500 mL + 20 mL for each kilogram greater than 20

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