LOSSES

External losses (e.g., blood, ascites) should be replaced to maintain normal blood volume and normal composition of the ECF volume. Blood loss is replaced initially with 3 mL of balanced salt solution or 0.9% NaCl for each milliliter of blood loss. For each milliliter of blood lost, 1 mL of colloid solution should be administered to provide improvement of filling pressures, arterial blood pressure, and heart rate. PRBC infusions are used at roughly 1 mL for each 2 mL of blood lost plus crystalloid or colloid, as previously described. Although the effect on volume is 1:1, the hematocrit of PRBCs (60% to 70%) is about twice the necessary hematocrit of the patient. In patients with reasonable cardiac reserve and without compromised regional circulations (e.g., coronary, cerebral, renal, intestinal), hemoglobin levels of 7.5 g/dL or greater are usually well tolerated.^[126] If blood volume is normal and cardiac failure is not a problem, signs of sympathetic activation, mixed venous oxygen desaturation, or electrocardiogram signs of myocardial ischemia suggest the need for RBC administration. The following equation is used to calculate the necessary volume of RBCs to be infused, and weight is measured in kilograms. Because PRBCs have a hematocrit of 60%, the volume to be infused is found by dividing the volume of RBCs desired by the hematocrit (Hct) of the unit of PRBCs.

PRBC_infused = (Hct_desired × 55 × Weight − Hct_observed × 55 × Weight)/0.60

Ascites and pleural effusions drained during surgery reform at extremely variable rates. The electrolyte composition is the same as that at the ECF volume, but it also contains protein at concentrations 30% to 100% of the plasma value. Balanced salt solutions are most appropriate for replacement, but colloid should be added when dilution of the patient's colloidal osmotic pressure (COP) becomes severe (<15 to 17 mm Hg) and the apparent volume of redistribution of crystalloid begins to increase.

The electrolyte composition of gastrointestinal tract losses is site dependent (see Table 46-22 ). Most gastrointestinal losses removed at the time of surgery entered the bowel lumen preoperatively and should be considered to be part of the deficit. Evaporation from exposed viscera is entirely water, but the electrolyte is left behind, leading to a need for free water. The amount evaporated is directly proportional to temperature and exposed surface area and inversely proportional to relative humidity. Excess urine due to diuretics, glycosuria, or diabetes insipidus should be replaced with a solution based on urinary electrolyte measurements. In general, the sodium concentration ranges between 50 and 100 mEq/L, and the potassium concentration ranges between 20 and 60 mEq/L.

Redistribution

Redistribution (so-called third-space losses) primarily results from tissue edema and transcellular fluid displacement. Functionally, this fluid is not available to the vascular space.^{[127] [128]} The volume of edema formed is governed by the principles discussed earlier. Colloid enters the injured

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Table 46-23 describes the fluid management, starting with a hemoglobin level of 15 g/dL, for a 70-kg patient undergoing gastrectomy who has been fasting for 10 hours. The maintenance rate is 110 mL/hour, producing a deficit of 1100 mL.

During the first and second hours of intra-abdominal activity, 100 mL of blood was lost and replaced at 3:1 rate with a balanced salt solution. By the fourth hour, the deficit had been replaced, and because the abdomen was being closed during part of that hour, the estimate of third-space losses was likewise reduced, and no further blood loss occurred. The assumption is made that urine flow was 50 to 80 mL/hour and that heart rate and arterial blood pressure were in an acceptable range and the central venous pressure (CVP) remained 6 to 9 mm Hg. Had the CVP or urine output begun to increase, the rate of fluid administration would have been slowed. If oliguria and tachycardia had occurred, a fluid bolus would have been administered.