Correct maintenance of ﬂuid and electrolyte balance in the postoperative period starts with the preoperative assessment, with emphasis on establishing normal ﬂuid and electrolyte parameters before surgery. Postoperatively, close monitoring of daily weight, urine output, serum hematocrit, serum electrolytes, and hemodynamic parameters will yield the necessary information to make correct adjustments in crystalloid replacement. The normal daily ﬂuid and electrolyte requirements must be met and any unusual ﬂuid and electrolyte losses, including those from the gastrointestinal tract, lungs, or skin, must be compensated. After the first few postoperative days, third-space ﬂuid begins to return to the intravascular space, and ADH and aldosterone levels revert to normal. The excess ﬂuid retained perioperatively is mobilized and excreted through the kidneys, and exogenous ﬂuid requirements decrease. Patients with inadequate cardiovascular or renal reserve are prone to ﬂuid overload during this time of third-space reabsorption, especially if intravenous ﬂuids are not appropriately reduced.
The most common ﬂuid and electrolyte disorder in the postoperative period is ﬂuid overload. Fluid excess can occur concomitantly with normal or decreased serum sodium. Large amounts of isotonic ﬂuids are usually infused intraoperatively and postoperatively to maintain blood pressure and urine output. Because the infused ﬂuid is often isotonic with plasma, it will remain in the extracellular space. Under such conditions, serum sodium will remain within normal levels. Fluid excess with hypotonicity (decreased serum sodium) can occur if large amounts of isotonic ﬂuid losses (e.g., blood and gastrointestinal tract) are inappropriately replaced with hypotonic ﬂuids. The predisposition toward retention of free water in the immediate postoperative period compounds the problem. An increase in body weight occurs concomitantly with the ﬂuid expansion. In the patient who is not allowed anything by mouth, catabolism should induce a daily weight loss as great as 300 g per day. The patient who is gaining weight in excess of 150 g per day is in a state of ﬂuid expansion. Simple ﬂuid restriction will correct the abnormality. When necessary, diuretics can be used to increase urinary excretion.
States of ﬂuid dehydration are uncommon but will occur in patients who have large daily losses of ﬂuid that are not replaced. Gastrointestinal losses should be replaced with the appropriate ﬂuids. Patients with high fevers should be given appropriate free water replacement, because up to 2 L per day of free water can be lost through perspiration and hyperventilation. Although these increased losses are difficult to monitor, a reliable estimate can be obtained by monitoring body weight.
A variety of metabolic, respiratory, and electrolyte abnormalities in the postoperative period can result in an imbalance in normal acid-base homeostasis, leading to alkalosis or acidosis. Changes in the respiratory rate directly affect the amount of carbon dioxide that is exhaled. Respiratory acidosis will result from carbon dioxide retention in patients who have hypoventilation from central nervous system depression. This condition can result from oversedation with narcotics, particularly in the presence of concurrent severe chronic obstructive pulmonary disease. Respiratory alkalosis can result from hyperventilation caused by excitation of the central nervous system by drugs, pain, or excess ventilator support. Numerous metabolic derangements can result in metabolic alkalosis or acidosis. Proper ﬂuid and electrolyte replacement as well as maintenance of adequate tissue perfusion will help prevent most acid-base disorders that occur during the postoperative period.