Patients who have ﬂuid or electrolyte abnormalities preoperatively can pose a diagnostic challenge. The correct diagnosis and therapy is contingent on a correct assessment of total body ﬂuid and electrolyte status. The management of hyponatremia, for example, may be either ﬂuid restriction or ﬂuid replacement. The choice of treatment depends on whether there is overall extracellular ﬂuid excess and normal body sodium stores or decreased overall total body sodium stores and extracellular ﬂuid. A detailed history is necessary to disclose any underlying medical illness and to assess the amount and duration of any abnormal ﬂuid losses or intake. Initial evaluation should include an assessment of hemodynamic, clinical, and urinary parameters to determine the overall level of hydration as well as the ﬂuid status of the extracellular ﬂuid compartment. The patient who has good skin turgor, moist mucosa, stable vital signs, and good urinary output is well hydrated. Nonpitting edema is indicative of extracellular ﬂuid excess, whereas patients with orthostasis, sunken eyes, parched mouth, and decreased skin turgor have extracellular volume contraction. A patient’s overall extracellular ﬂuid status does not always reﬂect the hydration status of the intravascular compartment. A patient can have increased interstitial ﬂuid and yet be intravascularly dry, requiring replacement with isotonic ﬂuid.
The laboratory workup for patients who may have preexisting ﬂuid problems should include assessment of blood hematocrit, serum chemistry, glucose, blood urea nitrogen (BUN) and creatinine, urine osmolarity, and urine electrolyte levels. Serum osmolarity is mainly a function of the concentration of sodium and is given by the following equation: 2[Na+] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8
Normal serum osmolarity is typically 290 to 300 mOsm. Blood hematocrit will rise or fall inversely at a rate of 1% per 500-mL alteration of extracellular ﬂuid volume. The BUN:creatinine ratio is typically 10:1 but will rise to a ratio of greater than 20:1 under conditions of extracellular ﬂuid contraction. Under conditions of extracellular ﬂuid deficit, urine osmolarity will typically be high (>400 mOsm), whereas urine sodium concentration is low (<15 mEq/L), indicative of an attempt by the kidney to conserve sodium. Under conditions of extracellular ﬂuid excess or in cases of renal disease in which the kidney has impaired ability to retain sodium and water, urine osmolarity will be low and urine sodium will be high (>30 mEq/L). Changes in sodium can give insight into the degree of extracellular ﬂuid excess or deficit. In the average person, the serum sodium rises by 3 mmol/L for every liter of water deficit and falls by 3 mmol/L for each liter of water excess. One must be careful in making these estimates because patients with prolonged water and electrolyte loss can have low serum sodium levels and marked water deficits.
Because sodium is the major extracellular cation, shifts in serum sodium levels are usually inversely correlated with the hydration state of the extracellular ﬂuid compartment. The pathophysiology of hyponatremia is usually expansion of body ﬂuids leading to excess total body water (27,33). Symptomatic hyponatremia usually does not occur until the serum sodium is below 120 to 125 mEq/L. The severity of the symptoms (nausea, vomiting, lethargy, seizures) is related more to the rate of change of serum sodium than to the actual serum sodium level.