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The pathophysiology of all electrolyte disorders is rooted in basic principles of total body water and its distribution across fluid compartments. The optimal evaluation and treatment of fluid and electrolyte disorders requires a careful interpretation of serum and urine chemistries in conjunction with a thorough history and physical examination. While classic teaching has focused on physical examination to determine a patient’s volume status, such an approach can be challenging because of limitations in accurate bedside analysis of volume.

A. Body Water and Fluid Distribution

Total body water depends on the relative proportions of muscle and fat in the body. Total body water is typically estimated as 50% of body weight in women and 60% in men, as women, on average, have a higher proportion of fat to body weight (Table 21–1). Total body water also tends to decrease with age due to declining muscle mass. Approximately two-thirds of total body water is located in the intracellular compartment and one-third is located in the extracellular compartment. The extracellular compartment is further divided into the interstitial fluid volume (15% of body weight) and the plasma fluid volume (5% of body weight). Changes in total body water content are best evaluated by documenting changes in body weight. Extracellular volume (ECV) may be assessed by physical examination (eg, blood pressure, pulse, jugular venous distention, edema). Quantitative assessments of ECV and intravascular volume may be invasive (ie, central venous pressure or pulmonary wedge pressure) or noninvasive (ie, inferior vena cava diameter and right atrial pressure by echocardiography). Intracellular volume (ICV) is assessed using the serum sodium concentration.

Table 21–1.Total body water (as percentage of body weight) in relation to age and sex.

B. Serum Electrolytes

In health, serum electrolytes are maintained within a narrow range by the kidneys (homeostasis). The serum level of an individual electrolyte may be normal, elevated, or decreased, but may not correlate with the total body levels of that electrolyte due to shift of water or electrolytes into and out of cells.

C. Evaluation of Urine

The urine concentration of an electrolyte is helpful to determine whether the kidney is excreting or retaining the electrolyte in response to high or low serum levels. A 24-hour urine collection for daily electrolyte excretion remains the gold standard for assessment of renal electrolyte handling; however, it can be cumbersome, as well as technically challenging in certain patients. A more convenient method to determine renal electrolyte handling is the use of fractional excretion (FE) of an electrolyte X (FEx) calculated from a spot urine sample and serum sample, using creatinine ...

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