Approach to Fluid and Electrolyte Disorders and Acid-Base Problems
Section snippets
Is the hyponatremia representative of a hypoosmolar state?
There are two general causes of hyponatremia in which it is not associated with a hypoosmolar state. The first of these is pseudohyponatremia, which involves an abnormal measurement of the serum sodium (Na). This occurs in patients with hyperglobulinemia or hypertriglyceridemia, in whom plasma water relative to plasma solids is decreased in blood, leading to less sodium in a given volume of blood. In general, hyperglobulinemia sufficient to cause pseudohyponatremia is rare and occurs only in
Step 2: Examine the serum chloride concentration
One should always examine the serum chloride concentration with respect to the serum sodium concentration. As the serum sodium concentration either increases or decreases because of disorders in tonicity, the serum chloride concentration will change in parallel and to the same extent. Whenever the serum chloride concentration moves in a direction opposite or changes disproportionately to the change in serum sodium concentration, an acid-base disorder is suggested.
Step 3: Calculate the anion gap
Calculation of the anion gap should be a routine part of the examination of every set of electrolytes, no matter how normal the individual values may appear. The anion gap is equal to the difference between the plasma concentrations of the major cation (Na+) and the major measured anions (Cl− + HCO3−).
The normal value of the anion gap is approximately 12 plus or minus 2. Most of the unmeasured anions consists of albumin, and therefore the normal anion gap changes in
Step 4: Is the measured HCO3− concentration equal to the predicted HCO3−?
In the setting of an increased anion gap, one needs to determine whether the measured serum HCO3− is equal to the predicted serum HCO3−. In general, the serum HCO3− concentration will fall by an amount equal to the increase in the anion gap. Sometimes an increased anion gap may be the only clue to a metabolic acidosis. For instance, consider a patient with a measured plasma HCO3− concentration of 22 mEq/L in the setting of an anion gap of 22. In this instance the anion gap has increased by 10,
Step 5: Interpret the arterial blood gas
Before one can approach a patient with an acid-base disorder, it is important to distinguish the suffixes “-emia” and “-osis.” The suffix, “-emia” refers to the concentration in the blood. Thus, acidemia means excess acid in the blood (low blood pH), and alkalemia means excess alkali in the blood (high blood pH). The suffix “-osis” refers to a process. Thus, acidosis refers to a process that adds acid to the blood, while alkalosis refers to a process that adds alkali to the blood. A patient
Is the hypokalemia due to a cell shift?
In the absence of physical and historical evidence of gastrointestinal or renal potassium (K) losses, either a redistribution of K at the cellular level or laboratory error will account for a low serum K. The regulation of K distribution between the intracellular and extracellular space is referred to as internal K balance. While the kidney is ultimately responsible for maintenance of total body K, factors that modulate internal balance are important in the disposal of acute K loads. Cell