What is metabolic alkalosis?

Updated: Dec 10, 2020
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Metabolic alkalosis is defined as elevation of the body's pH above 7.45. [1] Metabolic alkalosis involves a primary increase in serum bicarbonate (HCO3-) concentration, due to a loss of H+ from the body or a gain in HCO3-. As a compensatory mechanism, metabolic alkalosis leads to alveolar hypoventilation with a rise in arterial carbon dioxide tension (PaCO2), which diminishes the change in pH that would otherwise occur.

Normally, arterial PaCO2 increases by 0.5-0.7 mm Hg for every 1 mEq/L increase in plasma bicarbonate concentration, a compensatory response that is very quick. If the change in PaCO2 is not within this range, then a mixed acid-base disturbance occurs. For example, if the increase in PaCO2 is more than 0.7 times the increase in bicarbonate, then metabolic alkalosis coexists with primary respiratory acidosis. Likewise, if the increase in PaCO2 is less than the expected change, then a primary respiratory alkalosis is also present.

The first clue to metabolic alkalosis is often an elevated bicarbonate concentration that is observed when serum electrolyte measurements are obtained. Remember that an elevated serum bicarbonate concentration may also be observed as a compensatory response to primary respiratory acidosis. However, a bicarbonate concentration greater than 35 mEq/L is almost always caused by metabolic alkalosis.

Metabolic alkalosis is diagnosed by measuring serum electrolytes and arterial blood gases. If the etiology of metabolic alkalosis is not clear from the clinical history and physical examination, including drug use and the presence of hypertension, then a urine chloride ion concentration can be obtained. Calculation of the serum anion gap may also help to differentiate between primary metabolic alkalosis and metabolic compensation for respiratory acidosis. (See Workup.)

The management of metabolic alkalosis depends primarily on the underlying etiology and on the patient’s volume status. Direct treatment of the alkalosis itself (eg, administration of acidic intravenous solutions) may be indicated in some cases (see Treatment).

An algorithmic approach to metabolic alkalosis is depicted in the image below.

Algorithm for metabolic alkalosis. Algorithm for metabolic alkalosis.

For a discussion of metabolic alkalosis in children, see Pediatric Metabolic Alkalosis. For a general review of acid-base regulation, see Metabolic Acidosis.


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