Evaluation of metabolic alkalosis

Definition

Metabolic alkalosis is indicated by an increase in plasma bicarbonate (HCO₃) level. Arterial pH >7.45 defines alkalosis.

It is the consequence of disorders that cause either a loss of hydrogen ions from the body or an increase in plasma HCO₃. The severity of alkalosis depends on the severity of the underlying disorder; it may be more severe if both metabolic and respiratory alkalosis are present.

Pathophysiology

The main mechanisms involved can be one, or a combination, of the following:

• Loss of hydrogen ions from the body. Hydrogen ions can be lost from the body either through the gastrointestinal tract or through the kidneys. In the body, hydrogen ions may shift from the extracellular fluid into the cells. If the loss of hydrogen ions exceeds production by the diet and metabolism, the serum HCO₃ level increases, leading to metabolic alkalosis.​ Loss of hydrogen ions through the stomach and kidneys is accompanied by the production of HCO₃.

• Administration of HCO₃, addition of HCO₃-generating substances, or intake of HCO₃ or substances (citrate, acetate, or lactate) that increase HCO₃ production in excess of hydrogen ion production will lead to metabolic alkalosis. This is usually compensated by the kidneys with normal function by renal excretion of HCO₃.​

• Severe circulating volume contraction. This leads to loss of extracellular fluid and relative increase in HCO₃ concentration.

Metabolic alkalosis generally requires an initiation factor that starts the process and a maintenance factor that continues the imbalance by preventing renal excretion of excess HCO₃. Sometimes, the same factor may be responsible for both initiation and maintenance.[1]Galla JH. Metabolic alkalosis. J Am Soc Nephrol. 2000 Feb;11(2):369-75. http://jasn.asnjournals.org/content/11/2/369.full http://www.ncbi.nlm.nih.gov/pubmed/10665945?tool=bestpractice.com [2]Garella S, Chang BS, Kahn SI. Dilution acidosis and contraction alkalosis: review of concept. Kidney Int. 1975 Nov;8(5):279-85. http://www.ncbi.nlm.nih.gov/pubmed/536?tool=bestpractice.com