Assessment 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 underlying disorder; it may be more severe if both metabolic and respiratory alkalosis are present.

Pathophysiology

The main mechanisms involved can be either one or a combination of 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 bicarbonate 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 or addition of HCO3-generating substances

  Intake of bicarbonate or substances (citrate, acetate, or lactate) that increase bicarbonate 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 bicarbonate.

• Severe circulating volume contraction.

  This leads to loss of extracellular fluid and relative increase in bicarbonate 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):369-375. 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