Contraction alkalosis

Contraction alkalosis refers to the increase in blood pH that occurs as a result of fluid losses (volume contraction). The change in pH is especially pronounced with acidic fluid losses caused by problems like vomiting.

Pathophysiology

Extracellular fluid (ECF) volume contraction is associated with decreased blood volume and decreased renal perfusion pressure. Three compensation mechanisms engage as a result:

1. renin secretion is increased,
2. production of angiotensin II is increased, and
3. secretion of aldosterone is increased.

Increases in angiotensin II cause increased Na⁺-H⁺ exchange in the proximal tubule and increased HCO₃^- (bicarbonate) reabsorption in the proximal tubule due to increased luminal H⁺. Increased aldosterone secretion stimulates the H-ATPase of alpha-intercalated cells of the collecting duct, which causes 1) increased distal tubule H⁺ secretion, worsening the metabolic alkalosis, and 2) increased generation of "new" bicarbonate within these same cells, which will be reabsorbed.

Additionally, increased aldosterone secretion causes increased distal tubule K⁺ secretion, in turn causing the hypokalemia seen with contraction alkalosis. Alkalosis also induces H⁺-efflux from cells through the K⁺-H⁺ exchanger, leading to hypokalemia.

Treatment

Treatment consists of NaCl infusion to correct ECF volume contraction and administration of K⁺ to replace urinary losses.

References

http://www.nature.com/ki/journal/v54/n6/full/4490448a.html