Contraction alkalosis

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

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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 causes increased Na⁺-K⁺ exchange and increased HCO₃^- (bicarbonate) reabsorption in the proximal tubule. Increased aldosterone secretion causes increased distal tubule H⁺ secretion, worsening the metabolic alkalosis. Additionally, increased aldosterone secretion causes increased distal tubule K⁺ secretion, causing the hypokalemia seen with contraction alkalosis.

Treatment

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

References