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For further information, see CMDT Part 21-22: Respiratory Alkalosis

Key Features

  • Occurs when hyperventilation reduces the PCO2, increasing serum pH

  • The most common cause is hyperventilation syndrome (Table 21–16)

  • Bacterial septicemia and cirrhosis are other common causes

  • Pregnancy is another cause of chronic respiratory alkalosis; progesterone stimulates the respiratory center, producing an average PCO2 of 30 mm Hg

Table 21–16.Causes of respiratory alkalosis.

Clinical Findings

  • Acute respiratory alkalosis

    • Light-headedness

    • Anxiety

    • Paresthesias

    • Perioral numbness

    • Tetany in severe alkalosis from low ionized calcium


  • Elevated arterial blood pH, low PCO2

  • Serum HCO3 is decreased in chronic respiratory alkalosis

  • Although serum HCO3 is frequently < 15 mEq/L in metabolic acidosis, such a low level in respiratory alkalosis is unusual and may represent a concomitant primary metabolic acidosis


  • Treatment is directed toward the underlying cause

  • Rapid correction of chronic respiratory alkalosis may result in metabolic acidosis as PCO2 is increased in the setting of previous compensatory decrease in HCO3

  • In acute hyperventilation syndrome from anxiety,

    • Breathing into a paper bag should be discouraged because it does not correct PCO2 and may decrease PO2

    • Sedation may be necessary if the process persists

  • The severity of hypocapnia in critically ill patients has been associated with adverse outcomes

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