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

Key Features

  • Elevated arterial blood pH (> 7.45), low PCO2 (< 32 mm Hg)

  • Occurs when hyperventilation reduces the PCO2, increasing serum pH

  • Causes (Table 21–16)

    • Hypoxia

      • Decreased inspired oxygen tension

      • Ventilation-perfusion inequality

      • Hypotension

      • Severe anemia

    • CNS-mediated disorders

      • Voluntary hyperventilation

      • Anxiety-hyperventilation syndrome

      • Cerebrovascular accident (infarction, hemorrhage)

      • Infection

      • Trauma

      • Tumor

    • Pulmonary disease

      • Interstitial lung disease

      • Pneumonia

      • Pulmonary embolism

      • Pulmonary edema

    • Mechanical overventilation

  • In pregnancy progesterone stimulates the respiratory center, producing an average PCO2 of 30 mm Hg

Table 21–16.Causes of respiratory alkalosis.

Clinical Findings

  • Light-headedness

  • Anxiety

  • Paresthesias

  • Perioral numbness

  • Tetany in severe alkalosis from low ionized calcium


  • Arterial blood pH is elevated, and PCO2 is low

  • Serum bicarbonate is decreased in chronic respiratory alkalosis


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

  • Treatment is directed toward the underlying cause

  • In acute hyperventilation syndrome from anxiety, rebreathing into a paper bag increases PCO2. Sedation may be necessary if the process persists

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