Alkalosis, Respiratory

KEY FEATURES

• Elevated arterial blood pH, low PCO₂

• Always a disorder of hyperventilation, which reduces the PCO₂ and increases the serum pH

• Causes (Table 23–16)

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

• Salicylates

  • Directly stimulate respiration

  • Suspect aspirin toxicity when both respiratory alkalosis and an anion gap metabolic acidosis are present, particularly with alkalemia

• Symptoms are related to decreased cerebral blood flow

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

• Determination of appropriate metabolic compensation may reveal an associated metabolic disorder

  • Metabolic compensation is greater if the respiratory alkalosis is chronic

  • In acute respiratory alkalosis, HCO₃– decreases by 2 mEq/L for every 10 mm Hg decrease in PCO₂

  • In chronic respiratory alkalosis, HCO₃– decreases by 4 mEq/L for every 10 mm Hg decrease in PCO₂

CLINICAL FINDINGS

• Light-headedness

• Anxiety

• Paresthesias

• Perioral numbness

• Tetany in severe alkalosis from low ionized calcium

DIAGNOSIS

• Arterial blood pH is elevated, and PCO₂ is low

• Serum bicarbonate is decreased in chronic respiratory alkalosis

TREATMENT

• Treatment is directed toward the underlying cause

• In acute hyperventilation syndrome from anxiety, breathing into a paper bag should be discouraged because it does not correct PCO₂ and may decrease PO₂

• Reassurance may be sufficient for the anxious patient, but sedation may be necessary if the process persists

• Hyperventilation is usually self-limited since muscle weakness caused by the respiratory alkalemia will suppress ventilation

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