21-22: Respiratory Alkalosis

GENERAL CONSIDERATIONS

Respiratory alkalosis occurs when hyperventilation reduces the PCO₂, increasing serum pH. The most common cause of respiratory alkalosis is hyperventilation syndrome (Table 21–16), but bacterial septicemia and cirrhosis are other common causes. In pregnancy, progesterone stimulates the respiratory center, producing an average PCO₂ of 30 mm Hg and respiratory alkalosis. Symptoms of acute respiratory alkalosis are related to decreased cerebral blood flow induced by the disorder.

Determination of appropriate metabolic compensation may reveal an associated metabolic disorder (see Mixed Acid-Base Disorders).

As in respiratory acidosis, the metabolic compensation is greater if the respiratory alkalosis is chronic (see Table 21–11). Although serum HCO₃^– is frequently less than 15 mEq/L in metabolic acidosis, such a low level in respiratory alkalosis is unusual and may represent a concomitant primary metabolic acidosis.

CLINICAL FINDINGS

A. Symptoms and Signs

In acute cases (hyperventilation), there is light-headedness, anxiety, perioral numbness, and paresthesias. Tetany occurs from a low ionized calcium, since severe alkalosis increases calcium binding to albumin.

B. Laboratory Findings

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, the traditional treatment of 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^–. The severity of hypocapnia in critically ill patients has been associated with adverse outcomes.