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.
Table 21–16.Causes of respiratory alkalosis. |Favorite Table|Download (.pdf) Table 21–16. Causes of respiratory alkalosis.
Decreased inspired oxygen tension
Cerebrovascular accident (infarction, hemorrhage)
Pharmacologic and hormonal stimulation
Recovery from metabolic acidosis
Interstitial lung disease
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.
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.
Arterial blood pH is elevated, and PCO2 is low. Serum bicarbonate is decreased in chronic respiratory alkalosis.
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.
et al. Metabolic acidosis or respiratory alkalosis? Evaluation of a low plasma bicarbonate using the urine anion gap. Am J Kidney Dis. 2017 Sep;70(3):440–4.