respiratory alkalosis

Introduction

Decreased pCO2 due to hyperventilation. It may be primary or compensatory to metabolic acidosis.

Etiology

• hypoxemia
  • cyanotic heart disease
  • chronic heart failure
  • pulmonary parenchymal disease
  • high altitude exposure
  • severe anemia
  • hypotension
• pulmonary disease (thoracic stretch receptor stimulation)
  • asthma
  • pulmonary fibrosis
  • pneumonia
  • pulmonary embolism
  • pulmonary edema
  • interstitial lung disease
  • ARDS
  • pneumothorax, hemothorax
• enhanced respiratory drive
  • anxiety, psychosis
  • infection, gram-negative sepsis
  • liver disease (severe)
  • pregnancy
  • excessive mechanical ventilation
  • subarachnoid hemorrhage
  • brainstem tumor
  • pharmaceutical agents
    • salicylates*
    • theophylline
    • catecholamines
    • nicotine
    • progesterone, medroxyprogesterone
    • angiotensin II
• rapid correction of chronic metabolic acidosis may result in persistence of respiratory alkalosis (longer persistence of CNS acidosis)

* most important cause; also associated with increased anion gap metabolic acidosis

Clinical manifestations

• light-headedness
• syncope
• seizures
• palpitations
• cardiac arrhythmias
• symptoms resembling hypocalcemia
  • paresthesias
  • carpopedal spasm
  • tetany
  • muscle cramps

Laboratory

• arterial blood gas
  • increased pH
  • decreased pCO2
• basic metabolic panel
  • serum bicarbonate is slightly decreased
  • chronic respiratory alkalosis mildly increases the anion gap

* Rules for predicting [[[A18927|HCO3-]]] compensation for pure respiratory alkalosis

• acute: 2 meq decrease in [[[A18927|HCO3-]]] for each 10 mm Hg decrease in pCO2 (PaCO2, arterial)
• chronic: 4-5 meq decrease in [[[A18927|HCO3-]]] for each 10 mm Hg decrease in pCO2
• use 24 meq/L as starting/normal [[[A18927|HCO3-]]]
• failure of the [[[A18927|HCO3-]]] to decrease by the expected value suggests complicating metabolic alkalosis
• excessive decrease in the [[[A18927|HCO3-]]] suggests complicating metabolic acidosis
• [[[A18927|HCO3-]]] does not fall below 15 meq/L unless a concomitant metabolic acidosis is present

Management

• correct underlying disorder
• acute therapy is generally not necessary unless the pH is > 7.50
• if hypoxemia is not present
  • reassurance
  • breathing into a paper bag
• rapid correction of chronic respiratory alkalosis with compensatory metabolic acidosis will result in persistence of the metabolic acidosis
• CO2 rebreathing apparatus for persistent hyperventilation due to CNS disease

More general terms

• alkalosis