Respiratory acidosis

Respiratory acidosis is acidosis (abnormal acidity of the blood) due to decreased ventilation of the pulmonary alveoli, leading to elevated arterial carbon dioxide concentration. more...

Respiratory acidosis is a clinical disturbance that is due to alveolar hypoventilation. Production of carbon dioxide occurs rapidly, and failure of ventilation promptly increases the level of PaCO₂. Alveolar hypoventilation leads to an increased PaCO₂ (ie, hypercapnia). The increase in PaCO₂ in turn decreases the HCO₃^-/PaCO₂ and decreases pH. Hypercapnia and respiratory acidosis occur when impairment in ventilation occurs and the removal of CO₂ by the lungs is less than the production of CO₂ in the tissues.

Types of respiratory acidosis

Respiratory acidosis can be acute or chronic. In acute respiratory acidosis, the PaCO₂ is elevated above the upper limit of the reference range (over 6.3 kPa) with an accompanying acidemia (pH <7.35). In chronic respiratory acidosis, the PaCO₂ is elevated above the upper limit of the reference range, with a normal or near-normal pH secondary to renal compensation and an elevated serum bicarbonate (HCO₃^- >30 mm Hg).

Acute respiratory acidosis occurs when an abrupt failure of ventilation occurs. This failure in ventilation may be caused by depression of the central respiratory center by cerebral disease or drugs, inability to ventilate adequately due to neuromuscular disease (eg, myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barré syndrome, muscular dystrophy), or airway obstruction related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.

Causes

Chronic respiratory acidosis may be secondary to many disorders, including COPD. Hypoventilation in COPD involves multiple mechanisms, including decreased responsiveness to hypoxia and hypercapnia, increased ventilation-perfusion mismatch leading to increased dead space ventilation, and decreased diaphragm function secondary to fatigue and hyperinflation.

Chronic respiratory acidosis also may be secondary to obesity hypoventilation syndrome (ie, Pickwickian syndrome), neuromuscular disorders such as amyotrophic lateral sclerosis, and severe restrictive ventilatory defects as observed in interstitial fibrosis and thoracic deformities.

Lung diseases that primarily cause abnormality in alveolar gas exchange usually do not cause hypoventilation but tend to cause stimulation of ventilation and hypocapnia secondary to hypoxia. Hypercapnia only occurs if severe disease or respiratory muscle fatigue occurs.

Physiological response

Metabolism rapidly generates a large quantity of volatile acid (CO₂) and nonvolatile acid. The metabolism of fats and carbohydrates leads to the formation of a large amount of CO₂. The CO₂ combines with H₂O to form carbonic acid (H₂CO₃). The lungs excrete the volatile fraction through ventilation, and acid accumulation does not occur. A significant alteration in ventilation that affects elimination of CO₂ can cause a respiratory acid-base disorder. The PaCO₂ is maintained within a range of 39-41 mm Hg in normal states.

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