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For further information, see CMDT Part 21-: Respiratory Acidosis (Hypercapnia)

Key Features

  • Low arterial pH (< 7.35), increased PCO2 (> 48 mm Hg)

  • Respiratory acidosis results from decreased alveolar ventilation and subsequent hypercapnia

  • Acute respiratory acidosis

    • Associated with only a modest increase in bicarbonate since serum bicarbonate is an ineffective buffer because of impaired elimination of carbon dioxide

    • HCO3 increases by 1 mEq/L for every 10 mm Hg increase in PCO2

  • Chronic respiratory acidosis

    • Generally seen in patients with underlying lung disease, such as chronic obstructive pulmonary disease

    • HCO3- increases by 3 mEq/L for every 10 mm Hg increase in PCO2

Clinical Findings

  • Acute respiratory acidosis: somnolence, confusion, mental status changes, myoclonus, asterixis

  • Severe hypercapnia

    • Increases cerebral blood flow, cerebrospinal fluid pressure, and intracranial pressure

    • Papilledema and seizures may be seen


  • Arterial pH is low and PCO2 is increased

  • Serum HCO3 is elevated but does not fully correct the pH

  • Respiratory etiologies of respiratory acidosis usually have a wide A-a difference; a relatively normal A-a difference in the presence of respiratory acidosis is highly suggestive of global hypoventilation


  • Consider a diagnostic and therapeutic trial of intravenous naloxone

    • If opioid overdose is a possible diagnosis or

    • There is no other obvious cause for hypoventilation

  • Noninvasive or mechanical ventilation may be necessary

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