Acidosis, Respiratory

KEY FEATURES

• Low arterial pH, increased PCO₂

• Results from hypoventilation and subsequent hypercapnia

• Both pulmonary and extrapulmonary disorders can cause hypoventilation

• 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

  • Renal excretion of acid as NH₄Cl results in a compensatory metabolic alkalosis; complete renal metabolic compensation takes several days

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

CLINICAL FINDINGS

• Acute onset 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

DIAGNOSIS

• 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

TREATMENT

• 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