(See also Acid-Base Regulation and Acid-Base Disorders.)
Respiratory acidosis is carbon dioxide (CO2) accumulation (hypercapnia) due to a decrease in respiratory rate and/or respiratory volume (hypoventilation). Causes of hypoventilation (discussed under Ventilatory Failure) include
Hypoxia typically accompanies hypoventilation.
Respiratory acidosis may be
Distinction is based on the degree of metabolic compensation; carbon dioxide is initially buffered inefficiently, but over 3 to 5 days the kidneys increase bicarbonate reabsorption significantly.
Symptoms and Signs
Symptoms and signs depend on the rate and degree of Pco2 increase. CO2 rapidly diffuses across the blood-brain barrier. Symptoms and signs are a result of high CO2 concentrations and low pH in the CNS and any accompanying hypoxemia.
Acute (or acutely worsening chronic) respiratory acidosis causes headache, confusion, anxiety, drowsiness, and stupor (CO2 narcosis). Slowly developing, stable respiratory acidosis (as in COPD [chronic obstructive pulmonary disease]) may be well tolerated, but patients may have memory loss, sleep disturbances, excessive daytime sleepiness, and personality changes. Signs include gait disturbance, tremor, blunted deep tendon reflexes, myoclonic jerks, asterixis, and papilledema.
Diagnosis
Recognition of respiratory acidosis and appropriate renal compensation (see Diagnosis of Acid-Base Disorders) requires ABG determination and measurement of serum electrolytes. Causes are usually obvious from history and examination. Calculation of the alveolar-arterial (A-a) O2 gradient (inspired Po2 − [arterial Po2+5⁄4 arterial Pco2]) can help distinguish pulmonary from extrapulmonary disease; a normal gradient essentially excludes pulmonary disorders.
Treatment
Treatment is provision of adequate ventilation by either endotracheal intubation or noninvasive positive pressure ventilation (for specific indications and procedures, see Overview of Respiratory Failure). Adequate ventilation is all that is needed to correct respiratory acidosis, although chronic hypercapnia generally must be corrected slowly (eg, over several hours or more), because too-rapid Pco2 lowering can cause a posthypercapnic “overshoot” alkalosis when the underlying compensatory hyperbicarbonatemia becomes unmasked; the abrupt rise in CNS pH that results can lead to seizures and death. Any potassium and chloride deficits are corrected.
Sodium bicarbonate is almost always contraindicated, because of the potential for paradoxical acidosis within the CNS. One exception may be in cases of severe bronchospasm, in which bicarbonate may improve responsiveness of bronchial smooth muscle to beta-agonists.
Key Points
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Respiratory acidosis involves a decrease in respiratory rate and/or volume (hypoventilation).
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Common causes include impaired respiratory drive (eg, due to toxins, CNS disease), and airflow obstruction (eg, due to asthma, COPD [chronic obstructive pulmonary disease], sleep apnea, airway edema).
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Recognize chronic hypoventilation by the presence of metabolic compensation (elevated bicarbonate [HCO3−]) and clinical signs of tolerance (less somnolence and confusion than expected for the degree of hypercarbia).
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Treat the cause and provide adequate ventilation, using tracheal intubation or noninvasive positive pressure ventilation as needed.
