Laboratory Studies
Failure to consider a mixed acidosis can lead to missed therapies and diagnosis. Always critically analyze acid-base values by assessing the pH, the arterial partial pressure of carbon dioxide (Pa CO2), and the bicarbonate (HCO3–) concentration.
Diagnosis of respiratory acidosis is established with arterial blood gas values. [4] Insight into the underlying cause and etiopathologic mechanisms requires further investigations, as detailed below. The serum HCO3– level and pH can be helpful in distinguishing acute hypercapnia from chronic hypercapnia. If the pH is greater than 7.45, elevated Pa CO2 may compensate for metabolic alkalosis and not a primary process.
In the setting of acute respiratory acidosis, pH decreases by 0.08 for every 10-mmHg increase in Pa CO2. The HCO3– concentration increases by 1 mEq/L for every 10-mmHg increase in Pa CO2. If Pa CO2 increases acutely to 80 mmHg, the pH is 7.12, and the HCO3– value is 28 mEq/L.
In the setting of chronic respiratory acidosis, pH decreases 0.03 for every 10-mmHg increase in Pa CO2. HCO3– concentration equals 24 mmol/L ± 4 for every 10-mmHg increase in Pa CO2 greater than 40 mmHg. For example, if the Pa CO2 is 80 mmHg, the pH is 7.28, and the HCO3– value is 40 mEq/L ± 4.
The HCO3– -resorption process is efficient. If a patient with chronic hypercapnia has a pH higher than 7.20, a superimposed acute-on-chronic respiratory acidosis or a concomitant metabolic acidosis is most likely occurring as well.
A toxicology screen for narcotics, benzodiazepines, alcohol, or tricyclic antidepressants should be performed if indicated. Electrolyte assessment is indicated for abnormalities associated with muscle weakness (eg, hypophosphatemia, hypokalemia, hypomagnesemia, and hypocalcemia).
Other tests
In children undergoing flexible bronchoscopy, monitoring alveolar ventilation with transcutaneous CO2 (TcCO2 ) levels appears to be useful in detecting significant hypoventilation (elevated TcCO2 ) during these procedures, particularly when patients may require large doses of sedation and for those at risk of complications from respiratory acidosis. [7]
S100B protein is an accurate biomarker for neuroapoptosis and brain damange. Respiratory acidosis is associated with high levels of S100B in umbilical cord at birth; measuring umbilical cord pH and pCO2 may help identify neonates at risk of neuroapoptosis. [6]
Radiography, CT, and MRI
Chest radiography findings may help in the diagnosis.
Computed tomography (CT) scanning of the chest is indicated if the history and physical findings suggest primary pulmonary disease. CT angiography may be indicated to rule out pulmonary embolus.
CT scanning or magnetic resonance imaging (MRI) of the brain is indicated if the history and physical findings suggest signs of an intracranial process. MRI of the spine may be indicated by the history and physical findings.
Other Tests
Additional studies that may be considered include the following:
-
Pulmonary function tests, including spirometry, if the child can cooperate
-
Electromyography (EMG), if indicated to evaluate neuromuscular disease
-
Polysomnography (sleep study) to evaluate for obstructive or central sleep apnea, if indicated
