Pediatric Respiratory Acidosis Workup
- Author: Mithilesh K Lal, MD, MBBS, MRCP, FRCPCH, MRCPCH(UK); Chief Editor: Timothy E Corden, MD more...
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. 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-mm Hg increase in Pa CO2. The HCO3– concentration increases by 1 mEq/L for every 10-mm Hg increase in Pa CO2. If Pa CO2 increases acutely to 80 mm Hg, 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-mm Hg increase in Pa CO2. HCO3– concentration equals 24 mmol/L ± 4 for every 10-mm Hg increase in Pa CO2 greater than 40 mm Hg. For example, if the Pa CO2 is 80 mm Hg, 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).
Radiography, CT, and MRI
Chest radiography findings may help in the diagnosis.
Computed tomography (CT) 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 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.
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
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