Laboratory Studies
An essential laboratory analysis is as follows:
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Arterial blood gas determination: Alkalemia is documented by the presence of an increased pH level (>7.45) on arterial blood gas determinations. The presence of a decreased PaCO2 level (< 35 mm Hg) indicates a respiratory etiology of the alkalemia.
The following laboratory studies may be helpful:
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Serum chemistries: Acute respiratory alkalosis causes small changes in electrolyte balances. Minor intracellular shifts of sodium, potassium, and phosphate levels occur. A minor reduction in free calcium occurs due to an increased protein-bound fraction. Compensation for respiratory alkalosis is by increased renal excretion of bicarbonate. In acute respiratory alkalosis, the bicarbonate concentration level decreases by 2 mEq/L for each decrease of 10 mm Hg in the PaCO2 level. In chronic respiratory alkalosis, the bicarbonate concentration level decreases by 5 mEq/L for each decrease of 10 mm Hg in the PaCO2 level. Plasma bicarbonate levels rarely drop below 12 mm Hg secondary to compensation for primary respiratory alkalosis.
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Complete blood count (CBC): An elevation of the white blood cell (WBC) count may indicate early sepsis as a possible etiology of respiratory alkalosis. A reduced hematocrit value may indicate severe anemia as the potential cause of respiratory alkalosis.
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Liver function test: Findings may be abnormal if hepatic failure is the etiology of the respiratory alkalosis.
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Cultures of blood, sputum, urine, and other sites: These should be considered, depending on information obtained from the history and physical examination and if sepsis or bacteremia are thought to be the cause of the respiratory alkalosis.
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Thyroid testing: Thyroid-stimulating hormone and thyroxine levels may be indicated to rule out hyperthyroidism.
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Beta-human chorionic hormone levels may be helpful in ruling out pregnancy.
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Drug screens and theophylline and salicylate levels may be useful to determine whether drugs or medications are the cause.
Imaging Studies
Consider the following imaging studies:
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Chest radiography: Chest radiography should be performed to help rule out pulmonary disease as a cause of hypocapnia and respiratory alkalosis. Potential etiologies that may be confirmed based on chest radiography findings include pneumonia, pulmonary edema, aspiration pneumonitis, pneumothorax, and interstitial lung disease.
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Computerized tomography (CT) scanning: CT scanning of the chest may be performed if chest radiography findings are inconclusive or a pulmonary disorder is strongly considered as a differential diagnosis. CT scanning is more sensitive for helping detect disease, and findings may reveal abnormalities not seen on the chest radiograph. Consider spiral CT angiography of the chest if pulmonary embolism is suggested. Consider CT scanning of the brain if a central cause of hyperventilation and respiratory alkalosis is suspected. Specific etiologies that may be diagnosed based on brain CT scan findings include cerebrovascular accident, central nervous system tumor, and central nervous system trauma.
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Ventilation perfusion scanning: Consider this scan in patients who are unable to undergo an intravenous contrast injection associated with CT scanning to assess the patient for pulmonary embolism.
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Brain magnetic resonance imaging (MRI): If a central cause of hyperventilation and respiratory alkalosis is suggested and the initial brain CT scan findings are negative or inconclusive, an MRI of the brain can be considered. MRIs may reveal abnormalities not seen on CT scans, particularly lesions of the brain stem. Possible etiologies based on MRIs include cerebrovascular accident, central nervous system tumor, and central nervous system trauma.
Other Tests
Echocardiography can be performed to assess myocardial and valvular function. A "bubble" study is helpful when assessing patients for unexplained hypoxemia and right-to-left shunting of blood.
Procedures
Perform a lumbar puncture if the history and physical examination findings are suggestive of a CNS infectious process.
