Respiratory Acidosis Workup

In patients without an obvious source of hypoventilation and respiratory acidosis, a drug screen should be performed. The effects of sedating drugs such as narcotics and benzodiazepines in depressing the central ventilatory drive and causing respiratory acidosis should also be considered. These sedative drugs should be avoided in patients with respiratory acidosis.

Chest and brain imaging studies as well as tests for pulmonary function, nerve function, and transdiaphragmatic pressure, when available, may also be helpful.

Arterial blood gas (ABG) analysis is necessary in the evaluation of a patient with suspected respiratory acidosis or other acid-base disorders. The bicarbonate level reported on the blood gas analysis is calculated from the Henderson-Hasselbalch equation. Thus, a serum bicarbonate level must also be obtained. Other test that may be helpful include serum electrolytes and biochemistries, thyroid studies, a complete blood cell (CBC) count, and drug and toxicology screens.

ABG level

Acidemia is documented by the presence of a decreased pH (< 7.35) on ABG analysis. The presence of an increased partial arterial pressure of carbon dioxide (PaCO₂) (> 45 mm Hg) indicates a respiratory etiology of the acidemia.

Hypoxemia may be present and is frequently associated with pulmonary diseases that can cause respiratory acidosis.

Serum electrolytes and biochemistries, and thyroid studies, and CBC count

The most common abnormal serum electrolyte finding in chronic respiratory acidosis is the presence of a compensatory increase in serum bicarbonate concentration.

Some patients with hypothyroidism hypoventilate. In addition, hypothyroidism may cause obesity, leading to obstructive sleep apnea and sleep apnea–related hypoventilation. Obesity hypoventilation syndrome also leads to chronic respiratory acidosis. A thyrotropin and free T4 level should therefore be considered in selected patients.

Many patients with chronic hypercapnia and respiratory acidosis are also hypoxemic. These patients may have secondary polycythemia as demonstrated by elevated hemoglobin and hematocrit values.

Drug and toxicology screens

Drug and toxicology screens should be performed in patients presenting with unexplained hypercapnia and respiratory acidosis. Screening for specific drugs, including opiates, barbiturates, and benzodiazepines, should be performed.

Radiography, computed tomography (CT) scanning, and fluoroscopy of the chest may provide helpful information in determining causes of respiratory acidosis.

Radiography

Chest radiography should be performed to help rule out pulmonary disease as a cause of hypercapnia and respiratory acidosis.

Findings on chest radiographs that may help determine an etiology of respiratory acidosis include: (1) hyperinflation and diaphragmatic flattening due to severe obstructive airway disease, (2) infiltrates secondary to pneumonias, (3) elevated diaphragm related to diaphragmatic weakness or paralysis, (4) pneumothorax, (5) atelectasis, and (6) thoracic skeletal deformaties.

If complicating pulmonary hypertension is present, the hilar vascular shadows are prominent and the cardiac silhouette may show evidence of right ventricular enlargement.

CT scanning

A CT scan of the chest may be obtained if the results of chest radiography are inconclusive or if a pulmonary disorder remains high on the differential diagnosis. CT scanning is more sensitive for detecting pulmonary diseases than plain radiography and may reveal abnormalities not observed on chest radiographs.

Fluoroscopy

A fluoroscopic "sniff test," in which paradoxical elevation of the paralyzed diaphragm is observed with inspiration, can confirm diaphragmatic paralysis, even in the presence of a normal appearance on chest radiographs. However, this test is not as useful in bilateral diaphragmatic paralysis compared with unilateral diaphragmatic paralysis.

Radiologic studies of the brain should be considered if a central cause of hypoventilation and respiratory acidosis is suspected.

CT scanning

Specific etiologies that may be diagnosed using brain CT scanning include stroke, central nervous system (CNS) tumor, and CNS trauma. Pay particular attention to the brainstem for lesions in the pons and medulla.

MRI

If a central cause of hypoventilation and respiratory acidosis is suspected and initial findings after brain CT scanning are negative or inconclusive, consider magnetic resonance imaging (MRI) of the brain. The MRI may disclose abnormalities not observed on CT scans, particularly in the brainstem.

Pulmonary function test measurements are required for the diagnosis of obstructive lung disease and for assessment of the severity of disease. Forced expiratory volume in 1 second (FEV₁) is the most commonly used index of airflow obstruction. The ratio of FEV₁ to forced vital capacity (FVC) (ie, FEV₁/FVC), is reduced and is the diagnostic variable in airflow obstruction.

Lung volume measurements may document an increase in total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). Measurement of maximal inspiratory and expiratory pressures may be useful in screening for respiratory muscle weakness.

Electromyography (EMG) and nerve conduction velocity (NCV) are useful in diagnosing neuromuscular disorders (eg, myasthenia gravis, Guillain-Barré syndrome, amyotrophic lateral sclerosis), which can cause ventilatory muscle weakness. These studies may reveal a neuropathic or a myopathic pattern, depending on the etiology of the diaphragmatic and respiratory muscle dysfunction.

Measurement of transdiaphragmatic pressure is a useful diagnostic test to document respiratory muscle weakness, but it is not only difficult to perform but is usually performed only in specialized pulmonary function laboratories.

The test is performed by placing an esophageal catheter with an esophageal balloon and a gastric balloon. The difference between the pressures measured at the 2 balloons is the transdiaphragmatic pressure. Patients with diaphragmatic dysfunction and paralysis have a decrease in maximal transdiaphragmatic pressure.