Approach Considerations
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 be considered. These sedative drugs should be avoided, if possible, in patients with respiratory acidosis.
Radiography, computed tomography (CT) scanning, and fluoroscopy of the chest may provide helpful information in determining causes of respiratory acidosis. Radiologic studies (CT scanning and magnetic resonance imaging [MRI]) of the brain should be considered if a central cause of hypoventilation and respiratory acidosis is suspected. Tests for pulmonary function, nerve function, and transdiaphragmatic pressure (when available), may also be helpful.
Laboratory Tests
Arterial blood gas (ABG) analysis is necessary in the evaluation of a patient with suspected respiratory acidosis or other acid-base disorders. [4] The bicarbonate level reported on the blood gas analysis is calculated from the Henderson-Hasselbalch equation. Thus, a measured serum bicarbonate level must also be obtained. Other tests that may be helpful include serum electrolytes and biochemistries, thyroid studies, a complete blood count (CBC), and drug and toxicology screens.
Acidemia is documented by the presence of a decreased pH (< 7.35) on ABG analysis. The presence of an increased partial pressure of arterial 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 cause respiratory acidosis.
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 (OSA) and sleep apnea–related hypoventilation. Obesity hypoventilation syndrome (OHS) also leads to chronic respiratory acidosis. A thyrotropin and a 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 should be performed in patients presenting with unexplained hypercapnia and respiratory acidosis. Screening for specific drugs, including opiates, barbiturates, and benzodiazepines, should be performed.
A study by Sadot et al found alveolar hypoventilation to be frequent among children undergoing flexible bronchoscopy. The investigators stated, therefore, that during the procedure children, especially those susceptible to complications from respiratory acidosis or who are expected to need a large amount of sedation, should be monitored for a rise in transcutaneous carbon dioxide, an indicator of alveolar hypoventilation. The study included 95 children. [22]
Plain Radiography and Fluoroscopy
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 the following:
-
Hyperinflation and diaphragmatic flattening due to severe obstructive airway disease
-
Infiltrates secondary to pneumonias
-
Elevated diaphragm related to diaphragmatic weakness or paralysis
-
Pneumothorax
-
Atelectasis
-
Thoracic skeletal deformities
If complicating pulmonary hypertension is present, the hilar vascular shadows may be prominent and the cardiac silhouette may show evidence of right ventricular enlargement.
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 as it is in unilateral diaphragmatic paralysis.
CT and MRI
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 than plain radiography for detecting pulmonary diseases and may reveal abnormalities not observed on chest radiographs.
Specific etiologies that may be diagnosed by 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.
If a central cause of hypoventilation and respiratory acidosis is suspected and after initial findings brain CT imaging is negative or inconclusive, a MRI of the brain should be perfromed. MRI may disclose abnormalities not observed on CT scans, particularly in the brainstem.
Pulmonary Function Testing
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 (FEV1.0) is the most commonly used index of airflow obstruction. The ratio of FEV1.0 to forced vital capacity (FVC) (ie, FEV1.0/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) in obstructive airway diseases. TLC is decreased in restrictive lung diseases. Measurement of maximal inspiratory and expiratory pressures may be useful in screening for respiratory muscle weakness.
EMG and Nerve Conduction Velocity
Electromyography (EMG) and measurement of nerve conduction velocity (NCV) are useful in diagnosing neuromuscular disorders (eg, myasthenia gravis, Guillain-Barré syndrome, and amyotrophic lateral sclerosis [ALS]), which can cause ventilatory muscle weakness. These studies may reveal a neuropathic pattern or a myopathic pattern, depending on the etiology of the diaphragmatic and respiratory muscle dysfunction. Some centers can perform phrenic nerve conduction studies and diaphragmatic EMG in the workup of diaphragmatic dysfunction.
Measurement of Transdiaphragmatic Pressure
Measurement of transdiaphragmatic pressure is a useful diagnostic test for documenting respiratory muscle weakness. However, it is difficult to perform, and it 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.
Other Tests
Capnography, or end-tidal carbon dioxide monitoring
Capnography is a noninvasive bedside diagnostic tool for measurement of the partial pressure of carbon dioxide in exhaled breath, especially in the operating room, endoscopy suite settings, and the emergency department (ED) setting. A meta-analysis of 13 randomized, controlled trials showed that with capnography monitoring, employed in combination with visual assessment and pulse oximetry, there was a reduction in respiratory compromise during procedural sedation and analgesia administered for ambulatory surgery, in comparison with the use of visual assessment and pulse oximetry alone. [23]
