eMedicine Specialties > Pulmonology > Acid-Base Disorders

Respiratory Acidosis: Differential Diagnoses & Workup

Author: Wael El Minaoui, MBBS, Fellow in Pulmonary/Critical Care Medicine, East Tennessee State University
Coauthor(s): Ryland P Byrd Jr, MD, Professor, Department of Internal Medicine, Division of Pulmonary Medicine and Critical Care Medicine, James H Quillen College of Medicine, East Tennessee State University; Chief of Pulmonary Medicine, Medical Director of Respiratory Therapy, Intensive Care Unit, Program Director of Pulmonary Diseases and Critical Care Medicine Fellowship, James H Quillen Veterans Affairs Medical Center
Contributor Information and Disclosures

Updated: Apr 2, 2009

Differential Diagnoses

Asthma
Emphysema
Botulism
Obesity
Chronic Bronchitis
Opioid Abuse
Chronic Obstructive Pulmonary Disease
Sedative, Hypnotic, Anxiolytic Use Disorders
Diaphragm Disorders
Diaphragmatic Paralysis

Other Problems to Be Considered

Amyotrophic lateral sclerosis
Muscular dystrophy
Severe kyphoscoliosis
Guillain-Barré syndrome
Myasthenia gravis

Workup

Laboratory Studies

  • Arterial blood gas determination: Acidemia is documented by the presence of a decreased pH (<7.35) shown on arterial blood gas analysis. The presence of an increased PaCO2 (>45 mm Hg) indicates a respiratory etiology of the acidemia.
  • Hypoxemia: This may be present and frequently is associated with pulmonary diseases that can cause respiratory acidosis.
  • Serum chemistries: The most common finding in chronic respiratory acidosis is the presence of a compensatory increase in serum bicarbonate concentration.
  • Hypothyroidism: Some patients have hypothyroidism as a cause for their obesity (and resulting hypoventilation due to associated obstructive sleep apnea) leading to chronic respiratory acidosis. A thyrotropin and free T4 level should be obtained.
  • Complete blood cell count: Many patients with chronic hypercapnia and respiratory acidosis also are hypoxemic. These patients may have secondary polycythemia.
  • Drug 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.

Imaging Studies

  • Chest 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 hyperinflation and diaphragm flattening secondary to severe obstructive airway disease, infiltrates secondary to pneumonias, elevated diaphragm related to diaphragmatic weakness or paralysis, pneumothorax, and atelectasis.
    • With complicating pulmonary hypertension, the hilar vascular shadows are prominent and the cardiac silhouette may show evidence of right ventricular enlargement.
  • CT scanning of the chest: 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 disease and may reveal abnormalities not observed on chest radiographs.
  • CT scanning of the brain: Perform imaging of the brain if a central cause of hypoventilation and respiratory acidosis is suspected. Specific etiologies that may be diagnosed using brain CT scanning include stroke, CNS tumor, and CNS trauma. Pay particular attention to the brainstem for lesions in the pons and medulla.
  • MRI of the brain: If a central cause of hypoventilation and respiratory acidosis is suspected and initial findings after brain CT scanning are negative or inconclusive, consider MRI of the brain. The MRI may disclose abnormalities not observed on CT scans, particularly in the brainstem.
  • 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. This test is not as useful in bilateral diaphragmatic paralysis compared with unilateral diaphragmatic paralysis.

Other Tests

  • Pulmonary function testing
    • These 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, functional residual capacity, and residual volume.
    • Measurement of maximal inspiratory and expiratory pressures may be useful in screening for respiratory muscle weakness.
  • Electromyography and nerve conduction velocity: 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
    • This diagnostic test is useful in documenting respiratory muscle weakness, but it is difficult to perform and usually is performed only in specialized pulmonary function laboratories.
    • This 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.

More on Respiratory Acidosis

Overview: Respiratory Acidosis
Differential Diagnoses & Workup: Respiratory Acidosis
Treatment & Medication: Respiratory Acidosis
Follow-up: Respiratory Acidosis
References
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Further Reading

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Keywords

respiratory acidosis, hypoventilation, hypercapnia, alveolar hypoventilation, impaired ventilation, central respiratory depression, myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barre syndrome, muscular dystrophy, asthma, airway obstruction, chronic obstructive pulmonary disease, COPD, increased ventilation-perfusion mismatch, decreased diaphragm function, diaphragm dysfunction, obesity hypoventilation syndrome, obesity-hypoventilation syndrome, pickwickian syndrome, respiratory muscle fatigue, emphysema, chronic bronchitis, bronchitis, amyotrophic lateral sclerosis, diaphragm paralysis, kyphoscoliosis

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Contributor Information and Disclosures

Author

Wael El Minaoui, MBBS, Fellow in Pulmonary/Critical Care Medicine, East Tennessee State University
Disclosure: Nothing to disclose.

Coauthor(s)

Ryland P Byrd Jr, MD, Professor, Department of Internal Medicine, Division of Pulmonary Medicine and Critical Care Medicine, James H Quillen College of Medicine, East Tennessee State University; Chief of Pulmonary Medicine, Medical Director of Respiratory Therapy, Intensive Care Unit, Program Director of Pulmonary Diseases and Critical Care Medicine Fellowship, James H Quillen Veterans Affairs Medical Center
Ryland P Byrd Jr, MD is a member of the following medical societies: American College of Chest Physicians, American Thoracic Society, and Southern Medical Association
Disclosure: Nothing to disclose.

Medical Editor

Oleh Wasyl Hnatiuk, MD, Program Director, National Capital Consortium, Pulmonary and Critical Care, Walter Reed Army Medical Center; Associate Professor, Department of Medicine, Uniformed Services University of Health Sciences
Oleh Wasyl Hnatiuk, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and American Thoracic Society
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

Gregg T Anders, DO, Medical Director, Great Plains Regional Medical Command , Brook Army Medical Center; Clinical Associate Professor, Department of Internal Medicine, Division of Pulmonary Disease, University of Texas Health Science Center at San Antonio
Gregg T Anders, DO is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and American Thoracic Society
Disclosure: Nothing to disclose.

CME Editor

Timothy D Rice, MD, Associate Professor, Departments of Internal Medicine and Pediatrics and Adolescent Medicine, Saint Louis University School of Medicine
Timothy D Rice, MD is a member of the following medical societies: American Academy of Pediatrics and American College of Physicians
Disclosure: Nothing to disclose.

Chief Editor

Zab Mosenifar, MD, Director, Division of Pulmonary and Critical Care Medicine, Director, Women's Guild Pulmonary Disease Institute, Executive Vice Chair, Department of Medicine, Cedars Sinai Medical Center; Professor of Medicine, David Geffen School of Medicine at UCLA
Zab Mosenifar, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Federation for Medical Research, and American Thoracic Society
Disclosure: Nothing to disclose.

 
 
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