Supine spirometry

If you are not already performing supine spirometry you should consider adding it to your arsenal of tests. Other than an exam table no new equipment is needed to perform it and it is a simple technique that can provide useful information towards diagnosing and monitoring diaphragmatic dysfunction. It is non-invasive when compared to a transdiaphragmatic pressure test (which requires an esophageal balloon), does not require ionizing radiation (fluoroscopy) and is likely more accurate and better tolerated than MIP and MEP tests. Candidates for this test include patients with neuromuscular diseases, suspected or known diaphragmatic paralysis or any patient complaining of dyspnea that cannot be explained by other routine testing.

Vital capacity is dependent on a number of factors, an important one being the range of motion of the diaphragm. The initial position of the diaphragm and the distance it can move is determined by effect of gravity on the abdomen and its contents. For this reason vital capacity is greatest when performed in the upright position and lower when performed in the supine position.

Persons with normal lung function usually see a decrease in FVC from upright to supine of about 3% to 8%. In individuals with diaphragmatic dysfunction this decrease is usually over 10%. Patients with unilateral diaphragmatic paralysis tend to have a decrease of at least 15% whereas those with bilateral diaphragmatic paralysis tend to have decreases greater than 25%.

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