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%.
The change in FVC from upright to supine may be less sensitive than the actual percent predicted FVC, however. As a rule of thumb, therefore, an FVC that decreases more than 15% in the supine position or that is less than 75% of predicted in the supine position should be considered as a positive indicator of diaphragmatic dysfunction.
Since this test requires the patient to be in a supine position an exam table that can be set in a completely horizontal position is necessary, and one that can be raised and lowered is ideal. We have used a folding massage table for a time, but I do not recommend using an actual bed as the softness and/or springiness of a mattress may interfere with the ability of the patient to perform the FVC maneuver. Although a pillow may make the patient more comfortable, it may also cause the upper airway to bend or constrict and for this reason should be avoided.
Before performing a supine spirometry test some thought should be taken regarding the patient’s suitability for testing. Patient and technician safety should not be compromised. Since many candidate patients have neuromuscular disease they may have limited mobility and may not be able to transfer to an exam table without assistance. Given that back injuries from lifting patients is one of the most common employee injuries in a hospital, patients that require more assistance than a technician is able to safely provide should not be tested. It should also be remembered that a patient is allowed to refuse testing if they don’t feel comfortable with the test. Lab staff should always take the time to explain the purpose of the test and the reason it should be performed, but after that it is the patient’s decision whether or not to proceed.
Supine spirometry should be performed using the same ATS/ERS criteria used for upright spirometry. We use the pre-drug mode for the upright spirometry and post-drug mode for the supine spirometry and are able to label the post-drug mode as Supine.
There is no CPT code for upright and supine spirometry so you can’t bill more than plain spirometry for the test. We are still on a mostly manual billing system and so we can control how the test is billed but if you have an automated billing system you should make sure that it will not take the presence of pre-drug and post-drug spirometry results as a flag to bill for pre/post bronchodilator testing.
When reviewing results it is useful to differentiate between diagnosing and monitoring. Patients sent for diagnosis are often in the early stages of their disease process and so the FVC criteria of 15% change and less than 75% of predicted is relatively easy to apply. When monitoring patients trends in the supine FVC is likely more important.
The Neurology department at our hospital has an ALS clinic so we routinely see a dozen or more patients a month with ALS, most often for monitoring the progress of their disease. ALS patients tire easily and this can limit the number of times they can perform a spirometry effort. There is also often upper airway involvement with ALS and spirometry results in both the upright and supine positions often shows pauses, plateaus and early termination and this can make interpretation difficult.
Even though supine spirometry requires extra effort for the PFT Lab and is not specifically reimbursed I think that it is a critical test for a portion of our patient population and for this reason alone it needs to be performed.
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