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%.

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.


Fromageot C, Lofaso F, Annane D, Falaize L, Lejaille M, Clair B, Gajdos P, Raphael JC. Supine fall in lung volumes in the assessment of diaphragmatic weakness in neuromuscular disorders. Arch Phys Med Rehabil 2001; 82:123-128.

Lechtzin N, Wiener CM, Shade DM, Clawson L, Diette GB. Spirometry in the supine position improves the detection of diaphragmatic weakness in patients with Amyotrophic Lateral Sclerosis. Chest 2002; 121:436-442.

Meysmann M, Vincken W. Effect of body posture on spirometric values and upper airway obstruction indices derived fro the flow-volume loop in young nonobese subjects. Chest 1998; 114:1042-1047.

Patel AS, O’Donnell C, Parker MJ. Diaphragm paralysis definitively diagnosed by Ultrasonography and postural dependence of dynamic lung volumes after seven decades of dysfunction. Lung 2007; 185:15-20.

Shepard JW, Burger CD. Nasal and oral flow-volume loops in normal subjects and patients with obstructive sleep apnea. Am Rev Resp Dis 1990; 142:1288-1293

Vilke GM, Chan TC, Neuman T, Clausen JL. Spirometry in normal subjects in sitting, prone and supine positions. Respir Care 2000; 45:407-410

Wade OL, Gilson JC. The effect of posture on diaphragmatic movement and vital capacity in normal subjects with a note on spirometry as an aid in determining radiological chest volumes. Thorax 1951; 6:103-124.

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8 thoughts on “Supine spirometry

  1. First of all let me thank you for the time and effort you put forth on this website! I look forward to each edition.
    We are going to start doing supine spirometry in our lab, but I’m not having any luck finding reference values. Can you please advise?

    Thanks again!
    Susie Roeder RT/RPFT

    • As far as I know there are no supine reference values per se, just the comparison with upright results. Normal is <10% decrease in FVC from upright to supine. A decrease in FVC >15% from upright to supine suggests diaphragmatic dysfunction. In-between is suspicious but not definitive. The test is not perfect because some patients with upper airway problems will have positive results but you can often tell this from the flow-volume loop. Its value is that it is specific to the diaphragm while a reduced MIP/MEP just says muscle weakness (or poor effort).

  2. Thank you so for this information! We had our first order ever for one of these today and I am going to use this as our initial guideline.

  3. I am a respiratory clinical physiologist and have previously attended a course on muscle function delivered by out uk professional body ARTP. At this point I was informed that a 15% fall in FVC was suggestive of diaphragmatic weakness and 25% was diagnostic. I have searched the net trying to find the latest guidelines on this to no avail. Could you possibly be of any assistance? Any help you can provide is much appreciated .
    Kind Regards

    • Lisa –

      There are no official standards for upright and supine spirometry. The 2002 ATS/ERS statement on respiratory muscle testing (Am J Respir Crit Care Med 2002; 166: 518-624) mentions it in passing saying “In most normal subjects, VC in the supine position is 5–10% less than when upright and a fall of 30% or more is generally associated with severe diaphragmatic weakness” and in the section’s conclusions they only state that “A fall in VC in the supine position, compared with when upright, suggests severe diaphragm weakness or paralysis.”

      My lab uses 15% as the cutoff and my personal experience is that by the time somebody has a 25% decrease they’re already noticeably compromised in other ways. Interpreting upright/supine changes in FVC also has to be tempered with the fact that many patients with neuromuscular disease perform spirometry poorly and often have notably short expiratory times.

      Regards, Richard

  4. Hi- Love your site
    Curious on your interpretation of upright/supine PFTs in patients with very low lung volumes. We recently tested a CF patient with a pre-FVC of 1.57, whose FVC dropped 12% to 1.38. Only a volume change of 190 mL. Is the interpretation of testing affected by low baseline volumes?

    • Phil –

      Good question. The literature on supine spirometry, such as it is, does not address that issue. Many patients with diaphragmatic dysfunction already have a reduced FVC so I’m going to have to say that a change of 15% is probably a large enough signal to suspect diaphragmatic dysfunction regardless of the FVC volume, as long as you are sure that both upright and supine tests are good quality. Your patient however, only changed by 12%, so suspicious perhaps, but not definitive.

      Regards, Richard

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