Mouthpieces and test quality

Mouthpieces serve at least two purposes. The first is to prevent cross-contamination among patients using PFT equipment. The ATS Statement on General Considerations for Lung Function Testing, (Eur Resp J, 2005; 26: 153-161) discusses this and it is clear that mouthpieces should be used whenever and wherever pulmonary function tests are performed.

The second purpose however, is to prevent patients from leaking air during testing. Leaks are a chronic problem for all pulmonary function tests and my experience has been that when they occur it is almost always the patient that is leaking and not the equipment.

Although all pulmonary function tests can be affected by patient leaks the helium dilution and nitrogen washout lung volume measurements are particularly prone to leaks due of the length of time it takes for them to be performed. These tests are also sensitive to small leaks because the accuracy of the measurements is based on relatively small changes in gas concentrations.

A flanged rubber (or soft plastic) mouthpiece usually isn’t needed for most spirometry testing and may not be necessary for Diffusion Capacity testing but there will always be patients that cannot maintain a seal with any kind of simple round mouthpiece and so a flanged mouthpiece should always be an option.

I am a strong advocate of the use of flanged mouthpieces and I am concerned that over the last decade, and probably longer, there has been a wholesale movement towards the use of small flanged mouthpieces. It has, in fact, gotten quite difficult to find large mouthpieces.

I know I will be dating myself when I say that when I say that for the first twenty years I spent in a pulmonary function lab there were only two sizes of mouthpieces: adult and child. Almost all flanged mouthpieces being sold today I would consider to be the same as what the child size used to be.

I am not sure how or why this trend began. It may be easier to get a patient onto a small mouthpiece, but once there they have to work harder to keep their lips tight around it and are more likely to leak during testing. Patients often complain that their lips are tired after a lung volume test when a small mouthpiece is used. The advantage of the large (adult) sized mouthpieces is that once it’s in place, it is very hard for a patient to leak around it even if they don’t keep their lips tight. There are patients, like those with TMJ or scleroderma, that cannot open their mouths wide enough for a large mouthpiece but I’ve always been able to get a large mouthpiece into almost every patient I’ve tested.

I’ve talked to a couple manufacturer’s representative about this and they have said this is what their customers want. At the same time though most labs are not offered any alternative to small mouthpieces or may not even be aware there are alternatives. There are only a couple manufacturers (A-M Systems, Vacumed) that still make large mouthpieces and adapters for different brands of equipment and in-line filters can be hard to find or nonexistent.

I am concerned about this trend because I think that small mouthpieces are not benefiting our patients. They may be easier to use but they are far more likely to produce inaccurate results.

I am even more concerned, however that I’ve heard that some PFT labs are not using flanged mouthpieces at all and are instead using cardboard mouthpieces for all of their tests. This is inexcusable both in terms of patient cross-contamination and in terms of accurate testing. If it is being done to save money it is a false economy and the real price being paid is in inaccurate test results and patient safety. If it is being done because the staff don’t know any better, then the medical director needs to be taken to task.

Personally I’d recommend that flanged rubber mouthpieces should be available for any type of pulmonary function testing, and mandated for lung volume measurements. Labs should also stock at least a couple different sizes of mouthpieces and always use the largest one a patient is able to accommodate. There are already a sufficient number of pitfalls involved in performing accurate pulmonary function tests without making it more difficult for the patient to maintain a tight seal during testing.

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PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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