Student spirometers, what can they teach?

It’s been several decades since I last saw water-seal bell spirometers being used in a Pulmonary Function lab. They have been displaced mostly by systems that use flow sensors of one type of another and the small handful of equipment manufacturers that still make volume-displacement spirometers use rolling seals. This isn’t to say that this kind of spirometer isn’t being manufactured any more and in fact there appears to be a modestly thriving market in water-seal spirometers intended for use by students.

In the low-end of the market (under $1000) there are several different systems that would likely never make it into a PFT Lab. None of the manufacturers or distributors provide any claims about their accuracy and considering they are mostly made of injection-molded plastic it is hard to see what level of accuracy they could ever offer. Moreover, the volume readout for these spirometers is a dial that is moved by the chain attached between the bell and the counter-weight. The gradation on these dials allows you to measure (somewhat optimistically, I’d say) differences in exhaled volume of 0.1 liter.

Carolina_Student_Wet_Spirometer 

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At the higher end of the market (~$10,000) there at least one precision bell teaching spirometer with a kymograph that would probably meet ATS-ERS standards for volume, but since all counter-weighted bell spirometers have a limited frequency response (the bell can rise faster than the counter-weight and kymograph pen-assembly can fall) it may not meet standards for flow rates. This system has an in-line canister for soda-lime which allows it to measure oxygen uptake as well as being able to perform basic spirometry (it would probably also be able to measure lung volumes if it had a helium analyzer but that option is not offered).

Harvard_Apparatus_Recording_SpirometerSo, why would you want to use an old-fashioned counter-weighted, water-seal, bell spirometer to teach students about the respiratory system? I think the first answer would be their durability and simplicity. Spirometers of this type can last for years despite the constant abuse of indifferent (and occasionally hostile) students and there is not much that can go wrong with them that isn’t relatively easy to fix.

Another reason though, is the causal and conceptual link that can be shown between exhaled air and the movement of the spirometer bell. This makes it clear that exhaled air is something that can be measured.  Most PFT lab systems are pretty much a black box between the patient and what shows up on the computer screen. This can make it hard to show somebody that has had no prior experience with pulmonary function testing what the link is between what the patient is doing and what the results of the tests look like. I will admit this is a personal bias since I was taught on bell spirometers (not that there was any other type way back when). My lab still has several bell spirometer systems and when I taught PFTs I almost always started with one of these systems.

So what can you teach with a student spirometer? Not much, but more than you might think at first. Vital capacity, of course, and I am sure that more than one enterprising teacher has had his class perform their vital capacity and then had their students line up in order based on their vital capacity volume. This almost always provides a clear lesson about the relationship between height, gender and vital capacity.

The other thing you can teach is something about the lung volume subdivisions. Even though these systems are mostly equipped with just a dial and are intended to measure only exhalation, you can have a student note the volume before they’ve exhaled as much as they can and then, while the student stays on the mouthpiece, have them return to their normal breathing level (FRC) and then note the volume again. Simple math will let the student calculate their vital capacity, inspiratory capacity, expiratory reserve volume and possibly even tidal volume.

What the low-end spirometers can’t teach is anything about expiratory flow rates. With the high-end spirometer, students can perform a timed vital capacity and from the pen tracing they can extract FEV1 and FEF25-75, but it is not possible to derive peak flow or a flow-volume curve from a kymograph tracing.

Having said that these system can’t show flow rates there is at least one student pneumotach-based spirometer.

Neulog_Student_Flow_Sensor 

Given the dominance of flow-based PFT systems this is a bit surprising there aren’t more spirometer systems like this, but even more surprising, the software used to analyze the flow signal is only designed to integrate the area under the flow curve and measure vital capacity (it may be able to show a flow-volume curve but the manual only talks about measuring vital capacity).

In one sense, the inability to teach expiratory flow rates is disappointing but student spirometers are intended for basic anatomy and physiology courses and expiratory flow rates are more about disease processes. I don’t think these student spirometers are in any way adequate to teach pulmonary function testing other than possibly a few of the most basic concepts but as a teaching adjunct for anatomy and physiology I think they serve a good purpose.

It also shows that good designs never go away. Almost 170 years after Hutchinson invented it, the water-seal bell spirometer still has a role to play.

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

2 thoughts on “Student spirometers, what can they teach?

  1. Where is this “modestly thriving market” in water-seal spirometers intended for use by students? I have several to sell!

    • Christy –

      There is a minor on-going market for very simple water seal spirometers in high school and college physiology classes. There are a number of educational distributors that sell these. They do not in any way meet the ATS/ERS spirometer quality standard and mostly just measure FVC. I have been contacted a number of times however, by educators looking for old Collins spirometers but only for ones that are mostly intact. In particular what they are looking for are spirometers with a functional kymograph so that they can produce volume-time tracings. Collins spirometers occasionally come up for sale on Ebay and similar sites and ones that are intact generally sell pretty quickly.

      Regards, Richard

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