A report of a patient’s complete set of PFTS came across my desk yesterday. There were a lot of inconsistencies in the report and I ended up looking at the raw data for every single test. When I looked at the spirometry results I was surprised to see which effort the technician had selected to report the FEV1.

The patient’s spirometry efforts were highly variable and not terribly reproducible. The FEV1 that had been selected came from an effort with an expiratory pause that occurred before the first second and with a number of cough artifacts. There was another spirometry effort effort however, that did not have a pause and had a larger FEV1. The reason the other effort had not been selected was that it did not meet ATS-ERS criteria for back-extrapolation whereas the first effort did.

The technician that performed the test and selected the effort has over a half-dozen years of experience in our lab and has worked in other PFT labs as well. I learned that she had thought very hard about which effort to select before choosing the one she did. An interesting side note to this is that when the test software was asked to select which FEV1 should be reported it also chose the same effort as our technician.

When we train technicians in our lab back-extrapolation is one of the spirometry quality issues we emphasize. My experience has been that efforts with large amounts of back-extrapolation tend to overestimate the FEV1. When we teach how to select spirometry efforts we try to make the point that you can’t select the highest FEV1 without also looking at test quality. It is this point that caused the technician to select the FEV1 she did.

We have some pretty extensive technician training materials that we developed over the years, but teaching about a sub-one second expiratory pause is not in there and this is because I never once thought it was necessary. To me it is self evident that if a patient does not have a clean exhalation during the first second then the FEV1 is not accurate. I think, however, this highlights a personal blind spot. I think it also highlights a difference in how technicians learn about PFTs now and how I learned about PFTs 40 years ago.

I learned spirometry using a counterweighted water-seal bell spirometer with a kymograph. A felt-tipped pen on the counterweight drew a line on graph paper that had been taped to the kymograph drum. After a set of tests I had to take the graph paper and use a plastic overlay to determine where the FEV1 was on the tracing. I then had to take the numbers from the graph paper and multiply them by a bell correction factor and BTPS to get the actual FVC and FEV1. Similarly I had to calculate the FEV1/FVC ratio and percent predicted for all the test values.

I am not in any way advocating we go back to the level of technology nor is this a diatribe about how easy technicians have it now (yes, I walked six miles to school every day, uphill both ways, through the drifts of snow and the blinding heat) but the point is that it made a direct connection for me between what a patient did during the test and what the results were. Now everything is handled by a computer and although a technician can see the patient perform the test and what the results look like afterward, everything that happens in-between those two events is now a black box to them. This means that technicians are far more likely now to learn rules about testing without really understanding where those rules come from and what limits the rules have.

So, using this as a teaching moment, I showed the technician that since the volume-time curves of the two efforts overlapped almost completely up until the pause, this means that the back-extrapolation did not likely affect the FEV1. I also showed that since the patient paused their exhalation before the first second there was no way the selected effort could have an accurate FEV1. Finally I pointed out that if the first FEV1 had been selected the patient would have been diagnosed with an obstructive lung disease when the other FEV1 made it clear they did not have one.

This will be added to our teaching material. I would like to think that it will help our technicians develop a deeper understanding of the testing process but {sigh} it is more likely this will be just another rule to remember.

As I mentioned earlier, however, the testing software also rejected the larger FEV1. This also appears to be because of back-extrapolation and highlights some serious deficiencies in the software. The full paragraph from the ATS-ERS spirometry standard on spirometry quality says (italics are mine):

The following conditions must also be met: 1) without an unsatisfactory start of expiration, characterised by excessive hesitation or false start extrapolated volume or EV 5% of FVC or 0.150 L, whichever is greater (fig. 2); 2) without coughing during the first second of the manoeuvre, thereby affecting the measured FEV1 value, or any other cough that, in the technician’s judgment, interferes with the measurement of accurate results [3]; 3) without early termination of expiration (see End of test criteria section); 4) without a Valsalva manoeuvre (glottis closure) or hesitation during the manoeuvre that causes a cessation of airflow, which precludes accurate measurement of FEV1 or FVC [3]; 5) without a leak [3]; 6) without an obstructed mouthpiece (e.g. obstruction due to the tongue being placed in front of the mouthpiece, or teeth in front of the mouthpiece, or mouthpiece deformation due to biting); and 7) without evidence of an extra breath being taken during the manoeuvre.

In this particular case, it is clear that sentences #2 and #4 are not implemented in the software. I realize that not all of the sentences in this statement can be implemented in software (sentences #5 and #6 for example) but pauses or hesitations within the first second should be amenable to software analysis. Is this also a blind spot on the part of the manufacturer? Do they also think that a sub-one second pause is self-evident? I don’t have an answer to that nor do I know how wide-spread a problem this is. Answers to technical questions like this are hard to get because oftentimes the answer may only be known to a single programmer or project manager as well as the fact that manufacturers tend to be disinclined to answer this kind of question in the first place.

I think the lesson is that neither your staff nor your test systems may be aware of the effect that sub-one second pauses have on FEV1. You and I may think it is self-evident, but this may be a blind spot for us, our staff training and our equipment.

Reference:

Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of spirometry. Eur Respir J 2005; 26: 319-338.

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