What’s normal about the FEV1/FVC ratio?

The FEV1/FVC ratio is used to estimate the presence and degree of airway obstruction. For well over thirty years my lab has used an FEV1/FVC ratio of 95% of predicted as the cutoff for normalcy. This value (carved onto a stone tablet by the way) had been brought to the lab by a founding physician who had come to the department from the NIH in the 1970’s. Since the software and hardware upgrade this summer our PFT Lab has switched to the NHANES III spirometry reference equations but we have so far resisted changing our 95% cutoff to the lower limit of normal (LLN). This is due in part to inertia but also in part to a mistrust in the concept of LLN. We have been steadily re-evaluating all of our testing criteria and have turned again to the FEV1/FVC ratio with the question as to whether our 95% cutoff is over-zealous or whether the LLN is too lax.

Strictly speaking LLN is a statistical concept. In the NHANES III study (and most others) it is computed as the mean predicted value minus 1.645 times the standard estimate of error. Unlike the reference equations for FVC and FEV1 which use both height and age as factors, the NHANES III reference equations for the FEV1/FVC ratio are derived solely from age. It is not clear to me this is completely correct and I have discussed some of the discrepancies between the NHANES III predicted FEV1/FVC ratio and height in a prior posting but it does make analyzing the LLN for the ratio easy. For adult, Caucasian males the reference equations are:

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N2 washout switch-in error

I have been taking a close look at the raw data from all lung volume tests lately in large part because N2 washouts are still relatively new to my PFT Lab and we’re continuing to learn from our mistakes. When I saw this N2 washout test I knew that there was something wrong with it. The patient had performed three N2 washout tests and the TLC, FRC and RV for this one test were significantly larger than for the other tests. The most common problem we’ve been having with N2 washouts has been with patient leaks during the washout period which almost always show up as an upwards drift in the tidal baseline. This test did not show any drift however, and it took me a little while before I could see what was wrong with it.

The N2 washout maneuver has the patient start by breathing tidally for a short period of time in order to determine where end-exhalation (FRC) is located. The patient then performs a slow vital capacity maneuver by steadily inhaling maximally to TLC and then exhaling maximally to RV. The technician then switches the patient into the washout breathing circuit at maximal exhalation and the patient resumes breathing tidally for the remainder of the test.

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Height and arm-span

The PFT Lab I am associated with has been making a point of having the technicians re-measure patient height with each visit. Part of the reason for this is that several years ago the medical assistants in the pulmonary outpatient clinic were tasked with obtaining patient heights, weights and blood pressures. For a period of time the technicians used these heights when entering the patient demographic information but it was soon noticed that patient heights often changed by several inches from visit to visit. For this reason we have asked the technicians to re-measure patient height instead.

One possible cause for the fluctuation in heights was that the medical assistants were measuring patient height using the height rod attached to the scale while also taking their weight. The PFT Lab has wall-mounted stadiometers in or near all of the lab’s testing rooms so that patient height can be taken with their back against the wall rather than free-standing.

Another reason to regularly re-measure patient height is that the lab’s population has a significant number of patients that have routinely been seen by the lab and the pulmonary physicians for years. The lab’s patient and results database now goes back over twenty years and patients that were seen 15 and 20 years ago have been referred again for pulmonary function testing and their height has changed in the meantime.

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