Why haven’t computerized interpretations gotten any better?

Almost all pulmonary function test systems seem to come with a module that can perform a computerized interpretation of PFT results. Their accuracy has been studied occasionally, often by the developers of a particular algorithm and just as often a rosy picture is painted. Given their limited (and likely pre-cleaned) data sets I am sure this is accurate as far as it goes. I have done my own admittedly very unscientific comparison and would say that for two-thirds of the patients tested the results are probably okay. The other third? Varying degrees of not so much.

This concerns me because the very locations that could use the expert assistance of computerized interpretation, small clinics and doctor’s offices where inexperienced and under-trained staff are usually tasked to perform the tests and where this would be most useful, cannot rely on it. This fact was highlighted in a recent report in the European Respiratory Journal which showed that computerized interpretation did not improve the quality of care in general practitioners offices.

Computerized interpretation of pulmonary function tests have been around for at least 40 years. At one time or another developers have used expert systems, branching logic, fuzzy logic and neural networks. Algorithms have been tweaked and updated as our understanding of pulmonary function testing has improved but none are essentially any better or more accurate now than in the 1970’s.

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FEV1/FVC ratio and height

The PFT Lab I work with has recently gone through a major software and hardware upgrade. As part of this process we made the decision to switch our spirometry predicted equations to NHANESIII. The lab has been using the Morris predicteds for at least the last 25 years and this switch has led us to re-visit some of the issues involved in interpreting spirometry results.

More than one person that I’ve known and respected has said that spirometry is all about FEV1 and I think this is a true statement. There is a lot of other information you can get from a Forced Vital Capacity but it always comes back to FEV1.

Stepping aside from the mechanical and patient issues involved in obtaining an FEV1, once you have an acceptable FEV1 measurement how do you assess it? There is always the percent predicted and the lower limit of normal (LLN) but a reduced or normal FEV1 by itself cannot differentiate between an obstructive, restrictive or normal pattern. This is where the FEV1/FVC ratio comes in and an interesting question is where the predicted values for this ratio come from.

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Single-breath TLC measurements

I was reviewing the specifications of different pulmonary function test systems recently and saw that several manufacturers were advertising that some of their test systems are able to measure TLC, FRC and RV from a single-breath maneuver. This is true, but only to a very limited degree and I think it is reasonable to ask how accurate and clinically useful these measurements are and whether it is legitimate to bill for the test.

The measurement is made by having a patient exhale to RV and then inhale a gas mixture containing a tracer gas (an insoluble gas like helium or methane) to TLC. When the patient exhales, the degree by which the tracer has been diluted is then used to calculate the patient’s TLC. The math is quite simple and as is expressed as:

TLC = (inspired volume x (Fitrace/Fetrace)) – machine deadspace

This is done routinely as part of the DLCO test and there it is referred to as alveolar volume (VA) and it should be noted that all manufacturers are using the VA from the DLCO test as a substitute for TLC and are not performing single-breath TLC as a separate measurement.

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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.

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Why isn’t PAO2 measured during DLCO tests?

Most pulmonary technicians and physicians are aware that a patient’s hemoglobin and carboxyhemoglobin levels are factors that affect DLCO test results. What may be less well appreciated is the degree to which DLCO varies with the alveolar oxygen concentration.

Oxygen and carbon monoxide both compete for uptake by hemoglobin so when alveolar O2 is low or high the measured DLCO will also vary accordingly. The relationship between DLCO and PAO2 has been studied both in a regards to FIO2 and to barometric pressure. Both empirically and mathematically DLCO is estimated to change by approximately 0.35% per mmHg PAO2 change.

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