Measuring respiratory resistance with the APD

Inspiratory and expiratory flow rates are a function of driving pressure (i.e. the pressure difference between the alveoli and the atmosphere) and airway resistance. For this reason it would seem that airway resistance should be one of the most commonly performed pulmonary function tests but instead it is the outcome of airway resistance and driving pressure, i.e. the expiratory and inspiratory flow rates that are measured almost exclusively. One reason for this is that resistance measurements requires relatively expensive equipment such as a body plethysmograph or an impulse oscillometer as well as a fair amount of technical expertise.

The airflow perturbation device (APD) is a potentially inexpensive system for measuring respiratory resistance during tidal breathing. The device itself is mechanically simple, the concepts and mathematics that permit it to work are, however, a bit more complicated.

The APD consists of a mouth pressure transducer and a pneumotach whose end is attached to a rotating wheel. The wheel has open segments and segments with a mesh that partially obstructs airflow through the pneumotach. The rotation of the wheel causes a series of perturbations to the airflow through the pneumotach.

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Social Security Disability Evaluations

I was contacted recently by an individual with some questions about the pulmonary function testing needed for a Social Security Disability evaluation. With a small amount if research I was able to answer their questions but this brought up an interesting point and that is that despite the number of patients we see every year my lab only rarely performs any pulmonary function testing for disability evaluations. The reason I know this is because the Social Security Administration (SSA) has very specific requirements for the content and form of pulmonary function reports and we are very rarely asked for these reports.

The pulmonary function tests the SSA uses as part of a disability evaluation are:

  • Spirometry
  • Diffusing Capacity (DLCO)
  • ABG
  • Pulse Oximetry

Interestingly, lung volume measurements are not included. This is not specifically explained but it appears to be because evaluation for restriction is covered by the criteria for FVC and FEV1.

For all pulmonary function tests the SSA requires that the individual be medically stable, which they define as not:

  • Within 2 weeks of a change in prescribed respiratory medication.
  • Experiencing, or within 30 days of completion of treatment for, a lower respiratory tract infection.
  • Experiencing, or within 30 days of completion of treatment for, an acute exacerbation of a chronic respiratory disorder.
  • Hospitalized, or within 30 days of a hospital discharge, for an acute myocardial infarction (heart attack).

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CPT Codes

About a month or so ago I was corresponding with the manager of a small PFT lab and in response to one of their questions I had mentioned that there were no CPT codes for MIP/MEP. They responded with “what’s a CPT code?” so I guess this means that CPT codes aren’t as well known as I thought they were.

CPT stands for Current Procedural Terminology and is managed by the American Medical Association. CPT codes are a relatively universal way to classify and describe all medical tests and procedures. They are also used by all insurance companies for medical billing so one downside to this is if there isn’t a CPT code for a test or a procedure, you can’t bill for it. CPT codes also include conditions that limit performing (or at least billing for) some tests in various combinations and to some extent this drives the way PFT tests are ordered and performed.

The CPT codes are reviewed, revised and updated annually. There have been a number of additions and changes to PFT CPT codes during the last five to ten years, and I’d say that with a few notable exceptions, most current PFT testing is adequately covered by the CPT codes. The current PFT CPT codes are:

CPT: Description: Exclusions:
94010 Spirometry, including graphic record, total and timed vital capacity, expiratory flow measurement(s), with or without maximum voluntary ventilation. Do not report in conjunction with 94150, 94200, 94375, 94728.
94011 Measurement of spirometry forced expiratory flows in an infant or child through 2 years of age
94012 Measurement of spirometry forced expiratory flows, before and after bronchodilator, in an infant or child through 2 years of age.
94013 Measurement of lung volumes (i.e., functional residual capacity (FRC); forced vital capacity (FVC), and expiratory reserve volume (ERV) in an infant or child through 2 years of age.
94014 Patient-initiated spirometry recording per 30 day period of time; includes reinforced education, transmission of spirometry tracing, data capture, analysis of transmitted data, periodic recalibration and review and interpretation by a physician or other qualified health professional.
94015 [patient-initiated spirometry] recording (includes hook-up, reinforced education, data transmission, data capture, trend analysis, and periodic recalibration).
94016 [patient-initiated spirometry] review and interpretation only by a physician or other qualified health professional.
94060 Bronchodilator responsiveness, spirometry as in 94010, pre- and post-bronchodilator administration. Do not report in conjunction with 94150, 94200, 94375, 94728. For prolonged exercise test for bronchospasm with pre- and post-spirometry use 94620.
94070 Bronchspasm provocation evaluation, multiple spirometric determination s as in 94010, with administered agents (eg. antigen(s), cold air, methacholine).

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Another post-BD FVC conundrum

Okay, this may be wrong but at the moment I’m can’t seem to find a reason why it should be. A report like this came across my desk a couple of days ago.

Observed: %Predicted: Post-BD: %Predicted: %Change:
FVC: 4.59 94% 4.87 100% +6%
FEV1: 3.38 89% 3.58 94% +6%
FEV1/FVC: 73.6 95% 73.5 95% 0

Not particularly unusual and it would usually be interpreted as being within normal limits without a significant post-BD change. If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction. But you’re not supposed to use the post-BD FVC this way, are you?

Well, why not?

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When is it an expiratory plateau?

Over the last couple of weeks I’ve had an unusual number of patients with expiratory plateaus on their flow-volume loops. Expiratory plateaus are usually considered to be a sign of an intrathoracic central or upper airway obstruction and several of these patients had a diagnosis of tracheomalacia but many of them didn’t. Expiratory (and inspiratory) plateaus are mentioned in the ATS/ERS standards for interpretation but since there isn’t a specific definition (other than “plateau”), an expiratory plateau is a “know it when you see it” sort of thing.

The word plateau tends to imply that the flow-volume loop is both flat and level. Most textbook examples of an expiratory plateau tend to show a flow-volume loop that has been perfectly truncated, usually something like this:

or this:

but it usually isn’t that simple. An expiratory plateau is a consequence of a flow limitation, but during a forced exhalation the diameter of the airways decreases as the lung volume decreases from TLC towards RV. Depending on what is causing the flow limitation the plateau isn’t necessarily flat or level.

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Normal or obstruction?

I had finished reviewing a pre- and post-BD spirometry report yesterday and was about to toss it on my out pile when I noticed something a bit odd about the post-BD results. I pulled it back and spent some time trying to decide if the interpretation needed to be changed but after a lot of internal debate I finally let it go as it was. I’ve continued to think about it however, and although I’m not sure that was the right decision I still haven’t come up with a clear answer.

Here’s what I saw:

Observed: %Predicted: Post-BD: %Predicted: %Change:
FVC: 3.70 97% 3.91 103% +6%
FEV1: 2.82 94% 2.79 93% -1%
FEV1/FVC: 76 95% 71 89% -6%
PEF: 6.62 94% 7.19 102% +9%
Exp. Time: 10.92 11.15

The reported pre-BD and post-BD results were from good quality tests and met the criteria for repeatability. My problem is that the baseline results were normal but if I had seen the post-BD results by themselves I would have considered them to show mild airway obstruction.

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An unusual error in helium dilution lung volumes

Recently I was reviewing a report that included helium dilution lung volumes. What caught my eye was that the TLC and the FRC didn’t particularly fit in with the results from the other tests the patient had performed.

Test: Observed: %Predicted:
FVC: 2.83 114%
     
TLC: 3.03 71%
FRC: 0.88 39%
RV: 0.09 5%
SVC: 2.93 118%
     
VA: 3.64 88%

When compared to the FVC and the VA (from the DLCO test) the lung volumes are significantly lower. In particular the FRC and RV are markedly reduced. This is somewhat unusual for helium dilution lung volume since most errors usually cause FRC, RV and TLC to be over-estimated instead of being under-estimated. When I checked the other reports for the day I found that two other patients that had had their lung volumes measured on the same test system also had a TLC, FRC and RV that were noticeably reduced. Obviously we had some kind of equipment problem with that test system but it took a bit of sleuthing before I found out what had happened.

Like all lung volume tests, the helium dilution technique produces a lot of numbers, most of which are not included on the report. One of the first things I did was to call up the within-test data (our test systems store data every 15 seconds during the test and re-calculate FRC each time).

Time: FRC, Liters He conc. (%) Ve (L./min.) Vt, Liters
0:15 -1.00 9.71 6.16 0.21
0:30 0.06 8.87 10.1 0.59
0:45 0.43 8.61 11.76 0.78
1:00 0.69 8.44 9.05 0.72
1:15 0.76 8.39 8.18 0.74
1:30 0.79 8.37 8.32 0.59
1:45 0.82 8.36 8.15 0.62
2:00 0.83 8.35 7.79 0.65
2:15 0.86 8.33 5.51 0.62
2:30 0.87 8.32 5.34 0.63
2:45 0.88 8.32 0 0

When looking at this it was immediately evident there was a problem because the initial FRC was negative and this shouldn’t be possible. About the only way that helium dilution lung volumes can normally be underestimated is if the test is terminated way too early and the negative FRC ruled this out. It also narrows down the possible problems, but I had to think for a while and in doing so had to go back to the basics of the helium dilution test.

Helium dilution used to be the most common method for measuring lung volumes, but it requires a closed-circuit test system with a volume displacement spirometer. Most current test systems are open-circuit flow sensor-based systems and lung volumes are usually measured by nitrogen washout (or by plethysmography). Nevertheless, there are a couple of closed-circuit systems still being manufactured and there are a fair number of these systems still in service.

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2017 ERS DLNO standards

The European Respiratory Society has just published the first standards for DLNO testing. This is a signal that DLNO is moving from a research setting into routine clinical testing. Although it is unlikely that most PFT labs will immediately jump into DLNO testing, the standard is still interesting because of an extensive discussion of DLNO, DLCO, DMCO and Vc measurements and physiology. The DLNO standards (and their supplementary material) are open-access and can be downloaded from the European Respiratory Journal.

DLNO is performed in the same manner as a single-breath DLCO and it is specifically recommended that DLCO and DLNO tests be performed simultaneously. There are however, specific test system requirements based both on the properties of NO and on the two types of NO analyzers:

  • Nitric Oxide reacts with oxygen to form NO2 and at the levels used for DLNO testing (40-60 ppm) does so at a rate of approximately 1.2 ppm per minute. DLNO test gas is therefore usually stored as 400-1200 ppm NO in N2 and mixed into the DLCO test gas mixture (0.3% CO, 21% O2) ≤2 min before the DLCO/DLNO test. This would seem to require that the DLCO/DLNO test gas mixture to be held in a reservoir of some kind and to preclude the use of a demand valve but this was not specifically discussed. Because of uncertainties that occur when mixing the DLCO/DLNO gas mixture and in how long the mixture may be held in the reservoir the inspired NO concentration must also be measured immediately before the DLCO/DLNO test is performed.
  • The type of NO gas analyzer will determine how the expiratory gas concentrations are measured. Chemiluminescent analyzers usually have a response time on the order of ≤70 msec, and for these reasons can be used to perform a real-time analysis of exhaled air. Chemiluminescent analyzers are expensive however, and can add significantly to the cost of a test system. Electrochemical cells are significantly less expensive but have a response time on the order of 10 seconds and are therefore suitable only to test systems that mechanically collect an alveolar sample.

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Some DLCO errors the 2017 standards will probably fix

Last week I ran across a couple errors in some DLCO tests that I don’t remember seeing before, or at least not as distinctly as they appeared this time. If I hadn’t been looking carefully I could have missed them but both sets of errors will be a lot more evident when the 2017 ERS/ATS DLCO standards are implemented.

The first error has to do with gas analyzer offsets. What alerted me was a set of irreproducible DLCO results.

Test 1: Test 2: Test 3: Test 4:
DLCO (ml/min/mmHg): 24.53 17.21 12.91 6.74
Inspired Volume: 1.99 2.06 2.32 2.26
VA (L): 3.83 3.52 3.63 2.60
Exhaled CH4: 43.27 49.19 54.80 74.14
Exhaled CO: 16.09 23.15 31.39 49.46

When I first looked at the graphs for each test, there wasn’t anything particularly evident until I pulled up the graph for the fourth DLCO test:

This graph showed that the baseline CH4 and CO readings were significantly elevated, but this hadn’t been evident in the previous tests.

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What’s the frequency, plethysmograph?

Once again we’ve had some staff turnover. Rightly or wrongly, the pattern we follow in staffing the lab is to hire people with a science degree and then train them ourselves. Our hires are usually interested in a career in medicine but often haven’t decided what specifically interests them. We look for individuals with people skills on top of their education and ask for a minimum of a year’s commitment with the requirement that they get their CPFT certification by the end of the year. Sometimes our staff only stays a year, sometimes a couple years, and most of the time when they leave they go back to college for a more advanced degree and become nurses or physician assistants and occasionally even physicians (a couple of our pulmonary fellows were former PFT lab alumni).

We do this mostly because it’s very hard to find anybody with prior experience in pulmonary function testing. I’d like to say this is a recent occurrence but realistically it’s been this way for decades. One of the reasons for this is that there are no college level courses on pulmonary function testing. Although the training programs for respiratory therapists often include some course work on PFTs this is almost always a one semester lecture course with no hands-on training (when it is included at all).

Another reason is that trained individuals often do not stay in this field. This is partly because there isn’t much of a career path since the most you can usually aspire to is being a lab manager but even then I know of many small PFT labs where the manager is somebody outside the field such as a nurse or administrator with no experience in pulmonary function testing so often that isn’t even an option. Another reason though, is that the PFT Lab pay scale, although adequate, is often noticeably less than other allied health professions such as radiology techs, ultrasound techs and sleep lab techs.

Anyway, the downside of this hiring pattern is that it seems like we’re always hiring and training new staff (however untrue that may actually be). We do have a fairly good training program however, so new staff usually come up to speed and become reasonably productive in a short period of time. Even so, it takes at least a year before a new technician is reasonably proficient not just in performing the tests, but in understanding the common testing problems and errors. This is at least one reason why I spend much of my time reviewing raw test data and sending annoying emails to the lab staff.

It also means that we frequently revisit basic testing issues.

Recently, a report with a full panel of tests (spirometry, lung volumes, DLCO) came across my desk. The patient had had a full panel a half a year ago and when I compared the results between the two sets of tests there had been no significant change in FVC, FEV1 and DLCO but the TLC was over a liter higher than it had been last time.

Jan, 2017 June, 2016
Observed: %Predicted: Observed: %Predicted:
FVC: 2.04 85% 2.38 97%
FEV1: 0.58 32% 0.62 34%
FEV1/FVC: 28 38% 26 36%
TLC: 7.27 152% 6.10 126%
FRC: 6.16 222% 4.83 174%
DLCO: 8.12 51% 8.91 55%

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