Sharing opinions with Paul Enright

SDr. Paul Enright is a well-known name in the field of Pulmonary Function testing. He is the lead author or co-author of over a hundred articles and has served on many of the ATS/ERS standards committees.


We both retired in southern Arizona and live a couple of towns apart from each other. We have corresponded for a while but met face-to-face only recently. We both drive small red vehicles, Richard a Ford Transit Van and Paul a Prius Compact. We both love to visit National Parks; Richard’s favorite is Canyonlands while Paul’s favorite is Jasper, with many large wild animals. This posting is based on a set of suggestions by Paul.

In which hospital-based PFT labs have you worked?

Richard: St. Elizabeth’s then Beth Israel Deaconess Medical Center, both in Boston.

Paul: I started a very small PFT lab at the Kuakini Hospital in Honolulu; then the basement lab of the National Jewish Hospital in Denver, Colorado; then the Plummer Building of the Mayo Clinic in Rochester Minnesota; then the University Medical Center in Tucson, Arizona; then a NIOSH van running out of Morgantown, West Virginia.

Which is the largest PFT lab that you ever visited?

Richard: the PFT Lab at Mass General in Boston.

Paul: INER in Mexico City, where they test more than 10,000 patients per year. The medical director of the lab is my friend Laura G. One year a guard with a shotgun stood outside the lab because the payroll with bonuses for the institution was stolen the previous month (December).

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N2 washout is affected by N2 excretion and other factors

The Lung Clearance Index (LCI) was first described in 1952 by Margaret Becklake, and is defined as the number of lung volume turnovers required to reduce the concentration of a tracer gas by a factor of 40. LCI is calculated as the cumulative exhaled volume (CEV) during the washout divided by the functional residual capacity (FRC).

Clinically LCI has been used most often in individuals with cystic fibrosis and this is because the LCI has been repeatably shown to be sensitive to changes in airway status that are not reflected in the FEV1. LCI has shown similar results in patients with primary ciliary dyskinesia. As expected LCI has also been tested on patients with COPD, bronchiectasis and asthma although these patients tend to show a better correlation between FEV1 and LCI.

LCI has been performed using a wide variety of tracer gases including helium, methane, argon, nitrogen and sulfur hexaflouride (SF6). The commercial systems that are currently available use either N2 or SF6. N2 washout LCI has recently received a great deal of criticism and some of these criticisms seem to apply to N2 washout lung volumes as well.

Most specifically, a number of studies have noted that the N2 washout FRC is routinely higher than the SF6 FRC and plethysmographic FRC. In addition, the N2 washout LCI tends to be significantly higher than the SF6 LCI and this difference increases as LCI increases.

As examples in a study of patients with COPD the N2 washout FRC averaged 14% higher than the plethysmographic FRC. In other studies of normal subjects the N2 washout FRC was on average 0.20 to 0.21 L higher than plethysmographic FRC. Finally, a study that performed N2 and SF6 washouts simultaneously on CF patients and healthy controls showed the N2 washout LCI to be on average 7.93% higher than SF6 in the healthy controls and 29.13% higher than SF6 in the CF patients. In the same study N2 washout FRC was 12.66% higher than SF6 FRC in the healthy controls and 30.09% higher than SF6 FRC in CF patients.

So why is there such a discrepancy?

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FEV1 and VC should be measured separately

The FEV1 and VC both provide quite different information about a patient’s lungs. Unfortunately, spirometry as it is currently practiced is optimized towards generating an accurate FEV1 more than an accurate VC. This is partly due to limitations in the maneuver itself and partly due to the lack of accurate end-of-test criteria for an adequate VC. In one sense this is okay since more than one person that I’ve known and respected has said that “it’s all about the FEV1”.

Having said that, an accurate FEV1/VC ratio is essential for detecting and quantifying airway obstruction and an SVC maneuver is more likely to obtain a more accurate VC. This matters because the current ATS/ERS spirometry guidelines recommend that the FEV1/VC be reported, where the VC is the largest value obtained from any test and reference equations indicate that the SVC is routinely larger than the FVC:

So, shouldn’t we be routinely performing both FVC and SVC maneuvers when we do spirometry on our patients? And why aren’t we?

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The chemo did a real number on me. I was included in a study of a new drug and it literally almost killed me. Twice. For this reason I have left the study and have been put on a gentler and more normal regime of chemo. I have lost a lot of stamina (and strength and weight) but have recovered somewhat. I have returned to catch up on comments as best I can and apologize for not responding for so long.

If possible I will post something new but it will probably be a while before I feel up to doing so.

Thanks to everybody for their kind and supportive thoughts.


This is probably the last post I will be able to write.  I was diagnosed less than two months ago with a very nasty cancer with a poor prognosis.  I thought I could power through it but a serious infection with sepsis and a week and a half stay in the hospital has convinced me that it’s time to quit and focus on other things.

Sucks, but that’s life.

There are many pulmonary function topics I would have liked to discuss but time has run out.  I will leave you to ponder the two biggest elephants in the room; that of height and that of ethnicity.  The relationship between height and FVC, TLC, etc. is inexact and yet nobody seems to think about any alternate anthropomorphic measurements.  Sitting height is only marginally better but it is better.  Is anybody using it?  No.  C’mon people, it’s way past time that we found better anthropomorphic correlations for FVC, FEV1, TLC and DLCO.

And what the heck is ethnicity?  Where is there a definition for it?  Although I applaud the GLI efforts for more universal FVC reference values they included fudge factors for ethnicity.  Fudge factors!!??.  It’s time that the concept of ethnicity was dropped and better (see above) anthropomorphic correlations were made.

Keep learning.  Keep questioning.



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A modest proposal for a clinical spirometry grading system

A while back I reviewed the spirometry grading system that was included in the 2017 ATS reporting standards. My feeling was, and continues to be, that its usefulness is very limited because it’s mostly a reproducibility grading system that relies on a few easy-to-measure parameters. This doesn’t mean that a grading system can’t be helpful, just that it needs to be focused differently.

In a clinical PFT lab many patients have difficulty performing adequate and reproducible spirometry, but that doesn’t mean the results aren’t clinically useful. Moreover, suboptimal quality results may be the very best the patient is ever able to produce. So what’s more important in a grading system than reproducibility is the ability to assess the clinical utility of a reported spirometry effort.

The two most important results that come from spirometry are the FEV1 and the FVC, and I strongly believe that they need to be assessed separately. For each of these values there are two aspects that need to be determined. First, is there a reliable probability that the reported value is correct? Second, are any errors causing the reported value to be underestimated or overestimated? The two are inter-related since a value with excellent reliability is not going to have any significant errors, but if there are errors then a reviewer needs to know which direction the result is being biased.

The current ATS/ERS standards contain specific thresholds for certain spirometry values such as expiratory time and back-extrapolation. Although these are certainly indications of test quality they are almost always used in a binary [pass | fail] manner. In order to assess clinical usefulness however, you instead need to grade these on a scale. For example an expiratory time of 5.9 seconds for spirometry from a 60 year-old individual would mean that there is a small probability that the FVC is underestimated, but with an expiratory time of 1.9 seconds the FVC would have a very high probability of being underestimated and this needs to be recognized in order to assess clinical utility.

Note: Although the A-B-C-D-F grading system is rather prosaic it is still universally understandable, so I will use it for grading reliability. An A grade or an F grade are probably easy to assign but differentiating between B-C-D may be more subjective, particularly since reliability depends on multiple parameters and judging their relative contribution is always going to be subjective at some point. For bias, I will be using directional characters (↑↓) to show the direction of the bias (i.e. positive or negative), so ↑ will indicate probable overestimation, ↓ will indicate probable underestimation, and ~ indicates a neutral bias.

FEV1 / Back extrapolation:

Back-extrapolation is a way to assess the quality of the start of a spirometry effort and the accuracy of the timing of the FEV1. The ATS/ERS statement says that the back-extrapolated volume must be less that 5% of the FVC or less than 0.150 L, whichever is greater.

My experience is that an elevated back-extrapolation tends to cause FEV1 to be overestimated far more often than underestimated. So a suggested grading system for back-extrapolation would be (and I’ll be the first to admit these are off the top of my head and open for discussion):

Back-Extrapolation: Reliability: Bias:
Within standards: A ~
> 1 x standard, < 1.5 x standard: B
> 1.5 x standard, < 2 x standard C ↑↑
> 2 x standard, < 2.5 x standard: D ↑↑↑
> 2.5 x standard F ↑↑↑↑

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Infection Control

The issue of infection control has been a topic of a couple of discussions I’ve had lately. In particular, it was reported to me that a PFT lab had come under fire from a Joint Commission inspector who did not believe that filter mouthpieces were adequate and that “patient valves and circuits need to be sterilized between each patient”.

Unfortunately with all the other things we have to worry about it’s all too easy to become blasé about infection control. This despite the fact that every hospital I’ve visited in the last dozen or so years has posted numerous signs about hand washing and the safe disposal of contaminated supplies. But maybe it’s because we’re inundated with reminders that we’ve developed a blind spot about it.

The 2005 ATS/ERS statement on general considerations has two pages devoted to infection control (pages 155-157). The ATS procedure manual also has four pages devoted to infection control (pages 34-38), although much of this is devoted to a discussion of tuberculosis, cystic fibrosis and sterilization procedures. Of necessity, the ATS/ERS statement and ATS procedure manual discuss infection control in generalities and any given lab will need to have a policy tailored for their specific circumstances. Even so, either or both of these (as well as Kendrick et al’s 2003 review) should be the basis for your lab’s policy on infection control (and you do have one, don’t you?).

So what are the issues?

Diseases can be transmitted by direct contact (saliva) or indirect contact (airborne particles). PFT Labs need to prevent cross-transmission of diseases by the use of barrier devices (gloves, filter mouthpieces) and proper cleaning procedures.

So yeah, it’s as simple as that, but as usual the devil is in the details and in particular there are trade-offs between expense, time and efficacy. Continue reading

Is gas trapping more common than we think it is?

Over the last couple of years I’ve run across a number of test systems that do not include tidal loops along with the maximal flow-volume loop. I’ve wondered why this was done and because of this I’ve thought a lot about tidal flow-volume loops and what additional information, if any, they add to spirometry interpretation.

One of my thoughts has been about the relationship between obesity and the IC and ERV. FVC and TLC are often reasonably preserved even with relatively severe obesity. FRC, on the other hand, is often noticeably affected with even minor changes in BMI (and interestingly this applies to reduced as well as elevated BMI’s). When FRC decreases because of obesity the IC usually increases and the ERV decreases and for this reason the IC/ERV ratio has been suggested as a way to monitor changes in FRC without having to actually measure lung volumes.

IC and ERV are not measured as part of spirometry but the position of the tidal loops gives at least a general indication of their magnitude and I’ve noticed that there’s a moderately good correlation between BMI and the position of the tidal loop.

With this in mind, I see up to a dozen reports a week with restrictive-looking spirometry (i.e. symmetrically reduced FVC and FEV1 with a normal FEV1/FVC ratio) on patients with a diagnosis of asthma. This is nothing new and there have probably been at least 10 articles in the last decade about the Restrictive Spirometry Pattern (RSP). Interpreting these kinds of spirometry results is always problematic, particularly when there are no prior lung volume measurements to rule-in or rule-out restriction. I’ve noticed however, that patients with a restrictive spirometry pattern almost always have the tidal loop on the far right-hand side of the flow-volume loop (zero or near zero ERV). For example:

Observed: %Predicted:
FVC: 1.65 74
FEV1: 1.21 73
FEV1/FVC: 73 100

But there doesn’t seem to be any relationship between this observation and the patient’s BMI and in fact, this is seen even when BMI is normal or somewhat reduced. Continue reading

Telling the right story

The 2005 ATS/ERS spirometry standard make it permissible and even recommends that the FVC and FEV1 be selected from different efforts. I disagree somewhat with their criteria for selecting the FEV1 but overall reporting composite results makes a lot of sense. In an ideal world we’d always get the best FVC and FEV1 in a single effort but what we more often get is a good FEV1 with a poor FVC or a poor FEV1 with a good FVC. So, it best serves the clinical needs of the patient to report the best elements from multiple spirometry efforts.

However, I was disappointed that the 2017 ATS reporting standards did not in any way address how to indicate that composite results are being reported, nor does it resolve the selection of the flow-volume loops and volume-time curves that accompany the numerical results. That leaves it to us to decide how to do this but this in turn is often limited by the capabilities of our equipment’s software.

One test system that I routinely take to a free spirometry screening clinic will only report the three “best” efforts based solely on the largest combined FVC + FEV1. Admittedly, to some extent this follows the 2005 ATS/ERS spirometry standards selection criteria but other than deleting a specific test effort I cannot override these selections nor can I mix and match the FVC and FEV1 values. This means that what it reports as the “best” effort doesn’t always agree with what in reality are the best results.

My lab’s software however, allows us to select which test efforts the FVC and FEV1 come from. In addition we can select which test effort the ancillary measurements (Peak Flow, Expiratory Time, FIVC, FEF50, etc.) and which effort the flow-volume loop and volume-time graphs comes from.

It is therefore possible to select the FVC, FEV1, ancillary measurements and the graphs from entirely different test efforts. Thankfully, this almost never done but when I review reports what I see most frequently is that the FVC is selected from one test effort, but the FEV1, ancillary measurements and graphs are selected from another. To some extent this makes sense because I’d usually agree that the Peak Flow should always be associated with the FEV1, and if that’s the case, then so should the flow-volume loop. The problem with this is that the FVC often comes from a test effort with a substantially longer expiratory time and when results are selected this the volume-time curve and expiratory time are instead reported for the effort the FEV1 came from.

This leads to a report that look like this:

Observed: Predicted: %Predicted:
FVC: 2.62 3.65 72%
FEV1: 2.01 2.58 78%
FEV1/FVC: 77 72 107%
Peak Flow: 8.83 6.73 131%
Exp. Time: 1.20

with graphs like:

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I’ve got the old back-extrapolation blues

A couple days ago I pulled my copy of the Intermountain Thoracic Society manual on pulmonary function testing off the bookshelf and thumbed through it a bit. It was first published in 1975 and was the first major attempt towards standardizing the performance and interpretation of PFTs.

My first thought was that we’ve come a long way since then. Most importantly our understanding of what spirometry can (and cannot) tell us has improved dramatically.

Equipment too, has advanced since 1975, most particularly due to the first equipment standards that were published in that decade. As a reminder, spirometer accuracy was not a given and there are number of studies dating from that time period that detailed just how woefully inaccurate many of them were.

In 1975 computerized spirometers were exceptionally rare and I was reminded of this because 141 pages (two-thirds!) of the ITS manual is filled with look-up tables for predicted values and ATPS – BTPS – STPD conversion factors.

Most spirometry systems were entirely manual and the majority of us measured FVC and FEV1 manually from pen tracings on kymograph paper. The results were then hand-calculated and then hand-written onto report forms. Since our equipment is so much more accurate and our computers acquire and calculate test results automatically, everything is so much better now, isn’t it?

Overall, I’d have to say yes. Testing is much quicker and more accurate than it used to be in 1975, and no, I’m not particularly nostalgic about those days.

{Arrrhh, gather round lads and lasses and let me tell you of the days when coal-fired steam-powered spirometers rumbled and hissed in basement labs everywhere; when you had to solve regression equations with your slide rule on the fly or risk the horror of ripped kymograph paper, exploding alveolar sample bags and spirometer bells gone ballistic without warning. The toll this daily physical and mental trauma took amongst the lowly pulmonary techs was terrifying and only the bravest continued the daily battle against gnarly patients, sneering doctors, black-hearted administrators and monopolistic manufacturers…

…Oops! Wrong time-line; those are memories from the universe one north and two left of ours. Too much steampunk sci-fi late at night and too little sleep left me momentarily confused}

I ran across an error today that reminded me that although computerized test systems are essential to our ability to run efficient and accurate labs, at the same time the limitations of software that comes along with them hinders our ability to detect and correct errors.

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