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

IC, ERV and the FVC

While reviewing reports today I ran across a couple of lung volume tests from different patients where the SVC was over a liter less than the FVC. Suboptimal SVC measurement can affect both the TLC and the RV and in one case the TLC was slightly below normal (78% of predicted) and in the other the TLC was within normal limits but the RV was over 150% of predicted. Both patients had had lung volume measurements previously and the current TLC was significantly different than it had been before.

I seem to run across this problem at least once a week so I am reasonably used to making manual corrections. I’ve discussed this previously but basically I use the position of the tidal loop within the maximal flow-volume loop obtained during spirometry to determine IC and ERV and then re-calculate TLC and RV accordingly.

fvl_tvl_4

Anyway, for this reason I had tidal loops, and IC and ERV on my mind while I was reviewing other reports. Shortly after this I came across a report that had “fair FVC test quality and reproducibility” in the tech notes so I pulled up the raw spirometry test data and took a closer look.

What I found was that the patient had performed five spirometry efforts and that the FVC and FEV1 was different on each test. All five spirometry efforts met the ATS/ERS criteria for back-extrapolation, expiratory time and end-of-test flow rates. I clicked back and forth between the different spirometry efforts to make sure the right FVC and FEV1 had been selected and when I did I noticed that the position of the tidal loop was shifting left and right and that the closer it was to TLC, the lower the FVC and FEV1 were and vice versa.

fvl_tvl_1

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Does the FEV1/SVC ratio over-diagnose airway obstruction?

A low FEV1/VC ratio is the primary indication for airway obstruction.

ATS_ERS_Interpretation_Algorithm

From ATS/ERS Interpretive Strategies for Lung Function tests, page 956.

The ATS/ERS statement on interpretation says

The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function (fig. 2). Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”

I understand and in general agree with the idea of using the largest VC regardless of where it comes from and this is because the FVC is often underestimated for any number of good (and not so good) reasons. When this happens the FEV1/FVC ratio will be overestimated and airway obstruction will be under-diagnosed. However the ATS/ERS statement is also grounded in the notion that all vital capacities (FVC, SVC, IVC) are the same and this isn’t necessarily true. The problem comes from the fact that the predicted values and lower limit of normal (LLN) for the FEV1/VC ratio always come from reference equations for FEV1/FVC ratios. Because the SVC (and IVC) are usually larger than the FVC this means there is at least the potential for airway obstruction to be over-diagnosed.

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COPD and the FEV1/FVC ratio. GOLD or LLN?

Everyone uses the FEV1/FVC ratio as the primary factor in determining the presence or absence of airway obstruction but there are differences of opinion about what value of FEV1/FVC should be used for this purpose. Currently there are two main schools of thought; those that advocate the use the GOLD fixed 70% ratio and those that instead advocate the use the lower limit of normal (LLN) for the FEV1/FVC ratio.

The Global Initiative for Chronic Obstructive Lung Disease (GOLD) has stated that a post-bronchodilator FEV1/FVC ratio less than 70% should be used to indicate the presence of airway obstruction and this is applied to individuals of all ages, genders, heights and ethnicities. The official GOLD protocol was first released in the early 2000’s and was initially (although not currently) seconded by both the ATS and ERS. The choice of 70% is partly happenstance since it was one of two fixed FEV1/FVC ratio thresholds in common use at the time (the other was 75%) and partly arbitrary (after all why not 69% or 71% or ??).

The limitations of using a fixed 70% ratio were recognized relatively early. In particular it has long been noted that the FEV1/FVC ratio declines normally with increasing age and is also inversely proportional to height. For these reasons the 70% threshold tends to over-diagnose COPD in the tall and elderly and under-diagnose airway obstruction in the short and young. Opponents of the GOLD protocol say that the age-adjusted (and sometimes height-adjusted) LLN for the FEV1/FVC ratio overcomes these obstacles.

Proponents of the GOLD protocol acknowledge the limitation of the 70% ratio when it is applied to individuals of different ages but state that the use of a simple ratio that is easy to remember means that more individuals are assessed for COPD than would be otherwise. They point to other physiological threshold values (such as for blood pressure or blood sugar levels) that are also understood to have limitations, yet remain in widespread use. They also state that it makes it easier to compare results and prevalence statistics from different studies. In addition at least two studies have shown that there is a higher mortality of all individuals with an FEV1/FVC ratio below 70% regardless of whether or not they were below the FEV1/FVC LLN. Another study noted that in a large study population individuals with an FEV1/FVC ratio below 70% but above the LLN had a greater degree of emphysema and more gas trapping (as measured by CT scan), and more follow-up exacerbations than those below the LLN but above the 70% threshold.

Since many of the LLN versus GOLD arguments are based on statistics it would be useful to look at the predicted FEV1/FVC ratios in order to get a sense of how much under- and over-estimation occurs with the 70% ratio. For this reason I graphed the predicted FEV1/FVC ratio from 54 different reference equations for both genders and a variety of ethnicities. Since a number of PFT textbooks have stated that the FEV1/FVC ratio is relatively well preserved across different populations what I initially expected to see was a clustering of the predicted values. What I saw instead was an exceptionally broad spread of values.

Male_175cm_Predicted

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An FVC is not an SVC

I’ve discussed the issue of inserting a predicted FVC into the predicted lung volumes several times now. At the risk of beating this issue to death I’d like to put to rest the notion that an FVC and an SVC are the same thing.

A Forced Vital Capacity (FVC) maneuver is designed to measure the maximum expiratory flow rates, in particular the expired volume in 1 second (FEV1). It has long been recognized that the effort involved in the FVC maneuver can cause early airway closure, even in individuals with normal lungs, and that for this reason the vital capacity can be underestimated due to gas trapping. This effect is usually magnified with increasing age and in individuals with obstructive lung disease.

A Slow Vital Capacity (SVC) maneuver is designed to measure the lung volume subdivisions Inspiratory Capacity (IC) and Expiratory Reserve Volume (ERV), and to maximize the measured volume of the vital capacity. Due to the more relaxed nature of the SVC maneuver there is significantly less airway closure and for this reason the SVC volume is usually larger than the FVC, again even in individuals with normal lungs.

Comparing individual reference equations can be difficult but in general the reference equations for SVC and FVC agree with this. Taking the available SVC and FVC reference equations (unfortunately limited to Caucasian because there are almost no SVC equations for other ethnicities) it is apparent that the average predicted SVC is larger than the average predicted FVC, and that the magnitude of this difference increases with age:

SVC_vs_FVC_Male

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The importance of an earnest SVC

A report came across my desk today and at first glance it looked fairly straightforward. There was a mildly reduced TLC and FVC, and although the SVC was slightly lower than the FVC it looked like this patient had mild restriction.

Observed: %Predicted: Predicted:
FVC: 1.73 68% 2.56
FEV1: 1.23 65% 1.89
FEV1/FVC: 71 97% 73
TLC: 3.58 73% 4.89
FRC: 2.07 75% 2.78
RV: 1.94 83% 2.33
RV/TLC: 54 114% 48
SVC: 1.69 66% 2.56
IC: 1.51 72% 2.11
ERV: 0.13 30% 0.45

In addition, the flow-volume loop looked fairly typical for restriction, with a normal peak flow and a reduced volume.

SVC_TLC_Under_FVL_redacted

When I looked at the DLCO results however, I suddenly got a different picture. Specifically, the VA from the DLCO was larger than the TLC and the inspired volume (Vinsp) was significantly larger than both the FVC and the SVC.

Observed: %Predicted: Predicted:
DLCO: 13.51 83% 16.23
VA: 3.87 82% 4.73
Vinsp: 2.26

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The ERV Effect

I used to think that spirometry and diffusion capacity tests were hard and that lung volumes were easy. That may have been true in terms of getting patients to do the tests but I’ve long since come to the conclusion that it is easier to assess the quality of spirometry and diffusing capacity tests and know whether you have reasonably accurate results than it is to do this for lung volumes regardless of which lung volume measurement technique you use.

I was reviewing a set of plethysmographic lung volume tests when I noticed something very odd about the reported results. I usually look at just the VTG loops and the volume-time graphs in order to assess test quality. The testing software automatically selects and averages all VTG efforts and when I reviewed them there were a couple loops that were poor quality and I manually de-selected them. I was reviewing this report because the reported lung volume results didn’t quite match what the spirometry results were saying so this time I also took a close look at the numbers after I removed the low-quality loops. That’s when I realized that the reported TLC was larger than the two tests it was averaged from.

Pleth Math

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SVC and the FEV1/FVC Ratio

The Slow Vital Capacity (SVC) maneuver is usually performed as part of lung volume measurements. It is not unusual for the SVC to be larger than the FVC, particularly in patients with airway obstruction. This can have a bearing on the FEV1/FVC ratio and in fact the ATS-ERS recommendations for PFT interpretation say that the largest vital capacity value regardless of which test it comes from should be used to calculate the FEV1/VC ratio. When I review a full panel of tests (FVC, lung volumes, DLCO) I always check to see if the SVC or IVC (from the DLCO test) are larger than the FVC and then re-calculate the FEV1/FVC ratio and its percent predicted if they are. Test results that at first glance look normal will instead show airway obstruction often enough when this has been done that the time spent going through this process is worthwhile.

This only works however, when I have a full panel of tests to extract other vital capacities from. Patients that show airway obstruction when their FEV1/VC ratio is re-calculated have often had only spirometry performed on prior visits and their spirometry results were considered to be within normal limits at those times. Our lab software lets us select and report the “best” FVC and FEV1 from a series of spirometry efforts so this raises an interesting question and that is when and how often should a SVC maneuver be performed instead of a FVC maneuver during a spirometry session in order to get and report the largest VC?

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