Making Assumptions about TGV and FRC

When lung volumes are measured in a plethysmograph the actual measurement is called the Thoracic Gas Volume (TGV). This is the volume of air in the lung at the time the shutter closes and the subject performs a panting maneuver. Ideally, the TGV measurement should be made at end-exhalation and should be approximately equal to the Functional Residual Capacity (FRC). For any number of reasons in both manual and automated systems this doesn’t happen and the point at which the TGV is measured is either above or below the FRC.

Testing software usually corrects for the difference in TGV and FRC by determining the end-exhalation baseline that is present during the tidal breathing at the beginning of the test. Using this value the software can determine where the TGV was measured relative to the tidal breathing FRC and then either subtracts or adds a correction factor to derive the actual FRC volume.

One problem with this is that leaks in either the subject or the mouthpiece and valve manifold can occur during the panting maneuver and the end-exhalation baseline can shift and this will affect the calculation of RV and TLC. I’ve discussed this previously and as a reminder, RV is calculated from:

RV = [average FRC] – [average ERV]

where the FRC is determined from the corrected TGV and ERV is determined from SVC maneuvers. TLC is then calculated from:

TLC = RV + [largest SVC]

When the post-shutter FRC baseline shifts upwards (higher lung volumes relative to the pre-shutter FRC):

ERV is underestimated, which in turn causes both RV and TLC to be overestimated. When the post-shutter FRC baseline shifts downwards (lower lung volumes relative to the pre-shutter FRC):

ERV is overestimated, which in turn causes both RV and TLC to be underestimated.

I’ve been aware of this problem for quite a while and use this as a guideline when selecting the FRCs and SVCs from specific plethysmograph tests. All of these assumptions are based on the fact that FRC is derived from the pre-shutter end-exhalation tidal breathing. Well, you know what they say about assuming…

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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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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:


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What’s abnormal about FRC?

I’ve had a number of reports across my desk in the last couple of weeks with both elevated and reduced FRC’s that were associated with a more-or-less normal TLC. I reviewed the raw data from all of these tests (I review the raw data from all lung volume tests) and in only a few instances did I make any corrections to the report. This made me think however, about what, if anything, is an abnormal FRC trying to tell us?

The answers to that question range from “a whole bunch” to “not much” to “darned if I know”. When you measure lung volumes TLC is really the only clinically important result. RV can be useful at times but although the other lung volume subdivisions may play a role in the measurement process they have only a limited diagnostic value. All lung volume measurements start with FRC, however, and if you don’t know you have an accurate FRC how do you know that TLC is accurate?

FRC is a balance point of opposing forces in the lung and thorax. Lung tissue wants to collapse, the rib cage wants to spring open and the diaphragm wants to do whatever muscle tone, gravity and the abdomen allows it to do. All of these forces are to one extent or another dynamic and can change over time. These changes can occur both slowly and rapidly, and are the primary reason why isolated changes in FRC don’t tend to have a lot of clinical significance. For all lung volume measurements however, one primary assumption is that FRC does not change during the test and this isn’t necessarily true.

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