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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Flow-volume loops are timeless

Recently I’ve been trying to help somebody whose spirometry results changed drastically depending on where their tests were performed. When their spirometry was performed on an office spirometer their FVC was less than 60% of predicted and when they were performed in a PFT lab on a multi-purpose test system their FVC was closer to 90% of predicted. Part of the reason for this was that different predicted equations are being used in each location but even so there was about a 1.5 liter difference in FVC.

One important clue is that the reports from the office spirometer showed an expiratory time of around 2 to 2-1/2 seconds while the reports from the PFT lab showed expiratory times from 9 to 12 seconds. The reports from both locations however, only had flow-volume loops and reported expiratory time numerically. There were no volume-time curves so it isn’t possible to verify that the spirometry being performed at either location was measuring time correctly or to say much about test quality.

The shape of a flow-volume loop is often quite diagnostic and many lung disorders are associated with very distinct and specific contours. Volume-time curves, on the other hand, are very old-school and are the original way that spirometry was recorded. The contours of volume-time curves are not terribly diagnostic or distinctive and I suspect they are often included as a report option more because of tradition than any thing else. But volume-time curves are actually a critically important tool for assessing the quality of spirometry and one of the most important reasons for this is because there is no time in a flow-volume loop.

With this in mind, the following flow-volume loop came across my desk yesterday. The FVC, FEV1 and FEV1/FVC ratio were all normal and it was the best of the patient’s efforts.


The contour of this flow-volume loop is actually reasonably normal, except possibly for the little blip at the end.
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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.


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.


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When flow-volume loops get kinky

One of the more recognizable flow-volume loop contours is the one associated with severe airway obstruction. Specifically, this type of loop shows an abrupt decrease in flow rate following the peak flow with a more gradual decrease in flow rates during the remainder of the exhalation.


This abrupt decrease in flow rates was first described on a volume-time curve and the inflection point was called a “kink” but this point also corresponds with the inflection point on the flow-volume loop. This feature has also been called a “notch” or a “spike” but a number of researchers have called this the Airway Collapse pattern (AC) and it is more formally defined as a sharp decrease in flow rate from peak flow to a discontinuity point at less than 50% of the peak flow and occurring within the first 25% of the exhaled vital capacity.

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What’s a normal Flow-volume Loop?

Dozens of articles have been written about the correlation between different abnormal flow-volume loop contours and pulmonary disorders. In contrast very little has ever been written about what constitutes a normal flow-volume loop and what this looks like has been primarily anecdotal.

Interestingly, the ATS/ERS standard for spirometry includes an example of a “normal” flow-volume loop but its source and what makes it normal is not explained.


From the ATS/ERS standard on spirometry, page 327.

One feature that is commonly seen as a feature of normal flow-volume loops has been variously called a ‘shoulder’ or ‘knee’.


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Exercise and the IC, EELV and Vt/IC ratio

Determining whether a subject has a ventilatory limitation to exercise used to be fairly simple since it was based solely on the maximum minute ventilation (Ve) as a percent of predicted. There has been some mild controversy about how the predicted maximum ventilation is derived (FEV1 x 35, FEV1 x 40 or measured MVV) but these don’t affect the overall approach. Several decades ago however, it was realized that subjects with COPD tended to hyperinflate when their ventilation increased and that this hyperinflation could act to limit their maximum ventilation at levels below that predicted by minute ventilation alone.

The fact that FRC could change during exercise was hypothesized by numerous investigators but the ability to measure FRC under these conditions is technically difficult and this led to somewhat contradictory results. About 25 years ago it was realized that it wasn’t necessary to measure FRC, just the change in FRC and that this could be done with an Inspiratory Capacity (IC) measurement.

The maximum ventilatory capacity for any given individual is generally limited by their maximum flow-volume loop envelope. When a person with normal lungs exercises both their tidal volume and their inspiratory and expiratory flow rates increase.



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Short efforts, gas trapping and leaks

Outside the pulmonary lab there is this notion that spirometry is supposed to be so easy that anyone can do it. You just tell the patient to take a deep breath in and blow out fast and to keep blowing until they’re empty. What’s so hard about that?

Sheesh. GIGO. I keep finding ways in which the patient, their physiology and our equipment can conspire in ways to produce errors that even experienced technicians can miss. I’ve been paying a lot of attention to flow-volume loops lately and maybe it’s for this reason that I’ve seen a steady stream of spirometry tests that have something wrong with the FVC volume.

What I’ve been seeing are flow-volume loops where the end of exhalation is to the left of either the start of the FVC inhalation or of the tidal loop. Taken at face value this means that the patient did not exhale as much as they inhaled (and that the FVC is therefore underestimated) but there are several reasons why this happens and it takes a bit of detective work to figure out the cause and what to do about it.

The simplest reason is a short expiratory time. Flow-volume loops however, do not show time, only flow and volume. Sometimes when a patient stops exhaling abruptly it’s easy to see that the effort is short.


Other times it isn’t as clear:


and you need to look at the volume-time curve as well.

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Graphical Analysis of Flow-Volume Loops

I’ve been thinking a bit about the shape of flow-volume loops lately. In part this has been about ways to accurately describe them in reports; in part speculation about the information that may be embedded in them that isn’t in any of the routinely reported spirometry values; and in part about how the human eye perceives and categorizes them in a way that is difficult to simplify and put into a computer algorithm. A couple days ago I found a recent article where a geometrical analysis was applied to flow-volume loops in individuals with COPD and this got me curious about what other graphical techniques have been used to analyze flow-volume loops.

Given how long flow-volume loops have been around (over 50 years) the graphical analysis of flow-volume loops has been attempted remarkably few times. Excluding a handful of strictly numerical approaches (based primarily on MEF and MIF ratios) I was only able to find three graphical analysis techniques. I think this small number says volumes about the difficulty of analyzing flow-volume loop shapes meaningfully. Despite different degrees of sophistication the reality is that none of these techniques has ever seen any kind of common usage. Even so these attempts are both interesting and instructive.

The most recent technique is a fairly straightforward geometric approach from Lee et al and its use appears to be limited primarily to individuals with airway obstruction.


The flow-volume loop is analyzed primarily to determine what the authors call the Area of Obstruction (Ao). To do this, a diagonal line is drawn from peak flow to the end of exhalation. The area that exists between the actual flow-volume loop contour and this diagonal line is defined as the area under the diagonal (Au). The area of Au is then compared to the area of a triangle (At) defined by the peak flow, the exhaled volume at the time of the peak flow, and the end of exhalation. The area of obstruction, which is actually a ratio, is then calculated as:

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Hidden FIVC and FVC. When all the data is relevent.

For the first dozen or so year that I worked in a pulmonary function lab it was with counter-weighted, volume-displacement water-seal spirometers more or less like this:


Patients would do a series of tests and I’d end up with a bunch of pen traces on kymograph paper that I’d have to measure with a ruler and use a desktop calculator (it was about a foot square, weighed a couple of pounds and had a nixie tube digital display) to create a hand-written report. I’m not going to suggest that these spirometers were in any way better than what we’re using now but I have to say that I would have seen the following problems more or less immediately.

Recently I was reviewing a report from a patient with very severe obstruction and noticed something a bit off about the flow-volume loop. Specifically, the end-exhalation of the tidal loop looked like it was at a significantly higher volume than the end of the FVC effort.


Because the high-frequency sawtooth pattern (from the patient, not the equipment) makes it a little hard to see if this is what was really happening, I downloaded the raw data and re-graphed the volume-time curve with a spreadsheet.

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Goiter, upper airway obstruction and the flow-volume loop

The thyroid gland is located across the front of the upper airway a short distance below the larynx. An enlarged thyroid gland is known as a goiter. The most common worldwide cause of goiter is an iodine deficiency. This is much less common in the western nations where factors such as Hashimoto’s thyroiditis, Graves’ disease, multi-nodular thyroid disease, thyroid cancer, pregnancy and the side effects of some medications are the its primary causes. Common respiratory complaints associated with goiter include cough, hoarseness, shortness of breath and stridor.


[illustration from HealthyThyro.com]

When a goiter is large enough it can press against the trachea and cause a narrowing or deviation of the upper airway. My lab usually gets at least a couple of patients referred to us every year with a diagnosis of goiter and a request that we assess whether it is causing any significant airway obstruction. Decades ago I was taught by my medical director that when this occurs it shows up as an expiratory plateau on a flow-volume loop.


The reality (as usual) is more complex and this is mostly because the thyroid gland lies close to the boundary between the extrathoracic and intrathoracic sections of the trachea. Depending on its size and the which direction the thyroid expands towards, goiter can show up as an extrathoracic or intrathoracic airway obstruction. Even more importantly, as a recent article in Chest showed, the airway obstruction from goiter can be dependent on body position as well.

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