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.

fvl_timeless

The contour of this flow-volume loop is actually reasonably normal, except possibly for the little blip at the end.
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Which DLCO should be reported?

I like to think my lab is better than most but every so often something comes along that makes me realize I’m probably only fooling my self.

Earlier this week I was reviewing the DLCO test data for a patient with interstitial lung disease. At first glance the spirometry and DLCO results pretty much matched the diagnosis and I had already seen they weren’t significantly different from the last visit. The technician had written “fair DLCO reproducibility” which was reason enough to review the test data but I’ve actually been making a point of taking a careful look at all DLCO tests, not just the questionable ones, for the last couple of weeks. I took one look at the test data, put my head in my hands, and counted to ten before continuing.

Reported: %Predicted: Test #1: Test #2: Test #3:
DLCO: 13.22 66% 10.08 92.17 16.36
Vinsp (L): 2.17 2.20 2.15
VA (L): 3.45 66% 2.89 2.93 4.02
DL/VA: 3.78 91% 3.49 31.5 4.07
CH4: 60.84 60.94 43.15
CO: 34.46 0.51 23.13

Even though the averaged DLCO results were similar to the last visit, the two tests they were averaged from were quite different. Reproducibility was not fair, it was poor. But far more than that, something was seriously wrong with the second test and the technician hadn’t told anybody that they’d had problems with the test system. {SIGH}. It’s awful hard to fix a problem when you don’t even know there is one in the first place.

I usually review reports in the morning the day after the tests have been performed, so the patient was long gone by the time I saw the results. This left me with a problem that I’m sure we’ve all had at one time or another and that was whether any of the DLCO results were reportable.
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A change that probably isn’t a change

Recently a report came across my desk from a patient being seen in the Tracheomalacia Clinic. The clinic is jointly operated by Cardio-Thoracic Surgery and Interventional Pulmonology and among other things they stent airways. The patient had been stented several months ago and this was a follow-up visit. Given this I expected to see an improvement in spirometry, which had happened (not a given, BTW, some people’s airways do not tolerate stenting), but what I didn’t expect to see was a significant improvement in lung volumes and DLCO.

When I took a close look at the results however, it wasn’t clear to me that there really had been a change. Here’s the results from several months ago:

Observed: %Predicted: Predicted:
FVC: 1.19 50% 2.38
FEV1: 0.64 35% 1.79
FEV1/FVC: 53 71% 76
TLC: 3.21 76% 4.22
FRC: 2.34 96% 2.43
RV: 2.11 113% 1.85
RV/TLC: 66 150% 44
SVC: 1.15 48% 2.37
IC: 0.87 48% 1.80
ERV: 0.25 41% 0.58
DLCO: 6.59 38% 16.18
VA: 1.78 43% 4.12
IVC: 1.04

Change_that_isnt_change_2015_FVL_redacted_2

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Assessing changes in DLCO

We have a number of patients who have spirometry and DLCO testing performed at regular intervals. I’ve noticed that every so often DLCO results change significantly without a change in spirometry (or lung volumes) or there’s a modest change in spirometry and a marked change in DLCO. I’ve been concerned that this may be a symptom of problems with our DLCO (CO/CH4) gas analyzers and at least once recently this kind of discrepancy did lead to having an analyzer being serviced. Realistically though, the gas analyzers are routinely passing their calibrations and when I look at the trends in calibration there hasn’t been any systematic drift. This doesn’t rule out intermittent problems however, so in order to find out whether these changes in DLCO are “real” or an artifact of our testing systems I decided to see if taking a closer look at the results would help resolve this.

First, what constitutes a significant change in DLCO?

My lab’s current working definition is an increase or decrease in DLCO that is 2.0 ml/min/mmHg or 10%, whichever is greater. This is slightly different from the ATS/ERS DLCO intra-session repeatability requirements (3.0 ml/min/mmHg or 10%) and may mean that we’re setting the bar too low but there’s a difference between intra-session and inter-session variability. Specifically, we average the two closest results (assuming there are at least two tests of good quality) from one testing session to another and it is the inter-session average we are comparing, not individual tests and for this reason we feel that a smaller change can be relevant.

Note: The ATS/ERS statement on interpretation does discuss inter-session DLCO variability but there it is expressed as >7% within the same day and >10% year to year without setting an upper limit. The year to year value is based solely on a study from 1989 on eight individuals using a manually operated testing system (Collins Modular Lung Analyzer) that used a semi-automated alveolar sampling bag and for this reason it’s hard to be sure it is still relevant.

Second, which test parameters have the greatest effect on calculated DLCO?

As a reminder, the DLCO formula is:

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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.

Abrupt_Termination_02_FVL

Other times it isn’t as clear:

Short_Exhalation_FVL

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

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A real fixer-upper

I was reviewing reports today when I ran across one with some glaring errors. There were several things that immediately told me that the reported plethysmographic lung volumes were way off; the VA from the DLCO was almost a liter and a half larger than the TLC and the SVC was only about half the volume of the FVC.

Table1

When I took a look at the raw test data I saw at least part of the reason why the technician had selected these results to be reported and that was because the SVC quality from most of the efforts was poor. They mostly looked like this:

Fixer_Upper_01

It is apparent that the patient leaked while panting against the closed shutter and this caused the FRC baseline to shift upwards. I’ve discussed this problem previously, but when this happens the RV is larger than the FRC, there is a negative ERV and the TLC is overestimated. There is no way to fix this problem from within the software. The FRC is determined by the tidal breathing before the shutter closes and cannot be re-measured afterward.

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Tidal flow-volume loops

I was reviewing a spirometry report and noticed something odd about the flow-volume loop, or more specifically the tidal loop, and this got me to thinking about what tidal loops can tell us about test quality, patient physiology and the ability of the technician to coach a spirometry test.

FVC_with_no_IC_Redacted2

There are at least a couple things wrong with this FVC test effort. First the exhalation time was only about 3 seconds so the FVC volume was likely underestimated by a fair amount. Second, it wasn’t reproducible and this was actually the patient’s the best test effort. What I noticed however, was that the tidal loop was shifted almost completely to the left.

There are a number of criteria for assessing the quality of a forced vital capacity. Exhalation quality can be determined reasonably well by back extrapolation, expiratory time and the terminal expiratory flow rate. When it comes to assessing the completeness of the inspiration that precedes the exhalation however, there really isn’t much to go on other than the reproducibility of an individual’s spirometry efforts.

When I measured the tidal loop what I saw was that IRV was about 0.10 L and the ERV, although likely underestimated by a fair amount, was at least 0.80 L. What I actually think this tidal loop is saying is that the patient didn’t take as deep a breath as they could at the start of the test, but what other things could affect the position of the tidal loop?

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What’s up with Peak Flow?

Two PFT reports came across my desk recently and comparing them got me to thinking about Peak Expiratory Flow (PEF). The FEV1 from both tests were mildly reduced with an FEV1/FVC ratio that was moderately reduced and an FVC that was within normal limits.

Peak_Flow_1

Observed: %Predicted:
FVC (L): 3.72 96%
FEV1 (L) 2.17 78%
FEV1/FVC (%): 58 80%
PEF (L/sec): 2.97 41%

Peak_Flow_2

Observed: %Predicted:
FVC (L): 2.46 88%
FEV1 (L) 1.66 75%
FEV1/FVC (%): 67 83%
PEF (L/sec): 7.65 128%

Both tests also showed mild airway obstruction but despite this the Peak Flows were quite different. One test had a PEF that was moderately to severely reduced and the other had a PEF that was elevated. It’s fascinating that two such completely different flow-volume loops are so numerically similar.

In another sense, though, how can these two different spirometry efforts both be labeled as mild airway obstruction? Or more importantly, they both may be mild obstruction but isn’t the quality of the obstruction different?

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Hidden FIVC

I was about to put a PFT report I’d been reviewing in my outbox when I noticed something odd about the flow-volume loop.

Hidden FIVC_redacted 1

What I saw was that the final inspiration of the FVC maneuver had ended to the left of the initial inspiration. This means a couple of thing, first and foremost that the FIVC was larger than the FVC and that the FVC was likely underestimated because the patient hadn’t really taken a full inspiration prior to exhaling. I had already looked at the raw data for the patient’s spirometry results for other reasons but I pulled them up again to see if I had missed something.

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Creepy decimal points

This is one of my pet peeves. It started for me back in the 1970’s when the Intensive Care Units where I was working were evaluating thermal dilution cardiac output meters. This was at a time when digital displays were just starting to become common. One of the meters showed cardiac output with two digits after the decimal point (i.e. 0.12) and the other one had three digits after the decimal point (i.e. 0.123).

Thermal dilution cardiac output works by threading a catheter (a Swan-Ganz is what was used at the time) with a thermistor in its tip through the right side of a patient’s heart into their pulmonary artery. A small amount of iced saline is then injected through the catheter and the system times how long it takes for this cold pulse to go around the patient’s body and return. There are a number of uncertainties involved so it’s not a terribly accurate technique and the very best you could ever expect would be a precision of about 1/10th of a LPM and that’s being very generous.

We had no ability to actually determine if either meter was accurate and the best we could see was that both meters gave similar results on the same patient. Neither meter was particularly harder or easier to use than the other. Nevertheless the cardiac output meter with 3 digits after the decimal point won the evaluation hands down because everybody said it had to be more accurate. This may say something about human nature but it’s also just nonsense. Simply because somebody places extra digits after the decimal point doesn’t make the measurement more accurate.

I’ve seen many times where a test result is reported with more digits after the decimal point than you could reasonably expect to get from the equipment or the measurement. When new devices (software or smartphones for example) add new features that are of little value (other than probably as ad copy) it is called feature creep. When this happens with digits I think this should be called decimal creep.

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