Why DIY CPET reports?

When I first started performing CPETs in the 1970’s a patient’s exhaled gas was collected at intervals during the test in Douglas bags and I had a worksheet that I’d use to record the patient’s respiratory rate, heart rate and SaO2. After the test was over I’d analyze the gas concentrations with a mass spectrometer and the gas volumes with a 300 liter Tissot spirometer and then use the results from these to hand calculate VO2, VCO2, Rq, tidal volume and minute volume. These results were then passed on to the lab’s medical director who’d use them when dictating a report.

Around 1990 the PFT lab I was in at the time acquired a metabolic cart for CPET testing. This both decreased the amount of work I had to do to perform a CPET and significantly increased the amount of information we got from a test. The reporting software that came with the metabolic cart however, was very simplistic and neither the lab’s medical director or I felt it met our needs so I created a word processing template, manually transcribed the results from the CPET system printouts and used it to report results.

Twenty five years and 3 metabolic carts later I’m still using a word processing template to report CPET results.


Well, first the reporting software is still simplistic and using it we still can’t get a report that we think meets our needs (and it’s also not easy to create and modify reports which is a chronic complaint I have about all PFT lab software I’ve ever worked with). Second, there are some values that we think are important that the CPET system’s reporting software does not calculate and there is no easy way to get it on a report as part of the tabular results. Finally, and most importantly, I need to check the results for accuracy.

You’d think that after all these years that you wouldn’t need to check PFT and CPET reports for mathematical errors but unfortunately that’s not true. For example, these results are taken from a recent CPET:

Time: VO2 (LPM): VCO2 (LPM): Reported Rq: “Real” Rq:
Baseline: 0.296 0.220 0.74 0.74
00:30 0.302 0.214 0.77 0.71
01:00 0.363 0.277 0.77 0.76
01:30 0.395 0.306 0.78 0.77
02:00 0.424 0.353 0.99 0.83
02:30 0.459 0.403 0.92 0.88
03:00 0.675 0.594 0.89 0.88
03:30 0.618 0.584 0.94 0.94
04:00 0.836 0.822 1.00 0.98

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The FVC/DLCO ratio. Will the real percent predicted please stand up?

Recently a reader asked me a question about the FVC/DLCO ratio. To be honest I’d never heard of this ratio before which got me intrigued so I spent some time researching it. What I found leaves me concerned that a lack of understanding about reference equations may invalidate several dozen interrelated studies published over the last two decades.

Strictly speaking the FVC/DLCO ratio is the %predicted FVC/%predicted DLCO ratio (which is usually abbreviated FVC%/DLCO%) and it appears to be used exclusively by specialists involved in the treatment of systemic sclerosis and related disorders. Specifically, the ratio is used to determine whether or not a patient has pulmonary hypertension. The basic idea is that (quoting from a letter to the editor):

“As we know, in ILD both FVC and DLCO fall and their fall is proportionate, whereas in pulmonary arterial hypertension DLCO falls significantly and disproportionately to FVC.”

A variety of algorithms using the FVC%/DLCO% have been devised. Inclusion factors are usually an FVC < 70% of predicted and a DLCO (corrected for hemoglobin) < 60% of predicted. A number of different threshold values for FVC%/DLCO% have been published ranging from 1.4 to 2.2 with the differences appearing to be dependent on study population characteristics and the type of statistical analysis performed. It is thought that individuals meeting the inclusion factors with an FVC%/DLCO% ratio above the threshold most probably have pulmonary hypertension.

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When no change is a change, or is it?

I was reviewing a spirometry report last week and when I went to compare the results with the patient’s last visit I noticed that the FVC and FEV1 hadn’t changed significantly. However, the previous results were from 2009 and when the percent predicted is considered there may have been a significant improvement.

2009 Observed: %Predicted:
FVC: 2.58 87%
FEV1: 1.60 72%
FEV1/FVC: 62 82%
2016 Observed: %Predicted:
FVC: 2.82 104%
FEV1: 1.65 82%
FEV1/FVC: 59 79%

The answer to whether or not there was an improvement would appear to depend on what changes you’d normally expect to see in the FVC and FEV1 over a time span of 7 years. The FVC and FEV1 normally peaks around age 20 to 25 and then declines thereafter.



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Top 10 spirometry errors and mistakes

A couple of days ago my medical director and I had a short discussion about teaching pulmonary fellows to read PFTs and agreed that in order to be good at interpreting PFTs it isn’t the basic algorithms that are hard, it’s gaining an understanding of test quality and testing problems. My medical director then suggested this topic. At first I wasn’t sure I could find 10 errors but after spending a couple hours digging through my teaching files I managed to come up with just a few more than that. So strictly speaking it’s not a top 10 list but I kept the title because I liked it.

Spirometry errors and mistakes seem to fall into four categories: demographics, reference equations, testing and interpretation.


Normal values are based on an individual’s age, height and gender. When this information is entered incorrectly the normal reference values will also be incorrect. These errors often go uncaught because whoever reviews and interprets reports usually isn’t the same person who sees the patient and performs the tests. This type of error often doesn’t get corrected until the results are uploaded into a hospital information system or the patient returns for a second (or third or fourth) visit.

1. Wrong gender.

Pulmonary function reference equations are gender specific and for individuals with the same age and height, men will have a larger FVC and FEV1 than women do. When a patient’s demographics information is manually entered into a PFT system it’s always possible for somebody to enter the wrong gender. When this happens the predicted values will be either over- or under-estimated. This happens in my lab at least a half a dozen times a year and it’s why when I review reports I try to check the patient’s gender right after reading their name.

This is also a problem area for individuals who have gone through gender reassignment (transsexuals). An individual’s physiologic/developmental gender needs to be used to generate predicted values but this may be at odds with their gender recorded in a hospital’s information system. Some PFT lab systems populate their demographics information from their hospital’s information system when an order is received and it may or may not be possible to alter gender once this has happened. In other cases, an individual’s demographics may be cross-referenced when PFT results are uploaded into hospital information system and may throw an error if the wrong gender is present.

2. Wrong height

All lung volumes and capacities scale with height. Like any other manual entry height can be mis-entered and the most common error I’ve seen is for somebody to enter 60 inches when they meant 6 feet 0 inches.

Height can also be mis-measured if the patient isn’t asked to remove their shoes or to stand straight, or if the patient is asked for their height and it isn’t even measured. An error of an inch or two probably won’t make a big difference in a patient’s predicted values (particularly given the discrepancies between different reference equations) but for somebody who’s on the edge of normal and abnormal it can make a significant difference in how a report is interpreted.

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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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When is a change in FVC significant?

Most of the COPD patients that are seen in my lab tend to have little change in their FEV1 from visit to visit but their FVC often changes significantly. A change in FVC is usually related to how long a patient is able to exhale and this in turn is usually related to how well they are feeling at the time. This would seem to imply that a significant change in FVC, particularly for a patient with COPD, is, if not clinically significant, at least clinically important even when the FEV1 hasn’t changed.

The problem with this is that expiratory time can be affected by things other than how the patient is feeling. Dyspnea and fatigue, of course. As importantly some technicians are better at motivating patients than other technicians so it can also be related to which technician is performing their tests. Even when the same technician is involved however, there is no guarantee that the level of motivation or a patient’s response to that motivation will be the same.

So how do you know if a change in FVC clinically significant or not?

Recently a spirometry report from a patient with very severe COPD came across my desk. When comparing the results to those of the last visit I could see that there had been a small (but not significant) increase in FEV1 but at the same time there had been a large (and significant) increase in FVC.

Visit 1: Observed: %Predicted:
FVC (L): 1.28 36%
FEV1(L): 0.53 19%
FEV1/FVC: 41 53%
Visit 2: Observed: %Predicted:
FVC (L): 1.93 55%
FEV1(L): 0.60 22%
FEV1/FVC: 31 40%

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Six-Minute Walk with Helium-Oxygen

We recently performed a 6-minute walk test with helium-oxygen (heliox) for a patient of one of the physicians that specializes in airway stenting. His reasons for the test weren’t particularly clear (and he hasn’t bothered to try to clarify them with me) but most probably it has to do with differentiating between central and peripheral airway obstruction. Interestingly, he predicted the patient would have a significant improvement in 6-minute walk distance and instead there was little difference between the heliox 6MWT and one performed with 3 LPM supplemental O2.

6MWT: SaO2: Distance:
80% Helium – 20% O2, by mask 95% 440 meters
3 LPM O2, by nasal cannula 98% 457 meters

Helium is an inert, insoluble, low mass gas and both its therapeutic use and its use in physiological measurements has to do with it’s low density (and the fact that it’s highly insoluble, but that’s for purposes different than those discussed here).

  Density (g/m3)
He 0.179
N2 1.251
O2 1.429
Air (78% N2, 21% O2) 1.293
Heliox (80% He, 20% O2) 0.429

A typical way to assess its effect is by comparing air and heliox flow-volume loops:


Interestingly, despite an apparent increase in flow rates there is usually no significant difference in FEV1 (one study showed a range of +2% to +7% in a group of over 1500 subjects). The most common heliox FVL measurements are the change in expiratory flow at 50% of the FVC (ΔMEF@50%) and the Volume of Isoflow (which is the point at which the air and heliox expiratory flows become equivalent). Many of the earlier studies with heliox also measured ∆MEF@75% and ∆FEF25-75, and a tiny handful of studies (particularly given the technical difficulties) have measured ∆RAW and ∆sGAW.
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Ventilatory response to hypoxia and hyperoxia

While reading a recently published article I found they had performed response to hypoxia and hyperoxia testing as part of the study. At one time or another in the past I’ve read about response to hypoxia testing but I’d never heard about hyperoxia testing before. I had some difficulty understanding their interpretation of the study’s results and for this reason I’ve spent some time reading up on the subject. I’m not sure this helped because there appears to be a lack of consensus not in only how to perform these tests but also in how they are interpreted, except perhaps in the most simplistic sense. Hypoxia and hyperoxia testing has been performed primarily to gain a deeper understanding of the way in which the peripheral (carotid) and central chemoreceptors function. There are a variety of sensor-feedback network models and results are often presented in terms of one model or another and this makes comparing results from different studies difficult. Interpretation and comparison is further complicated by the fact that results depend not only on the length of time that hypoxia or hyperoxia is maintained but whether the subject was exposed to hypoxia, hyperoxia or hypercapnia previously.

The ventilatory response to hypoxia tends to have three phases. First, once a subject begins breathing a hypoxic gas mixture within several seconds there is a rapid increase in minute ventilation known as the Acute Hypoxic Ventilatory Response (AHVR). Second, after several minutes there is a decrease in ventilation and this is usually called the Hypoxic Ventilatory Depression (HVD). Third, there is a progressive rise in ventilation after several hours which is related to acclimatization to altitude. It is the first phase, AHVR, that is most commonly measured during a hypoxic ventilatory response test. The actual length of time that is spent in any of these phases is widely variable between individuals and there is also a relatively large day-to-day variability within the same individual.
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When hypoventilation is the primary CPET limitation

Hypoventilation is defined as ventilation below that which is needed to maintain adequate gas exchange. It can be a feature in lung diseases as diverse as chronic bronchitis and pulmonary fibrosis but determining whether it is present of not is often complicated by defects in gas exchange. When desaturation occurs during a CPET (i.e. a significant decrease in SaO2 below 95%) this is a strong indication that the primary exercise limitation is pulmonary in nature and from that point the maximum minute ventilation and the Ve-VCO2 slope can show whether the limitation is ventilatory or instead due to a gas exchange defect. But in this circumstance what what does it mean when both the maximum minute ventilation and Ve-VCO2 slope are normal?

Recently a CPET came across my desk for an individual with chronic SOB. The individual recently had a full panel of pulmonary function tests:

Observed: %Predicted:
FVC (L): 1.73 62%
FEV1 (L): 1.39 66%
FEV1/FVC: 80 106%
TLC (L): 2.99 62%
DLCO (ml/min/mmHg): 14.66 84%
DL/VA: 5.45 124%
MIP (cm H2O): 11.5 18%
MEP(cm H2O): 21.3 24%

The reduced TLC showed a mild restrictive defect. At the same time the relatively normal DLCO indicates that the restriction is probably not due to interstitial lung disease and more likely either a chest wall or a neuromuscular disorder, both of which can prevent the thorax from expanding completely but where the lung tissue remains normal. The reduced MIP and MEP tends to suggest that a neuromuscular disorder is the more likely of the two.

I take this with a grain of salt however, and that is because this individual never had pulmonary function tests before and for this reason there is no way to know what their baseline DLCO was prior to the restriction. At the same time far too many individuals perform the MIP/MEP test poorly and low results are not definitive, and in this case in particular the results are so low the individual should have been in the ER, not the PFT Lab.

The CPET results were somewhat complicated, in that a close inspection showed both pulmonary and cardiovascular limitations.
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Asleep at the wheel

During this last week I was contacted by two different individuals who were asking for help in understanding their PFT results. In both cases they had a markedly elevated TLC and the interpretation included the notation that they had gas trapping and hyperinflation. Even though the amount of information they provided was minimal I am extremely skeptical that the TLC measurements were correct.

Gas trapping usually only occurs with severe airway obstruction. Hyperinflation, which at minimum consists of a chronically elevated FRC and RV, usually only occurs after prolonged gas trapping. An elevated TLC usually occurs only with prolonged hyperinflation and given the improvements in the care and treatment of COPD I’ve seen over the last several decades, has become relatively uncommon.

But one individual had perfectly normal spirometry:

FVC: 107%
FEV1: 112%
FEV1/FVC: 105%
TLC: 143%

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