OK, so I wrote a really (overly) long piece on how to interpret a 2-day CPET report from Workwell Foundation with an eye toward understanding these documents for disability purposes. To understand something I pretty much need to delve into the science and write about it. While writing that piece certainly helped with my understanding of CPET, I am not sure how helpful it was to others!
I promised to write an easier (and shorter!) guide to CPET reports. Here it is.
(ME/CFS) is a disabling neuroimmune disease that affects over 20 million people
worldwide. Approximately 25% of patients with this disease are housebound
and 80-90% are unable to work.
One of the only tests that shows the disability experienced in ME/CFS is the 2-day cardiopulmonary exercise test (CPET).
While CPET is a very powerful tool for winning disability cases and understanding how to pace activities, CPET reports, and the science underlying them, are difficult to absorb.
There is a lot of nuanced information in these reports, much of which doctors, disability evaluators, and judges don’t understand very well because it is so technical. For this reason, it is important that patients understand their results to ensure that key aspects of their disability are not missed or misrepresented.
This overview provides a brief(er) explanation of the various sections of a CPET report from Workwell Foundation (other clinics adopting Workwell’s methods include much of the same information in their reports).
First…why a 2-day test?
The most important reason for a 2-day CPET (vs. a 1-day CPET) is that some of the unique impairments found in ME/CFS don’t show up until the second day of testing. A big difference between ME/CFS and other conditions is that (most) people with ME/CFS cannot repeat their results on the second day of testing. This does not occur in other conditions that have been looked at. Even patients with severe heart failure can repeat their results, albeit not very good ones.
People with ME/CFS typically show a drop in function on the second day of CPET testing. Day-to-day test variability is less than 8% for healthy individuals. This is not from deconditioning as even couch potatoes can repeat their numbers on the second day. Values greater than 8% indicate that people simply cannot sustain activities from day to day. These values can be easily found in your CPET report.
If you do not show a drop in function on day 2 of CPET testing, it could be because you arrived at the test in post-exertional malaise (PEM). PEM is a worsening of symptoms in response to exertion and is a hallmark of ME/CFS.
If you do not want to read any further, the take-home message is this: If you show a significant drop in function in any of the following categories on the second day of your CPET, it is likely that you cannot sustain work safely.
For information about what a 2-day CPET entails, see my original piece. You can also find a sample CPET report at the end of the article (it is not included here).
What gets measured in the 2-day CPET?
Assessment of effort
Quantifying effort is essential for debunking the myth that ME/CFS patients are malingerers (people who fake being sick). To address this, Workwell Foundation (and others) measure the peak respiratory exchange ratio (RER), which is the ratio between the amount of carbon dioxide produced and oxygen used. RER is an objective measure and cannot be faked. Values greater than 1.1 indicate excellent effort.
Workwell Foundation (and others) also use a patient-reported scale that measures perceived exertion throughout the test, with anything above 17 points (out of 20-point scale) indicating that maximal effort was reached. According to Workwell Foundation, most people are able to achieve this.
So, if anyone is implying you are faking your illness, simply point to this section of your report.
VO2 is a fancy way of saying the volume of oxygen consumed, or, put more plainly, oxygen consumption. VO2 max is reached when oxygen levels remain at a steady level despite an increase in workload on the exercise bike.
VO2 max is a direct measure of disability because it places a cap on the level of work a person is capable of doing. You simply cannot do more than your ability to consume oxygen allows.
The average VO2 max is around 35–40 mL/kg/min in healthy sedentary males and 27–31 mL/kg/min in healthy sedentary females and declines by ~10% each decade after the age of 30. It is approximately double for endurance athletes. You can reference the table in my original piece that breaks down VO2 max by gender and age to guage your level of impairment.
The ability to consume oxygen (VO2) to fuel a certain level of activity is fundamental to the idea of functional capacity. Functional capacity is an important concept in disability and is calculated as the maximum oxygen consumption (VO2 max) that is measured during a CPET, divided by the predicted VO2 max based on age and gender (= VO2 max/VO2 predicted). A functional capacity greater than 85% is considered normal.
Patients may have a normal test on day 1 of exercise testing but often dip below this 85% threshold on day 2. Again, this is why the second day of testing is required to demonstrate disability.
No one lives or works at their maximum oxygen consumption, certainly not people with ME/CFS! The more relevant measure of disability for ME/CFS patients is the ventilatory/anaerobic threshold (V/AT) because it shows the amount of work that can reasonably be sustained.
V/AT, hereafter called the anaerobic threshold (AT) for the sake of simplicity (this is brushing the ventilatory threshold under the rug a bit), corresponds to the point at which lactate and/or lactic acid begins to accumulate exponentially and cannot be cleared from muscles and the bloodstream faster than it is generated. This represents the point at which anaerobic metabolism – an inefficient form of energy production – kicks in.
There are two relevant numbers to focus on for disability and pacing purposes:
1) The amount of oxygen consumed at the threshold
2) The heart rate at the threshold
If the AT occurs at a low rate of oxygen consumption and/or at a low heart rate, normal daily activities may be more than a person with ME/CFS can manage.
Most daily activities use aerobic metabolism. However, when the anaerobic threshold is low, it means there is reliance on anaerobic metabolism to do basic tasks of daily living that would normally be met with aerobic metabolism.
Being above AT can only be sustained for short periods of time and results in delayed recovery, a feature of PEM. Workwell Foundation cautions against spending more than 2 minutes with a heart rate above AT to minimize PEM.
When we cross this threshold we might as well be sprinting around (but, of course, cannot!). In other words, even simple activities, like brushing teeth or making a smoothie, are the metabolic equivalent of sprinting for some people with ME/CFS. No wonder this disease is so exhausting.
In the general population, average AT occurs at about 63% of VO2 max in men and 58% of VO2 max in women. In ME/CFS patients, the percentage can be vastly lower. It is impossible to know the actual number without doing a 2-day CPET.
How metabolic responses translate to disability
The metabolic measures from the 2-day CPET come in handy for disability cases because they provide objectivemeasures of how much work can be safely done in a day.
For example, according to Workwell Foundation, the International Labor Organization considers 30% of VO2 max as the threshold for acceptable physiological demands over an 8-hr work day. CPET reports from Workwell Foundation provide this number – be sure to show it to your doctor and disability lawyer (if you have one). This places a quantitative limit on what you can safely be expected to do in a day.
VO2 max and VO2 can easily be converted to the metabolic equivalent (MET). A MET is a physiological measure of the cost of doing activities. One MET is equivalent to oxygen use at a rate of 3.5 mL/kg/min. METs < 3 are considered light activities, METs between 3 and 6 are considered moderate levels of activity, and METs > 6 correspond to vigorous activities.
VO2 and METs can also be matched against the New York Heart Association’s (NYHA) Functional Classification to determine level of disability.
You can also use the METs listed in the Compendium of Physical Activities to convert back to VO2 values to see how activities stack up against your VO2 measured at the anaerobic threshold. Simply multiply the METs value by 3.5 to convert it back to VO2.
Understanding METs prior to going into a disability hearing is a good idea as this topic may come up in discussions with the vocational expert, the person who determines if you are able to do any type of job.
Make sure that your lawyers and doctors understand that oxygen consumption and METs at the anaerobic threshold – especially on the 2nd day of your test – are the relevant numbers to look at, as they indicate the types of work and activities that can be reasonably sustained.
CPET quantifies “dynamic loss of function”
I recently read a transcript of one of Dr. Paul Cheney’s talks (Dr. Cheney, along with Dr. Daniel Peterson, was on the scene at the Lake Tahoe outbreak in the mid-1980s). In his talk, he explained perfectly why a drop in function after activity is so disabling for people with ME/CFS.
Cheney said, in reference to how patients describe fatigue, “They talk about that their energy is impaired. And it’s not just that they’re fatigued. There’s a dynamic quality to this energy impairment.
People describe it as, “It isn’t so much that I have no energy, but if I do something, I’ll just crash. I’ll collapse.”
And that collapse of function is dynamic collapse of function. An hour later or a day later or a week later – it lasts for days or weeks or months.
It’s really what characterizes this fatigue state. A dynamic loss of function.”
It is this dynamic loss of function that the 2-day CPET quantifies and that must be conveyed to the doctors, lawyers, and judges working on your disability case.
Work in Watts
Work is measured in Watts (W), which is a rate expressed as energy per unit time. For example, a 100 W lightbulb turned on for 1 hour will use 100 Watts/hour. A human’s ability to perform work can also be thought of in this way. The average human produces/uses about 100 W (the same as a lightbulb!) in a day, with about 20% of this used to operate the brain.
Workload at AT can provide a stark picture of how ill ME/CFS patients are. Workload measured at AT (vs. at VO2 max) is the most relevant number, one that can be cross-referenced with activities of daily living to see where a person falls on the disability spectrum.
For example, let’s say your workload at AT was 30 W. Just for scale, very light housework requires about 30 W. Ironing and cooking range from 35-44 W. Thus, this aspect of a CPET directly quantifies your ability to do work, literally!
However, workload is just one aspect of disability – even if you could iron or cook based on your workload results, a low VO2 at AT may prevent you. Workload is just another aspect of disability, for some more than others.
The cardiovascular data from a CPET also provide insights into your level of disability. As above, the anaerobic threshold is the point at which anaerobic metabolism occurs. Spending much time above your AT will likely throw you into PEM.
ME/CFS patients can use the anaerobic threshold as a benchmark for pacing, with the goal to spend as much of the day possible below this value.
Although there are rules of thumb to help guide pacing decisions, the 2-day CPET is the only way to know your actual AT and the change in AT between day 1 and 2.
Chronotropic insufficiency (CI)
A common response to exercise is an increase in heart rate. Failure of the heart to keep pace with increased demand is called chronotropic incompetence (CI), a disabling condition that causes an intolerance to activity and often leads to poor health outcomes.
In theory, the maximum heart rate (HR max) that can be achieved is estimated to be around 220 – age. Workwell Foundation cautions against pacing plans based on age-predicted heart rate thresholds because the chronotropic response is impaired in so many people with ME/CFS. This argues for using even lower pacing thresholds when CI is a part of the picture.
A recent paper from Workwell Foundation found that CI is common in ME/CFS and can make a bad situation worse. A blunted heart rate in response to exercise could exacerbate the already low oxygen consumption found in this disease and further limit levels of activity. CI further compounds PEM.
If your report indicates that you have CI, it is a good idea to get it checked out by your doctor.
Respiratory responses during the 2-day CPET may also show different aspects of disability in ME/CFS patients. During exercise, CO2 builds up and ventilation (breathing) rate increases. As with VO2, the CPET measures VCO2, or the volume of CO2 exhaled per minute. Another measure, VE, or minute ventilation, is a measure of the O2 inhaled and the CO2 exhaled from a person’s lungs per minute. VE provides a measure of ventilatory efficiency and is measured in mm Hg (mm mercury).
VE/VCO2 is a measure of CO2 elimination. The normal range for VE/VCO2 is between 20-30; values above this are commonly found in ME/CFS. Elevated VE/VCO2 values are associated with diseases which have an increased breathing requirement for a given level of exercise or workload, such as heart failure. Get this checked out if yours is off.
Pet CO2 (partial pressure of end-tidal CO2)
Pet CO2 is another valuable marker of disease severity and corresponds to the partial pressure (measured in mm Hg) detected at the end of exhalation when CO2 is highest. Values <40 mm Hg are low and values <30 mm Hg are very low. Low Pet CO2 may be an indication of hyperventilation, decreased cardiac output, or poor pulmonary blood flow and should be assessed.
Low Pet CO2 is associated with orthostatic hypocapnia in ME/CFS. Hypocapnia is the term for low CO2 in the blood and is caused by deep or rapid breathing (hyperventilation).
A retrospective study found evidence for orthostatic hypocapnia in a subset of ME/CFS patients, caused by hyperventilation upon being tilted upright in a tilt table test.
This is another yet form of orthostatic intolerance that contributes to the disability picture found in ME/CFS patients. If you have orthostatic intolerance due to POTS or hypotension, this will only add to your disease burden (and disability). Be sure to highlight this finding if you have it.
Following a 2-day CPET, patients are asked to fill out a recovery form for one week. This is an opportunity to be very explicit about how the CPET impacted you, at least in the initial week after the test. This gets incorporated into your CPET report and can further bolster your disability claim if you had an abnormal response (guessing pretty much everyone does).
Ultimately, it is a very personal choice for a ME/CFS patient to either chose or forgo a 2-day CPET. Many patients report permanent losses to health following a CPET, yet many more appear to recover eventually, with 50% of Workwell patients recovering within 1 week. It is hard to predict who will recover and who will not. Just like with any treatment advice, we are all different and ultimately cannot predict how our bodies will respond to an extreme stressor like the CPET.
A 2-day CPET report provides the grist of your disability case and a roadmap for how to avoid activities that lead to PEM.