Reprinted with the kind permission of Caroline Christian and Health Rising.
By Caroline Christian
( In this article Caroline, a former biologist, explains the ins and out of one the most difficult to understand but important tests done in ME/CFS – maximal cardiopulmonary exercise CPET) testing. In doing so she’s produced a document that both patients and doctors can benefit from (and provides a copy of an actual CPET workup).
Thanks to Caroline for her willingness to do the work to shine a light on a very complex subject and for sharing her work on Health Rising. (For this overview a printout might be helpful and a button to produce a document / PDF file is available at the lower left hand part of the page of the original article here. – Cort)
Myalgic/encephalomyelitis (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.
Despite these staggering statistics, ME/CFS is one of the more challenging conditions for which to receive disability benefits in the United States (and elsewhere). The reasons for this are complex, but stem largely from a mischaracterization of the disease by the Centers for Disease Control and Prevention (CDC) and a long history of poor research funding from the National Institutes of Health (NIH).
Poor funding has slowed research and left patients without a definitive test (i.e. biomarker) for this disease, let alone any FDA-approved treatments. The absence of a diagnostic test and accepted treatments has allowed deeply flawed stereotypes to take hold in medicine, a fact not lost on long-term disability (LTD) companies and the Social Security Administration (SSA). This places the burden of proof on patients.
One of the only tests that shows the disability experienced in ME/CFS is the 2-day cardiopulmonary exercise test (CPET). The CPET directly measures an individual’s capacity for work and is considered the gold standard for measuring disability – not only for ME/CFS but for a variety of other conditions. A second day of testing is needed for ME/CFS patients because it is the only way to demonstrate the unique impairments underlying the disease.
Having a poorly understood condition means that ME/CFS patients are faced with a tough choice. The 2-day CPET helps to increase the chances of being awarded disability benefits, but undergoing the test increases the risk of getting sicker, as most exercise is strongly contraindicated for ME/CFSpatients. No one should have to face such difficult choices, but this is the reality for people with this disease.
Fortunately, ME/CFS patients have strong allies at the Workwell Foundation, Ithaca College, Cornell University, and other research institutions. These groups are focused on understanding the functional impairments found in ME/CFS. In fall 2017, Drs. Maureen Hanson and Betsy Keller, in collaboration with the Workwell Foundation, received one of the three Collaborative Research Center grants awarded by NIH to nail down the immunological, neurological and metabolic changes that underlie post-exertional malaise (PEM) using the 2-day CPET. PEM is a worsening of neuroimmune symptoms that follows exertion and is the cardinal component of ME/CFS.
Using a sample 2-day CPET evaluation (see Appendix 1), the aim of this article is to help patients better understand how to read a CPET report.
Testing for Disability at the Workwell Foundation
Researchers at the Workwell Foundation pioneered the 2-day CPET for ME/CFS and are the experts at writing reports that assess the level of disability experienced by ME/CFS patients during and after physical exertion. A 2-day CPET can also confirm a ME/CFS diagnosis by demonstrating PEM. They have published extensively on exercise intolerance in ME/CFS, lending further credibility to their work.
People with ME/CFS often show a marked drop in functional capacity on the second day of exercise testing, which is not found in other conditions. According to Workwell, even patients with severe heart failure can repeat their results on the second day of exercise testing. Day-to-day test variability is less than 8% for healthy individuals. The abnormal response between the two tests provides evidence of impaired recovery after exertion (aka PEM).
It is impossible to fake a CPET, so the results tend to carry far more weight than a list of subjective symptoms.
The CPET is also called a maximal graded exercise test. The “maximal” part refers to exercising until it is no longer possible, while seated on a cycle ergometer (stationary exercise bike). A CPET can be done on a treadmill, although this is not recommended by the Workwell Foundation due to the risk of falling and other reasons.
A maximal effort is required to measure the aerobic energy impairments that are unique to ME/CFS (submaximal testing is not sufficient).
The graded exercise part of the test (not to be confused with Graded Exercise Therapy!) involves a 10 watt/minute ramping protocol; i.e., for each minute of the test, pedal resistance increases by 10 W (this amount may vary depending on age, gender, and level of illness).
Prior to the test, patients are hooked up to a 10-lead ECGand vitals are recorded. When ready, patients mount a stationary exercise bike that is fitted with equipment that captures and analyzes expired gasses to determine oxygen consumption, carbon dioxide production, and pulmonary ventilation. Heart rate and blood pressure are measured before, during, and after the test. Two exercise tests are performed on consecutive days to determine the response to exertion.
Prior to the test, patients are provided with a chart showing different levels of perceived exertion, with 20 being the highest level of difficulty. Patients must reach a minimum level of 17 for the test to be considered a valid maximal exercise test.
The tests last for 8-12 minutes and the vast majority of Workwell clients are able to successfully complete it on both days.
Patients are told that if they try to stop before the relevant data are collected, they will be asked to continue. However, patients are not told when to stop so as to not influence the test. Most patients go beyond the point necessary in an effort to make the test as meaningful as possible.
After reaching the maximum amount of exercise tolerated, patients are given a cool down period during which they continue pedaling without resistance.
The test is repeated the following day at the same time.
What gets measured in the 2-day CPET?
Assessment of effort
Quantifying effort is essential for addressing the misguided belief that ME/CFS patients are deconditioned couch potatoes, or worse, malingerers. The Workwell Foundation has two ways of measuring effort. First, they measure 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. According to the American Heart Association, a RER greater than 1.1 indicates excellent effort. Second, they use the patient-reported scale for perceived exertion, with anything above 17 points (out of 20-point scale) indicating that maximal effort was reached during the test.
The Workwell Foundation states that nearly all ME/CFS patients give it their all during testing and maximum effort is nearly always reached.
VO2 max: VO2 is a measure of the body’s efficiency at doing work. VO2 max is the maximum rate of oxygen consumption during incremental exercise. It is expressed in milliliters (mL) of oxygen (O2)/kilogram(kg)/minute and depends on age and gender. VO2 max is reached when O2 levels remain at a steady level despite an increase in workload.
VO2 max is a direct measure of disability because it captures the functional capacity, or level of work, a person is capable of doing. 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 (Table 1) and declines by ~10% each decade after the age of 30. VO2 max is approximately double for endurance athletes.
A functional capacity (VO2 max actual/ VO2 max predicted) greater than 85% is considered normal.
Table 1. VO2 max by gender and age.
Using the example in Appendix 1, VO2 max was 21.7 mL/kg/min on day 2 of CPET testing. At 50 years of age, this is classified as “poor function” (Table 1).
V/AT: No one lives or works at their VO2 max. 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 corresponds to the point at which lactate and/or lactic acid begin to accumulate exponentially and cannot be cleared from muscles and the blood stream faster than it is generated. This represents the point at which anaerobic metabolism – an inefficient form of energy production that produces toxic by-products – kicks in. This happens, in part, because there is not enough oxygen to keep up with demand.
The aerobic energy production system provides long periods of sustained energy. Problems with that system make sustained activity difficult or impossible for people with ME/CFS.
V/AT can be expressed either as the amount of oxygen (VO2) that is consumed at the anaerobic threshold or as the heart rate at which the crossover into anaerobic metabolism occurs. If this 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 of our energy demand is met using aerobic metabolism, which typically covers energy for activities such as walking, seated tasks, and other basic activities of daily living. According to Workwell, the reduction in V/AT following exertion demonstrates that there is a greater reliance on anaerobic metabolism to support lower intensity work that would normally be met with aerobic metabolism.
Energy expenditure at or above V/AT is fatiguing, can only be sustained for short periods of time and results in the delayed recovery seen during PEM. This is why Workwell cautions against spending more than 2 minutes with your heart rate above V/AT and minimizing the number of times V/AT is crossed in a day.
According to Workwell, VO2 and work intensity at V/AT are important measures of the capacity to do continuous work.
As Staci Stevens from the Workwell Foundation puts it, aerobic and anaerobic metabolism are like the tortoise and the hare. Aerobic metabolism is the long and slow system (the tortoise), but one that nets 36 ATP molecules per molecule of glucose. Anaerobic metabolism (the hare) is typically reserved for shorter and more intense bursts of activity, with only a net gain of 2 ATP molecules produced using this system. In ME/CFS, the tortoise is sick, requiring the hare to do basic daily activities that should be covered by aerobic metabolism. Overreliance on anaerobic metabolism causes PEM.
In the example (Appendix 1), VO2 max didn’t tell the full story. The heart rate at the anaerobic threshold on the second day occurred at a mere 69 beats per minute (bpm). That’s the maximal heart rate this person could muster before hitting their anaerobic threshold and beginning to incur damage in the form of lactic acid accumulations, pain, fatigue, etc. That low heart rate indicated that one day after a short exercise test, there was little aerobic capacity left. Any moderate physical activity would put them over their anaerobic threshold.
The anaerobic energy production system (the hare) provides short bursts of energy that cannot be sustained. An inability to use the aerobic energy production systems for sustained activities, forces people with ME/CFS to use their anaerobic energy production system – resulting in pain and fatigue.
For scale, the suggested target heart rate zone for exercise from the American Heart Association for a healthy 50-year-old person is 85-145 bpm.
Similarly, VO2 at the anaerobic threshold is 10.2 mL/kg/min, which presents a far more disabling picture because it places another low ceiling on the level that can be sustained.
In the general population, average V/AT occurs at about 63% of VO2 max in men and 58% of VO2 max in women. In ME/CFS patients, that number 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 objective measures of how much work can be safely done in a day. For example, according to Workwell, the International Labor Organization considers 30% of VO2 max as the threshold for acceptable physiological demands over an 8-hr workday.
In the example (Appendix 1), VO2 max on day 2 was 21.7 mL/ kg/min, putting the acceptable amount of physical demand for a typical workday at about 6.5 mL/minute/kg (VO2 max x 0.3). This number can then be compared to the demands that different jobs and activities place on the human body. For example, showering takes 7 mL/ kg/min. ME/CFS patients are advised to not engage in activities that exceed their oxygen consumption threshold.
VO2 max can easily be converted to the metabolic equivalent (MET). The metabolic equivalent 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, which approximates the rate of energy expenditure at rest for a 40-year old man who weighs 70 kg. METs < 3 are considered light activities, METs between 3 and 6 are considered moderate levels of activity, and METs > 6 correspond to vigorous activities.
Using the example in Appendix 1, a VO2 max of 21.7 mL O2/kg/min could be expressed in METs by dividing through by 3.5 mL O2/kg/min, or 21.7 mL O2/kg/min ÷ 3.5 mL O2/kg/min = 6.2 METs in this example. Thus, the most a person with a VO2 max this low can do is moderate activity, keeping in mind that this is based on VO2 max, which corresponds with the peak activity that is attainable; most people with ME/CFS stay far away from that threshold. For ME/CFS patients, the relevant value is VO2 at V/AT, which is always less than VO2 max.
In the example, V/AT was 10.2 mL O2/kg/min. In this case, the corresponding MET value is 10.2 mL O2/kg/min ÷ 3.5 mL O2/kg/min = 2.9 METs, a substantially lower number. This value can be cross-referenced to the Compendium of Physical Activities to see the types of activities that are safe, given the upper value of METs that can be performed based on V/AT. Since many daily activities fall within the 3-5 METs range, ME/CFS patients may continually exacerbate symptoms associated with PEM by doing simple activities of daily living like brushing teeth, showering, or preparing a quick meal.
A limitation of the METs approach is that it does not account for age and body mass differences and does not consider the fact that some people have a lower level of fitness than others. Research has shown that METs underestimate energy expenditure from 12-22% in people with lower fitness. This is especially true in ME/CFS, in which impaired oxidative metabolism is the norm. Walking at 3 mph requires 4 METs and is classified to be a moderate-intensity activity, but to a person with ME/CFS such activity levels cannot safely be sustained (due to PEM) and can carry a heavy cost of recovery. For this reason, corrected MET values that account for age, weight, and gender are preferable when determining a safe level of energy expenditure for an individual.
VO2 and METs can also be matched against the New York Heart Association’s (NYHA) Functional Classification to determine disability according to how limited a person is during activity. Using these criteria, the data in Appendix 1 show moderate-to-severe disability.
Work in Watts
Work is measured in Watts (W), which is a rate expressed as energy per unit time. For example, a 100 W light bulb turned on for 1 hour will use 100 Watts/hour. A 50 W light bulb could remain on for 2 hours using the same energy. 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 (in Watts) at V/AT provides a stark picture of how ill ME/CFS patients are. If it takes 100 W to power the human body, and the workload at V/AT is a fraction of that, it would show another aspect of disability. In Appendix 1, workload at V/AT ranged between 36 Watts on day 1 and 31 Watts on day 2, which is equivalent to very light housework. Ironing and cooking range from 35-44 Watts, suggesting that even these light activities are too demanding.
The cardiovascular data also provide insights about level of disability. As above, the V/AT represents the point at which anaerobic metabolism (and acidosis from lactic acid production) occurs. ME/CFS patients use the V/AT as a benchmark for pacing, with the goal being to spend as much of the day as possible below this value.
(If you can’t do a two-day CPET you can use a multiplier (.6, .5, .4, .3 ) and then assess your response to get an idea of your safe heart rate using this formula: (220 bpm – age) x 0.6 or 0.5 or 0.4 or 0.3 = safe heart rate.)
Although there are rules of thumb to help guide pacing decisions, the 2-day CPET is the only way to know actual V/AT and the change in V/AT between days 1 and 2. Using the example in Appendix 1, and applying a multiplier commonly used by ME/CFS patients (0.6), a 50-year-old’s AT would be = (220 bpm – age) x 0.6 = (220 bpm – 50) x 0.6 = 102 bpm. Actual AT, as measured by a 2-day CPET, was 69 bpm in the example – a far more disabling picture.
One way to approximate AT is to do the calculations using different multipliers (0.4, 0.5, and 0.6) and get familiar with how your body responds to exceeding each of these different thresholds. Many people learn to recognize symptoms that occur when AT has been crossed (e.g., dizziness, shortness of breath). If you are using an estimated AT of 100 bpm and feel horrible at 85 bpm, this might be an indication that the multiplier used is too high. Better safe than sorry.
Chronotropic insufficiency (CI): Another instructive measure from the 2-day CPET is the peak heart rate obtained during the test. In theory, the maximum heart rate (HR max) that can be achieved is estimated to be around 220 – age. A discrepancy between HR max and actual peak heart rate achieved during the test might indicate chronotropic insufficiency.
Chronotropic insufficiency is the inability of the heart to increase its rate commensurate with demand and is often expressed as the percentage of HR max that is obtained during a maximal exercise test. The ability to perform work depends on an ability to increase oxygen uptake (VO2) and is key to the ability to sustain aerobic exercise. Because the heart rate must increase dramatically during exercise in order to propel enough oxygen-loaded blood to the muscles for them to do their work, an inability to do that properly (CI) can be very disabling.
Chronotropic incompetence or the inability to get the heart rate up to speed during exercise appears to be common in ME/CFS appears to be quite common in ME/CFS, as well as in patients with cardiovascular disease, and produces severe exercise intolerance in its own right, having nothing to do with disorders of the autonomic nervous system that are common in ME/CFS, such as postural orthostatic tachycardia syndrome (POTS), and neurally-mediated hypotension (NMH).
Interestingly, because patients are seated and moving during the CPET, the large rises in HR seen in POTS patients often do not occur during CPET testing, according to the Workwell Foundation. If anything, these patients have a weaker than predicted heart rate response, possibly due to chronotropic insufficiency. The Workwell Foundation is currently preparing a paper looking at CI in ME/CFS.
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).
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VE/VCO2: VE/VCO2 is a measure of CO2 elimination. VE is modulated by CO2 and has a tight, linear relationship with VCO2. The slope of that relationship during exercise – also called VE/VCO2 – is used as an index of ventilatory efficiency. The normal range for the VE/VCO2 is 20-30; values above this are commonly found in ME/CFS. Elevated VE/VCO2 values are associated with diseases that have an increased ventilatory requirement for a given level of exercise or workload, such as heart failure.
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 yet another form of orthostatic intolerance that contributes to the disability picture found in ME/CFS patients.
Each of the variables measured in the CPET provides an opportunity to compare a patient’s values against that which would be predicted based on the general population. The drop in functional capacity between day 1 and day 2 presents a stark, and often bleak, picture for many ME/CFS patients.
The value of the test is two-fold. First, it provides irrefutable and quantitative evidence on the functional capacity of a person, and thus their ability to perform work. Second, the metabolic, cardiac, work, and respiratory measures provide actual (vs. theoretical) benchmarks for pacing. VO2 max, METs, and workload at V/AT all offer ways to deepen our pacing efforts. We likely know the types of activities that cause us to crash, yet do them anyway. By understanding how activities are classified in terms of corrected METs, Watts, or VO2, patients can better learn how to avoid activities that are all but certain to lead to PEM because such activities will likely invoke anaerobic metabolism.
Ultimately, it is a very personal choice for an ME/CFS patient to either choose or forgo a 2-day CPET for disability purposes. 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.
Some people bristle at the idea of calling the 2-day CPET a biomarker for ME/CFS because it is unethical to ask patients to knowingly cause harm to themselves. Hopefully, with the advent of a more straightforward biomarker, like a blood test, the 2-day CPET may not be required to win disability cases in the future. However, given that the CPET provides direct measures of functional capacity, it would stand to reason that the utility of the CPET to ME/CFS patients is not going to lessen anytime soon.
Many patients have abnormal results that show up on day one. Can the test be stopped at this point and still be used to demonstrate disability?
Patients with a very low VO2 max and V/AT may have the numbers to demonstrate disability (Table 1). However, many patients have found that their results from a 1-day test were dismissed as deconditioning, while others have succeeded in being awarded disability with a 1-day test, presumably because VO2 max was very low and/or there were other factors that helped with their case.
A second test is necessary to document the atypical recovery response and prolonged fatigue associated with PEM. This information shows the lack of ability to sustain work.
Couldn’t the CPET be adapted to be done from bed, especially for severely ill patients, who might reach their anaerobic threshold upon rolling over?
For the CPET to be valid it must be a maximal exercise test. This is not possible while in bed. A maximal test is required for VO2 max and to ensure V/AT has occurred during the test. These variables form the basis for assessing functional capacity and therefore disability.
I did an exercise test at my cardiologist’s office recently. Couldn’t I use the results from that test rather than putting myself through a 2-day CPET?
While a stress test provides valuable information about certain heart conditions, it is not considered a maximal exercise test and therefore VO2 max and V/AT cannot be derived. A 2-day CPET is required to capture the drop in function that is often seen between days 1 and 2.
Is a test invalid if patients arrive already in PEM from the stress of travel?
PEM will lower the anaerobic threshold, so the starting point might be lower than it would be when not in a crash. However, repeating the test often yields a significant drop in function, even though the starting place was low to begin with.
What is the best way to prepare for the test?
The best information on how to prepare for the 2-day CPET comes directly from the Workwell Foundation.
What is the best airport to fly into?
Sacramento is the closest airport, but San Jose and San Francisco are within 3 hours of Ripon, CA, where the Workwell Foundation is located. However, it is always a good idea to double check where your test is scheduled; higher-risk cardiac patients are required to do their test in Newport Beach, CA, where an MD helps to oversee the test.
How long do I need to stay for the test?
A minimum of 2 nights is required, and it would be wise to arrive the night before (to minimize PEM on day of test) and to stay the night of the second test, and possibly a third, depending on travel distance. Patients are not allowed to drive themselves after the test.
How can I help speed up my recovery?
In addition to the information from Workwell on how to best prepare for the test, they also recommend that patients arrange for IV saline after the test. Saline can still be beneficial even if administered a few days after the test, when PEM really sets in.
How have other patients responded to a 2-day CPET?
Everyone responds differently to the CPET, but it is still worth exploring other patients’ experiences.
Where else can I get a 2-day CPET besides Workwell?
There are a few places that use the same or similar methodology as Workwell, including Betsy Keller at Ithaca College and Laura Black at Hunter-Hopkins Center in North Carolina. Other sites can be found here.
The Workwell Foundation and colleagues recently published a paper outlining their methodology for assessing ME/CFS patients with a 2-day CPET. This paper could be shared with sports medicine physiologists, or others who perform CPET testing, to inform them of how the 2-day test is done for ME/CFS patients. Workwell uses methodologies that adhere to cardiopulmonary testing standards established by the American College of Sports Medicine.
Acknowledgements: Deep gratitude to the Workwell Foundation for being champions of the ME/CFS community and Staci and Jared Stevens for their contributions to this article. Many thanks to Cort Johnson for his excellent editing suggestions.
More on Workwell from Health Rising:
- The Disability Defenders: The Workwell Foundation, Chronic Fatigue Syndrome (ME/CFS) and Fibromyalgia
- How Physical Therapists are Getting it About Chronic Fatigue Syndrome (ME/CFS): The Workwell Foundation Pt. I
- The Workwell Foundation Resource Page for Chronic Fatigue Syndrome
Caroline’s Health Rising Articles
- Learn more about Caroline and check out more Health Rising articles from her here.
Appendix 1. Sample 2-day CPET report.
Cardiopulmonary Exercise Test (CPET) Evaluation Report
Patient demonstrates cardiopulmonary anomalies*, low function and delayed recovery with severe symptom exacerbation post-exertion. This will severely limit the patient’s ability to engage in normal activities of daily living and precludes employment of even a sedentary/stationary nature. *Treating physician please take note.
The patient was referred to our lab for global functional evaluation examining metabolic, cardiovascular and pulmonary function after experiencing physical stress. The patient underwent a cardiopulmonary exercise test-retest over a two-day period. She is 50 years-old, 69 inches tall and weighs 117.4 pounds.
The patient performed symptom limited 10 W/min ramping protocols on a bicycle ergometer while expired gases were collected for determination of oxygen consumption, carbon dioxide production and pulmonary ventilation. Two exercise tests were performed on consecutive days. The heart rate, blood pressure and arterial oxygen saturation were assessed throughout the tests. Appropriate measures were taken to calibrate and test the accuracy and reliability of the testing equipment on both days. These tests were performed to determine functional capacity and assess the recovery response to a standardized physical stressor.
In the fields of exercise science and medicine, cardiopulmonary exercise testing (CPET) is considered the gold standard for measuring and evaluating functional capacity and fatigue. Position statements and/or guidelines for the performance of this testing are available from the American College of Sports Medicine, American Heart Association, American College of Chest Physicians, American Thoracic Society and the American Medical Association, among others. All endorse this method of testing and acknowledge peak oxygen consumption, only available with CPET, as the most accurate measurement of functional capacity. Workwell Foundation has adopted this standardized, reliable and accurate tool to evaluate disability in fatigue-related disorders.
(1) Assessment of Effort: Normal
The patient was cooperative and there is no evidence of malingering. Respiratory exchange ratio (RER) and rating of perceived exertion (RPE) values met criteria for maximal effort on both tests. Cardiopulmonary exercise testing provides objective measures that can clearly distinguish between indolence and true disability. See page 4, #1 Assessment of Effort.
2) Metabolic Responses: Abnormal
Peak and submaximal oxygen consumption was abnormally reduced between tests (11%, 16%). Day to day test variability for metabolic processes is less than 8% for healthy individuals. Abnormally high variability for these measures indicates a disruption of homeostasis during physical activity. Test 2 measured value for oxygen consumption at the ventilatory/anaerobic threshold indicates severe to moderate disability using the Weber/New York Heart Association criteria. See page 4, #2 Metabolic Responses.
3) Workload: Abnormal
Submaximal workload was abnormally reduced between tests (14%). This indicates a diminished capacity for activity post exertion. Submaximal workload of 36 to 31 watts is equivalent to light housework activities such as ironing or cooking which range from 35 to 44 watts. See page 4, #3 Work in Watts.
4) Cardiovascular Responses: Abnormal
Peak heart at only 71% to 65% of predicted value may indicate chronotropic incompetence. Diminished maximum heart rate is a significant functional impairment. Certain patient medications may affect heart rate. See also Respiratory Responses below. See page 5, #4 Cardiovascular Responses.
5) Respiratory Responses: Abnormal
Values for exhaled carbon dioxide were reduced, i.e., low PetCO2 (<40 mmHg) and elevated VE/VCO2(>30). These measures show impaired ventilatory efficiency and are considered indicators for chronic metabolic acidosis, and/or cardiovascular disease, and/or pulmonary disease. Low exhaled CO2 levels can also be indicative of orthostatic hypocapnia. See page 5, #5 Respiratory Responses.
6) Recovery Response: Abnormal
A recovery time of 24 hours or less and minor muscle soreness is considered normal following exercise testing. This patient’s recovery time of 7+ days along with excessive fatigue and symptom exacerbation should be considered an extreme reaction to physical activity. See page 5, #6
- Assessment of Effort
The American Heart Association cite peak respiratory exchange ratio (RER) as the most accurate and reliable gauge of subject effort during cardiopulmonary exercise testing. A peak RER ≥1.10 is generally considered an indication of excellent patient effort. An RER between 1.0 and 1.09 indicates good effort.
|Test Criteria||Test 1||Test 2|
|RER >= 1.10||1.19||1.17|
|RPE >= 17||20||20|
RER = Respiratory Exchange Ratio
RPE = Rate of Perceived Exertion
- Metabolic Responses
|Peak Values||Oxygen Consumption(mL/min)||Oxygen Consumption(mL/min/kg)||Percent Predicted(%)|
|V/AT||Oxygen Consumption(mL/min)||Oxygen Consumption(mL/min/kg)||Percent Predicted%|
|mL/min – milliliters per minutemL/kg/min – milliliters per kilogram per minute
V/AT – ventilatory/anaerobic threshold
(determined using V-slope, ventilatory equivalents, and end-tidal pressure methods)
- Work in Watts
|Workload||V/AT (W)||Peak (W)||Percent Predicted%|
W – Watts
V/AT – Ventilatory/Anaerobic Threshold
- Cardiovascular Response
|Heart Rate||Resting seated(bpm)||V/AT(bpm)||Peak(bpm)||Percent Predicted(%)|
|Blood Pressure||Resting Seated(mmHg)||Resting Supine(mmHg)||Peak(mmHg)|
- bpm – beats per minute
- mmHg – millimeters of mercury
- V/AT – ventilatory/anaerobic threshold
- Respiratory Response
|VE/VCO2||PETCO2(mmHg)||VE(L/min)||VE % Predicted|
- Recovery Response
A post exercise test log was maintained by the patient. Following testing the patient experienced profound fatigue, weakness, difficulty walking, dizziness/lightheadedness, nausea, pressure headache, increased thirst, burning sensation in limbs and head, erratic heart rate, and cognitive difficulties. Patient was not recovered 7 days post-testing.
The patient’s low peak oxygen consumption, reduced oxygen consumption at the ventilatory/anaerobic threshold (V/AT)* and symptom exacerbation post activity indicates significant impairment. Energy expenditures at, or close to the V/AT represent vigorous activity and can be sustained for only short periods of time. The International Labor Organization regard 30% or less of maximal oxygen consumption (VO2 max) as the threshold for acceptable physiological demands over an 8-hour work day. Estimated energy expenditures for most occupations and life activities can be found in the Compendium of Physical Activities.
Based upon a test 2 measured peak exercise capacity of 21.7 ml.kg-1.min-1, the safe limit for sustained activity is an oxygen consumption of around 6.5 ml.kg-1.min-1. The estimated oxygen requirement for seated office or computer work is 5.25 ml.kg-1min-1. However, even sedentary work involves more than just sitting at a desk. For normal office tasks, the energy cost rises to 10.5 ml.kg-1.min-1 which is above the V/AT. Driving to and from work would require 8.75 ml.kg-1.min-1 of oxygen. In addition, every day activities such as showering (7.0 ml.kg-1.min-1) or making the bed (11.6 ml.kg-1.min-1) represent significant energy demands. The patient should limit activities requiring oxygen consumption above 6.5 ml.kg-1.min-1 and avoid, if possible, activities requiring oxygen consumption beyond the test 2 V/AT of 10.2 ml.kg-1.min-1. Maximal heart rate should not exceed 69 beats per minute. Energy expenditures at or close to this level will likely result in symptom exacerbation and delayed recovery.
Even a sedentary job would require more energy than can be safely sustained.