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What’s the Real Cause of Exercise Intolerance?

Exercise Intolerance

Many people with illnesses such as Chronic Fatigue Syndrome (CFS), Fibromyalgia (FM), and POTS report similar experiences and frustrations with exercise. They are told that exercise and staying active will help their condition and yet attempts at aerobic exercise often lead to breathlessness, dizziness, and post-exertional fatigue. Many times, their doctors blame these symptoms on deconditioning, or simply being “out of shape.” However, these patients - especially those who were active or athletic before becoming chronically ill - can attest to the fact that their symptoms go beyond the adjustment period normally experienced when starting an exercise program. Research validates that you’re not simply “out of shape”.

New research findings have discovered distinct physiological reasons for this phenomenon known as “exercise intolerance,” validating the experiences of these chronically ill patients.

Insights from the field of Pulmonology

A pulmonologist out of Brigham and Women’s hospital in Boston recently published the results of his study, which included over 600 of his patients over the course of nine years. All reported dyspnea (breathlessness) following exercise. These patients had no history of heart failure, cystic fibrosis, or arrhythmias that might otherwise explain their exercise intolerance.

While Dr. Systrom does not specifically study CFS and related disorders, he sought to shed light on a large subset of patients who are often left without answers after undergoing the litany of non-invasive cardiac tests typically run by pulmonologists. Interestingly though, many of these patients appear to fall into the CFS, FM, and POTS category, leading experts in both fields to take notice of Systrom’s findings.

The study participants underwent invasive Cardio Pulmonary Exercise Testing (iCPET), which involves placing catheters into the radial and pulmonary arteries. Data was gathered as they were exercised (using a bicycle) to the point of exhaustion. In about 10% of them, low QT and ventricular filling was found. This essentially means that the heart was not filling adequately with blood when it is supposed to (diastole).

Ironically, this is actually the opposite of what is seen when a de-conditioned or “out of shape” person exercises! The researchers attributed these findings to inadequate expansion of the heart muscle during exercise, which is required for diastole. This is called “low preload” and explains low energy production that has been previously demonstrated in patients with Chronic Fatigue syndrome.

The Link to Chronic Fatigue Syndrome and Related Disorders

The patients were treated with IV saline solution in order to increase blood volume. While this did improve the low QT somewhat, it did not completely remedy the problem. This led the researchers to conclude that low blood volume is not the primary cause of exercise intolerance in these individuals. Instead, Systrom believes that the main problem is related to “impaired capacitance,” or decreased constriction of the veins during exercise.

Constriction is necessary to pump blood up to the heart. This can lead to decreased heart size. Problems with both blood flow and an atrophied heart muscle have been found in previous studies related to autonomic dysfunction, adrenal insufficiency, and autoimmune processes – all things proposed to be at play in CFS, FM, and POTS.

Decreased oxygen utilization was seen in the study participants as well. This is consistent with circulation issues seen in these disorders due to blood pooling in other areas of the body, such as the abdominal region.

Conclusions and Considerations

Identifying that these patients are most likely suffering from a circulation problem rather than a blood volume problem is a huge step in helping to ameliorate the symptoms of exercise intolerance in these individuals. Unfortunately, it still does not answer the question of why these dysfunctions occur in the first place.

Systrom observed in his practice that the onset of symptoms often appeared after a bout with an infectious disease. This would correspond to the research of those who propose an inflammatory or immune/autoimmune response in these disorders. A small group of POTS patients in part of the study improved with increased fluid intake, compression stockings, exercise training, and medications to down-regulate the sympathetic nervous system. Some also responded well to a vaso-constricting drug.

While being “out of shape” may contribute to exercise intolerance in these patients to some degree, these recent findings confirm that deconditioning is secondary to the circulatory and other problems seen in CFS, FM, and POTS.

Although this particular study does not provide concrete answers on the underlying causes of the disorders themselves, hopefully the findings will lead to increased awareness about the pathophysiology of exercise intolerance seen in these patients. This should, in turn, result in more compassion from physicians and targeted treatments that will allow these patients to safely experience the many benefits of exercise.


1. Johnson, J. The Exercise Intolerance in POTS, ME/CFS and Fibromyalgia Explained? Health Rising: Finding Answers for ME/CFS and FM. Available at: http://www.healthrising.org/blog/2016/07/04/exercise-intolerance-fibromyalgia-chronic-fatigue-pots-explained/

2. Nijs, J et al. Altered immune response to exercise in patients with chronic fatigue syndrome/myalgic encephalomyelitis: a systemic literature review. Exerc Immunol Rev. 2014; 20: 94-116.

What’s the Real Cause of Exercise Intolerance? was last modified: November 8th, 2017 by Holtorf Medical Group

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