Insights From Invasive Cardiopulmonary Exercise Testing of Patients With Myalgic Encephalomyelitis/Chronic Fatigue Syndrome.
Joseph, Phillip, Arevalo, Carlo, Oliveira, Rudolf K F et al. · Chest · 2021 · DOI
Quick Summary
This study used specialized heart and lung testing during exercise to understand why people with ME/CFS struggle with physical activity. Researchers found that ME/CFS patients had two different problems during exercise: some had reduced blood flow returning to the heart, while others couldn't efficiently extract oxygen from their blood. About 31% of patients also showed evidence of small nerve fiber damage in their skin biopsies.
Why It Matters
This research provides objective physiological evidence for two distinct mechanisms of exercise intolerance in ME/CFS, moving beyond functional descriptions to identify measurable abnormalities in cardiac output and oxygen utilization. The identification of a subgroup with small fiber pathology suggests potential neuropathic contributions to exertional dysfunction, opening avenues for mechanistically targeted interventions. These findings validate that ME/CFS exertional intolerance has biological underpinnings rather than purely psychological causes.
Observed Findings
ME/CFS patients showed significantly lower right atrial pressures during exercise (1.9 ± 2 mmHg) compared to controls (8.3 ± 1.5 mmHg).
Peak VO₂ was substantially lower in ME/CFS patients (80% ± 21% predicted) versus controls (101.4% ± 17% predicted).
The low-flow tertile exhibited reduced peak cardiac output (88.4% ± 19% predicted) compared to normal- and high-flow groups (~99-100% predicted).
The high-flow tertile showed impaired systemic oxygen extraction (0.74 ± 0.1) versus low- and normal-flow groups (0.88 ± 0.11 and 0.86 ± 0.1, respectively).
Small fiber neuropathy was identified in 31% of the 160 ME/CFS patients based on skin biopsy analysis.
Inferred Conclusions
Two distinct peripheral neurovascular dysregulation patterns contribute to ME/CFS exertional intolerance: reduced cardiac output from impaired venous return, and impaired peripheral oxygen extraction.
In patients with small-fiber pathology, neuropathic dysregulation causing microvascular dilation may reduce oxygen delivery to tissues and cardiac return, thereby limiting exercise capacity.
Small fiber neuropathy is a common but not universal pathological finding in ME/CFS and may represent a distinct pathophysiologic subtype.
Remaining Questions
Does the presence or severity of small fiber neuropathy predict response to specific therapeutic interventions targeting vasomotor or autonomic dysfunction?
What This Study Does Not Prove
This study does not establish causation between small fiber neuropathy and exertional intolerance, as the severity of nerve fiber denervation did not correlate with exercise limitation measures. It does not determine whether the observed neurovascular abnormalities cause ME/CFS or result from it, nor does it prove these mechanisms apply to all ME/CFS patients. The study population was selected from those undergoing clinical iCPET, which may not represent the broader ME/CFS patient population.
Are the identified cardiopulmonary phenotypes (low-flow vs. high-flow with impaired extraction) stable over time, or do they change with disease progression or recovery?
What mechanisms drive the development of these two distinct patterns—are they different disease processes, or do they represent different stages of the same underlying pathology?
Can non-invasive testing methods reliably identify and stratify patients into these cardiopulmonary phenotypes for clinical use?