E2 ModerateModerate confidencePEM unclearObservationalPeer-reviewedMachine draft
Hypocapnia is a biological marker for orthostatic intolerance in some patients with chronic fatigue syndrome.
Natelson, Benjamin H, Intriligator, Roxann, Cherniack, Neil S et al. · Dynamic medicine : DM · 2007 · DOI
Quick Summary
This study compared how ME/CFS patients and healthy volunteers responded to standing up for 8 minutes. Researchers measured blood pressure, heart rate, breathing rate, and carbon dioxide levels in the blood. ME/CFS patients were more likely than healthy people to have abnormal responses, particularly low carbon dioxide levels (hypocapnia), which was linked to feeling more ill and short of breath.
Why It Matters
This study identifies orthostatic hypocapnia as a potential biological marker that could help distinguish a physiologically measurable subgroup of ME/CFS patients, reducing the clinical heterogeneity that complicates research and treatment. Understanding specific physiological abnormalities in ME/CFS may lead to targeted diagnostic tools and interventions. The association between hypocapnia and symptom severity suggests a mechanism worth investigating further.
Observed Findings
- 53% of ME/CFS patients showed abnormal orthostatic test results compared to 20% of controls (p<0.002).
- Orthostatic hypocapnia occurred in 20.6% of ME/CFS patients versus 2.9% of controls (p<0.02).
- Rates of orthostatic tachycardia, hypotension, and hypertension did not significantly differ between ME/CFS patients and controls.
- ME/CFS patients with hypocapnia reported significantly more feelings of illness and shortness of breath than both controls and ME/CFS patients with normal test results.
Inferred Conclusions
- A substantial proportion of ME/CFS patients have orthostatic intolerance, but this manifests primarily as hypocapnia rather than the blood pressure or heart rate changes seen in other conditions.
- Orthostatic hypocapnia may represent a distinct physiological subtype of ME/CFS, allowing better patient stratification and reduced heterogeneity in research samples.
- Hypocapnia during orthostatic stress correlates with increased symptom severity and may serve as a biological marker for a specific ME/CFS phenotype.
Remaining Questions
- What mechanisms drive the development of orthostatic hypocapnia in ME/CFS? Is hypocapnia a primary feature or secondary to another underlying abnormality?
- Do ME/CFS patients with orthostatic hypocapnia respond differently to treatments compared to other ME/CFS subtypes?
What This Study Does Not Prove
This cross-sectional study cannot establish causation—it shows that hypocapnia is associated with ME/CFS and orthostatic symptoms, but does not prove hypocapnia causes the condition. The study does not demonstrate whether hypocapnia is primary, secondary, or merely a marker of other underlying dysfunction. The findings apply specifically to patients with positive orthostatic testing and cannot be generalized to all ME/CFS patients.
Tags
Symptom:Orthostatic IntoleranceFatigue
Biomarker:Blood Biomarker
Method Flag:Weak Case DefinitionSmall SampleExploratory Only
Metadata
- DOI
- 10.1186/1476-5918-6-2
- PMID
- 17263876
- Review status
- Machine draft
- Evidence level
- Single-study or moderate support from human research
- Last updated
- 8 April 2026
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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