What a CPET shows and why it matters

A cardiopulmonary exercise test (CPET) is the most comprehensive investigation available for unexplained exercise intolerance, breathlessness, or fatigue with exertion. It measures not just how much exercise you can do, but why you can’t do more — identifying whether the limiting factor is pulmonary, cardiovascular, muscular, or functional.

In the context of post-viral conditions, CPET has emerged as one of the most diagnostically useful tests available. It consistently reveals abnormalities that standard testing (lung function, echocardiogram, bloods) misses, and it provides a quantitative map of what the body is doing during exertion that has direct implications for management.

What a CPET measures

A CPET is conducted on a cycle ergometer or treadmill while wearing a face mask that measures inhaled and exhaled gases. The workload is increased progressively — typically 1 watt every 10 seconds on a cycle — while measuring:

VO2 (oxygen consumption) — reflects how much oxygen the body is using, which is a proxy for overall metabolic work. VO2max is the maximum oxygen uptake and is the gold standard measure of cardiorespiratory fitness.

VCO2 (carbon dioxide production) — tracks metabolic CO2 generation. Rises predictably during aerobic exercise; rises sharply when anaerobic metabolism begins.

Respiratory exchange ratio (RER) — the ratio of VCO2 to VO2. At rest, around 0.8. During aerobic exercise, gradually rises. When anaerobic threshold is crossed, shoots toward 1.0 and above as bicarbonate buffering generates additional CO2.

Heart rate response — tracks how heart rate changes with increasing workload. Normal heart rate reserve (maximum predicted HR minus resting HR) is used fully by maximum exercise. In dysautonomia, this can be profoundly abnormal.

Oxygen pulse — VO2 divided by heart rate, a proxy for stroke volume times oxygen extraction. A flat or falling oxygen pulse during exercise suggests impaired cardiac output augmentation.

Ventilatory efficiency (VE/VCO2 slope) — how much ventilation is required per unit of CO2 production. Elevated in pulmonary hypertension, heart failure, and various other pathologies.

The anaerobic threshold

The anaerobic threshold (AT) — also called the ventilatory anaerobic threshold or VT1 — is the exercise intensity above which the body can no longer meet energy demands purely through aerobic metabolism. Energy demand begins to outstrip aerobic supply, and anaerobic glycolysis kicks in, producing lactate and excess CO2.

In healthy, fit individuals, the AT occurs at around 50–65% of VO2max. In sedentary but healthy people, it may be at 40–50%. In trained endurance athletes, it can be at 80–90%.

In post-viral conditions, particularly ME/CFS and long COVID, the AT is often strikingly low — sometimes at 30–40% of predicted VO2max, or even lower. This means the body tips into anaerobic metabolism at minimal effort levels. Walking at a normal pace or climbing a single flight of stairs can push workload above the AT.

This has direct practical implications. Exercising above the AT appears to trigger the metabolic and immune events associated with post-exertional malaise. Staying below it — what some researchers call “aerobic envelope” or “AT-guided pacing” — is the physiological basis for the pacing approach used in ME/CFS management. For recumbent exercise, knowing your anaerobic threshold from a CPET gives you a heart rate ceiling to work within.

What CPET shows in post-viral conditions

Several groups have published CPET data from ME/CFS and long COVID cohorts. The patterns found repeatedly include:

Reduced VO2max — maximum oxygen uptake is below age- and sex-predicted norms. This is the direct measure of impaired aerobic capacity, and it’s consistently found in ME/CFS, with severity roughly correlating with disease severity.

Low anaerobic threshold — as described above, the AT occurs very early in the exercise protocol, often at workloads well below what would produce symptoms in healthy people.

Impaired heart rate response — in some patients, heart rate fails to rise appropriately with increasing workload (chronotropic incompetence). In POTS-phenotype presentations, heart rate may rise too fast too soon, exceeding predicted values at low workloads.

Flat oxygen pulse — suggesting failure of cardiac output to augment appropriately during exercise. This is the cardiovascular signature of impaired stroke volume response.

The 2-day CPET finding — perhaps the most striking feature of ME/CFS is the reproducibility failure. In healthy people, performing the same maximal CPET on two consecutive days produces near-identical results. In ME/CFS, the second-day CPET shows significantly lower VO2max and AT than the first. This is unique to ME/CFS (and some long COVID patients) and suggests that the physiological impairment worsens after exertion — the biological signature of post-exertional malaise.

Long COVID patterns — studies of long COVID patients without pre-existing cardiopulmonary disease have found similar but often milder patterns: reduced VO2max, early AT, and sometimes impaired heart rate recovery. Some long COVID CPET studies have also found reduced skeletal muscle oxygen extraction despite normal cardiac output, suggesting a peripheral (mitochondrial or microvascular) mechanism.

Why it matters for diagnosis

Standard investigations in post-viral fatigue — full blood count, thyroid function, echocardiogram, spirometry — are usually normal. This is appropriate but frustrating: normal results rule out obvious alternative explanations but don’t explain what’s actually happening.

CPET provides objective, quantified evidence of physiological impairment in people whose standard tests are normal. A CPET showing VO2max of 60% predicted and AT at 35% predicted is objective evidence of significant functional impairment that cannot be attributed to deconditioning alone (which doesn’t produce this specific pattern) or anxiety (which doesn’t either).

This matters enormously for validation, for disability assessment, and for guiding treatment. It’s the difference between “we can’t find anything wrong” and “here is the specific physiological mechanism of your exercise intolerance.”

Why it matters for management

The anaerobic threshold value from a CPET provides a target heart rate for exercise management. If your AT corresponds to a heart rate of 95 bpm, then exercising below 90 bpm keeps you reliably in the aerobic zone. This is actionable and specific in a way that generic advice to “pace yourself” is not.

Several post-viral and ME/CFS specialists now advocate for AT-guided pacing as the practical management approach most supported by the physiological evidence. It’s more conservative than the Levine exercise programme (which targets gradual escalation of capacity) but less restrictive than complete rest.

Access in the UK

CPET is performed in specialist cardiopulmonary or respiratory centres. In the NHS, it’s most commonly requested for:

Pre-operative cardiac/respiratory risk assessment
Unexplained dyspnoea
Heart failure assessment
Pulmonary hypertension workup

Post-viral patients need to specifically request it for exercise intolerance evaluation, framing it in terms of unexplained dyspnoea and exercise limitation. Some long COVID clinics are now offering CPET as part of their assessment pathway. It’s harder to access outside of these contexts.

Private CPET testing is available at some centres for approximately £300–500. If you’re significantly limited by exercise intolerance and standard tests are normal, it’s one of the more useful investments available.

References

Davenport TE, Stevens SR, VanNess MJ, et al. Conceptual model for physical therapist management of chronic fatigue syndrome/myalgic encephalomyelitis. Phys Ther. 2010;90(4):602–614.

Keller BA, Pryor JL, Giloteaux L. Inability of myalgic encephalomyelitis/chronic fatigue syndrome patients to reproduce VO2peak indicates functional impairment. J Transl Med. 2014;12:104.

Systrom DM, Malhotra R, Mikkelsen ME. Unexplained exertional dyspnea caused by low ventricular filling pressures: results from clinical right heart catheterization studies. Eur Respir J. 2022;59(3):2101734.

Vermeulen RCW, Vermeulen van Eck IWM. Decreased oxygen extraction during cardiopulmonary exercise test in patients with chronic fatigue syndrome. J Transl Med. 2014;12:20.

Mancini DM, Brunjes DL, Lala A, et al. Use of cardiopulmonary stress testing for patients with unexplained dyspnea post-coronavirus disease. JACC Heart Fail. 2021;9(12):927–937.