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A cardiopulmonary exercise test, or CPET, is an exercise test on a bike or treadmill while you breathe through a mouthpiece that measures the air going in and out. Ordinary tests look at your heart or your lungs while you are sitting still. This one watches your heart, lungs, circulation and muscles all working together while you exercise — which is when your symptoms actually happen. That lets the team see which part of the system runs out first, and therefore what is worth treating. The test is meant to be hard and you stop when you have had enough; it is closely supervised throughout. Sometimes the most useful result is a normal one, because it points the answer somewhere treatable, such as breathing pattern or fitness, rather than to disease.
What CPET is
Cardiopulmonary exercise testing is a maximal or symptom-limited exercise test performed on a cycle ergometer or treadmill while expired gas is analysed breath by breath. It measures simultaneously what the lungs, the heart, the circulation and the muscles are doing at every workload — and because it captures all of them at once, it can identify which of them is the reason a person has to stop.
That is the whole point of the test. Almost every other investigation examines one organ at rest. CPET examines the integrated system under load, which is the state in which patients actually experience their symptoms.
Why not a six-minute walk test?
| Field walking tests | CPET | |
|---|---|---|
| What it gives | Functional capacity, desaturation, a responsive outcome measure | Peak oxygen uptake, ventilatory threshold, gas exchange efficiency, cardiac and ventilatory reserve |
| Effort | Self-paced and submaximal in most patients | Maximal, with an objective marker of effort |
| Answers “how far?” | Yes | Yes |
| Answers “why so far and no further?” | No | Yes |
| Setting | A corridor, a stopwatch, a pulse oximeter | Metabolic cart, ECG, medical supervision |
The six-minute walk and the incremental shuttle walk remain the right tests for most rehabilitation settings: they are cheap, repeatable, responsive to intervention and meaningful to patients. CPET is the test to reach for when the question is diagnostic, when risk must be quantified before surgery, or when exercise must be prescribed precisely in someone whose physiology is unusual.
What is measured
| Variable | What it means | Typical use |
|---|---|---|
| V̇O2 peak not V̇O2 max — see below | The highest oxygen uptake achieved during the test; the integrated ceiling of the oxygen transport chain as far as this test reached it | Prognosis, transplant listing, surgical risk, training prescription |
| Gas exchange threshold (GET) also VT1, ventilatory threshold, or “anaerobic threshold” | The work rate above which lactate begins to accumulate and ventilation rises disproportionately to oxygen uptake | Effort-independent measure of fitness; anchor for training intensity; preoperative risk |
| V̇E/V̇CO2 slope | Ventilation required per unit of carbon dioxide cleared — ventilatory efficiency | Strongly prognostic in heart failure and pulmonary vascular disease |
| O2 pulse (V̇O2/HR) | Surrogate for stroke volume × arteriovenous oxygen difference | A plateau or fall suggests a cardiac limitation or ischaemia |
| Breathing reserve | Peak ventilation as a proportion of maximum voluntary ventilation | Exhausted reserve indicates a ventilatory limit |
| Heart rate reserve and chronotropic response | Whether the predicted maximum heart rate is approached | Chronotropic incompetence; effect of beta-blockade |
| RER (V̇CO2/V̇O2) | Marker of effort; >1.10 in adults indicates a genuinely maximal test | Validates every other number on the report |
| SpO2, and arterial gases if sampled | Gas exchange under load; dead space fraction if arterial line in situ | Interstitial and pulmonary vascular disease |
| Flow–volume loops during exercise | Dynamic hyperinflation, expiratory flow limitation | COPD, and the inspiratory capacity manoeuvre that quantifies it |
V̇O2 peak and V̇O2 max are not the same number
V̇O2 max is a physiological maximum, and it has to be demonstrated: oxygen uptake plateaus despite a rising work rate, showing that the oxygen transport chain has genuinely run out. V̇O2 peak is simply the highest value reached before the test stopped, with no claim that a ceiling was found.
The distinction matters clinically because a plateau is rarely seen in patients. Most clinical tests are symptom-limited — they end with breathlessness, leg fatigue, chest pain or an ECG change, not with a physiological ceiling — so what is measured is a peak, and reporting it as a maximum claims something the test did not show. A clinical CPET report should say peak.
It also changes how a result is read. A low V̇O2 peak means this patient stopped here, for this reason, which is a different statement from this is the limit of their oxygen transport. This is why RER is checked first: below 1.05–1.10 the test was submaximal and the peak understates true capacity, which is also why prognostic cut-offs are unreliable in that situation. Where a genuine maximum is required — sports physiology, research — a verification phase or a second test is used to confirm the plateau, which is not part of routine clinical testing.
Indications
- Unexplained breathlessness or exercise intolerance where resting tests are normal or discordant with symptoms — the classic indication.
- Preoperative risk assessment before major intra-abdominal, thoracic, vascular or lung resection surgery.
- Heart failure — prognostication and timing of transplantation or mechanical support.
- Pulmonary hypertension and chronic thromboembolic disease — detecting a pulmonary vascular limitation and following response to treatment.
- Interstitial lung disease — quantifying gas exchange impairment and oxygen requirement under load.
- Exercise prescription in cardiac and pulmonary rehabilitation where a precise, physiologically anchored intensity is needed.
- Evaluation of treatment — before and after surgery, rehabilitation, transplantation or a pulmonary vasodilator.
- Sports and occupational assessment, and fitness-to-work or fitness-to-dive decisions.
How the test is performed
The standard adult protocol is an incremental ramp on a cycle ergometer, individualised so that the incremental phase lasts 8–12 minutes. Too steep a ramp truncates the test before a threshold can be identified; too shallow and peripheral fatigue or boredom terminates it before a cardiopulmonary limit is reached. The test comprises rest, unloaded pedalling, the ramp, and an active recovery period.
Cycling is preferred for most clinical testing: work rate is quantifiable, the artefact burden is lower, arterial lines and blood sampling are practical, and the risk of falling is removed. Treadmill testing yields a V̇O2 peak roughly 5–10% higher and is used where walking is the relevant task, or in patients unable to cycle.
The patient breathes through a mouthpiece or mask with continuous 12-lead ECG, blood pressure and oximetry. Symptoms are rated at each stage — usually breathlessness and leg fatigue on the Borg CR10 scale — and the reason the patient stopped is recorded, because it is part of the result.
Reading the test: pattern recognition
The same point on the test is called the anaerobic threshold, the first ventilatory threshold (VT1), the lactate threshold and the gas exchange threshold (GET), and many exercise physiologists now prefer GET. The objection to “anaerobic” is physiological rather than pedantic: muscle does not become anoxic at this point. Lactate rises because production outpaces clearance while oxidative metabolism continues, so the name describes a mechanism that is not occurring. GET also states what is actually being measured — a change in the relationship between expired gases, not a metabolic state inferred from it.
All four names refer to the same measured event, so reports and the literature use them interchangeably. The threshold values quoted below, including the 11 mL/kg/min surgical figure, come from studies that used “anaerobic threshold” and remain directly comparable.
Interpretation is not a single number but a pattern. The following are the common ones:
| Pattern | V̇O2 peak | Key features |
|---|---|---|
| Cardiac / circulatory limitation | Reduced | Low anaerobic threshold, O2 pulse plateau, heart rate reserve exhausted, breathing reserve preserved, ECG changes may appear |
| Ventilatory limitation | Reduced | Breathing reserve exhausted, dynamic hyperinflation with falling inspiratory capacity, heart rate reserve preserved, expiratory flow limitation on loops |
| Pulmonary vascular limitation | Reduced | High V̇E/V̇CO2 slope, low end-tidal CO2, desaturation, low O2 pulse, preserved breathing reserve |
| Gas exchange (parenchymal) limitation | Reduced | Desaturation with exercise, high V̇E/V̇CO2, rapid shallow breathing pattern, preserved cardiac reserve |
| Deconditioning | Reduced | Low anaerobic threshold, everything else proportionate, all reserves preserved, normal efficiency |
| Dysfunctional breathing / EILO | Often normal | Erratic ventilation, high and variable V̇E/V̇CO2 without desaturation, hypocapnia, symptoms out of proportion; inspiratory flow truncation in laryngeal obstruction |
| Submaximal / non-diagnostic | Variable | RER <1.05, no threshold identified, early volitional stop — report as such rather than over-interpret |
The numbers that change decisions
- Peak V̇O2 <14 mL/kg/min (or <12 on beta-blockade) has historically identified heart failure patients whose prognosis justifies transplant assessment.1
- V̇E/V̇CO2 slope >35–36 is a powerful and effort-independent prognostic marker in heart failure — in several cohorts a stronger predictor than peak V̇O2 itself.2
- Anaerobic threshold <11 mL/kg/min before major surgery identifies substantially elevated perioperative risk; the finding by Older and colleagues that cardiopulmonary reserve outperformed clinical assessment underpins modern preoperative CPET services.3,4
- RER <1.05–1.10 means the test was submaximal, and every prognostic threshold above becomes unreliable.
These thresholds inform a conversation; they do not make a decision. A number derived from a poorly performed, prematurely terminated or badly calibrated test is worse than no number at all.
Safety, contraindications and stopping
CPET in appropriately screened patients is safe, with serious adverse event rates comparable to standard exercise ECG testing. Medical supervision, resuscitation equipment and a clear escalation plan are prerequisites.
Absolute contraindications include acute myocardial infarction within 3–5 days, unstable angina, uncontrolled arrhythmia causing symptoms or haemodynamic compromise, symptomatic severe aortic stenosis, decompensated heart failure, acute pulmonary embolism, acute myocarditis or pericarditis, aortic dissection, and uncontrolled asthma. Desaturation below 85% on room air at rest is a relative contraindication requiring supplemental oxygen and specialist decision.
Reasons to terminate a test: chest pain suggestive of ischaemia, ischaemic ECG changes, sustained ventricular arrhythmia, a fall in systolic blood pressure below baseline with increasing workload, severe hypertensive response, SpO2 below 80%, pallor, ataxia, confusion, or the patient's request. See ECG basics for the rhythms that stop a session.
What the test means for the patient
Patients are often told they are being tested “to see how fit they are”, which undersells it and can make a poor result feel like a personal failure. It is more accurate and more useful to explain that the test finds out which part of the system runs out first — and that this determines the treatment. A person who learns that their limitation is deconditioning rather than heart disease often experiences it as genuine reassurance; a person whose test shows a pulmonary vascular limitation has an explanation for symptoms that may have been dismissed for years.
Practical preparation: a light meal 2–3 hours beforehand, no vigorous exercise on the day, no caffeine or smoking for several hours, comfortable clothing and shoes, usual medications unless the referrer has said otherwise, and a clear warning that the test is designed to be hard and to be stopped at the limit.
Role of the physiotherapist
In most Australian services, physiotherapists and exercise physiologists conduct CPET, and physiotherapists are the largest group of users of its results. The contribution is threefold.
Conducting the test well: selecting an appropriate ramp, coaching without leading, recognising a submaximal effort, and recording symptoms and reason for cessation accurately. A technically poor test is not a neutral event — it generates numbers that will be acted upon.
Translating the result into a prescription: a training intensity anchored to the ventilatory threshold or to a percentage of peak work rate is more defensible than a percentage of predicted maximum heart rate, particularly in patients on rate-limiting drugs, in atrial fibrillation, or with chronotropic incompetence. CPET also defines whether interval or continuous training is more appropriate, whether supplemental oxygen is needed, and where the safe ceiling lies.
Knowing when not to test: in suspected post-exertional symptom exacerbation — in long COVID and ME/CFS — a maximal test can provoke a prolonged deterioration lasting days to weeks. Screen for PESE first, and if it is present, do not proceed to maximal testing as a routine assessment.5
Evidence summary
Framing. CPET is the reference standard for exercise assessment and also the investigation most often over-ordered and under-interpreted. Its value lies not in producing a peak V̇O2 but in resolving the diagnostic question that resting investigations cannot: which link in the oxygen transport chain fails first. Its limitations are equally specific — the test is effort-dependent, it requires a technically sound ramp and calibration, and the widely quoted prognostic thresholds derive from cohorts assembled before contemporary therapy.
Prognosis in heart failureMancini's cohort established peak V̇O2 as a transplant-selection variable, with values below 14 mL/kg/min identifying patients whose survival with transplantation exceeded survival with medical therapy of the era.1 The threshold has not aged neatly: contemporary medical therapy, devices and beta-blockade have improved survival at any given peak V̇O2, and current guidance treats it as one input among several. Ventilatory efficiency has proved more durable. The V̇E/V̇CO2 slope predicts mortality and hospitalisation independently of peak V̇O2, is effort-independent, and can be derived from a submaximal test — a practical advantage in patients who cannot or will not go to exhaustion.2 Guideline statements from the AHA and EACPR set out the multivariable approach now recommended.6
Preoperative risk stratificationOlder and colleagues demonstrated in older patients undergoing major abdominal surgery that an anaerobic threshold below 11 mL/kg/min, particularly with myocardial ischaemia, identified a group with markedly higher cardiovascular mortality, and that CPET outperformed clinical assessment in risk prediction.3 Subsequent systematic review has confirmed an association between CPET-derived variables — anaerobic threshold, peak V̇O2 and V̇E/V̇CO2 — and postoperative morbidity across surgical populations, while noting heterogeneity in thresholds and outcome definitions.4 The more interesting question for physiotherapy is not risk prediction but modification: prehabilitation trials have shown improvements in CPET variables preoperatively, with less consistent effects on clinical outcomes.
Exercise prescription and the ventilatory thresholdAnchoring training intensity to the first ventilatory threshold rather than to a heart rate percentage is physiologically defensible and clinically useful, because the same relative heart rate corresponds to widely differing metabolic loads between individuals and is unusable in atrial fibrillation, with pacing, or under beta-blockade. In pulmonary rehabilitation, higher-intensity prescription derived from a maximal test produces greater physiological training effects than symptom-limited self-paced walking, though the difference in patient-reported outcomes is smaller.7 The pragmatic position is that CPET-anchored prescription is most valuable where the physiology is atypical: pulmonary vascular disease, complex congenital heart disease, post-transplant, and unexplained exercise intolerance.
Physiotherapy implications- Read the RER before anything else. Below 1.05–1.10 the test is submaximal and peak values must not be used for prognostic thresholds — but the V̇E/V̇CO2 slope and the ventilatory threshold may still be valid.
- Prescribe from the ventilatory threshold or peak work rate, not predicted maximum heart rate, in anyone rate-limited or in atrial fibrillation.
- A normal CPET in a symptomatic patient is a positive finding, not a negative one. It reframes the problem towards breathing pattern disorder, laryngeal obstruction, deconditioning or a non-cardiorespiratory cause — all of which have treatments.
- Screen for post-exertional symptom exacerbation before referring for maximal testing in long COVID or ME/CFS; two-day CPET protocols exist as a research tool and should not be used as a routine clinical assessment given the risk of prolonged deterioration.5
- Record the symptom that stopped the test and its Borg rating. It is frequently the most clinically actionable line in the report.
Approach the report as a hierarchy: was the test maximal; which reserve was exhausted; was gas exchange preserved; was efficiency normal; and does the pattern match the history. Discordance is informative — a patient with severe symptoms, a normal peak V̇O2 and an erratic ventilatory pattern is telling you something specific. Beware attributing all limitation to the organ named in the referral: it is common for a patient referred with “COPD breathlessness” to have preserved breathing reserve and a low anaerobic threshold, which is deconditioning and is treatable in eight weeks.
Evidence gapsPrognostic thresholds require recalibration to contemporary therapy, and most were derived in predominantly male, predominantly non-Indigenous cohorts with limited applicability to Australian practice. Whether prehabilitation-induced improvements in CPET variables translate into reduced postoperative complications remains unproven in adequately powered trials. Reference values for V̇O2 peak in older adults and in ethnically diverse populations are inadequate. The role of CPET in long COVID is unresolved and complicated by the harm associated with maximal testing in patients with PESE. Finally, the incremental value of CPET-guided over standard prescription in routine pulmonary and cardiac rehabilitation has not been established in a trial with patient-reported outcomes.
Prescribing exercise from the result
CPET gives the most precise prescription available because it identifies the physiological transitions rather than estimating them: train at 60–80% of peak work rate for continuous work, at or just above the ventilatory threshold where the test quality supports it, or by heart-rate reserve when the chronotropic response is intact and not drug-limited. Equally important, CPET identifies why the patient stopped — ventilatory, cardiac, peripheral or effort-limited — which shapes the programme as much as the intensity does. See prescribing exercise from test results.
References & evidence base
- Mancini DM, Eisen H, Kussmaul W, et al. Value of peak exercise oxygen consumption for optimal timing of cardiac transplantation in ambulatory patients with heart failure. Circulation 1991;83:778–86.
- Arena R, Myers J, Aslam SS, et al. Peak V̇O2 and V̇E/V̇CO2 slope in patients with heart failure: a prognostic comparison. Am Heart J 2004;147:354–60.
- Older P, Hall A, Hader R. Cardiopulmonary exercise testing as a screening test for perioperative management of major surgery in the elderly. Chest 1999;116:355–62.
- Moran J, Wilson F, Guinan E, et al. Role of cardiopulmonary exercise testing as a risk-assessment method in patients undergoing intra-abdominal surgery: a systematic review. Br J Anaesth 2016;116:177–91.
- Stussman B, Williams A, Snow J, et al. Characterization of post-exertional malaise in patients with myalgic encephalomyelitis/chronic fatigue syndrome. Front Neurol 2020;11:1025.
- Guazzi M, Arena R, Halle M, et al. 2016 focused update: clinical recommendations for cardiopulmonary exercise testing data assessment in specific patient populations. Circulation 2016;133:e694–711.
- Spruit MA, Singh SJ, Garvey C, et al. An official ATS/ERS statement: key concepts and advances in pulmonary rehabilitation. Am J Respir Crit Care Med 2013;188:e13–64.
References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.
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