Cardiopulmonary Function Assessment Scores (CPET)

RER Peak Index (Respiratory Exchange Ratio) Calculator

Clinical calculation support · Verify results before clinical use

Peak Exercise Gas Exchange

Enter peak carbon dioxide production and oxygen uptake measured during CPET.

CPET · Ratio

—Peak RER

Effort interpretation

Result will appear here

The ratio is calculated as V̇CO₂ divided by V̇O₂.

Clinical notePeak RER is an effort indicator, not a standalone measure of exercise capacity or a reason by itself to terminate exercise. Interpret it in the context of the entire CPET.

About Peak Respiratory Exchange Ratio

The respiratory exchange ratio (RER) is the ratio of carbon dioxide production (V̇CO₂) to oxygen uptake (V̇O₂), measured from expired gases during cardiopulmonary exercise testing (CPET).

Peak RER	General interpretation
<1.00	May indicate submaximal effort; consider symptoms, limiting physiology, and test termination reason.
1.00–<1.10	Interpret alongside other effort indicators and the clinical context.
≥1.10	Commonly supports high or near-maximal effort, but does not prove maximal effort in every patient.

Important considerations

Use V̇CO₂ and V̇O₂ values measured at the same peak-exercise time point and in compatible units.
RER can rise above 1.0 because of bicarbonate buffering of exercise-related acid production and hyperventilation, not simply because carbohydrate oxidation predominates.
In patients with heart failure or other exercise-limiting conditions, failure to reach a target RER does not by itself invalidate the CPET; assess the full test and its limitations.

References

American Thoracic Society; American College of Chest Physicians.

ATS/ACCP Statement on cardiopulmonary exercise testing. Am J Respir Crit Care Med. 2003;167(2):211–277.

doi:10.1164/rccm.167.2.211 · PMID: 12524257

Guazzi M, et al.

Clinical recommendations for cardiopulmonary exercise testing data assessment in specific patient populations. Use peak RER as one component of integrated CPET interpretation rather than in isolation.

JACC review (2019)

Clinical disclaimer: This calculator performs a simple ratio calculation and provides general interpretive context. It is not a validated standalone prognostic score and must not replace professional interpretation of CPET, the patient's clinical status, or relevant local protocols.