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    96 Respiratory exchange ratio (RER)
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    96 Respiratory exchange ratio (RER)

    Respiratory exchange ratio: fuel use, maximal effort in exercise testing and nutrition in intensive care.

    The respiratory exchange ratio (RER) is the ratio of carbon dioxide exhaled to oxygen taken up (VCO₂/VO₂), measured in exhaled gas. In a steady state at rest it equals the respiratory quotient of cell metabolism and shows which fuels are being burned: about 0.7 for fat, about 0.8 for a mixed diet and 1.0 for carbohydrate. During hard exercise, lactic acid is buffered by bicarbonate, releasing extra CO₂, and RER rises above 1.0. An RER of 1.10 or more at peak exercise indicates that a person has made a near-maximal effort. In this application, the value also reflects the balance between oxidative processes and lipid peroxidation.

    Gas exchangeExercise testingMetabolism
    FormulaRER = VCO₂ / VO₂
    At rest≈ 0.7–0.9
    Maximal effort≥ 1.10
    Measured byexhaled gas analysis
    Respiratory exchange ratio: below 0.7 suggests error or ketosis, 0.7 to 1.0 typical at rest and moderate exercise, 1.0 to 1.1 hard exercise, 1.1 and above maximal effort Respiratory exchange ratio (VCO₂ / VO₂) Below 0.7 error, ketosis 0.7 Rest, moderate 0.7–1.0 1.0 Hard 1.0–1.1 1.1 Maximal ≥ 1.10
    Figure 1. RER zones. Values above 1.0 at rest suggest hyperventilation or excess carbohydrate intake.

    Clinical significance

    • Exercise testing: RER confirms the adequacy of effort, so that a low peak VO₂ is not mistaken for disease when the person stopped early
    • Nutrition: in intensive care, an RER persistently above 1.0 may indicate overfeeding with carbohydrate (fat synthesis); values near 0.7 occur with starvation and ketosis
    • Hyperventilation temporarily raises RER by washing out CO₂ stores; hypoventilation lowers it
    Why it matters

    RER helps interpret cardiopulmonary exercise tests and adjust nutrition in critically ill patients. A single value should be read together with ventilation and the clinical situation.

    References

    1. 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
    2. Weir JB de V. New methods for calculating metabolic rate with special reference to protein metabolism. J Physiol. 1949;109(1–2):1–9. doi:10.1113/jphysiol.1949.sp004363

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 5 May 2024