What is VO2 (oxygen consumption)?

What is VO2 (Oxygen Consumption)?

VO2 (oxygen uptake) is the volume of oxygen extracted from inspired air in a given period of time, expressed in milliliters per minute or liters per minute. 1

Core Definition and Measurement

• VO2 represents the volume of O2 extracted from inspired air per unit time, typically measured in mL/min or L/min using standard temperature, pressure, and dry (STPD) conditions 1

• In steady-state conditions, oxygen uptake equals oxygen consumption, which is the amount of O2 utilized by the body's metabolic processes 1

• These terms can differ under non-steady-state conditions when oxygen is flowing into or being utilized from the body's stores 1

Clinical Measurement Context

Direct Measurement Methods

• VO2 is most accurately measured through expired gas analysis during cardiopulmonary exercise testing, where it reflects the integrative response of pulmonary, cardiovascular, hematopoietic, neuropsychological, and skeletal muscle systems 1

• The measurement requires stable inspired oxygen concentration (FiO2) for accurate metabolic assessment, particularly in mechanically ventilated patients 2

Calculated vs. Measured VO2

• VO2 calculated using the Fick equation (cardiac output × arteriovenous oxygen content difference) systematically underestimates true VO2 with approximately 20% error in absolute values, though it may reflect directional changes 3

• Independent measurement via spirometry is more accurate than Fick-calculated VO2 for research purposes, though calculated VO2 can be used clinically with discretion 3

Key VO2-Related Terms

VO2 max (Maximal Oxygen Uptake)

• VO2 max is the highest attainable O2 uptake for a given subject, determined during dynamic exercise when VO2 plateaus despite continued increases in work rate 1

• VO2 max is considered the best measure of cardiovascular fitness and exercise capacity, representing the maximal achievable level of oxidative metabolism 1

• It is determined by the product of maximum cardiac output and maximum arteriovenous oxygen difference 1

VO2 peak (Peak Oxygen Uptake)

• VO2 peak is the highest VO2 achieved on a test performed to the limit of tolerance, without requiring evidence of plateauing 1

• In clinical practice, VO2 peak is often used interchangeably with VO2 max because patients frequently cannot achieve a clear plateau due to symptom limitation 1

• VO2 peak should be expressed in absolute values (L/min) and as a percentage of predicted value 1

Factors Affecting VO2

Physiological Determinants

• VO2 max is influenced by age, sex, exercise habits, heredity, and cardiovascular clinical status 1

• The main determinants of normal VO2 max are genetic factors and quantity of exercising muscle 1

• VO2 max declines progressively with age, with the rate of decline accelerating dramatically in the 70s and beyond, exceeding 20% per decade 4

Normalization Methods

• VO2 is commonly normalized by body weight (mL/kg/min), though this may produce deceptively low values in obese individuals 1

• In obesity, normalization by height (VO2/ht) may better correlate with lean body mass 1

• Small but normal subjects have higher VO2 per kilogram than larger subjects, making weight-based normalization imperfect 1

Clinical Applications

Exercise Testing

• The VO2/work rate slope (normally 8.5-11 mL/min per watt) reflects metabolic and mechanical efficiency during incremental cycle ergometry 1

• A reduced VO2/work rate slope most often indicates inadequacies of O2 transport from heart, lung, or circulatory disease 1

Critical Care Context

• Mechanical ventilation with muscle relaxation reduces VO2 by more than 20% compared to spontaneous breathing, preserving oxygen delivery for vital organs 5

• VO2 remains independent of oxygen delivery until delivery falls below approximately 13 mL/kg/min, after which VO2 becomes linearly dependent on delivery 6

• In septic shock, splanchnic VO2 increases early, while in hemorrhagic shock it remains unchanged, suggesting different pathophysiological mechanisms 7

Important Clinical Caveats

• VO2 estimation from resting indices, work rate, or submaximal protocols is unreliable due to physiologic mechanisms and methodologic inaccuracies 1

• Direct measurement of VO2 max is reliable and reproducible in both normal subjects and patients 1

• Preexisting left ventricular dysfunction or exercise-induced myocardial ischemia can greatly affect VO2 max, making prediction from age and exercise habits alone difficult 1