18.7
mL O2/dL
14.2
mL O2/dL
4.5
mL O2/dL
223
mL/min
18.7
mL O2/dL
14.2
mL O2/dL
4.5
mL O2/dL
223
mL/min
The O2 Consumption Calculator (VO2) determines whole-body oxygen consumption using the reverse Fick principle. Oxygen consumption, often written as VO2, represents the total amount of oxygen extracted from the blood by the tissues each minute and serves as one of the most fundamental parameters in critical care medicine, exercise physiology, and hemodynamic monitoring. Normal resting VO2 in adults is approximately 200 to 250 mL per minute, or about 3.5 mL per kilogram per minute, a value known as one metabolic equivalent (MET).
The Fick equation states that VO2 equals cardiac output multiplied by the arteriovenous oxygen difference (CaO2 minus CvO2). Oxygen content in blood depends primarily on hemoglobin concentration and its oxygen saturation, since each gram of fully saturated hemoglobin carries 1.34 mL of oxygen (Huffner's constant). The arterial oxygen content (CaO2) is calculated as 1.34 times hemoglobin times arterial saturation, while the venous oxygen content (CvO2) uses mixed venous saturation obtained from a pulmonary artery catheter. The dissolved oxygen contribution (0.003 times PaO2) is typically omitted in clinical calculations due to its negligible magnitude under normal conditions.
Understanding VO2 is essential in multiple clinical contexts. In the intensive care unit, VO2 helps assess the balance between oxygen delivery (DO2) and oxygen demand. When VO2 falls despite adequate DO2, it may indicate mitochondrial dysfunction as seen in septic shock. Conversely, when tissues extract increasingly more oxygen to compensate for inadequate delivery, the arteriovenous difference widens and the mixed venous saturation drops. A critically low SvO2 below 65% often signals that oxygen delivery is insufficient to meet metabolic demands, prompting interventions such as blood transfusion, fluid resuscitation, or inotropic support.
In exercise physiology, VO2 max represents the maximum rate of oxygen consumption achievable during maximal exertion and serves as the gold standard measure of aerobic fitness. Elite endurance athletes may achieve VO2 max values exceeding 70 mL/kg/min, while sedentary individuals may measure below 30 mL/kg/min. During exercise, VO2 increases linearly with workload up to the VO2 max plateau, with the increase driven by both rising cardiac output and widening arteriovenous oxygen difference.
The clinical measurement of VO2 can be performed directly through indirect calorimetry, which measures inspired and expired gas volumes and concentrations, or calculated using the Fick method as implemented in this calculator. The Fick method requires knowledge of cardiac output (measured by thermodilution or other techniques) and blood gas values. While indirect calorimetry is considered more accurate for determining actual metabolic rate and guiding nutritional support, the Fick calculation provides a rapid bedside estimate using data already available from hemodynamic monitoring.
Elevated VO2 occurs during fever, sepsis, burns, agitation, shivering, and exercise, reflecting increased metabolic demand. Decreased VO2 is seen in hypothermia, deep sedation, and pathological supply dependency where tissues cannot extract adequate oxygen despite need. The oxygen extraction ratio (VO2/DO2) normally ranges from 0.22 to 0.30, meaning tissues normally extract about 25% of delivered oxygen. When this ratio exceeds 0.40, critical oxygen delivery thresholds may be approaching, warranting aggressive hemodynamic optimization.
This calculator applies the Fick equation: VO2 = CO x (CaO2 - CvO2) x 10. Arterial oxygen content (CaO2) is calculated as 1.34 x Hb x SaO2/100, and venous oxygen content (CvO2) as 1.34 x Hb x SvO2/100. The multiplication by 10 converts the result from mL O2/dL to mL O2/L, allowing proper unit matching with cardiac output in L/min. The final VO2 is expressed in mL/min.
Normal VO2 at rest is 200-250 mL/min (approximately 3.5 mL/kg/min). The arteriovenous O2 difference (normally 3.5-5.0 mL O2/dL) reflects tissue oxygen extraction. A widened A-V difference suggests increased extraction (possibly from inadequate delivery), while a narrowed difference may indicate poor tissue extraction (as in sepsis) or excessive delivery. CaO2 below 16 mL O2/dL or CvO2 below 12 mL O2/dL warrants clinical attention.
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Normal values: CaO2 ~18.4, CvO2 ~14.1, A-V difference ~4.3, VO2 ~215 mL/min — all within the expected normal range at rest.
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Low Hb reduces oxygen content despite compensatory high CO. Narrow A-V difference and low VO2 suggest impaired tissue oxygen extraction typical of distributive shock.
The Fick equation relates oxygen consumption (VO2) to cardiac output (CO) and the arteriovenous oxygen difference: VO2 = CO x (CaO2 - CvO2). It is a fundamental principle in cardiovascular physiology first described by Adolf Fick in 1870 and remains central to hemodynamic assessment.
Huffner's constant represents the volume of oxygen (in mL) that can be carried by one gram of fully saturated hemoglobin. The theoretical value is 1.39 mL/g, but the clinically used value is 1.34 mL/g, which accounts for the small percentage of hemoglobin that exists as methemoglobin and carboxyhemoglobin in normal blood.
Dissolved oxygen (calculated as 0.003 x PaO2) contributes only about 0.3 mL O2/dL at a normal PaO2 of 100 mmHg, compared to approximately 18-20 mL O2/dL carried by hemoglobin. This negligible contribution is typically omitted in clinical calculations for simplicity, though it becomes more significant during hyperbaric oxygen therapy.
Normal mixed venous oxygen saturation (SvO2), measured from the pulmonary artery, is 65-75%. Values below 65% suggest increased oxygen extraction due to inadequate delivery (low cardiac output, anemia, or hypoxemia) or increased metabolic demand. SvO2 above 80% may indicate impaired tissue extraction, as seen in sepsis or cyanide poisoning.
Cardiac output can be measured invasively using thermodilution via a pulmonary artery catheter, or non-invasively using echocardiography (Doppler-based), arterial pulse contour analysis, or bioimpedance/bioreactance methods. The thermodilution technique remains the clinical reference standard, though non-invasive methods are increasingly used.
The oxygen extraction ratio (O2ER) equals VO2 divided by oxygen delivery (DO2), or equivalently (SaO2 - SvO2) / SaO2. Normal O2ER is 0.22-0.30 (22-30%). Values above 0.40 suggest critical oxygen supply-demand imbalance and are associated with tissue hypoxia and lactic acidosis.
While the Fick principle applies during exercise, direct measurement of VO2 during exercise typically uses expired gas analysis (indirect calorimetry) rather than invasive blood gas measurements. This calculator is most applicable to critical care settings where cardiac output and mixed venous gases are available from invasive monitoring.
CaO2 is primarily determined by hemoglobin concentration and arterial oxygen saturation. Anemia reduces CaO2 even with normal saturation. Hypoxemia reduces CaO2 by lowering SaO2. Carbon monoxide poisoning reduces functional hemoglobin without affecting measured SaO2 on pulse oximetry, leading to falsely normal CaO2 calculations.
VO2 increases approximately 10% for each degree Celsius of fever above normal body temperature. A patient with a temperature of 40 degrees C may have a VO2 30% above baseline. This increased metabolic demand must be met with increased oxygen delivery to prevent tissue hypoxia.
VO2 index normalizes oxygen consumption to body surface area (BSA): VO2I = VO2 / BSA. Normal VO2 index is 120-160 mL/min/m2. Indexing to BSA allows comparison between patients of different body sizes and is considered more clinically relevant than absolute VO2 in critical care assessment.
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