Oxygen Extraction Ratio

Reviewed and revised 29 August 2014

OVERVIEW

• Oxygen extraction ratio (O2ER) is the ratio of oxygen consumption (VO2)  to oxygen delivery (DO2)
  • global oxygen delivery (DO2) is the total amount of oxygen delivered to the tissues per minute, irrespective of the distribution of blood flow
  • oxygen consumption (VO2) is the total amount of oxygen removed from the blood due to tissue oxidative metabolism per minute
• Under resting conditions with normal distribution of cardiac output, DO2 is more than adequate to meet VO2 and ensure that aerobic metabolism is maintained
• Oxygen that is not extracted returns to the mixed venous circulation
• ScvO2 of 70% indicates oxygen delivery is adequate (assuming normal microcirculatory function)

CALCULATION

O2ER = VO2 / DO2 = (SaO2-SvO2) / SaO2

NORMAL VALUES

In a normal 75 kg adult undertaking routine activities:

• VO2 is approximately 250 ml/min (cf. VO2max in a non-athlete 75kg person is about 3L/min)
• DO2 is approx 1 L/min
• O2ER is 25% (increases to ~70% during maximal exercise in an athlete)
• SvO2 70%

O2ER varies for different organs:

• cardiac O2ER = >60%
• hepatic O2ER = 45-55%
• renal O2ER = <15%

RELATIONSHIP BETWEEN VO2 and DO2

• Initially, as metabolic demand (VO2) increases, or DO2 diminishes, O2ER rises to maintain aerobic metabolism and consumption remains independent of delivery.
• However, at a point called critical DO2 (cDO2)—the maximum O2ER is reached. This is believed to be ~70%.
• Beyond cDO2 any further increase in VO2, or decline in DO2, must lead to tissue hypoxia and anaerobic metabolism (lactate production is a surrogate for this)

In reality each tissue / organ has its own cDO2 — the higher the O2ER for a given tissue , the greater the dependence on DO2 (supply dependence).

INTERPRETATION OF ABNORMAL O2ER

High O2ER suggests inadequate oxygen delivery (OH CRAP; shock)

• oxygen (hypoxic hypoxia: low FiO2 gas or high altitude; lung disease)
• hemoglobin (anemia)
• contractility
• rate/ rhythm
• afterload
• preload
• shock/ hypoperfusion due to other causes

or increased oxygen consumption (VO2)

• fever and inflammatory states, e.g. sepsis, burns, trauma, surgery
• increased metabolic rate, e.g. hyperthyroidism, adrenergic drugs, hyperthermia, burns
• increased muscular activity, e.g. exercise, shivering, seizures, agitation/anxiety/pain, weaning from ventilation/ increased respiratory effort

Low O2ER suggests increased oxygen delivery

• hyperoxia, e.g high FiO2 gas, hyperbaric oxygen or ECMO

or decreased oxygen consumption

• decreased metabolic rate, e.g. hypothyroidism, sedatives/ hypnotics, hypothermia
• decreased muscular activity e.g. sedation/analgesics, muscle paralysis, ventilatory support
• antipyretics
• Starvation/hyponutrition
• Sepsis due to shunting and histotoxic hypoxia
• Histotoxic hypoxia, e.g. cyanide poisoning

References and links

Journal articles

• McLellan SA, Walsh TS. Oxygen delivery and haemoglobin. Contin Educ Anaesth Crit Care Pain (2004) 4 (4): 123-126. [Free Full Text]
• Nebout S, Pirracchio R. Should We Monitor ScVO2 in Critically Ill Patients? Cardiol Res Pract. 2012;2012:370697. PMC3177360.