SvO2, or mixed venous oxygen saturation, measures the percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart. It indicates how much oxygen is "left over" after tissues extract what they need. A decreased SvO2 means cardiac output is not high enough to meet tissue oxygen demands, while an increased SvO2 obtained before and after therapy changes can show if the treatment improved the patient. SvO2 is useful for evaluating if ventilator changes or other therapies adequately increased oxygen delivery to tissues.

2. Mixed Venous
Oxygen Saturation
By
Dr Hania Afzal
Resident Medicine

3. What is it?
• SvO2 is the percentage of oxygen bound to
hemoglobin in blood returning to the right side of
the heart.
• The amount of oxygen "left over" after the
tissues remove what they need.
• Used to recognize when a patient's body is
extracting more oxygen than normally.
• An increase in extraction is the body’s way to
meet tissue oxygen needs when the amount of
oxygen reaching the tissues is less than needed.

4. How to get blood
• Pulmonary artery catheter
• Central venous oxygen saturation(ScvO2)
from IJ/ subclavian vein (though not
appropriate)

5. What does it tell us?
• whether the CO and O2 delivery is high
enough to meet a patient's needs.
• useful if measured before and after
changes are made to cardiac medications
or mechanical ventilation,

6. What are the normal values?
• Normal SvO2 60-80%.
• Normal ScvO2 (from an internal jugular or
subclavian vein) is > 70%.

7. How do I use this information
clinically?
• ATP (energy) is needed for cell function and
survival.
• Tissues require oxygen to make ATP (energy).
• If oxygen being received by the tissues falls
below the oxygen required (because of an
increased need, or decreased supply), the body
attempts to compensate as follows:

8. Body’s compensation to dec oxygen
supply
• First : Cardiac Output increases
Oxygen Delivery (DO2) =
Cardiac Output (HR X Stroke Volume)
X
Oxygen Content (Hb X SaO2)

9. Second: Tissue oxygen extraction increases.
• Tissues begin to extract a higher percentage of
oxygen from the arterial blood.
• This results in a reduced amount of oxygen
remaining in the blood as it returns to the right
side of the heart (decreased SvO2).

10. • Third: Anaerobic Metabolism increases
Inadequate O2 supply leads to anaerobic
metabolism for ATP producing lactic acid and
causing lactic acidosis/metabolic acidosis

11. Why measure it?
Decreased SvO2 indicates
• CO is not high enough to meet tissue O2
needs
Improving/Normal SvO2 indicates
• Patient improvement

12. • Rise in SvO2 with increasing lactate
‘ Is inappropriate’
As pt having increased lactate or 3rd
compensation must have 1st 2 compensatory
mechanisms i.e Increase CO and Increased tissue
extraction(dec SvO2)
• It tells that tissues can’t extract as in late septic
shock or cell poisoning/cyanide

13. Normal SvO2 with normal lactate
means adequate CO
Measuring SvO2 before and after a change in
therapy can assist in determining whether the
therapy made patient better or worse.

14. SvO2 can also be useful when evaluating
changes to ventilator therapy, especially in
unstable patients.
• Changes are made in ventilator to increase
blood oxygen content
• Increase PEEP to increase oxygen
• But increase PEEP decreases CO
• The "best" PEEP is the level that improves
the SaO2 without causing the SvO2 to fall.

15. • Tissue oxygen need is met when the amount of
oxygen being delivered to the tissues is
sufficient to meet the amount of oxygen being
consumed (VO2).
• When the oxygen delivery falls below oxygen
consumption needs, lactic acidosis develops.
VO2 (Oxygen Consumption) = Cardiac
Output X Hb X (SaO2 - SvO2)

16. Causes of drop in SvO2
• cardiac output is not high enough to
meet tissue oxygen needs
• Hb is too low
• SaO2 is too low
• Oxygen consumption has increased
without an increase in oxgyen
delivery