This document discusses parameters used to evaluate tissue oxygenation, including oxygen content and saturation of blood, oxygen delivery and uptake, and oxygen extraction ratio. It provides normal levels and ranges for these parameters. Oxygen uptake is the most vital value that is maintained by adjusting oxygen extraction ratio. A critical oxygen delivery level exists below which oxygen uptake becomes supply dependent. Elevated blood lactate and low oxygen uptake can indicate threatened or impaired tissue oxygenation.

2. Dr Fakhir Raza

3. Concept of Tissue Oxygenation
• Parameters to evaluate tissue oxygenation
• Concentration of oxygen in blood
• The rate of O2 transport in arterial blood
• The rate of O2 uptake into tissues
• The extent of Hemoglobin desaturation in capillary blood (extraction)

4. A. Oxygen content of blood
• The sum of Hb-O2 and Plasma bound Oxygen
1.Hemoglobin-Bound O2
HbO2=1.34xHbxSO2
Hb-O2 = ml of Oxygen in 100ml whole blood
1.34 is O2 binding capacity of Hemoglobin (1.34 ml O2 per gram of Hb)
SO2 is saturation of Hb (1=100%)
For arterial blood SO2 is SaO2
2. Dissolved O2 (ml of O2 /100ml of blood)
Dissolved O2 =0.003x P O2
It shows that Oxygen is relatively insoluble in plasma. (one liter of Blood will
contain 3ml of Oxygen.

5. 3. Total O2 content of blood (ml of O2 /100ml of blood)
O2 Content= (1.34xHbxSO2)+ (0.003xP O2)
Normally we ignore plasma bound O2 content of blood and O2 content is
considered equivalent to the Hb-bound fraction.

6. Normal Levels of Oxygen in Arterial and
Venous Blood
Parameter Arterial blood Venous blood
PO2 90 mmHg 40mm Hg
O2 Saturation of Hb 0.98 0.73
HbO2 19.7mL/dL 14.7mL/dL
Dissolved O2 0.3mL/dL 0.1mL/dL
Total O2 content 20mL/dL 14.8mL/dL
Blood Volume 1.25L 3.75
Volume of O2 250mL 555mL

7. B. Oxygen Delivery (DO2)
• The rate of transport in arterial blood is known as O2 delivery (DO2)
DO2 =Q x CaO2x10
Q is cardiac output
CaO2is the O2content of arterial blood (mL/100mL).
Multiplier 10 is used to convert CaO2from mL/100mL to mL/L, which
allows the DO2 to be expressed as mL/min.
Or
DO2=Q x (1.34xHbxSaO2)x10

8. Normal Ranges of Oxygen Transport
Parameters
Parameter Absolute Range Size-Adjusted Range
Cardiac output 5-6L/min 2.4 – 4.0L/min/m2
O2 delivery 900 – 1100mL/min 520 – 570mL/min/m2
O2 Uptake 200 – 270mL/min 110 – 160mL/min/m2
O2 Extraction Ratio 0.20 – 0.30

9. C. Oxygen Uptake (VO2)
The rate at which O2 moves out of the capillaries and into the tissues is called the
O2 uptake (VO2)
VO2 can be calculated or measured
Calculated VO2
VO2 = Q x CaO2 -CvO2) x 10
CaO2 -CvO2 is the difference of oxygen content between arterial and venous blood.
CaO2 -CvO2 shares the common term (1.34 x Hb) so above equation can be written
as:
VO2 = Q x (1.34 x Hb) x (SaO2 - SvO2) x 10
Where SaO2 - SvO2 is the difference in Oxyhemoglobin saturation between arterial
and mixed venous blood.
In this method we need invasive monitoring to know about cardiac output and SvO2

10. • As Oxygen is not stored in the tissues so VO2 is also the measure of
Oxygen consumption of the tissues.

11. • Measured VO2 = VE x FiO2 – FEO2
is the volume of gas that is exhaled per minute (L/min)
FiO2 and FEO2 are the fractional concentration of O2 in inhaled and
exhaled gas, respectively.
which method is better and why ?

12. Oxygen Extraction
• The ratio of O2 uptake to O2 delivery is called oxygen extraction ratio
(O2ER).
O2ER = VO2 / DO2
Since the equation VO2 / DO2 shares the common term
(Q x 1.34xHb x 10) so the equation can be expressed as
O2ER = SaO2 - SvO2 )/ SaO2
When SaO2 is close to 1.0 (which is usually the case), the O2ER is roughly
equal to SaO2 - SvO2:
O2ER = SaO2 - SvO2

13. VO2 is most vital value to be maintained
• The Oxygen Extraction adjust itself to maintain the VO2 which is the
most vital value to be maintained.
• O2ER =VO2 / DO2This equation can be rearranged like this
• VO2 = O2ER x DO2
• In case of Decreasing Oxygen Supply (DO2) The extraction of O2 (O2ER)
increases to maintain the normal VO2

14. Concept of critical DO2
• The DO2 at which the VO2 becomes supply-dependent is called the
critical oxygen delivery (critical DO2)
• It means no more Extraction of Oxygen is possible below that level of
Oxygen.

15. Graphical representation of relation of
DO2, VO2, SaO2 and SvO2
SaO2 = 98%
SvO2 = 73%
SaO2 = 98%
SvO2 = 50%
Normal
Maximum
O2 Extraction
Dysoxia
VO2 = DO2 x (SaO2 – SvO2)
DO2
VO2

17. Clinical Markers of Threatened or
Impaired Tissue Oxygenation
• Oxygen Transport Variables
1.VO2 < 100mL/min/m (when not due to hypometabolism)
2.(SaO2 - SvO2) > 50%
3. SvO2 < 50%
Lactate Concentration in blood
1.> 2mEq/L is abnormal
2.> 4mEq/L has prognostic implications.

18. Blood Lactate
• Impaired Tissue Oxygenation
• Other causes
• Hepatic insufficiency (which impairs lactate clearance)
• Thiamine deficiency ( which inhibits pyruvate entry into mitochondria)
• Severe sepsis (same as thiamine)
• Intracellular alkalosis (which stimulates glycolysis)
In severe sepsis there is cytokine mediated inhibition of pyruvate
dehydrogenase, an enzyme involved in pyruvate transport into mitochondria.
So despite of good oxygen there is accumulation of lactate in severe sepsis.

19. references

20. • Conclusions: In patients in the ED with a sepsis diagnosis, early
lactate normalization during the first 6 h of resuscitation was the
strongest independent predictor of survival and was superior to other
measures of lactate kinetics