Systemic Oxygenation

Dr Fakhir Raza

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)

A. Oxygen content of blood

• The sum of Hb-O2 and Plasma bound Oxygen1.Hemoglobin-Bound O2

HbO2=1.34xHbxSO2 Hb-O2 = ml of Oxygen in 100ml whole blood1.34 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.

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.

Normal Levels of Oxygen in Arterial and Venous BloodParameter 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

B. Oxygen Delivery (DO2)

• The rate of transport in arterial blood is known as O2 delivery (DO2)

DO2 =Q x CaO2 x10Q is cardiac output

CaO2 is the O2 content of arterial blood (mL/100mL).

Multiplier 10 is used to convert CaO2 from mL/100mL to mL/L, which allows the DO2 to be expressed as mL/min.

Or

DO2 =Q x (1.34xHbxSaO2)x10

Normal Ranges of Oxygen Transport ParametersParameter 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

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 measuredCalculated VO2

VO2 = Q x CaO2 -CvO2) x 10CaO2 -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 10Where 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

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

• 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 ?

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

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

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.

Graphical representation of relation of DO2, VO2, SaO2 and SvO2

SaO2 = 98%SvO2 = 73%

SaO2 = 98%SvO2 = 50%

NormalMaximum O2 Extraction

Dysox

ia

VO2 = DO2 x (SaO2 – SvO2)

DO2

VO2

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 blood1.> 2mEq/L is abnormal2.> 4mEq/L has prognostic implications.

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.

references

• 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