Systemic oxygenation
- 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