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Trauma Surgery / Surgical Critical Care Ryder Trauma Center 8/4/2010

Oxygen Delivery, Consumption and Extraction Formulas. Trauma Surgery / Surgical Critical Care Ryder Trauma Center 8/4/2010 Nir Hus, M.D., Ph.D. Oxygen and Carbon Dioxide Transport Parameters :.

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Trauma Surgery / Surgical Critical Care Ryder Trauma Center 8/4/2010

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  1. Oxygen Delivery, Consumption and Extraction Formulas. Trauma Surgery / Surgical Critical Care Ryder Trauma Center 8/4/2010 Nir Hus, M.D., Ph.D. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  2. Oxygen and Carbon Dioxide Transport Parameters: Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  3. O2 Binding Basics • Each gram of physiologic hemoglobin will bind 1.34 mL oxygen. • One gram of hemoglobin potentially binds 1.39 mL oxygen. • But, a small fraction (3% to 5%) of circulating hemoglobin is present as methemoglobin and carboxyhemoglobin • Since these forms of Hb have a reduced O2-binding capacity, the lower value of 1.34 mL/g is considered more representative of the O2-binding capacity of the total hemoglobin pool Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  4. Dissolved O2 • Dissolved O2 (ml/100ml plasma) = 0.003(ml O2/100ml plasma/mmHg) x PO2 (mmHg) • So…in 100ml of blood at 100% O2 sat  3ml of dissolved O2 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  5. Dissolved O2 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  6. Arterial O2 Content (CaO2) • CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) • There are ~200 mL oxygen in each liter of Arterial blood, and only 1.5% (~3 mL) is dissolved in the plasma. • The oxygen consumption of an average-sized adult at rest is 250 mL/min Thus: • If we were forced to rely solely on the dissolved O2 in plasma, a cardiac output of 89 L/min would be necessary to sustain aerobic metabolism. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  7. Arterial O2 Content (CaO2) CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  8. Arterial O2 Content (CaO2) CaO2 = (1.34 xHbx SaO2) + (0.003 x PaO2) Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  9. O2 SAT vs PaO2 Effects of equivalent (50%) reductions in hemoglobin concentration (Hb) and arterial Po2 (Pao2) on the oxygen concentration in arterial blood (Cao2). Normal Hypoxemia CaO2 ml/L Anemia Hb 15 15 7.5 PaO2 90 45 90 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  10. O2 SAT vs PaO2 • A 50% reduction in hemoglobin (from 15 to 7.5 g/dL) is accompanied by an equivalent 50% reduction in Cao2 (from 200 to 101 mL/L), • A similar 50% reduction in the PaO2 (from 90 to 45 mm Hg) results in only an 18% decrease in Cao2 (from 200 to 163 mL/L). Thus Anemia has a much more profound effect on blood oxygenation than hypoxemia. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  11. O2 SAT vs PaO2 • It should also serve as a reminder to avoid using the Pao2 to assess arterial oxygenation! • The Pao2 should be used to evaluate the efficiency of gas exchange in the lungs Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  12. The Paucity of O2 in Blood • The combined volume of O2 in arterial and venous blood is ~805 mL. • To appreciate what a limited volume this represents, consider that the whole-body O2 consumption of an average-sized adult at rest is about 250 mL/min. • This means that the total volume of O2 in blood is enough to sustain aerobic metabolism for only 3 to 4 minutes. Thus, • we have only a precious few minutes to begin assisted breathing maneuvers before the oxygen stores in the blood are completely exhausted. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  13. The Paucity of O2 in Blood Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  14. The Paucity of O2 in Blood • The limited quantity of O2 in blood can also be demonstrated by considering the oxidative metabolism of glucose. C6H12O6 + 6O2 6CO2 + 6H2O. This formula indicates that complete oxidation of one mole of glucose utilizes 6 moles of oxygen. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  15. The Paucity of O2 in Blood • For a Glucose level of 90 mg/dl, in a 70kg adult : Total glucose in blood (90mg/dl) = 25 mmol Total O2 in blood (805ml) = 36.3 mmol O2 need for glucose metabolism 25mmol x 6 = 150 mmol Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  16. The Paucity of O2 in Blood • This shows that the O2 in blood is only about 20% to 25% of the amount needed for the complete oxidative metabolism of the glucose in blood. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  17. Why so Little O2? Who Knows ??? • Hypothesis: Limiting the oxygen concentration in the vicinity of cells may be a mechanism for protecting cells from oxygen-induced cell injury. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  18. Oxygen Delivery (DO2) • DO2 = volume of O2(ml) that reaches the systemic capillaries each minute. • It is equivalent to the product of the O2 content in arterial blood (CaO2) in mL/L and the cardiac output (Q) in L/min. Do2 = Q x CaO2 x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  19. Oxygen Delivery (DO2) • Do2 = Q x CaO2 x 10 • CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) Thus: • Do2 = Q x CaO2 x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  20. Oxygen Delivery (DO2) • Do2 = Q x CaO2 x 10 • CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) Thus: • Do2 = Q x (1.34 x Hb x SaO2) x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  21. Oxygen Delivery (DO2) • The normal Do2 in adults at rest is: • 900–1,100 mL/min, • 500–600 mL/min/m2 when adjusted for body size Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  22. Oxygen Uptake (VO2) • When blood reaches the systemic capillaries, oxygen dissociates from hemoglobin and moves into the tissues. • The rate at which this occurs is called the oxygen uptake (VO2) in ml/min. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  23. Oxygen Uptake (VO2) • Since oxygen is not stored in tissues, the VO2 is also a measure of the oxygen consumption of the tissues. • The VO2 (mL/min) can be calculated as the product of the cardiac output (Q) and the delta arterio-venous oxygen content (CaO2 - CvO2). VO2 = Q x (CaO2 - CvO2) x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  24. Oxygen Uptake (VO2) • CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) • CvO2 = (1.34 x Hb x SvO2) + (0.003 x PvO2) • VO2 = Q x (CaO2 - CvO2) x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  25. Oxygen Uptake (VO2) • Since the CaO2 and CvO2 share a common term: (1.34 x Hb x10), • The equation: VO2 = Q x (CaO2 - CvO2) x 10 • can be restated as: Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  26. Oxygen Uptake (VO2) • CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2) • CvO2 = (1.34 x Hb x SvO2) + (0.003 x PvO2) • VO2 = Q x (CaO2 - CvO2) x 10 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  27. Oxygen Uptake (VO2) • Since the CaO2 and CvO2 share a common term: (1.34 x Hb x10), • The equation: VO2 = Q x (CaO2 - CvO2) x 10 • can be restated as: VO2 = Q x 13.4 x Hb x (SaO2 – SvO2) Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  28. Oxygen Uptake (VO2) • This equation expresses VO2 using variables that can be measured in clinical practice. VO2 = Q x 13.4 x Hb x (SaO2 – SvO2) • The normal range for VO2 in healthy adults at rest is 200–300 mL/min, or 110–160 mL/min/m2 when adjusted for body size Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  29. Oxygen Uptake (VO2) - CAUTION • The VO2 is not equivalent to the whole-body VO2 because it does not include the O2 consumption of the lungs! • Normally, the VO2 of the lungs represents less than 5% of the whole-body VO2, but it can make up 20% of the whole-body VO2 in patients with inflammatory conditions in the lungs (which are common in ICU patients) Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  30. Oxygen Uptake (VO2) - CAUTION • This discrepancy can be important when VO2 is used as an end-point of hemodynamic management because an underestimate of whole-body VO2 could lead to overaggressive management to augment the VO2. • Direct measurement of the VO2 is a more accurate representation of the whole-body VO2. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  31. Oxygen-Extraction Ratio (O2ER) • The fraction of the oxygen delivered to the capillaries that is taken up into the tissues is an index of the efficiency of oxygen transport. • This is monitored with a parameter called the oxygen extraction ratio (O2ER), which is the ratio of O2 uptake to O2 delivery. O2ER = VO2 / DO2 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  32. Oxygen-Extraction Ratio (O2ER) • The O2ER is normally ~0.25 • Range = 0.2–0.3 • O2ER = VO2 / DO2 Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  33. Oxygen-Extraction Ratio (O2ER) • Only 25% of the O2 delivered to the systemic capillaries is taken up into the tissues. • Although O2 extraction is normally low, it is adjustable and can be increased when oxygen delivery is impaired. • The adjustability of O2 extraction is an important factor in the control of tissue oxygenation. Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  34. Oxygen-Extraction Ratio (O2ER) Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

  35. The End They will probably change these couches after I retire… Nir Hus MD., PhD. Ryder Trauma Center Jackson Memorial Hospital

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