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Oxygen Debt

TRAUMA-ICU NURSING EDUCATIONAL SERIES. Oxygen Debt. Critical Care Medicine Boston Medical Center Boston University School of Medicine Bradley J. Phillips, M.D. Tissue O 2 -Balance. Oxygen supply to the tissues is the rate of O 2 uptake from the microcirculation VO 2 & ER

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Oxygen Debt

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  1. TRAUMA-ICU NURSING EDUCATIONAL SERIES Oxygen Debt Critical Care Medicine Boston Medical Center Boston University School of Medicine Bradley J. Phillips, M.D.

  2. Tissue O2-Balance • Oxygen supply to the tissues is the rate of O2 uptake from the microcirculation • VO2 & ER • The metabolic requirement for oxygen is the rate at which oxygen is metabolized to water within the mitochondria • MRO2 • Because oxygen is NOT stored in the tissues, VO2 must match MRO2 if aerobic metabolism is to continue when matching occurs, glucose is completely oxidized to yield 36 moles of ATP

  3. Oxygen Balance when matching occurs, glucose is completely oxidized to yield 36 moles of ATP • When matching is not equal (VO2 is less than MRO2), a portion of the glucose is diverted to the production of lactate in an attempt to salvage energy • Per mole of glucose converted through anaerobic metabolism, 2 moles of ATP are gained (47 kcal)

  4. Dysoxia the condition in which the production of ATP is limited by the supply of oxygen when cell dysoxia leads to a measurable change in organ function….SHOCK

  5. VO2 & MRO2

  6. VO2 Deficit • In ICU patients, a VO2 that falls below the normal range (i.e. below 100 ml/min), can be used as evidence of impaired tissue oxygenation • Studies have shown a direct relationship between the magnitude of the VO2 deficit and the risk of multiorgan failure [Dunham et al. CCM 1991;19:231-243] [Shoemaker et al. Chest 1992;102:208-215]

  7. Oxygen Debt The cumulative VO2 deficit is referred to as the “oxygen debt” In ICU patients, there may be a progressive and linear relationship between VO2 & DO2

  8. Monitoring of O2 Transport The transport variables provide no information about the ADEQUACY of tissue (cellular) oxygenation… because that requires a measurement of metabolic rate.

  9. Interpreting the Transport Variables • Low VO2: • Indicates a tissue oxygen deficit • “Oxygen Debt” • The total VO2 deficit over time • Remember the direct relationship exists between magnitude of the oxygen debt and subsequent risk of multiorgan failure • Normal VO2: • Requires a blood lactate level to determine the adequacy of global tissue oxygenation

  10. Correcting a VO2 Deficit (1) • Step 1: CVP or PWP • If low, infuse volume to normalize filling pressure • If normal or high, go to step 2 • Step 2: CO • If low & filling pressures not optimal…infuse volume • If low & filling pressures high, start DOBUTAMINE & titrate keep CI > 3 L/min/m2 (some believe 5) • If blood pressure is also low, start DOPAMINE or LEVOPHED • If CI > 3, proceed to Step 3.

  11. Correcting a VO2 Deficit (2) • Step 3: VO2(Oxygen Uptake) • If VO2 is less than 100 ml/min/m2, use VOLUME • to goal of CVP 8 – 12; PWP 18 – 20 • inotropic therapy to achieve a CI > 4.5 L/min/m2 • Correct Hb if less than 8 g/dl (some say 10 g/dl) • If VO2 is greater than 100 ml/min/m2, proceed to Step 4. • Step 4: Blood Lactate • Lactate > 4 with other signs of shock (i.e. organ failure, low BP), decrease METABOLIC RATE – via sedation or paralysis (? Pentobarbital coma) • Lactate 2 – 4...controversial ! • Lactate < 2…observe

  12. VO2 & DO2 vs. Time

  13. Role of Serum Lactate (1) • An elevated lactate indicates that VO2 is less than the metabolic rate • The approach must then be to either decrease the metabolic rate or increase the VO2 • achieving a supranormal level of VO2 may be difficult • and carries risks

  14. Serum Lactate (2) Aduen, et al. JAMA 1994;272:1678-1685

  15. Serum Lactate & Cardiac Index

  16. Serum Lactate & Cardiac Index

  17. Optimizing Oxygen Transport: The Steps Filling Pressures Cardiac Output VO2 Serum Lactate

  18. Carbon Dioxide (1) An increase in PCO2 of 5 mmHg can result in a twofold increase in minute ventilation… to produce the same increment in ventilation, the PaO2 must drop to 55 mmHg The ventilatory control system keeps a close eye on CO2 but pays little attention to PaO2…while clinicians keep a close eye on PaO2 and pay little attention to PCO2 “I just don’t understand….”

  19. Carbon Dioxide (2) • The CO2 “Sink” • Ready source of ions (H+ & HCO3-) • Buffering capacity of Hb • (6x that of all the plasma • proteins combined)

  20. CO2 Extraction

  21. The Respiratory Quotient RQ = VCO2 / VO2 • VCO2 normally 10 mEq/min (14,400 mEq/24 hrs) • Exercise: lung excretion can reach 40,000 mEq/24 hrs. • The kidneys normally excrete 40 – 80 mEq acid /24 hrs

  22. Oxygen Transport Variables ParameterNormal Range Delivery (DO2)500 - 800 ml/min Uptake (VO2) 110 - 160 ml/min Extraction Ratio (ER) 22 - 32 % Mixed Venous PO2 33 - 53 mmHg Mixed Venous SO2 68 - 77 % ** DO2 & VO2 can be indexed to body surface area

  23. Oxygen Debt “it would be a most difficult task to explain” Any Questions ?

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