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

2. Tissue O 2 -Balance Oxygen supply to the tissues is the rate of O 2 uptake from the microcirculation –VO 2 & ER The metabolic requirement for oxygen is the rate at which oxygen is metabolized to water within the mitochondria –MRO 2 Because oxygen is NOT stored in the tissues, VO 2 must match MRO 2 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 (VO 2 is less than MRO 2 ), 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. VO 2 & MRO 2

6. VO 2 Deficit In ICU patients, a VO 2 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 VO 2 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 VO 2 deficit is referred to as the “oxygen debt” In ICU patients, there may be a progressive and linear relationship between VO 2 & DO 2

8. Monitoring of O 2 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 VO 2 : –Indicates a tissue oxygen deficit –“Oxygen Debt” The total VO 2 deficit over time Remember the direct relationship exists between magnitude of the oxygen debt and subsequent risk of multiorgan failure Normal VO 2 : –Requires a blood lactate level to determine the adequacy of global tissue oxygenation

10. Correcting a VO 2 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 VO 2 Deficit (2) Step 3: VO 2 (Oxygen Uptake) –If VO 2 is less than 100 ml/min/m 2, 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 VO 2 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. VO 2 & DO 2 vs. Time

13. Role of Serum Lactate (1) An elevated lactate indicates that VO 2 is less than the metabolic rate The approach must then be to either decrease the metabolic rate or increase the VO 2 achieving a supranormal level of VO 2 may be difficult and carries risks

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

15. Serum Lactate & Cardiac Index

17. Optimizing Oxygen Transport: The Steps Filling Pressures Cardiac Output VO 2 Serum Lactate

18. Carbon Dioxide (1) An increase in PCO 2 of 5 mmHg can result in a twofold increase in minute ventilation… to produce the same increment in ventilation, the PaO 2 must drop to 55 mmHg The ventilatory control system keeps a close eye on CO 2 but pays little attention to PaO 2 …while clinicians keep a close eye on PaO 2 and pay little attention to PCO 2 “I just don’t understand….”

19. Carbon Dioxide (2) The CO 2 “Sink” Ready source of ions (H + & HCO 3 - ) Buffering capacity of Hb (6x that of all the plasma proteins combined)

20. CO 2 Extraction

21. The Respiratory Quotient RQ =VCO 2 / VO 2 VCO 2 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 Parameter Normal Range Delivery (DO 2 ) 500 - 800 ml/min Uptake (VO 2 ) 110 - 160 ml/min Extraction Ratio (ER) 22 - 32 % Mixed Venous PO 2 33 - 53 mmHg Mixed Venous SO 2 68 - 77 % ** DO 2 & VO 2 can be indexed to body surface area

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