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Oxygen: Consumption and Delivery

Oxygen: Consumption and Delivery. PICU Resident Talk Stanford School of Medicine Pediatric Critical Care Medicine June 2010. Learning Objectives. After this lesson, the participant will be able to:

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Oxygen: Consumption and Delivery

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  1. Oxygen: Consumption and Delivery

    PICU Resident Talk Stanford School of Medicine Pediatric Critical Care Medicine June 2010
  2. Learning Objectives After this lesson, the participant will be able to: List the factors that affect oxygen delivery, including blood flow, hemoglobin, and O2 saturation. List the possible reasons for a low mixed venous O2 saturation and possible treatments.
  3. Why Is Oxygen Important? Used in cellular respiration Needed for energy production by cells and tissues 2 ATP Glucose Oxygen KREB’S CYCLE + ELECTRON TRANSPORT GLYCOLYSIS Pyruvate 34 ATP
  4. How is energy produced without oxygen? 2 ATP Glucose Oxygen X KREB’S CYCLE + ELECTRON TRANSPORT GLYCOLYSIS Pyruvate Lactate 34 ATP
  5. Measuring Oxygen Oxygen capacity: maximal amount of oxygen that can be taken up by hemoglobin in the blood Oxygen saturation: the proportion of O2 combined with hemoglobin to the total amount of O2 that can be taken up by hemoglobin. Oxygen content: the amount of oxygen present in both hemoglobin and dissolved O2 CaO2 = (Hgb g/dL x 1.36 ml O2/g Hgb x SaO2) + (0.003 x PaO2)
  6. The Oxygen Supply and Demand Balance Oxygen Consumption (VO2) “demand” Oxygen Delivery (DO2) “supply”
  7. Oxygen Delivery Amount of oxygen delivered (DO2) to the body is the product of systemic blood flow (Q) and oxygen content (CaO2) of systemic arterial blood. DO2 = Q x CaO2 (Q = flow = cardiac output) ● ● ●
  8. Poor Oxygen Delivery Remembering these two equations: DO2 = CO x CaO2 CaO2 = (Hgb g/dL x 1.36 mlO2/gHgb x SaO2)+(0.003 x PaO2) It follows that there are 3 reasons for poor O2 delivery: anoxic anoxemia--(low SaO2) anemic anoxemia--(low Hgb) stagnant anoxemia--(low CO) ●
  9. Oxygen Demand Oxygen requirement determined by metabolic activity. High O2 requirement: Heat production (maintain temperature) Fever: 13% ↑ in BMR and oxygen consumption for every 1 degree ↑ in temperature* Physical activity Moderate O2 requirement: Cardiac contraction, normal respiration Low O2 requirement: Basic cellular function (ionic transport, electrical activity) *DuBois, EF. JAMA 77:352-355, 1921
  10. Oxygen Extraction Oxygen extraction is the proportion of oxygen delivered that is consumed. CaO2 - CvO2 Normally about 30% Delivery is about 500 ml/min/m2, consumption is about 150 ml/min/m2 For normal arterial oxygen saturation of 100%, mixed venous saturation should be 70% Oxygen extraction can be increased to ~70% under conditions of stress
  11. Ways to measure Oxygen supply/demand imbalance Mixed venous saturation Lactate
  12. Mixed Venous Saturation Ideally measured where venous blood is completely mixed (ie. pulmonary artery). Reflects balance of O2 extraction by tissues and O2 delivery=> ↓ SvO2 reflects ↑ O2 extraction or inadequate O2 delivery or both
  13. Lactate Product of anaerobic mitochondrial metabolism which occurs under conditions of inadequate oxygen delivery Can be measured by any blood sample Cleared by liver so level can be affected by liver failure Lactate can also be elevated by increase in pyruvate production or inhibition of pyruvate metabolism Serial measurements can determine lactate clearance which correlates with mortality
  14. When there is an imbalance you can . . . Increase oxygen delivery Or Decrease oxygen demand
  15. Possible ways to ↑ oxygen delivery Remembering these two equations: DO2 = CO x CaO2 CaO2 = (Hgb g/dL x 1.36 mlO2/gHgb x SaO2)+(0.003 x PaO2) Increase Heart Rate or Stroke volume => ↑ Cardiac Output Provide oxygen or establish airway to improve saturation Transfuse blood to increase Hemoglobin
  16. Possible ways to ↓ oxygen demand Regulating temperature—keep “cool” Decrease activity—sedation and/or paralysis Decrease work of breathing—pressure support or intubation with mechanical ventilation Hold enteral feeds
  17. Key Learning Points DO2 = CO x CaO2 = (HR x SV) x (Hgb x SaO2 x 1.36) Reasons for ↓ MVSO2 ↓DO2 Poor CO => Tx w/ cardiovascular meds Anemia => Tx w/ transfusion Hypoxemia => Tx w/ oxygen ↑ VO2 Fever => Tx w/ cooling WOB => Tx w/ mechanical ventilation Exercise/ Agitation => Tx w/ sedation/ paralysis
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