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Oxygen Delivery

Oxygen Delivery. Jenny Boyd, MD. Case #1. 12 mo male with a history of truncus arteriosus type I s/p repair with placement of a RV-PA conduit as a newborn who is now s/p conduit replacement. Patient is being admitted to the PCCU post-operatively. What are the goals of our care?.

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Oxygen Delivery

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  1. Oxygen Delivery Jenny Boyd, MD

  2. Case #1 • 12 mo male with a history of truncus arteriosus type I s/p repair with placement of a RV-PA conduit as a newborn who is now s/p conduit replacement. Patient is being admitted to the PCCU post-operatively. • What are the goals of our care? 12 mo male who just had heart surgery and is very sick. We have to take care of him until 7AM when the morning crew arrives. Images from American Heart Association

  3. Care of the PCCU Patient • 2 main goals of critical care: • Ensure adequate oxygen delivery! • Buy time!

  4. 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

  5. Case #1 (cont.) • Initial assessment: PERRL, clear BS bilaterally, RRR, soft belly, warm extremities, well-perfused, 2+ pulses, brisk cap refill. • Initial CXR looks good, all tubes and lines in expected places. • Initial ABG: pH 7.32, pCO2 52, pO2 142, BE -0.2, lactate 3.9 (nl <2) • Initial elevation of lactate very common post-bypass, should resolve within 4 hours

  6. Case #1 (cont.) • Over the next few hours, patient is hemodynamically stable with good perfusion, decent UOP and minimal bleeding from surgical site. • Repeat ABGs are normal except the lactate rises from 3.9  4.4  5.1 • Are you worried? • Is an elevated lactate harmful?

  7. Where Does Lactate Come From? • So, why is our patient’s lactate elevated? 2 ATP Glucose Oxygen X KREB’S CYCLE + ELECTRON TRANSPORT GLYCOLYSIS Pyruvate Lactate 34 ATP

  8. . . DO2 = CaO2* Q Oxygen Delivery • O2 delivery dependent on cardiac output and O2 content of the blood • O2 content is primarily due to hemoglobin saturation with little contribution of dissolved O2 in blood CaO2 = (SaO2* Hb * 1.34)+(0.003 * PaO2)

  9. Oxygen Delivery (cont.) • From previous equations, we can simplify to: • So, there are 3 reasons for poor O2 delivery: 1) anemic anoxemia (low Hgb) 2) anoxic anoxemia (low SaO2) 3) stagnant anoxemia (low Q) . O2 Delivery ≈ Hgb x SaO2 x Q . How much O2 delivery does our patient need?

  10. Oxygen Consumption • Goal: O2 delivery > O2 consumption • Adequate O2 delivery may become insufficient if tissue O2 consumption increases! • Fever increases O2 consumption 10% per degree • Agitation can increase O2 consumption by 40%

  11. Back to the Patient! • Due to the elevated lactate, we minimize O2 consumption by ensuring our patient is afebrile and well sedated. However, our next lactate has risen to 7.0. • What’s wrong with our patient? • Anemic? • Low sats? • Low cardiac output?

  12. Our Patient (cont.) • Since return from the OR, our patient’s Hgb has been > 10 and SaO2 has been >95% • How do we know what our cardiac output is? • What determines cardiac output?

  13. Measuring Cardiac Output • Thermodilution • Need cardiac catheterization • Echocardiography • Need an echocardiographer • Shortening fraction • Surrogate markers • Oxygen extraction

  14. Oxygen Extraction • Measure O2 consumption by looking at O2 extraction: SaO2 – SvO2 • Should be ~20 - 30 mmHg • Need arterial line and right atrial line • Increased O2 extraction can be due to increased O2 consumption (hungry mouths) or decreased O2 delivery (not enough food)

  15. Regional Oxygen Extraction • NIRS (Near-Infrared Spectroscopy) Monitoring • Measures organ-specific oxygen extraction • Kidney – Surrogate for cardiac output • ≈ SaO2 – 15 • Brain – Because the brain is important! • ≈SaO2 – 30 Image from Children’s Hospital of Wisconsin

  16. Understanding Cardiac Output (Q) . . • Q = Heart Rate x Stroke Volume • What determines stroke volume? • Preload • Contractility • Afterload

  17. Stroke Volume Preload Frank-Starling Curve • Increasing preload increases myosin-actin overlap, resulting in increased stroke volume • Increasing contractility increases stroke volume for a given preload • Increasing afterload decreases stroke volume for a given preload

  18. Increasing cardiac output (Q) . . • Remember: Q = Heart Rate x Stroke Volume • Increase heart rate • Pacing • Inotropes • Increase preload • Preload ≈ CVP • Increase contractility • Inotropes • Decrease afterload • Vasodilators

  19. Where were we? • Our patient was having rising lactates despite minimizing O2 consumption and having normal Hgb and SaO2. As we check on him, we note that he is normotensive, warm and well-perfused, with good peripheral pulses and brisk capillary refill. He has had adequate urine output since return from the OR. What other information do you want/need? • Arterial SO2 = 100% • Mixed venous SO2 = 75% • Renal SO2 = 90% • Cerebral SO2 = 80% • CVP = 14

  20. So why is our lactate so high? • Increased production • Dead tissue? • Decreased clearance • Liver failure?

  21. Conclusion • As the nurse is drawing a hepatic function panel, your patient begins to seize. After terminating his seizure, an emergent head CT is performed, revealing left-sided cerebral infarction, probably a bypass-related complication. • Patient discharged to home on POD #8 on Keppra with weakness of RUE

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