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Dusan Hanidziar, MD Department of Anesthesiology Tufts Medical Center

O 2 transport O 2 physical solubility HbO 2 saturation HbO 2 dissociation curve P50 2,3-diphosphoglycerate Hb as a buffer. Dusan Hanidziar, MD Department of Anesthesiology Tufts Medical Center. O 2 transport. Oxygen delivery (DO 2 ) is a function of: 1. Cardiac output

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Dusan Hanidziar, MD Department of Anesthesiology Tufts Medical Center

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  1. O2 transportO2 physical solubilityHbO2 saturationHbO2 dissociation curveP502,3-diphosphoglycerateHb as a buffer Dusan Hanidziar, MD Department of Anesthesiology Tufts Medical Center

  2. O2 transport • Oxygen delivery (DO2) is a function of: 1. Cardiac output 2. O2 content in arterial blood DO2 = Q x CaO2 Q=cardiac output Q=stroke volume x heart rate CaO2=oxygen content in arterial blood CaO2= (1.34 x Hb x SaO2) + (0.003 x PaO2) Adapted from Marino PL: ICU book, 2007

  3. O2 transport • O2 is carried in the blood in two forms: 1. bound to hemoglobin within erythrocytes 2. dissolved in plasma • 1 L of arterial blood contains ~ 200 mL O2 • Arterial and venous blood together contain ~ 800 mL O2 Adopted from Marino PL: ICU book, 2007

  4. O2 physical solubility • O2 is poorly soluble in plasma (3 mL O2/ 1 L plasma at 100 mm Hg PO2) Dissolved O2= 0.003 x PaO2 0.003=solubility coefficient • 100% FiO2 (PaO2 500-600 mmHg) increases amount of dissolved O2 to ~18 mL/L plasma Adapted from Marino PL: ICU book 2007

  5. Oxygen saturation of hemoglobin • SaO2 SaO2 = HbO2/ (HbO2 + reduced Hb + COHb + MetHb) obtained by ABG, 97.5% at 100 mmHg PO2 • SpO2 SaO2 = HbO2/ (HbO2 + reduced Hb) obtained by pulse oximetry • SvO2 SvO2 = HbO2/ (HbO2 + reduced Hb + COHb + MetHb) obtained by VBG, 75% at 40 mmHg PO2 Hb + 4O2 ↔ nH+ Hb(O2) 4 Adapted from Barash PG et al: Clinical Anesthesia, 2006

  6. O2 partial pressures • Partial pressure of oxygen drops as oxygen moves from inhaled air to tissues • O2 tension clinically measured by ABG (normal PaO2 >80 mm Hg) or VBG (40-50 mm Hg) West JB: Respiratory Physiology 2012

  7. HbO2 dissociation curve • Rightward shift (by ↑CO2, ↑H+, ↑Temp, ↑2,3-DPG) facilitates O2 unloading in the tissues • Permissive hypercapnia during GA may improve tissue oxygenation and protect from stroke, MI, AKI Adapted from Miller RD: Basics of Anesthesia, 2011

  8. 2,3-diphosphoglycerate (2,3-DPG) • End-product of erythrocyte metabolism, located in the central cavity of hemoglobin • 2,3-DPG promotes release of O2 molecules from Hb https://iwasa.hms.harvard.edu/project_pages/hemoglobin.html • High altitude, COPD, CHF increase 2,3-DPG content in erythrocytes Adapted from West JB: Respiratory Physiology 2012

  9. Hemoglobin as a buffer • Amino acid residues (-COO-, -NH2) of Hb bind H+ generated by hydration of CO2 in erythrocytes • Reduced hemoglobin is a better buffer than oxygenated Hb Adapted from Marino PL: ICU book, 2007 and West JB: Respiratory Physiology 2012

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