Oxygen-Hemoglobin Dissociation Curve

Oxygen-Hemoglobin Dissociation Curve

David Ray Velez, MD

Also Known As: “Oxyhemoglobin Dissociation Curve” or “Oxygen Dissociation Curve”

Oxygen-Hemoglobin Dissociation

  • Compares the Partial Pressure of Oxygen (PaO2) to Oxygen Saturation of Hemoglobin (SaO2)
  • Shape:
    • Sigmoidal (S-Shaped)
    • Steep Slope at Low Oxygen Tensions – Increasing PaO2, When Low, Can Significantly Improve SaO2
    • Gradual Slope at High Oxygen Tensions – Continually Increasing PaO2, When High, Has Minimal Effect on SaO2
  • The Curve Can Be Shifted Left or Right Depending on Various Factors that Affect Oxygen and Hemoglobin Binding

Oxygen-Hemoglobin Dissociation Curve

Oxygen Delivery (DO2)

  • Oxygen Delivery (DO2) = CaO2 x CO
  • Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)
  • Arterial Oxygen Content (CaO2) = (Hgb x SaO2 x 1.34) + (PaO2 x 0.003)
  • SaO2: Percent Arterial Oxygen Saturation of Hgb
    • SpO2: Peripheral Oxygen Saturation is Detected on Pulse Oximeter and Used to Estimate SaO2
  • PaO2: Partial Pressure of Dissolved Oxygen in Blood (Oxygen Tension)
  • *The Majority Delivered Oxygen is Carried Bound to Hemoglobin (SaO2) and the Remaining Oxygen Dissolved in Plasma (PaO2) Only Contributes 1-2% and is Much Less Clinically Significant
  • *See Oxygen Delivery (DO2)

Left-Shift

  • Left Shift – Increased Affinity of Oxygen to Hemoglobin (Supports O2 Binding)
  • Factors that Cause a Leftward Shift:
    • Decreased CO2
    • Decreased Temperature
    • Decreased H+ (Increased pH)
    • Decreased 2,3-Diphosphoglycerate (DPG)
    • Carbon Monoxide Exposure – Increases Affinity at Other Binding Sites
  • In the Lungs, Where CO2 is Ventilated and pH Increases, the Curve Shifts to the Left and Increases Affinity to Grab Oxygen for Delivery
  • *Older Studies Reported a Left-Shift in Sepsis But this Has Since Been Found Preventable by Avoiding Hypophosphatemia, Transfusion with 2,3-DPG Depleted Blood, and Treatment of Acidemia

Right-Shift

  • Right Shift – Decreased Affinity of Oxygen to Hemoglobin (Causes O2 Unloading)
  • Factors that Cause a Rightward Shift:
    • Increased CO2
    • Increased Temperature
    • Increased H+ (Decreased pH)
    • Increased 2,3-DPG (2,3-Diphosphoglycerate)
  • In the Tissues, Which Produce CO2 and Decrease pH, the Curve Shifts to the Right and Decreases Affinity to Unload Oxygen at the Tissues

What is 2,3-DPG (2,3-Diphosphoglycerate)?

  • 2,3-DPG is a Compound in Red Blood Cells that Binds to Deoxygenated Hemoglobin to Stabilize the Structure State
  • Hypoxemia Causes Increased 2,3-DPG Levels that Further Decreases Oxygen Affinity to Promote Unloading and Can Further Propagate Hypoxemia
  • Fetal Hemoglobin Has a Low Affinity for 2,3-DPG Resulting in Overall Higher Affinity for Oxygen