Surgical Critical Care: Oxygen Delivery & Monitoring

Oxygen-Hemoglobin Dissociation Curve

• Compares Partial Pressure of Oxygen (PO2) vs Percent Oxygen Saturation of Hemoglobin
• Shifts:
  • Rightward Shift – Causes Decreased Affinity & Increased O2 Unloading
  • Leftward Shift – Causes Increased Affinity & Support O2 Binding
• Factors that Cause Rightward Shift:
  • Increased CO2
  • Increased Temperature
  • Increased H+ (Decreased pH)
  • Increased 2,3-DPG (2,3-Diphosphoglycerate)
    • Hypoxia Causes Decreased Affinity for Oxygen Due to 2,3-DPG Having Increased Affinity for Deoxygenated Hemoglobin
    • Fetal Hemoglobin Has a Low Affinity for 2,3-DPG Resulting in Overall Higher Affinity for Oxygen
• Factors that Cause a Leftward Shift:
  • Decreased CO2
  • Decreased Temperature
  • Decreased H+ (Decreased pH)
  • Decreased 2,3-DPG (2,3-Diphosphoglycerate)

Oxygenation Parameters

• CaO2: Arterial O2 Content
• SaO2: Percent Arterial Saturation of Hgb
• SpO2: Peripheral Capillary Oxygen Saturation
  • Estimates SaO2
  • Detected by Pulse Oximeter
  • *May Have a Bias of +2% with Darkly-Pigmented Skin
• PaO2: Partial Pressure of Dissolved O2 in Blood (Arterial “Oxygen Tension”)
  • Measured on ABG
• Hypoxic Definitions:
  • Hypoxemia: Low Oxygen Content in Blood
  • Hypoxia: Low Oxygen Content in Tissues

Arterial O2 Content (CaO2)

• CaO2 = (Hgb x SaO2 x 1.34) + (PaO2 x 0.003)
  • Oxygen Carrying Capacity = 1.34
• Most Important Determinant: Hgb
  • SaO2 Has Less Variance
• PaO2 Only Contributes 1-2%

Pulse Oximetry vs Arterial Blood Gas

• Pulse Oximetry is Generally Superior to ABG in Measuring Oxygenation
• ABG Challenges:
  • Expensive, Painful & Time Consuming
  • Blood Loss
  • May be Contaminated with Venous Blood
  • Although a Better Measure of Lung Function, Pulse Ox Better Measures Systemic Delivery
  • Often Misinterpreted – (SpO2 of 88% Correlates with PaO2 of 55 mmHg)
• When ABG is Preferred:
  • If Pulse Oximeter Waveform is Unreliable
  • P/F Ratio Calculation in ARDS
  • Diagnosis of Methemoglobinemia

Oxygen Extraction

• Oxygen Extraction Ratio = (CaO2 – CvO2) / CaO2
  • Average: 0.25-0.30
  • Highest in Coronary Circulation & Brain Tissue
  • Increases as O2 Consumption Increases
• Oxygen Delivery:Consumption Ratio: 4:1
  • Oxygen Delivery Dependent on: Cardiac Output & CaO2
• Determinants of Myocardial Oxygen Consumption: Wall Tension (#1) & Heart Rate

Venous Oxygen Saturation

• Mixed Venous Oxygen Saturation (SvO2): 70-75%
  • Generally Measured from Pulmonary Artery
  • True Value Would Be at IVC & Coronary Sinus
• Venous Oxygen Saturation:
  • Highest: Renal Veins (80%)
  • Lowest: Coronary Sinus (30%)