Oxygen Delivery (DO2)

Oxygen Delivery (DO2)

David Ray Velez, MD

Table of Contents

Oxygen Delivery (DO2)
Oxygen Consumption (VO2) and Oxygen Extraction Ratio (O2ER)
Evaluating Oxygenation

Oxygen Delivery (DO2)

Oxygen Delivery (DO2) = CaO2 x CO

Cardiac Output (CO)

  • CO = Heart Rate (HR) x Stroke Volume (SV)
  • Factors that Increase Stroke Volume:
    • Increased Preload (End-Diastolic Volume)
    • Increased Contractility
    • Decreased Afterload
  • *See Hemodynamic Physiology

Arterial Oxygen Content (CaO2)

  • CaO2 = (Hgb x SaO2 x 1.34) + (PaO2 x 0.003)
  • Hgb: Hemoglobin
  • Oxygen Carrying Capacity = 1.34
  • 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)

Level of Importance

  • Hemoglobin is Generally the Most Clinically Significant Determinant of Arterial Oxygen Content (CaO2)
  • SaO2 Has the Same Impact as Hemoglobin but Generally Has Less Variance
  • PaO2 is the Least Important Measure and Only Contributes 1-2%

Oxygen-Hemoglobin Dissociation Curve

  • Compares Partial Pressure of Oxygen (PaO2) to Percent Oxygen Saturation of Hemoglobin (SaO2)
  • Shifts:
    • Left Shift – Increased Affinity Supports O2 Binding
    • Right Shift – Decreased Affinity Causes O2 Unloading
  • 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
  • Factors that Cause a Rightward Shift:
    • Increased CO2
    • Increased Temperature
    • Increased H+ (Decreased pH)
    • Increased 2,3-DPG (2,3-Diphosphoglycerate)
      • Hypoxia Further Decreases Oxygen Affinity 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
  • *See Oxygen-Hemoglobin Dissociation Curve

Alveolar-Arterial (A-a) Gradient

  • Evaluates the Degree of Shunting and V/Q Mismatch to Help Determine the Cause of Hypoxia
  • A-a Gradient = PAO2 – PaO2
    • PAO2: Alveolar Partial Pressure of Oxygen
    • PaO2: Arterial Partial Pressure of Oxygen
  • PAO2 = FiO2 x (PATM – PH2O) – PaCO2/RQ
    • PATM = Atmospheric Pressure
    • PH2O = Water Vapor Pressure (Usually 47 mmHg)
    • RQ = Respiratory Quotient (Usually 0.8)
  • *See Alveolar-Arterial (A-a) Gradient

Oxygen-Hemoglobin Dissociation Curve

Oxygen Consumption (VO2) and Oxygen Extraction Ratio (O2ER)

Oxygen Consumption (VO2)

  • VO2 = (CaO2 – CvO2) x CO
    • CO: Cardiac Output
    • CaO2: Arterial Oxygen Content
    • CvO2: Venous Oxygen Content

Venous Oxygen Content (CvO2)

  • CaO2 = (Hgb x SvO2 x 1.34) + (PvO2 x 0.003)
  • Hgb: Hemoglobin
  • Oxygen Carrying Capacity = 1.34
  • SvO2: Mixed Venous Oxygen Saturation of Hgb
    • Generally Measured from a Pulmonary Artery Catheter
    • True Value Would Be at IVC and Coronary Sinus
    • Normal: 70-75%
      • Highest in the Renal Veins (80%)
      • Lowest in the Coronary Sinus (30%)
    • ScvO2: Central Venous Oxygen Saturation
      • Drawn from a Central Venous Catheter to Estimate SvO2
      • Approximately 3-5% Higher than SvO2 (Does Not Include Coronary Sinus Return)
  • PvO2: Partial Pressure of Dissolved Oxygen in Venous Blood

Oxygen Extraction Ratio (O2ER)

  • Oxygen Extraction Ratio Describes the Ratio of Oxygen Consumption (VO2) to Delivery (DO2)
  • O2ER = VO2 / DO2
    • VO2 = (CaO2 – CvO2) x CO
    • DO2 = CaO2 x CO
    • Therefore: O2ER = (CaO2 – CvO2) / CaO2
  • Normal Values:
    • VO2: 120-170 mL/min/m2
    • DO2: 500-600 mL/min/m2
      • Critical Value: 330 mL/min/m2 – Level at Which Oxygen Uptake Begins to Decline Causing an “Oxygen Debt”
    • O2ER: 25-30%
  • O2ER is Highest in the Coronary Circulation (> 60%) and Brain Tissue
  • Elevated O2ER Indicates Inadequate Oxygen Delivery or Increased Consumption
  • Oxygen Delivery:Consumption Ratio: 4:1

Evaluating Oxygenation

Arterial Blood Gas (ABG)

  • Generally Measures and Reports PaO2 (What is Used in P:F Ratio, etc)
  • May Also Measure SaO2
  • ABG Challenges:
    • Expensive, Painful, and 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)
  • *See Reading an Arterial Blood Gas (ABG)

Pulse Oximeter (Pulse Ox)

  • Detects SpO2
  • Pulse Ox Challenges:
    • May Have a Bias of +2% in Dark Skin
    • Requires Pulse Detection in Peripheral Circulation – May Be Abnormal in Shock or Hypothermia
    • Generally Not Accurate Once Below 70%
    • Falsely Elevated in Carbon Monoxide Poisoning

ABG vs Pulse Ox

  • Pulse Oximetry (SpO2) is Generally Superior to ABG (PaO2) in Measuring Oxygenation
  • When ABG is Preferred:
    • If Pulse Oximeter Waveform is Unreliable
    • For P/F Ratio Calculation in ARDS
    • Diagnosis of Methemoglobinemia

Venous Blood Gas (VBG)

  • Sources:
    • Peripheral – From Peripheral Veins
    • Central Venous – From a Central Line
    • Mixed Venous – From a Pulmonary Artery Catheter
  • Benefits Over ABG:
    • Easier to Obtain
    • Less Painful
    • Can Be Obtained While Sampling Other Lab Tests
    • Lower Risk of Complications (Hematoma, Dissection, Thrombosis)
  • Challenges:
    • ABG is Still Considered the Gold Standard Blood Test in Evaluating Oxygenation, Ventilation, and Acid-Base Status
    • VBG is Generally Less Well Validated than an ABG
    • VBG and ABG May Have Worse Agreement in the Presence of Severe Circulatory Failure/Hypotension
    • VBG May Correlate to ABG Better for pH and pCO2 than for PaO2 or SaO2

General Target Values

  • SaO2/SpO2 > 92% (88% in COPD or ARDS)
  • PaO2 > 60 mmHg (55 mmHg in COPD or ARDS)
  • ScvO2 > 70%

Hypoxic Definitions

  • Hypoxemia: Low Oxygen Content in Blood
  • Hypoxia: Low Oxygen Content in Tissues