Alveolar-Arterial (A-a) Gradient

Alveolar-Arterial (A-a) Gradient

David Ray Velez, MD

Table of Contents

Definition/Equations
Clinical Use/Classification

Definition/Equations

Definition

  • Definition: The Difference Between Alveolar and Arterial Oxygen Tension
  • Used to Evaluate 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 (Determined by the Alveolar Gas Equation)
    • PaO2: Arterial Partial Pressure of Oxygen (Measured by Arterial Blood Gas/ABG)
  • *See Oxygen Delivery (DO2)

Alveolar Gas Equation

  • PAO2 = PiO2 – PaCO2/RQ
  • PiO2: Partial Pressure of Oxygen Inspired in the Alveoli
    • PiO2 = FiO2 x (PATM – PH2O)
    • FiO2: Fraction of Inspired Oxygen
    • PATM: Atmospheric Pressure (760 mmHg at Seal Level, 235 mmHg on Mount Everest)
    • PH2O: Water Vapor Pressure (47 mmHg Approximately)
  • PaCO2: Arterial Partial Pressure of Carbon Dioxide
  • RQ: Respiratory Quotient (Usually 0.8)
  • Long Form: PAO2 = FiO2 x (PATM – PH2O) – PaCO2/RQ + f
    • f: 2-3 mmHg to Represent Partial Pressure of Nitrogen (Generally Ignored)

Clinical Use/Classification

Clinical Use

  • The A-a Gradient Evaluates the Degree of Shunting and V/Q Mismatch to Help Determine the Cause of Hypoxia
  • Short Form (Assuming at Sea-Level):
    • A-a Gradient = PAO2 – PaO2
    • A-a Gradient = (FiO2 x 713) – (PaCO2 / 0.8) – PaO2
  • Generally Just Need an ABG (PaO2/PaCO2) and Ventilator Settings (FiO2) for Approximation
  • *Note: The Normal Gradient May Be Higher with Increased Age
    • Estimated Normal A-a Gradient < (Age/4) + 4

Normal Gradient (5-10 mmHg)

  • Indicates that the Partial Pressure of Oxygen in the Alveoli is Similar to the Blood
  • Insufficient Oxygen Delivery to the Alveoli
  • Causes:
    • Hypoventilation (High PACO2)
    • High Altitude (Low Barometric Pressure)
    • Low FiO2

Elevated Gradient (> 10 mmHg)

  • Indicates that the Partial Pressure of Oxygen in the Alveoli is Higher than in the Blood
  • Causes:
    • Ventilation-Perfusion (V/Q) Mismatch (Pneumonia, CHF, ARDS, Atelectasis, PE, etc.)
    • Right-to-Left Shunt (Intrapulmonary or Cardiac)
    • Elevated Oxygen Extraction Ratio (O2ER): Ratio of Oxygen Consumption to Delivery
    • Diffusion Defect (Rare)