A-a Gradient

Aka: A-a Gradient, Alveolar-Arterial Oxygen Tension Difference, (A-a) PO2 difference, (A-a) DO2, Alveolar Oxygen Equation

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II. Indications

  1. Hypoxemia

III. Physiology

  1. Efficiency of gas exchange between alveolus and artery
    1. Reflects the status of the oxygen exchange in the distal lungs (alveoli) and blood
  2. A-a Gradient is the "report card of the lungs"
    1. An abnormal A-a Gradient suggests the lungs as possible cause of Hypoxemia
    2. A normal A-a Gradient suggests causes external to the lungs for hypoexemia (e.g. Bellows Failure, CNS depression)
    3. Swadron (2019) Pulmonary, National Emergency Board Review, CCME, accessed 5/24/2019

IV. Calculation

  1. Alveolar Oxygen Equation
    1. PAO2 = FIO2 x (Pb - 47) - PaCO2/0.8
      1. where PAO2 is the predicted alveolar oxygen based on the arterial CO2
      2. where FIO2 on room air = 0.21
      3. where Pb is barometric pressure (760 mmHg at sea level)
      4. where vapor pressure = 47
      5. where PaCO2 is the measured arterial CO2 (nearly identical to alveolar CO2 at end-expiration)
      6. where respiratory quotient = 0.8 (based on relative rates of CO2 production and Oxygen Consumption)
  2. A-a Gradient (at sea level)
    1. A-a Gradient = PAO2 - PaO2
      1. where PAO2 is predicted alveolar oxygen pressure (based on Alveolar Oxygen Equation above)
      2. where PaO2 is measured arterial oxygen pressure
    2. A-a Gradient = FIO2 x (760 - 47) - (PaCO2/0.8) - PaO2
      1. where FIO2 on room air = 0.21
      2. where atmospheric pressure = 760
      3. where water vapor pressure = 47
      4. where PaO2 is the measured arterial pressure of oxygen
  3. A-a Gradient on room air (FIO2 0.21)
    1. A-a Gradient = 150 - (PaCO2/0.8) - PaO2

V. Interpretation: Calculating a normal A-a Gradient

  1. A-a Gradient = (Age/4) + 4
  2. Young person at sea level
    1. A-a increases 5 to 7 mmHg for every 10% increase FIO2
    2. Room Air: 10 to 20 mmHg
    3. 100% oxygen: 60 to 70 mmHg
  3. Increased age affects A-a Gradient (at sea level, on room air)
    1. Age 20 years: 4 to 17 mmHg
    2. Age 40 years: 10 to 24 mmHg
    3. Age 60 years: 17 to 31 mmHg
    4. Age 80 years: 25 to 38 mmHg

VI. Interpretation: Hypoxemia causes differentiated by A-a Gradient

  1. Increased A-a Gradient
    1. Right to Left Intrapulmonary Shunt (e.g. fluid filled alveoli), decreased PaO2 and PCO2
      1. Congestive Heart Failure
      2. Adult Respiratory Distress Syndrome (ARDS)
      3. Lobar Pneumonia
      4. Atelectasis
    2. V/Q Mismatch (due to lung dead space), increased PCO2 reflecting widespread severe lung disease
      1. Severe Obstructive Lung Disease (e.g. Asthma, COPD)
      2. Severe Interstitial Lung Disease
  2. Normal A-a Gradient
    1. Hypoventilation (PaCO2 increased)
      1. Neuromuscular disorders
      2. Central Nervous System disorder
    2. Low inspired FIO2 (e.g. high altitude)

VII. References

  1. Davies (1986) Acute Respiratory Failure, Cyberlog, Cardinal Health Systems, p. 22-3

Calculation (FPnotebook.com: DrBits) Open in New Window

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