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Equations

Scores

A-a O2 Gradient

  Units: US SI
Atmospheric Pressure (760 mm Hg or 101.33 kPa @ sea level) mm Hg
PaO2 (arterial O2) mm Hg
PaCO2 (arterial CO2) mm Hg
FIO2 (fraction inspired O2 - room air is 21%) %
Alveolar O2 mm Hg
A-a O2 Gradient (Alveolar - Arterial) mm Hg

A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG

Normal Gradient Estimate = 2.5 + Age * 0.21

The 5 Causes of Hypoxemia, #1-3 have an elevated A-a Gradient:

  1. V/Q Mismatch (ex: PNA, CHF, PE ,ARDS, atelectasis, etc)
  2. Shunt (ex: PFO, ASD, pulmonary AVMs)
  3. Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA)
  4. Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc)
  5. Low FiO2 (ex: high altitude)