Defined as PaO2 <60 mm Hg
Amount of oxygen available is a function of the arterial oxygen content and blood flow
Occurs in states of low CO, low hemoglobin, or low SaO2 (arterial oxygen saturation)
Relative<unk> 
PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
Can calculate amount of relative<unk>  by A-a gradient
P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)
Amount of oxygen available is a function of the arterial oxygen content and blood flow
PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
Can calculate amount of relative<unk>  by A-a gradient
P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)
P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)
Hypoventilation
Always associated with increased PaCO2
Normal A-a gradient
Right-to-left shunt
Occurs when blood enters the systemic arteries with out traversing ventilated lung
Occurs with pulmonary consolidation, atelectasis
Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
V/Q mismatch
PE, pneumonia, asthma, COPD
Improves with supplemental O2
A-a gradient increased
Diffusion impairment
Improves with supplemental O2
A-a gradient increased
Low inspired O2
Improves with supplemental O2
A-a gradient normal
Always associated with increased PaCO2
Normal A-a gradient
Occurs when blood enters the systemic arteries with out traversing ventilated lung
Occurs with pulmonary consolidation, atelectasis
Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
Occurs with pulmonary consolidation, atelectasis
PE, pneumonia, asthma, COPD
Improves with supplemental O2
A-a gradient increased
Improves with supplemental O2
A-a gradient increased
Improves with supplemental O2
A-a gradient normal
Hypoxemia
Pulmonary
Hypercapnia
Airway obstruction
Metabolic alkalosis
Anaphylaxis
Metabolic acidosis
Angioedema