Hypoxemia

Background

• 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 Hb, or low SaO2 (arterial oxygen saturation)
• Relative Hypoxemia
  • PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
  • Can calculate amount of relative hypoxemia by A-a gradient
    • P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy pts)

Pathophysiology

Causes

1. Hypoventilation
   1. Always associated w/ increased PaCO2
   2. Normal A-a gradient
2. Right-to-left shunt
   1. Occurs when blood enters the systemic arteries w/o traversing ventilated lung
      1. Occurs with pulmonary consolidation, atelectasis
   2. Hallmark is failure of arterial O2 levels to increase w/ supplemental O2 (incr A-a)
3. V/Q mismatch
   1. PE, PNA, asthma, COPD
   2. Improves w/ supplemental O2
   3. A-a gradient increased
4. Diffusion impairment
   1. Improves w/ supplemental O2
   2. A-a gradient increased
5. Low inspired O2
   1. Improves w/ supplemental O2
   2. A-a gradient normal

Compensation

1. Increased minute ventilation
2. Pulmonary arterial vasoconstriction
3. Increased sympathetic tone

Source

Tintinalli