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
- Hypoventilation
- Always associated w/ increased PaCO2
- Normal A-a gradient
- Right-to-left shunt
- Occurs when blood enters the systemic arteries w/o traversing ventilated lung
- Occurs with pulmonary consolidation, atelectasis
- Hallmark is failure of arterial O2 levels to increase w/ supplemental O2 (incr A-a)
- Occurs when blood enters the systemic arteries w/o traversing ventilated lung
- V/Q mismatch
- PE, PNA, asthma, COPD
- Improves w/ supplemental O2
- A-a gradient increased
- Diffusion impairment
- Improves w/ supplemental O2
- A-a gradient increased
- Low inspired O2
- Improves w/ supplemental O2
- A-a gradient normal
Compensation
- Increased minute ventilation
- Pulmonary arterial vasoconstriction
- Increased sympathetic tone
Work-Up
DDx
Treatment
Disposition
See Also
Source
Tintinalli