16: Hypoxemia Flashcards
Define Hypoxemia
PaO2 < 80 mm Hg, or
SaO2/SpO2 < 95%
Define severe hypoxemia
PaO2 < 60 mm Hg, or
SaO2/SpO2 < 90%
What is the normal PaO2 at sea level and room air FiO2?
80-110 mm Hg
What is the shape of the Oxygen-hemoglobin dissociation curve?
sigmoid curve
At what wavelengths does a pulse oximeter measure the oxygenated hemoglobin fraction?
660 nm
940 nm
List the corresponding SaO2 % values for these PaO2 values
List 4 limitations of SpO2 measurements
- may measure carboxyhemoglobin and methemoglobin as oxygenated Hb
- severe anemia may lead to hypoxemia but if the residue Hb are well oxygenated SpO2 will be normal
- periperhal vasoconstriction may cause low SpO2 readings without hypoxemia
- SpO2 cannot detect PaO2 changes at 100-500 -> important when monitoring patients on supplemental O2
Define cyanosis
subjective
5 g/dL deoxygenated Hb
i.e., SpO2 of 67% or PaO2 of 37 mm Hg
What are the 3 causes of hypoxemia?
- low inspired oxygen
- hypoventilation
- venous admixture
different to previous “5 causes” -> combines venous admixture
Explain how circulatory shock or high O2 demand may lead to hypoxemia
shock/high O2 demands -> venous O2 decreased -> takes more time and O2 for blood to be arterialized in lungs -> lowers PAO2 -> lowers PaO2
usually doesn’t happen, lungs can offset this by decreasing shunt fract
How do end-tidal CO2 and central venous CO2 compare to PaCO2?
etCO2 5 mm Hg lower and central venous 5 mm Hg higher than PaCO2
What are the 4 main gases within the alveoli?
- O2
- CO2
- water vapor
- nitrogen
What is the PAO2 equation
What is venous admixture?
includes any ways in which blood gets from right side of circulation to left side without being properly oxygenated and then mixes with the arterialized blood reducing overall PaO2
What are 4 causes for venous admixture leading to hypoxemia and how do they respond to O2 supplementation?
Low V/Q regions (e.g., moderate to severe edema, pneumonia, hemorrhage) -> responsive to O2
Zero V/Q regions/Atalectasis (e.g., severe to very severe edema, pneumonia, hemorrhage) -> not responsive to O2
diffusion defects (e.g., O2 toxicity, smoke inhalation, ARDS) -> partially responsive to O2
Anatomic right-to-left shunts (e.g., PDA) -> not O2 responsive