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
What is the normal percentage of venous admixture in healthy animals?
< 5%
What may lead to regions or low V/Q ratio
small airway narrowing or alveolar fluid accumulation -> will impair ventilation but some gas exchange still occurs (no zero V/Q)
e.g., bronchospasm, fluid accumulation along walls of lower airways, epithelial edema, pneumonia, hemorrhage
What are the causes of zero V/Q?
small airway collapse or alveolar collapse (atalectasis) -> no ventilation but perfusion -> zero V/Q -> physiologic shunt
e.g., airway fluids, airway collapse, severe edema/pneuminia, hemorrhage
Is hypoventilation responsive to O2 supplementation?
Yes
very responsive, hypoventilation on supplemental O2 almost impossible to cause hypoxemia -> increased FiO2 will sufficiently improve PAO2 -> increase PaO2
Is low V/Q responsive to O2 supplementation?
Yes
Is zero V/Q responsive to O2 supplementation?
No -> O2 cannot get to the gas exchange surface
PPV and PEEP may help
Is diffusion impairment responsive to O2 supplementation?
partially responsive
Name examples of causes for diffusion impairment
- oxygen toxicity
- smoke inhalation
- ARDS
Is an anatomic shunt responsive to oxygen supplementation?
No
Explain the 120 rule and how it is utilized
at sea level and room air (21% O2) PaCO2 and PaO2 should add up to ~ 120 mm Hg
e.g., if hypoventilating -> PaCO2 60 mm Hg –> PaO2 would decrease from 80 mm Hg to 60 mm Hg due to decrease O2 delivery to alveoli (hypoventilation)
if e.g., PaCO2 60 mm Hg and PaO2 40 mm Hg -> indicates underlying oxygenation issue additionally to hypoventilation
What is the normal PAO2-PaO2 gradient?
< 10 mm Hg at room air
values will be much higher at increased FiO2, only useful on room air
values > 20 -> decreased oxygenation efficiency
What is the abbreviated PAO2 equation?
150-PaCO2
at sea level and 21% O2
in what conditions should you not use the P/F ratio and why?
at room air –> changes in PaCO2 will affect PAO2 and PaO2 at room air
What is a normal P/F ratio?
~ 500
What is the oxygenation index?
index to evaluate oxygenation in ventilated patients
OI = mean airway pressure x FiO2 x 100/PaO2
What is the equation to determine CaO2?
(1.34 x Hb x SO2) + (0.003 x PO2)
Atalectatic lungs are not O2 responsive. What other treatment can be used to improve hypoxemia from atalectasis?
PPV
How does pulmonary diffusion impairment develop?
inflammation -> during healing process type II pneumocytes proliferate -> thickening of the gas exchange barrier -> diffusion of O2 impaired until type II mature to type I pneumocytes
