16: Hypoxemia

Question 1

Define Hypoxemia

Answer 1

PaO2 < 80 mm Hg, or
SaO2/SpO2 < 95%

Question 2

Define severe hypoxemia

Answer 2

PaO2 < 60 mm Hg, or
SaO2/SpO2 < 90%

Question 3

What is the normal PaO2 at sea level and room air FiO2?

Answer 3

80-110 mm Hg

Question 4

What is the shape of the Oxygen-hemoglobin dissociation curve?

Answer 4

sigmoid curve

Question 5

At what wavelengths does a pulse oximeter measure the oxygenated hemoglobin fraction?

Answer 5

660 nm
940 nm

Question 6

List the corresponding SaO2 % values for these PaO2 values

Answer 6

Question 7

List 4 limitations of SpO2 measurements

Answer 7

1. may measure carboxyhemoglobin and methemoglobin as oxygenated Hb
2. severe anemia may lead to hypoxemia but if the residue Hb are well oxygenated SpO2 will be normal
3. periperhal vasoconstriction may cause low SpO2 readings without hypoxemia
4. SpO2 cannot detect PaO2 changes at 100-500 -> important when monitoring patients on supplemental O2

Question 8

Define cyanosis

Answer 8

subjective
5 g/dL deoxygenated Hb

i.e., SpO2 of 67% or PaO2 of 37 mm Hg

Question 9

What are the 3 causes of hypoxemia?

Answer 9

• low inspired oxygen
• hypoventilation
• venous admixture

different to previous “5 causes” -> combines venous admixture

Question 10

Explain how circulatory shock or high O2 demand may lead to hypoxemia

Answer 10

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

Question 11

How do end-tidal CO2 and central venous CO2 compare to PaCO2?

Answer 11

etCO2 5 mm Hg lower and central venous 5 mm Hg higher than PaCO2

Question 12

What are the 4 main gases within the alveoli?

Answer 12

• O2
• CO2
• water vapor
• nitrogen

Question 13

What is the PAO2 equation

Answer 13

Question 14

What is venous admixture?

Answer 14

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

Question 15

What are 4 causes for venous admixture leading to hypoxemia and how do they respond to O2 supplementation?

Answer 15

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

Question 16

What is the normal percentage of venous admixture in healthy animals?

Answer 16

< 5%

Question 17

What may lead to regions or low V/Q ratio

Answer 17

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

Question 18

What are the causes of zero V/Q?

Answer 18

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

Question 19

Is hypoventilation responsive to O2 supplementation?

Answer 19

Yes

very responsive, hypoventilation on supplemental O2 almost impossible to cause hypoxemia -> increased FiO2 will sufficiently improve PAO2 -> increase PaO2

Question 20

Is low V/Q responsive to O2 supplementation?

Answer 20

Yes

Question 21

Is zero V/Q responsive to O2 supplementation?

Answer 21

No -> O2 cannot get to the gas exchange surface

PPV and PEEP may help

Question 22

Is diffusion impairment responsive to O2 supplementation?

Answer 22

partially responsive

Question 23

Name examples of causes for diffusion impairment

Answer 23

• oxygen toxicity
• smoke inhalation
• ARDS

Question 24

Is an anatomic shunt responsive to oxygen supplementation?

Answer 24

No

Question 25

Explain the 120 rule and how it is utilized

Answer 25

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

Question 26

What is the normal PAO2-PaO2 gradient?

Answer 26

< 10 mm Hg at room air

values will be much higher at increased FiO2, only useful on room air
values > 20 -> decreased oxygenation efficiency

Question 27

What is the abbreviated PAO2 equation?

Answer 27

150-PaCO2

at sea level and 21% O2

Question 28

in what conditions should you not use the P/F ratio and why?

Answer 28

at room air –> changes in PaCO2 will affect PAO2 and PaO2 at room air

Question 29

What is a normal P/F ratio?

Answer 29

~ 500

Question 29

What is the oxygenation index?

Answer 29

index to evaluate oxygenation in ventilated patients
OI = mean airway pressure x FiO2 x 100/PaO2

Question 30

What is the equation to determine CaO2?

Answer 30

(1.34 x Hb x SO2) + (0.003 x PO2)

Question 31

Atalectatic lungs are not O2 responsive. What other treatment can be used to improve hypoxemia from atalectasis?

Answer 31

PPV

Question 32

How does pulmonary diffusion impairment develop?

Answer 32

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