SM_158a: Hypoxemia

Question 1

The alveolar to arterial PO₂ difference (A-aPO₂) indicates ________

Answer 1

The alveolar to arterial PO₂ difference (A-aPO₂​) indicates gas exchange problem while breathing room air

Question 2

The P-F ratio (P_aO₂/F_IO₂) indicates _______

Answer 2

The P-F ratio (P_aO₂/F_IO₂​) indicates gas exchange problem while breathing elevated O₂

Question 3

What are the causes of low P_aO₂?

Answer 3

• Ambient hypoxia
• Diffusion impairment
• Hypoventilation
• Shunt
• V·/Q· inequality

Question 4

What causes ambient hypoxia?

Answer 4

• Low P_B (high altitude)
• Low F_IO₂ (enclosed space with another gas displacing O₂)

Question 5

What casues diffusion impairment?

Answer 5

• Thickened respiratory membrane (idiopathic pulmonary fibrosis)
• Increased diffusion distance (emphysema, minor contribution to hypoxemia)

Question 6

In which condition is a thickened respiratory membrane the cause for diffusion impairment?

Answer 6

Idiopathic pulmonary fibrosis

Question 7

In which condition is increased diffusion distance responsible for diffusion impairment?

Answer 7

Emphysema

Question 8

When a thickened respiratory membrane causes diffusion impairment, increasing F_IO₂ _______

Answer 8

When a thickened respiratory membrane causes diffusion impairment, increasing F_IO₂​ increases driving force for O₂ diffusion, which corrects hypoxemia

Question 9

Diffusion impairment due to increased diffusion distance is ______ to an increase F_IO₂

Answer 9

Diffusion impairment due to increased diffusion distance is responsive to an increase F_IO₂​

Question 10

Alveolar hypoventilation causes a(n) _______ in P_aCO₂

Answer 10

Alveolar hypoventilation causes an increase in P_aCO₂​

Question 11

What can cause alveolar hypoventilation?

Answer 11

• CNS depression (brain injury, disease, drug abuse)
• Neuromuscular disorders (ALS, myasthenia gravis)
• Obstructive pulmonary disease (COPD, obesity-hypoventilation syndrome)
• Restrictive pulmonary disease (kyphoscoliosis)

All increase P_aCO₂ (hallmark of hypoventilation)

Question 12

Hypoventilation is defined by _______

Answer 12

Hypoventilation is defined by elevated P_aCO₂

Question 13

What are the types of shunt?

Answer 13

• Anatomic shunt
• Capillary shunt

Question 14

What are the causes of anatomic shunting?

Answer 14

• Bronchial and thebesian veins (very small amounts)
• VSD or ASD
• PDA

Question 15

What characterizes capillary shunting?

Answer 15

Zero V·/Q· lung regions

(Blood flow through capillaries past collapsed alveoli (atelectasis) or unventilated alveoli)

Question 16

Why does capillary shunting involve V·/Q· lung regions?

Answer 16

Blood flow through capillaries past collapsed alveoli (atelectasis) or unventilated alveoli

Question 17

Shunting is relatively ______ to increased F_IO₂

Answer 17

Shunting is relatively refractory to increased F_IO₂

Question 18

By adding mixed venous blood to the arterial circulation, a shunt tends to ______ P_aO₂ and ______ P_aCO₂

Answer 18

By adding mixed venous blood to the arterial circulation, a shunt tends to reduce P_aO₂ and increase P_aCO₂

Question 19

Increasing alveolar ventilation will often resolve ______ but not ______ resulting from a shunt

Answer 19

Increasing alveolar ventilation will often resolve hypercapnia but not the hypoxemia resulting from a shunt

Question 20

Why is it difficult to correct the hypoxemia due to a shunt by elevating the F_IO₂?

Answer 20

The HbO₂ curve flattens at PO₂ > 60 mmHg

• Increasing V·_A or F_IO₂ will not appreciably increase O₂ content in ventilated portions of the lung
• The lack of an increase in O₂ content in ventilated portions of the lung means that addition of venous blood reduces P_aO₂

Question 21

Increasing V·_A or F_IO₂ will _______ O₂ content in ventilated portions of the lung, so addition of venous blood _______ P_aO₂

Answer 21

Increasing V·_A or F_IO₂ will not appreciably increase O₂ content in ventilated portions of the lung, so addition of venous blood reduces P_aO₂

Question 22

Compared to O₂, why is it less likely that a shunt will elevate P_aCO₂?

Answer 22

CO₂ curve does not flatten

• Increasing ventilation will decrease P_aCO₂ in ventilated portions of the lung
• This offsets the effect of the shunt in raising P_aCO₂

Question 23

Increasing ventilation will _______ P_aCO₂ in ventilated portions of the lung, which ______ the effect of the shunt in ______ P_aCO₂

Answer 23

Increasing ventilation will decrease P_aCO₂ in ventilated portions of the lung, which offsets the effect of the shunt in raising P_aCO₂

Question 24

A shunt tends to ______ P_aO₂, while the tendency for a shunt to ______ P_aCO₂ can be compensated for by a(n) ______ in ventilation

Answer 24

A shunt tends to decrease P_aO₂, while the tendency for a shunt to increase P_aCO₂ can be compensated for by a(n) increase in ventilation

Question 25

\_\_\_\_\_\_\_ is the most common cause of hypoxemia clinically

Answer 25

V·/Q· inequality is the most common cause of hypoxemia clinically

Question 26

Describe what is happening with Goat X

Answer 26

* Hypoxemic while breathing room air * Hypoxemia corrected but with difficulty to elevated F_IO₂ * Elevated A-aPO₂ difference breathing room air * Low P-F ratio * Normocapnic (normal P_aCO₂)

Question 27

Describe the ventilation/perfusion model

Answer 27

1. At a constant flow (Q), the faster the dye is added (V), the higher the effluent dye concentration 2. At a constant V, the faster the Q, the lower the effluent dye concentration 3. Dye concentration is dependent on the V·/Q· ratio

Question 28

V·/Q· = 1 is \_\_\_\_\_

Answer 28

V·/Q· = 1 is ideal | (PO₂ = 100 mmHg, PCO₂ = 100 mmHg)

Question 29

When V·/Q· = 0, V· = \_\_\_\_, indicative of a \_\_\_\_ The alveolus has ____ PO₂, PCO₂

Answer 29

When V·/Q· = 0, V· = 0, indicative of a shunt The alveolus has mixed venous PO₂, PCO₂

Question 30

When V·/Q· = infinity, Q· = \_\_\_\_\_, indicative of \_\_\_\_\_ The alveolus has ____ PO₂, PCO₂

Answer 30

When V·/Q· = infinity, Q· = 0, indicative of dead space The alveolus has inspired PO₂, PCO₂

Question 31

V·_A/Q· line represents all possible PO₂ and PCO₂ values for a \_\_\_\_\_\_\_\_\_\_

Answer 31

V·_A/Q· line represents all possible PO₂ and PCO₂​ values for a single unit in a healthy lung

Question 32

Decreased V·/Q· indicates a \_\_\_\_\_\_

Answer 32

Decreased V·/Q· indicates a shunt

Question 33

Increased V·/Q· indicates a \_\_\_\_\_

Answer 33

Increased V·/Q· indicates a dead space

Question 34

V·/Q· is ____ at the apex

Answer 34

V·/Q· is high at the apex

Question 35

\_\_\_\_ V·/Q· at the apex means _____ PO₂ and _____ PCO₂

Answer 35

High V·/Q· at the apex means high PO₂ and low PCO₂​

Question 36

V·/Q· is ____ at the base

Answer 36

V·/Q· is low at the base

Question 37

\_\_\_\_ V·/Q· at the base means _____ PO2 and _____ PCO2

Answer 37

Low V·/Q· at the base means low PO2 and high PCO2

Question 38

Lung units with _____ V·/Q· add relatively _____ O₂ compared to decrement in O₂ caused by units with low V·/Q·

Answer 38

Lung units with high V·/Q· add relatively little O₂ compared to decrement in O₂ caused by units with low V·/Q·

Question 39

V·/Q· inequality ______ P_aO₂

Answer 39

V·/Q· inequality decreases P_aO₂

Question 40

Describe what is happening with Goat X

Answer 40

Hypoxemia, elevated A-aPO₂ difference, low P-F ratio, normocapnia * Severe V·/Q· maldistribution * Capillary shunt (atelectasis)

Question 41

What is the effect of positive end expiratory pressure?

Answer 41

* Raises pressure in airway -\> prevents alveolar collapse * Reduces cardiac output

Question 42

What is the effect of periodic hyperinflation/sigh?

Answer 42

Elevated tidal volume

Question 43

How can the hypoxemia, elevated A-aPO2 difference, low P-F ratio, normocapnia of Goat X be prevented?

Answer 43

* Positive end expiratory pressure: prevents alveolar collapse by raising pressure in airway, reduces cardiac output * Periodic hyperinflation/sigh: elevates tidal volume

Question 44

A-aPO₂ is _____ in a healthy young adult

Answer 44

A-aPO₂ is \<10 mmHg in a healthy young adult A-aPO₂ = 2.5 + 0.25(age in years) * Broncial and thebesian circulations * V·/Q· mismatching

Question 45

A-aPO₂ is elevated in \_\_\_\_\_\_\_, \_\_\_\_\_\_\_, and \_\_\_\_\_\_\_

Answer 45

A-aPO₂ is elevated in diffusion impairment, shunt, V·/Q· maldistribution

Question 46

A-aPO₂ is not elevated in ________ and \_\_\_\_\_\_\_\_

Answer 46

A-aPO₂ is not elevated in ambient hypoxia or hypoventilation