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Pulm > SM_158a: Hypoxemia > Flashcards

SM_158a: Hypoxemia Flashcards

1
Q

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

A

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

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2
Q

The P-F ratio (PaO2/FIO2) indicates _______

A

The P-F ratio (PaO2/FIO2​) indicates gas exchange problem while breathing elevated O2

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3
Q

What are the causes of low PaO2?

A
  • Ambient hypoxia
  • Diffusion impairment
  • Hypoventilation
  • Shunt
  • V·/Q· inequality
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4
Q

What causes ambient hypoxia?

A
  • Low PB (high altitude)
  • Low FIO2 (enclosed space with another gas displacing O2)
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5
Q

What casues diffusion impairment?

A
  • Thickened respiratory membrane (idiopathic pulmonary fibrosis)
  • Increased diffusion distance (emphysema, minor contribution to hypoxemia)
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6
Q

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

A

Idiopathic pulmonary fibrosis

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7
Q

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

A

Emphysema

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8
Q

When a thickened respiratory membrane causes diffusion impairment, increasing FIO2 _______

A

When a thickened respiratory membrane causes diffusion impairment, increasing FIO2​ increases driving force for O2 diffusion, which corrects hypoxemia

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9
Q

Diffusion impairment due to increased diffusion distance is ______ to an increase FIO2

A

Diffusion impairment due to increased diffusion distance is responsive to an increase FIO2​

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10
Q

Alveolar hypoventilation causes a(n) _______ in PaCO2

A

Alveolar hypoventilation causes an increase in PaCO2​

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11
Q

What can cause alveolar hypoventilation?

A
  • 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 PaCO2 (hallmark of hypoventilation)

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12
Q

Hypoventilation is defined by _______

A

Hypoventilation is defined by elevated PaCO2

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13
Q

What are the types of shunt?

A
  • Anatomic shunt
  • Capillary shunt
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14
Q

What are the causes of anatomic shunting?

A
  • Bronchial and thebesian veins (very small amounts)
  • VSD or ASD
  • PDA
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15
Q

What characterizes capillary shunting?

A

Zero V·/Q· lung regions

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

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16
Q

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

A

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

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17
Q

Shunting is relatively ______ to increased FIO2

A

Shunting is relatively refractory to increased FIO2

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18
Q

By adding mixed venous blood to the arterial circulation, a shunt tends to ______ PaO2 and ______ PaCO2

A

By adding mixed venous blood to the arterial circulation, a shunt tends to reduce PaO2 and increase PaCO2

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19
Q

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

A

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

20
Q

Why is it difficult to correct the hypoxemia due to a shunt by elevating the FIO2?

A

The HbO2 curve flattens at PO2 > 60 mmHg

  • Increasing V·A or FIO2 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 PaO2
21
Q

Increasing V·A or FIO2 will _______ O2 content in ventilated portions of the lung, so addition of venous blood _______ PaO2

A

Increasing V·A or FIO2 will not appreciably increase O2 content in ventilated portions of the lung, so addition of venous blood reduces PaO2

22
Q

Compared to O2, why is it less likely that a shunt will elevate PaCO2?

A

CO2 curve does not flatten

  • Increasing ventilation will decrease PaCO2 in ventilated portions of the lung
  • This offsets the effect of the shunt in raising PaCO2
23
Q

Increasing ventilation will _______ PaCO2 in ventilated portions of the lung, which ______ the effect of the shunt in ______ PaCO2

A

Increasing ventilation will decrease PaCO2 in ventilated portions of the lung, which offsets the effect of the shunt in raising PaCO2

24
Q

A shunt tends to ______ PaO2, while the tendency for a shunt to ______ PaCO2 can be compensated for by a(n) ______ in ventilation

A

A shunt tends to decrease PaO2, while the tendency for a shunt to increase PaCO2 can be compensated for by a(n) increase in ventilation

25
Q

_______ is the most common cause of hypoxemia clinically

A

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

26
Q

Describe what is happening with Goat X

A
  • Hypoxemic while breathing room air
  • Hypoxemia corrected but with difficulty to elevated FIO2
  • Elevated A-aPO2 difference breathing room air
  • Low P-F ratio
  • Normocapnic (normal PaCO2)
27
Q

Describe the ventilation/perfusion model

A
  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
28
Q

V·/Q· = 1 is _____

A

V·/Q· = 1 is ideal

(PO2 = 100 mmHg, PCO2 = 100 mmHg)

29
Q

When V·/Q· = 0, V· = ____, indicative of a ____

The alveolus has ____ PO2, PCO2

A

When V·/Q· = 0, V· = 0, indicative of a shunt

The alveolus has mixed venous PO2, PCO2

30
Q

When V·/Q· = infinity, Q· = _____, indicative of _____

The alveolus has ____ PO2, PCO2

A

When V·/Q· = infinity, Q· = 0, indicative of dead space

The alveolus has inspired PO2, PCO2

31
Q

V·A/Q· line represents all possible PO2 and PCO2 values for a __________

A

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

32
Q

Decreased V·/Q· indicates a ______

A

Decreased V·/Q· indicates a shunt

33
Q

Increased V·/Q· indicates a _____

A

Increased V·/Q· indicates a dead space

34
Q

V·/Q· is ____ at the apex

A

V·/Q· is high at the apex

35
Q

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

A

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

36
Q

V·/Q· is ____ at the base

A

V·/Q· is low at the base

37
Q

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

A

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

38
Q

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

A

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

39
Q

V·/Q· inequality ______ PaO2

A

V·/Q· inequality decreases PaO2

40
Q

Describe what is happening with Goat X

A

Hypoxemia, elevated A-aPO2 difference, low P-F ratio, normocapnia

  • Severe V·/Q· maldistribution
  • Capillary shunt (atelectasis)
41
Q

What is the effect of positive end expiratory pressure?

A
  • Raises pressure in airway -> prevents alveolar collapse
  • Reduces cardiac output
42
Q

What is the effect of periodic hyperinflation/sigh?

A

Elevated tidal volume

43
Q

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

A
  • Positive end expiratory pressure: prevents alveolar collapse by raising pressure in airway, reduces cardiac output
  • Periodic hyperinflation/sigh: elevates tidal volume
44
Q

A-aPO2 is _____ in a healthy young adult

A

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
45
Q

A-aPO2 is elevated in _______, _______, and _______

A

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

46
Q

A-aPO2 is not elevated in ________ and ________

A

A-aPO2 is not elevated in ambient hypoxia or hypoventilation

Pulm (32 decks)

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