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Pulmonary > PATHOPHYS: Gas Exchange II > Flashcards

PATHOPHYS: Gas Exchange II Flashcards

1
Q

What is the equation for partial pressure of a gas?

A

Partial Pressure = Atmospheric Pressure X Fraction of gas

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2
Q

What is the fractional percent of oxygen in air?

A

21%

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3
Q

What is the partial pressure of oxygen in air?

A

160 mmHg

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4
Q

Why is does the air we breathe in not have a partial pressure of oxygen at 160 mmHg?

A

Because air is inspired, rapidly warmed, and saturated with H2O (so you must account for water vapor)

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5
Q

What is the partial pressure of water vapor at body temp?**

A

47 mmHg

this will be on test

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6
Q

What is total gas pressure (accounting for water vapor)?

A

760-47= 713

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7
Q

What is the partial pressure of oxygen that we breathe into the lungs?

A 
150 mmHg
(760-46) X 0.21= 150
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8
Q

True or False: water vapor varies greatly with altitude.

A

FALSE: water vapor does NOT vary with altitude

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9
Q

What is the definition of minute ventilation?

A

the volume exhaled per minute, calculated as:

Tidal Volume X Respiratory Rate

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10
Q

What is the definition of anatomic dead space?

A

the air which remains int he conducting airways at the end of inspiration and DOES NOT participate in gas exchange

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11
Q

What is alveolar dead space?

A

alveoli that are ventilated but not perfused (so they cannot participate in gas exchange)

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12
Q

How do alveoli with reduced but not absent ventilation behave?

A

They act as if a portion was normal and a portion was dead space

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13
Q

What is physiologic dead space?

A

the sum of anatomic and alveolar dead space

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14
Q

What is the equation for alveolar ventilation?

A

Minute Ventilation - Dead Space = Alveolar ventilation (effective ventilation)

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15
Q

How can you approximate anatomic dead space?

A

it is around lean body weight in pounds (typically 150-180 mL)

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16
Q

True or false: dead space is a static volume.

A

FALSE!

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17
Q

What will increase the anatomic dead space?

A

increasing lung volume

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18
Q

What will decrease the alveolar dead space?

A

exercise (increases blood flow to alveoli at the apex of the lung!)

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19
Q

True or false: increased alveolar dead space is ALWAYS pathologic.

A

TRUE! (ex. pulmonary embolism)

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20
Q

What does the alveolar gas equation tell us?

A

It allows us to:

1) estimate the alveolar partial pressure of oxygen
2) Determine if hypoxemia is due to lung disease or not
3) Determine the causes of hypoxemia

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21
Q

CO2 is eliminated from the body solely via what process?

A

VENTILATION

22
Q

Alveolar oxygen is a balance of what two processes?

A 
Oxygen delivery
Oxygen removal (from alveolus by capillary blood)
23
Q

What determines the rate of oxygen removal at the alveolus?

A

tissue oxygen consumption (usually around 250 mL/min

24
Q

What is the respiratory exchange ratio (R)? What is the normal value of R?

A

Volume of carbon dioxide produced for volume of oxygen removed (usually 0.8)

25
Q

What is the alveolar gas equation?

A

PAO2= PiO2- (PaCO2/R)

26
Q

What is PAO2 at sea level with normal R and PaCO2 of 40 mmHg (normal)?

A

PAO2= 150 - (40/0.8)= 100 mmHg

27
Q

Which gives a greater PAO2: normal breathing, hyperventilation, or hypoventilation?

A

Hyperventilation (have a drastically lower PaCO2 because you “blow off” carbon dioxide with deep breathing)

28
Q

How is arterial oxygen tension (PaO2) measured?

A

arterial blood gas test

29
Q

How is alveolar oxygen tenxion (PAO2) measured?

A

NOT measured–calculated from the alveolar gas equation

30
Q

What is the A-a difference?

A

Calculation of PAO2-PaO2

31
Q

What change in A-a will tell you if you have lung disease?

A

Widened A-a (or larger number) will tell you that your hypoxemia is due to lung problem

32
Q

What is the formula for a normal A-a gradient?

A

(Age + 4)/4

33
Q

True or false: A-a gradient decreases with age.

A

FALSE: A-a gradient increases with age, because arterial oxygen tension decreases with age due to decreased oxygen transfer to blood (alveolar oxygen tension remains the same!)

34
Q

What is hypoxemia?

A

decreased oxygen tension int he circulating blood compared to normal

35
Q

What are two broad categories of hypoxemia?

A

Defective exchange of oxygen in the lungs (widened A-a) OR from decreased delivery of oxygen to the alveolus (normal A-a in absence of lung disease)

36
Q

List the 5 causes of hypoxemia.

A

Normal A-a

1) Decreased PiO2 (high altitude)
2) Hypoventilation

Widened A-a

3) Diffusion limitation
4) R to L shunt
5) V/Q mismatch

37
Q

How might you measure hypoventilation?

A

PACO2 is inversely proportional to alveolar ventilation, so PaCO2 is a good measure of alveolar ventilation.

38
Q

What is the least clinically significant cause of A-a widening? Why?

A

Diffusion limitation, because diffusion capacity would have to be SEVERELY decreased (lower than 1/3 of normal) or have extremely decreased transit time (from really high cardiac output)

39
Q

True or false: diffusion capacity tests can tell you about the mechanism (decreased surface area, increased membrane thickness, etc.) of diffusion limitation.

A

FALSE: if can’t separate the mechanisms

40
Q

True or false: abnormal diffusion is more likely to cause problems during exercise than during rest.

A

TRUE: it may lead to a lower PaO2 during exercise due to lowered transit time

41
Q

How might you treat hypoxemia due to diffusion limitation?**

A

Give 100% supplemental oxygen

42
Q

Name 2 types of R-to-L shuts.

A 
Cardiac Shunts
Pulmonary shunts (unventilated alveoli with preserved perfusion)
43
Q

How might you treat hypoxemia due to R-to-L shunt?**

A

TRICK QUESTION! Does not respond to supplemental oxygen like the other causes of A-a widening!

44
Q

What is R when FiO2 is 100%?

A

1

45
Q

What is the most common cause of hypoxemia in hospitalized patients?

A

V/Q mismatch

46
Q

True or false: shunts have very large V/Q ratios.

A

FALSE! They are at the extreme of LOW V/Q

47
Q

How might you treat hypoxemia due to V/Q defect?

A

supplemental oxygen

48
Q

What diseases may be associated with low V/Q?

A
  • Obstructive diseases (asthma, COPD)

- Pulmonary edema

49
Q

What diseases may be associated with high V/Q?

A
  • Pulmonary embolism

- Dead space (extreme)

50
Q

True or False: must lung diseases cause V/Q mismatch.

A

TRUE!

51
Q

How is water cleared from alveoli (after edema)?

A

Sodium channels! (water follows sodium)

Pulmonary (34 decks)

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