Ch. 5 West Flashcards

1
Q

T/F: The alveolar PO2 is largely determined by the level of alveolar ventilation

A

True

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2
Q

The PO2 of inspired air is, at sea level, with a body temperature of 37*C and a water vapor pressure of about 50 mmHg is _______ mmHg

A

150 mmHg

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3
Q

Five causes of hypoxemia:

A

Low inspired FiO2
Hypoventilation
Diffusion impairment
R-to-L shunt
V/Q mismatch

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4
Q

Alveolar PO2 is determined by a balance between the rate of _____ of O2 by the blood, and the rate of ____ of the O2 by alveolar ventilation.

A

Rate of removal of O2 by the blood
Rate of replenishment of O2 by alveolar ventilation

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5
Q

Based on the relationship between ventilation and CO2 level seen in the alveolar ventilation equation, decreasing the ventilation by 50% will result in the PCO2 _____ under steady state conditions.

A

Doubling

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6
Q

In the alveolar gas equation, “R” represents the respiratory quotient. Explain what that is.

A

The respiratory exchange ratio, i.e. the ratio between CO2 production and O2 consumption, which is determined by the metabolism of the tissues in a steady state and varies based on the balance of fuels consumed (carbs, fats, proteins etc.).

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7
Q

Alveolar gas equation

A

P(AO2) = P(IO2) - [P(ACO2)/R]

Normal value of R is about 0.8

Shows that the normal fall in alveolar PO2 is slightly greater than the rise in PCO2 during hypoventilation

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8
Q

Why does alveolar CO2 take longer to reach equilibrium if a patient goes from hypoventilation to hyperventilation?

A

Because the body stores of CO2 are pretty high in the form of bicarb and therefore it takes longer to come to equilibrium

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9
Q

T/F: Hypoventilation always increases the alveolar and arterial PCO2

A

True

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10
Q

T/F: Hypoxemia associated with hypoventilation is rectified by adding additional oxygen to the inspired gas.

A

True

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11
Q

Why does diffusion limitation rarely cause hypoxemia?

A

Because the red blood cells spend enough time in the pulmonary capillary to allow nearly complete equilibrium with the alveolar O2.

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12
Q

What does the term “shunt” indicate as pertains to pulmonary circulation?

A

Shunt indicates that blood is entering the arterial system without having gone through ventilated areas of the lung.

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13
Q

What are two sources of partially oxygenated/unoxygenated blood that is normally re-introduced to the systemic circulation?

A

Some of the bronchial artery blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partially depleted.
Another source is a small amount of coronary venous blood that drains directly into the LV through the thebesian veins.

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14
Q

In healthy individuals, the normal % shunt that occurs from drainage of the bronchial and thebesian circulation is:

a. 1%
b. 5%
c. 10%
d. 15%

A

b. 5%

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15
Q

Which of the following is an important feature of a shunt?

a. Provision of 100% oxygen can completely abolish hypoxemia
b. With provision of oxygen, the shunt can be overridden (i.e. the PaO2 experiences a typical linear response to oxygen therapy)
c. Hypoxemia cannot be completely abolished by provision of 100% oxygen, and in general the response to oxygen therapy is poor.
d. Most patients are not oxygen responsive at all.

A

c. Hypoxemia cannot be completely abolished by provision of 100% oxygen, and in general the response to oxygen therapy is poor.

Hypoxemia cannot be completely abolished by provision of 100% oxygen. The shunt will always continue to depress the PaO2, however the patient may still benefit from oxygen therapy since it will hyper-oxygenate the non-shunted blood and can tip the scale in the patient’s favor. The smaller the shunt, the more oxygen-responsive a patient will be.

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16
Q

Why do some patients with a shunt have a low arterial CO2? And why doesn’t a shunt cause increased PCO2?

A

Chemoreceptors sense any elevation of arterial PCO2 and respond by increasing ventilation. This reduces the PCO2 of un-shunted blood until the arterial PCO2 is normal.

Some patients with shunt will have a low PCO2 because their respiratory rates are higher (increased drive due to hypoxemia)

17
Q

In a setting of V/Q of zero (i.e. no ventilation, but still has perfusion), the alveolar gas and end-capillary blood will start to look similar to _____

A

Mixed venous blood

18
Q

In the setting of V/Q of infinity (lots of ventilation but no perfusion), the alveolar gas and end-capillary blood will start to look similar to ____

A

Inspired gas

19
Q

T/F: Ventilation increases slowly from top to bottom of the lung, and blood flow increases more rapidly

A

True

20
Q

T/F: The PO2 of the alveoli decreases markedly down the lung, whereas the PCO2 INCREASES proportionally less.

A

True

21
Q

If ventilation is less at the top of the lung, why is the V/Q ratio high at the top of the lung?

A

Ventilation is less at the top than the bottom, but the differences in blood flow are more marked and there is less perfusion at the top of the lung.

22
Q

The changes in PO2 from top to bottom of the lung vary over ___ mmHg whereas the PCO2 varies much less.

A

over 40 mHg

23
Q

The difference between the CO2 level at the top and bottom varies less because the CO2 level relies on ____ more than circulation.

A

Ventilation

24
Q

T/F: Lung units with high V/Q (i.e. high ventilation with low perfusion) do not increase the oxygen concentration of the patient’s blood as much, despite their relatively high PO2.

A

True

The net result is a depression of the arterial PO2 below that of the mixed alveolar PO2 i.e. the alveolar-arterial gradient

25
Q

What is a normal V/Q ratio?

A

About 1.0

26
Q

T/F: Lung units with abnormally high V/Q are inefficient at decreasing the CO2

A

True, because while they can ventilate, they cannot uptake CO2 from the blood since there is no blood flow

27
Q

Why is the CO2 often normal despite V/Q mismatch clinically in our patients?

A

Chemoreceptors trigger increase RR to clear the CO2

28
Q

Why is increased RR less useful in raising the PaO2 (as compared to its ability to clear the CO2)?

A

Because the oxygen dissociation curve is almost flat at the top meaning that units with moderately low V/Q ratios will benefit from appreciably from the increased ventilation. However, some hypoxemia will remain.

29
Q

T/F: The different behavior of the two gases results from the different shapes of their dissociation curves.

A

True

30
Q

T/F: Hypoventilation results in no A-a difference, and responds well to oxygen.

A

True
No A-a gradient/no abnormal gradient
Responds well to O2

31
Q

T/F: Diffusion impairment results in an increased A-a gradient and responds well to oxygen.

A

True
A-a gradient elevated
Responds well to O2

32
Q

T/F: Shunt results in an increased A-a gradient and responds poorly to oxygen but clinically can be useful in determining the degree of shunt (subjectively).

A

True

33
Q

T/F: V/Q mismatch results in increased A-a gradient and typically does respond well to oxygen.

A

True

34
Q

Normal A-a gradient value

A

10-15 mmHg

35
Q

What are the two causes of hypercapnia?

A

Hypoventilation
VQ mismatch

36
Q
A