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Pulmonology > Hypoxemia > Flashcards

Hypoxemia Flashcards

1
Q

What does CaO2 mean?

A

arterial oxygen CONTENT

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2
Q

What does CvO2 mean ?

A

venous oxygen Content

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3
Q

What does DaO2 mean?

A

arterial oxygen delivery

DvO2 = venous oxygen delivery

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4
Q

PAO2 means what?

A

alveolar oxygen tension

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5
Q

PaO2 means what?

A

arterial oxygen tension

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6
Q

What does Q(E) mean?

A

edema fluid flow

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7
Q

What does SpO2 mean?

A

Peripheral oxygen saturation(measured with a pulse oximeter at the finger)
normal value: 97%

SaO2: arterial oxygen sat

ScvO2: central venous oxygen saturation

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8
Q

What does SvO2 or MvO2 mean?

A

mixed venous oxygen saturation

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9
Q

Why are PAO2 and PaO2 never equal?

A

the shape of the oxy-hb curve

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10
Q

Above what value of PO2 is the Hb curve flat?

A

60

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11
Q

Can you equate PACO2 with PaCO2 or PAO2 with PaO2?

A

PACO2 = Pa CO2 because atmospheric tension of CO2 is essentially zero

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12
Q

What is the P/F ratio and what is it useful for?

A

The P/F ratio is the ratio of PaO2 to FiO2 in supplemental oxygen by cannula, and it gives you the severity of hypoxemia in situations in which FiO2 does not equal 0.21 (i.e. the alveolar gas equation changes).

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13
Q

Is FiO2 certain when oxygen is given by mechanical ventilation or by cannula?

A

mechanical ventilation ONLY

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14
Q

What is a normal P/F ratio?

A

> 400

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15
Q

Is hypoxemia more or less severe when P/F is low?

A

more severe

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16
Q

What situation is the P/F ratio important for in acute diagnosis?

A

ARDS

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17
Q

What do conditions that increase the A-a difference in room air do to the P/F for patients on supplemental oxygen?

A

lower it

A wide A-a difference and a low P/F both indicate a diseased lung

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18
Q

With a normally positioned curve, what saturation does a PaO2 of 95 mmHg indicate?

A

97%

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19
Q

With a normally positioned curve, what PaO2 does a saturation of 97% indicate?

A

95 mmHg

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20
Q

A PaO2 of 60 mmHg is associated with what saturation?

A

90%

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21
Q

A saturation of 90% is associated with what PaO2?

A

60 mmHg

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22
Q

What is the saturation of mixed venous blood with a PvO2 of 40 mmHg?

A

75%

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23
Q

How do you identify a right shifted curve when looking at arterial saturation?

A

Arterial saturation must be lower than expected for any given PaO2

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24
Q

Cyanide poisoning is an example of hypoxia or hypoxemia?

A

hypoxia only

25
Q

What is the first step in evaluating a hypoxemic patient?

A

A-a difference or PaO2/FiO2 ratio if on supplemental oxygen

26
Q

How do you determine the A-a difference?

A

subtract the PaO2 (given in lab results) from the PAO2 (calculated by alveolar gas equation)

27
Q

What are the 3 main pathophysiologic states that cause hypoxemia with a large A-a difference or a low P/F

A
  1. Shunt (Va/Q = 0)
  2. Low Va/Q that is NOT zero
  3. Diffusion limitation
28
Q

What is the quick differential for a shunt, in which Va/Q is zero and the A-a difference is big?

A

blood, pus, water, atelectasis

29
Q

Is a shunt oxygen responsive or unresponsive?

A

unresponsive

30
Q

With a small shunt, will you be more or less responsive to O2?

A

more responsive, due to less hypoxemia

31
Q

In the Starling equation, what increases KF (the filtration coefficient)?

A

permeability and surface area

32
Q

What part of the Starling equation do processes like diuresis, NO, morphine and phlebotomy lower?

A

Pmy (hydrostatic pressure of the microvasculature)

These decrease edema formation

33
Q

What does successful edema clearance rely on besides hydrostatic and oncotic pressure?

A

active sodium transport

34
Q

What do these criteria describe?

  1. occurs within one week of clinical insult
  2. bilateral alveolar opacities on Xray or CT
  3. presentation predominately caused by elevated left-sided filling pressures
  4. PaO2/FiO2 < 300
A

ARDS

35
Q

Protein rich edema fluid in the setting of direct lung injury OR severe inflammation is an example of what syndrome? What does this due to the shunt fraction?

A

ARDS

increased shunt fraction

36
Q

What does diffuse alveolar damage indicate?

A

ARDS (early)

37
Q

What condition is classically defined by hyaline membranes?

A

ARDS

38
Q

In a pulmonary embolism, what happens to the VA/Q ratio?

A

it is lowered because Q is increased for any given VA due to redirected blood from blocked arteries to other arterial branches

39
Q

Is hypoxemia due to low VA/Q oxygen responsive?

A

yes

40
Q

Will a low MvO2/SVO2 cause hypoxemia?

A

yes

41
Q

If a patient has a shunt, low VA/Q or diffusion limitation, what will lowering MvO2 do?

A

further impair SaO2

42
Q

Low cardiac output, anemia, arterial hypoxemia (SaO2 and PaO2) and increased O2 consumption do what?

A

lower MvO2, thus impairing SaO2 further and worsening hypoxemia

43
Q

In a blood transfusion to an anemic patient, dobutamine in a patient with a low CO, or deep sedation with medication that lowers V ̇O_2, what will happen to MvO2? SaO2?

A

MvO2 will increase, and so will SaO2

44
Q

How doe we measure MvO2? (2)

A

Swan-Ganz catheter or by using ScvO2 as a proxy

45
Q

How do we measure ScvO2, which is aproxy for MvO2?

A

central venous cather at the internal jugular or subclavian

46
Q

What are the four conditions that cause hypoxemia with a normal A-a difference?

A

low barometric pressure
low FiO2
hypoventilation
Low R

47
Q

In hypoxemia with a normal A-a difference, what do we know about gas exchange?

A

it is normal

48
Q

What are situations that can lower FiO2 below 0.21?

A

fires, industrial accidents, bioterrorism, and mining - situations where other gases dilute the fraction of inspired oxygen by displacing it

49
Q

What is by far the most common cause of hypoxemia with a normal A-a in clinical practice?

A

hypoventilation = acute elevation of PaCo2 in the setting of suppressed respiration or decreased VA

50
Q

In hypoxemia with a normal A-a caused by hypoventilation, what is the therapeutic approach in most scenarios?

A

increased alveolar ventilation with naloxone

51
Q

A 44 year-old man presents to NMH with heroin overdose. An arterial blood gas demonstrates a PaO2 of 40 mmHg, a PaCO2 of 60 mmHg and a pH of 7.24. The most likely cause of hypoxemia is:

a) Aspiration pneumonitis
b) Hypoventilation
c) Hypoventilation with aspiration pneumonitis
d) Pneumothorax
A

Answer: C
This patient certainly has hypoventilation (as evidenced by the PaCO2 of 60 mmHg). However, he also has a wide A – a difference (PAO2 = 150 – 60/0.8 = 75 mmHg; PaO2 = 40 mmHg; 75 – 40 = 35 mmHg). Hypoventilation alone will not widen the A – a difference, you need a #1, 2, or 3 from our list above. Aspiration pneumonitis paired with hypoventilation would explain the above findings.

52
Q

Which of the following statements is true regarding a patient with a large intrapulmonary shunt?

  a) Hypoxemia is relatively responsive to supplemental oxygen
  b) A drop in mixed venous oxygen saturation is unlikely to worsen hypoxemia.
  c) Hypoxemia is associated with a decreased P/F ratio
         d) Hypoxemia occurs with a normal A-a difference
A

A large shunt is associated with a large A – a difference and a low PaO2 / FiO2 ratio. Large shunts are not responsive to supplemental oxygen and will be impacted by a drop in MvO2.

Answer: C

53
Q

A 29 year-old medical student is camping at an altitude of 10,000 feet where the barometric pressure is 530 mmHg. Her resting respiratory rate is slightly increased, which she believes correctly is her body’s attempt to elevate PAO2 and thereby PaO2. Assuming she has a PaCO2 of 28 mmHg, an A-a difference of < 10 mmHg and a normal value for R, which of the following is the best estimate of her PaO2?

a) 91 mmHg
b) 85 mmHg
c) 74 mmHg
d) 64 mmHg

A

PAO2 = (530 – 47) x 0.21 – 28 / 0.8 = ~ 66 mmHg. Her PaO2 would need to be slightly below this to account for a small normal A – a difference.

54
Q

A patient with a long-standing history of alcohol abuse is intubated and mechanically ventilated on 100% supplemental oxygen (FiO2 1.0) for pneumonia. His arterial saturation is 87 to 88% (which gives him a PaO2 of ~ 53 mmHg). His mixed venous saturation is 75%. Assuming that the blood leaving the remaining “normal” alveolar capillary units and heading to the left atrium via a pulmonary vein has a saturation of 100%, what is best estimate of the percent of the cardiac output that goes through V/Q zero (shunt) units?

a) 20%
b) 30%
c) 40%
d) 50%

A

This question is asking you determine a patient’s shunt fraction (how much of a patient’s CO is traveling through V/Q = 0 units). A 50% shunt means that half of the patient’s CO exits the lung with a saturation of 75%, the other half with a saturation of 100%. Remember, when mixing blood you can average content and saturation but not PaO2. The average of 75% and 100% is 87.5%.

55
Q

A patient with a long-standing history of alcohol abuse is intubated and mechanically ventilated on 100% supplemental oxygen (FiO2 1.0) for pneumonia. His arterial saturation is 87 to 88% (which gives him a PaO2 of ~ 53 mmHg). His mixed venous saturation is 75%.
The same patient in question 4 develops an acute upper GI bleed from alcoholic gastritis. The venous saturation falls from 75% to 60%. Assuming all other cardiopulmonary parameters remain the same, what would happen to the patient’s arterial saturation?
a) it would increase to 90%
b) it would decrease to 85%
c) it would decrease to 80%
d) it would decrease to 75%

A

A 50% shunt means that half of the patient’s CO exits the lung with a saturation of 75%, the other half with a saturation of 100%. Remember, when mixing blood you can average content and saturation but not PaO2. The average of 75% and 100% is 87.5%.

Now you are averaging 60% with 100% (the venous blood that is 60% saturated leaves V/Q = 0 units still 60% saturated). The patient’s saturation therefore falls to 80%

56
Q

Four 30 year-old patients are being evaluated for respiratory distress in a Florida emergency department. Room air arterial blood gases are measured in each. Which patient most likely has pneumonia?

a) PaO2 100 PaCO2 32 pH 7.47
b) PaO2 95 PaCO2 36 pH 7.43
c) PaO2 90 PaCO2 28 pH 7.43
d) PaO2 85 PaCO2 44 pH 7.35

A

Answer: C
Pneumonia is an alveolar filling disease (leading to V/Q = 0 units) and should widen the A – a difference. For C, PAO2 = (760 – 47) x 0.21 – 28/0.8 = 115; PaO2 = 90; A – a difference = 115 – 90 = 25 which is wide. This would be consistent (but not diagnostic of) pneumonia.

57
Q

What is the PaO2 cut off for hypoxemia?

A

80 mm Hg

58
Q

what is the PaCO2 cut off for hyperventilation?

A

45 mmHg

Pulmonology (35 decks)

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