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Pulm by Minnie: AH Update > 151, 157, 159, 160: Hypercapnia/Hypoxemia I, II, III > Flashcards

151, 157, 159, 160: Hypercapnia/Hypoxemia I, II, III Flashcards

1
Q

Why is PAO2 > PaO2?

A
  • The non-linear shape of the oxygen-hemoglobin dissociation curve
    • PaO2 does not increase proportionally to PAO2 when PAO2 > ~60 mmHg
  • Small, physiologic vascular shunts that mix deoxygenated blood with oxygenated blood leaving the pulmonary capillary bed
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2
Q

What are the most common causes of respiratory acidosis with decreased V̇E?

A

Decreased V̇E = most common cause of respiratory acidosis

  • The patient won’t breathe (CHOCH)
    • CNS tumor or infection
    • Head trauma w/ increased intracranial pressure
    • Obesity (-> Hypoventilation)
    • CNS depressant drugs
    • Hypothyroidism
  • The patient can’t breathe (CORN)
    • Chest wall deformity
    • Obstructive or Restrictive lung disease
    • Neuromuscular disease
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3
Q

What pathologies could cause diffusion-limitation hypoxemia?

A
  • Increased interstitial thickening
    • ILD
  • Decreased area available for diffusion
    • Emphysema
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4
Q

What is the equation for PAO2 in terms of Patm, PH2O, FiO2,
and PaCO2?

A

Normal values:

  • Patm = 760 mmHg
  • PH2O = 47 mmHg
  • PaCO2 = 40 mmHg
  • FiO2 = 0.21
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5
Q

Why is tuberculosis more likely to infect the apex of the lung than the base?

A

Mycobacterium tuberculosis live best in a high-oxygen environment. PO2 in the apices of the lungs is highest because they are not perfused as wall as the bases; therefore, PCO2 will be lower and PO2 will be higher in the apices.

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6
Q

What are the 4 major causes of hypoxemia with a normal A-a difference?

A
  • Low barometric pressure
  • Low FiO2
  • Hypoventilation
  • Low RQ
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7
Q

What is the equation for PaCO2?

A

Denominator is an approximation for V̇A

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8
Q

Regions of the lung with a low V̇A/Q̇ will have a larger impact on the final PO2 than regions of the lung with a high V̇A/Q̇.

Why?

A
  • Regions with high V̇A/Q̇ don’t do much to increase PaO2 above what is found in the normal lung regions
    • Normal lung regions are already in the flat part of the oxygen-hemoglobin binding curve
    • Small increases in PAO2 do not have a substantial impact on PaO2
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9
Q

Why does V̇A/Q̇ vary among different lung regions in normal lungs?

A

Gravitational forces

  • V̇A/Q̇ decreases from the top of the lung to the bottom
  • From top to bottom of the lung…
    • V̇A increases
    • Q̇ increases
    • However, gravity increases Q̇ more than it increases V̇A, resulting in lower V̇A/Q̇ at the bases of the lung than the apices
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10
Q

A 72 year-old man with severe smoking-related emphysema presents with acute breathlessness from pulmonary embolism. He is anti-coagulated with low molecular weight heparin but is eventually intubated and mechanically ventilated for acute hypercapnic respiratory failure. He is sedated and unresponsive and afebrile. He has decreased breath sounds with expiratory phase prolongation and faint expiratory wheezes. PaO2 is 90 mmHg on 30% supplemental oxygen. PaCO2 is 60 mmHg with a respiratory rate of 20/min and a tidal volume of 600ml. These data suggest:

a) increased CO2 production
b) increased dead space
c) decreased dead space to tidal volume ratio
d) increased alveolar ventilation

A

b) increased dead space

Elevated PaCO2 despite large minute ventilation => increased dead space

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11
Q

In mixed venous blood, PvO2 = 40 mmHg , and SaO2 is ~

A

In mixed venous blood, PvO2 = 40 mmHg , and SaO2 is ~75%

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12
Q

Increasing VD/VT leads to [hypercapnia/hypocapnia]

A

Increasing VD/VT leads to hypercapnia

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13
Q

How is MvO2 (aka SvO2) measured?

A

MvO2 can be measured using a Swan-Ganz cather

However, usually ScvO2 (Central venous Hb saturation) is measured via triple lumen catheter instead, because you avoid messing with the heart

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14
Q

In normal arterial blood, when PaO2 = , SaO2 is ~90%

A

In normal arterial blood, when PaO2 = 60 mmHg , SaO2 is ~97%

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15
Q

In normal arterial blood, when PaO2 = 60 mmHg , SaO2 is ~

A

In normal arterial blood, when PaO2 = 95 mmHg , SaO2 is ~90%

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16
Q

If a patient is hypoxemic and hypercapnic, what is the likely cause of hypoxemia?

A

Alveolar hypoventilation due to…

  • Inadequate ventilator settings
  • Low lung compliance
  • Depresion of the brainstem respiratory osscillator
  • Muscle paralysis or fatigue
  • High airway resistance

Shunt is also possible, but hypercapnia is the “hallmark of alveolar hypoventilation”

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17
Q

Which variable is most commonly out of equillibrium in hypercapnia (respiratory acidosis)? How is it changed?

  1. V̇CO2
  2. V̇E
  3. VD/VT
A

b. V̇E is too low

Increased V̇CO2 and inceased VD/VT can also cause hypercapnia

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18
Q

What pathologies could cause hypoxemia with a low V̇A/Q̇?

A
  • Ventilation to perfused lung units is decreased but not absent
    • Asthma
    • COPD
    • Interstitial lung disease
  • Q̇ is high for a given V̇A
    • Pulmonary embolism: Blood is redirected from blocked pulmonary arteries to other arterial branches
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19
Q

In normal arterial blood, when PaO2 = 95 mmHg , SaO2 is ~

A

In normal arterial blood, when PaO2 = 95 mmHg , SaO2 is ~97%

20
Q

What is the body’s compensatory response for local hypocapina in the lung (refelcting a high V̇A/Q̇)?

A

Bronchoconstriction

Limits ventilation to the alveolar unit that is poorly perfused

This typically only occurs in regions with VERY high V̇A/Q̇

21
Q

Normal V̇A/Q = ______

A

Normal V̇A/Q = 1 L/min

Ventilation and perfusion should match

22
Q

Why might over-oxygenation in patients with COPD cause hypercapnia?

A

Increasing oxygen makes ventilation/perfusion matching even worse, due to impaired hypoxic pulmonary vasoconstriction

(When this is impaired, non-working alveoli are still being perfused?)

23
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?

  1. PaO2 100, PaCO2 32, pH 7.47
  2. PaO2 95, PaCO2 36, pH 7.43
  3. PaO2 90, PaCO2 28, pH 7.43
  4. PaO2 85, PaCO2 44, pH 7.35
A

c. PaO2 90, PaCO2 28, pH 7.43

Pneumonia is an alveolar filling disease (V/Q -> 0)

Choose the patient with the widest A-a difference

24
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:

  1. Aspiration pneumonitis
  2. Hypoventilation
  3. Hypoventilation with aspiration pneumonitis
  4. Pneumothorax
A

c. Hypoventilation with aspiration pneumonitis

A-a difference is large (calculate). Hypoventilation alone will not cause an increase in the A-a difference

25
Q

In normal arterial blood, when PaO2 = , SaO2 is ~97%

A

In normal arterial blood, when PaO2 = 95 mmHg , SaO2 is ~97%

26
Q

Which of the following would decrease PaCO2?

a) an increase in CO2 production
b) a decrease in minute ventilation
c) an increase in the dead space to tidal volume ratio
d) an increase in alveolar ventilation

A

d) an increase in alveolar ventilation

27
Q

Below PaO2 = ______, SaO2 falls rapidly

A

Below PaO2 = 60 mmHg , SaO2 falls rapidly

This is known as the “knee” of the hemoglobin-oxygen dissociation curve

28
Q

What are the most common causes of increased VD/VT?

A
  • Pulmonary Embolism
  • Emphysema
  • Low VT during mechanical ventilation
  • Shallow breathing from illness
  • Pulmonary hyperinflation

Note: Tachypnea does not imply hyperventilation! Tachypnea with low VT = hypoventilation

29
Q

What is wrong with this lung?

A

Atelectasis

30
Q

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

  1. Hypoxemia is relatively responsive to supplemental oxygen
  2. A drop in mixed venous oxygen saturation is unlikely to worsen hypoxemia.
  3. Hypoxemia is associated with a decreased P/F ratio
  4. Hypoxemia occurs with a normal A-a difference
A

c. Hypoxemia is associated with a decreased P/F ratio

(Decreased P/F ratio correlates with an increased A-a difference)

31
Q

List the 4 major causes of an intrapulmonary shunt

A
  • Blood - Alveolar hemorrhage
  • Pus - Pneumonia
  • Water - Pulmonary edema
    (may be cardiogenic or non-cardiogenic)
  • Atelectasis - Alveolar collapse
32
Q

How does Acute Respiratory Distress Syndrome (ARDS) lead to hypoxemia? Describe the pathophysiology

A

ARDS is a type of intrapulmonary shunt caused by either direct injury or non-pulmonary insult

  • -> Systemic release of inflammatory mediators
  • -> Drainage to alveolar epithelium and capillary endothelium
  • -> Protein-rich edema fills interstitial and alveolar spaces
  • -> Large shunt fraction (
  • -> Hypoxemia
33
Q

What is the body’s compensatory response for local hypoxia and hypercapnia in the lung (refelcting a low V̇A/Q̇)?

A

Vasoconstriction

Limits perfusion to the alveolar unit that is poorly ventilated

34
Q

What are the most common causes of respiratory acidosis with increased V̇CO2?

A

FOSH

  • Fever
  • Overfeeding
  • Seizures
  • Hypermetabolic state (ex: hyperthyroid)
35
Q

Which variable is most commonly out of equillibrium in hypocapnia (respiratory alkylosis)? How is it changed?

  1. V̇CO2
  2. V̇E
  3. VD/VT
A

b. V̇E is too high

36
Q

If a patient is hypoxemic and V̇A/Q = 0, what is causing the hypoxemia?

Will giving oxygen help?

A
  • *Shunt:** intracardiac or intrapulmonary.
  • *Giving oxygen will not help**
  • Intracardiac: R->L shunt
    • Patent foramen ovale
    • Atrial septal defect
    • AV malformations
  • Intrapulmonary
    • Blood - Alveolar hemorrhage
    • Pus - Pneumonia
    • H2O - Pulmonary edema
      (may be cardiogenic or non-cardiogenic)
    • Atelectasis - Alveolar collapse
37
Q

How would you calculate VD/VT?

A

PECO2 = mixed, expired CO2

38
Q

How would you calculate the shunt fraction, Qs/Qt?

A
39
Q

Describe the early histopathology and timing of Acute Respiratory Distress Syndrome (ARDS)

A

Diffuse alveolar damage (DAD), hyaline membranes are a classic finding

Occurs within 1 week of clinical insult

40
Q

In health, the difference between PAO2 and PaO2 is ____

A

In health, the difference between PAO2 and PaO2 is <10 mmHg

(PAO2 > PaO2)

41
Q

Why is the PAO2 - PaO2 difference useful only in patients breathing room air?

A

PAO2 - PaO2 increases with supplemental oxygen

Patients recieving supplemental oxygen will have an artificially increased PAO2 - PaO2. In this case, use PaO2/FiO2 instead (normal >400)

42
Q

In which patients would a PaO2/FiO2 be most relevant? Why?

What is the normal value?

A

Patients recieving supplemental oxygen; PAO2 - PaO2​ will be artificially elevated, so use PaO2/FiO2 instead

Normal: PaO2/FiO2 > 400

Lower numbers = more severe hypoxemia

Conditions that increase the PAO2 - PaO2​ difference also decrease PaO2/FiO2

43
Q

What are the 4 most common causes of low MvO2 (aka SvO2​)?

A
  • Low cardiac output
  • Anemia
  • Arterial hypoxemia
  • Increased O2 consumption by tissues
44
Q

How does low MvO2 (aka SvO2​) cause hypoxemia?

A

If there is less oxygen in the blood returning to the heart, there will likely be less oxygen in the arterial blood leaving the heart

Low MvO2 usually exacerbates hypoxemia caused by shunt, low V̇A/Q̇, or diffusion limitation, rather than acting as a primary cause of hypoxemia

45
Q

In mixed venous blood, PvO2 = , and SaO2 is ~75%

A

In mixed venous blood, PvO2 = 40 mmHg , and SaO2 is ~75%

46
Q

What are the 4 causes of hypoxemia that cause an increased A-a difference?

Which one is refractory to increased O2?

A
  • Shunt
    • Refractory to increased O2
  • Low V̇/Q̇
  • Diffusion Limitation
  • Low MvO2 (aka SvO2)

Pulm by Minnie: AH Update (34 decks)

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