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

Question 1

Why is P_AO2 > P_aO2?

Answer 1

• The non-linear shape of the oxygen-hemoglobin dissociation curve
  
  • P_aO2 does not increase proportionally to P_AO2 when P_AO2 > ~60 mmHg
• Small, physiologic vascular shunts that mix deoxygenated blood with oxygenated blood leaving the pulmonary capillary bed

Question 2

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

Answer 2

Decreased V̇_E = 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

Question 3

What pathologies could cause diffusion-limitation hypoxemia?

Answer 3

• Increased interstitial thickening
  
  • ILD
• Decreased area available for diffusion
  
  • Emphysema

Question 4

What is the equation for P_AO2 in terms of P_atm, P_H2O, F_iO2,
and P_aCO2?

Answer 4

Normal values:

• P_atm = 760 mmHg
• P_H2O = 47 mmHg
• P_aCO2 = 40 mmHg
• F_iO2 = 0.21

Question 5

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

Answer 5

Mycobacterium tuberculosis live best in a high-oxygen environment. P_O2 in the apices of the lungs is highest because they are not perfused as wall as the bases; therefore, P_CO2 will be lower and P_O2 will be higher in the apices.

Question 6

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

Answer 6

• Low barometric pressure
• Low F_iO2
• Hypoventilation
• Low RQ

Question 7

What is the equation for P_aCO₂?

Answer 7

Denominator is an approximation for V̇_A

Question 8

Regions of the lung with a low V̇_A/Q̇ will have a larger impact on the final P_O2 than regions of the lung with a high V̇_A/Q̇.

Why?

Answer 8

• Regions with high V̇_A/Q̇ don’t do much to increase P_aO2 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 P_AO2 do not have a substantial impact on P_aO2

Question 9

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

Answer 9

Gravitational forces

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

Question 10

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

Answer 10

b) increased dead space

Elevated P_aCO2 despite large minute ventilation => increased dead space

Question 11

In mixed venous blood, P_vO2 = 40 mmHg , and S_aO2 is ~

Answer 11

In mixed venous blood, P_vO2 = 40 mmHg , and S_aO2 is ~75%

Question 12

Increasing V_D/V_T leads to [hypercapnia/hypocapnia]

Answer 12

Increasing V_D/V_T leads to hypercapnia

Question 13

How is M_vO2 (aka S_vO2) measured?

Answer 13

M_vO2 can be measured using a Swan-Ganz cather

However, usually S_cvO2 (Central venous Hb saturation) is measured via triple lumen catheter instead, because you avoid messing with the heart

Question 14

In normal arterial blood, when P_aO2 = , S_aO2 is ~90%

Answer 14

In normal arterial blood, when P_aO2 = 60 mmHg , S_aO2 is ~97%

Question 15

In normal arterial blood, when P_aO2 = 60 mmHg , S_aO2 is ~

Answer 15

In normal arterial blood, when P_aO2 = 95 mmHg , S_aO2 is ~90%

Question 16

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

Answer 16

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”

Question 17

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

1. V̇_CO2
2. V̇_E
3. V_D/V_T

Answer 17

b. V̇_E is too low

Increased V̇_CO2 and inceased V_D/V_T can also cause hypercapnia

Question 18

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

Answer 18

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

Question 19

In normal arterial blood, when P_aO2 = 95 mmHg , S_aO2 is ~

Answer 19

In normal arterial blood, when P_aO2 = 95 mmHg , S_aO2 is ~97%

Question 20

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

Answer 20

Bronchoconstriction

Limits ventilation to the alveolar unit that is poorly perfused

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

Question 21

Normal V̇_A/Q = ______

Answer 21

Normal V̇_A/Q = 1 L/min

Ventilation and perfusion should match

Question 22

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

Answer 22

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?)

Question 23

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

Answer 23

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

Question 24

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

Answer 24

c. Hypoventilation with aspiration pneumonitis

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

Question 25

In normal arterial blood, when P_aO2 = , S_aO2 is _~97%_

Answer 25

In normal arterial blood, when P_aO2 = **_95 mmHg_** , S_aO2 is _~97%_

Question 26

Which of the following would decrease P_aCO2? ## Footnote 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

Answer 26

d) an increase in alveolar ventilation

Question 27

Below P_aO2 = \_\_\_\_\_\_, S_aO2 falls rapidly

Answer 27

Below P_aO2 = **_60 mmHg_** , S_aO2 falls rapidly This is known as the "knee" of the hemoglobin-oxygen dissociation curve

Question 28

What are the most common causes of increased V_D/V_T?

Answer 28

* **Pulmonary Embolism** * **Emphysema** * Low V_T during mechanical ventilation * Shallow breathing from illness * Pulmonary hyperinflation Note: Tachypnea does not imply hyperventilation! Tachypnea with low V_T = hypoventilation

Question 29

What is wrong with this lung?

Answer 29

Atelectasis

Question 30

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

Answer 30

c. Hypoxemia is associated with a decreased P/F ratio (Decreased P/F ratio correlates with an increased A-a difference)

Question 31

List the 4 major causes of an intrapulmonary shunt

Answer 31

* **Blood** - Alveolar hemorrhage * **Pus** - Pneumonia * **Water** - Pulmonary edema (may be cardiogenic or non-cardiogenic) * **Atelectasis** - Alveolar collapse

Question 32

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

Answer 32

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**

Question 33

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

Answer 33

Vasoconstriction Limits perfusion to the alveolar unit that is poorly ventilated

Question 34

What are the most common causes of respiratory acidosis with increased V̇_CO2?

Answer 34

FOSH * Fever * Overfeeding * Seizures * Hypermetabolic state (ex: hyperthyroid)

Question 35

Which variable is most commonly out of equillibrium in hypocapnia (respiratory alkylosis)? How is it changed? 1. V̇_CO2 2. V̇_E 3. V_D/V_T

Answer 35

b. V̇_E is too high

Question 36

If a patient is hypoxemic and V̇_A/Q = 0, what is causing the hypoxemia? Will giving oxygen help?

Answer 36

* *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

Question 37

How would you calculate V_D/V_T?

Answer 37

P_ECO2 = mixed, expired CO2

Question 38

How would you calculate the shunt fraction, Q_s/Q_t?

Answer 38

Question 39

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

Answer 39

Diffuse alveolar damage (DAD), hyaline membranes are a classic finding Occurs within 1 week of clinical insult

Question 40

In health, the difference between P_AO2 and P_aO2 is \_\_\_\_

Answer 40

In health, the difference between P_AO2 and P_aO2 is **_\<10 mmHg_** (P_AO2 \> P_aO2)

Question 41

Why is the P_AO2 - P_aO2 difference useful only in patients breathing room air?

Answer 41

P_AO2 - P_aO2 increases with supplemental oxygen Patients recieving supplemental oxygen will have an artificially increased P_AO2 - P_aO2. In this case, use P_aO2/F_iO2 instead (normal \>400)

Question 42

In which patients would a P_aO2/F_iO2 be most relevant? Why? What is the normal value?

Answer 42

Patients recieving supplemental oxygen; P_AO2 - P_aO2​ will be artificially elevated, so use P_aO2/F_iO2 instead Normal: P_aO2/F_iO2 \> 400 Lower numbers = more severe hypoxemia Conditions that increase the P_AO2 - P_aO2​ difference also decrease P_aO2/F_iO2

Question 43

What are the 4 most common causes of low M_vO2 (aka S_vO2​)?

Answer 43

* Low cardiac output * Anemia * Arterial hypoxemia * Increased O₂ consumption by tissues

Question 44

How does low M_vO2 (aka S_vO2​) cause hypoxemia?

Answer 44

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 M_vO2 usually **exacerbates hypoxemia** caused by shunt, low V̇_A/Q̇, or diffusion limitation, rather than acting as a primary cause of hypoxemia

Question 45

In mixed venous blood, P_vO2 = , and S_aO2 is _~75%_

Answer 45

In mixed venous blood, P_vO2 = **_40 mmHg_** , and S_aO2 is _~75%_

Question 46

What are the 4 causes of hypoxemia that cause an **increased A-a difference**? Which one is refractory to increased O2?

Answer 46

* Shunt * Refractory to increased O2 * Low V̇/Q̇ * Diffusion Limitation * Low M_vO2 (aka S_vO2)