V/Q Relationship & Hypoxemia

Question 1

The Alveolar Gas Equation for PAO2

Answer 1

[FiO2(barometric-47)]-(PACO2/R)

Question 2

Causes of hypoxemia

Answer 2

Low PiO2 (high altitude)
Shunts
Hypoventilation
Diffusion impairment (infiltrates)
V/Q mismatch

Question 3

If the atmospheric pressure is 486 mmHg, patient PACO2 is 40 mmHg, what is their PAO2 with no supplemental oxygen?

Answer 3

~42

0.21*(486-47)-(40/0.8)

Question 4

What does the human body do to acclimatize to higher altitudes (hypoxic conditions)

Answer 4

Hyperventilation
Polycythemia
Disphosphoglycerate

Question 5

Can supplemental oxygen help in hypoventilation?

Answer 5

Yes, rectifies hypoxemia but can still have high CO2 and low pH that needs attention

Question 6

Hemoglobin is almost saturated even at what PaO2?

Answer 6

70-75

Question 7

How does diffusion impairment across the alveoli cause an inability to saturate completely?

Answer 7

Reduced equilibration reserve with thickened diffusion barrier (edema or fibrosis), so doesn’t completely saturate in time

Question 8

Can supplemental oxygen help in a case of diffusion impairment (edema or fibrosis)?

Answer 8

Yes, increases pressure gradient and aids equilibration reserve

Question 9

Single most important cause of hypoxemia

Answer 9

V/Q mismatch

Question 10

Is supplemental oxygen helpful in a case of an anatomic shunt?

Answer 10

No, won’t respond (Hb from good alveoli is already saturated and “extra” oxygen dissolved in blood is not enough to compensate)

Question 11

If the V/Q value lies closer to 0, then that means you have what abnormality?

Answer 11

Poor ventilation (Bronchitis, CF, etc.)

Question 12

If the V/Q value lies closer to infinity, then that means you have what abnormality?

Answer 12

Alveolar dead space that creates physiologic dead space (poorly perfused: emphysema, PE)

Question 13

Why does V/Q rise dramatically during exercise?

Answer 13

Ventilation and perfusion both increase, but ventilation much more so

Question 14

Which cause of hypoxemia does not result in an increase in the A-a gradient?

Answer 14

Hypoventilation (and also carbon monoxide poisoning)

Question 15

Is supplemental oxygen helpful in a case of V/Q mismatch (bronchitis, asthma, cystic fibrosis)?

Answer 15

Yes

Question 16

Anatomic vs. physiologic dead space

Answer 16

Anatomic dead space: portion of lung that doesn’t exchange gases (is fixed)
Physiologic dead space: includes anatomic dead space and any alveolar dead space (variable)

Question 17

What is alveolar/pathologic dead space?

Answer 17

Alveoli are being ventilated, but there is no perfusion (so it is wasted); seen in the apex of the lung and with PE

Question 18

Alveolar dead space (increases/decreases) with exercise

Answer 18

Decreases: upper lobes who usually receive minimal perfusion (alveolar dead space) are now more heavily perfused

Question 19

Why are minute ventilation and alveolar ventilation different?

Answer 19

Minute ventilation is RR x TV

Alveolar ventilation is RR x (TV-anatomic dead space)

Question 20

The normal movement tendency of the lung is (inward/outward)

Answer 20

Inward

Question 21

The normal movement tendency of the chest wall is (inward/outward)

Answer 21

Outward

Question 22

If you see an A-a gradient of >15 in the presence of hypoxemia, you should be thinking of what pathologies?

Answer 22

Right-to-left shunt
V/Q mismatch
Diffusion limitation

Question 23

Relationship of pressures for alveoli/arterioles/venules in the apex of the lung

Answer 23

PA>Pa>Pv (blood flow is least, so pinched off)

Question 24

Relationship of pressures for alveoli/arterioles/venules in the base of the lung

Answer 24

Pa>Pv>PA (highest blood flow)

Question 25

V/Q= 0 means what?

Answer 25

Complete shunt (perfusion but no ventilation) (asthma or obstruction) (inc. oxygen won’t help)

Question 26

V/Q= infinity means what?

Answer 26

Dead space ventilation (ventilation but no perfusion) (PE) (oxygen will help)

Question 27

A physiologist divides the lung into three zone (apex, middle, and base). In an experiment, he applies a small amount of positive pressure ventilation while studying the blood flow in the different zones. What will he note in the apex (assuming the patient is standing)?

Answer 27

Blood flow will be reduced in the apex. Blood flow is already the least in this area (V/Q close to infinity) and inc. pressure ventilation will inc. alveolar pressure and compress capillaries even further