Hypoxemia

Question 1

Easy shortcut approximation of the alveolar air equation?

Answer 1

pAO2 = fiO2 * 7 * 100 - pCO2
(on room air, 147 - pCO2)
edit I really think you should still multiply the pCO2 * 1.2, though… that’s how it was done our lab cases. The above should be fine for the exam, though…

Question 2

What’s the A-a gradient? (called here AaDO2..)

Answer 2

AaDO2 = pAO2 - pAO2

Question 3

Compensations for hypoxemia? (4ish were listed)

Answer 3

Hyperventilation - with increased sensitivity to hypoxia.
Polycythemia.
Right shift in O2 binding (more 2,3-BPG -> Hb releases O2 more easily), then later left shift.
Decreased plasma volume.

Question 4

5 layers that O2 must diffuse through in the lung?

Answer 4

Surfactant.
Alveolar membrane.
Interstitial fluid.
Capillary membrane.
RBC membrane.

Question 5

Equation describing the bulk diffusion of a gas (Vgas) across a membrane?
Which variable do we want to asses clinically?

Answer 5

Vgas = A / T * D * (P1 - P2)
Where A = area, T = thickness, D = diffusion coefficient, and P = pressure on either side of membrane.

D is what we’re interested in.

Question 6

What’s the equation for the diffusion coefficient for CO?

Answer 6

D(L-CO) = V(CO) / pACO

where V(CO) = volume of diffused CO

Question 7

Does D(L) only reflect the diffusion capacity of the alveolar membrane itself?

Answer 7

No. It also includes contribution from O2’s or CO’s ability to complex with Hb.

Question 8

4 things that suggest diffusion impairment as the cause of hypoxemia?

Answer 8

Widened A-a gradient.
Hypoxemia only with exertion, or disease.
Normal paCO2.
Hypoxia responds to increased fiO2.

Question 9

3 things that suggest shunt / shunt-like effect as the cause of hypoxemia?

Answer 9

Widened A-a gradient.
Normal paCO2.
Hypoxemia does not respond to increased fiO2. (important)

Question 10

Why doesn’t “shunt” improve with increased fiO2?

Answer 10

The part of the lung without shunts will already have saturated its hemoglobin

Question 11

3 things that suggest alveolar hypoventilation as the cause of hypoxemia?

Answer 11

A-a gradient is normal.
Hypoxemia responds to increased fiO2.
paCO2 is always high in hypoventilation. (important)

Question 12

Causes of hypoventilation?

Answer 12

CNS, neuromuscular diseases.
Drugs (benzos, opiates)

Chronic O2 use.

Question 13

Major lab value difference between shunt and dead space causes of hypoxemia?

Answer 13

Dead space causes hypercapnia (aka. hypercarbia).

Question 14

What does “V/Q mismatch” as a cause of hypoxemia refer to?

Answer 14

Something intermediate between the extremes of shunt and dead space.
There will be widened A-a, typically hypoxemia will respond to increased fiO2, and paCO2 will be normal or low.

Question 15

Decreased cardiac output is a cause of hypoxemia. What is the A-a gradient like?

Answer 15

The A-a gradient is wider because of the low pO2 of mixed venous blood coming into the pulmonary capillaries.

Question 16

Limitations of masks for delivering increased fiO2?

Answer 16

Not all of them can get fiO2 that high… some can only go up to 60%, for example.

Question 17

What’s PEEP?

Answer 17

Positive end-expiratory pressure - it keeps the alveoli open, for better ventilation.

Question 18

How can you significantly increase the amount of dissolved O2 in the bloodstream?

Answer 18

Hyperbaric therapy.

Good for CO poisoning

Question 19

4 complications of O2 therapy?

Answer 19

Acute lung injury.
Hypercarbia, when used chronically (because drive to ventilate is suppressed).
Absorption atelectasis. (see next card)
Retrolental fibroplasia. (when O2 is given to infants, can cause permanent blindness)

Question 20

What is absorption atelectasis, and why does O2 therapy cause it?

Answer 20

Atelectasis = alveolar collapse.
Atelectasis can happen distal to obstructions like mucus plugs as gases trapped in the alveoli diffuse into the bloodstream. Normally this doesn’t happen because nitrogen diffuses very slowly.
If there’s high % O2, it can “wash out” the nitrogen, then the O2 is absorbed, and the alveoli collapse.