Pulmonary Edema - Mechanisms

Question 0

What’s special about the thickness of the alveolar capillary walls?

Answer 0

They’re thin on the side facing the alveoli for gas exchange.
They’re thick on the the other side for fluid exchange.

Question 1

Two broad types of pulmonary edema?

Answer 1

Hydrostatic.

Permeability. (ARDS)

Question 2

How do alveolar epithelial cells help keep fluid out of the alveoli?

Answer 2

Tight junctions keep out solutes bigger than urea…

Active Na+ transport across Type II epithelial cells drives out the small amount of fluid that does get into airspaces.

Question 3

90% of fluid in the alveoli gets pumped out. What happens to the rest?

Answer 3

It’s reabsorbed via lymphatics.

Question 4

Mild edema vs. severe edema?

Answer 4

Mild edema is confined to the interstitial space.

Severe edema is in the airspaces.

Question 5

What are the 2 parts of the Earnest Starling equation that describes the driving force for fluid movement across a membrane?

Answer 5

Hydrostatic pressure.

Oncotic pressure.

Question 6

2 situations in which the microvascular oncotic pressure is reduced, leading to edema?

Answer 6

ARDS.

Hypoalbuminemia.

Question 7

What’s the Sieving Effect, and how is it a protective factor against edema when hydrostatic pressure is elevated?

Answer 7

When hydrostatic pressure is high, solutes will build up in the capillaries - mitigating movement of fluid into alveoli.

Question 8

What are 3 “anatomic sinks” where edema fluid can go initially?

Answer 8

Interstitium.
Bronchovascular bundles.
Pleural space.
(this keeps the fluid away from the gas exchange areas)

Question 9

What Na+ transporters are involved in the Type II alveolar epithelial cells?

Answer 9

ENaC, apical (which, recall, is amelioride sensitive)
Na+/K+ ATPase, basolateral.

(Pulmonary edema is a very rare side effect of amelioride…)

Question 10

Do the Na+ pumps on Type II epithelial cells like being bathed in gastric acid?

Answer 10

Nope, not at all.

This makes the pulmonary edema caused by ARDS/aspiration worse.

Question 11

What’s a drug that conveniently seems to upregulate ENaC?

Answer 11

Albuterol…

Question 12

As a general rule, is edema worse in ARDS or in CHF?

Answer 12

It’s usually worse in ARDS.
In CHF, the safety mechanisms against edema are upregulated.
In ARDS, those safety mechanisms are impaired.

Question 13

5 edema safety factors?

Answer 13

1. Decreased interstitial oncotic P, increased capillary oncotic P.
2. Increased interstitial hydrostatic P, decreased cap hydrostat P.
3. Anatomic sinks.
4. Increase in transepithelial Na+ transport.
5. Reserve capacity of lymphatic drainage.

Question 14

2 things that cause hydrostatic pulmonary edema other than CHF?

Answer 14

Fluid overload (esp. in IV resuscitation, for eg. sepsis).
Acute or chronic kidney injury.

Question 15

What causes the edema in ARDS?

Answer 15

Increase in capillary permeability, due to widespread endothelial injury.

Question 16

Why doesn’t the Sieving Effect help in ARDS?

Answer 16

Endothelial injury allows free flow of solutes, so there’s no increased oncotic pressure of capillaries.

Question 17

How can edema from CHF vs. ARDS be determined?

Answer 17

CHF will probably have a gradual onset history, have other symptoms like leg swelling.
Also CHF will have high BNP, echo findings.

Question 18

How long does recovery take from hydrostatic pulmonary edema vs. CHF?

Answer 18

Hydrostatic pulmonary edema can be rapidly reversed pharmacologically.
ARDS doesn’t remit until the endothelium/epithelium heals, which can take days to weeks.

Question 19

What precipitates ARDS? (there are lots of things - 2 main categories)

Answer 19

Direct lung injury - pneumonia, aspiration, others.

Indirect - sepsis, severe trauma and shock.

Question 20

4 phases of ARDS?

Answer 20

Exudative (permeability pulmonary edema).
Fibroproliferative.
Recovery.
Residual Damage.

Question 21

What’s the broad term for what’s seen in histology of ARDS?

Answer 21

Diffuse alveolar damage.

Question 22

What’s a pathognomonic histologic finding in early ARDS?

Answer 22

Hyaline membranes.

Question 23

What kind of pattern of inflammation is seen in histology of ARDS?

Answer 23

Chronic inflammatory cells.

then granulation tissue, collagen deposition, then reepithelialization

Question 24

Can ARDS and hydrostatic pulmonary edema exist at the same time?

Answer 24

yup

Question 25

How might you increase the capillary oncotic pressure in hydrostatic pulmonary edema?

Answer 25

Give albumin.

this provides transient benefit in setting of hypoalbuminemia

Question 26

Does albuterol help in ARDS?

Answer 26

Nope. The Type II epithelial cells are too injured.

It causes tachycardia without benefit.

Question 27

Limitations to using a ventilator for ARDS?

Answer 27

Want to keep pressures low so that further damage isn’t caused.