DSA07: Causes of Hypoxemia (V-Q mismatch)

Most common cause of hypoxemia?

V-Q mismatch

Normal V/Q>

0.8

V stands for

Q stands for

Ventilation

perfusion

With obstructed ventilation, what happens to V/Q?

It goes down

B/C ventilation is significantly decreased

With obstructed ventilation, what does alveolar/arterial O2 gas equal?

venous gas because no O2 is getting into arterial blood

With obstructed blood supply (ex. embolism), what happens to V/Q?

It goes way up

-perfusion goes way down, causing V/Q to go up

-ventilation increases b/c total dead space increases

recall:

dead space = anatomic dead space + physiologic deadspace

-if alveoli are blocked, we increase physiologic and total dead space

-thus, more ventilation occurs, but less perfusion

What does O2 look like with obstructed blood supply?

150

-same as inspired air

Where is V/Q the lowest due to gravity?

Base of lung

B/C perfusion is the highest here because most blood flow

Where is V/Q highest due to gravity?

Apex of lung

B/C perfusion is lowest here, thus V/Q increases

PO2/PCO2 at apex of lung

high PO2

low PCO2

PO2/PCO2 at base of lung

low PO2

high PCO2

What can cause low V/Q?

-anything that decreases ventilation or increases perfusion

ex.

emphysema (obstructed ventilation)

bronchitis (obstructed ventilation)

asthma

edema

Which causes of hypoxemia increase A-a gradient?

anything that causes PaO2 to decrease

-recall: hypoventilation causes PAO2 and PaO2 to decrease so A-a does not change

Thus,

diffusion defect (less PaO2)

V/Q mismatch (less PaO2)

Shunt (less PaO2 that cannot be corrected even with 100% O2)

Normal A-a gradient?

12-15

Altitude effect on hypoxemia?

less atm

PaO2 = (Patm-47)(0.21) - PaCO2/R

PaO2 will be less due to decreased atmospheric pressure

Compensation to high altitude hypoxemia?

hyperventilation

-decreases PaCO2 to increase overall PaO2

What type of breathing in respiratory alkalosis (hyperventilation)?

Cheyne-Stokes breathing

Renal comp for respiratory alkalosis?

excrete HCO3-

Effect of high altitude on plasma volume?

decreases it (polycythemia)

Effect of high altitude on 2,3 DPG?

increases it to shift O2 curve rightwards and promote O2 release to tissues