Ventilation and Perfusion

Question 1

What is the effect of low PaO2 on pulmonary circulation and why?

Answer 1

Vasoconstriction; shunts blood away from poorly oxygenated areas of lung to well-ventilated regions of lung.

Question 2

What does it mean for gas to be perfusion limited?

Answer 2

Gas equilibrates early along the length of the capillary to alveolar air; diffusion can only be Increased if blood flow increases.

Question 3

What gases are exchanged in a “perfusion limited” manner?

Answer 3

O2 (normally), CO2, and N20.

Question 4

What does it mean for gas to be diffusion limited?

Answer 4

Gas does not equilibrate by the time blood reaches the end of the capillary.

Question 5

In what conditions is gas exchange diffusion limited?

Answer 5

fibrosis, emphysema; CO.

Question 6

What is the formula for diffusion?

Answer 6

Vgas = Area/Thickness * D(P1-P2)

*D(P1-P2) = difference in partial pressures.

Question 7

What variable of the diffusion formula changes in fibrosis?

Answer 7

T increases (thicker wall)

Question 8

What variable of the diffusion formula changes in emphysema?

Answer 8

Area for diffusion decreases.

Question 9

What can cause oxygen to become diffusion limited?

Answer 9

Fibrosis (increase thickness)
High altitude (changes the P1-P2 gradient)
Exercise - more Hb needing to be saturated for a given Area of exchange.

Question 10

What is the Ohms law equivalent for vascular resistance?

Answer 10

Change in pressure = Flow * Resistance,

or R = change in pressure/flow

Question 11

What is the formula for pulmonary vascular resistance in relation to pressure and flow?

Answer 11

(P pulmonary artery - P atrium)/cardiac output

Question 12

How does pulmonary vascular resistance relate to viscosity, vessel length, and vessel radius?

Answer 12

R = 8viscositylength/r^4

• resistance increases with viscosity and with length
• resistance decreases to the 4th power with increase in radius

Question 13

What determines alveolar gas?

Answer 13

PAO2 = PIO2 - (PaCO2/R)

PIO2 = oxygen in inspired air
PaO2 = arterial PaCO2
R = resp. quotient; CO2 produced/O2 consumed

Question 14

What are normal values for PAO2 formula?

Answer 14

= 150mmg - PaCO2/0.8

Question 15

What is the A-a gradient? What is it useful for?

Answer 15

PAlveolarO2 - ParterialO2 = 10-15mmg.
The A-a gradient tells you the difference between how much oxygen is in the alveolus, and how much is in the blood. An abnormally increased A-a gradient can identify a source of hypoxemia

Question 16

Under what hypoxemic conditions is A-a gradient increased?

Answer 16

Right to left shunting (causes abnormally poorly oxygenated blood)
V/Q mismatch (abnormally matched perfusion and ventilation)
Fibrosis (diffusion limitation)

Question 17

What are causes of hypoxemia with a normal A-a gradient?

Answer 17

High altitude, hypoventilation (eg opiod use)

Question 18

What is the definition of hypoxemia?

Answer 18

Decreased PaO2 (pressure of oxygen in arteries)

Question 19

What is the definition of hypoxia?

Answer 19

Decreased oxygen delivery to tissues

Question 20

What are three causes of hypoxia other than hypoxemia?

Answer 20

Decrease in cardiac output, anemia, Co poisoning

Question 21

What is V/Q and what is the ideal V/Q?

Answer 21

V = ventilation, Q = perfusion, ideally V/Q = 1

Question 22

Where is there wasted ventilation in the lung?

Answer 22

Apex of lung, where V/Q = 3. (Zone 1)

Question 23

Where is there wasted perfusion of the lung?

Answer 23

Bases of lungs, where V/Q = 0.6 (Zone 3)

Question 24

How does ventilation and perfusion change from top to bottom of the lung?

Answer 24

Both ventilation and perfusion are greater at the base of the lung. However, there is a greater drop in perfusion at the apex of the lung than there is in ventilation; hence there is wasted ventilation at the apex.

Question 25

How does exercise change ventilation/perfusion relationships in the lung?

Answer 25

Increases cardiac output, and vasodilation of apical capillaries; greater perfusion of the apices, V/Q ratio approaches 1.

Question 26

What is going on when V/Q = 0

Answer 26

V has dropped to “0”, ie 0 ventilation. This represents an airway obstruction. Blood flowing past it would not be picking up any oxygen, similar to as if it were skipping the lungs altogether, like in R-L heart shunt. This is called SHUNT.

Question 27

Does shunt correct with oxygen?

Answer 27

No.

Question 28

What is going on when V/Q = infinity?

Answer 28

Q has dropped to 0, ie, perfusion block. Assuming < 100% dead space, 100% O2 improves PaO2.