30. Ventilation/Perfusion Relationship

Question 1

What is the % oxygen in inspired, expired and alveolar air?

Answer 1

21
16
14

Question 2

What is the % carbon dioxide in inspired, expired and alveolar air?

Answer 2

0
4
6

Question 3

What is the % nitrogen in inspired, expired and alveolar air?

Answer 3

79
80
80

Question 4

What is the PO2 in inspired, expired and alveolar air?

Answer 4

150
120
100

Question 5

What is the PCO2 in inspired, expired and alveolar air?

Answer 5

0
30
40

Question 6

How is the partial pressure found?

Answer 6

Take atmospheric pressure (760) and take away vapour pressure (47)
Then multiply by the percentage the gas takes up

Question 7

Why is the PO2 in pulmonary veins slightly higher than arteries?

Answer 7

Blood supply to the lungs themselves (ie bronchial and thebesian veins) rejoin into arteries

Question 8

How can arterial blood gas be measured?

Answer 8

Arterial blood sample
Blood gas analyser
Resulting graph is a capnogram

Question 9

What factors affect diffusion?

Answer 9

Pressure difference
Surface area
Distance the gas has to travel

Question 10

What is the diffusing capacity?

Answer 10

Rate of transfer of gas from lung to blood/partial pressure difference
Measure of a patient’s ability to diffuse passively

Question 11

Why is CO used to find the partial pressure difference?

Answer 11

Binds to haemoglobin so effectively that the partial pressure difference is equal to the alveolar PCO

Question 12

In what cases would the diffusing capacity be reduced?

Answer 12

Lung fibrosis
Pneumonia
Oedema
Emphysema

Question 13

What is the alveolar gas equation?

Answer 13

PAO2= PiO2-PaO2/R

Question 14

What is the closing volume?

Answer 14

Volume at which airways start collapsing

Higher intrapleural pressure at base will cause it to collapse first

Question 15

Why is there high nitrogen after closing volume?

Answer 15

Air coming out is preferentially from apex

Has more nitrogen because inspiration went primarily to base

Question 16

What is wasted ventilation?

Answer 16

Increased V/Q ratio
Higher alveolar dead space
Respiratory muscles are carrying out useless work
No gas exchange
Embolism, emphysema

Question 17

What is shunting?

Answer 17

Decreased V/Q ratio
Deoxygenated blood bypasses exchange area and rejoins left heart
Arterial hypoxia
COPD, asthma, bronchitis

Question 18

What causes hypoxic pulmonary vasoconstriction?

Answer 18

Low PAO2 or high PACO2 causes relaxation of airway smooth muscle
Constriction of pulmonary arterioles- directs oxygen elsewhere