Ventilation perfusion matching and dead space

Question 1

What is dead space?

Answer 1

Volume of air in the respiratory passages that does not undergo gas exchange with the blood

Question 2

Usual volume of dead space

Answer 2

150ml

Question 3

What are the types of dead space?

Answer 3

Anatomical dead space
Alveolar dead space
Physiological dead space

Question 4

What is anatomical dead space?

Answer 4

Volume of air present in the conducting zone where gaseous exchange does not occur

Question 5

What is alveolar dead space?

Answer 5

Dead space in non-functional alveoli with poor blood supply

In normal physiological conditions, no alveolar dead space exists

Question 6

What is physiological dead space?

Answer 6

Anatomical dead space + Alveolar dead space

Since in normal physiological conditions, alveolar dead space = 0
Physiological dead space = Anatomical dead space

Question 7

What are the factors affecting the volume of dead space?

Answer 7

Sympathetic stimulation = Bronchodilatation → increase dead space volume
Parasympathetic stimulation = Bronchoconstriction → decrease dead space volume

Question 8

What is the importance of dead space?

Answer 8

WARMF CREAP
Warm, moist, and filter the inspired air
Causes difference in composition between the alveolar air and expired air
Regulates the resistance to air flow
Responsible for the difference between pulmonary ventilation and alveolar ventilation

Question 9

What is pulmonary ventilation/minute ventilation/minute respiratory volume?

Answer 9

Volume of air breathed in or out in one minute
Pulmonary ventilation = Tidal volume x Respiratory rate

Tidal volume = 500ml
Respiratory rate = 12/min
Therefore, pulmonary ventilation = 500 x 12 = 6L/min at rest

Question 10

What is alveolar ventilation?

Answer 10

It is the volume of air reaching the alveoli per minute
Alveolar ventilation = (Tidal volume - Dead space) x Respiratory rate

Tidal volume = 500 ml
Dead space = 150 ml
Respiratory rate = 12/min
Alveolar ventilation = (500-150) x 12 = 4.2L/min at rest

Question 11

What are the factors affecting alveolar ventilation?

Answer 11

Rate and depth of breathing (shallow rapid breathing = decrease alveolar ventilation)
Volume of dead space (increased dead space = decreased alveolar ventilation)

Question 12

Why is alveolar ventilation more important than pulmonary ventilation?

Answer 12

Alveolar ventilation is an indicator of gas exhange

Question 13

Hypoxia’s effect on pulmonary and alveolar ventilation

Answer 13

Pulmonary ventilation is constant because of increased respiratory rate
Alveolar ventilation decreases because of increased alveolar dead space

Question 14

Ventilation Perfusion Ratio (Va/Q)

Answer 14

Ventilation perfusion ratio = Alveolar ventilation (Va)/Pulmonary Perfusion (Q)

Question 15

How does the ventilation perfusion ratio vary across the lung?

Answer 15

Lung apex - More ventilation than perfusion
Lung base - More perfusion than ventilation

Question 16

Pathological causes of ventilation perfusion ratio changes

Answer 16

High ventilation perfusion ratio
Pulmonary thromboembolism → less perfusion → decreased gas exchange.
Emphysema → high ventilation → decreased gas exchange
Low ventilation perfusion ratio
Bronchial asthma → less ventilation → decreased gas exchange

For health lung, the body needs adequate ventilation and perfusion in order to maintain adequate gas exchange