L86: Ventilation (V), Perfusion (Q) & (V/Q) Relationship

Question 1

Define ventilation (V):

Answer 1

Process by which air moves in and out of the lungs

Question 2

Define perfusion (Q):

Answer 2

Process by which deoxygenated blood passes through the lungs and becomes oxygenated

Question 3

What is the (V/Q) ratio?

Answer 3

The relationship between ventilation and perfusion (blood flow)

Question 4

What does the (V/Q) ratio determine?

Answer 4

Normal gas exchange and thus, the level of PO2 and PCO2 in the blood.

Question 5

Which forces affect ventilation?

Answer 5

• Gravity;
• Compliance;
• Resistance.

Question 6

How does gravity affect ventilation?

Answer 6

Gravity leads to a non-uniform distribution of ventilation within the lungs

Question 7

Explain how gravity affects ventilation within the lungs:

Answer 7

In an upright position:

• Alveoli at the apex are more expanded than those at the base;
• Therefore greater alveolar volume at the apex;
• Pleural pressure is more negative at the apex than at the base;
• Transpulmonary pressure is greater at the apex than the base.

Question 8

What is compliance, with regards to the lungs?

Answer 8

Effort required to stretch the lungs

Question 9

High compliance is:

Answer 9

Easy expansion of the lungs

Question 10

In pulmonary conditions, compliance is:

Answer 10

Decreased

Question 11

What is resistance, with regards to the lungs?

Answer 11

Narrowing or obstruction of the airway that may reduce air flow

Question 12

Large diameter airways have:

Answer 12

Decreased resistance

Question 13

In pulmonary conditions, resistance is:

Answer 13

Increased

Question 14

What is anatomical dead space?

Answer 14

Volume of gas during each breath that fills the conducting airways

Question 15

What is physiological dead space?

Answer 15

Total volume of gas in each breath that does not participate in gas exchange

Question 16

What are the blood supplies for the lungs?

Answer 16

• Pulmonary Circulation (heart and lungs);

- Bronchial Circulation (O2 to parenchyma)

Question 17

What are the main features of pulmonary circulation?

Answer 17

• Low pressure;
• Low resistance;
• Affected by gravity more than systemic circulation.

Question 18

How is the V/Q ratio calculated for a single alveolus?

Answer 18

Alveolar ventilation: Capillary flow

Question 19

How is the V/Q ratio calculated for entire lung?

Answer 19

Total alveolar ventilation: Cardiac output

Question 20

In healthy individuals, the typical V/Q range for lung is:

Answer 20

0.8-1.2

Question 21

When ventilation exceeds perfusion:

Answer 21

V/Q > 1

Question 22

When perfusion exceeds ventilation:

Answer 22

V/Q < 1

Question 23

In a perfect model…

Answer 23

inspired gas and cardiac output are shared equally between alveoli, V/Q = 1. Alveolar and arterial PO2 are the same.

Question 24

What is arterial hypoxemia?

Answer 24

Arterial PO2 < 80 mmHg

Question 25

What is hypoxia?

Answer 25

When insufficient O2 to carry out metabolic functions, PO2 < 60 mmHg

Question 26

What is hypercapnia?

Answer 26

Increase in arterial PCO2 > 40 mmHg

Question 27

What is hypocapnia?

Answer 27

Decrease in arterial PCO2 < 35 mmHg

Question 28

What is anatomical shunt?

Answer 28

When a proportion of the deoxygenated blood entering the lungs, is shunted away from the alveoli and is not oxygenated before leaving the lungs in the pulmonary veins. This leads to a lower PO2, hypoxemia, in blood leaving the lungs.

Anatomical shunt can also happen in the heart e.g. R to L shunt.

Question 29

How can hypoxemia caused by anatomical shunt be confirmed, clinically?

Answer 29

Treatment using 100% oxygen will not increase the overall PO2 (by much).

Question 30

What is physiological shunt?

Answer 30

Blood perfusing non-ventilated alveoli in the lungs due to obstructions etc.

Question 31

What is V-Q mismatching?

Answer 31

Variation of V/Q ratios in alveoli. Some V/Q <1, some V/Q >1.

Question 32

What is COPD?

What are the symptoms of COPD?

Answer 32

A condition where airflow is obstructed. Includes emphysema and bronchitis;

Chronic cough, chest tightness, shortness of breath, mucous.

Question 33

What are the features of emphysema?

What are the symptoms of emphysema?

Answer 33

Structures in alveoli are over inflated;

Patients can inhale but exhalation is difficult due to decreased elasticity.

Question 34

What are the features of chronic bronchitis?

What are the symptoms of chronic bronchitis?

Answer 34

Inflammation of bronchi causing excessive mucous production and swelling;

Shortness of breath and mild exertion.

Question 35

Give an example and the features of a restrictive lung disease:

Answer 35

Pulmonary fibrosis;

Scarring and thickening of tissue, decreased elasticity and gas exchange.

Question 36

When measuring lung volumes, what are the 4 major recorded values?

Answer 36

• Forced vital capacity (FVC);
• Forced expiratory volume in 1 second
  (FEV1);
• Ratio of FEV1 to FVC (FEV1/FVC);
• Average midmaximal expiratory flow (FEF25-75).

Question 37

In a healthy individual, what is FEV1/FVC?

Answer 37

> 70%, i.e. >70% of lung volume is expired in 1 second

Question 38

In an individual with obstructive lung disease, what is FEV1/FVC?

Answer 38

<70%, i.e. <70% of lung volume is expired in 1 second

Question 39

In an individual with restrictive lung disease, what is FEV1/FVC?

Answer 39

> 70%, because lung volume (FVC) is much smaller than that of a healthy individual

Question 40

What is PO2 at rest?

Answer 40

100 ± 2 mmHg

Question 41

What is PCO2 at rest?

Answer 41

40 ± 2 mmHg

Question 42

What is the rate of ventilation at rest?

Answer 42

~6-7 L/min (12-15 breaths/min, 500ml/breath)

Question 43

What is the rate of gas exchange at rest?

Answer 43

~ 250 mL/ O2 consumed;

~ 200 mL/ CO2 expired.