Ventilation-perfusion Relationships

Question 0

What is the hypothetical ideal lung situation ?

Answer 0

No difference between partial pressure of alveolar oxygen and arterial oxygen
The diffusion across the membrane proceeds to equilibrium in all lung units and all blood passing through is exposed to alveolar air
NEVER EXISTS: but difference is extreme,ly small

Question 1

What is ventilation ?

Answer 1

Movement of air from the atmosphere through ventilatory system

Question 2

What is hypoxaemia?

Answer 2

Any situation where partial pressure of oxygen in arterial blood is abnormally low

Question 3

What are 4 basic causes of hypoxaemia ?

Answer 3

Hypoventilation
Diffusion limitation
Shunt
Ventilation-perfusion inequality

Question 4

What 2 factors balance the partial pressure of oxygen in the alveoli ?

Answer 4

Rate of removal of oxygen by blood- determined by metabolic demands
Rate of replenishment of oxygen by alveolar ventilation.

Question 5

What happens if the rate of replenishment at the alveoli falls but the rate of oxygen removal remains the same…

Answer 5

Partial pressure of oxygen in both arterial and alveoli falls while the carbon dioxide increases

Question 6

What is hypoventilation ?

Answer 6

Situation where ventilation falls below demands of internal respiration

Question 7

What are some causes of hypoventilation ?

Answer 7

Drugs which depress centra respiratory drive - opiates and barbiturates
Damage to the chest wall
Paralysis of respiratory muscles

Question 8

When can the diffusion difference at the lungs get larger ?

Answer 8

During exercise due to reduced time and if the blood gas barrier thickens

Question 9

What is shunted blood ?

Answer 9

Is blood that enters the arterial system without going through ventilated areas of lung

Question 10

How do shunts occur and what do they cause ?

Answer 10

They reduce the amount of oxygen in the systemic arteries compared to pulmonary capillaries
Pulmonary arteriovenous fistula- some patients have abnormal vascular connections between pulmonary veins and pulmonary arteries
Cardiac atrial septal defect- causes arterial blood and venous blood to mix

Question 11

How do we know that pulmonary vascular resistance is much lower ?

Answer 11

MAP = CO * TPR
The CO is the same and the MAP is much lower s therefore the resistance must be lower- a tenth of the systemic resistance

Question 12

If the pulmonary arterial pressure rises it causes the pulmonary vascular resistance to decrease. Why ?

Answer 12

Recruitment: as pressure rises previously closed capillaries open and start conducting blood
Distension: pressure rises causes capillary lumen radius to increase which causes a decrease in resistance

Question 13

How does blood flow exist in an upright lung ?

Answer 13

It decreases almost linearly from the base to the apex

Due to hydrostatic pressure difference within blood vessels - more recruitment and distension at the base

Question 14

What are the 3 main forces that affect blood flow through the pulmonary capillary bed ?

Answer 14

Hydrostatic pressure
Arteriovenous pressure difference
Alveolar pressure

Question 15

What is zone1 of the 3 flow zones in an upright lung ?

Answer 15

At the top of the lung
Capillaries are squashed flat so there is no flow
Alveolar pressure > arterial pressure > venous pressure

Question 16

What is zone 2 of the 3 flow zones in an upright lung ?

Answer 16

Flow is determined by the difference between the arterial and alveolar pressures
Arterial pressure > alveolar pressure > venous pressure

Question 17

What is zone 3 of the 3 flow zones ?

Answer 17

Flow is determined by the arteriovenous pressure difference

Arterial pressure > venous pressure > alveolar pressure

Question 18

Why is ventilation greater at the base of the lungs compared to the apex ?

Answer 18

Due to the weight of the lungs and effects of gravity
- intrapleural pressure at the base is less negative so alveoli at base are less expanded - in inspiration there is a greater volume change in the alveoli at the base - this is partly due to reduced surfactant lining the alveoli

Question 19

What is the ventilation-perfusion ratio ?

Answer 19

Ratio between alveolar ventilation and pulmonary blood flow

Question 20

What is the ventilation perfusion ratio at the base ?

Answer 20

The pulmonary blood flow > alveolar ventilation so ratio is < 1

Question 21

As you move up the lungs what happens to the ventilation- perfusion ratio ?

Answer 21

Increases towards 1 - > 1 towards top
Because as you move up the lungs the pulmonary blood flow and alveolar ventilation decrease but blood flow decreases faster

Question 22

What happens if ventilation and blood flow are mismatched ?

Answer 22

Impairment of both oxygen and carbon dioxide transfer

Question 23

What happens to the ventilation- perfusion ratio if ventilation decreases ?

Answer 23

Ratio decreases causing partial pressure of oxygen to decrease and partial pressure of carbon dioxide to increase

Question 24

What happens to ventilation-perfusion ratio if perfusion decreases ?

Answer 24

Increases ratio causing an increase in partial pressure of oxygen and a decrease in partial pressure of carbon dioxide BUT it doesn’t lead to a huge increase in oxygen transport because hb is almost fully saturated

Question 25

What happens when there is a low alveolar partial pressure of oxygen ?

Answer 25

Pulmonary arterioles constrict

Redirect blood to better ventilated areas

Question 26

What happens in areas where perfusion is low but there is high alveolar partial pressure of oxygen ?

Answer 26

Pulmonary arterioles dilate
Increased pulmonary blood flow
Increased oxygen uptake

Question 27

What happens in alveolar area withs high partial pressures of carbon dioxide ?

Answer 27

Bronchodilation - increase elimination of carbon dioxide

Question 28

What happens in alveolar areas with low partial pressure of carbon dioxide ?

Answer 28

Bronchoconstriction - redirects blood to better perfused areas

Question 29

What happens in high altitude pulmonary oedema ?

Answer 29

Vasoconstriction of arterioles is uneven
Produces an increase in pulmonary MAP with significant increases in pressure in capillaries downstream of non constricted arterioles
Initiates an inflammatory response which increase pulmonary capillary permeability making situation worse

Question 30

What are the treatments of altitude sickness ?

Answer 30

To descend
Supplemental oxygen
Hyperbaric bag
Vasodilator drug. nifedipine- blocks calciu channels in vascular smooth muscle to reduce extent of pulmonary hypoxia vasoconstrictor response