Ventilation/Perfusion Abnormalities

Question 1

Consider the regional differences in ventilation between the apex and base of the lung. In relation to these differences:

Resting lung volume is lower at the bases

Answer 1

True. Due to the weight of the lung and higher intrapleural pressure, the resting lung volume is lower at the base than the apex.

Question 2

Consider the regional differences in ventilation between the apex and base of the lung. In relation to these differences:

Compliance is lower at the bases

Answer 2

False. The slope of the pressure-volume curve is known as compliance. The lung is easier to inflate at low lung volumes, so compliance is higher at the base.

Question 3

Consider the regional differences in ventilation between the apex and base of the lung. In relation to these differences:

The weight of the lung is uniform

Answer 3

True. The weight of the lung at the base is greater than at the apex.

Question 4

Consider the regional differences in ventilation between the apex and base of the lung. In relation to these differences:

Intrapleural pressure is higher at the apex

Answer 4

False. Intrapleural pressure is higher, i.e. less negative, at the bases to support the weight of the lungs.

Question 5

Consider the regional differences in ventilation between the apex and base of the lung. In relation to these differences:

Ventilation is greater at the bases

Answer 5

True. Regional ventilation is defined as change in volume per unit resting volume. Inflation pressure is lower at the bases as is resting volume, hence a greater change is seen when compared with the apices.

Question 6

With the body in the erect position, going from the apex to the base of the lung:

Ventilation decreases

Answer 6

False. Due to lower resting volume and better compliance, ventilation is greater at the base of the lung.

Question 7

With the body in the erect position, going from the apex to the base of the lung:

Compliance increases

Answer 7

True. The base of the lung is more compliant due to its position on the pressure-volume curve.

Question 8

With the body in the erect position, going from the apex to the base of the lung:

Ventilation/perfusion ratio remains constant

Answer 8

False. It decreases from the apex to the base.

Question 9

With the body in the erect position, going from the apex to the base of the lung:

Blood flow increases

Answer 9

True. Blood flow increases due to differences in hydrostatic pressures, i.e. West zones.

Question 10

With the body in the erect position, going from the apex to the base of the lung:

Intrapleural pressure increases

Answer 10

True. Due to the increased weight of the lung at the base, an equal, i.e. higher, pressure is required to balance this.

Question 11

Regarding the relationship between ventilation and perfusion (V/Q):

Ventilation changes more than perfusion

Answer 11

False. Perfusion changes more than ventilation.

Question 12

Regarding the relationship between ventilation and perfusion (V/Q):

Ventilation increases from the apex to the base

Answer 12

True. Resting volume at the base is lower than at the apex. Ventilation is change in volume per unit volume, so ventilation is greater at the base.

Question 13

Regarding the relationship between ventilation and perfusion (V/Q):

Perfusion increases from base to apex

Answer 13

False. Perfusion increases due to gravity and hydrostatic pressure differences.

Question 14

Regarding the relationship between ventilation and perfusion (V/Q):

The ventilation/perfusion ratio at the apex is greater than at the base

Answer 14

True. V/Q is >1 at the apex and <1 at the base.

Question 15

A reduction in PaO2 can be caused by:

Hypoventilation

Answer 15

True. PaO2 is determined by the rate of removal of oxygen, through metabolism, and rate of replenishment of oxygen, through alveolar ventilation.

Question 16

A reduction in PaO2 can be caused by:

Breathing an hypoxic mixture

Answer 16

True. This is correct unless there is compensation such as hyperventilation.

Question 17

A reduction in PaO2 can be caused by:

An increase in dead space

Answer 17

True. An example of this is a pulmonary embolus. The alveolus is ventilated but not perfused so there is no blood to oxygenate, i.e. wasted ventilation.

Question 18

A reduction in PaO2 can be caused by:

Hypermetabolic states

Answer 18

True. This is correct unless ventilation is increased to compensate.

Question 19

A reduction in PaO2 can be caused by:

An increase in shunt

Answer 19

True. An example of this is pneumonia. Shunted blood is not exposed to oxygen and therefore depresses arterial oxygen content.

Question 20

Are these statements true or false?

Hypoventilation causes an increase in PaCO2

Answer 20

True. This can be predicted from the alveolar gas equation.

Question 21

Are these statements true or false?

Shunt causes an increase in PaCO2

Answer 21

False. Any rise is detected by chemoreceptors resulting in an increase in ventilation.

Question 22

Are these statements true or false?

The hypoxia caused by shunt can be abolished by increasing inspired oxygen

Answer 22

False. Shunted blood is never exposed to the increase in FiO2 and therefore the reduction in PaO2 persists.

Question 23

Are these statements true or false?

The hypoxia caused by hypoventilation can be abolished by increasing inspired oxygen

Answer 23

True. The increase in inspired oxygen compensates for the hypoventilation.

Question 24

Are these statements true or false?

Alveolar (PAO2) and arterial (PaO2) oxygen are usually the same

Answer 24

False. Under normal circumstances there is a small difference between the PAO2 and PaO2 due to incomplete diffusion.

Question 25

In the shunt equation, used to calculate the degree of shunt, which of these are required?

A. Cardiac output
B. Arterial oxygen content
C. Mixed venous oxygen content
D. End-capillary oxygen content
E. Alveolar carbon dioxide concentration
F. Inspired oxygen concentration

Answer 25

A - D

Qs/Qt = (CcO2 - CaO2)/(CcO2 - CvO2)

E&F PACO2 and FiO2 do not feature in the equation.

Question 26

Regarding a lung unit:

A normal ventilation/perfusion ratio is about 2.5

Answer 26

False. A normal / ratio is around 1.0.

Question 27

Regarding a lung unit:

The ventilation/perfusion ratio increases as ventilation falls

Answer 27

False. V/Q decreases and approaches zero.

Question 28

Regarding a lung unit:

If there is no ventilation, PAO2 and PACO2 are the same as inspired air

Answer 28

False. They become the same as mixed venous blood (shunt with raised CO2). Usually an increase in CO2 is not seen due to compensatory hyperventilation.

Question 29

Regarding a lung unit:

The ventilation/perfusion ratio approaches infinity (∞) as perfusion increases

Answer 29

False. It approaches ∞ as perfusion decreases.

Question 30

Regarding a lung unit:

If perfusion is completely abolished, PAO2 and PACO2 are the same as mixed venous levels

Answer 30

False. They become the same as inspired gas (dead space with no increase in CO2).