West's 9th ed - Chapter 2 - Ventilation (1)

Question 1

What is a normal tidal volume?

Answer 1

Approx 500mL

Question 2

What is meant by the Vital Capacity?

Answer 2

This is the volume exhaled from the point of maximal inspiration to the point of maximal exhalation. It is the Total Lung Capacity excluding the Residual Volume.

Question 3

What is the Functional Residual Capacity?

Answer 3

This is the volume of gas remaining in the lung at the end of a tidal exhalation. It is larger than the Residual Volume.

Question 4

How much volume does the anatomical dead space hold?

Answer 4

About 150mL

Question 5

What is Boyle’s Law?

Answer 5

Boyle’s Law states that the Pressure multiplied by the Volume will always be constant, assuming constant temperature. P1V1 = P2V2.

Question 6

What is the difference between the Alveolar Ventilation and the Total Ventilation?

Answer 6

The alveolar ventilation equals the total ventilation minus the ventilation of the anatomical dead space.

Question 7

If you measure the concentration of CO2 in expired air, where can you assume it has come from?

Answer 7

The entire CO2 concentration in expired air has come from the alveolar space, and none from the dead space. This helps to calculate the volume of these two spaces.

Question 8

In healthy people, what is the difference between the alveolar P_CO2 and the arterial P_CO2?

Answer 8

They are virtually identical.

Question 9

What is the difference in volume between anatomical dead space and physiological dead space?

Answer 9

Anatomical: This is measured by Fowler’s method (nitrogen sampling of air at mouth).

Physiological: This is measured by Bohr’s method (concentration of CO₂ in expired air).

In healthy subjects, the two volumes are the same, but in patients with lung disease, physiological dead space may be considerably larger because of inequality of blood flow and ventilation.

Question 10

What is the definition of anatomic dead space?

Answer 10

Anatomic dead space is the volume of the conducting airways.

Question 11

What is the definition of the physiologic dead space?

Answer 11

Physiologic dead space is the volume of gas that does not eliminate CO₂. It includes non-perfused alveoli.

Question 12

Which regions of the lung are better ventilated?

Answer 12

The dependent regions. Gravity helps ventilation. In upright posture, the lower regions are ventilated better than the apical regions. Lying supine, the posterior lung is better ventilated than the anterior lung.

Question 13

When you exhale against a closed glottis, what happens to the volume in your lungs and the pressure in your airways?

Answer 13

Volume decreases slightly and pressure increases slightly. This is important to note when using a body plethysmograph.

Question 14

When using the helium dilution method to measure FRC, which equation do you use?

Answer 14

Question 15

When using Fowler’s method of measuring anatomic dead space, which equation do you use?

Answer 15

You don’t use an equation. You plot the N₂ concentration (y-axis) against the expired volume (x-axis). You end up with a sigmoid curve, and basically the volume at the midpoint of the sigmoid is the volume of dead space.

Question 16

When using a body plethysmograph (body box) to measure FRC, which equation do you use?

Answer 16

P₁V₁ = P₂(V₁ - change in volume)

Question 17

When using Bohr’s method to measure physiologic dead space, which equation do you use?

Answer 17

Question 18

Explain how Fowler’s method works (to measure FRC).

Answer 18

A patient makes one inhalation of 100% oxygen, then connects their lips to a one-way valve that samples the nitrogen of their expired air repeatedly as they exhale. The first part of the exhaled air contains no nitrogen, because it is simply the air from the anatomical dead space that had just breathed in the 100% oxygen.

Question 19

In a normal tidal breath, how much air is inspired? How much of this is alveolar ventilation, and how much is dead space ventilation?

Answer 19

Tidal breath = 500mL.

Alveolar ventilation = 350mL.

Dead space = 150mL.

Question 20

In a healthy subject, what happens to arterial P_CO2 when you double the ventilation?

Answer 20

When you double the ventilation, you halve the alveolar P_CO2, and since the alveolar P_CO2 and the arterial P_CO2 are virtually identical, this means that you also halve the arterial P_CO2.