Ventilation and Lung Volumes

Question 1

What is total lung capacity (TLC)?

Answer 1

It is the maximum volume of gas that the lungs can contain. Normally it is 6-7L

Question 2

What is tidal volume (VT)?

Answer 2

It is the volume of gas which flows into and then out of the lung in one breath. VT is normally 500-600 mL and increases with exercise.

Question 3

What is Inspiratory reserve volume (IRV)?

Answer 3

It is the maximum volume of gas that can be inhaled from the end-tidal inspiratory position.

Question 4

What is expiratory reserve volume (ERV)?

Answer 4

It is the volume of gas that can be exhaled from the end-tidal expiratory position.

Question 5

What is residual volume (RV)?

Answer 5

It is the volume of gas contained in the lungs after a maximal forced expiration. The residual volume can not be exhaled.

Question 6

What is vital capacity (VC)? How can you solve for it?

Answer 6

It is the maximum volume of gas that can be exhaled after a maximal inspiration. VC = IRV + VT + ERV = TLC - RV

Question 7

What is Inspiratory reserve capacity (IC)? What is the formula to solve for it?

Answer 7

It is the maximum volume of gas that can be inhaled from the resting expiratory position. IC = VT + IRV = TLC - FRC

Question 8

What is functional residual capacity (FRC)? What is the formula to solve for it?

Answer 8

It is the volume of gas in the lungs after a normal expiration when the diaphragm and chest muscles are relaxed, that is, when the lungs and chest wall are at mechanical equilibrium. FRC cannot be measured with a spirometer since this capacity includes RV.

Question 9

What happens to FRC if lung compliance is increased?

Answer 9

FRC increases too

Question 10

What is the effect on FRC if lung compliance is decreased?

Answer 10

FRC decreases

Question 11

What happens to lung compliance and FRC with aging?

Answer 11

They both increase during aging.

Question 12

There are three ways FRC can be measured, what are they?

Answer 12

Open-circuit Nitrogen washout.
Closed-circuit helium dilution for measuring FRC.
Body Plethysmograph.

Question 13

Describe open-circuit Nitrogen washout:

Answer 13

First, while the subject is breathing air, an alveolar gas sample is taken and the initial N2 fraction is measured. Then at the end of eupneic exertion, with the lung at FRC, the subject breathes 100% O2 for at least 7 min to wash out all of the N2 from the lung. The expired gas is collected in a large spirometer. The volume expired and the N2 fraction in the collected gas are measured. Conservation of mass determines FRC.

Question 14

What is the equation for Nitrogen washout?

Answer 14

FRC = F_N2 sp x V sp / F_N2 lung

Question 15

What does the plethysmograph allow you to measure? What is the formula?

Answer 15

It measures the total amount of gas in the lung at FRC. V = FRC = -Pt∆V / ∆P

Question 16

What does the dilution method allow you to measure? What can you calculate by combining the two?

Answer 16

Dilution methods only measure exchangeable air. By combining the two methods, the amount of trapped air can be estimated.

Question 17

How do you calculate Alveolar and Dead space ventilation?

Answer 17

fVt = fVd + fVa.

Use tidal volume times breaths per minutes. Subtract the anatomical dead space to give alveolar gas exchange.

Question 18

Alveolar ventilation sets the partial pressures of O2 and CO2. What effect does hypoventilation have? How about hyperventilation?

Answer 18

Hypo - results in alveolar hypercapnea and hypoxia.

Hyper - results in alveolar hypocapna and hyperoxia

Question 19

What is the alveolar gas equation for CO2?

Answer 19

P_A CO2 = V_CO2 P_T/ V_A (not the V’s have dots meaning rate.)

Question 20

Alveolar gas equation for O2 is the basis for Oxygen therapy what is the formula?

Answer 20

P_A O2 = P_l O2 - (V_O2 P_T / V_A)
Influx = efflux in steady state
P_A O2 = P_l O2 - (P_A CO2 / R)

Question 21

Increasing the partial pressure of oxygen in the inspired gas causes what in the alveoli?

Answer 21

Increases alveolar oxygen.

Question 22

VT is only about 10% of TLC, what consequence does this have on alveolar gas composition?

Answer 22

It does not change much with each breath during eupneic breathing. The large amount of air that is not exchanged acts as a buffer to minimize these oscillations.

Question 23

Describe the single breath analysis of Dead space (Fowler method)

Answer 23

A subject inspires air containing negligible CO2 and then exhales into a spirometer while the fraction of CO2 in the expired gas is being measured. This allows a value of V_D that can be used to estimate V_A. The principle of the Fowler method is that the expired CO2 comes exclusively from the alveoli and not from the anatomic dead space.