Lung volumes and capacities

Question 1

Inspiratory reserve volume

Answer 1

Differences between the top of tidal volume and maximum inspiratory volume.
3.2L

Question 2

Expiratory reserve volume

Answer 2

Difference between the bottom of tidal volume and the minimal volume after expiration.
1.1L

Question 3

Residual volume

Answer 3

The volume of air left in the lungs after you maximally expire.
1.2L

Question 4

Tidal volume

Answer 4

Change in volume on normal breathing.

0.5L

Question 5

Inspiratory capacity

Answer 5

Tidal volume + inspiratory reserve volume

3.7L

Question 6

Functional residual capacity

Answer 6

Expiratory reserve volume + residual volume

2.3L

Question 7

Total lung capacity

Answer 7

Sum of all volumes

6L

Question 8

Vital capacity

Answer 8

Inspiratory reserve volume + tidal volume + expiratory reserve volume.
4.6L

Question 9

Difference between new and original spirometers?

Answer 9

The new version measures flow and then calculates volume from this, whereas the old directly measures volume.

Question 10

Why is spirometry limited?

Answer 10

Residual volume is missing, so functional residual volume and functional capacity cannot be measured.

These are important for COPD.

Question 11

How can you measure RV?

Answer 11

Plethysmography

Gas washout/dilution

Question 12

What are the two most common ways of gas washout/dilution?

Answer 12

1. Nitrogen washout

2. Helium dilution

Question 13

Difference in FRC between plethesmography and gas dilution.

Answer 13

In plethesmography, we are measuring the change in pressure in the airways (one sealed box vs another sealed box). The thoracic cavity is the whole sealed chamber that you’re comparing the box to – there is some air in the thoracic cavity, so you’re measuring the total volume of air in this chamber. In gas dilution, you’re only measuring the volume of the lungs.

This difference is magnified when there is an obstruction. This is because of gas trapping. With the pleth measurement, it still measures the volume of air behind the obstruction, whereas the dilution technique only measures the air communicating with the external environment (not the air behind the obstruction).

If you’re interested in assessing a obstruction, use the gas dilution technique rather than pleth.

Question 14

What happens to lung volumes in asthma?

Answer 14

Asthma is characterised by:

1. airway inflammation
2. airway remodelling
3. airway obstruction

These three things contribute to increase in FRC and TLC.

This is because WOB has increased, so the respiratory system compensates by breathing shallowly and at high volumes. This tends to dynamically hyperinflate the lungs.

Additionally, you have airway inflammation causing obstruction and therefore gas trapping.

If you have someone with chronic, long term asthma they tend to be breathing at a higher lung volume which has impacts on the outer forces (muscle weakness, muscle fatigue).

Question 15

What happens to lung volumes in COPD?

Answer 15

Characterised by two overlapping structural issues:

1. Emphysema - destruction of the parenchyma causing enlargement of the airway spaces
2. Small airway inflammation

Same shifts as asthmatic, increased work of breathing, increased volume of lung breathing and shallow breaths, so increase TLC and FRC.

The biggest change is a decrease in elastic recoil - increasing propensity for lung to collapse. This results in hyperinflation of the lung.

Additionally, decreased tethering forces on airways, which tends to exacerbate tendency to collapse upon expiration. So you get an increase in FRC.

Question 16

What happens to lung volumes in pulmonary fibrosis?

Answer 16

Fibrotic lesions in the parenchyma, which leads to increased lung stiffness (resists outward stretch - increase elastic recoil).

Proliferation of fibrotic lesions throughout the lung, in particular the lung parenchyma obliterating the lung alveoli - resisting elastic stretch (opposite to COPD) - because of this FRC and TLC are decreased, as there is an increased propensity for the lungs to collapse and recoil.

Question 17

FEV1

Answer 17

Forced expiratory volume in 1s = volume of air expired in first second of maximum expiratory effort from TLC.

Dependent on flow (Q).
Q = change in P/ resistance
Resistance is influenced by airway radius.

Question 18

FVC

Answer 18

Forced vital capacity = total volume of air expired maximum expiratory effort from TLC.

Independent of flow.

FVC = TLC - RV

Question 19

FEV1 and FVC in airway obstruction

Answer 19

FEV1 will be reduced
FVC will be normal (it may take longer, and require more effort, but should still be normal).

FEV1:FVC ratio decreases.

Question 20

FEV1, FVC and FEV1/FVC ratio in COPD

Answer 20

Decrease in FEV1.
Normal or slight decrease in FVC (due to dynamic remodelling causing gas trapping)
Decrease in FEV1/FVC ratio
Obstructive phenotype

Question 21

FEV1, FVC and FEV1/FVC ratio in asthma

Answer 21

Decrease in FEV1
Normal or slight decrease in FVC (due to dynamic remodelling causing gas trapping)
Decrease in FEV1/FVC ratio
Obstructive phenotype

Question 22

FEV1, FVC and FEV1/FVC ratio in fibrosis

Answer 22

Decrease in FEV1
Decrease in FVC
Normal FEV1/FVC ratio
Restrictive phenotype.

TLC will also be decreased.

Question 23

Obstructive respiratory condition on flow-volume loop

Answer 23

Peak flow unchanged, decaying flow will be faster (characteristic bow in the flow volume curve), essentially get to the same point as a health person on FVC.

Question 24

Restrictive respiratory condition on flow-volume loop

Answer 24

Decrease in FVC, decrease peak flow - uniform decaying flow.