9. Lung volumes

Question 1

Draw a spirometer trace to illustrate the various lung

volumes.

Answer 1

Spirometry is the standard method for

measuring most relative lung volumes.

However, it is incapable of providing information
about absolute volumes of air in the lung.

Thus a different approach is required
to measure residual volume,
functional residual capacity
and total lung capacity.

Two of the most
common methods of obtaining information about these volumes are

1. gas dilution tests
2. body plethysmography

Question 2

Draw spirometry trace from page 28

What the capacities
volumes

Answer 2

All values quoted are approximate for a 70-kg man:
TLC Total lung capacity (6000 mL)
VC Vital capacity (4800 mL)
TV Tidal volume (400–600 mL)
IRV Inspiratory reserve volume (2500 mL)
ERV Expiratory reserve volume (1200 mL)
RV Residual volume (1200–1500 mL)
FRC Functional residual capacity (3000 mL standing up or 2000 mL
supine).

Question 3

Which lung volumes can be measured with a spirometer?

Answer 3

Any lung volume that incorporates

residual volume
cannot be measured with simple spirometry
(i.e. TLC, RV and FRC),

the rest can be measured:
> Vital capacity –
the maximum volume expired after a maximal inspiration

> Tidal volume – normal resting breath volume

> Inspiratory reserve volume –
volume of air that can be inspired
over and above the resting tidal volume

> Expiratory reserve volume –
volume of air that can be expired
from end of normal tidal volume

Question 4

How can absolute lung volumes

be measured?

Answer 4

The absolute lung volumes are:

1. > Residual volume –
   volume of air remaining in the lungs
   after maximal expiration

2.
> Total lung capacity – 
total volume of air
in the lungs 
after maximal inspiration

3.
> Functional residual capacity –
volume of air 
remaining in the lungs
after a normal expiratory breath

These cannot be measured
by simple spirometry,

but require the use of more
advanced techniques such as

1. gas dilution

or

2. body plethysmography.

Question 5

Describe how nitrogen washout may be used to measure RV and FRC.

Answer 5

> The fractional lung nitrogen concentration (FLN2)
is constant at 79%
(i.e. 790 mL per 1000 mL air).

1. The subject rebreathes several times
   from a bag of known volume
   containing a nitrogen-free gas.

2
Thus, the nitrogen from the patient’s lungs
equilibrates with gas in the bag
and so the nitrogen concentration
will decrease as the volume of distribution
has increased.

3. 
> In order to measure RV, 
the rebreathing process is 
started from the end
of a maximal expiration (i.e. from RV).

4.
> In order to measure FRC,
the rebreathing process is started
from the end of a normal tidal breath (i.e. from FRC).

5. 
> The principle behind the nitrogen washout method
is that the amount of nitrogen 
at the start of the determination 
(nitrogen in the patient’s lungs only) 

is the same amount that ultimately is
distributed between the lung and the bag.

6. As the volume of the bag and the fractional concentration of nitrogen in the lungs are known,
   the fractional concentration of nitrogen
   in the bag at equilibration is measured,

allowing calculation of either RV or FRC

Question 6

Disadvantage of Gas dilution methods

Answer 6

Both the nitrogen washout
and helium wash in methods
measure only communicating gas.

This is a disadvantage compared to the

total body plethysmography method,

which is able to measure communicating
and non-communicating gas
(i.e. gas trapped behind closed airways).

Question 7

What is the forumla for N wash out to calculate Frc

Answer 7

VB x FxN2
VL = ____________

        FLN2– FxN2 

Where:
VL Volume of lung (FRC or RV)
VB Volume of bag
FxN2 Fractional concentration of N2 in bag
FLN2 Fractional concentration of N2 in the lung

Exactly the same principle is used
for the helium wash-in method of
determining RV or FRC.

Question 8

Describe how body plethysmography may be used to

measure RV and FRC.

Answer 8

1. > The subject sits inside an
   airtight chamber equipped
   to measure pressure, flow or volume changes.
2. > The subject inhales or exhales to a
   particular volume (usually FRC),
   and then a shutter drops across their breathing tube.

3. 
> The subject makes respiratory efforts 
against the closed shutter, 
causing their chest volume to expand 
and decompressing the air in their lungs.

4.
> T he increase in their chest volume
slightly reduces the box volume and
thus slightly increases the pressure in the box.

5. > The most common measurements made
   using the body plethysmograph

are

1. thoracic gas volume and
2. airway resistance.

The following equation is then used:

Pressure 1 · Volume 1 = Pressure 2 ⋅ (Volume 1 − Volume 2)

(It uses Boyle’s law – at a constant temperature, within a closed system, pressure is inversely proportional to volume.)

Question 9

What determines FRC?

Apnoea - how much o2 stored in FRC

Answer 9

FRC is dependent on the balance
of the tendency of the lungs to recoil and
the thoracic cage to expand.

Under conditions of apnoea,
FRC represents the pulmonary oxygen store.

If FRC is 2500 mL, breathing 21% O2
the oxygen store is 500 mL,
but this can be increased by preoxygenation (denitrogenation) to 2500 mL.

If resting total body O2 requirement is 250 mL/min, FRC represents a 10 minute O2 store during apnoea.

Question 10

> FRC is increased by:

Answer 10

> FRC is increased by:
• Standing position
• COPD
• Asthma
• PEEP.

Question 11

> FRC is reduced by:

Answer 11

> FRC is reduced by:
• Supine position
• General anaesthesia
• Pregnancy
• Obesity.

Question 12

What is closing capacity?

Answer 12

The closing capacity is the
volume of the lungs at which
the small airways begin to collapse and close off.

If FRC is less than the closing capacity,
areas of the lung will be perfused but not ventilated, resulting in an increase in
V /Q mismatch.