Functional residual capacity (FRC)

What are the physiological consequences of decreasing functional residual capacity by one litre in an adult? (01A8)

Definition of FRC

Definition

Functional residual volume
= residual volume + expiratory reserve volume

where, residual volume is the volume of gas remaining in lung at the end of a maximal expiration.

 

Alternatively,

Functional residual volume is the volume of gas remaining in lung at the end of expiration during normal tidal breathing.

Alternatively,

FRC is essentially the balance point where the tendency for lung to recoil equals to the tendency for chest wall to spring out.

 

[See diagram 20050306(6) - Elastic force - lung vs chest wall]

NB: Relaxation pressure is the pressure taken after the subjects inspires or expires to a certain volume, with respiratory muscles relaxed and spirometry valve closed.

Normal value

FRC = 30mL/kg
= 2100 mL/kg in a 70kg person.

 

Measurement of volumes

2 methods

1. Helium dilution technique
2. Body plethysmograph

Helium dilution technique

Helium is used because it is almost insoluble in blood

1. Subject rebreathing from a closed circuit with known initial volume (V1) and concentration of helium (C1)
2. After a period of rebreathing, the final helium concentration (C2) is taken

V1 x C1 = (V1 + FRC) x C2

=> FRC = V1x (C1/C2 -1)

Body plethysmograph

Based on Boyle's law

1. Airtight box
2. Subject makes inspiratory effort from FRC, against a closed mouth piece

As a result,

• Pressure in lung and mouth piece drops
  - pressure in box before and after = P1, P2
  - mouth pressure before and after = P3, P4
• Lung volume goes up
  - change in volume = V'
• Box pressure goes up and volume drops
  - box volume = Vbox

Boyle's Law = P x V is constant given constant temperature

Thus,

A. Gas in box: P1 x Vbox = P2 x (Vbox - V')

B. Gas in lung: P3 x FRC = P4 x (FRC+V')

(P1, P2, P3, P4 can be measured)

(Vbox can also be measured)

=> Volume change (V') can be calculated from the first equation

=> FRC can be calculated

Difference between the two methods

In healthy people there is very little difference.

Gas dilution technique measures only the communicating gas volume.

Thus,

• Gas trapped behind closed airways
• Gas in pneumothorax

=> are not measured by gas dilution technique, but measured by body plethysmograph.

Effect of decreased FRC

 

Airway resistance (AWR)

Airways are held open by the radial traction of the surrounding lung parenchyma.

Thus,

Decreased FRC

=> reduced airway calibre

=> increased airway resistance

NB: Lung volume vs AWR is downward sloping. Not U-shaped.

Pulmonary vascular resistance (PVR)

Extra-alveolar vessels are also held open by radial traction of the surrounding lung parenchyma.

Thus,

Decreased FRC

=> reduced calibre of extra-alveolar capillary

=> increased pulmonary vascular resistance

NB: Lung volume vs PVR is U-shaped

Compliance

Compliance is reduced at low volume because of

1. collapsed airway/alveoli
2. positive intrapleural pressure

Thus,

Decreased FRC

=> reduced compliance

Work of breathing

Because of reduced compliance
AND

Because of increased airway resistance

Decreased FRC

=> Work of breathing is increased

Venous admixture

As FRC is reduced toward the closing capacity,

=> ventilation of the basal alveoli is reduced, or intermittent, or stopped.

=> V/Q scatter AND/OR shunt is increased

=> Ateletasis may occur

=> PaO2 is decreased

 

 

Factors affecting FRC

1. Height
2. Weight
3. Posture/position
4. Disease
5. Muscle relaxation

FRC increases with

• Increased height
• Erect position (30% more than in supine)
• Decreased lung recoil (e.g. emphysema)

FRC decreases with

• Obesity
• Muscle paralysis (especially in supine)
• Supine position
• Restrictive lung disease (e.g. fibrosis)
• Pregnancy
• Anaesthesia

FRC does NOT change with age.

 

Functions of FRC

1. Oxygen store
2. Buffer for maintaining a steady arterial PO2
3. Partial inflation helps prevent atelectasis
4. Minimise the work of breathing
5. Minimise pulmonary vascular resistance
6. Minimised V/Q mismatch
   - only if closing capacity is less than FRC
7. Keep airway resistance low (but not minimal)

Additional notes

Examiner's comment

• definition, normal value
• decreased compliance, increased airway resistance, increased pulmonary vascular resistance
• increased work of breathing (due to airway collapse, decreased compliance)
• V/Q mismatch => arterial hypoxemia
• (extra) functions of FRC
• (extra) common cause for such a fall
• (common errors) confusing shunt with dead space, mislabeled diagram, confusing diagram volume vs AWR and volume vs PVR.

To be added later