What are the physiological consequences of decreasing functional residual capacity by one litre in an adult? (01A8)
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.
Functional residual volume is the volume of gas remaining in lung at the end of expiration during normal tidal breathing.
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.
FRC = 30mL/kg
= 2100 mL/kg in a 70kg person.
Helium is used because it is almost insoluble in blood
V1 x C1 = (V1 + FRC) x C2
=> FRC = V1x (C1/C2 -1)
Based on Boyle's law
As a result,
Boyle's Law = P x V is constant given constant temperature
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
In healthy people there is very little difference.
Gas dilution technique measures only the communicating gas volume.
=> are not measured by gas dilution technique, but measured by body plethysmograph.
Airways are held open by the radial traction of the surrounding lung parenchyma.
=> reduced airway calibre
=> increased airway resistance
NB: Lung volume vs AWR is downward sloping. Not U-shaped.
Extra-alveolar vessels are also held open by radial traction of the surrounding lung parenchyma.
=> reduced calibre of extra-alveolar capillary
=> increased pulmonary vascular resistance
NB: Lung volume vs PVR is U-shaped
Compliance is reduced at low volume because of
=> reduced compliance
Because of reduced compliance
Because of increased airway resistance
=> Work of breathing is increased
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
FRC does NOT change with age.