FRC - functional residual capacity
The FRC, the volume of gas contained in the lung after a normal expiration, is mainly determined by the interaction between elastic recoil of the chest and lungs (animation on the left).
In the newborn both the thorax and lung are very compliant, so that the FRC is very small. Particularly in the supine posture, when the diaphragm is pushed up by the abdominal contents, gas transport is hampered by the occurrence of airway closure. Newborns elevate their FRC by glottis closure and by postinspiratory stimulation of inspiratory muscles during expiration.
Increased FRC in airway obstruction
In severe airway obstruction many lung compartments may be
incapable of emptying due to airway closure.
In addition expiratory flow may be so limited that insufficient
time is available to reach the lung volume that would be obtained
in the case of elastic equilibrium between lung and chest.
This gives rise to a higher endexpiratory volume and a concomitant
increase in elastic recoil pressure (pleural pressure falls),
slight widening of the airway due to the larger distending
pressure and hence some benefit to expiratory flow. The endexpiratory
volume increases to the point where a new dynamic equilibrium
is reached between inspiratory and expiratory tidal volume.
It follows that severe airway obstruction is associated with
an increase in FRC (hyperinflation). If FRC is normal in a
patient with airway obstruction when at rest, it may increase
during exercise; due to flow limitation the time available
for lung emptying may not suffice at the increased tidal volume.
Diminished FRC
A low FRC occurs in restrictive ventilatory defects. The FRC
also diminishes in the supine posture because the abdominal
contents the push the diaphragm upwards; this phenomenon is
most pronounced with space occupying intra-abdominal processes
(e.g. pregnancy,
hepatosplenomegaly, ascites). Unilateral paralysis of the
diaphragm is usually not associated with a change in the FRC;
bilateral paralysis of the diaphragm is associated with a
smaller FRC in both the sitting and supine posture.
Recommended procedures
- Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Official Statement of the European Respiratory Society. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.