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.
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.
- 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.