Functional residual capacity
|This article does not cite any references or sources. (November 2014)|
Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.
FRC is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV) and measures approximately 2400 mL in an 80 kg, average-sized male. It can not be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.
A lowered or elevated FRC is often an indication of some form of respiratory disease. For instance, in emphysema, the lungs are more compliant and therefore are more susceptible to the outward recoil forces of the chest wall. Emphysema patients often have noticeably broader chests because they are breathing at larger volumes. In healthy humans, FRC changes with body posture. Obese patients will have a lower FRC in the supine position.
The predicted value of FRC was measured or large populations and published in several references. FRC was found to vary by a patient's age, height, and sex. An online calculator exists that will calculate FRC for a patient using these references.
- R.O. Crapo, A.H. Morris, R.M. Gardner, Reference Spirometric Values using Techniques and Equipment that meet ATS recommendations. American Review of Respiratory Disease, Volume 123, pp.659-664, 1981.
- P.H. Quanjer. Lung Volumes and Forced Ventilatory Flows. Eur Respir J, Vol 6, Suppl 16, pp. 5-40, 1993.
- H. Hedenström, P. Malmberg, K. Agarwal. Reference Values for Lung Function tests in Females. Bull. Eur. Physiopathol. Respir. 21, pp. 551-557, 1985.
- A. Zapletal, T. Paul, M. Samanek. Die Bedeutung heutiger Methoden der Lungenfunktionsdiagnostik zur Feststellung einer Obstruktion der Atemwege bei Kindern und Jugendlichen. Z. Erkrank. Atm.-Org., Volume 149, pp.343-371, 1977.
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