FRC or Functional Residual Capacity is the volume of lung that exists at the end of normal exhalation after a normal VT and when there is no muscle activity or pressure difference between alveoli and atmosphere¹. From the picture below, FRC is equal to a summation of Residual volume and expiratory reserve volume.

Picture from Miller Anesthesia 5th Ed.

Other lung volumes can be measured by a simple spirometry but the residual volume which is a part of FRC need special methods. There are 3 techniques to measure the FRC clinically.

1. O2 wash out method : by having the patient breathing 100% O2 for several minutes. The N2 in the lungs will be eliminated and then measure the quantity of the N2. The FRC can be calculated from by the product of N2 measured and 1.2 (the interpolation of 79% N2 content in room air)

2. Helium wash in/ helium dilution: by using a helium as a tracer gas. The CO2 will be absorbed by Soda lime and O2 will be replaced as it's used. After equilibration, the He concentration is measured. He is uptake very minimal to the blood stream. By comparing the initial concentration and measured concentration, the FRC can be calculated.

3. Body plethysmography : by using Boyle's law that the product of P (Pressure) and V (volume) is a constant. The subject will be placed in the gas-tight container box, the changes of lung volume then will be reflected by pressure changes.

*the disparity between FRC measured by He dilution and body plethysmography is used to detect large, non-ventilating lung blebs. ¹

Normal FRC is 2.5-3 L for an average male. Click to see other normal values of lung volumes. The FRC is normally above the closing capacity (which is the sum of closing volume and residual volume). But if CC exceeds FRC, the small airway closure occurs and leads to V/Q mismatch and hypoxia. The FRC is also considered as an O2 reserve, so in the patients who has decreased FRC will developed hypoxemia quicker if the airway complications occur.

FRC is reduced in :

• General anesthesia : 20% with spontaneous breathing and 16% with mechanical breathing².
• Supine position
• obesity
• pregnancy
• other lung diseases e.g. restrictive lungs, pulmonary edema, pneumonia, ARDS, atelectasis.

FRC is increased in :

• PEEP and CPAP, thus PEEP will increase FRC and helps reduce the airways closure.
• asthma