Functional Residual Capacity
Also written as: FRC — Functional Residual Capacity
The volume of air remaining in the lungs at the end of a passive, tidal exhalation, representing the resting equilibrium point of the respiratory system.
Full Definition
Functional residual capacity (FRC) is the lung volume at which the inward elastic recoil of the lungs is balanced by the outward recoil of the chest wall, yielding a passive resting position. In pediatric patients, FRC is particularly important because infants have highly compliant chest walls, placing them at greater risk of dynamic airway closure. FRC is measured by body plethysmography (FRCpleth) or gas dilution methods (FRCgas); in obstructive disease, FRCpleth exceeds FRCgas due to non-communicating trapped gas. Editors should note that FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV), and must not be used interchangeably with TLC or RV.
Usage
Usage note: Specify measurement technique (plethysmography vs. gas dilution) on first reference. Do not use interchangeably with residual volume or total lung capacity.
In Context
- "Elevated functional residual capacity measured by plethysmography indicated air trapping in the asthmatic child, consistent with dynamic hyperinflation." — Pulmonary function test report
- "Authors frequently write 'FRC' without clarifying the measurement method; editors reviewing lung function manuscripts should query whether plethysmographic or gas-dilution FRC is reported." — Journal manuscript review note