Why is the functional residual capacity (FRC) important in oxygenation?
Answer:
In a healthy young adult, FRC is above CC (FRC > CV + RV). Increases in CC and/or decreases in FRC can result in CC exceeding FRC and subsequent closure of the small airways during certain periods of normal tidal ventilation. Airway closure produces shunting, with perfusion of unventilated alveoli. Therefore, shunt (QS/QT) is increased and arterial oxygenation is decreased.
Perfusion to the lung occurs in both inspiratory and expiratory phases of respiration. The FRC, or lung volume remaining at end expiration, provides surface area for gas exchange during this phase of respiration. The greater the FRC, the more oxygenation occurs. Patients in the supine position, under general anesthesia, or with acute respiratory distress syndrome all have decreased FRC to varying degrees. PEEP increases FRC, improving oxygenation in the expiratory phase and reducing small airway collapse.
FRC can be measured by helium dilution, nitrogen washout, or body plethysmography.
Reference(s):
- Buist AS. New tests to assess lung function. The single-breath nitrogen test. N Engl J Med. 1975;293:438-440.
- Lumb AB. Nunn’s Applied Respiratory Physiology. 8th ed. Philadelphia, PA: Elsevier; 2017:37-38.