From the Guidelines
The functional residual capacity (FRC) is crucial for maintaining lung function and preventing complications, as decreased FRC can lead to atelectasis, increased heterogeneity of lung expansion, and cyclic lung overstress, ultimately resulting in postoperative pulmonary complications 1. The physiological significance of FRC can be understood by its role in maintaining open airways and preventing alveolar collapse during normal breathing, ensuring continuous gas exchange even when we're not actively inhaling. Key aspects of FRC include:
- Serving as a crucial oxygen reservoir between breaths
- Maintaining lung compliance by keeping alveoli partially inflated
- Providing time for proper gas mixing and diffusion
- Helping to stabilize blood oxygen levels by providing a buffer against fluctuations in breathing patterns As noted in a study on lung-protective ventilation for surgical patients, intraoperative atelectasis results in decreased FRC, which can persist for weeks after operation and is associated with increased postoperative pulmonary risk 1. Factors that can affect FRC levels include:
- Obesity
- Pregnancy
- Supine positioning
- Respiratory diseases like COPD, which often increases FRC due to air trapping
- Restrictive lung diseases, which typically reduce FRC Understanding FRC is essential for proper ventilator management in critical care settings, as maintaining appropriate FRC levels helps prevent atelectasis and optimize oxygenation, ultimately reducing morbidity, mortality, and improving quality of life 1.
From the Research
Physiological Significance of Functional Residual Capacity (FRC)
- FRC is a critical parameter in respiratory physiology, representing the volume of air present in the lungs at the end of a passive expiration 2, 3, 4, 5, 6
- It plays a significant role in maintaining oxygenation and preventing lung injury in patients with acute respiratory distress syndrome (ARDS) 2, 3, 4, 5
- FRC is correlated with trans-pulmonary pressure, and both parameters have predictive value for 28-day mortality in ARDS patients 2
- Measurements of FRC can guide optimal positive end-expiratory pressure (PEEP) settings, improving lung recruitment and oxygenation 2, 3, 4, 5
- FRC is a sensitive indicator of PEEP-induced aeration and recruitment of lung tissue, and may be a useful adjunct to PaO2 monitoring 4
- Changes in FRC can reflect the degree of lung aeration and consolidation, making it a valuable parameter for monitoring lung injury 4, 6
Clinical Applications of FRC Measurements
- FRC measurements can be used to optimize PEEP settings and improve lung recruitment in mechanically ventilated patients 2, 3, 4, 5
- FRC can be used to monitor alveolar derecruitment and recruitment, and to guide ventilatory strategies 6
- Measurements of FRC can provide important information on the pulmonary situation in critically ill patients, particularly when combined with other parameters such as arterial oxygenation and respiratory compliance 6
- FRC may be useful in guiding the weaning process from mechanical ventilation, as well as in monitoring the effects of prone positioning and endotracheal suctioning 6