Functional residual capacity

Lung Volumes

Functional Residual Capacity (FRC) refers to the volume of air remaining in the lungs, specifically within the parenchyma tissues, after a passive exhalation. It represents a critical physiological parameter in understanding lung function and evaluating potential respiratory ailments.

Definition

At the FRC stage:

• The elastic recoil forces exerted by both the lungs and chest wall counterbalance each other.
• There is no active engagement from the diaphragm or any other respiratory muscles.

Composition and Measurement

FRC consists of two primary volumes:

• Expiratory Reserve Volume (ERV): The extra amount of air that can be expired from the lungs through forced breathing out after regular exhalation.
• Residual Volume (RV): The amount of air that remains in the lungs even after the most forceful expiration.

In an average 80 kg male, FRC is typically around 2400 mL. It is worth noting that:

• FRC cannot be estimated via spirometry since it includes the RV.
• Accurate assessment of RV necessitates specialized techniques such as nitrogen washout, helium dilution, or body plethysmography.

Clinical Significance

Deviations in FRC, be it elevated or diminished, can be indicative of respiratory disorders:

• Emphysema: In this condition, the lungs become more compliant. Hence, they are more influenced by the outward recoil forces from the chest wall. Consequently, patients with emphysema often exhibit broader chest profiles because they breathe at greater volumes.
• Body Posture & FRC: In healthy individuals, the FRC value varies with changes in body posture. For example, obese individuals typically manifest a reduced FRC when lying supine.

Measurement Techniques

Two predominant methods for FRC measurement include:

• Helium Dilution Technique: This method involves the patient breathing in a known volume of helium. The concentration of helium is then measured after equilibration, allowing for the estimation of lung volumes.
• Pulmonary Plethysmograph: A body box method where changes in pressure and volume are monitored to calculate lung volumes.

Both these techniques enable medical practitioners to gauge lung functionality, facilitating the diagnosis of an array of respiratory conditions. An accurate evaluation of FRC is pivotal in discerning the extent of a patient's respiratory ailment and in determining suitable therapeutic interventions.

See Also

• Spirometry
• Lung Volumes
• Respiratory System
• Emphysema