Functional residual capacity (FRC)

Functional residual capacity (FRC) is a lung volume after expiration. The more lung volume is in the expiration phase, the more pulmonary residual volume and the worse the functional state of the lungs will be.

Functional Residual Capacity (FRC) = Expiratory Reserve Volume (ERV ~1500-2000cm³) + Residual Volume (RV ~800-1000cm³)

Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.

 FRC is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV) and measures approximately 2400 mL in an 80 kg, average-sized male. It cannot be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.

A Reduced or elevated FRC is often an indication of some form of respiratory disease. For instance, in emphysema, FRC is increased, because the lungs are more compliant and the equilibrium between the inward recoil of the lungs and outward recoil of the chest wall is disturbed. As such, patients with emphysema often have noticeably broader chests due to the relatively unopposed outward recoil of the chest wall. Total lung capacity also increases, largely as a result of increased functional residual capacity. In healthy humans, FRC changes with body posture. Obese patients will have a lower FRC in the supine position due to the added tissue weight opposing the outward recoil of the chest wall.

The helium dilution technique and pulmonary plethysmograph are two common ways of measuring the functional residual capacity of the lungs.

The predicted value of FRC was measured for large populations and published in several references. FRC was found to vary by a patient's age, height, and sex. An online calculator exists that will calculate FRC for a patient using these references.

The FRC is:

The volume of gas present in the lung at end-expiration during tidal breathing

Composed of ERV and RV

This is usually 30-35 ml/kg, or 2100-2400ml in a normal-sized person

It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure.

The measurement of FRC is an important starting point for the measurement of other lung volumes

The FRC is important because:

At FRC, the small airway resistance is low.

At FRC, lung compliance is maximal

FRC maintains a oxygen reserve which maintains oxygenation between breaths

At FRC, pulmonary vascular resistance is minimal

Where closing capacity is greater than the FRC, gas trapping and atelectasis can develop

The FRC is affected by:

Factors which influence lung size (height and gender)

Factors which influence lung and chest wall compliance (emphysema, ARDS, PEEP or auto-PEEP , open chest, increased intraabdominal pressure, pregnancy, obesity, anaesthesia and paralysis)

Posture (FRC is lower in the supine position)

If the FRC decreases (say, by 1000ml) the consequences are:

Decreased lung compliance

Increased airway resistance

Increased work of breathing

Decreased tidal volume and increased respiratory rate

Decreased oxygen reserves

Increased atelectasis

Increased shunt

Increased pulmonary vascular resistance

Increased right ventricular afterload

Residual volume (RV)

Residual volume (RV) is the volume of air remaining in the lungs after maximum forceful expiration. In other words, it is the volume of air that cannot be expelled from the lungs, thus causing the alveoli to remain open at all times. The residual volume remains unchanged regardless of the lung volume at which expiration was started. Reference values for residual volume are 1 to 1.2 L, but these values are dependent on factors including age, gender, height, weight, and physical activity levels.

The residual volume is an important component of the total lung capacity (TLC) and the functional residual capacity (FRC). TLC is the total volume of the lungs at maximal inspiration which is about 6 L on average, though true values are dependent on the same factors that affect residual volume. FRC is the amount of air remaining in the lungs after a normal, physiologic expiration. The TLC, FRC, and RV are absolute lung volumes and cannot be measured directly with spirometry. Instead, they must be calculated using indirect measurement techniques such as gas dilution or body plethysmography. Calculating the residual volume can give an indication of lung physiology and pathology.

https://pubmed.ncbi.nlm.nih.gov/29763183/#:~:text=Functional%20residual%20capacity%20(FRC)%2C,expansion%20are%20balanced%20and%20equal.

https://pubmed.ncbi.nlm.nih.gov/29630222/

https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20054/functional-residual-capacity