Understanding Functional Residual Capacity (FRC)

Measuring FRC and Condtions Which Cause an Increase

What is functional residual capacity (FRC) and why is it important in COPD?. Istockphoto.com/Stock Photo©janulla

Definition: Functional Residual Capacity (FRC)

Functional residual capacity (FRC) refers to the volume of air left in the lungs after a normal, passive exhalation. It is mainly determined by the balance between the elastic forces of the lung and chest wall.

Purpose and Function of the Functional Residual Volume

You may wonder why some air is left in the lungs after exhalation.  In order to understand this, it can be helpful to think about blowing up a balloon.

  It is easier to fill the balloon if it is already slightly inflated.  With the lungs, the functional residual volume allows the lungs to be inflated again with less effort.

Without the residual volume, the amount of oxygen and carbon dioxide in the alveoli - the small units in the lungs where oxygen and carbon dioxide exchange takes place - would also vary significantly with each breath. So the functional reserve volume serves to keep some of these small units open.

This reserve volume is maintained by a balance of elastic forces.  On the lung side, there is the elastic recoil of the alveoli, the small sacs at the very end of the airways where air exchange.  Externally there is the elasticity of the chest wall - the skin, muscles, and connective tissue of the chest wall which move with each breath.

To get a picture and better visualize this process, FRC has been compared to the windbag of a bagpipe.

Determinants of Functional Reserve Capacity (FRC)

The FRC is a measure of the balance between the elastic forces of the lungs and the chest wall; the point in which the elastic forces inward of the lung balance the tendency of the chest wall to move outward. Expiration is a passive process caused by the passive recoil of the lungs.

  This recoil depends on elasticity.  You can picture this as a piece of elastic in a pair of boxer shorts.  During inhalation, the elastic is stretched.  Exhalation is like letting go after stretching the waistline of the shorts.  It goes back to where it began by itself - passively - but does not completely close up.  When you hold up a pair of these boxers off of your body, there is still a "waistline" so they can slip easily onto your body.

Measurement of Functional Reserve Capacity (FRC)

FRC is measured during normal breathing, also called "tidal breathing."  This is in contrast to tests such as spirometry that measure forced exhaling efforts such as forced vital capacity FVC) and forced expiratory reserve volume (FEV.)  In that sense, it is a better measure of what is really happening in your lungs, since most breaths are passively rather than forcefully exhaled.

Functional Residual Capacity in COPD

In chronic obstructive pulmonary disease, there is a loss of elastic recoil of the lungs.

  This changes the balance between the inward forces of the lungs relative to the outward forces of the chest wall, and leads to an increased functional residual capacity (hyperinflation.)

Physical deconditioning and muscle weakening, meanwhile, can result in less elasticity of the chest wall.  The combination adds further increases to an already increased functional residual capacity, and is one of the reasons that physical conditioning is so important for people with COPD.  It is also the reason that physical activity can lead to a rapidly increasing sense of shortness of breath.

Stated another way, hyperinflation alone can result in shortness of breath.  When an increased residual capacity is combined with a decrease in air flow during exercise this can worsen further.  Imagine blowing up a balloon and letting out some but not all of the air with each breath.  When this occurs in the lungs, it becomes more and more difficult to breathe in enough air above the residual volume to bring oxygen to the alveoli.

Functional Residual Capacity (FRC) Variations with Exercise  in COPD

As noted above, FRC increases further in those with COPD, which both increases dyspnea (the sensation of shortness of breath) and reduces the ability to exercise.

Functional Residual Capacity and Position

FRC can vary significantly with different position of the body.  FRC decreases when people lie on their backs due the pressure of the abdominal organs upward on the diaphragm.

Effect of Other Medical Conditions on FRC from the Chest Wall Side

FRC can also decrease when there is more pressure on the diaphragm from the abdomen, as in pregnancy, enlargement of the liver or spleen, or if there is a fluid accumulation in the abdomen (ascites) due to liver disease or cancer.

Conditions with an Increased Functional Residual Capacity (FRC) from the Lung Side

An increase in FRC is associated with severe airway obstruction or anything which reduces the elasticity of the lungs, such as the extreme airway obstruction in emphysema

Treatment

Treatment for a reduced FRC focuses on the underlying cause.  If the cause is related to chest wall forces - treatments such as treating ascites or position changes may help.

Most of the time a decrease in FRC is related to decreases in elastic recoil in the lungs.  Treatment will then be focused on treating the underlying condition.

Physical activity can improve FRC by improving chest wall elasticity.

When severe, positive end-expiratory pressure (PEEP) - non-invasive ventilation - can increase FRC.

Also Known As: FRC

Sources:

Gagnon, P., Guenette, J., Langer, D. et al. Pathogenesis of hyperinflation in chronic obstructive pulmonary disease. International Journal of Chronic Obstructive Pulmonary Disease. 2014. 9:187-201.

Rossi, A., Aisanov, Z., Avdeev, S. et al. Mechanisms, assessment, and therapeutic implications of lung hyperinflation in COPD. Respiratory Medicine. 2015. 109(7):785-802.

Thomas, M., Decramer, M., and D. O’Donnell. No room to breathe: the importance of lung hyperinflation in COPD. Primary Care Respiratory Journal. 2013. 22(1):101-11.

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