Functional Residual Capacity and COPD

Doctor listening to woman's cough with stethoscope
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Functional residual capacity (FRC) refers to the volume of air left in the lungs after a normal, passive exhalation. It is used to evaluate the elasticity of the lungs and chest wall in persons with respiratory illnesses like chronic obstructive pulmonary disease (COPD).

Why Functional Residual Capacity Is Important

When you exhale, you don't expel all of the air from your lungs. Some will remain after exhalation in order to maintain elasticity of the organ.

Think of it like a balloon which it easier to fill if it's already half inflated. The same principle applies to the lungs. The retained air (known as functional residual volume) allows the lungs to fill with less effort while keeping the elastic forces of inhalation and exhalation in balance. Without this balance, the interchange between oxygen and carbon dioxide in our alveoli would vary, sometimes significantly.

Purpose of Functional Reserve Capacity

The FRC measures the point in which the inward forces of the lung compete with the tendency of the chest wall to move outward.

If the inhalation and exhalation are in balance, there is no competition and respiration is considered normal. On the other hand, if they aren't in balance, our ability to either absorb oxygen molecules in our blood or remove carbon dioxide from our blood is compromised.

FRC is just one test a doctor will use to evaluate your COPD. The others include a forced expiratory reserve volume (FEV), which measures how much air you can forcefully exhale in one second, and forced vital capacity (FVC), which measures the total volume of air forcefully expelled from the lungs.

FRC may, in some ways, be a better measure of what is really happening in your lungs since most breaths are passively rather than forcefully exhaled.

The Connection Between COPD and Functional Residual Capacity

COPD is characterized by a loss of elastic recoil of the lungs. This alters the balance in the way we breathe and leads to an increased FRC (hyperinflation).

Hyperinflation, in turn, leads to a condition we call dyspnea, or shortness of breath. When this happens, you have to breathe faster in order to get enough air into your lungs. It limits your ability to exercise or do strenuous work because there is simply not enough oxygen being inhaled to service your muscle, heart, and brain cells.

The FRC can also change as a result of other conditions beyond COPD. It can decrease when there is persistent pressure on the diaphragm, as can happen during pregnancy when your liver or spleen is enlarged, or if there is an accumulated fluid in the abdomen (ascites) caused by cirrhosis or liver cancer. By contrast, it can increase in the presence of a severe airway obstruction as seen in people with emphysema.

How FRC Informs COPD Treatment

COPD causes an overall physical de-conditioning that impacts both muscle strength and the elasticity of the chest wall. These deficits only exacerbate the symptoms of COPD and are the main reason why physical conditioning is so vital to people living with the disease.

Based on your physical condition and the severity of your symptoms, your doctor may prescribe a course of physical therapy in association with a fitness program tailored to your limitations. In this instance, an FRC would be used to monitor progress.

In severe cases where exercise is not possible, positive end-expiratory pressure (PEEP) (a non-invasive form of ventilation) can be used to aid respiration.

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