What Determines Residual Volume?
Residual volume is determined by two competing factors.
The strength of the expiratory muscles and the inwards pull of the lungs tends to decrease residual volume. The outward pull of the chest wall tends to increase residual volume.
Why Do We Need Residual Volume?
The lungs are never completely empty; there is always some air left in the lungs after a maximal exhalation. The air that remains in the lungs is needed to help keep the lungs from collapsing.
If this residual volume did not exist and the lungs were completely empty, the tissues that make up the lung would could and stick together, making it almost impossible for us to re-inflate and force them open with effort.
Residual volume is necessary for breathing and proper lung function. This air that remains in the lungs is also important for preventing large fluctuations in respiratory gases—oxygen (O2) and carbon dioxide (CO2).
The residual volume is the only lung volume that cannot be measured directly because it is impossible to completely empty the lung of air. This volume can be calculated, rather than directly measured.
How Is Residual Volume Measured?
There is no way to measure residual volume directly, but it can be calculated indirectly with a pulmonary function test. Other lung volumes and capacities must first be measured directly, usually with spirometry, before RV can be calculated.
The first step in calculating RV is to determine the functional residual capacity (FRC). Measurement of the FRC can be done using one of the following three tests.
Helium Dilution Test
During a helium dilution test, you would breathe from a container containing a known fraction of helium gas. A spirometer measures the change in the concentration of the gases in the container. This test is conducted in a closed system circuit. It is used less frequently than body plethysmography.
Body Plethysmography
The body plethysmography test measures the total amount of air the lungs can hold. During this test, you would sit inside an airtight plethysmograph booth and breathe through a mouthpiece while pressure and airflow measurements are collected.
As you exhale, the volume of your thoracic cavity can be calculated by recording the change in pressure of the entire chamber. Body plethysmography is the most frequently used method for calculating FRC and residual volume and it yields the most accurate measurements—but it is also the most expensive.
Nitrogen Washout
Another closed-circuit test, a nitrogen washout, can also be used to determine FRC, but this method is rarely used.
Clinical Significance
Residual volume can be used to help diagnose or measure the severity of obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD), asthma, and bronchiectasis. These conditions are characterized by airway inflammation, collapsible airways, expiratory flow obstruction, and air trapping.
In obstructive lung disease, inflammation and decreased elastic recoil cause increased airway resistance and lead to premature small airway closure during expiration. As a result, excess air is trapped and left in the lung after you breathe, leading to an increase in residual volume.
On the other hand, restrictive lung diseases such as pulmonary fibrosis and sarcoidosis, or extrinsic processes like kyphosis and obesity, restrict lung expansion. As a result, less air is retained in the lungs after maximal expiration and therefore less residual air than normal is left in the lungs after expiration.
If you are having trouble breathing, see a healthcare professional promptly. You may be asked to undergo lung function testing that includes calculations of residual volume and other factors that impact lung function—such as total lung capacity, functional residual capacity, tidal volume, and vital capacity.
In acute, severe cases, difficulty breathing can be an emergency. Call 911 immediately, as some causes of shortness of breath may be life-threatening.
