The residual volume functions to keep the alveoli open even after maximum expiration. In healthy lungs, the air that makes up the residual volume is utilized for continual gas exchange to occur between breaths. Inspiration draws atmospheric oxygen into the lungs to replenish the oxygen-depleted residual air for gas exchange in the alveoli.

Although breathing mechanics are complex, it is important to remember that air will flow from high-pressure areas to low-pressure areas. During tidal breathing, the inspiration and expiration at physiologic rest, the volume of air entering and leaving the lungs is known as the tidal volume (TV). On tidal inspiration, inspiratory muscle contraction increases the volume of the chest causing the intrapleural pressure (Ppl) to drop from -5 cm H2O to -8 cm H2O. The decreased Ppl causes the alveolar pressure (Palv) to decrease 1 cm H2O below atmospheric pressure. As a result, air from the relatively high-pressure atmosphere flows into the low-pressure alveoli. Inspiration is an active process requiring the rhythmic contraction of inspiratory muscles that work to expand the chest cavity. Tidal expiration is a passive process that works in reverse. The inspiratory muscles relax, decreasing the size of the chest cavity, and increasing Ppl and Palv. Once Palv is greater than atmospheric pressure, air flows out of the lungs. 

Residual volume can be understood by investigation of breathing that exceeds tidal volumes. Following maximal inspiration, the volume of air that leaves the lungs during a maximal force expiration is known as the vital capacity (VC). VC is composed of the tidal volume, expiratory reserve volume (ERV), and inspiratory reserve volume (IRV). The ERV is the volume of air that can be forcefully exhaled after a normal resting expiration, leaving only the RV in the lungs. Forcefully exhaling the ERV is an active process requiring the contraction of expiratory muscles in the chest and abdomen. This increases Ppl and Palv above atmospheric pressure. Due to the elastic recoil of the alveoli, the pressure inside of the alveoli remains higher than that of the pleura, and the alveoli remain open. The pressure inside the airways (Paw) slowly decreases as you move up from the alveoli to the trachea as a result of increased airway resistance. In sections of small, non-cartilaginous airways, pleural pressure is greater than airway pressure and causes a collapse of the airway (Figure 1B). The air that remains in the lungs after the collapse of all small airways is the residual volume.

There are no methods to measure residual volume directly. Other lung volumes and capacities must first be measured directly before RV can be calculated. The first step in calculating RV is to determine the FRC. Measurement of the FRC can be done using one of the following three tests.

Helium Dilution Test

In this test, the patient inhales a known volume of air (V1) containing a known fraction of helium (FHe1) at end-expiration of tidal breathing, where the volume of air left in the lungs is equal to FRC. A spirometer measures the fraction of helium after equilibration in the lungs (FHe2).

• FRC = V1(FHe1-FHe2) / FHe2

Nitrogen Washout

The nitrogen washout test utilizes the nitrogen that makes up 78% of atmospheric air. A patient breathes through a 2-way valve connected to 100% oxygen on inspiration and a collection spirometer on expiration. The spirometer measures the volume of air and fraction of nitrogen expired with each breath. Once the fraction of nitrogen is below 1.5% for 3 consecutive breaths, the test is complete. The initial amount of nitrogen in the lungs must be equal to the total amount of nitrogen exhaled, and thus the FRC can be calculated.

• FRC = V exhaled * C exhaled N2 / C initial alveolar N2

Body Plethysmography

Plethysmography is based on Boyle’s law of gases. In a closed system at a constant temperature, the product of pressure and volume of a known mass of gas is constant. That is to say, pressure and volume are inversely proportional.

• P1V1 = P2V2

To conduct the test, a patient is placed inside an enclosed chamber and breathes through a spirometer that can measure changes in pressure and volume. After a period of tidal breathing, the spirometer is closed at end-expiration, and the patient breathes against it. Changes in pressure at the mouthpiece are recorded. As the patient exhales, the volume of the thoracic cavity can be calculated by recording the change in pressure of the entire chamber. This test is the most accurate measure of FRC, but also the most expensive.

Once the FRC has been measured using one of these three methods, the expiratory reserve volume (ERV) and vital capacity (VC) are measured using standard spirometry. Calculations of TV and TLC can be made using the measured FRC, ERV, and VC values and the following equations:

• RV = FRC - ERV

• TLC = VC + RV

Obstructive Lung Disease (OLD)

Obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD), asthma, and bronchiectasis, are characterized by airway inflammation, easily collapsible airways, expiratory flow obstruction, and air trapping. In obstructive lung disease, inflammation and decreased elastic recoil cause increase airway resistance and lead to earlier small airway closure during expiration. The premature airway closure increases the volume of air retained in the lungs at the end of expiration; this is referred to as air trapping. This trapped air results in pulmonary hyperinflation. Therefore patients with obstructive lung disease have elevated TLC, FRC, and RV (Figure 1C). [4][5][6]

Body plethysmography yields a higher FRC in patients with obstructive lung disease than those measured by gas dilution techniques because it includes both well-ventilated and poorly ventilated areas of the lung. RV is generally the first volume to increase in obstructive lung disease and can be a good measure to evaluate early disease states.

The RV/TLC ratio is used as a measure of resting pulmonary hyperinflation in patients with COPD. In a study by Shin et al., and elevated RC/TLC ratio was shown to be a significant risk factor for all-cause mortality in COPD patients. [7]

Restrictive Lung Disease (RLD)

Restrictive lung diseases are a result of processes that restrict pulmonary expansion. The restriction can be related to intrinsic diseases such as pulmonary fibrosis and sarcoidosis, or extrinsic processes like kyphosis and obesity. In either case, the result is restricted expansion, decreased lung volumes, and inadequate ventilation. TLC, FRC, and RV will all be decreased in restrictive lung disease.

The effects of obesity on lung function are a growing concern as the prevalence and severity of obesity increase. Studies have shown that increasing body mass index (BMI) correlates with lower VC, TLC, and RV, but that these values remain within normal limits. Significant decreases in FRC and ERV are seen as BMI increases to the extent that FRC approaches RV. [8][9][10]       

Drowning

An interesting clinical use for residual volume is applied during post-mortem autopsies of drowning victims. The residual volume of air in the lungs can only be removed if it is replaced by another substance. In the case of drowning victims, water will replace the residual air in the lungs. During autopsies, medical examiners can clamp the trachea and submerge the lungs in water. If the lungs sink, no residual air remains, so it is likely the person drowned after inhaling large amounts of water. However, if the lungs float, the residual volume of air remains in the lungs. The residual volume was not replaced by water, so it is likely the person died before entering the water.