Residual volume (RV) is the volume of air that remains in the lungs after maximum forceful expiration. In other words, it is the volume of air that cannot be expelled from the lungs. This volume remains unchanged regardless of the lung volume at which expiration was started. Reference values for residual volume are considered to be 1 to 1.2 L, but these values are dependent on many 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. FRC is the amount of air remaining in the lungs after a normal, physiologic expiration (Figure 1A). The TLC, FRC, and RV are absolute lung volumes and cannot be measured directly with spirometry. Instead, they must be calculated using techniques such as gas dilution or body plethysmography. Measuring the residual volume can give an indication of lung physiology and pathology.
The residual volume functions to keep the alveoli open even after maximum expiration. In healthy lungs, the air that makes up the residual volume allows for continual gas exchange to occur between breaths. The oxygen-depleted residual air is then mixed with newly inhaled air to improve gas exchange at the alveoli.
The mechanics of breathing may seem complicated, but it is important to remember that air will flow from higher pressure to lower pressure. 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 the pleural pressure (Ppl) drops from -5 cmHO to -8 cmHO, leading the alveolar pressure (Palv) to decrease 1 cmHO below atmospheric pressure. As a result, air flows into the alveoli. This 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, the air flows out of the lungs.
To understand residual volume, however, it is important to look at breathing above the tidal limits, that is, breathing at maximal inspiration and expiration. During this type of breathing, the volume of air entering and leaving the lungs 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 increasing 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 ways to measure the residual volume of a patient directly. As such, other lung volumes and capacities must be measured first, and then the 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).
The nitrogen washout test utilizes the fact that the air we breathe is roughly 80% nitrogen. 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.
Plethysmography is based on Boyle’s Law of gases. In a closed system at 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.
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. Using the measured FRC, ERV and VC we can calculate the RV and TLC with the simple equations below.
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 OLD, inflammation and decreased elastic recoil increase airway resistance and lead to earlier small airway closure during expiration. That is, the pleural pressure exceeds the airway pressure earlier, trapping air in the lungs. This trapped air results in pulmonary hyperinflation. Therefore patients with OLD have increase TLC, FRC and RV (Figure 1C).
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 OLD and can be a good measure to determine 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.
Restrictive Lung Disease (RLD)
Restrictive lung diseases are a result of processes that restrict pulmonary expansion. These can be an intrinsic disease such as pulmonary fibrosis and sarcoidosis, or extrinsic processes like kyphosis and morbid obesity. In either case, the result is a restricted expansion, decreased lung volumes, and inadequate ventilation. In the case of RLD, the TLC, FRC, and RV will all be decreased.
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 that point that FRC approaches RV. 
An interesting clinical use for residual volume is applied during post-mortem autopsies of drowning victims. The only way that the residual volume of air in the lungs can be removed is to be replaced. In the case of drowning victims, water will replace residual air in the lungs. During autopsies, medical examiners can clamp the trachea and submerge the lungs in water. If the lungs sink, there is no residual air, and it is likely this person drowned after inhaling large amounts of water. If the lungs float, the residual volume of air remains in the lungs, and it is likely this person died before entering the water.
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