Residual Volume (RV) is the air remaining in the lungs following a maximal, forceful exhalation. This volume cannot be expelled because it keeps the delicate air sacs, the alveoli, from collapsing, maintaining continuous gas exchange. Unlike other lung volumes, RV cannot be measured directly using standard spirometry, which only measures the air that moves in and out of the lungs. Specialized, indirect techniques are necessary to determine this trapped volume and fully assess an individual’s total lung capacity (TLC). Measuring RV is essential for comprehensively evaluating respiratory function and diagnosing pulmonary conditions.
The Clinical Necessity of Measuring Residual Volume
Measuring Residual Volume is a foundational step in pulmonary function testing because it provides information that simple spirometry cannot. RV is an absolute lung volume and a component of both the Total Lung Capacity (TLC) and the Functional Residual Capacity (FRC). FRC is the volume of air left in the lungs after a normal, relaxed exhalation, while RV is the air remaining after a maximal exhalation.
An accurate measurement of RV is necessary to calculate the Total Lung Capacity, the sum of all lung volumes. The calculated TLC helps differentiate between the two major categories of lung disease: obstructive and restrictive. For instance, obstructive diseases like emphysema often increase RV, elevating the TLC (hyperinflation). Conversely, restrictive diseases, such as pulmonary fibrosis, usually reduce TLC.
Determining Residual Volume Through Gas Dilution
The inert gas dilution technique, typically using helium (He), is one established method for indirectly measuring lung volume. This method relies on the principle of conservation of mass. The patient breathes a known volume of gas from a spirometer containing a specific concentration of helium, which is used because it is inert and not absorbed into the bloodstream.
The patient breathes normally within this closed circuit until the helium concentration equilibrates between the spirometer and the air volume in the lungs. This mixing dilutes the initial helium concentration, and the final, lower concentration is measured. Technicians use the initial and final concentrations and volumes to calculate the Functional Residual Capacity (FRC).
Residual Volume is then determined by subtracting the Expiratory Reserve Volume (ERV)—measured separately via spirometry—from the calculated FRC. A significant limitation is that this method only measures lung volume that communicates with the airways. If a patient has severe airway obstruction, trapped air will not mix with the helium, leading to an underestimation of the true RV.
Determining Residual Volume Through Body Plethysmography
Body plethysmography, often called the “body box,” is an alternative method considered the most accurate way to measure total gas volume in the chest. This technique is based on Boyle’s Law: for a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional. The test involves the patient sitting inside a small, airtight chamber.
The patient breathes through a mouthpiece, and a shutter is closed, temporarily occluding the airway. When the patient attempts to inhale against the closed shutter, the chest wall expands, causing the gas in the lungs to decompress. This results in a measurable drop in pressure at the mouth and an increase in pressure inside the sealed box. These corresponding pressure changes are used in the Boyle’s Law calculation.
This calculation determines the Thoracic Gas Volume (TGV), which is equivalent to the Functional Residual Capacity (FRC) when measured after a normal exhalation. The major advantage is that plethysmography measures all gas within the chest cavity, including air trapped behind obstructed airways. For patients with significant air trapping, this measurement is more accurate than the gas dilution result. As with the helium method, RV is calculated by subtracting the Expiratory Reserve Volume from the measured FRC.
Clinical Significance of High and Low Residual Volume
Measured Residual Volume is interpreted by comparing it to predicted normal values based on a patient’s age, sex, and height. An abnormally high RV often indicates an obstructive lung disease, such as emphysema or severe asthma. This elevation occurs because airways collapse prematurely during exhalation, trapping a greater volume of air than normal. This air trapping is a hallmark of pulmonary hyperinflation, which impairs the efficiency of subsequent breaths.
Conversely, an abnormally low RV is observed in certain restrictive conditions. In these cases, the lungs or chest wall may be stiff or restricted, as seen in pulmonary fibrosis or some neuromuscular diseases. A low RV suggests a reduced overall lung volume, contributing to the diagnosis of a restrictive ventilatory defect.