Lung volume measurements provide an assessment of respiratory system function, offering insights into the mechanics of breathing and the elastic properties of the lungs and chest wall. The process of inhaling and exhaling is broken down into discrete components representing the amount of air held within the lungs during the respiratory cycle. The Expiratory Reserve Volume (ERV) is a key measurement used in a complete respiratory health evaluation, helping clinicians identify subtle changes in lung function.
Defining and Measuring Expiratory Reserve Volume
The Expiratory Reserve Volume (ERV) is defined as the maximum amount of air a person can forcefully exhale after completing a normal, quiet breath out. This volume represents a reserve of air not typically used during resting, tidal breathing, but accessed when greater expiratory effort is required. For an average healthy adult, this volume is estimated to be 1.0 to 1.2 liters in men and 0.7 to 0.9 liters in women, though these values are highly variable based on factors like age, height, and body composition.
ERV is measured using spirometry, a common non-invasive pulmonary function test. The patient breathes normally for a few cycles, and then, after a normal exhalation, pushes out all the remaining air they possibly can.
The forceful expulsion of the ERV is an active process relying on the contraction of expiratory muscles. The abdominal muscles and the internal intercostal muscles contract to forcefully reduce the volume of the thoracic cavity, driving this extra volume of air out.
Relationship to Other Lung Volumes
Expiratory Reserve Volume is one of four primary lung volumes that make up the total capacity of the lungs. These volumes combine to form various lung capacities, offering a broader view of respiratory function. The other primary volumes are Tidal Volume (TV), the air exchanged during normal breathing, and Inspiratory Reserve Volume (IRV), the air inhaled beyond a normal breath.
The ERV plays a direct role in defining two significant lung capacities.
Functional Residual Capacity (FRC)
ERV combines with the Residual Volume (RV) to form the Functional Residual Capacity (FRC). FRC is the volume of air remaining in the lungs at the end of a normal, passive exhalation, representing the equilibrium point between the lungs’ tendency to collapse and the chest wall’s tendency to expand.
Vital Capacity (VC)
ERV is a component of the Vital Capacity (VC), the total volume of air that can be forcibly exhaled after a maximal inhalation. Mathematically, VC is the sum of IRV, TV, and ERV (VC = IRV + TV + ERV).
Clinical Significance of ERV Changes
Changes in the Expiratory Reserve Volume indicate certain respiratory and systemic conditions. A reduced ERV is common in conditions that restrict the downward movement of the diaphragm or compress the lungs from outside the chest wall.
Impact of Obesity
The most common cause for a significant reduction in ERV, even in healthy lungs, is obesity. The accumulation of adipose tissue around the abdomen and chest mechanically impedes the diaphragm’s ability to descend during inspiration and the chest wall’s ability to recoil during expiration.
This external pressure causes the Functional Residual Capacity (FRC) to decrease. Since Residual Volume (RV) tends to remain stable or increase, the decrease in FRC is primarily reflected as a sharp drop in ERV. The reduction in ERV forces the individual to breathe at a lower lung volume, which can lead to the collapse of small airways, causing inefficient gas exchange and shortness of breath.
Other Restrictive Conditions
ERV is also frequently reduced in other conditions that affect the chest wall or diaphragm, such as advanced pregnancy, severe scoliosis, or neuromuscular diseases.
A decrease in ERV is a hallmark of a restrictive pattern caused by extrapulmonary factors. This contrasts with obstructive lung diseases like emphysema, which cause air trapping and an increase in residual volume. Measuring ERV provides clinicians with a specific tool to distinguish between mechanical restriction of the chest wall and internal airway obstruction.