The Volume of Air Moved with Maximum Exhalation After Deep Inhalation
The volume of air moved with maximum exhalation after deep inhalation measures vital capacity (VC).
Understanding Vital Capacity
Vital capacity (VC) is defined as the volume change at the mouth between the position of full inspiration and complete expiration, expressed in liters at BTPS (body temperature, ambient pressure, saturated with water vapor) 1. This measurement represents the maximum amount of air a person can expel from their lungs after taking the deepest possible breath.
The vital capacity can be measured in two ways:
- Expiratory vital capacity (EVC): The maximal volume of air exhaled from the point of maximal inhalation 1
- Inspiratory vital capacity (IVC): The maximal volume of air inhaled from the point of maximal exhalation 1
Differentiating Vital Capacity from Other Lung Volumes
To understand why the answer is vital capacity, it's important to distinguish it from other lung volume measurements:
Inspiratory reserve volume: The additional volume of air that can be inhaled after a normal tidal inspiration (not the complete volume moved during maximum exhalation after deep inhalation)
Tidal volume: The volume of air moved during normal quiet breathing (much smaller than vital capacity)
Residual volume: The volume of air remaining in the lungs after maximal exhalation (cannot be measured by spirometry alone)
Total lung volume: The total amount of air in the lungs after maximal inspiration (equals vital capacity plus residual volume)
Measurement Technique
The vital capacity maneuver is performed with the subject using a mouthpiece and wearing a nose clip. The procedure involves:
- The subject exhales completely to residual volume (RV)
- Then inhales to total lung capacity (TLC)
- Finally exhales completely to RV again 1
This maneuver is performed in a relaxed manner, except near end-inspiration and end-expiration where extra effort is required to reach maximum volumes 1.
Clinical Significance
Vital capacity measurements are commonly used for:
- Diagnosis of restrictive lung diseases
- Assessment of respiratory muscle strength
- Monitoring disease progression in neuromuscular disorders
- Evaluating response to therapeutic interventions
Variations in vital capacity can be observed in relation to height, age, gender, and ethnicity 2. For instance, taller individuals typically have higher vital capacities due to increased lung surface area 2.
Technical Considerations
For accurate measurement of vital capacity, the spirometer must comply with specific requirements:
- Capability to accumulate volume for at least 30 seconds
- Ability to display both inspiratory and expiratory maneuvers
- Provision of a display showing the entire recorded VC maneuver 1
The European Respiratory Society recommends that proper quality control measures be implemented to ensure accurate measurements, including appropriate back extrapolated volume (BEV) calculations and proper patient instruction 3.