- Clinical assessment of lung function is aided by the determination of several parameters derived from usage of a spirometer. This device allows for accurate measurement of the rate and volume of air flowing into and out of a patient's lungs during particular maneuvers. "Lung Volumes" are those parameters which can be measured directly by usage of the spirometer whereas "Lung Capacities" are theoretical values determined from spirometer data.
- Tidal Volume
- The Tidal Volume is the volume of air breathed in and out by a person during normal breathing at rest.
- Inspiratory Reserve Volume (IRV)
- The Inspiratory Reserve Volume (IRV) is the volume of air beyond the tidal volume that an individual can breathe in when asked to breathe in to his or her maximum capacity.
- Expiratory Reserve Volume (ERV)
- The Expiratory Reserve Volume (ERV) is the volume of air beyond the Tidal Volume that a person can breathe out when asked to breathe out to his or her maximum capacity.
- Residual Volume
- The Residual Volume is the volume of gas remaining in the lung after a person has breathed out to his or her maximum capacity. Naturally, the residual volume cannot be determined directly using spirometry since it is not possible to measure the remaining air volume after maximum expiration. Consequently, more advanced techniques must be used to determine this parameter.
- Forced Expiratory Volume (FEV)
- The Forced Expiratory Volume (FEV) is the volume of air that can be forcibly exhaled in one second. The ratio of FEV to the Forced Vital Capacity (FVC), known as the (FEV/FVC), is used diagnostically to distinguish Obstructive Lung Diseases and Restrictive Lung Diseases. In a healthy lung the FEV/FVC is about about 0.8 (i.e. 80% of the total FVC is forcibly exhaled in the first second). However, in those with obstructive lung disease the FEV/FVC ratio is reduced meaning that less of the total FVC is forcibly exhaled in the first second than in a normal lung. In those with restrictive lung disease the FEV/FVC ratio is increased meaning that more of the total FVC is forcibly exhaled in the first second than in a normal lung.
|The above graph shows the volume of air inspired and expired by an individual over time using a spirometer. After a period of initial quite breathing the individual takes a maximal inspiration and a maximal expiration. Lung volumes can be calculated from the graph above.|
- Vital Capacity
- The Vital Capacity is the volume of air a person can breathe in and out when breathing in and out to his or her maximum capacity.
- Derivation: Vital Capacity = Inspiratory Reserve Volume + Tidal Volume + Expiratory Reserve Volume
- Forced Vital Capacity
- The Forced Vital Capacity (FVC) is the amount of air that can forcibly be exhaled after a maximal inspiration. In reality, this should be the same as the vital capacity but is termed separately to indicate the 'forced' nature of the exhalation. The length of time it takes for an individual to breathe out to FVC is a useful diagnostic tool and is used together with the Forced Expiratory Volume to distinguish Restrictive Lung Diseases from Obstructive Lung Diseases.
- Functional Residual Capacity
- The Functional Residual Capacity (FRC) is the volume of air remaining in the lung when a person breathes out after a normal breath
- Derivation: Functional Residual Capacity = Expiratory Reserve Volume + Residual Volume
- Total Lung Capacity
- Total Lung Capacity (TLC) corresponds to the total volume of air that the lung can contain. The TLC cannot be measured directly using spirometry as it includes the air remaining in the lung after maximal expiration (i.e. the Residual Volume).
- Derivation: Total Lung Capacity = Vital Capacity + Residual Volume