Lung Volumes

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Description

Lung volumes are also known as respiratory volumes. It refers to the volume of gas in the lungs at a given time during the respiratory cycle. Lung capacities are derived from a summation of different lung volumes. The average total lung capacity of an adult human male is about 6 litres of air. Lung volumes measurement is an integral part of pulmonary function test. These volumes tend to vary, depending on the depth of respiration, ethnicity, gender, age and in certain respiratory diseases. A number of the lung volumes can be measured by Spirometry- Tidal volume, Inspiratory reserve volume, and Expiratory reserve volume. However, measurement of Residual volume, Functional residual capacity, and Total lung capacity is through body plethysmography, nitrogen washout and helium dilution.

Lung Volumes

• Tidal Volume(TV)

It is the amount of air that can be inhaled and exhaled during normal quiet breathing. Normal adult value is 5ooml.

• Inspiratory Reserve Volume(IRV)

It is the maximum amount of air that can be inhaled after a normal tidal volume. Normal adult value is 1900-3300ml.

• Expiratory Reserve Volume(ERV)

It is the maximum amount of air that can be exhaled after exhalation of normal tidal volume. Normal adult value is 700-1200ml.

• Residual Volume(RV)

It is the amount of air remaining in the lungs after maximal exhalation. Normal adult value is averaged at 1200ml.

Lung capacities

• Inspiratory capacity(IC)

It is the maximum volume of air that can be inhaled following a resting state. It is calculated from the sum of inspiratory reserve volume and tidal volume. IC = IRV+TV

• Total Lung Capacity(TLC)

it is the maximum volume of air the lungs can accommodate. Value is about 6,000mL. It is calculated by summing tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume. TLC = TV+IRV+ERV+RV

• Vital Capacity(VC)

It is the total amount of air exhaled after maximal inhalation. Value is about 4800mL and it varies according to age and body size. It is calculated by summing tidal volume, inspiratory reserve volume, and expiratory reserve volume. VC = TV+IRV+ERV

• Function Residual Capacity(FRC)

It is the amount of air remaining in the lungs at the end of a normal exhalation. It is calculated by adding together residual and expiratory reserve volumes. Value is about 1800 – 2200 mL. FRC = RV+ERV

Measurement of Lung Volumes

Measurements of lung volume are important for a correct physiological diagnosis, however, its role in the assessment of disease severity, functional disability, the course of disease and response to treatment remains controversial. Measurement can be done using Spirometry, Body plethysmography, Nitrogen washout and Helium dilution.

For more on Spirometry, see Incentive Spirometry

1. Body plethysmography: primarily measures FRC using Boyle's law. Body plethysmography is particularly appropriate for patients who have air spaces within the lung that do not communicate with the bronchial tree.
2. Nitrogen washout: This technique is based on washing out the N2 from the lungs, while the patient breathes 100% O2 using dilution properties of gases.
3. Helium dilution: The method for measuring lung volumes is based on the equilibration of gas in the lung with a known volume of gas containing helium.

Resources

European Respiratory Journal http://erj.ersjournals.com/content/26/3/511

References

The method for measuring lung volumes is based on the equilibration of gas in the lung with a known volume of gas containing helium