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. The ratio of FRC to TLC is an index of hyperinflation. FRC = RV+ERV.įRC does not rely on effort and highlights the resting position when inner and outer elastic recoils are balanced. The normal value is about 1800 – 2200 mL. It is calculated by adding together residual and expiratory reserve volumes. It is the amount of air remaining in the lungs at the end of a normal exhalation. VC is sometimes reduced in obstructive disorders and always in restrictive disorders VC indicates ability to breathe deeply and cough, reflecting inspiratory and expiratory muscle strength.VC should be 3 times greater than TV for effective cough. It is calculated by summing tidal volume, inspiratory reserve volume, and expiratory reserve volume. The value is about 4800mL and it varies according to age and body size. It is the total amount of air exhaled after maximal inhalation. TLC may be increased in patients with obstructive defects such as emphysema and decreased in patients with restrictive abnormalities including chest wall abnormalities and kyphoscoliosis. TLC is calculated by summation of the four primary lung volumes (TV, IRV, ERV, RV). The normal value is about 6,000mL(4‐6 L). It is the maximum volume of air the lungs can accommodate or sum of all volume compartments or volume of air in lungs after maximum inspiration. It is calculated from the sum of inspiratory reserve volume and tidal volume. It is the maximum volume of air that can be inhaled following a resting state. The RV can also be expressed as a percentage of total lung capacity and values in excess of 140% significantly increase the risks of barotrauma, pneumothorax, infection and reduced venous return due to high intra thoracic pressures as noticed in patients with high RV who require surgery and mechanical ventilation thus needs high peri-operative inflation pressures. In obstructive lung diseases with features of incomplete emptying of the lungs and air trapping, RV may be significantly high. Normal adult value is averaged at 1200ml(20‐25 ml/kg) .It is indirectly measured from summation of FRC and ERV and cannot be measured by spirometry. It is the volume of air remaining in the lungs after maximal exhalation. ERV is reduced with obesity, ascites or after upper abdominal surgery It is the volume of air that can be exhaled forcibly after exhalation of normal tidal volume. It is the amount of air that can be forcibly inhaled after a normal tidal volume.IRV is usually kept in reserve, but is used during deep breathing. The normal adult value is 10% of vital capacity (VC), approximately 300-500ml (6‐8 ml/kg) but can increase up to 50% of VC on exercise This depicts the functions of the respiratory centres, respiratory muscles and the mechanics of the lung and chest wall. It is the amount of air that can be inhaled or exhaled during one respiratory cycle. Image: Lungs in situ - anterior view Lung Volumes However, measurement of Residual volume, Functional residual capacity, and Total lung capacity is through body plethysmography, nitrogen washout and helium dilution technique. A number of the lung volumes can be measured by Spirometry- Tidal volume, Inspiratory reserve volume, and Expiratory reserve volume. These volumes tend to vary, depending on the depth of respiration, ethnicity, gender, age, body composition and in certain respiratory diseases. Lung volumes measurement is an integral part of pulmonary function test. The average total lung capacity of an adult human male is about 6 litres of air. Lung capacities are derived from a summation of different lung volumes. It refers to the volume of gas in the lungs at a given time during the respiratory cycle. Lung volumes are also known as respiratory volumes.
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