Vital capacity of lungs (VC)

This is a value, representing lung ability to receive blood circulation minute volume, which defines alveolar surface area, participating in respiration.

Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume.

A person's vital capacity can be measured by a wet or regular spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease. Furthermore, the vital capacity is used to determine the severity of respiratory muscle involvement in neuromuscular disease, and can guide treatment decisions in Guillain–Barré syndrome and myasthenic crisis.

A normal adult has a vital capacity between 3 and 5 litres. A human's vital capacity depends on age, sex, height, mass, and ethnicity.

Lung volumes and lung capacities refer to the volume of air associated with different phases of the respiratory cycle. Lung volumes are directly measured, whereas lung capacities are inferred from volumes.

Role in Diagnosis

The vital capacity can be used to help differentiate causes of lung disease. In restrictive lung disease, the vital capacity is decreased. In obstructive lung disease, it is usually normal or only slightly decreased.

Vital capacity increases with height and decreases with age. Formulas to estimate vital capacity are

where vc is approximate vital capacity in cm3, a is age in years, and h - is height in cm.

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, body composition^[1] 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 technique.

Image: Lungs in situ - anterior view ^[2]

• Tidal Volume (TV)

It is the amount of air that can be inhaled or exhaled during one respiratory cycle^[3]. This depicts the functions of the respiratory centres, respiratory muscles and the mechanics of the lung and chest wall^[4].

The normal adult value is 10% of vital capacity (VC), approximately 300-500ml (6‐8 ml/kg)^[4]; but can increase up to 50% of VC on exercise^[5]

• Inspiratory Reserve Volume (IRV)

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 1900-3300ml.

• Expiratory Reserve Volume (ERV)

It is the volume of air that can be exhaled forcibly after exhalation of normal tidal volume. The normal adult value is 700-1200ml. ERV is reduced with obesity, ascites or after upper abdominal surgery^[4]

• Residual Volume (RV)

It is the volume of air remaining in the lungs after maximal exhalation. 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.

In obstructive lung diseases with features of incomplete emptying of the lungs and air trapping, RV may be significantly high. 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. ^[6]

• 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 or sum of all volume compartments or volume of air in lungs after maximum inspiration. The normal value is about 6,000mL (4‐6 L). TLC is calculated by summation of the four primary lung volumes (TV, IRV, ERV, RV).

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^[7].

• Vital Capacity(VC)

It is the total amount of air exhaled after maximal inhalation. The 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.

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^[8]. VC is sometimes reduced in obstructive disorders and always in restrictive disorders^[8]

• 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. The normal value is about 1800 – 2200 mL. FRC = RV+ERV.

FRC does not rely on effort and highlights the resting position when inner and outer elastic recoils are balanced. FRC is reduced in restrictive disorders. The ratio of FRC to TLC is an index of hyperinflation^[9]. In COPD, FRC is upto 80% of TLC^[4].

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https://en.wikipedia.org/wiki/Vital_capacity