Introduction:
An average adult breathes approximately 12 to 15 times per minute under normal physiologic conditions. A single breath is one complete respiratory cycle consisting of one inspiration and one expiration. Lung volumes are an important aspect of pulmonary function tests as they provide information about the physical conditions of the lungs. The summation of two or more lung volumes gives the lung capacity. Spirometry assesses lung function by measuring the volume of air the patient can expel from the lungs after maximal inspiration. It is a simple, safe, and non-invasive method that measures lung function with a graphical display.
What Are the types of lung volumes?
Four types of lung volumes:
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Tidal Volume (TV):
It is the volume of air inhaled or exhaled in one breath. It is approximately 500 ml.
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Inspiratory Reserve Volume (IRV):
It is the amount of air over the tidal volume that is inhaled with a maximum inspiratory effort at the end-inspiratory tidal position. It is approximately 3000 ml. It is usually kept in reserve but used during deep breathing.
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Expiratory Reserve Volume (ERV):
The air exhaled with maximum expiratory effort. It is approximately 1100/1200 ml. It is usually reduced with obesity, ascites, and upper abdominal surgery.
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Residual Volume (RV):
The volume of air remaining in the lungs at the end of forceful or complete expiration. It is approximately 1100/1200 ml. It cannot be measured by spirometry. It indirectly measures the FRC and ERV.
Summation of the specific lung volume produces the following lung capacities:
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Total Lung Capacity (TLC):
It is the maximum amount of air that can fill the lungs. (TLC = TV + IRV + ERV + TV). It is approximately 6000 ml.
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Vital Capacity ( VC):
It is the air exhaled with maximum effort after maximum inspiration. (VC = IRV + ERV + TV). It is approximately 4800 ml.
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Inspiratory Capacity (IC):
It is the maximum amount of air that can be inspired. It is approximately 3600 ml.
(IC = IRV + TV).
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Functional Residual Capacity (FRC):
The air remaining in the lungs after a normal expiration. It is approximately 2400 ml. (FRC = ERV + RV). It cannot be measured by spirometry.
What Are the Factors Affecting Lung Volume and Capacity?
Factors affecting lung volume and capacity include
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Age.
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Sex.
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Height.
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Weight.
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Race.
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Certain respiratory diseases.
Age: With aging, forced vital capacity (FVC) and forced expiratory volume at one second (FEV1) decrease, whereas lung volume and capacities such as RV and FRC increase.
Gender: Usually, males have a longer airway than females, thereby increasing the specific resistance in the respiratory tract. The increased work of breathing to increased ventilation in females confers a higher oxygen consumption in females than in males.
Height: All the lung capacities, such as TLC, FVC, RV, VC, and FEV1, are equally affected by the individual's height since they are directly proportional to the body size.
Weight: FRC and ERV decrease with an increase in weight, so much so that in morbidly obese patients, the tidal volume becomes equal to the residual volume.
What Is the Significance of Vital Capacity?
It indicates the ability to breathe deeply and cough and directly reflects the strength of the inspiratory and expiratory muscles.
Physiological Factors:
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Gender: It is usually more in males than females.
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Age: It is usually decreased in old age.
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Position: It is decreased in a supine position as there is an increased venous return to the heart and increased in an erect position.
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Pregnancy: It is decreased in cases of pregnancy.
Pathological Factors:
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Ascites: Itis decreased as the diaphragm is pulled downwards.
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Pulmonary Diseases: It is reduced in cases of pulmonary fibrosis, edema, pneumonia, chronic bronchitis, asthma, and emphysema.
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Space Occupying Lesion: It is decreased in case of a tumor, pleural or pericardial effusion, and kyphoscoliosis.
What Is the Significance of Functional Residual Capacity?
The functional residual capacity helps to maintain the exchange of gases even at the end of expiration and is therefore known to be a buffer. It is also known as the relaxation volume because at FRC, the elastic recoil of the lungs is equal to that of the chest wall. It minimizes the work of breathing and pulmonary vascular resistance. It also reduces ventilation-perfusion mismatch.
Factors affecting the functional residual capacity:
Increased FRC:
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Height.
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Erect position.
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Decreased lung recoil as in emphysema.
Decreased FRC:
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Obesity.
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Muscle paralysis.
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Supine position.
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Restrictive lung disease such as pulmonary fibrosis.
FRC is independent of age.
How Are Lung Volume and Capacity Measured?
Spirometers available in the market can help in checking the lung function. All the lung volumes and capacities can be measured by spirometer except
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Residual volume.
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Total lung capacity.
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Functional residual capacity.
These can be calculated by body plethysmography, nitrogen washout, and helium (He) dilution method.
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Body Plethysmography: It is an instrument used to measure the changes in the lung volume within an organ or entire body. The word plethysmos is derived from a Latin word meaning enlargement. The patient sits in an airtight plastic chamber. At the end of a normal exhalation , the shuttle of the mouthpiece is closed, and the patient is asked to make respiratory efforts. As the patient inhales, gas volume in the lungs expands, so the lung volume also increases. The pressure in the chamber rises, whereas the volume is decreased. This method is based on Boyle's law, where at a constant temperature, P x V = constant, P = pressure, and V = volume. It is indicated in
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To monitor chronic lung diseases like chronic obstructive lung disease (COPD), cystic fibrosis, and asthma.
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To detect the early changes in lung function.
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To assess the environmental impact, such as exposure to noxious agents at home as well as workplace.
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To compare the pulmonary function with known standards that reflect how the lung works.
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Nitrogen Washout: after a maximal expiration (RV) or normal expiration (FRC), the patient inspires 100 % oxygen and then expires into a spirometer free of nitrogen. Over the next few minutes, approximately six to seven minutes till all the nitrogen is washed out from the lungs. The nitrogen concentration of the spirometer is calculated, followed by the total volume of air exhaled. As air has 80 % nitrogen, actual FRC / RV is calculated. The only disadvantage of this method is that it takes a lot of time for washout and does not estimate the volume of poorly ventilated regions of the lungs.
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Helium Dilution Method: it is a closed-circuit technique in which a patient breathes in and out of the spirometer filled with 10 % of helium and 90 % of oxygen till the concentration in the spirometer and lungs equalizes. As helium is insoluble in blood and there is no loss of helium, it can be calculated by C1 x V1 = C2 x(V1 + V2); therefore, V2 = V1(C1-C2)/ C2, where V1 = volume of spirometer V2 = functional residual capacity C1 = concentration of helium in the spirometer before equilibrium. C2 = concentration of helium in the spirometer after equilibrium.
Conclusion:
Lung volume and capacity provide information about the physical condition of the lungs. It measures how quickly an individual can move air in and out of the lungs. Lung volume and capacity may be affected by various factors such as age, gender, height and weight, and certain respiratory diseases. However, all the lung volume and capacity can be measured by spirometer except residual volume, functional residual capacity, and total lung capacity.
