![]() ![]() As the subject breathes, exhaled gas is collected until the concentration of nitrogen reaches a plateau. For the nitrogen washout technique, the test subject inhales 100% oxygen beginning at FRC. Exhaled air contains a lower concentration of oxygen, usually 14 to 16%, plus 3 to 5% carbon dioxide and water. The air that we breathe consists of approximately 21% oxygen, 1% argon, 0.04% carbon dioxide, and a variable amount of water vapor. This method underestimates lung volumes when portions of the lung communicate poorly with the central airways, particularly in patients with emphysematous bullae. The equation can be solved for lung volume. The final concentration of helium equals the initial helium concentration times the initial volume of the device divided by the final volume of the lungs plus the device, adjusting for oxygen consumption and carbon dioxide production during the test. Lung volumes also can be measured by having the patient rebreathe from a device containing a known volume and concentration of an inert gas (e.g., helium, neon, argon, or methane), which does not react with elements in the blood or tissues, until equilibrium is achieved. A plethysmographic total lung capacity greater than 150% of the reference value should be viewed with suspicion. Although body plethysmography is generally the most accurate method for measurement of lung volumes, particularly in patients with airway obstruction, it can overestimate lung volumes if panting is too rapid. A similar panting maneuver with the shutter open is used to calculate airway resistance. The body plethysmograph, a sealed box in which the patient sits, measures the changes in lung volume during panting pressure measured at the mouth represents the pressure changes within the lung during these volume changes. The process of measuring lung volume by plethysmography consists of panting against a closed shutter to compress and to rarify gas in the chest. Body Plethysmographyīody plethysmography, the preferred method for measuring lung volumes, is based on Boyle’s law: at a given temperature, the product of the pressure and volume of a quantity of gas at one time will be equal to the product of the pressure and volume of the gas at another time (P1 × V1 = P2 × V2). Measurement of residual volume or any of the capacities that include it, so-called absolute lung volumes, requires more sophisticated methods, such as body plethysmography, the inert gas dilution technique, or the nitrogen washout technique. ![]() Three of the volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume) can be measured with a spirometer. Total lung capacity is the sum of residual volume plus expiratory reserve volume plus tidal volume plus inspiratory reserve volume. Vital capacity is the sum of tidal volume plus inspiratory reserve volume plus expiratory reserve volume. Inspiratory capacity is the sum of tidal volume plus inspiratory reserve volume. FRC is the sum of expiratory reserve volume plus residual volume. The volume that can be inhaled above tidal volume is the inspiratory reserve volume.Ī series of capacities consist of the sum of two or more different volumes. The volume exhaled in a normal breath is the tidal volume. The difference between FRC and residual volume is the expiratory reserve volume. The amount of air in the lungs at the relaxation point, when muscle effort is minimized and the inward recoil of the lung is balanced by the outward recoil of the chest wall, is the functional residual capacity (FRC). The air that remains in the lung after a maximal expiratory effort is the residual volume. The volume of air in the lung at any given time can be partitioned ( Fig. Lee Goldman MD, in Goldman-Cecil Medicine, 2020 Lung Volumes ![]()
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