![]() ![]() A similar panting maneuver with the shutter open is used to calculate airway resistance. 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 plethysmographic total lung capacity greater than 150% of the reference value should be viewed with suspicion. 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. 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. The equation can be solved for lung volume. This method underestimates lung volumes when portions of the lung communicate poorly with the central airways, particularly in patients with emphysematous bullae. The air that we breathe consists of approximately 21% oxygen, 1% argon, 0.04% carbon dioxide, and a variable amount of water vapor. Exhaled air contains a lower concentration of oxygen, usually 14 to 16%, plus 3 to 5% carbon dioxide and water. For the nitrogen washout technique, the test subject inhales 100% oxygen beginning at FRC. ![]() As the subject breathes, exhaled gas is collected until the concentration of nitrogen reaches a plateau. ![]()
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