Functional Residual Capacity specifically measures the volume of air remaining in the lungs after normal expiration. It is known as one of the capacities due to the equation: FRC = RV + ERV. Functional Residual Capacity = Residual Volume + Expiratory Reserve Volume.
Measuring FRC via body piethysmography is measuring the volume of the lungs by pressure change. This can be measured when a patient sits in an airtight box and begins to breathe normal. Then a shutter drops across the patient's breathing valve causing the chest volume to expand and decompress the air in the lungs. The volume in the box is reduced due to the increased chest volume. FRC is measured through this method and additionally measures other conducting pathways such as abdominal gas.
Helium is a gas that is lighter than air; it is non-toxic, tasteless, odorless and colorless. Patients can breathe from a volume of helium gas for a time of four to seven minutes. For patients with COPD, the oxygen concentration in the mixture of helium is set to 30 percent so they feel comfortable during the test. According to Morgan Scientific, "A carbon dioxide absorber is situated in line with expired breath to keep the closed-circuit CO2 level below 0.5 percent and avoid discomfort and hyperventilation."
After oxygen is added and seven minutes has passed by, the FRC is specifically measured with this equation: FRC = (percent helium initial - percent helium final) / percent helium final x system volume.
Nitrogen recovery is a gas dilution technique that can measure the lungs' volume. Nitrogen has an infinite time to reach an airway since it is present in the lungs at all times. While performing the Carbon Monoxide Diffusion Capacity, a patient exhales all the way out and then inhales diffusion gas until they are full. Once this has happened, the new nitrogen from the DLCO mixture immediately mixes with the nitrogen that was in the residual volume and measures the TLC.