Method and device for determination of the functional residual capacity (FRC)

A method for determination of the functional residual capacity (FRC) of lungs and lung volume by introduction of helium or another inert gas, using an oscillatory determination means, wherein a gas mixture containing helium or another inert gas is delivered to a patient or a subject in an open system via a respirator or an appliance supporting respiration, and measuring the concentration and quantity of this gas mixture containing helium or another inert gas by a measurement apparatus connected to a tube or to a mask, wherein the concentration or the density of the gas mixture in the lung volume, is determined by comparing the red values of the gas mixtures on inhalation and exhalation, wherein these steps are repeated until the difference between inhalation and exhalation gas concentration in one respiratory cycle falls below a preselected threshold value, or until the future changes in the gas concentrations can be predicted from the measurement of the gas concentrations in the inhalation and exhalation gas concentrations, wherein measuring the concentration is by an oscillatory pressure sensor, and the values measured by the pressure sensor are collected and processed in an algorithm.