(EN) The invention relates to a device for determining the partial pressure of at least one gas in an end expiratory gas mixture, comprising a receiving unit for a volume of end expiratory gas, at least one measuring device to determine the volume of an end expiratory gas received therein and to output measuring signals, in addition to display and/or storage and evaluation unit to receive and process the signals of the at least one measuring device. The invention also relates to a method for monitoring fluctuations in respiratory metabolism of the human or animal body and to the use thereof in determining respiratory function.