The invention provides a method for obtaining and evaluating spirographic and gas exchange data, involving measuring a concentration of a breathing gas gc(Vx) for a plurality of volumes Vx of breathing gas mix, wherein the measuring is performed on inhaled breathing gas mix, further involving determining an apparatus dead space ApDS, of breathing gas mix in the apparatus not taking part in gas exchange, by evaluating the measured concentrations gc(Vx) as a function of the volume V of the inhaled breathing gas mix. The use of inhaled breathing gas mix allows to determine the apparatus dead space, which is an important parameter when evaluating ventilation efficiency and the like. Furthermore, the invention provides a method in which inhalation and exhalation parts of a spirogram are synchronized, by matching features thereof, in order to further improve the accuracy of gas exchange volumes and derived data. Finally, the invention provides a spirographic device and a patient ventilation apparatus, incorporating the present method.