Abstract: The invention involves a device and method for ascertaining the functioning of the respiratory system and determining whether a pulmonary embolism is present. The device comprises an apparatus containing sensors for measuring the oxygen and carbon dioxide concentrations as well as the volume of air inhaled and exhaled by a patient. From this data, a processor computes the ratio of carbon dioxide to oxygen for the volume of expired air and displays the results on a screen. By comparing the results to predetermined normal values, an accurate determination can be made regarding the presence of a pulmonary embolism.