The single-breath diffusing capacity of the lung (DLCO) is a commonly performed pulmonary function test. This standardized method for DLCO measurement is based on a full inspiration of a DLCO test gas mix followed by a breath hold phase and a full expiration. The DLCO test gas mix normally contains 10% helium (He) and 0.3% carbon monoxide (CO). Alveolar volume and CO diffusion are determined by measuring the reduction of He and CO concentrations from inspiration to expiration. Using the volume of inspiration, volume of expiration, gas concentrations of in- and expiration phases, breath hold time and environmental conditions the DLCO value can be computed (see [5]). The method and the device of the present invention measure the He concentration with a main-stream molar mass sensor, or with a side-stream molar mass sensor. The CO concentration is measured with an independent CO sensor in the side-stream. The supply with DLCO test gas to the patient is solved with a small valve system directly mounted to the flow sensor. The valve system can easily be cleaned by exchanging the parts that are exposed to the expiratory gas.
