Fig 1. Schematic design of the N2MBW setup.
The figure illustrates the patient interface and measurement points of the current nitrogen multiple-breath washout (N2MBW) setup device (Exhalyzer D). Flow (and derived volume) are measured by a mainstream ultrasonic flowmeter. Gas concentrations are measured by the side-stream laser O2 sensor and the main-stream infra-red CO2 sensor. The N2 fraction is measured indirectly by FN2 = 1 –FO2 –FCO2 –FArgon. The gas sampling port divides pre- from post-capillary dead space. Star symbols give approximates of volume and thus delay times (off set) between gas and flow sampling points. The gas supply illustrates the open bypass; during the N2MBW the patient breathes 100% O2 through the mouthpiece.