A measurement device and a method for analyzing a sample gas by infrared absorption spectroscopy are described. The measurement device comprises a narrowband laser having a line width of less than 0.2 cm−1 and being smaller than a width of an infrared absorption line to be measured of a sample gas. The measurement device is suited and can be arranged to measure the respiratory gas of a human or animal as sample gas, wherein the respiratory gas exchanges in the measurement chamber only by the respiration of the human or animal, and the respiratory resistance of the measurement device is less than 60 mbar.