Abstract: A device (10) for detecting a substance in a patient's expired breath (88) such as adenosine indicative of cardiovascular distress. The device (10) shines an infrared light (68) through a chamber (14) containing the breath (88) to a detector (26). The detector (26) measures the intensity of the light (68) at selected frequency corresponding to absorption bands of the substance. Multiple photodetectors (112, 114, 116, 118) at different absorption band frequencies increase the specificity of detection. A reference chamber (16) containing a reference gas and coupled to an infrared photodetector (118) may be used to provide a baseline reference for a quantitative analysis.