Development of a photoacoustic sensor for monitoring propofol concentration in breath gas

Propofol is an intravenously administered drug used for general anesthesia. While its application has many advantages compared to volatile anesthetic agents, no real-time monitoring method exists yet for controlling the dosage, and hence patients face a considerable risk of over- or underdosage. With the overall objective of increasing patient safety, our team developed a sensor based on photoacoustic principles for the measurement of propofol concentration in the breath gas during anesthesia. In this paper we present the proposed detection method as well as solutions to a series of challenges associated with the in situ application of the sensor. Using a differential technique, our sensor evaluates the concentrations of both propofol and acetone from the measured photoacoustic signals. By applying mid-power UV LEDs as light sources and MEMS microphones as electroacoustic transducers, the sensor reaches a 1 𝜎 limit of detection of 0.7 ppb propofol in laboratory conditions. A frequency tracker is realized, enabling the detection of the two trace gases as the O2 and CO2 content of the breath gas varies. Using ion molecule reaction mass spectrometry as the reference method, the average root mean square concentration differences during surgeries are found as 2.7 ppb for propofol and 0.12 ppm for acetone in ten patients under clinical conditions.