Fiber Bragg Grating Probe for Relative Humidity and Respiratory Frequency Estimation: Assessment during Mechanical Ventilation

Fiber Bragg grating (FBG) sensors have gained popularity in medicine for some valuable features, such as small size, immunity to electromagnetic interferences, and good metrological properties. Among several applications, this technology has been used in the analysis of gases in mechanical ventilation. In this field, in particular during invasive ventilation, the gas delivered by the ventilator must be warmed and humidified in order to reach the patients with optimal conditions (100% of relative humidity and 37 °C). The most popular device used to accomplish this task is the heated wire humidifiers (HWHs). Unfortunately, their performances are influenced by ventilatory settings (e.g., respiratory frequency) and environmental conditions. The aim of this paper is to fabricate a probe based on an FBG sensor able to monitor both the gas relative humidity and the respiratory frequency. This information can be used as a feedback to improve the performance of the HWHs. The probe consists of a needle, which houses an FBG sensor coated by hygroscopic material (i.e., agar). This solution allows an easy insertion of the probe within the ducts connecting the ventilator to the patient. The proposed system has been assessed during mechanical ventilation at different respiratory frequencies. Future testing will be focused on the development of a system able to monitor other parameters that influence the HWHs performances (e.g., gas temperature and minute volume).