Respiratory monitoring during mechanical ventilation is constrained by procedure complexity, operator dependency, and delayed response. This study aimed to develop an intraoperative lung monitoring system that calculates respiratory rate (RR) from lung sounds acquired via an esophageal stethoscope, providing a simple alternative to conventional monitoring techniques. An acoustic signal processing algorithm was developed to analyze lung sounds and detect RR in real time. The system integrated a microphone sensor into a standard esophageal stethoscope and applied preprocessing, dual-threshold peak detection, signal-to-noise ratio filtering, and RR confirmation logic. To evaluate the system under clinical conditions, three experiments were conducted: to assess measurement accuracy by comparing RR measurements with reference values independently derived from the CO2 waveform, to compare measurement error by analyzing error metrics in subgroups stratified by ventilation mode, and to measure detection latency relative to a conventional capnography monitor. Lung sound signals were recorded from 37 adults under general anesthesia. The acoustic algorithm showed higher measurement accuracy, with a mean difference of 0.00 ± 0.11 compared to 0.04 ± 0.34 for capnography. Measurement error metrics were similar between volume-controlled and pressure-controlled ventilation modes (mean difference = 0.00 ± 0.12 and 0.00 ± 0.08, respectively). The acoustic system detected RR changes more rapidly (9.76 ± 2.78 s) than capnography (48.12 ± 5.64 s), reflecting significantly shorter detection latency. The proposed system demonstrated higher measurement accuracy than conventional capnography, exhibited similar measurement errors in both ventilation modes, and achieved significantly shorter detection latency. These results support its feasibility for real-time intraoperative respiratory rate monitoring.
Keywords: Acoustic signal processing; Esophageal stethoscope; Intraoperative lung monitoring; Lung sound analysis; Respiratory rate monitoring.
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