A piezopolymer pressure sensor has been developed for service in a portable fetal heart rate monitor, which will permit an expectant mother to perform the fetal nonstress test, a standard predelivery test, in her home. Several sensors are mounted in an array on a belt worn by the mother. The sensor design conforms to the distinctive features of the fetal heart tone, namely, the acoustic signature, frequency spectrum, signal amplitude, and localization. The components of a sensor serve to fulfill five functions: signal detection, acceleration cancellation, acoustical isolation, electrical shielding, and electrical isolation of the mother. A theoretical analysis of the sensor response yields a numerical value for the sensor sensitivity, which is compared to experiment in an in vitro sensor calibration. Finally, an in vivo test on patients within the last six weeks of term reveals that nonstress test recordings from the acoustic monitor compare well with those obtained from conventional ultrasound.
A least-mean-square (lms) linear prediction algorithm has been developed to accomplish detection of fetal heart tones and thereby derive heart rate from a raw signal generated by a previously described passive acoustical sensor array. The desired heart tone signal has a characteristic signature but is of extremely low amplitude and contaminated with noise consisting of large-amplitude maternal heart tones, abdominal sounds, body motion, and environmental sounds, and also mild 60 Hz. The predictor coefficients were derived by adaptively “training” on ideal fetal heart tones recorded from several patients. The lms algorithm detects a heart tone event when the predictor mean-square error falls below an adaptively updated threshold level. The algorithm contains logic for correction of spurious and missed heart tones. A real-time working system was fabricated consisting of a sensor belt, front end electronics, a TMS320C25 digital signal processing board, an 80386 PC, and a strip chart recorder. The apparatus allows performance of the fetal nonstress test (NST) in a manner similar to that conventionally accomplished via ultrasound. The acoustical system was implemented in parallel with a commercial ultrasound unit on a series of patients undergoing NSTs. The heart rate records are compared.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.