Leadless transducers for cardiotocographs

Patient objection to monitoring during labour has led to a desire for leadless transducers for cardiotocographs (CTGs). This paper describes how radio telemetry has been used to convert an existing Doppler ultrasound based CTG into a leadless version. The use of radio has necessitated compression of the transmitted Doppler audio signal and a medium access control protocol in order to accommodate multiple CTGs. The encoder/decoder and protocol controller have been implemented on an 8 bit microcontroller. The protocol has a capacity for at least 12 CTGs with each transducer having a reliable range of around 20 m and a battery life of around 14 hours. Results show no noticeable degradation in CTG performance.