Towards multi-patient leadless and wireless cardiotocography via RF telemetry

During labour the condition of the fetus is monitored by a cardiotocograph which displays fetal heart rate and a measure of uterine contractions. Ultrasound and tocodynamometer transducers are placed on the mother's abdomen and connected to a bedside monitor and display unit via a cable. This paper describes a prototype wireless system aimed at demonstrating the potential elimination of the cable which is undesirable in a number of respects. The radio link utilised operates at a frequency of 418 MHz with data compression and time division multiplexing (TDM) employed to allow up to 10 units to operate simultaneously on the same frequency in the same area. Data compression is achieved by extracting the Doppler ultrasound signal envelope and representing the frequency content of the signal using a zero crossing counting technique. Two Medium Access Control (MAC) protocols were considered, with a synchronised time division multiplexing system shown to offer greater throughput and resistance to interference than Carrier Sense Multiple Access (CSMA). This wireless RF telemetry system could be readily adapted for other multi-patient monitoring applications.