Medical Engineering & Physics 20 (1998) 764–772 Towards multi-patient leadless and wireless cardiotocography via RF telemetry J. Taylor, B.R. Hayes-Gill * , J.A. Crowe, C.J. Paull Department of Electrical and Electronic Engineering, University of Nottingham, Nottingham, NG7 2RD, UK Received 11 December 1997; received in revised form 9 November 1998; accepted 9 November 1998 Abstract 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 elimin- ation 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.  1999 IPEM. Published by Elsevier Science Ltd. All rights reserved. Keywords: Telemetry; Radio frequency; Cardiotocography; Fetal monitoring 1. Introduction 1.1. The use of Doppler ultrasound for fetal monitoring During labour, the condition of the fetus is monitored by a cardiotocograph which displays fetal heart rate alongside a measure of uterine contractions. The heart rate is determined from either the fetal ECG or Doppler ultrasound. Ultrasound monitoring is the most com- monly used of these since it is non-invasive and also has the advantage that it can be employed antenatally. The contractions are usually obtained using an external toco- dynamometer (a strain gauge based device, commonly known as a ‘toco’). In use, the ultrasound and toco transducers are placed on the mother’s abdomen, fixed firmly in place by belts and connected via wires to the cardiotocograph at the bedside. The ultrasound transducer emits a beam of * Corresponding author. Tel.: 0115 951 5600; fax: 0115 951 5616; e-mail: barrie.hayes-gill@nottingham.ac.uk 1350-4533/99/$19.00  1999 IPEM. Published by Elsevier Science Ltd. All rights reserved. PII:S1350-4533(98)00086-1 ultrasound upon which moving objects such as the fetal heart impose a Doppler shift. The Doppler shift lies in the audible range (approximately 100 Hz to 700 Hz) and is therefore known as the Doppler audio signal. This gives the obstetrician or midwife useful audible feedback to help ensure that the transducer is correctly positioned to receive the strongest possible signal from the fetal heart. The fetal heart rate is calculated by performing a correlation process on the Doppler audio envelope. The cardiotocograph displays the heart rate and produces a print-out of heart rate and uterine contractions. This print-out, known as the cardiotocogram, forms an essen- tial part of fetal monitoring during labour [1]. 1.2. The desire for wireless transducers There are several advantages that wireless transducers offer in all areas of patient monitoring. One objection to fetal monitoring is the mother’s dislike of being ‘tied’ to a machine. The word ‘tied’ is chosen deliberately to highlight the difference in perception between the clini- cal view of a harmless non-invasive connection to a monitor and the mother’s view of an unnatural and