Biotelemetry Using Implanted Unit To Monitor Preterm Labor

Pressure changes are telemetered to the outside and analyzed to detect intrauterine contractions.

A biotelemetric system for monitoring key physiological parameters of a fetus and its uterine environment is undergoing development. The main purpose of the monitoring is to detect preterm labor in order to enable timely treatment. At the present stage of development, the system monitors pressure changes and temperature. The pressure changes serve as direct indications of intrauterine contractions that could be associated with the onset of preterm labor. Future versions of the system are expected to monitor additional parameters, including pH and the heart rate of the fetus.

The system (see Figure 1) includes a transmitting unit that contains a thermistor (the temperature sensor) and a piezoresistive transducer (the pressure sensor), a receiver, data-acquisition subsystem, and a digital signal-processing subsystem. The fully developed transmitting unit is projected to be small enough that it could be introduced into the uterine cavity through a 10-mm trocar during endoscopic fetal surgery; the surgical procedure for implanting this transmitter would be less invasive than are the hysterotomies performed to implant the transmitting units of telemetric systems developed previously for the same purpose.

The transmitter generates a pulsed signal at a carrier frequency between 174 and 214 MHz. The temperature and pressure information are conveyed by pulse-interval modulation (PIM): Pulses are transmitted in pairs at a pulse-pair-repetition frequency of about 1 to 2 Hz. The interval between the two pulses in each pair is proportional to the sensed pressure, while the interval between pairs is proportional to the departure of the sensed temperature from a predetermined nominal value. The low data rate is sufficient for monitoring intrauterine contractions, which typically occur over several minutes. The transmission range is 3 to 10 ft (1 to 3 m), depending on the position of the transmitter in the body.

Transmitter power is supplied by two silver oxide batteries; at an average power consumption <40 µW, the operational lifetime ranges from 4 to 6 months. The fully developed transmitting unit would be electronically identical to a larger prototype that has already been constructed (see Figure 2), but would be miniaturized by following the chip-on-board approach, in which unpackaged integrated-circuit chips are flip-chip bonded directly onto a printed-circuit board along with other components. The transmitter is encapsulated in biocompatible silicone rubber.

The receiver converts the PIM radio-frequency signal into a digital pulse stream, which is then decoded to obtain voltages proportional to the temperature and pressure readings. These voltages are digitized in the data-acquisition subsystem, which is a Personal Computer Memory Card Association (PCMIA) circuit card in a laptop computer. The digital data are then processed in the digital signal-processing system, which is the remainder of the laptop computer.

The processing is done by a LabVIEW® program that displays and stores the pressure and temperature data as functions of time, performs peak detection, and determines the frequency of contractions. The program integrates the pressure over time to obtain an index of the amount of preterm labor. A unique feature of the program is the application of statistical and frequency-analysis functions to the pressure data to help a pediatric surgeon detect preterm labor. Among other things, the program displays the frequency spectrum of intrauterine contractions. Another unique feature is a contraction-detecting algorithm.

This work was done by John W. Hines of Ames Research Center and Christopher J. Somps, Robert D. Ricks, and Carsten W. Mundt of Sverdrup Technology, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category. Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Ames Research Center; (650) 604-5104. Refer to ARC-14280.