ABSTRACT

This work presents closed loop, bidirectional telemetry hardware with a telecommunication system to carry data and images for remote control and monitoring of a chronically implanted device. While the uplink of the signals from the implanted unit to the external receiver was achieved via electromagnetic waves, infra-red signals were utilized for downlinking to the implanted device. Telemetry in both directions was capable of maintaining a range more than two and a half meters, although the design was not optimized for data rate. The video and data links with the remote site, which was 15 km away from the implant site, were established using intranet connections utilizing peer-to-peer communications.

INTRODUCTION

As implantable therapeutic devices such as pacemakers become more sophisticated, researchers have the option of utilizing them to measure physiologic parameters around the clock or to evaluate the therapy that is being delivered. Internal memory of the implanted device can be used for data storage, and reprogramming can be used to alter the therapy. However, limitations on the available memory and the need for frequent memory dumps and reprogramming reduce the practicality of this option. If the goal is to collect massive amounts of data or try new algorithms which are not optimized for the constraints of an implanted device, the only viable option is to have a bidirectional telemetry link between the implanted device and external resources.

During chronic animal studies, it is desirable to have sufficient telemetry range to cover the entire space of the cage. In a canine experiment, this could mean a floor space of approximately 1.2 x 1.8 meters. To prevent access by the animal to the external telemetry equipment, the telemetry range should be at least 2.5 meters.

If the site where the animal is located is not within easy reach of the researchers, then an internet or intranet link can be utilized to provide the communication between the investigators and the cage. A computer at either end of this link provides the interface between the researcher and the telemetry equipment. A block diagram of the system is shown in Figure 1.

Figure 1. System block diagram.

UPLINK

Frequency modulated (FM) electromagnetic waves were utilized for the telemetry uplink. A carrier frequency around 90ÿMHz was chosen to take advantage of commercially available FM receivers. Using an XFM-7 microminiature transmitter (Biotelemetrics, Inc., Boca Raton, FL), analog signals with bandwidths from DC to 3 KHz could be uplinked. A transmitter antenna was tunneled subcutaneously. It was found that this antenna had to be at least 10 cm, preferably 20 cm long for reliable signal reception. For this feasibility study, ventricular electrogram (EGM) signals were uplinked through the transmitter. A block diagram of the uplink device can be seen in Figure 2.

Figure 2. Uplink device block diagram.

DOWNLINK

Pulsed infra-red (IR) signals were used for downlink to the implanted device. IR was chosen since it is relatively harmless compared to ultraviolet rays, and penetrates well to a depth of 1 cm of tissue. A wide band IR source acted as the external transmitter. The implanted receiver was a 16ÿmmÿxÿ50ÿmm solar cell configured in its photo-electric mode. The cell was covered with Kodak gelatin filter No. 87C to prevent light in the visible spectrum from reaching the sensor, then hermetically sealed in Hysol casting compound prior to implant. A diagram of the custom IR downlink sensor is seen in Figure 3. IR signals received by the sensor were interpreted by an implanted pacemaker to change the pacing rate.

Figure 3. IR downlink sensor.

INTRANET LINK

The bidirectional link was established over an intranet, which is similar to the internet in principle, but access to it is limited to clients within an organization. Data packets consisting of headers and binary data were sent peer-to-peer, allowing up to eight analog channels of signals to be received at 1,000 samples/second each. Up to five eavesdroppers can monitor the experiment simultaneously. The same link also provided control of the IR source, hence closing the control loop. A video camera was placed over the animal cage and a "frame grabber" sent the images over the intranet to investigators at remote sites. This option enhanced the monitoring capabilities and provided explanations for aberrant data, as in the case of loss of contact while the animal was out of range during cage cleaning.

SUMMARY

This study demonstrated the feasibility of an implanted telemetry system with closed loop control. The long range was obtained using a mixture of electro-magnetic and optical signals as carriers. Furthermore, effective utilization of internet technology was demonstrated for data, command and image transmission to and from remote sites.

ACKNOWLEDGEMENTS

Special thanks to Pete Klager, Jim Kmiecik, Nels Nerison, Mike DeFranco, Paul Hsiao and Wes White for their work on the hardware and software developed specifically for this project.