DIGITAL RECEIVER RESPONSE CONTROL FOR THE RECEPTION OF TELEMETRY SIGNALS FROM IMPLANTED MEDICAL DEVICES

Abstract: A gain control system for verifying the programming of an implanted medical device is provided for an external programming unit which receives transmitted signals from the implanted device and verifies if these signals represent valid "1" and "0" data bits. If the signals of an encoded message unit are valid signals then processing continues. If some of the signals are invalid due to noise interference, the gain of the receiver of the programming unit is decremented in steps as long as the error rate exceeds a predetermined number of errors per unit time.

Description: Implanted medical devices such as heart pacemakers are often programmed by the use of telemetric signals that are generated by a remote programming unit. Verification of the programming of the implanted device is provided by the transmission of signals from the device to a receiving section of the programming unit. The programming signals are digital signals which are coded in some manner to signify logic 1 and 0 signals. Our European Patent No. 107483 discloses a programmer for a pacemaker which includes a telemetry receiver to receive pulse interval modulated radio-frequency signals, wherein the intervals between the bursts of high frequency pulses are long or short depending on the logic level of the data bit being transmitted. The received signals are decoded and the validity of the pulse intervals is assessed. Signals from implanted devices in a hospital environment are often subject to a relatively high level of noise, or interference and it may happen that the level of such interference exceeds the threshold of the programmer receiver, resulting in erroneous reception of the data from the implanted device. According to the invention, there is provided a telemetry receiver for receiving telemetry from an implanted device of the type which transmits information in the form of series of timed r.f. signals, said information being conveyed by the timing of said r.f. signals, said receiver including amplifier means for applying gain to said r.f. signals, a threshold level detector which detects the presence of said r.f. signals as amplified by said amplifier means when said amplified r.f. signals exceed a threshold level, and means for verifying the accuracy of the timing of said series of signals, counting means arranged to count the number of erroneously transmitted signals within one said series of signals and control means responsive to said counting means and arranged to prevent further processing of said information in response to one or more erroneous bits being counted and to decrease the level of said amplified r.f. signals relative to said threshold level in response to a predetermined number of erroneous signals being counted. The control means may be arranged to adjust the threshold level but preferably it is arranged to adjust the gain of the amplifier means. As long as the signal level is greater than the noise level, the gain or threshold of the receiver may be adjusted so that the noise level will be less than a threshold level. The information content of the coded signal is then employed to adjust the gain or threshold of the receiver. In the preferred embodiment, successive adjustment continues until all of the bits of the encoded information are received without error, and then further processing of received data from the implanted device is allowed to proceed. Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings. Fig 1. is a diagrammatic illustration of an incoming signal containing both telemetry information and noise at different gain settings relative to a threshold level; Fig 2. is a block diagram of apparatus according to the present invention; and Fig. 3 is a flow chart diagram which illustrates the operation of the gain control portion of the programming device of the present invention. The digital gain control implementation of the present invention may be utiiized in a remote programming device for programming an implanted medical device such as a heart pacemaker in order to confirm that the program transmitted to the device has been implemented. In a hospital environment, the electrical noise level is often very high, and signals transmitted from the implanted device must be received and detected despite a high level of background noise. Fig. 1 shows a series of information-containing signals which are labeled "signal" which extend above the background noise level, which is labeled "noise". However, the noise level does initially extend above the threshold level of the receiver, and therefore, the noise will cause errors in the received signal. The implanted device continually transmits an encoded predetermined message to the remote programming device. The gain will be decremented in 10 ms steps as long as the noise remains above the threshold level. When the noise drops below the threshold level so that only the signal is above the threshold the gain is no longer decremented. Fig. 2 shows a portion of the receiving section of the programming device which illustrates that the incoming signal 10, which may contain both signal and noise, is received by the antenna 12 and coupled on the line 14 to an amplifier 16. A reference voltage is coupled on the terminal 34 to establish a threshold level. The output of the amplifier 16 is coupled to a demodulator 18 which forms the pulses 20 consisting of the envelopes of the burst of the high frequency incoming signals. The pulses 20 provide the digital encoded information that is received from the implanted device which can be distinguished from the noise when the gain has been adjusted so that the noise falls below the threshold level. The spacing of the pulses 20 defines whether a 1 or a 0 bit has been transmitted. Information can be readily detected once the noise is below the threshold level. This signal is coupled onto line 22 to a microcomputer 24, where the pulse interval modulated signal is decoded to determine if the implanted device has been transmitting the correct signals. If the transmitted signals are correct, the programming device will indicate to the implanted device that processing of additional information may take place, and the implanted device will send further digitally encoded signals to the amplifier 16 which will be coupled through the demodulator 18 and the input line 22 to be processed. Gain control is achieved with the output lines 32 from the microcomputer 24 to the amplifier 16, which select various combinations of the input resistors in series with the lines which control the gain of the amplifier. Alternatively, the threshold level of the amplifier could be varied by the microcomputer instead of the gain. The flow chart of Fig. 3 illustrates an implementation of the digital gain control system. Initially the gain is set to a maximum and a sampling period is established, such as 10 ms, for example. The system is then activated to determine if any bursts of radio frequency (RF) pulses are present as indicated by step 42. It no RF pulses are present and the sampling, or initialization period of step 44 has expired, the gain will remain at a maximum, as indicated by step 40. However, if there are RF pulses present during the sampling period, the timing of the sampling period will be reset as indicated at step 46 so that the sampling period will be continually started following the termination of the previous sampling period as long as RF pulses are present. Once the RF pulses have been received and have been transmitted by the demodulator 18 to the microcomputer 24, the microcomputer verifies whether all of the pulses are associated with valid 1 and 0 data bits, as indicated by step 48. The manner in which this is done is currently employed in pulse interval modulation systems by utilizing a high frequency clock which produces pulses that are much narrower than the width of the pulses of the waveform 20. For example, the pulses in the waveform 20 may be 1000 times wider than the width of the clock pulse. The number of pulses from this high frequency clock which occur while the waveform 20 is at a high level are counted and this count indicates the width and the spacing between pulses of the waveform to a very precise degree. Spacing between the pulses of the waveform 20 may also be readily determined by the microprocessor utilizing either software control or hardware implementation by other methods well known to those skilled in the art. In the event that all of the pulses that are initially sampled in step 48 are valid "1's" and "0's", the programming unit will transmit a validation signal to the implanted device. The implanted device upon receipt of the validation signal will then transmit further signals to the programming device, and storage and processing of the transmitted data will continue as indicated by step 50. However, should there be errors in the encoded message transmitted from the implanted device, further processing will not continue until all of the pulses of an encoded message are validated. A second sampling period is employed during the reception of the encoded message at step 52, which is utilized to determine if the invalid pulse of step 48 has a time less than that of a valid 1 signal. This is done by measuring the interval between pulses, and if the pulse interval is less than that required for a valid "1", the pulse is counted as an error pulse. If the number of invalid pulses exceeds a predetermined number N over this period of time, the gain of the unit will be decremented one step as indicated by the steps 54 and 56. Further processing will then continue in the loop until the gain has been decremented to its minimum value, and either no verification is possible, or until the gain has been decremented to the point where the noise signal falls below the established threshold level and valid signals are being received. In the event that there are some invalid level transitions detected at step 52, but the error rate is not sufficiently high to meet the condition of step 54, the gain will remain at the set level, but further processing will not continue. This condition will be generally a temporary one since the background noise will generally either diminish or increase to change the signal to noise level and the response of the system. The present invention has been described with reference to a gain control system in which the gain is decremented. Furthermore, while the gain has been adjusted in the described embodiment, it will be apparent to those skilled in the art that the threshold level could alternatively be varied in accordance with the scope of the present invention.

Claims: A telemetry receiver for receiving telemetry from an implanted medical device of the type which transmits information in the form of series (10) of timed r.f. signals, said information being conveyed by the timing of said r.f. signals, said receiver including amplifier means (16) for applying gain to said r.f. signals, a threshold level detector which detects the presence of said r.f. signals as amplified by said amplifier means when said amplified r.f. signals exceed a threshold level (34), and means (48) for verifying the accuracy of the timing of said series of signals, characterized by counting means arranged to count the number of erroneously transmitted signals within one said series of signals and control means (54,56) responsive to said counting means and arranged to prevent further processing of said information in response to one or more erroneous signals being counted and to decrease the level of said amplified r.f. signals relative to said threshold level in response to a predetermined number of erroneous signals being counted. A telemetry receiver as claimed in claim 1 wherein said control means is arranged to adjust said threshold level. A telemetry receiver as claimed in claim 1 wherein said control means is arranged to adjust the gain of said amplifier means. A telemetry receiver as claimed in claim 3 wherein said control means is arranged successively to reduce said gain. A programmer for an implanted medical device, having means for verifying the programming of the implanted medical device,wherein the programmed implanted device transmits telemetry confirmation signals to the programmer after the programming of said implanted device and said means for verifying the programming comprises a telemetry receiver in accordance with any preceding claim.

Inventors: ANDERSON, KATHERINE H.
Applicants: MEDTRONIC, INC.
Application year: 1985
Main classification: H04B1
IPC codes: A61N1