Digital gain control for the reception of telemetry signals from implanted medical devices

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 unit 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.

BACKGROUND OF THE INVENTION 
Implanted medical devices such as heart pacemakers are often programmed to 
use telemetric signals that are generated by a remote programming unit. 
Verification of the programming of 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. In the disclosed 
embodiment, the encoding employs pulse interval modulation, wherein the 
intervals between bursts of high frequency pulses are long or short 
depending on the logic level of the data bit being transmitted. 
Signals from implanted devices in a hospital environment are often subject 
to a relatively high level of noise, or interference. As long as the 
signal level is greater than the noise level, however, the gain 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 of the receiver. When errors occur in the 
message being sent from the implanted device, the number of errors in the 
repeated signal are counted over a period of time. If more than a 
predetermined number of errors occur in that time, the gain will be 
adjusted by a predetermined amount, and the signal will be continually 
monitored to determine if the adjusted gain level has eliminated the 
errors. The 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.

TECHNICAL DESCRIPTION OF THE INVENTION 
The digital gain control implementation of the present invention may be 
utilized 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 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 6. 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 which forms the pulses 20 consisting of the envelopes of the 
burst of the high frequency incoming signals. The pulses 20 provides 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 allows the amplifier to select various 
combinations of the input resistors in series with the lines which control 
the gain of the amplifier. Alternately, 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 of the present invention. 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. If 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 programming 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 count the number of the pulses 20 
which have occurred which do not prove to be either a valid 1 or a 0 
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 exceed 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. It will be apparent to those 
skilled in the art that increasing the gain would also come within the 
scope of the present invention. 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 alternately be varied in 
accordance with the scope of the present invention.