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TELEMETRY SYSTEM FOR A MEDICAL DEVICE
This application is a continuation application of Ser. No. 418,925 filed Sept. 16, 1982, now abandoned, which 5 is a continuation of Ser. No. 194,807 filed Oct. 7, 1980, now abandoned.
BACKGROUND OF THE INVENTION
1. Field of the Invention 10 This invention relates to implantable medical devices
such as pacemakers, and more particularly, to a telemetry system for transmitting information from the pacemaker to a remote receiver for diagnostic purposes.
2. Description of the Prior Art 15 Pacemakers for providing stimulating pulses to the
heart in the absence of natural cardiac activity are wellknown. Originally, such pacemakers were fabricated from discrete analog components. More recently designed pacemakers employ digital circuitry realized in 20 monolithic form. The additional complexity resulting from monolithic digital implementation has been used to provided desirable pacemaker features, including programmability. One example of such prior art is U.S. Pat. No. 4,276,883 granted July 7, 1981 to McDonald et 25 al. This patent discloses a pacemaker having a number of programmable features including the pacing rate and pulse width. Information concerning these operating parameters is stored in digital form in the pacemaker's memory. After implantation it is desirable to read out 30 these memory locations for diagnostic purposes. Additional information which is useful for diagnostic purposes, such a lead impedance, battery voltage, and the patient's intracardiac electrogram are inherently analog in nature and not directly compatible with the other 35 digital information within the pacemaker. Consequently, conventional digital modems have not been applicable to pacemaker telemetry systems since their use would require the periodic conversion of the aforementioned analog data to a numerical value prior to transmission.
In contrast, the pulse interval telemetry system of the present invention is capable of transmitting analog data without conversion to a numerical value, and is capable of sequentially transmitting both digital and analog 45 data. This data is individually and serially transmitted in either an analog or digital format to a remote receiver.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the function elements of 5Q the system for encoding and transmitting information from the implanted medical device.
FIG. 2 is a truth table showing the relationship between the encoding scheme and the corresponding states of the various current sources of the system; 55
FIG. 3 is a waveform diagram showing the analog and digital data format; and
FIG. 4 is a schematic diagram showing the VFO, and current sources in a form suitable for implementation in a bipolar integrated circuit. 60
DESCRIPTION OF THE PREFERRED
EMBODIMENT
General
As previously described, the pulse interval modula- 65 tion telemetry system is used to transmit analog and digital information from the implanted medical device to a remote receiver. In the context of a pacemaker
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application the analog information may include battery voltage, lead impedance, or the patient's intracardiac electrogram. Similarly, typical digital data may include programmed pulse width and rate settings as well as identification information. An example of a pacemaker suitable for use as a source of digital information is taught by the previously mentioned U.S. Pat. No. 4,276,883 granted July 7, 1981, to McDonald et al. This application discloses a digitally implemented pacemaker having memory for storing digitally programmed information shown in FIG. 6H of the referenced patent. This information is stored in a parallel format as a sequence of binary digits.
A suitable source for analog information such as the patient's intracardiac electrogram may be found in U.S. Pat. No. 4,266,551 granted May 12, 1981 to Stein et al. The circuitry disclosed in this patent may be used to provide a source of intracardiac analog information to the telemetry system of the present invention.
As shown schematically in FIG. 1 the heart 10 has an indwelling catheter 11 for sensing cardiac depolarizations and for stimulating cardiac tissue. Pacer logic receives signals via sense amplifier 34 and delivers stimulating pulses by way of output amplifier 33. The pacer logic 12 shown operates under the control of parameter data stored in memory 15. The memory 15 contains the parameter data in parallel form which is serialized for data transmission by shift register 16 which forms a portion of the telemetry system.
In operation, the transmission of data is remotely initiated by the closure of a magnetically actuated reed switch within the pacemaker in the well-known manner. Digital data is then transmitted twice to a remote receiver where it is decoded and checked for errors. The digital data transmission is followed by the transmission of analog data in an analog format. The telemetry system is disabled by removing the magnet from the pacemaker site which opens the reed switch and disables the telemetry circuitry.
Additionally, the telemetry circuitry of the present invention includes a receiver blanking circuit which permits the transmission of analog or digital data to be interrupted by the remote programmer thus truncating the transmission of telemetry information so that the pacemaker may receive higher priority programming information from the remote programmer. This function is achieved by digital circuitry which detects the presence of a long duration burst of RF energy from the remote programmer which is received by the pacemaker and which is decoded to turn off the telemetry transmission systems and to prepare the digital circuitry for the reception of programming information from the remote programmer.
Oscillators
Referring to FIG. 1 the radio frequency carrier signal is developed by a radio frequency oscillator tank in FIG. 1. The tank circuit 14 is energized at periodic intervals determined by a variable frequency oscillator (VFO) 12. Radio frequency energy from the reasonant tank circuit 14 is coupled to antenna 16 which radiates this energy to a remote receiver (not shown).
The repetition rate of the variable frequency oscillator is set by a number of cooperating current sources which establish a net charging rate at the input node 18 of the VFO 12. When operating in the digital mode for the transmission of digital information the current
