Since the introduction of the first implantable pacemakers in the 1960's, there have been considerable advancements both in the field of electronics and the field of medicine, such that there is presently a wide assortment of commercially-available implantable medical devices. The class of implantable medical devices now includes not only pacemakers, but also implantable cardioverters, defibrillators, neural stimulators, and drug administering devices. Today's state-of-the-art implantable medical devices are vastly more sophisticated and complex than early pacemakers, capable of performing significantly more complex tasks. The therapeutic benefits of such devices have been well-proven.
As the functional sophistication and complexity of implantable medical devices has increased over the years, it has become increasingly more important for such devices to be equipped with a telemetry system for enabling them to communicate with an external unit.
For example, shortly after the introduction of the earliest fixed-rate, noninhibited pacemakers, it became apparent that it would be desirable for a physician to non-invasively exercise at least some amount of control over the device, e.g., to turn the device on or off or adjust the fixed pacing rate, after implant. In early devices, one way the the physician was able to have some control over implantable device operation was through the provision of a magnetic reed switch in the implantable device. After implant, the reed switch would be actuated by placing a magnet over the implant site. Reed switch closure could then be used, for example, to alternately activate or deactivate the device. Alternatively, the fixed pacing rate of the device could be adjusted up or down by incremental amounts based upon the duration of reed switch closure. Many different schemes utilizing a reed switch to adjust parameters of implanted medical devices have been developed. See, for example, U.S. Pat. No. 3,311,111 to Bowers, U.S. Pat. No. 3,518,997 to Sessions, U.S. Pat. No. 3,623,486 to Berkovits, U.S. Pat. No. 3,631,860 to Lopin, U.S. Pat. No. 3,738,369 to Adams et al., U.S. Pat. No. 3,805,796 to Terry, Jr., and U.S. Pat. No. 4,066,086 to Alferness et al.
As new, more advanced features are incorporated into implantable devices, it is typically necessary to convey correspondingly more information to the device relating to the selection and control of those features. For example, if a pacemaker is selectively operable in various pacing modes (e.g., VVI, VDD, DDD, etc. . . ), it is desirable that the physician or clinician be able to non-invasively select a mode of operation. Similarly, if the pacemaker is capable of pacing at various rates, or of delivering stimulating pulses of varying energy levels, it is desirable that the physician or clinician be able to select, on a patient-by-patient basis, appropriate values for such variable operational parameters.
Even greater demands are placed upon the telemetry system in implantable devices having such advanced features as rate adaptation based upon activity sensing, as disclosed, for example, in U.S. Pat. No. 5,052,388 to Sivula et al. entitled "Method and Apparatus for Implementing Activity Sensing in a Pulse Generator", in U.S. patent application Ser. No. 07/567,372 in the name of Sivula, et al. entitled "Rate Responsive Pacemaker and Method for Automatically Initializing the Same" [The "Duet auto-init" application], and in U.S. patent application Ser. No. 07/880,877 in the name of Shelton et al., entitled "Work-Modulated Pacing Rate Deceleration". The Sivula et al. '388 patent and the Sivula 07/567,372 and Shelton et al. 07/880,877 applications are each hereby incorporated by reference herein in their entireties.
The information which must be communicated to the implantable device in today's state-of-the-art pacemakers includes: pacing mode, multiple rate response settings, electrode polarity, maximum and minimum pacing rates, output energy (output pulse width and/or output current), sense amplifier sensitivity, refractory periods, calibration information, rate response attack (acceleration) and decay (deceleration), onset detection criteria, and perhaps many other parameter settings.
The need to be able to communicate more and more information to implanted devices quickly rendered the simple reed-switch closure arrangement insufficient. Also, it has become apparent that it would also be desirable not only to allow information to be communicated to the implanted device, but also to enable the implanted device to communicate information to the outside world.
For diagnostic purposes, for example, it is desirable for the implanted device to be able to communicate information regarding its operational status to the physician or clinician. State of the art implantable devices are available which can even transmit a digitized ECG signal for display, storage, and/or analysis by an external device.
As used herein, the terms "uplink" and "uplink telemetry" will be used to denote the communications channel for conveying information from the implanted device to an external unit of some sort. Conversely, the terms "downlink" and "downlink telemetry" will be used to denote the communications channel for conveying information from an external unit to the implanted device.
Various telemetry systems for providing the necessary communications channels between an external unit and an implanted device have been shown in the art. Telemetry systems are disclosed, for example, in the following U.S. Patents: U.S. Pat. No. 4,539,992 to Calfee et al. entitled "Method and Apparatus for Communicating With Implanted Body Function Stimulator"; U.S. Pat. No. 4,550,732 to Batty Jr. et al. entitled "System and Process for Enabling a Predefined Function Within An Implanted Device"; U.S. Pat. No. 4,571,589 to Slocum et al. entitled "Biomedical Implant With High Speed, Low Power Two-Way Telemetry"; U.S. Pat. No. 4,676,248 to Berntson entitled "Circuit for Controlling a Receiver in an Implanted Device"; U.S. Pat. No. 5,127,404 to Wyborny et al. entitled "Telemetry Format for Implanted Medical Device"; U.S. Patent No. 4,211,235 to Keller, Jr. et al. entitled "Programmer for Implanted Device"; U.S. Pat. No. 4,374,382 to Markowitz entitled "Marker Channel Telemetry System for a Medical Device"; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled "Telemetry System for a Medical Device".
Typically, telemetry systems such as those described in the above-referenced patents are employed in conjunction with an external programming/processing unit. One programmer for non-invasively programming a cardiac pacemaker is described in its various aspects in the following U.S. Patents to Hartlaub et al., each commonly assigned to the assignee of the present invention and each incorporated by reference herein: U.S. Pat. No. 4,250,884 entitled "Apparatus For and Method Of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart"; U.S. Pat. No. 4,273,132 entitled "Digital Cardiac Pacemaker with Threshold Margin Check"; U.S. Pat. No. 4,273,133 entitled "Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure"; U.S. Pat. No. 4,233,985 entitled "Multi-Mode Programmable Digital Cardiac Pacemaker"; and U.S. Pat. No. 4,253,466 entitled "Temporary and Permanent Programmable Digital Cardiac Pacemaker".
Aspects of the programmer that is the subject of the foregoing Hartlaub et al. patents (hereinafter "the Hartlaub programmer") are also described in U.S. Pat. No. 4,208,008 to Smith, entitled "Pacing Generator Programing Apparatus Including Error Detection Means" and in U.S. Pat. No. 4,236,524 to Powell et al., entitled "Program Testing Apparatus". The Smith '008 and Powell et al. '524 patents are also incorporated by reference herein in their entirety.
Although various different telemetry systems have been employed in the prior art, the present inventors believe that there remains a need for a telemetry system which is small and consumes relatively little power, both being extremely critical considerations in the context of battery-powered implantable medical devices. Many of the known telemetry systems (see, e.g., the above-referenced Calfee et al., Batty, Jr. et al., and Slocum et al. patents) are implemented with complex, energy-consuming circuits. Moreover, known telemetry systems are often implemented in hardwired, non-flexible circuitry not readily adaptable to more than one telemetry protocol.
Additionally, the very existence of so many different telemetry systems can itself be problematic, since even different devices from the same manufacturer may employ different and incompatible telemetry systems. From both a marketing standpoint and a manufacturing standpoint, it is costly and inefficient to require different programmers for each different device made by a given manufacturer.
For a given device, the uplink and downlink telemetry protocols may be entirely different and incompatible, since considerations of energy consumption and efficiency are different for an implanted device than for an external programming/control unit. From energy consumption and device cost standpoints, the need for two different telemetry circuits in an implanted device is clearly undesirable.