Patent Publication Number: US-6907291-B1

Title: Secure telemetry system and method for an implantable cardiac stimulation device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/160,980, filed May 30, 2002, now U.S. Pat. No. 6,600,952 which is a continuation of Ser. No. 09/410,517, filed Sep. 30, 1999, now U.S. Pat. No. 6,424,867. 

   FIELD OF THE INVENTION 
   The present invention is generally directed to an implantable stimulation device. The present invention is more particularly directed to such a device which provides telemetry communication with various different types of external devices and which is capable of detecting the type of external device with which is communicated and providing a corresponding appropriate response. 
   BACKGROUND OF THE INVENTION 
   Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers, which provide stimulation pulses to a heart to cause a heart, which would normally or otherwise beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators, which detect when the atria and/or the ventricles of the heart are in fibrillation and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functionalities of a pacemaker and a defibrillator. 
   Implantable cardiac stimulation devices have become extremely sophisticated. As is well known, they are able to provide therapy to a heart based upon numerous different programmable parameters. Such programmable parameters may, for example, control therapy modality, pacing rates, refractory periods, pacing and defibrillating pulse energies, detection sensitivities, and the like. 
   Programmable parameter selection is normally achieved with an external programmer, which establishes a communication channel via telemetry with the implanted device. To that end, the implantable devices include a telemetry circuit including a transmitter for transmitting device related information and electrograms to the external programmer and a receiver for receiving information, such as programmable parameter commands, from the external programmer. As a result of such external programmability, the implanted devices may be reprogrammed by the physician to continually adapt the device to a patient&#39;s changing condition. 
   More recently, with improvements in increased memory storage capacity, the implantable cardiac storage devices are now also seen as being capable of storing cardiac data which could not be previously stored. Such data may include, for example, device history data, cardiac data including stored electrograms to be used at follow-up visits, or cardiac data useful in diagnosing cardiac conditions other than those currently being treated by the implanted device. 
   As can thus be seen, the telemetry channel may be used to both read data from the implanted device and to send data into the device. Some of the data that is sent into the device is information that is passive and simply recorded in memory, to be read back at a later time. Other data that is sent into the device is active and causes changes in the behavior or operation of the device. Some of the active data, when sent to the device, will cause operational changes, which do not impact the therapy provided to the patient. Such data may include diagnostic data recording modes or real time data transmission channel selections. Active data sent to the implanted device does impact on the therapy received by the patient. Such data may include parameters such as pacing rate, mode, and rate responsive slope, for example. In some other cases, reading of data from an implanted device can cause active changes in the therapy being delivered to the patient. For example, temporary changes in pacing rate and modality may be required for reading measured data while more permanent changes are also possible results of reading data from a device. 
   Hence, there are a number of potential uses for the telemetry interface which can benefit from the ability to read data from an implanted device and to control active behavior of the device without affecting the therapy delivered to the patient. To ensure that the health and safety of the patient are maintained at all times, it is particularly desirable, if not necessary, to require that the possibility of affecting the therapy delivered be reduced to effectively zero. Examples of applications that could utilize this function are transtelephonic monitoring devices, patient activated data recording device, Holter-style recording instruments, stand-alone patient monitoring devices, or patient monitoring devices linked to a central station. 
   Unfortunately, these applications cannot use the same RF telemetry communications protocol as is typically used by programmers to communicate with and control the implanted device. This is because the potential exists for inadvertently affecting the therapy delivered by the device when errors enter the communications link, either as noise or interference in the RF link or by errors in the operation of the non-programming monitoring devices. 
   Prior systems have failed to provide adequate security to permit partial programming or non-programming devices to communicate with an implantable device. For example, systems are known wherein partial programming or non-programming external devices communicate with implanted devices using the same protocol as the devices full-featured programmer. These systems simply rely on the partial programming or non-programming device having limited or no programming ability. However, because they use the same protocol as the devices programmer, noise or interference with the communication link could cause the programmable parameters of the implanted device and thus the therapy delivered by the implanted device to be inadvertently affected by communication with a partial programming or non-programming device. 
   Other systems fail to recognize that partial programming or non-programming devices may be in communication with an implanted device. For example, U.S. Pat. No. 4,875,483 describes a system wherein an implantable stimulator uses a program access code to control who may change the programming of the device. Identification of an external device type by an implantable device is not contemplated. 
   U.S. Pat. No. 5,725,559 describes a system wherein an implantable stimulator uses codes to determine the current programming of the implanted device and the level of therapy the device is authorized to provide. While the system permits some level of personnel to provide maintenance type work on the device using the programmer, only a physician can upgrade the therapies given to the patient. However, the concept of the implanted device communicating with different types of external devices other than a programmer is not addressed. 
   U.S. Pat. No. 5,843,138 describes a system wherein a programmer is capable of programming the implanted device in a conventional way and also downloading new control software to the implanted device, subject to predetermined system conditions. Upon interrogating the implanted device, the programmer determines whether the device is of the type, which it is able to modify. Again, the described implanted device fails to respond to the need of being capable of different types of external devices and identifying the type of external device with which it is communicating for the purpose of providing an appropriate response. 
   Therefore, there is a need in the art for a new and improved implantable cardiac stimulation device which may communicate with various types of external devices including programmers which may change the operation of the implantable device and other external devices such as monitors which are intended to make minor modifications in the device operation and/or read data from the implanted device. More particularly, there is a need in the art for an implantable cardiac stimulation device which is capable of communicating with a plurality of different external devices, detecting the type of external device in which it is communicating, and then providing a suitable appropriate response to the external device. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an implantable cardiac stimulation device and method which provides secure communication between the implantable device and a plurality of different types of external devices such as programmers, intended to change therapy parameters of the implantable device, and monitors, intended to monitor device operation or patient condition. To that end, the implantable cardiac stimulation device detects they type of external device, of a plurality of different external devices, with which it is communicating for providing an appropriate response depending on the type of external device detected. When the external device is a programmer, an appropriate response includes permitting access to a portion or all of the programmable parameters of the device. When the external device is a monitor, not intended to program the device, an appropriate response includes locking out access to the programmable parameters while permitting access to readable memory portions containing, for example, data or other information concerning device operation or the patient&#39;s condition. 
   In accordance with the present invention, the implantable cardiac stimulation device includes a telemetry circuit which establishes communication with the external devices using, for each external device, a respective different communication protocol. A control circuit of the implantable device detects the external device based upon the communication protocol being used to support the communication. 
   The different protocols may be different encoding schemes, different data transmission rates, or different required access codes including access code verification or modification during the communication. At least one protocol may further require serial access codes for access to programmable parameters. Still further, each of the serial access codes may be different. 
   To ensure that a non-programmer does not gain access to the programmable parameters following communication with a programmer, the telemetry circuit may be deactivated upon completion of the communication with a programmer. The deactivation may be provided by a timer which automatically times out after completion of the communication. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features, and advantages of the present invention will be more apparent from the following more particular description thereof presented in conjunction with the following drawings wherein: 
       FIG. 1  shows a functional block diagram of an implantable dual-chamber pacemaker, which represents one type of implantable cardiac stimulation device with which the invention may be used; 
       FIG. 2  shows a functional block diagram of a telemetry circuit which may be used in the device of  FIG. 1  wherein the different protocols are different encoding schemes; 
       FIG. 3  shows a functional block diagram of another telemetry circuit which may be used in the device of  FIG. 1  wherein the different protocols are different data transmission rates; 
       FIG. 4  shows a functional block diagram of a further telemetry circuit which may be used in the device of  FIG. 1  wherein the different protocols include different access codes and access code verification; 
       FIG. 5  shows a functional block diagram of another telemetry circuit which may be used in the device of  FIG. 1  wherein the different protocols include different access codes and telemetry deactivation; 
       FIG. 6  shows a functional block diagram of another telemetry circuit which may be used in the device of  FIG. 1  wherein the different protocols include different access codes which are changed or modified during communication; 
       FIG. 7  is a general representation of an access code and a manner of modifying the access code during communication; 
       FIG. 8  shows a functional block diagram of the memory of the implantable cardiac stimulation device of  FIG. 1  including first and second programmable parameter portions and first and second data portions; and 
       FIG. 9  shows a functional block diagram of still another telemetry circuit which may be used in the device of  FIG. 1  wherein a programmer communication protocol requires two serial access codes and a non-programmer communication protocol requires one access code. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description is of the best mode presenting contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
   As indicated above, the present invention may be used with various types of implantable stimulation devices, including an implantable pacemaker configured to treat bradycardia and/or tachycardia, an implantable cardioverter-defibrillator (ICD), or a combination thereof. 
   To better understand the invention, it will first be helpful to have an understanding of the basic functions performed by the implantable stimulation device with which the invention is used, for example, a dual-chamber pacemaker. While a dual-chamber device has been chosen, this is for teaching purposes only. It is recognized that the present invention could be implemented into an ICD device or a single-chamber pacemaker, that one of skill in the art could readily adapt the dual-chamber device shown in  FIG. 1  to perform single-chamber functionality, and a single-chamber device or an ICD device is within the spirit of the invention. 
   In  FIG. 1 , a simplified block diagram of the circuitry needed for a dual-chamber pacemaker  70  is illustrated. The pacemaker  70  is coupled to a heart  28  by way of leads  74  and  76 , the lead  74  having an electrode  75  that is in contact with one of the atria of the heart, and the lead  76  having an electrode  77  that is in contact with one of the ventricles of heart. The leads  74  and  76  are electrically and physically connected to the pacemaker  70  through a connector  73  that forms an integral part of the housing wherein the circuits of the pacemaker are housed. 
   The connector  73  is electrically connected to a protection network  79 , which network  79  electrically protects the circuits within the pacemaker  70  from excessive shocks or voltages that could appear on the electrodes  75  and/or  77  in the event such electrodes were to come in contact with a high voltage signal, for example, from a defibrillation shock. 
   The leads  74  and  76  carry stimulating pulses to the electrodes  75  and  77  from an atrial pulse generator (A-PG)  78  and a ventricular pulse generator (V-PG)  80 , respectively. Further electrical signals from the atria are carried from the electrode  75  through the lead  74  to the input terminal of an atrial channel sense amplifier (P-AMP)  82 ; and electrical signals from the ventricles are carried from the electrode  77 , through the lead  76 , to the input terminal of the ventricular channel sense amplifier (R-AMP)  84 . Similarly, electrical signals from both the atria and ventricles are applied to the inputs of an IEGM (intracardiac electrogram) amplifier  85 . The amplifier  85  is typically configured to detect an evoked response from the heart  28  in response to an applied stimulus, thereby aiding in the detection of “capture.” Capture occurs when an electrical stimulus applied to the heart is of sufficient energy to depolarize the cardiac tissue, thereby causing the heart muscle to contract, or in other words, causing the heart to beat. Capture does not occur when an electrical stimulus applied to the heart is of insufficient energy to depolarize the cardiac tissue. 
   The dual-chamber pacemaker  70  is controlled by a control circuit or system  86  that typically includes a microprocessor programmed to carry out control and timing functions. The control system  86  receives the output signals from the atrial amplifier  82  over signal line  88 . Similarly, the control system  86  receives the output signals from the ventricular amplifier  84  over signal line  90 , and the other signals from the IEGM amplifier  85  over signal line  91 . These output signals are generated each time that a P-wave or an R-wave, or an evoked response is sensed within the heart  28 . The control system  86  also generates trigger signals that are sent to the atrial pulse generator  78  and the ventricular pulse generator  80  over lines  92  and  94 , respectively. These trigger signals are generated each time that a stimulation pulse is to be generated by the respective pulse generator  78  or  80 . The atrial trigger signal is referred to simply as the “A-trigger” and the ventricular trigger signal is referred to as the “V-trigger.” 
   During the time that either an A-pulse or V-pulse is being delivered to the heart, the corresponding amplifier, P-AMP  82  and/or R-AMP  84 , is typically disabled by way of a blanking signal presented to these amplifiers from the control system over signal lines  96  and  98 , respectively. This blanking action prevents the amplifiers  82  and  84  from becoming saturated from the relatively large stimulation pulses that are present at their input terminals during this time. This blanking action also helps prevent residual electrical signals present in the muscle tissue as a result of the pacemaker stimulation from being interpreted as P-waves or R-waves. 
   As shown in  FIG. 1 , the pacemaker  70  further includes a memory circuit  100  that is coupled to the control system  86  over a suitable data-address bus  102 . This memory circuit  100  allows certain control parameters, used by the control system  86  in controlling the operation of the pacemaker, to be programmably stored and modified by an external programmer  108 , as required, in order to customize the pacemaker&#39;s operation to suit the needs of a particular patient. Further, data sensed during the operation of the pacemaker may be stored in the memory  100  for later retrieval and analysis by either the programmer  108  or an external monitor  109 . 
   The memory  100  of the pacemaker  70  may take many forms, and may be subdivided into as many different memory blocks, portions, or sections as needed in order to allow desired data and control information to be stored. A feature of the present invention, in some embodiments thereof, is the ability to detect the type of external device communicating with the implantable device  70  and, depending on the type of device detected, provide an appropriate response. If the device  70  is communicating programmer  108 , for example, an appropriate response would include permitting access to the portions of memory  100  which store both readable data and programmable parameters. If, on the other hand, the device  70  is communicating with external monitor  109 , not intended to program the device  70 , an appropriate response may include precluding access to the programmable parameters while permitting access to those readable data portions which the monitor is intended to read. Still further, if the monitor requires temporary modification of some programmable parameters to permit the intended data to be generated and stored, an appropriate response may be to permit access to only those programmable parameters required for temporary modification to permit the intended data to be generated. 
   The pacemaker  70  further includes a telemetry/communications circuit  104 . In accordance with the present invention, the telemetry circuit  104  includes a first telemetry circuit  105  for communicating with the external programmer  108  in accordance with a first protocol and a second telemetry circuit  107  for communicating with the external monitor  109  in accordance with a second and different protocol. The telemetry circuit  104  is connected to the control system  86  by way of a suitable command/data bus  106 . The control system detects the type of external device communicating with the implantable device  70  and causes an appropriate response of the type discussed above. The telemetry circuits  105  and  107  may be selectively coupled to their respective external devices by means of appropriate communication links  110  and  111 , respectively, which communication links  110  and  111  may be any suitable electromagnetic link, such as an RF (radio frequency) channel, a magnetic link, an inductive link, an optical link, and the like. The external devices  108  and  109  may use the same type of link or different types of links with the different types of links being the different protocols. Advantageously, through the external devices  108  and  109  and the communication links  110  and  111  desired commands may be sent to the control system  86 . The control system  86  then responds according to the type of external device initiating the commands based upon the protocol used to establish the communication. Through communication links  110  and  111  the external devices  108  and  109  may send commands to be remotely received by the telemetry circuits  105  and  107 . Similarly, data initially sensed through the leads  74  or  76 , and processed by the microprocessor control circuit  86 , or other data measured within or by the pacemaker  70 , may be stored and uploaded along with command confirmations, such as programmable parameter modification confirmations, to the external devices  108  and  109 . In this manner, non-invasive communications can be established with the implanted pacemaker  70  from the remote, non-implanted, external devices  108  and  109 . 
   Although the telemetry circuits  105  and  107  are individually shown in  FIG. 1 , it will be understood by those skilled in the art that they are represented individually to generically illustrate that each is capable of communicating with its respective external device using respectively different protocols. Hence, it should be understood that the telemetry circuits  105  and  107  may be totally separate as illustrated or share the same circuit hardware while utilizing different protocol software to support the differing protocols. 
   A clock circuit  103  directs an appropriate clock signal to the control system  86 , as well as to other needed circuits throughout the pacemaker  70  by way of clock bus  105 . Additionally, the pacemaker  70  includes a battery  93 . It provides operating power to all of the circuits of the pacemaker  70  via a power signal line  95 . 
   It is noted that the pacemaker  70  in  FIG. 1  is referred to as a dual-chamber pacemaker because it interfaces with both the atria and ventricles of the heart. Those portions of the pacemaker  70  that interface with the atria, for example the lead  74 , the sense amplifier  82 , the atrial pulse generator  78 , and corresponding portions of the control system  86 , are commonly referred to as the “atrial channel.” Similarly, those portions of the pacemaker  70  that interface with the ventricles, for example the lead  76 , the R-wave sense amplifier  84 , the ventricular pulse generator  80 , and corresponding portions of the control system  86 , are commonly referred to as the “ventricular channel.” 
   As need for certain applications, the pacemaker  70  may further include at least one sensor  112  that is connected to the control system  86  of the pacemaker  70  over a suitable connection line  114 . While this sensor  112  is illustrated in  FIG. 1  as being included within the pacemaker  70 , it is to be understood that the sensor may also be external to the pacemaker  70 , yet still be implemented within or carried by the patient. A common type of sensor is an activity sensor, such as a piezoelectric crystal that is mounted to the case of the pacemaker. Other types of sensor are also known, such as sensors that sense the oxygen content of the blood, respiration rate, pH of blood, body motion, and the like. The type of sensor used is not critical to the present invention. Any sensor or combination of sensors capable of sensing a physiological or physical parameter relatable to the rate at which the heart should be beating and/or relatable to whether a tachyarrhythmia is likely to soon occur, can be used. Such sensors are commonly used with “rate-responsive” pacemakers in order to adjust the rate of the pacemaker in a manner that tracks the physiological or metabolic needs of the patient. 
   The pacemaker  70  further includes magnet detection circuitry  87 , coupled to the control system  86  over signal line  89 . It is the purpose of the magnet detection circuitry  87  to detect when a magnet is placed over the pacemaker, which magnet may be used by a physician or other medical personnel to perform various reset functions of the pacemaker  70  and/or to signal the control system  86  that an external device such as programmer  108  or external monitor  109  is in place to receive data from, or send data to, the pacemaker memory  100  or control system  86  through the telemetry communication circuits  105  or  107 . 
   The memory circuit  100 , and the circuits utilized in the atrial and ventricular channels may all be of common design as is known in the pacing art. The present invention is not concerned with the details of the circuitry utilized for each of these pacing elements. Rather, it is concerned with the manner in which all of these pacing elements cooperate with each other in order to provide a particular pacing mode of operation. Such cooperation is controlled by the control system  86 . 
   The control system  86  may be realized using a variety of different techniques and/or circuits. The preferred type of control system  86  is a microprocessor-based control system. It is noted, however, that the control system  86  could also be realized using a state machine. Indeed, any type of control circuit or system could be employed for the control system  86 . So long as the control system  86  controls the operation of the pacemaker (or other medical device) so that the desired functions are achieved as set forth herein, it matters little what type of control system is used. Those of skill in the implantable medical device art, given the teachings presented herein, should thus be able to fashion numerous different types of control systems or circuits that achieve the desired device control. 
   Representative of the types of control systems that may be used with the invention is the microprocessor-based control system described in U.S. Pat. No. 4,940,052 entitled MICROPROCESSOR CONTROLLED RATE-RESPONSIVE PACEMAKER HAVING AUTOMATIC RATE RESPONSE THRESHOLD ADJUSTMENT. Reference is also made to U.S. Pat. Nos. 4,712,555 and 4,944,298, wherein a state-machine type of operation for a pacemaker is described; and U.S. Pat. No. 4,788,980, wherein the various timing intervals used within the pacemaker and their inter-relationship are more thoroughly described. The &#39;052, &#39;555, &#39;298, and &#39;980 patents are incorporated herein by reference. 
   As previously mentioned, a feature of the present invention is the ability to detect the type of external device communicating with the implantable device  70  based upon the protocol used to support the communication and, depending on the type of device detected, provide an appropriate response. Reference will now be made to  FIGS. 2–9  which illustrate various embodiments and manners of implementing the present invention. Preliminarily, the telemetry circuit functional block diagrams of  FIGS. 2–6  and  9  illustrate certain protocol defining functionality to be resident in the telemetry circuits. While this may be a preferred implementation, it will be understood by those skilled in the art that some or all of the illustrated protocol defining functionality could alternatively reside within the control circuit  86  instead. Also, in these figures, individual telemetry circuits supporting different telemetry protocols are separately illustrated. However, as previously mentioned, the telemetry circuits may share telemetry circuitry hardware and instead implement the differing telemetry protocols through software or other separate control. 
   Referring now to  FIG. 2 , it shows a telemetry circuit  204  which may be used in the pacemaker  70  of  FIG. 1  in accordance with a first embodiment of the present invention. The telemetry circuit  204  includes a first telemetry circuit  205  for communicating with, for example, the external programmer  108  and a second telemetry circuit  207  for communicating with, for example, the external monitor  109 . The first circuit  205  includes telemetry circuit hardware  210  which may include a transmitter and a receiver and a first encoding scheme  212  corresponding to the encoding scheme of the programmer  108 . The second circuit  207  similarly includes telemetry circuit hardware  214  which may also include a transmitter and a receiver and a second encoding scheme  216  corresponding to the encoding scheme of the external monitor  109 . 
   The first and second encoding schemes are different resulting in first and second communication protocols, respectively. For example, one of the encoding schemes may be a pulse position modulation scheme while the other encoding scheme is a phase shift encoding scheme. As a further example, one encoding scheme may use complete or partial digital inversion while the other does not. Still further, one encoding scheme may utilize a cyclical redundancy code while the other encoding scheme uses any of the other encoding schemes mentioned above. 
   The differing encoding schemes and thus protocols of the telemetry circuit  204  permit the control circuit  86  to detect the type of external device communicating with the implantable device  70 . When the external programmer is communicating with telemetry circuit  205 , the control circuit  86  may provide access to the programmable parameters stored in the memory  100  along with the readable data. However, if the external monitor is communicating with telemetry circuit  207 , the control circuit  86 , may provide access to only the readable data stored in the memory  100  while blocking access to the programmable parameters. Hence, by virtue of the respective different protocols for communication with the different external devices, the type of external device communicating with the implantable device may be detected to facilitate an appropriate response while also providing enhanced security to assure that the implantable device will not inadvertently mistake one external device for another. This precludes the implantable device, for example, from providing access of programmable parameters to an external monitor, for example. 
     FIG. 3  shows another telemetry circuit  304  which may be used in the pacemaker of  FIG. 1  in accordance with another embodiment of the present invention. Again, the telemetry circuit  304  includes a first telemetry circuit  305  and a second telemetry circuit  307 . The telemetry circuits include telemetry hardware  310  and  314 , each including a transmitter and a receiver. The telemetry circuits  305  and  307  implement different respective protocols in the form of different data transmission rates under control of data rate controls  312  and  316 , respectively. The different transmission rates enable external device identification to facilitate appropriate responses while providing required security. 
   Referring now to  FIG. 4 , it illustrates still another telemetry circuit  404  embodying further aspects of the present invention. Here the telemetry circuit  404  includes a first telemetry circuit  405  and a second telemetry circuit  407 . The first telemetry circuit  405  includes telemetry circuit hardware  410  including a transmitter and a receiver. The second telemetry circuit  404  also includes telemetry hardware  412  including a transmitter and a receiver. The telemetry circuits  405  and  407  implement different respective protocols by requiring different access codes to establish communication with respective different external devices. To that end, the first telemetry circuit  405  includes a first access code control  414  while the second telemetry circuit  407  includes a second access code control  416 . 
   In use, for communication to be established with an external programmer, for example, the external programmer would provide the first access code for communication with the first telemetry circuit  405 . The external monitor, on the other hand, would be required to provide a second and different access code to be decoded by the second access code control  416  for communicating with the second telemetry circuit  407 . To ensure that there isn&#39;t an inadvertent mistake in access code identification, each of the telemetry circuits  405  and  407  includes an access code verifier  418  and  420 . The access code verifiers may be utilized to verify the access codes used by the respective telemetry circuits during a communication with an external device. As a communication with external device will most likely encompass a series of transmissions, the access code verifiers may preferably verify the respective access codes between each transmission of the external device to which its respective telemetry circuit is communicating. 
   As a result, the different access codes enable identification of the external device with which the implantable device is communicating. The different access codes and the access code verifiers provide security to assure that appropriate responses are provided and that a non-programming external device is not given access to programmable parameters of the implanted device. 
     FIG. 5  shows a similar telemetry circuit  504  in that it includes a first telemetry circuit  505  and a second telemetry circuit  507  wherein, in addition to the telemetry circuit hardware  510  and  512 , respectively, the telemetry circuits implement respective different protocols by requiring respective different access codes under control of access code controls  514  and  516 , respectively. The operation of the telemetry circuit  504  in that regard is the same as the telemetry circuit  404  of  FIG. 4 . 
   In contrast to the telemetry circuit  404  of  FIG. 4 , each of the first and second telemetry circuits  505  and  507  includes a deactivation circuit  518  and  520 , respectively. The first deactivation circuit  518  includes a timer  522  and the second deactivation circuit includes a second timer  524 . The deactivation circuits deactivate the communications channel of its respective telemetry circuit by deactivating its telemetry circuit following communication with its respective external device. The timers  522  and  524  may be utilized for automatically timing a timeout so that the telemetry circuits  505  and  507  are deactivated a predetermined time after completion of communication with its external device. This assures that if, for example, a programmer has completed communication with the implanted device, an external monitor is unable to gain access to the programmable parameters by virtue of the communication channel being deactivated. 
     FIG. 6  shows another telemetry circuit  604  embodying still further aspects of the present invention. The telemetry circuit  604  includes a first telemetry circuit  605  and a second telemetry circuit  607 . The first and second telemetry circuits  605  and  607  each includes first and second telemetry hardware  610  and  612 , respectively, each including a transmitter and a receiver. The first and second telemetry circuits  605  and  607  implement different protocols by requiring different respective access codes which, during transmission with their respective external devices, are changed or modified. To that end, the first telemetry circuit  605  includes a first access code generator  614  the second telemetry circuit  607  includes a second access code generator  616 . Preferably, the access codes are changed or modified between each transmission. The access codes may be modified in accordance with a pseudorandom-type generator in a manner as illustrated in  FIG. 7 .  FIG. 7  illustrates an illustrative access code  700 . The access code  700  includes six spaces wherein five of the six spaces contain a numerical value represented by a binary number. The first space  702 (X 1 ) may be a constant and depend on the type of external device. For example, the first digit  702  for an external programmer may always be a “5” while the first space  702  for an external monitor may be a “6.” After the first space  702 , there is a blank space  704  which is then followed by four consecutive spaces  706 ,  708 ,  710  and  712 . As will be noted by the arrow  714 , the access code  700  may be modified between transmissions by shifting the numerical value of the last space  712  to the third space  706  and then shifting the other numerical values to the right by one space. As the foregoing will be known by both the implantable device and the external device to which it is intended to communicate, even though the access code is modified between transmissions, the access code to which the implantable device will be looking at any one time will be known by the external device to which it is intended to communicate. However, other external devices will not have privy to the access code at any one time and will be precluded from evoking any response other than the response to which it is intended. 
     FIG. 8  shows a functional block diagram of the memory  100  and a manner in which it may be portioned for storing program parameters and readable data. Here it will be seen that the memory includes a first program parameter portion  150  for storing a first group of programmable parameters and a second memory portion  152  for storing a second group of programmable parameters. The memory  100  also includes a first data portion  154  for storing a first group of data and a second data portion  156  for storing a second group of data. When a programmer establishes communication with the implantable device, a suitable response may be the access of both program parameter portions  150  and  152  to the programmer along with access to data portions  154  and  156 . However, when the implantable device is communicating with an external monitor, an appropriate response may be the blocking out of the programmable parameter portions  150  and  152  while permitting access to one or both of the readable data portions  154  and  156 . In those instances where an external monitor requires temporary modification of therapy programmable parameters, an appropriate response may be permitting access to one of the program groups  150  or  152  while precluding access to the other on a temporary basis to permit the required data to be generated. Hence, by partitioning the memory  100  and providing selective access by the control circuit  86  responsive to the type of external device communicating with the implantable device based upon the protocol utilized, an appropriate response can be generated to assure all of the required functionality is provided to the external device without providing memory portions which would be inappropriate. 
   Referring now to  FIG. 9 , it shows another telemetry circuit  904  configured in accordance with a still further embodiment of the present invention. The telemetry circuit  904  includes a first telemetry circuit  905  and a second telemetry circuit  907 . The telemetry circuits  905  and  907  each include telemetry hardware  910  and  912 , respectively. The telemetry circuits  905  and  907  implement different communication protocols. To that end, the first telemetry circuit  905  has an access code control  914  which requires two serial access codes to be provided in order to establish communication with the telemetry circuit  905 . On the other hand, the telemetry circuit  907  requires a single access code under control of an access code control  916 . 
   The access code control  914  which requires successive serial access codes to be provided to gain entry may be utilized, for example, in permitting access to programmable parameters of an implantable device by a programmer. This renders the programming channel of the device more secure. In addition to the foregoing, one or both of the access codes may be modified during the communications, as for example between transmissions in a manner similar to that described previously with respect to  FIG. 7 . As a result, the type of external device communicating with the implantable device may be readily detected to facilitate the provision of an appropriate response. Further, by virtue of the different protocols, security is provided to the system to preclude a non-programming device from obtaining access to programmable parameters of the implantable device. 
   One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for the purpose of illustration and not of limitation, and the present invention is limited only by the claims that follows.