Abstract:
An implantable medical device (IMD) that can be wirelessly connected to user interface by which a patient can enter values of selected control parameters for controlling the IMD whereas other control parameters are not accessible via said user interface and can only be modified by a physician or other authorized personnel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an implantable medical device in general and to an implantable heart stimulator in particular. 
     The implantable heart stimulator preferably is an implantable pacemaker or an implantable cardioverter/defibrillator (ICD), or a device for cardiac resynchronization (CRT-D). 
     2. Description of the Related Art 
     Heart stimulators such as cardiac pacemakers are medical devices, usually implantable, that can be connected to or that are permanently connected to electrode leads for delivery of electrical stimulations pulses to the tissue (myocardium) of a human heart. Dual chamber pacemakers are capable of generating stimulation pulses for the atrium and the ventricle of a human heart. Biventricular pacemakers usually are capable to stimulate at least three chambers of a human heart that is the right atrium, the right ventricle and the left ventricle. 
     In a dual chamber pacemaker, this is usually realized by placing electrodes in both the right atrium and right ventricle of the heart. 
     In a demand-type pacemaker these electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization following a contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead as is used for sensing. 
     Separate stimulation pulse generators are usually provided for each heart chamber (atrium or ventricle) to be stimulated. 
     A control unit triggers the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses. 
     A stimulation pulse to the myocardium may cause a contraction of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse intensity is above the stimulation threshold of said myocardium. A sub-threshold stimulation pulse will not cause a cardiac contraction even if delivered to the myocardium in its non-refractory state. 
     Depending on the mode of operation, a pacemaker only delivers a stimulation pulse (pacing pulse) to a heart chamber (atrium or ventricle) if needed, that is, if no natural excitation of that chamber occurs. Such mode of operation is called an inhibited or demand mode of operation since the delivery of a stimulation pulse is inhibited if a natural excitation of the heart chamber is sensed within a predetermined time interval (usually called escape interval) so the heart chamber is only stimulated if demanded. 
     In a demand mode, the pacemaker monitors the heart chamber to be stimulated in order to determine if a cardiac excitation (heartbeat) has naturally occurred, such natural (non-stimulated) excitation, also referred to as “intrinsic” or “signs” cardiac activity, are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms. 
     In order to monitor the heart chamber and thus to determine whether or not a natural contraction of a heart chamber has occurred a pacemaker has a sensing stage which during operation of the pacemaker is connected to an electrode placed in a respective heart chamber. A natural contraction of a heart chamber can be detected by evaluating electrical potentials sensed by such sensing electrode. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a signal known as “P-wave”. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a signal known as “R-wave”. A P-wave or an R-wave represents an atrial event or a ventricular event, respectively, in the further course of this application. 
     In a demand mode of operation, the pacemaker monitors the heart for the occurrence of P-waves and/or R-waves. If such signals are sensed within a prescribed time period or time window, which is called atrial or ventricular escape interval, respectively, then the escape interval is reset (i.e., restarted) and generation of a stimulation pulse is inhibited and no unnecessary stimulation pulse is triggered. The escape interval is measured from the last heartbeat, i.e., from the last occurrence of an intrinsic (sensed) atrial event (P-wave, A-sense, AS) if the atrium is monitored, or an intrinsic (sensed) ventricular event (R-wave, V-sense, VS) if the ventricle is monitored, or the generation of a stimulation pulse (V-pace, VP; A-pace, AP) if no respective intrinsic event has occurred. If the escape interval “times-out”, i.e., if a time period equal to the escape interval has elapsed without the sensing of a P-wave and/or R-wave (depending upon which chamber of the heart is being monitored), then a stimulation pulse is generated at the conclusion of the escape interval, and the escape interval is reset, i.e., restarted. In this way, the pacemaker provides stimulation pulses “on demand,” i.e., only as needed, when intrinsic cardiac activity does not occur within the prescribed escape interval. 
     Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse is delivered to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other. 
     To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient. 
     Multiple-mode, demand-type, cardiac pacemakers shall allow a sequence of contractions of the heart&#39;s chamber which equals as far as possible a natural behavior of the healthy heart for damaged or diseased hearts that are unable to do so on their own. 
     In a healthy heart, initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall and acts as a natural “pacemaker” of the heart. In a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. The natural stimulus causing the atrium to contract is conducted to ventricle via the atrioventricular node (AV node) with a short, natural delay, the atrioventricular delay (AV-delay). Thus a short time after an atrial contraction (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 180 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. 
     Thus, in a healthy heart providing proper AV-synchrony an atrial contraction (A) is followed a relatively short time thereafter by a ventricle contraction (V), that in turn is followed a relatively long time thereafter by the next atrial contraction and so on. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump. 
     To mimic the natural behavior of a heart, a dual-chamber pacemaker, in conventional manner, defines a basic atrial escape interval (AEI) that sets the time interval for scheduling an atrial stimulation pulse. The atrial escape interval can be started by a ventricular event and end with an atrial event. A basic AV delay (AVD) or ventricular escape interval (VEI) sets the time interval or delay between an atrial event and a ventricular event. In such embodiment, AEI and AVD (or VEI) thus together define a length of a heart cycle which is reciprocal to the pacing rate at which stimulation pulses are generated and delivered to a patient&#39;s heart in the absence of sensed natural cardiac activity. 
     For the purpose of this application, a “ventricular event” may refer either a natural ventricular excitation (intrinsic ventricular event) which is sensed as an R-wave or a ventricular stimulation pulse (V-pulse, VP). Similarly, an atrial event shall refer to both, a P-wave or an atrial stimulation pulse (A-pulse, AP). 
     Since the atrial escape interval usually defines the time of delivery of a next scheduled atrial stimulation pulse, and since an atrial stimulation pulse may be timed from the latest ventricular event as well as from the latest atrial event, in some cases the atrial escape interval is an A-A interval. 
     One basic parameter of a heart stimulator&#39;s operation is stimulation rate. The stimulation rate is the V-V interval or the A-A interval the heart stimulator is applying. In modern heart stimulators the stimulation rate is often time variable in order to meet a hemodynamic demand of a patient that depends on the patient&#39;s physical activity. A hemodynamic sensor or activity sensor can be provided to adapt the actual stimulation rate to an actual hemodynamic demand. A heart stimulator allowing such rate adaption is called rate adaptive. Usually the actual stimulation rate is elevated compared to a base (minimum) stimulation rate. The base stimulation rate is applied whenever a patient is at rest. In order to mimic a natural circadian rhythm different base stimulation rates are provided for daytime (daytime base stimulation rate) when the patient is expected to be awake and night time when the patient is expected to sleep (nighttime stimulation rate). 
     Some parameters of an implantable medical device impact the lifestyle of a patient. An example is the time at which an implantable pulse generator (IPG) transitions from nighttime stimulation rate to daytime base stimulation rate. The former is generally lower than the later and thus provides less hemodynamic support. A pacemaker dependent person may thus feel less energetic if he or she wakes up before the programmed transition time. 
     All parameters may be programmed by the physician at follow-up using a device known as “physician programmer”. The follow-ups typically occur every three to six months. The physician may not have the time to discuss all the lifestyle impacting parameters with the patient. Even if time is allocated to this task during the follow-up, adapting the parameters to the lifestyle of the patient only two to four times a year may not provide sufficient granularity to react to changes that can potentially occur daily. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a heart stimulator that best fits the need of a patient. 
     The fundamental idea to achieve the object of the invention consists in allowing the patient to modify a subset of implantable medical devices parameters. 
     According to the invention the patient can modify selected parameters of its implantable medical device that impact his or her lifestyle more often, thereby improving quality of life. 
     This is achieved by an implantable medical device (IMD) that can be wirelessly connected to a user interface by which a patient can enter values of selected control parameters for controlling the IMD whereas other control parameters are not accessible via said user interface and can only be modified by a physician or other authorized personnel. 
     The term selected parameter or selected control parameter shall apply to those parameters that a patient can change. Non-selected parameters thus are parameters that only can be changed by authorized personnel such as a physician. 
     The selected subset of implantable medical devices parameters that a patient can modify comprises only parameters, with associated tuning ranges, that may safely be changed by someone without medical training. For example, an average patient can be trusted to adapt the transition time of an IPG from nighttime rate to daytime rate to its lifestyle, much like he or she would program an alarm clock. On the other hand, changing the atrioventricular delay interval (AV delay interval) requires knowledge not found outside the expert community and this parameter should therefore not be available for the patient to modify. 
     According to the invention, the implantable medical device comprises a telemetry unit connected to a memory and a control unit that control the implantable medical device&#39;s operation according to parameters stored in the memory. 
     The telemetry unit is adapted to wirelessly receive parameters for controlling the implantable medical device. 
     It is part of the invention to define a subset of implantable medical devices parameters that can be changed and the ranges within these selected parameters that can be changed, and to define other parameters that only can be amended by a physician. 
     In addition to adapt the heart stimulator so as to allow some selected parameters to be manipulated by a patient, a user interface is provided allowing the patient to modify the selected subset of the implantable medical device&#39;s parameters that can be patient modified. 
     According to alternative preferred embodiments of the invention two options are provided. 
     According to a first embodiment, the external device is provided with an interface that allows the patient to only modify the selected parameters. This external device can connect wirelessly to the implantable medical device. Optionally, the external device may be connected to a network where patient changes can be logged and analyzed. Optionally, the network may be connected to a physician network access so that the physician can receive notices of the patient changes to the parameters and review change history. 
     According to an alternative embodiment it is suggested to allow the patient to program selected parameters through a network access, for example a PC connected to the internet. The network access may be provided by a central service center that can connect to the implantable medical device via the external device. The central service center may thus provide a remotely accessible user interface that allows a user to only amend selected parameters. The central service center may further provide a second user interface with restricted access so only authorized personnel can access the second user interface. Via the second user interface a physician can amend other, non-selected parameters. The access to the first user interface (the patient interface) may be restricted so as to only allow a particular patient to access this interface in order to amend selected parameters of his own implantable medical device. 
     According to a particularly preferred embodiment according to the first alternative, the external device provides a wireless link to the implantable medical device and provides a user interface corresponding to a user interface of an alarm clock. In a particularly preferred embodiment, the external device is adapted to allow the patient to set the current time, program the wake-up alarm time and arm the wake-up alarm. 
     It is further preferred, that the external device is adapted to telemetrically instruct the implantable medical device to start the transition to nighttime rate when the patient arms the wake-up alarm. In addition, the external device may be adapted to telemetrically instruct the implantable medical device to program the wake-up alarm time that the patient entered as the transition time from nighttime rate to daytime rate. 
     Optionally, the external device may be connected to a central service center via a network where patient changes can be logged and analyzed. Optionally, the central service center provides a physician network access so that a physician can receive notices of the patient changes to the selected parameters and review change history. 
     Optionally, the external device may provide other features found in alarm clocks, such as a radio tuner and/or a CD player. 
     According to the invention this objective is achieved by a heart stimulation system comprising an implantable heart stimulator and an external transceiver device for wireless communication with said implantable heart stimulator. 
    
    
     
       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  is a schematic overview over a implantable device system comprising an implantable medical device, an external transceiver device and a service center. 
         FIG. 2  shows a three chamber bi-ventricular implantable cardioverter/defibrillator (ICD). 
         FIG. 3  is a schematic diagram of the device modules of the ICD of  FIG. 2 . 
         FIG. 4  is a schematic diagram of the external transceiver device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently 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. 
       FIG. 1  shows an implantable device system comprising an implantable medical device  10 , an external transceiver device  80  and a central service center  90 . The implantable medical device  10  is for example an implantable pacemaker or an implantable cardioverter/defibrillator or device for cardiac resynchronization. The implantable medical device  10  comprises an implant transceiver (not shown) for wireless communication with the external transceiver device  80 . The external transceiver device  80  comprises an external transceiver unit (not shown) for wireless communication with the implant transceiver unit and a data communication interface (also not shown) adapted to allow a data communication with the service center  90 . The data communication interface preferably is adapted to use a public data communication line as a telephone landline connection or wireless connection via GPRS/UMTS or SMS. 
     The central service center  90  comprises or is connected to a user interface allowing a physician or a team of physicians to interact with the central service center. The user interface may comprise a display for displaying data to the physician  100  and some input device allowing the physician  95  to enter instructions or data into the central service center  90 . The central service center  90  further comprises a central data base that is connected to said data communication interface (see  FIG. 4 ) and a data evaluation module that is connected to the data base that is adapted to evaluate data stored in said data base. 
     A patient having the medical device  10  implanted may communicate with the implantable medical device  10 , the central service center  90  or both by means of the external transceiver device  80 . For this purpose, the external transceiver device  80  may feature a user interface  108  as is illustrated in  FIG. 4 . The patient may also directly connect with the central service center  90  without using the external device but via the internet, for example. 
     In  FIG. 2  the implantable medical device is a three chamber biventricular pacemaker and cardioverter/defibrillator  10  that is connected to pacing/sensing leads placed in a heart  12  is illustrated. 
     Pacemaker  10  comprises a gas proof housing (case)  42  made from a biocompatible metal such as titanium. Pacemaker  10  comprises a transparent header  11  that is made from electrically insulating plastic and that encloses terminals to which electrode leads  16 ,  18  and  30  are connected detachably. Electrode leads  16 ,  18  and  30  each comprise a proximal connector (not shown) that is plugged into the connectors of header  13 . 
     The implantable medical device  10  is electrically coupled to heart  12  by way of leads  14 ,  16  and  30 . 
     Lead  14  is a right atrial electrode lead that has a pair of right atrial electrodes  22  and  24  that are in contact with the right atria  26  of the heart  12 . 
     Lead  16  is a right ventricular electrode lead that has a pair of ventricular stimulation and sensing electrodes  18  and  20  that are in contact with the right ventricle  28  of heart  12 . Further, a ventricular defibrillation shock coil  38  and an atrial defibrillation shock coil  40  are arranged on lead  16 . 
     Electrodes  22  and  18  are tip electrodes at the very distal end of leads  14  and  16 , respectively. Electrode  22  is a right atrial tip electrode RA Tip and electrode  18  is a right ventricular tip electrode. Electrodes  24  and  20  are ring electrodes in close proximity but electrically isolated from the respective tip electrodes  22  and  18 . Electrode  24  forms a right atrial ring electrode RA Ring and electrode  20  forms a right ventricular ring electrode RV Ring. Atrial cardioversion shock coil  40  is a coil electrode providing a relatively large geometric area when compared to the stimulation electrodes  18 ,  20 ,  22  and  24 . 
     Lead  30  is a left ventricular electrode lead passing through the coronary sinus of heart  12  and having a left ventricular ring electrode LV RING  32  a left ventricular tip electrode LV TIP  34 . Further, a left ventricular defibrillation shock coil  36  is arranged on lead  30 . 
     Implantable medical device  10  has a case  42  made from electrically conductive material such as titanium that can serve as a large surface electrode IMD CASE. 
     The plurality of electrodes  18 ,  20 ,  22 ,  24 ,  32 ,  34 ,  36 ,  38  and  40  connected to implantable medical device  10  together with case  42  allow for a number of different electrode configurations for measuring intrathoracic and intracardiac impedance. 
     Referring to  FIG. 3  a simplified block diagram of an implantable medical device  10  is illustrated. During operation of the pacemaker leads  14 ,  16  and  30  are connected to respective output/input terminals of pacemaker  10  as indicated in  FIG. 2  and carry stimulating pulses to the tip electrodes  18 ,  22  and  34  from a right ventricular pulse generator RV-STIM, a right atrial stimulation pulse generator RA-STIM and a left ventricular pulse generator LV-STIM, respectively. Further, electrical signals from the right ventricle are carried from the electrode pair  18  and  20 , through the lead  16 , to the input terminal of a right ventricular sensing stage RV-SENS; and electrical signals. from the right atrium are carried from the electrode pair  22  and  24 , through the lead  14 , to the input terminal of a right atrial channel sensing stage RA-SENS. Electrical signals from the left ventricle are carried from the electrode pair  32  and  34 , through the lead  30 , to the input terminal of a right ventricular sensing stage RV-SENS 
     The atrial channel sensing stage A-SENS and ventricular sensing stages RV-SENS and LV-SENS comprise analog to digital converter (ADC; not shown) that generate a digital signal from electric signals picked up in the atrium or the ventricle, respectively. 
     Controlling the implantable medical device  10  is a control unit CTRL  54  that is connected to sensing stages A-SENS and V-SENS, to stimulation pulse generators A-STIM and V-STIM and to an impedance determination unit  70 . Control unit CTRL  54  comprises a digital microprocessor forming a central processing unit (CPU; not shown) and is—at least in part—controlled by a program stored in a memory circuit MEM  56  that is coupled to the control unit CTRL  54  over a suitable data/address bus ADR. 
     Control unit CTRL  54  receives the output signals from the atrial sensing stage RA-SENS and from the ventricular sensing stages RV-SENS and LV-SENS. The output signals of sensing stages RA-SENS and RV-SENS are generated each time that a P-wave representing an intrinsic atrial event or an R-wave representing an intrinsic ventricular event, respectively, is sensed within the heart  12 . An As-signal is generated, when the atrial sensing stage RA-SENS detects a P-wave and a Vs-signal is generated, when the ventricular sensing stage RV-SENS detects an R-wave. 
     Control unit CTRL  54  also generates trigger signals that are sent to the atrial stimulation pulse generator RA-STIM and the ventricular stimulation pulse generators RV-STIM and LV-STIM, respectively. These trigger signals are generated each time that a stimulation pulse is to be generated by the respective pulse generator RA-STIM, RV-STIM or LV-STIM. The atrial trigger signal is referred to simply as the “A-pulse”, and the ventricular trigger signal is referred to as the “V-pulse”. During the time that either an atrial stimulation pulse or ventricular stimulation pulse is being delivered to the heart, the corresponding sensing stage, RA-SENS, RV-SENS and/or LV-SENS, is typically disabled by way of a blanking signal presented to these amplifiers from the control unit CTRL  54 , respectively. This blanking action prevents the sensing stages RA-SENS, RV-SENS and LV-SENS 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 pacer stimulation from being interpreted as P-waves or R-waves. 
     Furthermore, atrial sense events As recorded shortly after delivery of a ventricular stimulation pulses during a preset time interval called post ventricular atrial refractory period (PVARP) are generally recorded as atrial refractory sense event Ars but ignored. 
     Control unit CTRL  54  comprises circuitry for timing ventricular and/or atrial stimulation pulses according to an adequate stimulation rate that can be adapted to a patient&#39;s hemodynamic need as pointed out below. 
     Control unit CTRL  54  is connected to a memory circuit MEM  56  that allows certain control parameters, used by the control unit CTRL  54  in controlling the operation of the implantable medical device  10 , to be programmably stored and modified, as required, in order to customize the implantable medical device&#39;s operation to suit the needs of a particular patient. Such data includes the basic timing intervals used during operation of the pacemaker  10  and AV delay values and hysteresis AV delay values in particular. The stored control parameters in particular include an AV delay interval, a daytime base stimulation rate, a nighttime base stimulation rate and a night-to-day transition time and a day-to-night transition time. 
     Further, data sensed during the operation of the implantable medical device  10  may be stored in the memory MEM  56  for later retrieval and analysis. 
     A telemetry circuit TEL  58  is further included in the implantable medical device  10 . This telemetry circuit TEL  58  is connected to the control unit CTRL  54  by way of a suitable command/data bus. Telemetry circuit TEL  58  allows for wireless data exchange between the implantable medical device  10  and some remote programming or analyzing device which can be part of a centralized service center serving multiple pacemakers. Telemetry circuit  56  serves as a data interface for wireless data communication with external device  80  and for receiving values for selected control parameters to be stored in memory circuit MEM  56 , in particular. The selected control parameters include the night-to-day transition time and a day-to-night transition time. 
     The implantable medical device  10  in  FIG. 3  is referred to as a three chamber pacemaker/cardioverter/defibrillator because it interfaces with the right atrium  26 , the right ventricle  28  and the left ventricle of the heart  12 . Those portions of the pacemaker  10  that interface with the right atrium, e.g., the lead  14 , the P-wave sensing stage A-SENSE, the atrial stimulation pulse generator A-STIM and corresponding portions of the control unit CTRL  54 , are commonly referred to as the atrial channel. Similarly, those portions of the pacemaker  10  that interface with the right ventricle  28 , e.g., the lead  16 , the R-wave sensing stage V-SENSE, the ventricular stimulation pulse generator V-STIM, and corresponding portions of the control unit CTRL  54 , are commonly referred to as the ventricular channel. 
     In order to be able to detect periods of physical activity of a patient indicating that the patient is awake and in order to allow rate adaptive pacing in a DDDR or a DDIR mode, the pacemaker  10  further includes a physiological sensor ACT  60  that is connected to the control unit CTRL  54  of the pacemaker  10 . While this sensor ACT  60  is illustrated in  FIG. 2  as being included within the pacemaker  10 , it is to be understood that the sensor may also be external to the implantable medical device  10 , yet still be implanted within or carried by the patient. 
     The control unit CTRL  54  is adapted to determine an adequate heart rate or stimulation rate in any manner known as such. This includes application of a base application when a patient is at rest and applying an elevated stimulation rate, when the activity sensor  60  senses physical activity of a patient. Depending on the daytime, either a daytime base stimulation rate is applied or a nighttime base stimulation rate. 
     For impedance measurement, impedance determination unit  70  is provided. Impedance determination unit  70  comprises a constant current source  72  that is connected or can be connected to electrodes for intracorporeal placement as shown in  FIG. 2 . In order to allow for a plurality of impedance measurement electrode configurations, preferably some means of switching is provided between the constant current source  72  and the electrode terminals of the implantable medical device  10 . The switch is not shown in  FIG. 3 . Rather, particular impedance measurement configurations are shown as examples. 
     Similarly, a impedance measuring unit  74  for measuring a voltage corresponding to a current fed through a body by said constant current source is provided and can be connected to a number of electrodes although a switch for switching between these configurations is not shown in  FIG. 3 . 
     As an alternative to constant current source  72  a constant voltage source can be provided. Then, the measuring unit will be adapted to measure a current strength of a current fed through a body by said constant voltage source. 
     Both, constant current source  72  and impedance measurement unit  74 , are connected to an impedance value determination unit  76  that is adapted to determine an impedance value for each measuring current pulse delivered by the constant current source  72 . 
     The impedance value determination unit  76  comprises another analog to digital converter ADC in order to generate a digital impedance signal that is fed to the control unit CTRL  54 . 
     Further, a clock  78  is connected to control unit CTRL  54  in order to allow control unit  54  to control a base stimulation depending on the daytime. Depending on the output signal of clock  78  and the night-to-day transition time and a day-to-night transition time stored in Memory circuit  56 , control unit CTRL  54  either applies the daytime base stimulation rate or the nighttime base stimulation rate as stored in memory circuit MEM  56 . 
     Control unit CTRL  54  further comprises watchdog and reset units to provide safety when the CPU should fail. The watchdog units therefore are designed to operate independently from the CPU of the control unit CTRL  54 . In  FIG. 3 , the watchdog and reset units are not shown. 
       FIG. 4  is a more detailed representation of the external transceiver device  80 . The external device  80  comprises a telemetry circuit  100  adapted for wireless data transmission to the implantable medical device  10  and a data exchange interface  102  adapted to allow a data communication with service center  90 . Both, the telemetry circuit  100  and the data exchange interface  102  are connected to an external device control unit  104 . The external device control unit  104  is connected to an external device memory  106 . The external device memory  106  is adapted to store data received from or to be transmitted to either the central service center  90  or to the implant  10 . Further, the external device memory circuit  106  comprises data that can be entered via an external device user interface  108 . The external device user interface  108  is connected to the external device control unit  104  and comprises an input and display panel  110  and an interface circuit  112 . The input display panel  110  comprises a display  114  and two input buttons  116  and  118 . 
     The user interface  108  is adapted to display an actual day time on the display  114 . Further, an alarm time can be set via buttons  116  and  118 . Button  116  serves for entering the hour of alarm and button  118  serves for entering the minute of alarm. The alarm time thus set is displayed in display  114 . In order to enable or disable the alarm a toggle-button  120  is provided. The state of the alarm—enabled or disabled—is indicated on display  114  by an according icon. 
     Control unit  104  of the external device  80  is adapted to generate a data package to be sent to implant  10 , whenever an alarm is activated via user interface  108 . The data package comprises a night to day transition time corresponding to the alarm time set via user interface  108  and a day to night transition time corresponding to the actual day time when the alarm was activated. 
     This data package is received by telemetry circuit  58  of the implantable medical device  10  and the night to day transition time and the day to night transition time contained in the data package is stored in memory circuit MEM  56  of the implantable medical device  10 . The selected parameters thus generated, transmitted and stored in the implantable medical device  10  are used as control parameters for controlling the implantable medical device via the implantable medical device control unit CTRL  54 . 
     Although an exemplary embodiment of the present invention has been shown and described, it should be apparent to those of ordinary skill that a number of changes and modifications to the invention may be made without departing from the spirit and scope of the invention. This invention can readily be adapted to a number of different kinds of implantable medical devices by following the present teachings. All such changes, modifications and alterations should therefore be recognized as falling within the scope of the present invention.