Patent Publication Number: US-2007118187-A1

Title: Alerting method for a transvascular tissue stimulation system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims benefit of U.S. Provisional Patent Application No. 60/738,439 filed Nov. 21, 2005. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      Not Applicable  
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates to implantable medical devices that electrically stimulate tissue for therapeutic purposes, and more particularly to communication of data regarding operation of the implanted device to external monitoring equipment.  
      2. Description of the Related Art  
      Various physiological ailments have remedies that involve implanting a stimulation device which applies electrical pulses to an organ or other part of the patient&#39;s body associated with the ailment. The stimulation device includes an electronic pulse generator from which electrical leads extend to electrodes in contact with bodily tissue, which when electrically stimulated provide therapy to the patient.  
      For example, a common remedy for people with slowed or disrupted natural heart activity is to implant a cardiac pacing device, which is a small electronic apparatus that stimulates the heart to beat at regular rates. The pacing device typically is implanted in the patient&#39;s chest and has sensor electrodes that detect electrical impulses associated with heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to electrodes placed adjacent specific cardiac muscles, that when electrically stimulated contract the heart chambers.  
      U.S. Pat. No. 7,003,350 describes a cardiac pacemaker that is implanted in the vasculature of the patient. A power transmitter, located outside the patient, emits a radio frequency signal that is received by a pacing circuit on a stent embedded in a vein or artery near the patient&#39;s heart. The radio frequency signal induces a voltage pulse in an antenna of the pacing circuit, thereby conveying electrical power to the implanted circuitry. The pacing circuit senses electrical activity of the heart and determines when to apply that electrical power in the form of voltage pulses across a pair of electrodes in contact with blood vessel walls. The voltage pulses stimulate adjacent muscles, thereby contracting the heart.  
      These stimulation devices need to monitor and/or confirm overall treatment performance and efficacy. A cardiac pacing device, for example, monitors whether the pacing pulses are effective in improving or correcting heart rhythm. Other physiological parameters can be sensed to gather statistical data continuously or periodically which data can be compared against a baseline.  
      It is desired that physiological and device performance data be communicated from the implanted device to equipment outside the patient for review by medical personnel. It is further desirable that medical personnel be alerted automatically when the communicated data indicates adverse conditions. For example, the user and medical personnel must be alerted if the power transmitter is inadvertently removed or improperly positioned, so that the implanted device does not receive the radio frequency signal that provides operating power to the device.  
     SUMMARY OF THE INVENTION  
      The present system monitors an implanted medical device that stimulates tissue of a patient. This system can be configured to perform one or more alerting functions which include: warning the patient or a caregiver to perform action to correct an adverse condition detected by the monitoring, provide verification of proper placement of the medical device, and autonomously initiate communication with external, remotely located equipment.  
      The system for monitoring a medical patient and stimulating the patient&#39;s tissue includes a medical device for implantation entirely in vasculature of the patient and an external power source that is outside the patient. The medical device has a discriminator that receives and extracts energy from a first wireless signal which is used to power the medical device. A detector circuit produces data regarding a physiological characteristic or performance of the medical device and a feedback transmitter that sends information related to the data via a second wireless signal. That information can comprise the data or information derived from processing and analysis of the data.  
      The external power source transmits the first wireless signal and has a receiver that receives and extracts the information from a second wireless signal. A communication module is provided for communicating with a remote monitor. When the information indicates existence of a predefined condition, the communication module sends an alert message via a third wireless signal for reception by the remote monitor.  
      In one embodiment, the communication module has cellular telephone circuitry that produces the third wireless signal. When the data indicates existence of the predefined condition, the communication module dials a telephone number assigned to a remote monitor and sends an alert message for reception by the remote monitor.  
      In another aspect of the present invention, the medical device has a pair of electrodes for contacting the patient&#39;s tissue and a stimulation circuit applies electrical stimulation pulses to the pair of electrodes. The detector circuit also is connected to the pair of electrodes and senses a physiological characteristic of the medical patient simultaneously when an electrical stimulation pulse is being applied to those electrodes. In a preferred embodiment of this aspect, the detector circuit has an instrumentation amplifier with a variable gain and inputs connected to the pair of electrodes. The instrumentation amplifier is dynamically configured to have a lower gain while a stimulation pulse is being applied to the pair of electrodes than at other times. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is an illustration of a tissue stimulation system attached to a medical patient;  
       FIG. 2  is an isometric, cut-away view of a patient&#39;s blood vessels in which a receiver antenna, a stimulator, and an electrode of an intravascular medical device have been implanted at different locations; and  
       FIG. 3  is a schematic circuit diagram of the external and internal components for the tissue stimulation system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Although the present invention is being described in the context of and implanted tissue stimulation system and specifically a cardiac pacing system, it can be used with other implanted medical devices. Furthermore, the inventive concepts are not limited to devices implanted in the vascular system, but can be employed with components implanted elsewhere in the animal.  
      Initially referring to  FIG. 1 , a tissue stimulation system  10  for electrically stimulating a heart  12  to contract comprises an external power source  14  and a medical device  15  implanted in the blood circulatory system of a human medical patient  11 . The medical device  15  receives a radio frequency (RF) signal from the power source  14  worn outside the patient and the circuitry of the implanted device is electrically powered from the energy of that signal. At appropriate times, the medical device  15  delivers an electrical stimulation pulse into the surrounding tissue of the patient.  
      The power source  14  includes a radio frequency transmitter that is powered by a battery. The transmitter periodically emits a signal at a predefined radio frequency that is applied to a transmitter antenna in the form of a coil of wire within a band  22  that is placed around the patient&#39;s upper arm  23 . The radio frequency is received by an antenna assembly  24  implanted in the basilic vein  26  of the patient&#39;s upper right arm  23 , for example. In a basic version of the tissue stimulation system  10 , the radio frequency signal merely conveys energy for powering the medical device  15  implanted in the patient. In other systems, the transmitter modulates the radio frequency signal with commands that configure or control the operation of the medical device  15 .  
      Referring to  FIGS. 1 and 2 , the exemplary implanted medical device  15  includes an intravascular stimulator  16  located a vein or artery  18  in close proximity to the heart. Because of its electrical circuitry, the stimulator  16  is relatively large requiring a blood vessel that is larger than the arm vein  26 , that is approximately five millimeters in diameter. Therefore, the stimulator  16  is implanted in the superior or inferior vena cava, for example. However, it is contemplated that further miniaturization of components will reduce the size of the stimulator circuitry enabling placement in smaller veins and arteries. Electrical wires lead from the stimulator  16  through the cardiac vascular system to one or more locations in smaller blood vessels  19 , such as the coronary sinus vein, at which stimulation of the heart is desired. At those locations, the electrical wire  25 , from the stimulation circuit  32  is connected to a remote electrode  21  secured to the blood vessel wall.  
      Because the stimulator  16  of the medical device  15  is near the heart and relatively deep in the chest of the medical patient  11 , the RF antenna assembly  24  is implanted in a vein or artery  26  of the patient&#39;s upper right arm  23  at a location surrounded by the transmitter antenna within the arm band  22 . That arm vein or artery  26  is significantly closer to the skin and thus implanted antenna assembly  24  picks up a greater amount of the energy from the first radio frequency signal emitted by the power source  14 , than if that antenna assembly was located on the stimulator  16 . Alternatively, another limb, the neck or other area of the body with an adequately sized blood vessel close to the skin surface of the patient can be used. The implanted antenna assembly  24  comprises a receiver antenna  34  and a transmitter antenna  35  in the form of wire coils that are connected to the stimulator  16  by a cable  33 .  
      As illustrated in  FIG. 2 , the intravascular stimulator  16  has a body  30  constructed similar to well-known expandable vascular stents commonly employed to enlarge constricted blood vessels. The stimulator body  30  comprises a plurality of interwoven wires formed to have a memory defining a tubular shape or envelope. Those wires are heat-treated platinum, Nitinol, a Nitinol alloy wire, stainless steel, plastic wires or other materials which will provide the shape memory and not react with the tissue at the implantation site. Plastic or substantially nonmetallic wires may be loaded with a radiopaque substance to be visible with conventional fluoroscopy. The stimulator body  30  has a memory so that it normally assumes an expanded configuration when unconfined, but is capable of assuming a collapsed configuration when disposed and confined within a catheter assembly. In that collapsed state, the tubular body  30  has a relatively small diameter enabling it to pass freely through the vasculature of a patient while being guided on the catheter assembly. After being positioned in the desired blood vessel, the body  30  is released from the catheter assembly and expands to engage the blood vessel wall. The stimulator body  30  and other components of the medical device  15  are implanted in the patient&#39;s vasculature.  
      The body  30  has a stimulation circuit  32  mounted thereon and if implanted proximate the heart  12 , holds a first electrode  20  in the form of a ring that encircles the body. Alternatively, when the stimulator  16  is distant from the heart  12 , the first electrode  20  is remotely located in a small cardiac blood vessel, much the same as a second electrode  21 . Conventional circuitry within the stimulation circuit  32  detects the electrical activity of the heart  12  and determines when electrical pulses need to be applied so that the heart contracts at the proper rate. When stimulation is desired, the stimulation circuit  32  applies electrical voltage from an internal storage device across the electrodes  20  and  21 . The second electrode  21  and the first electrode, when located remotely from the stimulator  16 , can be mounted on a collapsible body of the same type as the stimulator body  30 . In all the examples cited with regard to the  FIG. 2 , it should be understood that the exemplary size limit is driving the decision on the placement of components. It is contemplated that miniaturization of components will lead to many more options for component placement.  
      Referring to  FIG. 3 , the electrical circuitry for the external power source  14  of the tissue stimulation system  10  includes a battery  46 , a radio frequency (RF) power transmitter  40 , a power feedback module  41 , an RF communication receiver  42 , an implant monitor  43 , and a control circuit  45 . In addition, a communication module  47  is provided to exchange data and commands via a communication link  48  with a remote monitor, such as a personal computer  70 , patient monitor  71 , cellular telephone  72 , pager, personal digital assistant (PDA), or similar wireless equipment. The communication link  48  preferably is wireless, such as a radio frequency signal or a cellular telephone call.  
      The battery  46  is rechargeable allowing patient mobility with periodic recharge cycles. Depending upon the type and size of the battery, the time between recharge cycles may be days, months or years. Power transmitter  40  and a first antenna  37  periodically transmit a radio frequency first wireless signal  36  that is pulse width modulated (PWM) in a variably controlled manner to convey different amounts of energy to the implanted medical device  15 . The stimulation circuit  32  is connected to the receiver antenna  34  that is tuned to pick-up the first wireless signal  36  which also carries control commands to the medical device  15 . The receiver antenna  34  is coupled to a discriminator  49  that separates the received signal into electrical power and commands. A rectifier  50  in the discriminator  49  extracts energy from the first wireless signal. Specifically, the radio frequency, first wireless signal  36  is rectified to produce a DC voltage (VDC) that is applied across the storage device  54 , e.g. a capacitor, which functions as a power supply by furnishing electrical power to the other components of the medical device.  
      The charge of the power storage device  54  is monitored and the stimulation circuit  32  sends data indicating its power needs via a second wireless signal  38  at a different radio frequency. The second wireless signal is received by a second antenna  39  and the RF communication receiver  42  in the external power source  14 . A power feedback module  41 , connected to the communication receiver, is part of closed control loop that receives the medical device&#39;s power needs data and responds by controlling the duty cycle of the first wireless signal  36  to ensure that the medical device  15  has a sufficient amount of electrical power.  
      As necessary, the first wireless signal  36  also carries control commands that specify operational parameters of the medical device  15 , such as the duration of the stimulation pulses to be applied to the electrodes  20  and  21 . These commands are sent digitally as a series of binary bits encoded on the first wireless signal  36  by fixed duration pulses of that signal. The receiver antenna  34  also is coupled to a data detector  51  within discriminator  49  that recovers the commands and other data from the first wireless signal. The recovered information is sent to a controller  53 , which controls the operation of a stimulation circuit  62 . Preferably, the controller  53  comprises a microcomputer that has analog and digital input/output circuits and an internal memory  55  that stores a software control program and data acquired and used by that program.  
      The controller  53  also receives signals from a detector circuit  56 , which includes a sensor  57  and an amplifier, that detect physiological characteristics, such as temperature, blood pressure, blood flow, blood volume, and blood glucose level of the patient  11 . The physiological data is stored by the controller  53  in the memory  55  from which it is periodically read and communicated to the external power source  14  or another external data gathering device.  
      The first and second electrodes  20  and  21  detect electrical activity of the heart and provide conventional electrocardiogram signals that are applied to inputs of a variable gain instrumentation amplifier  58  that also is part of the detector circuit  56 . The gain of the instrumentation amplifier  58  is varied by a signal from the controller  53 , as will be described. The output of the instrumentation amplifier  58  is coupled to an analog input of the controller  53  and to an input of a differentiator  59 . The differentiator  59  has another input which receives a reference level (REF) which enables signal transition detection to provide a signal to the controller  53  indicating events in the sensed cardiac activity. For example, the differentiator  59  in conjunction with software executed by the controller  53  determines the heart rate based on the number of transitions counted over a defined time interval. The controller  53  commences cardiac pacing when the heart rate goes out of a normal range for a given length of time. When the heart rate indicates fibrillation, the controller initiates defibrillation pulse to the electrodes  20  and  21 . A histogram of the electrocardiogram signals and pacing data related to usage of the medical device is stored in memory  55 .  
      Stimulation Signal Regulation  
      The software executed by the controller  53  analyzes the electrocardiogram signals from the first and second electrodes  20  and  21  and the other physiological signals from the sensors  57  to determine when and how to stimulate the patient&#39;s heart. When stimulation is required the controller  53  issues a command designating the voltage level, shape, and duty cycle of stimulation pulses to be applied to the first and second electrodes  20  and  21 . That command is sent to a stimulation signal generator  60  which responds by applying one or more pulses of voltage from the storage device  54  across the electrodes. The stimulation signal generator  60  controls the intensity and shape of the pulses. The output pulses from the stimulation signal generator  60  can be applied either directly to the first and second electrodes  20  and  21  or via an optional voltage intensifier  61 . The voltage intensifier  61  preferably is a “flying capacitor” inverter that charges and discharges in a manner that essentially doubles the power. However, other kinds of devices can be used to increase the stimulation voltage.  
      The first and second electrodes  20  and  21  also are used as sensors to provide feedback signals for regulating the stimulation. When stimulation is occurring, the instrumentation amplifier  58  has low gain (1× or lower) to avoid saturation and thus sense a physiological data simultaneously while a stimulation pulse is occurring. This is particularly useful to determine the impedance of the tissue between the electrodes  20  and  21 . The low gain setting allows measurement of the tissue and electrode interface impedance by using the known stimulation pulse duration and amplitude as a known source and the system impedance as a known impedance. From the sensed voltage and the known impedances, the tissue and electrode interface impedance can be determined. This information can also be logged into the memory  55  over time to monitor physiological changes that may occur.  
      When stimulation is inactive, the instrumentation amplifier  58  has a normal gain (100×-200×) to sense physiological characteristics, such as the electrical activity of the heart. At these times, the controller  53  analyzes the sensed physiological characteristics to calculate the actual heart rate and determine whether the heart is beating at the desired rate in response to pacing stimulation. If the heart is at the desired rate, the controller  53  decreases the stimulation pulse energy in steps until stimulation is no longer effective. The stimulation pulse energy then is increased until the desired rate occurs. Energy reduction is accomplished at least in two ways: (1) preferably, the duty cycle is reduced to linearly decrease that amount of energy dissipated in the tissue, or (2) the voltage amplitude is reduced in situations where energy dissipation might vary non-linearly because the tissue/electrode interface is unknown.  
      The stimulation is controlled by a functionally closed feedback loop. When stimulation commences, the sensed signal waveform can show a physiological response confirming effectiveness of that stimulation pulse. By stepwise increasing the stimulation pulse duration (duty cycle), a threshold can be reached in successive steps. When the threshold is reached, an additional duration can be added to provide a level of insurance that all pacing will occur above the threshold, or it may be sufficient to hold the stimulation pulse duration at the threshold.  
      After each successful stimulation pulse, a determination is made regarding the difference in duration existing between the last non-effective pulse and the present effective pulse. That difference in duration is added to the present time. The system then senses the effectiveness of subsequent stimulation pulses and remains at the same level for either an unlimited duration or backs off one step in pulse duration. When the effectiveness is maintained again after a preset time window, which could be a number of beats, minutes or hours, the system backs off one decrement at a time. As soon as the effectiveness of the stimulation pulses is lost, the system keeps incrementing the duration until an effective pulse is obtained. In summary, the sensing and stimulation is a closed loop system with two feedback responses: the first response is following an effective pulse and involves gradual reduction of duration after a predetermined number of beats or a predetermined time interval; and the second response is to an ineffective pulse and is immediate with pulse duration adjustment occurring within one beat.  
      Supplied Power Control  
      Another feedback control loop is employed to regulate the electrical power supplied to the implanted medical device  15  from the external power source  14 . As mentioned previously, the rectifier  50  in the discriminator  49  of the medical device  15  extracts energy from the received radio frequency first wireless signal  36  to charge the storage device  54 . The storage device  54  preferably is a super capacitor that is an electrochemical double layer capacitor (EDLC) hybrid between a conventional capacitor and a battery, and has a greater extend the life span and power capability than standard rechargeable batteries. However, a rechargeable battery can be employed as the storage device  54  instead of a capacitor. In either case, the circuitry of the medical device  15  receives power for an extended period even if the power source  14  is removed from the patient for short periods.  
      The DC voltage produced by rectifier  50  is regulated. For this function, the DC voltage is applied to a voltage detector  63  that senses and compares the DC voltage to a nominal voltage level desired for powering the medical device  15 . The result of that comparison is a control voltage which indicates the relationship of the actual DC voltage derived from the first wireless signal  36  to the nominal voltage level. The control voltage is fed to a feedback transmitter  64  and specifically to the input of a voltage controlled radio frequency oscillator  65  which produces an output signal at a radio frequency that varies as a function of the control voltage. For example, the radio frequency oscillator  65  has a center, or second frequency from which the actual output frequency varies in proportion to the polarity and magnitude of the control signal and thus deviation of the actual DC voltage from the nominal voltage level. For example, the radio frequency oscillator  65  has a first frequency of 100 MHz and varies 100 kHz per volt of the control voltage deviation with the polarity of the control voltage determining whether the oscillator frequency decreases or increases from the second frequency. For this exemplary oscillator, if the nominal voltage level is five volts and the output of the rectifier  50  is four volts, or one volt less than nominal, the output of the voltage controlled, radio frequency oscillator  65  is 99.900 MHz (100 MHz-100 kHz). That output is applied by an RF amplifier  66  to the transmitter antenna  35  in the implanted antenna assembly  24  which emits the second RF wireless signal  38 .  
      To control the energy of the first wireless signal  36 , the power source  14  contains a second antenna  39  that picks up the second wireless signal  38  from the implanted medical device  15 . Because the second wireless signal  38  indicates the level of energy received by medical device  15 , this enables power source  14  to determine whether medical device requires more or less energy to be adequately powered. The second wireless signal  38  is sent from the second antenna  39  to the power feedback module  41  which detects the frequency shift of that wireless signal from the second frequency and thus the thus deviation of the actual DC voltage from the nominal voltage level, which is an ERROR signal. That ERROR signal is used to control the duty cycle of the pulses of the first wireless signal  36  and thus the amount of energy that signal provides to the medical device  15 . By maintaining a constant voltage across storage device  54  in the medical device  15 , it is ensured that only the needed amount of power is transmitted.  
      Physiological Sensing  
      Referring still to  FIG. 3 , the first and second electrodes  20  and  21  detect electrocardiogram signals representing electrical activity of the heart and the sensors  57  provide signals related to other physiological characteristics, such as temperature, blood pressure, blood flow, blood volume, and blood glucose level. More sophisticated data analysis also can be performed to detect cardiac abnormalities, such as arrhythmias and atrial fibrillation. The controller  53  of the implanted medical device  15  receives and digitizes those signals and stores the resultant data in memory  55 . The sensors  57  may produce a signal that directly indicates a physiological characteristic, such as temperature or pressure, or the sensor signal may be processed in the controller  53  by software that implements a conventional algorithm to derive data, such as blood volume or blood glucose level, from that signal. Other data pertaining to operational conditions of the stimulation circuit  32  also are stored.  
      The data may be stored as trending logs that indicate patient and/or device conditions over time. Trending logs can be accumulated continuously with the implant monitor  43  keeping the highest time resolution for the most recent events in minutes, mid-range events in hours, and long-range events in days, weeks, etc. For example, it may be desired to take blood pressure readings every few minutes, whereas blood glucose levels can be recorded once an hour. In some instances the raw sensor data is averaged during a predefined time period by the controller and only the average is stored in the memory  55 . For other kinds of data, only a maximum or minimum value occurring in a given time period is retained. The storage time resolution for a given kind of data also may vary depending upon the recency of each item of that data, wherein more recently acquired items have a higher resolution than older items in order to conserve storage space in the memory. For example, every blood pressure reading acquired at five minute intervals during the last hour are held in the memory, and the data more than an hour old is culled with only every sixth data item (one per half hour) being retained. Alternatively, the culling process may average groups of data items (e.g. six blood pressure readings) and keep only the average in memory. The storage procedures, such as storage time resolution, averaging, etc., are user configurable by commands entered into the personal computer  70  and transmitted by the power source  14  via the radio frequency first wireless signal  36  to the implanted medical device  15 .  
      Alternatively, minimal data retention can occur in the implanted medical device  15  with the power source  14  performing the primary storage of data. Here the data acquired by the implanted medical device  15  is streamed in real-time via the radio frequency second wireless signal  38  to the power source  14  where the data is stored in the memory  44  of the implant monitor  43  or a memory of the control circuit. The raw sensor data can be sent for analysis by the implant monitor  43  to derive more complex data, such as blood volume and blood glucose level, and to detect cardiac abnormalities, such as arrhythmias and atrial fibrillation. Trend analysis also is performed on the raw sensor data and the complex data.  
      Regardless of the data processing and storage capacity of the implanted medical device  15 , data at some point in time is communicated to the power source  14  or another data gathering device that is external to the patient  11 . That data transfer may be at regular intervals based on a timer implemented by the controller  53 , upon the data having a predefined characteristic, e.g. blood pressure above a defined level or atrial fibrillation occurring, or in response to a request sent by the power source  14 . The request sent from the power source  14  may originate in its control circuit  45  or be relayed from the personal computer  70  or other remote monitor. When such transfer is initiated, the data is retrieved from the memory  55  in the medical device  15  and sent to a data modulator  67 . The data modulator  67  formats the data into a message packet that is applied to the RF amplifier  66 , which amplitude modulates the radio frequency signal from the voltage controlled RF oscillator  65  with that data packet. The modulated radio frequency signal is applied to the implanted transmitter antenna  35  from which it is emitted as the second wireless signal  38 .  
      When the power source  14  receives the second wireless signal  38 , the RF communication receiver  42  extracts modulated data which is transferred to the implant monitor  43  for storage in memory  44  and possible further processing. The power source  14  may also forward the data to the remote monitor, e.g. personal computer  70 , patient monitor  71  or cellular telephone  72 , via the communication module  47  and link  48 . The communication link  48  preferably is a wireless link, such as a radio frequency signal or a cellular telephone call, however it can be a cable that is occasionally plugged into the power source  14 .  
      If the data indicates a serious abnormality in the patient, the signal from the power source  14  on communication link  48  alerts a caregiver to that condition. For this function the implant monitor  43  in the power source  14  shown in  FIG. 3 , analyzes the data that either was transferred from the medical device or which was derived from that transferred data. That analysis compares the data to setpoints previously stored in memory  44  which designate a condition or event that requires alerting medical personnel. The setpoints can be stored by the manufacturer of the tissue stimulation system  10  or programmed into the power source  14  by the medical personnel. Some setpoints are thresholds of the data, such as a specific heart rate or blood pressure, while other setpoints are dependent variables such as a rate of change of a type of data, e.g. a maximum allowable heart rate change. When the setpoint comparison indicates an alert condition, the implant monitor  43  sends an alert signal to the control circuit  45  indicating the nature of the associated condition.  
      Other alert conditions relate to the performance of the tissue stimulation system  10 . For example, if the power feedback module  41  determines that the voltage on the implanted storage device  54  is below an acceptable level or that the second RF wireless signal has a signal strength below an given level or no longer is being received, as occurs when the arm band  22  is removed, the appropriate alert signal is sent to the control circuit  45  in the power source. The power feedback module  41  may calculate the power consumption of the medical device  15  and issue another alert signal when too much power is being consumed.  
      The control circuit  45  responds in several ways to these alert signals. A local alert is issued to the patient  11  from an annunciator such as an audible device  74  and a visible indicator  76  on the armband  22  on which the power source  14  is mounted. The audible annunciation is either a simple alarm tone or a voice message that is either pre-recorded or computer generated. Other types of annunciator displays can be provided for alphanumeric text and images related to the alert condition.  
      For example, an audible signal indicates when the power source  14  is at an optimal relative position with respect to the antenna assembly  24  of the implanted medical device  15 . This function is initiated by closing a switch  78  on the power source  14 . The RF communication receiver  42  in the power source  14  measures the strength of the second wireless signal  38  from the medical device  15  and a the audible device  74  emits a tone the loudness of which is varied in proportion to the strength of the second wireless signal. The best component positioning occurs when that signal strength is the greatest and is thus indicated when the tone is the loudest.  
      Remote alert annunciation also is provided to alert medical personnel such as a nurse, a caregiver, or a physician, or to alert a relative or another person. This further altering is carried out by the control circuit  45  forming a message based on the alert signal received from the implant monitor  43  or the power feedback module  41 . That message is customized for the remote monitor that is to receive the alert. For the personal computer  70  or the patient monitor  71  the message can simply be a number indicating the specific condition that triggered the alert, e.g. non-receipt of the second RF signal or high blood pressure. Alternatively, the alert message provides more specific information such as the patient&#39;s blood pressure measurement that was too high. Upon receiving the message, the personal computer  70  or the patient monitor  71  decodes the message contents using a data table stored in that recipient device and uses other stored information to present text on its display screen to inform a person about the nature of the alert. For the cellular telephone  72 , the control circuit formulates an audio message using pre-recorded announcements for the various alert conditions and sends that audio message to the communication module  47 , which in this case is a cellular telephone. The communication module  47  dials a predefined telephone number and when the recipient telephone  72  is answered the audio message is sent over the telephone link.  
      The alerting is a multi-tier system for certain conditions which trigger an alert. For example, as noted previously the power source  14  issues an alert when the radio frequency second wireless signal  38  is not received from the implanted medical device  15 , as occurs when the patient removes the arm band  22 . This event initially causes the power source  14  to issue local alerts by activating the audible device  74  and the visible indicator  76 . If within a given time period those alerts do not result in corrective action that reestablishes receiving the second wireless signal  38  (e.g. the patient putting on the arm band), the power source  14  issues an alert message via the communication module  47  to the remote monitors  70 - 72 .  
      The loss of the second wireless signal  38  is considered a serious condition of the patient as it may result from deactivation of the tissue stimulation system  10 . Examples of other serious conditions are excessively high blood pressure, absence of heartbeat for a prolonged time, and atrial fibrillation. In these cases, alert messages are issued immediately to the remote devices, without waiting to see if a local alert results in corrective action.  
      The present system provides impromptu situation-based, autonomous alerting by the tissue stimulation system  10  that allows corrective action at a tiered level, commensurate to the condition which triggered the alert. In autonomous alerting, the device takes action based on a set of criteria and circumstance. In some embodiments, environmental variables, such as air pressure, air temperature and skin temperature may be incorporated to correlate with physiological data prior to an alerting decision being made.  
      The alerting system is capable of self monitoring, physiological monitoring and autonomously alerting the patient, a bystander, a remote expert, a networked computer, a service person or a relative. Thus it is further intended to include alerting mechanism to communicate with different, independent communicable targets based on both the needs of the device and the patient based on predetermined conditions. In a first case, a caretaker can be alerted if internal and external components do not communicate with each other for a predetermined time. In a second case, the alerting mechanism may contact a medical service or physician if abnormal heart rhythms are observed. In a third example, the alerting mechanism may trigger a service call if communication is present but battery power is lower than a predetermined value.  
      The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.