Patent Publication Number: US-6990372-B2

Title: Programmable signal analysis device for detecting neurological signals in an implantable device

Description:
FIELD OF THE INVENTION 
   The present invention is generally directed to circuitry for use within implantable medical devices and in particular to such circuitry which senses neurological signals, e.g., from nerves or muscles, to detect intended or actual muscle stimulation. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to devices and systems of such devices for monitoring and/or affecting parameters of a patient&#39;s body for the purpose of medical diagnosis and/or treatment. More particularly, such devices, preferably battery powered, are configured for implanting within a patient&#39;s body, each device being configured to sense a body parameter, e.g., temperature, O 2  content, physical position, electrical potential, etc., and/or to affect a parameter, e.g., via nerve and/or muscle stimulation. 
   Commonly owned U.S. Pat. Nos. 6,164,284; 6,208,894; and 6,315,721; each entitled “System of Implantable Devices For Monitoring and/or Affecting Body Parameters” and U.S. Pat. No. 6,185,452 entitled “Battery Powered Patient Implantable Device”, each incorporated herein by reference in their entirety, describe devices configured for implantation within a patient&#39;s body, i.e., beneath a patient&#39;s skin, for performing various functions including: (1) stimulation of body tissue and/or sensing of body parameters, and (2) communicating between implanted devices and devices external to a patient&#39;s body. In an exemplary use, the implanted device is used to electrically stimulate a neural pathway and/or muscle and the same (and/or another) implanted device senses an evoked response from the intended muscle tissue and uses the detected signal to confirm that stimulation did occur and/or to achieve closed loop control. In general, the detected signal may exhibit a frequency component that corresponds to the intensity of the intended or actual muscle response and amplitude components that correspond to its proximity to the desired source tissue and/or other signal generating tissue. Depending on the application, there are various techniques that may be used to interpret the neurological signal. Such implantable devices are preferably powered using rechargeable batteries and it is desired that the time between rechargings be maximized by minimizing the power dissipation of such circuitry within these implantable devices. Accordingly, what is needed is a programmable signal analysis circuit that can be configured to interpret neurological signals using a plurality of analysis modes. Furthermore, such a circuit should minimize its power dissipation to thus enhance the battery life of the implantable devices. 
   SUMMARY OF THE INVENTION 
   The present invention is generally directed to circuitry for use within implantable medical devices and in particular to such circuits which sense neurological signals, e.g., from nerves or muscles, to detect intended or actual muscle stimulation. In an exemplary application for the present invention, such circuits may be used within implanted devices configured similarly to the devices described in the commonly owned U.S. Pat. No. 6,164,284. Such implanted devices typically comprise a sealed housing suitable for injection into the patient&#39;s body and preferably contain a power source, e.g., a battery, having a capacity of at least 1 microwatt-hour and power consuming circuitry preferably including a data signal transmitter and receiver and sensor/stimulator circuitry for driving an input/output transducer. In a typical application, such devices are used to stimulate a neural pathway or muscle and/or to block a neural pathway to alleviate pain or block stimulation of a muscle. The present invention is thus specifically directed to an implementation of the sensor circuitry for use in such an implantable device. 
   A preferred signal analysis device for use within an implantable device, wherein the implantable device is configured at least in part for sensing a biological signal within a patient&#39;s body and the implantable device is contained within a sealed elongate housing having an axial dimension of less than 60 mm and a lateral dimension of less than 6 mm, comprises: (1) sensing circuitry for receiving a biological signal within the implantable device and generating a sensed voltage output in response thereto; (2) event detection circuitry for detecting an attribute of the sensed output according to one or more designated criteria and designating the detected attribute as an event; (3) an event counter configured for accumulating detected events; (4) a clock counter for accumulating clock pulses; and (5) event analysis circuitry for determining a processed value corresponding to the detected events and the accumulated clock pulses. 
   In a further aspect of a preferred embodiment of the present invention, the event analysis circuitry is configurable to operate in a plurality of modes, e.g., a mode which determines the rate at which detected events occur, a mode which determines the number of events that occur within a designated time period, a mode which determines the amount of time between a start time and the first detected event, etc. 
   In a still further aspect of a preferred embodiment of the present invention, the event detection circuitry determines whether an event occurred according to one or according to two or more programmable criteria, e.g., amplitude threshold levels. Alternatively or additionally, a peak detector may be used to determine events. 
   Additionally, the event detection circuitry may be comprised in part of two digital to analog converters that convert programmable digital threshold values into analog threshold values that are compared with a sensed analog neurological signal value to determine when the neurological signal is above or below the programmed threshold values. These comparisons are then programmably used to identify (or to exclude) events. 
   The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of an exemplary system suitable for practicing the present invention, the system being comprised of implanted devices, e.g., microstimulators, microsensors and microtransponders, under control of an implanted system control unit (SCU). 
       FIG. 2  comprises a block diagram of the system of  FIG. 1  showing the functional elements that form the system control unit and implanted microstimulators, microsensors and microtransponders. 
       FIG. 3A  comprises a block diagram of an exemplary implantable device, as shown in U.S. Pat. No. 6,164,284, including a battery for powering the device for a period of time in excess of one hour in response to a command from the system control unit. 
       FIG. 3B  comprises a simplified block diagram of controller circuitry that can be substituted for the controller circuitry of  FIG. 3A , thus permitting a single device to be configured as a system control unit and/or a microstimulator and/or a microsensor and/or a microtransponder. 
       FIG. 4  shows an exemplary flow chart of the use of the exemplary system in an open loop mode for controlling/monitoring a plurality of implanted devices, e.g., microstimulators, microsensors. 
       FIG. 5  shows a simplified flow chart of the use of closed loop control of a microstimulator by altering commands from the system control unit in response to status data received from a microsensor. 
       FIG. 6  shows an exemplary injury, i.e., a damaged nerve, and the placement of a plurality of implanted devices, i.e., microstimulators, microsensors and a microtransponder, under control of the system control unit for “replacing” the damaged nerve. 
       FIG. 7  shows a simplified flow chart of the control of the implanted devices of  FIG. 6  by the system control unit. 
       FIG. 8  shows exemplary monophasic neurological signals that may be analyzed by the signal analysis device of the present invention. 
       FIG. 9  shows an exemplary biphasic neurological signal that may be analyzed by the signal analysis device of the present invention. 
       FIG. 10  shows a simplified block diagram of an exemplary signal analysis device of the present invention. 
       FIGS. 11A and 11B  show preferred and alternative implementations of the event criteria tester of the exemplary embodiment of  FIG. 10 . 
       FIG. 12  shows an alternative “fully” digital implementation of the exemplary embodiment of  FIG. 10 . 
       FIGS. 13A–D  show an exemplary flow chart for the operation of the signal analysis device of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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. 
   The present invention is generally directed to circuitry for use within implantable medical devices and in particular to such circuits which sense neurological signals, e.g., from nerves or muscles, to detect intended or actual muscle stimulation. In an exemplary application for the present invention, such circuits may be used within implanted devices configured similarly to the devices described in the commonly owned U.S. Pat. No. 6,164,284. Such implanted devices typically comprise a sealed housing suitable for injection into the patient&#39;s body and preferably contain a power source, e.g., a battery, having a capacity of at least 1 microwatt-hour and power consuming circuitry preferably including a data signal transmitter and receiver and sensor/stimulator circuitry for driving an input/output transducer. In a typical application, such devices are used to stimulate a neural pathway or muscle and/or to block a neural pathway to alleviate pain or block stimulation of a muscle. The present invention is thus specifically directed to an implementation of the sensor circuitry for use in such an implantable device. 
   In an exemplary system of devices which use the signal analysis device of the present invention, a system control unit (SCU) comprises a programmable unit capable of (1) transmitting commands to at least some of a plurality of implantable devices and (2) receiving data signals from at least some of those implantable devices. Such a system preferably operates, at least in part, in closed loop fashion whereby the commands transmitted by the SCU are dependent, in part, on the content of the data signals received by the SCU. 
     FIGS. 1 and 2  show an exemplary system  300  made of implanted devices  100 , preferably battery powered, under control of a system control unit (SCU)  302 , preferably also implanted beneath a patient&#39;s skin  12 . As described in the &#39;284 patent, potential implanted devices  100  (see also the block diagram shown in  FIG. 3A ) include stimulators, e.g.,  100   a  and  100   b , sensors, e.g.,  100   c , and transponders, e.g.,  100   d . The stimulators, e.g.,  100   a , can be remotely programmed to output a sequence of drive pulses to body tissue proximate to its implanted location via attached electrodes. The sensors, e.g.,  100   c , can be remotely programmed to sense one or more physiological or biological parameters in the implanted environment of the device, e.g., temperature, glucose level, O 2  content, nerve potential, muscle potential, etc. Transponders, e.g.,  100   d , are devices which can be used to extend the interbody communication range between stimulators and sensors and other devices, e.g., a clinician&#39;s programmer  172  and the patient control unit  174 . Preferably, these stimulators, sensors and transponders are contained in sealed elongate housings having an axial dimension of less than 60 mm and a lateral dimension of less than 6 mm. Accordingly, such stimulators, sensors and transponders are respectively referred to as microstimulators, microsensors, and microtransponders or referred to in general as battery-powered, implantable stimulator/sensor devices. Such microstimulators and microsensors can thus be positioned beneath the skin  12  within a patient&#39;s body using a hypodermic type insertion tool  176 . 
   As described in the &#39;284 patent, microstimulators and microsensors are remotely programmed and interrogated via a wireless communication channel, e.g., modulated AC magnetic, sound (i.e., ultrasonic), RF or electric fields, typically originating from control devices external to the patient&#39;s body, e.g., the clinician&#39;s programmer  172  or patient control unit  174 . Typically, the clinician&#39;s programmer  172  is used to program a single continuous or one time pulse sequence into each microstimulator and/or measure a biological parameter from one or more microsensors. Similarly, the patient control unit  174  typically communicates with the implanted devices  100 , e.g., microsensors  100   c , to monitor biological parameters. In order to distinguish each implanted device over the communication channel, each implanted device is manufactured with an address or identification code (ID)  303  specified in address storage circuitry  108  (see  FIG. 3A ) as described in the &#39;284 patent. 
   By using one or more such implantable devices in conjunction with the SCU  302  of the present invention, the capabilities of such implanted devices can be further expanded. For example, in an open loop mode (described below in reference to  FIG. 4 ), the SCU  302  can be programmed to periodically initiate tasks, e.g., perform real time tasking, such as transmitting commands to microstimulators according to a prescribed treatment regimen or periodically monitor biological parameters to determine a patient&#39;s status or the effectiveness of a treatment regimen. Alternatively, in a closed loop mode (described below in reference to  FIGS. 5–7 ), the SCU  302  periodically interrogates one or more microsensors and accordingly adjusts the commands transmitted to one or more microstimulators. 
     FIG. 2  shows a system  300  comprised of (1) one or more implantable devices  100  operable to sense and/or stimulate a patient&#39;s body parameter in accordance with one or more controllable operating parameters and (2) the SCU  302 . The SCU  302  is primarily comprised of (1) a housing  206 , preferably sealed and configured for implantation beneath the skin of the patient&#39;s body as described in the &#39;284 patent in reference to the implanted devices  100 , (2) a signal transmitter  304  in the housing  206  for transmitting command signals, (3) a signal receiver  306  in the housing  206  for receiving status signals, and (4) a programmable controller  308 , e.g., a microcontroller or state machine, in the housing  206  responsive to received status signals for producing command signals for transmission by the signal transmitter  304  to other implantable devices  100 . The sequence of operations of the programmable controller  308  is determined by an instruction list, i.e., a program, stored in program storage  310 , coupled to the programmable controller  308 . While the program storage  310  can be a nonvolatile memory device, e.g., ROM, manufactured with a program corresponding to a prescribed treatment regimen, it is preferable that at least a portion of the program storage  310  be an alterable form of memory, e.g., RAM, EEPROM, etc., whose contents can be remotely altered as described further below. However, it is additionally preferable that a portion of the program storage  310  be nonvolatile so that a default program is always present. The rate at which the program contained within the program storage  310  is executed is determined by clock/oscillator  312 . Additionally, a real time clock operating in response to clock/oscillator  312  preferably permits tasks to be scheduled at specified times of day. 
   The signal transmitter  304  and signal receiver  306  preferably communicate with implanted devices  100  using an RF signal, e.g., a propagated electromagnetic wave, modulated by a command data signal. Alternatively, an audio transducer may be used to generate mechanical vibrations having a carrier frequency modulated by a command data signal. In an exemplary system, a carrier frequency of 100 kHz is used which corresponds to a frequency that freely passes through a typical body&#39;s fluids and tissues. However, such sound means that operate at any frequency, e.g., greater than 1 Hz, are also considered to be usable with such devices. Alternatively, the signal transmitter  304  and signal receiver  306  can communicate using modulated AC, e.g., magnetic fields. 
   The clinician&#39;s programmer  172  and/or the patient control unit  174  and/or other external control devices can also communicate with the implanted devices  100 , as described in the &#39;284 patent, preferably using a modulated RF or AC magnetic field. Alternatively, such external devices can communicate with the SCU  302  via a transceiver  314  coupled to the programmable controller  308 . Since, the signal transmitter  304  and signal receiver  306  may operate using a different communication means, a separate transceiver  314  which operates using an alternative communication means may be used for communicating with external devices. However, a single transmitter  304 /receiver  306  can be used in place of transceiver  314  for communicating with the external devices and implanted devices if a common communication means is used. 
     FIG. 3A  comprises a block diagram of an exemplary implantable device  100  operable under control of controller circuitry  106  and includes a battery  104 , preferably rechargeable, for powering the device for a period of time in excess of one hour and responsive to command signals from a remote device, e.g., the SCU  302 . The controller circuitry  106  is primarily comprised of a controller  130 , configuration data storage  132  for prescribing its operation, and address storage circuitry  108  for storing the ID  303  of the device. As described in the &#39;284 patent, the implantable device  100  is preferably configurable to alternatively operate as a microstimulator and/or microsensor and/or microtransponder due to the commonality of most of the circuitry contained within. Such circuitry may be further expanded to permit a common block of circuitry to also perform the functions required for the SCU  302 . Accordingly,  FIG. 3B  shows an alternative implementation of the controller circuitry  106  of  FIG. 3A  that is suitable for implementing a microstimulator and/or a microsensor and/or a microtransponder and/or the SCU  302 . In this implementation, the configuration data storage  132  can be alternatively used as the program storage  310  when the implantable device  100  is used as the SCU  302 . In this implementation, XMTR  168  corresponds to the signal transmitter  304  and the RCVR  114   b  corresponds to the signal receiver  306  (preferably operable via electrodes  112   a  and  112   b  operating as an RF antenna) and the RCVR  114   a  and XMTR  146  correspond to the transceiver  314  (preferably operable via coil  116  for AC magnetic modes of communication). 
   In a preferred design, the contents of the program storage  310 , i.e., the software that controls the operation of the programmable controller  308 , can be remotely downloaded, e.g., from the clinician&#39;s programmer  172  using data modulated onto an RF signal or an AC magnetic field. In this design, it is preferable that the contents of the program storage  310  for each SCU  302  be protected from an inadvertent change. Accordingly, the contents of the address storage circuitry  108 , i.e., the ID  303 , is preferably used as a security code to confirm that the new program storage contents are destined for the SCU  302  receiving the data. This feature is significant if multiple patient&#39;s could be physically located, e.g., in adjoining beds, within the communication range of the clinician&#39;s programmer  172 . 
   In a further aspect of the present invention, it is preferable that the SCU  302  be operable for an extended period of time, e.g., in excess of one hour, from an internal power supply  316  (see  FIG. 2 ). While a primary battery, i.e., a nonrechargeable battery, is suitable for this function, it is preferable that the power supply  316  include a rechargeable battery, e.g., battery  104  as described in the &#39;284 patent, that can be recharged via an AC magnetic field produced external to the patient&#39;s body. Accordingly, power supply  102  of  FIG. 3A  is the preferred power supply  316  for the SCU  302  as well. 
   The battery-powered devices  100  of the &#39;284 patent are preferably configurable to operate in a plurality of operational modes, e.g., via a communicated command signal. In a first operational mode, device  100  is remotely configured to be a microstimulator, e.g.,  100   a  and  100   b . In this design (see  FIG. 3A ), controller  130  commands stimulation circuitry  110  to generate a sequence of drive pulses through electrodes  112  to stimulate tissue, e.g., a nerve or muscle, proximate to the implanted location of the microstimulator, e.g.,  100   a  or  100   b . In operation, a programmable pulse generator  178  and voltage multiplier  180  are configured with parameters (see exemplary Table I) corresponding to a desired pulse sequence and specifying how much to multiply (or divide) the battery voltage (e.g., by summing charged capacitors or similarly charged battery portions) to generate a desired compliance voltage V c . A first FET  182  is periodically energized to store charge into capacitor  183  (in a first direction at a low current flow rate through the body tissue) and a second FET  184  is periodically energized to discharge capacitor  183  in an opposing direction at a higher current flow rate which stimulates a nearby muscle or nerve. Alternatively, electrodes can be selected that will form an equivalent capacitor within the body tissue. 
   
     
       
         
             
           
             
               TABLE I 
             
             
                 
             
             
               Stimulation Parameters 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
               Current: 
               continuous current charging of storage 
             
             
                 
               capacitor 
             
             
               Charging currents: 
               1, 3, 10, 30, 100, 250, 500 μA 
             
             
               Current Range: 
               0.8 to 40 mA in nominally 3.2% steps 
             
             
               Compliance Voltage: 
               selectable, 3–24 volts in 3 volt steps 
             
             
               Pulse Frequency (PPS): 
               1 to 5000 PPS in nominally 30% steps 
             
             
               Pulse Width: 
               5 to 2000 μs in nominally 10% steps 
             
             
               Burst On Time (BON): 
               1 ms to 24 hours in nominally 20% steps 
             
             
               Burst Off Time (BOF): 
               1 ms to 24 hours in nominally 20% steps 
             
             
               Triggered Delay to BON: 
               either selected BOF or pulse width 
             
             
               Burst Repeat Interval: 
               1 ms to 24 hours in nominally 20% steps 
             
             
               Ramp On Time: 
               0.1 to 100 seconds (1, 2, 5, 10 steps) 
             
             
               Ramp Off Time: 
               0.1 to 100 seconds (1, 2, 5, 10 steps) 
             
             
                 
             
          
         
       
     
   
   In a next operational mode, the battery-powered implantable device  100  can be configured to operate as a microsensor, e.g.,  100   c , that can sense one or more physiological or biological parameters in the implanted environment of the device. In accordance with a preferred mode of operation, the system control unit  302  periodically requests the sensed data from each microsensor  100   c  using its ID  303  stored in the address storage circuitry  108 , and responsively sends command signals to microstimulators, e.g.,  100   a  and  100   b , adjusted accordingly to the sensed data. For example, sensor circuitry  188  can be coupled to the electrodes  112  to sense or otherwise used to measure a biological parameter, e.g., temperature, glucose level, O 2  content, voltage, current, impedance, etc., and provide the sensed data to the controller circuitry  106 . Preferably, the sensor circuitry  188  includes a programmable bandpass filter and an analog to digital (A/D) converter that can sense and accordingly convert the voltage levels across the electrodes  112  into a digital quantity. Alternatively, the sensor circuitry  188  may include one or more comparators for determining if the measured voltage exceeds a threshold voltage value or is within a specified voltage range, i.e., an amplitude window, and/or peak detectors for determining peaks of the sensed signals, i.e., zero slope points. Each of these attributes, i.e., qualified amplitudes or slopes, may be determined to be events. Furthermore, the sensor circuitry  188  of the present invention additionally includes counters, i.e., accumulators, which track the occurrences of these events to determine event rates, evoked response times, or the like and/or to determine if events occur within specified time periods, i.e., time windows. The sensor circuitry of the present invention will be discussed in detail further below. 
   The operational mode of the front end processing, e.g., the bandpass filter portion of the sensor circuitry  188  is remotely programmable via the device&#39;s communication interface (see exemplary Table II). 
   
     
       
         
             
           
             
               TABLE II 
             
             
                 
             
             
               Sensing Parameters 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
               Input voltage range: 
               5 μV to 1 V 
             
             
               Bandpass filter rolloff: 
               24 dB 
             
             
               Low frequency cutoff choices: 
               10, 30, 100, 300, 1000, 3000 Hz 
             
             
               High frequency cutoff choices: 
               300, 1000, 3000, 10000 Hz 
             
             
               Integrator frequency choices: 
               1 PPS to 100 PPS 
             
             
               Amplitude threshold 
               4 bits of resolution 
             
             
               for detection choices: 
             
             
                 
             
          
         
       
     
   
   Additionally, the sensing capabilities of a microsensor may include the capability to monitor the battery status via path  124  from the charging circuit  122  and can additionally include using an ultrasonic transducer (not shown) or the coil  116  to respectively measure the ultrasonic, magnetic or propagated RF signal magnitudes (or communication time delays) of signals transmitted between a pair of implanted devices and thus determine the relative locations of these devices. This information can be used to determine the amount of body movement, e.g., the amount that an elbow or finger is bent, and thus form a portion of a closed loop motion control system. 
     FIG. 4  shows a block diagram of an exemplary open loop control program, i.e., a task scheduler  320 , for controlling/monitoring a body function/parameter. In this process, the programmable controller  308  is responsive to the clock  312  (preferably a crystal controlled oscillator to thus permit real time scheduling) in determining when to perform any of a plurality of tasks. In this exemplary flow chart, the programmable controller  308  first determines in block  322  if it is now at a time designated as T EVENT1  (or at least within a sampling error of that time), e.g., at 1:00 AM. If so, the programmable controller  308  transmits a designated command to microstimulator A (ST A ) in block  324 . In this example, the control program continues where commands are sent to a plurality of stimulators and concludes in block  326  where a designated command is sent to microstimulator X (ST X ). Such a subprocess, e.g., a subroutine, is typically used when multiple portions of body tissue require stimulation, e.g., stimulating a plurality of muscle groups in a paralyzed limb to avoid atrophy. The task scheduler  320  continues through multiple time event detection blocks until in block  328  it determines whether the time T EVENTM  has arrived. If so, the process continues at block  330  where, in this case, a single command is sent to microstimulator M (ST M ). Similarly, in block  332  the task scheduler  320  determines when it is the scheduled time, i.e., T EVENTO , to execute a status request from microsensor A (SE A ). If so, a subprocess, e.g., a subroutine, commences at block  334  where a command is sent to microsensor A (SE A ) to request sensor data and/or specify sensing criteria. Microsensor A (SE A ) does not instantaneously respond. Accordingly, the programmable controller  308  waits for a response in block  336 . In block  338 , the returned sensor status data from microsensor A (SE A ) is stored in a portion of the memory, e.g., a volatile portion of the program storage  310 , of the programmable controller  308 . The task scheduler  320  can be a programmed sequence, i.e., defined in software stored in the program storage  310 , or, alternatively, a predefined function controlled by a table of parameters similarly stored in the program storage  310 . A similar process may be used where the SCU  302  periodically interrogates each implantable device  100  to determine its battery status. 
     FIG. 5  is an exemplary block diagram showing the use of the system of the present invention to perform closed loop control of a body function. In block  352 , the SCU  302  requests status from microsensor A (SE A ). The SCU  302 , in block  354 , then determines whether the present command given to a microstimulator is satisfactory and, if necessary, determines a new command and transmits the new command to the microstimulator A (ST A ) in block  356 . For example, if microsensor A (SE A ) is reading a voltage corresponding to the degree of contraction resulting from stimulating a muscle, the SCU  302  could transmit a command to microstimulator A (ST A ) to adjust the sequence of drive pulses, e.g., in magnitude, duty cycle, etc., and accordingly change the voltage sensed by microsensor A (SE A ). Accordingly, closed loop, i.e., feedback, control is accomplished. The characteristics of the feedback (position, integral, derivative (PID)) control are preferably program controlled by the SCU  302  according to the control program contained in program storage  310 . 
     FIG. 6  shows an exemplary injury treatable by the present system  300 . In this exemplary injury, the neural pathway has been damaged, e.g., severed, just above a patient&#39;s left elbow. The goal of this exemplary system is to bypass the damaged neural pathway to permit the patient to regain control of the left hand. An SCU  302  is implanted within the patient&#39;s torso to control a plurality of stimulators, ST 1 –ST 5 , implanted proximate to the muscles respectively controlling the patient&#39;s thumb and fingers (shown in the patient&#39;s hand for simplicity). Additionally, microsensor  1  (SE 1 ) is implanted proximate to an undamaged nerve portion where it can sense a signal generated from the patient&#39;s brain when the patient wants hand closure. Optional microsensor  2  (SE 2 ) is implanted in a portion of the patient&#39;s hand where it can sense a signal corresponding to stimulation/motion of the patient&#39;s pinky finger and microsensor  3  (SE 3 ) is implanted and configured to measure a signal corresponding to grip pressure generated when the fingers of the patient&#39;s hand are closed. Additionally, an optional microtransponder (T 1 ) is shown which can be used to improve the communication between the SCU  302  and the implanted devices. 
     FIG. 7  shows an exemplary flow chart for the operation of the SCU  302  in association with the implanted devices in the exemplary system of  FIG. 6 . In block  360 , the SCU  302  interrogates microsensor  1  (SE 1 ) to determine if the patient is requesting actuation of his fingers. If not, a command is transmitted in block  362  to all of the stimulators (ST 1 –ST 5 ) to open the patient&#39;s hand, i.e., to de-energize the muscles which close the patient&#39;s fingers. If microsensor  1  (SE 1 ) senses a signal to actuate the patient&#39;s fingers, the SCU  302  determines in block  364  whether the stimulators ST 1 –ST 5  are currently energized, i.e., generating a sequence of drive/stimulation pulses. If not, the SCU  302  executes instructions to energize the stimulators. In a first optional path  366 , each of the stimulators is simultaneously (subject to formatting and transmission delays) commanded to energize in block  366   a . However, the command signal given to each one specifies a different start delay time. Accordingly, there is a stagger between the actuation/closing of each finger. 
   In a second optional path  368 , the microstimulators are consecutively energized by a delay Δ. Thus, microstimulator  1  (ST 1 ) is energized in block  368   a , a delay is executed within the SCU  302  in block  368   b , and so on for all of the microstimulators. Accordingly, paths  366  and  368  perform essentially the same function. However, in path  366  the interdevice timing is performed by the clocks within each implanted device  100  while in path  368 , the SCU  302  is responsible for providing the interdevice timing. 
   In path  370 , the SCU  302  actuates a first microstimulator (ST 1 ) in block  370   a  and waits in block  370   b  for its corresponding muscle to be actuated, as determined by microsensor  2  (SE 2 ), before actuating the remaining stimulators (ST 2 –ST 5 ) in block  370   c . This implementation could provide more coordinated movement in some situations. 
   Once the stimulators have been energized, as determined in block  364 , closed loop grip pressure control is performed in blocks  372   a  and  372   b  by periodically reading the status of microsensor  3  (SE 3 ) and adjusting the commands given to the stimulators (ST 1 –ST 5 ) accordingly. Consequently, this exemplary system has enabled the patient to regain control of his hand including coordinated motion and grip pressure control of the patient&#39;s fingers. 
   Accordingly, to accomplish these tasks, such implantable devices must be able to interpret neurological signals from nerves and/or muscle tissues such that evoked muscular responses, neurologically sensed pressure or pain, e.g., touch, responses or the like may be detected and analyzed. The present invention is thus directed to an implementation of the sensor circuitry  188  for use in such an implantable device. Aspects of this invention are particularly directed to an implementation that is suitable for a small integrated circuit implementation that minimizes power, both of which features are particularly significant for use within the preferred microstimulator/microsensor environment. 
   Referring now to  FIGS. 8 and 9 , exemplary neurological waveforms are shown which have characteristics that are identifiable by embodiments of the present invention. For example, cases  1 A and  1 B of  FIG. 8  respectively show events that are identified by monophasic positive or negative signal components; while case  2  of  FIG. 9  shows an event that is identified by a positive, negative or positive and negative, i.e., biphasic, signal components. 
   As an initial step, small analog voltages are sensed from the electrodes  112   a ,  112   b  and amplified and filtered at a front end processor  1102  of the sensor circuitry  188  (see  FIG. 10 ). Next, this filtered analog input signal  1104  is processed according to various programmed criteria or modes. 
   In a first mode of operation (see  FIG. 8 ), this filtered signal is alternatively compared with one or two (or more) programmed threshold levels, shown as L 1  and L 2 . Voltage levels are shown with respect to a reference voltage V ref . These levels may be either positive or negative with respect to V ref . (It should be noted that the use of these threshold levels L 1  and L 2  is exemplary and the use of these threshold levels may be exchanged or substituted and the relationship of these threshold levels may be reversed as well. Furthermore, the use of only two threshold levels is exemplary and embodiments that identify events according to criteria including more than two thresholds is expressly recognized to be within the scope of the present invention.) In this first mode of operation, an event may be recognized by detecting an analog voltage signal extending above a first threshold level L 1  (see signal portions  1000  and  1002  of case  1 A) or extending below a second threshold level L 2  (see signal portion  1004  of Case  1 B). 
   Alternatively, in a second mode of operation, a second threshold level may be used to exclude signals from being detected as events. Thus, for detection in this mode, a signal must exceed a first threshold L 1  without exceeding a second threshold L 2 . Thus, in case  1 A, signal portion  1000  is identified as an event and signal portion  1002  is not. Case  1 B shows an alternative of this mode where events are identified when the signal extends below a second threshold L 2  without extending below a first threshold L 1 . Thus, in case  1 B, signal portion  1004  is identified as an event. 
   In a next mode of operation (see  FIG. 9 ), positive signal portions extending above a second threshold L 2  or negative signal portions extending below a first threshold L 1  are identified as events. Thus, case  2  shows two identifiable events corresponding to signal portions  1006  and  1008 . In a variation of this next mode of operation, an identifiable event may be when a signal passes above a second threshold L 2  and below a first threshold L 1 . Thus, in this variation, case  2  shows a single identifiable event. 
   In a further mode of operation, peaks, i.e., zero slope portions, of the sensed signal are detected as events by a peak detector  1106  (see  FIGS. 11A and 11B ). Thus, in this mode, case  1 A shows three identifiable events corresponding to signal peaks  1010 ,  1012  and  1014 . In a variation of this mode of operation, only peaks between the two programmed threshold levels L 1  and L 2  are identified as events, i.e., peaks that occur above or below the threshold levels are excluded. Thus, in this mode variation, case  1 A shows two identifiable events corresponding to peaks  1010  and  1012 . 
   Finally, the peak detector  1106  (see  FIGS. 11A and 11B ) may be used to adapt the threshold levels to changes in the average peak values, e.g., by monitoring changes in the average peak values and adapting the threshold levels by the actual or percentage changes of the average peak values. Such a feature may be useful to compensate for tissue encapsulation of the electrodes or automatically adapting to different signal strengths that may be measured during implantation. 
     FIG. 10  shows a block diagram of an exemplary embodiment of the signal analysis device  1100  of the present invention. The signal analysis device  1100  comprises at least a portion of the aforementioned sensor circuitry  188 . Signal analysis device  1100  is primarily comprised of: (1) sensing circuitry, i.e., front end processor  1102 , for receiving a biological signal within the implantable device  100  and generating a sensed voltage output, i.e., filtered analog input signal  1104 , in response thereto; (2) event detection circuitry, i.e., event criteria tester  1108 , that is alternatively configurable to determine whether an event occurred according to one or according to two or more programmable criteria for detecting an attribute of the filtered analog input signal  1104  and designating the detected attribute as an event; (3) an event counter  1110  configured for accumulating detected events; (4) a clock counter  1112  accumulating clock pulses, preferably from clocks associated with oscillator  312 ; and (5) event analysis circuitry, i.e., event analysis controller  1114 , configurable for operation in a plurality of modes of operation for determining a processed value corresponding to the detected events and the accumulated clock pulses. 
   Preferably, front end processor  1102  amplifies a neurological signal and bandpass filters the amplified signal according to programmable criteria transferred from controller circuitry  106  via mode control signal bus  1116 . The resulting filtered analog input signal  1104  is provided to event criteria tester  1108  which detects identifiable signal components, i.e., events, as previously described in reference to  FIGS. 8 and 9 . 
     FIGS. 11A and 11B  describe a preferred and an alternate implementation of the event criteria tester  1108  of the present invention. In the preferred implementation of  FIG. 11A , the event criteria tester  1108  is primarily comprised of (1) two or more digital to analog, i.e., D/A converters  1117  and  1118 ; (2) two or more analog comparators  1120  and  1122  and (3) programmable criteria logic  1124 . In operation, the D/A converters  1117 ,  1118  receive programmable digital threshold values from the controller circuitry  106  via mode control signal bus  1116  that are latched internally. The D/A converters  1117 ,  1118  generate analog threshold values that are input to analog comparators  1120 ,  1122 . Additionally, the comparators  1120 ,  1122  receive filtered analog input signal  1104  and generate signals  1126  and  1128  when the filtered analog input signal  1104  exceeds programmed threshold levels L 1  and L 2 , respectively. Preferably, signals  1126  and  1128  are processed subject to hysteresis. Programmable criteria logic  1124  then processes signals  1126 ,  1128  and determines events according to programmable criteria (as previously discussed) received by the criteria logic  1124  from control circuitry  106  via mode control signal bus  1116 . The programmable criteria logic generates an event detected signal  1130  according to the programmed criteria. This particular implementation provides the additional benefit that it minimizes power consumption. It is well known that for some “low power” digital logic, e.g., CMOS, the power consumption is a function of the number of logic transitions per unit time, e.g., frequency. By minimizing the number of digital logic transitions, the power consumption is accordingly minimized. D/A converters  1117 ,  1118  only experience digital logic transitions when they are initially loaded by the controller circuitry  106 . Accordingly, the associated power consumption is extremely low. The programmable criteria logic  1124  only experiences transitions when the filtered analog signal crosses the programmed threshold levels, i.e., when there are transitions of signals  1126 ,  1128 . These transitions occur at a fairly low rate and thus the associated power dissipation is also extremely low. 
   The implementation of  FIG. 11A  may additionally comprise a peak detector  1106  that receives an analog input from the filtered analog input signal  1104  and provides an output  1132  corresponding to detected peaks, i.e., points where the voltage slopes are zero, to the programmable criteria logic  1124 . As previously described, whether or not the detected peaks correspond to events may be qualified or excluded according to where the peaks occur in relation to the programmed thresholds. 
     FIG. 11B  shows an alternative implementation of the event criteria tester  1108  of  FIG. 10  that primarily operates in the digital domain. According to this implementation, the filtered analog input signal  1104  is converted to a digital signal  1134  by analog to digital converter (A/D)  1136 . This digital signal  1134  is then processed purely in the digital domain by the programmable criteria logic  1124 . In this implementation, the mode control signal bus  1116  provides digital threshold values to the programmable criteria logic. Preferably, the programmable criteria logic  1124  performs its threshold comparisons using hysteresis as discussed in reference to  FIG. 11A . 
   The implementation of  FIG. 11B  may additionally comprise a peak detector  1106  as described in relation to  FIG. 11A . Alternatively, the peak detector  1106  may operate on the digital signal  1134 . In a further alternative, the function of the peak detector  1106  may be incorporated into digital logic within the programmable criteria logic  1124 . 
   The implementation of  FIG. 11B  offers the advantage that since it may be totally implemented as digital logic, it can conveniently take advantage of transistor size reductions as semiconductor processes improve (along with associated power reductions). 
   Finally,  FIG. 12  shows an alternative “fully” digital embodiment of the present invention. In this embodiment, an optional A/D converter  1136  provides digital signal  1134  to a digital signal processor (DSP)  1138  where the aforedescribed functions are executed under software control. Advantageously in this embodiment, many DSPs include analog to digital converters and thus all or most of the functionality of the signal analysis device  1100  may be included within the DSP, optionally including the front end filtering of the front end processor  1102  (in which case the front end processor  1102  may solely consist of an amplifier). 
     FIGS. 13A–D  show an exemplary flow chart  2000  of the operation of the programmable signal analysis device  1100  of the present invention. Flow chart  2000  is directly applicable to the DSP implementation of  FIG. 12  but one of ordinary skill in the art should appreciate that it is equally applicable to hardware or hardware/software implementations as shown in  FIG. 10 ,  FIG. 11A , and  FIG. 11B . 
   In block  2002 , the neurological signal received from electrodes  112   a ,  112   b  are amplified and filtered by front end processor  1102 . According to the programmed attribute criteria, event criteria tester  1108  determines in block  2004  which criteria are enabled. In a first mode, block  2006  identifies events by determining whether the neurological signal has crossed a first threshold, e.g., L 1 . Alternatively in a second mode, block  2008  identifies events by determining whether the neurological signal has crossed a first threshold, e.g., L 1 , without crossing a second threshold, e.g., L 2 . In a variation of block  2008  (not shown), this mode may be accomplished by excluding mode  1  type events if the neurological signal has exceeded the second threshold. Alternatively, in a third mode, block  2010  identifies events by determining whether a peak has been detected. Alternatively, in a fourth mode, block  2012  identifies events by determining whether a peak has been detected between the first and second thresholds. In a variation of block  2012  (not shown), this mode may be accomplished by excluding mode  3  type events that do not occur between the two thresholds. 
   Once an event has been detected, block  2014  accumulates a count of signal events, e.g., in event counter  1110 . Event analysis controller  1114  then analyzes, in block  2016 , the accumulated events according to its programmed mode. In this example, three different exemplary modes are shown, mode  1  which determines an event rate (or clock count per event) by counting the clocks between events, mode  2  which counts a prescribed number of events and determines the rate by accumulating the clock counts for the prescribed number of events, and mode  3  which determines the number of clocks between a designated start time, e.g., corresponding to a stimulation pulse, and an event, e.g., an evoked response. Mode  3  is of particular use in analyzing cardiac responses. 
   When mode  1  is started, the event counter  1110  is reset in block  2020  and when the first event is detected in block  2022 , the clock counter  1112  is reset in block  2024 . Accordingly, when the next event is detected, a clock count between consecutive events is captured in block  2026 . This clock count may be output in block  2028  or it may be processed along with the clock rate to determine an event rate in block  2030 , e.g., a heart rate in beats per minute. Finally, in block  2032 , the clock counter  1112  is reset and the event counter  1110  is set to not be the first event, e.g., set to a value of 1. Accordingly, this process may redetermine the event rate as of the occurrence of the next event by using the last detected event as the “first” event. 
   When mode  2  is started, the event counter  1110  and the clock counter  1112  are reset in block  2034 . Additionally, an event threshold is loaded, i.e., the number of events that are to be counted before a result is reported. In block  2036 , event counts are ignored until the event threshold is reached. Then, in block  2038 , the clock counts are captured for the event threshold and this clock count value is optionally output in block  2040 . Alternatively or additionally, the captured clock count and event threshold values are used to calculate an event rate in block  2041 . Finally, in block  2042 , the clock counter  1112  and the event counter  1110  are reset. Accordingly, this process may redetermine the event rate when the event counter  1110  re-accumulates an event count equal to the event threshold. 
   When mode  3  is started, the clock counter  1112  is reset in block  2044 . This would typically correspond to the time that a stimulation pulse is emitted from the implantable device to stimulate muscle, e.g., cardiac, tissue. Preferably, in such a mode, there is a blanking period (not shown), in which the detection of events is suppressed, e.g., by blanking the response of the front end processor  1102  and/or suppressing accumulation or detection of events. Following this optional blanking period, the clock count is captured as of the first event, e.g., an evoked response, in block  2046  and the captured clock count value is output in block  2048 . 
   As previously discussed, the peak detector may alternatively or additionally be used to adjust the threshold levels, e.g., L 1  and L 2 . A background task to accomplish this feature is shown in blocks  2050 – 2054 . In block  2050 , it is determined whether a peak, a zero slope of the input signal, has been detected. If a peak has been detected, its voltage value is averaged with the prior peak values, e.g., with an analog or digital low pass filter, to achieve an average peak value. In block  2054 , the trend or change in the average peak value is analyzed and the programmed threshold values are adjusted accordingly. 
   In a next alternative mode, the duration of an event is detected, e.g., the amount of time that a neurological signal is greater than a first threshold, e.g., L 1 , without exceeding a second threshold, e.g., L 2 . A simplified example of this next alternative mode is described in relationship to blocks  2060 – 2070 . Following block  2040 , a fifth attribute criteria mode is used in block  2060 . In block  2060 , it is determined whether the sensed neurological signal exceeds a first threshold (preferably programmable to additionally determine that the neurological signal has not exceeded a second threshold). It the first threshold has been crossed, then, in block  2062 , the clock counter is reset and a duration count mode flag is enabled. The process then continues at block  2002 . The next time through in block  2060 , the leading edge of the neurological signal will not be detected, e.g., because the duration count mode flag is set and that flag identifies that the neurological signal already exceeds the first threshold. The process continues with block  2064  which determines if the trailing edge of the neurological signal has been detected while the duration count mode flag is set. Once this trailing edge is detected, the clock count is captured in block  2066  and output in block  2068 . The duration count mode flag is then reset in block  2070 . While the blocks  2060 – 2070  have essentially shown this function as being primarily implemented as a portion of the event criteria tester  1108 , it is recognized that portions of this function may alternatively or additionally be implemented as part of the event analysis controller  1114 . 
   While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. For example, while two threshold values have been expressly shown, three or more threshold values are considered to be a variation that is within the scope of the present invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced other than as specifically described herein.