Patent Publication Number: US-2023149716-A1

Title: Methods and Apparatus for Adjusting Neurostimulation Intensity Using Evoked Responses

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/165,678, filed Feb. 2, 2021, which is a continuation of U.S. application Ser. No. 16/178,106, filed Nov. 1, 2018 (now U.S. Pat. No. 10,940,316), which is a continuation of U.S. application Ser. No. 15/000,537, filed Jan. 19, 2016 (now U.S. Pat. No. 10,376,700), which is continuation of U.S. application Ser. No. 13/156,891, filed Jun. 9, 2011 (now U.S. Pat. No. 9,265,431), which is a Non-Provisional of U.S. Provisional Patent Application Ser. No. 61/356,251, filed on Jun. 18, 2010. These applications are incorporated herein by reference, and priority is claimed to them. 
     The following commonly assigned U.S. patent applications are related, and are herein incorporated by reference in their entirety: “METHODS AND APPARATUS FOR CONTROLLING NEUROSTIMULATION USING EVOKED NEURAL RESPONSES,” Ser. No. 13/156,879, filed on Jun. 9, 2011, now issued as U.S. Pat. No. 8,972,022 and “METHODS AND APPARATUS FOR ADJUSTING NEUROSTIMULATION INTENSITY USING EVOKED RESPONSES,” Ser. No. 13/156,914, filed on Jun. 9, 2011, now issued as U.S. Pat. No. 9,089,267. 
    
    
     TECHNICAL FIELD 
     This document relates generally to neurostimulation and more particularly to a neurostimulation system that detects evoked responses and uses the detected evoked responses for capture verification and parameter adjustment. 
     BACKGROUND 
     Vagus nerve stimulation has been applied to modulate various physiologic functions and treat various diseases. One example is the modulation of cardiac functions in a patient suffering heart failure or myocardial infarction. The myocardium is innervated with sympathetic and parasympathetic nerves including the cardiac branches of the vagus nerve. Activities in the vagus nerve, including artificially applied electrical stimuli, modulate the heart rate and contractility (strength of the myocardial contractions). Electrical stimulation applied to the vagus nerve is known to decrease the heart rate and the contractility, lengthening the systolic phase of a cardiac cycle, and shortening the diastolic phase of the cardiac cycle. This ability of vagus nerve stimulation is utilized, for example, to control myocardial remodeling. 
     In addition to treating cardiac disorders such as myocardial remodeling, vagus nerve stimulation is also known to be effective in treating disorders including, but not limited to, depression, anorexia nervosa/eating disorders, pancreatic function, epilepsy, hypertension, inflammatory disease, and diabetes. To ensure efficacy of a vagus nerve stimulation therapy, there is a need to verify that the stimulation activates the target branches of the vagus nerve and control the stimulation parameters to result in effective modulation of target functions. 
     SUMMARY 
     A neurostimulation system provides for capture verification and stimulation intensity adjustment to ensure effectiveness of vagus nerve stimulation in modulating one or more target functions in a patient. In various embodiments, stimulation is applied to the vagus nerve, and evoked responses are detected to verify that the stimulation captures the vagus nerve and to adjust one or more stimulation parameters that control the stimulation intensity. 
     In one embodiment, a system for delivering neurostimulation includes a stimulation output circuit, an evoked response detection circuit, and a control circuit. The stimulation output circuit delivers neurostimulation pulses to a vagus nerve. The evoked response detection circuit receives a physiological signal indicative of evoked responses being physiologic events evoked by the neurostimulation pulses and detects the evoked responses using the physiological signal and one or more detection thresholds. The control circuit includes a sensing parameter adjustor that adjusts the one or more detection thresholds using the detected evoked responses and a stored baseline response. 
     In one embodiment, a method for delivering neurostimulation is provided. Neurostimulation pulses are delivered to a vagus nerve. A physiological signal indicative of evoked responses is sensed. The evoked responses are each a physiologic event evoked by one of the neurostimulation pulses. The evoked responses are detected by comparing the physiological signal to one or more detection thresholds. The one or more detection thresholds are detected using the detected evoked neural responses. 
     This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the invention will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof. The scope of the present invention is defined by the appended claims and their legal equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate generally, by way of example, various embodiments discussed in the present document. The drawings are for illustrative purposes only and may not be to scale. 
         FIG.  1    is an illustration of an embodiment of a vagus nerve stimulation system and portions of an environment in which the system is used. 
         FIG.  2    is a block diagram illustrating an embodiment of a vagus nerve stimulation system providing for detection of evoked responses. 
         FIG.  3    is a block diagram illustrating another embodiment of the vagus nerve stimulation system of  FIG.  2   . 
         FIG.  4    is an illustration of an embodiment of an implantable system including the vagus nerve stimulation system and portions of an environment in which the implantable system is used. 
         FIG.  5    is a block diagram illustrating an embodiment of the implantable system of  FIG.  4   . 
         FIG.  6    is an illustration of evoked responses to a neurostimulation pulse. 
         FIG.  7    is an illustration of evoked responses to neurostimulation pulses of various amplitudes. 
         FIG.  8    is an illustration showing various stimulation thresholds. 
         FIG.  9    is a block diagram illustrating an embodiment of a circuit for detecting evoked neural responses. 
         FIG.  10    is an illustration of an evoked neural response and detection windows. 
         FIG.  11    is a block diagram illustrating an embodiment of a system for detecting evoked neural responses. 
         FIG.  12    is a block diagram illustrating an embodiment of a circuit for detecting evoked muscular responses. 
         FIG.  13    is a block diagram illustrating another embodiment of a system for detecting evoked muscular responses. 
         FIG.  14    is a block diagram illustrating an embodiment of a circuit for sensing various laryngeal signals. 
         FIG.  15    is a flow chart illustrating an embodiment of a method for automatic threshold adjustment for evoked response detection during vagus nerve stimulation. 
         FIG.  16    is a flow chart illustrating an embodiment of a method for adjusting stimulation intensity for vagus nerve stimulation. 
         FIG.  17    is an illustration showing an example of a relationship between stimulation thresholds for two types of fibers of the vagus nerve. 
         FIG.  18    is a flow chart illustrating an embodiment of a method for adjusting stimulation intensity for vagus nerve stimulation during implantation of an implantable medical device into a patient. 
         FIG.  19    is a flow chart illustrating an embodiment of a method for adjusting stimulation intensity for vagus nerve stimulation during follow-up visits by the patient using the implantable medical device. 
         FIG.  20    is a flow chart illustrating an embodiment of a method for automatic capture verification for vagus nerve stimulation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents. 
     It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment. 
     This document discusses a method and system for delivering neurostimulation to modulate one or more target functions and detecting one or more physiological responses evoked by the neurostimulation. The detection of an evoked physiological response indicates that the nerve is captured by the stimulation. In various embodiments, each evoked physiological response is detected from a neural signal sensed from the nerve and/or from another physiological signal capable of being a surrogate of the neural signal. While a vagus nerve stimulation is specifically discussed in this document as an example, the present method and system generally applies to stimulation of various target nerves. 
     The vagus nerve originates in the medulla and targets multiple organs in a person&#39;s body through a complex functional innervation pattern. There are both efferent and afferent nerve fibers within the vagus nerve trunk that convey neural activities to and from visceral organs such as the esophagus, gastrointestinal tract, kidney and pancreas (abdominal branch of vagus), thoracic organs such as the heart and lungs (thoracic branch of vagus), and voluntary muscles of the neck and multiple segments of the upper airway (recurrent laryngeal nerve, RLN). In one embodiment in which vagus nerve stimulation is delivered to modulate one or more cardiovascular functions, examples of evoked responses indicating that the vagus nerve is captured include neural signals sensed from the vagus nerve and signals indicative of laryngeal activities. 
     Fibers of the vagus nerve include A-fibers (myelinated fibers, also referred to as A-type fibers), B-fibers (myelinated parasympathetic fibers, also referred to as B-type fibers), and C-fibers (unmyelinated fibers, also referred to as C-type fibers), as summarized in Table 1. It is believed that functions of most of the visceral and thoracic organs are modulated by vagus nerve stimulation through activation of the B-fibers, while activation of the A-fibers results in evoked laryngeal activities. As verified by experimental data, the stimulation intensity required for activating the B-fibers (stimulation threshold for B-fibers) is higher than the stimulation intensity required for activating the A-fibers (stimulation threshold for A-fibers) because the diameters of the B-fibers are smaller than the diameters of the A-fibers. The stimulation intensity required for activating the C-fibers (stimulation threshold for C-fibers) is highest because the C-fibers have the smallest diameter among those three types of fibers. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Summary of Vagus Nerve Fiber Type Properties. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 A-Fibers 
                 B-Fibers 
                 C-Fibers 
               
               
                   
                   
               
               
                   
                 Diameter 
                  5-20 
                 1-3 
                 0.2-2   
               
               
                   
                 Myelinated 
                 Yes 
                 Yes 
                 No 
               
               
                   
                 Conduction Velocity (m/s) 
                  30-120 
                  3-20 
                 0.3-2   
               
               
                   
                 Per-Unit Latencies (ms/cm) 
                 0.08-0.3  
                 0.5-3.3 
                    5-33.3 
               
               
                   
                   
               
            
           
         
       
     
     The present method and system senses various signals indicative of the evoked neural and/or muscular responses to vagus nerve stimulation to ensure activation of the B-fibers. In various embodiments, vagus nerve stimulation is applied to modulate one or more cardiovascular functions, and one or more signals indicative of evoked neural responses of the vagus nerve and/or evoked muscular responses of the laryngeal muscles are sensed. Capture of the vagus nerve is verified by detected evoked responses, including the evoked neural and/or muscular responses. Stimulation parameters are set or adjusted to provide a stimulation intensity required for reliably activating the B-fibers. 
       FIG.  1    is an illustration of an embodiment of a vagus nerve stimulation system  100  and portions of an environment in which system  100  is used. System  100  includes a stimulation electrode  110 , a stimulation output circuit  111 , a control circuit  112 , a neural sensing electrode  114 A, a laryngeal activity sensor  114 B, and an evoked response detection circuit  116 . 
       FIG.  1    shows a portion of vagus nerve  102  of a patient. Vagus nerve  102  has branches including an RLN  106 . Stimulation electrode  110  is electrically connected to stimulation output circuit  111  and placed on vagus nerve  102  to allow for delivery of neurostimulation pulses from stimulation output circuit  111  to modulate functions of the patient&#39;s thoracic organs, including a heart  105 , and/or abdominal organs that are innervated by various branches of vagus nerve  102 . In the illustrated embodiment, stimulation electrode  110  is placed on the cervical vagus nerve (the portion of vagus nerve  102  cranial to where RLN  106  branches out). RLN  106  innervates laryngeal muscles (represented by a laryngeal muscle  107 ), which may contract in response to the neurostimulation pulses. 
     Responses evoked by the neurostimulation pulses are detected. In the illustrated embodiment, neural sensing electrode  114 A is placed on vagus nerve  102  to sense evoked neural responses, and laryngeal activity sensor  114 B is placed in or over laryngeal muscle  107  to sense evoked muscular responses. Evoked response detection circuit  116  detects the evoked neural and/or muscular responses. In other embodiments, system  110  includes either or both of neural sensing electrode  114 A and laryngeal activity sensor  114 B to sense either or both of the evoked neural responses and the evoked muscular responses. 
     Control circuit  112  controls delivery of the neurostimulation pulses from stimulation output circuit  111 . In one embodiment, control circuit  112  controls the delivery of the neurostimulation pulses using the detected evoked neural and/or muscular responses to ensure that vagus nerve  102 , or one or more of its branches, is activated as intended. 
     In various embodiments, the circuit of system  100 , including its various elements discussed in this document, is implemented using a combination of hardware and software. In various embodiments, control circuit  112  and/or evoked response detection circuit  116 , including their various elements discussed in this document, may be implemented using an application-specific circuit constructed to perform one or more particular functions or a general-purpose circuit programmed to perform such function(s). Such a general-purpose circuit includes, but is not limited to, a microprocessor or a portion thereof, a microcontroller or portions thereof, and a programmable logic circuit or a portion thereof. 
       FIG.  2    is a block diagram illustrating an embodiment of a vagus nerve stimulation system  200 . System  200  is an embodiment of system  100  and includes stimulation electrodes  210 , stimulation output circuit  111 , an evoked response sensor  214 , a sensor processing circuit  215 , an evoked response detection circuit  216 , a control circuit  212 , and a storage circuit  218 . 
     Stimulation electrodes  210  include one or more stimulation electrodes to be placed in the patient&#39;s body in one or more locations suitable for delivering neurostimulation pulses to activate vagus nerve  102 . In various embodiments, activation of vagus nerve  102  by the neurostimulation pulses includes activation of one or more portions or branches of vagus nerve  102 . In one embodiment, stimulation electrodes  210  include stimulation electrode  110 . In one embodiment, stimulation electrodes  210  include one or more implantable stimulation electrodes, which are incorporated into one or more implantable leads each including one or more conductors for electrically connecting the one or more stimulation electrodes to stimulation output circuit  111 . In one embodiment, stimulation electrodes  210  include one or more cuff electrodes to be placed on vagus nerve  102 . In one embodiment, stimulation electrodes  210  include at least a bipolar cuff electrode to be placed on vagus nerve  102 . In one embodiment, stimulation electrodes  210  include a monopolar cuff electrode to be placed on vagus nerve  102  and another stimulation electrode placed in or on the patient&#39; body. In one embodiment, stimulation electrodes  210  include at least a multi-contact electrode to be placed on or adjacent to vagus nerve  102 . 
     Evoked response sensor  214  is to be placed in or on the patient&#39; body in a location suitable for sensing a physiological signal indicative of evoked responses being physiologic events evoked by the neurostimulation pulses. In one embodiment, evoked response sensor  214  includes neural sensing electrode  114 A to sense evoked neural responses including action potentials in vagus nerve  102  evoked by the neurostimulation pulses. The evoked neural responses include evoked A-fiber responses and evoked B-fiber responses. The evoked A-fiber responses include action potentials in the A-fibers of vagus nerve  102  evoked by the neurostimulation pulses. The evoked B-fiber responses include action potentials in the B-fibers of vagus nerve  102  evoked by the neurostimulation pulses. In one embodiment, evoked response sensor  214  includes laryngeal activity sensor  114 B to sense an evoked muscular response including activities of laryngeal muscle  107  evoked by the neurostimulation pulses. In various embodiments, evoked response sensor  214  includes either or both of neural sensing electrode  114 A and laryngeal activity sensor  114 B. Sensor processing circuit  215  processes the physiological signal in preparation for detection of the evoked responses. Evoked response detection circuit  216  receives the processed physiological signal from sensor processing circuit  215 , detects the evoked responses using the processed physiological signal, and generates one or more response signals representative of the detected evoked responses. The one or more response signals includes information about, for example, whether vagus nerve  102  is captured by the neurostimulation pulses and measured characteristics of the evoked responses. 
     Control circuit  212  controls the delivery of the neurostimulation pulses using a stimulation intensity that is represented by one or more stimulation parameters such as a pulse amplitude and a pulse width. The stimulation intensity is the energy in each of the neurostimulation pulses as measured by the pulse amplitude and the pulse width. Control circuit  212  includes a parameter adjustor  220  to adjust the stimulation intensity by adjusting one or more stimulation parameters. In one embodiment, parameter adjustor  220  adjusts the one or more stimulation parameters using the one or more response signals generated by evoked response detection circuit  216 . 
     Storage circuit  218  stores the evoked responses in the form of one or more waveforms of the evoked responses and the one or more characteristic parameters of the evoked responses. In one embodiment, storage circuit  218  stores the stimulation intensity associated with detected evoked responses. 
     In various embodiments, the circuit of system  200 , including various embodiments of its elements as illustrated in  FIG.  2   , is programmed to perform the various functions discussed in this document. In various embodiments, such functions allow for performance of the methods including, but not limited to, those discussed with reference to  FIGS.  15 ,  16 , and  18 - 20   . 
       FIG.  3    is a block diagram illustrating an embodiment of a vagus nerve stimulation system  300 . System  300  is an embodiment of system  100  or  200  and includes stimulation electrodes  210 , stimulation output circuit  111 , evoked response sensor  214 , sensor processing circuit  215 , an evoked response detection circuit  316 , a control circuit  312 , and storage circuit  218 . 
     Evoked response detection circuit  316  is an embodiment of evoked response detection circuit  216  and detects the evoked responses using the physiological signal and generates one or more response signals representative of the detected evoked responses. Evoked response detection circuit  316  includes a detection timer  326 , a comparator  327 , and a measurement module  328 . Detection timer  326  controls timing of detection of the evoked responses. Examples of such timing include initiation of the detection according to a specified schedule and one or more detection windows within which the evoked responses are expected to be detected. Comparator  327  detects the evoked responses by comparing the physiological signal to one or more detection thresholds. In one embodiment, comparator  327  detects the evoked responses by comparing the physiological signal to one or more detection thresholds during the one or more detection windows. Measurement module  328  measures one or more characteristic parameters of the evoked responses. Examples of the one or more characteristic parameters include amplitude of the evoked responses, width of the evoked responses, and frequency characteristics of the evoked responses. In various embodiments, the one or more characteristic parameters are each a value measured from one of the evoked responses or being an average of values measured from a plurality of the evoked responses. In various embodiments, examples of the one or more response signals include a capture verification signal declaring capture of the vagus nerve by the neurostimulation pulses and one or more signals representative of the one or more characteristic parameters of the evoked responses. 
     Control circuit  312  is an embodiment of control circuit  212  and controls the delivery of the neurostimulation pulses using the stimulation intensity. Control circuit  312  includes a parameter adjustor  320 , which is an embodiment of parameter adjustor  220  and adjusts one or more parameters of the stimulation parameters using the one or more response signals. In the illustrated embodiment, parameter adjustor  320  includes a sensing parameter adjustor  321 , a sensing adjustment timer  322 , a stimulation parameter adjustor  323 , and a stimulation adjustment timer  324 . Sensing parameter adjustor  321  adjusts the one or more detection thresholds used by comparator  327  for detecting the evoked responses. Sensing adjustment timer  322  controls the timing of the adjustment of the one or more detection thresholds according to a specified schedule and/or in response to a user command. Stimulation parameter adjustor  323  adjusts the stimulation intensity by adjusting one or more of the stimulation parameters including either or both of the pulse amplitude and the pulse width of the neurostimulation pulses. In various embodiments, stimulation parameter adjustor  323  also adjusts other stimulation parameters such as pulse frequency, duty cycle, and stimulation duration. Stimulation adjustment timer  324  controls the timing of adjustment of the stimulation intensity according to a specified schedule and/or in response to a user command. 
       FIG.  4    is an illustration of an embodiment of an implantable system  430  and portions of an environment in which implantable system  430  is used. Implantable system  430  includes system  100  including its various embodiments as discussed in this document. 
     System  430  includes an implantable system  432  and an external system  436 . Implantable system  432  includes an implantable medical device (IMD)  434 . External system  436  and IMD  434  communicate via a telemetry link  435 . In various embodiments, implantable system  432  includes system  200  or system  300 . In various embodiments, IMD  434  integrates a cardiac rhythm management (CRM) device with a neural sensing and stimulation device including portions of system  200  or portions of system  300 . The CRM device senses cardiac electrical activities and delivers cardiac stimulation. Examples of the CRM device include pacemakers, cardioverter/defibrillators, combined pacemaker-cardioverter/defibrillators, cardiac resynchronization therapy (CRT) devices, and cardiac remodeling control therapy (RCT) devices. In various embodiments, neural activities are sensed to indicate a need for cardiac stimulation and/or to control the timing of pacing pulse deliveries. In various embodiments, cardiac activities are sensed to control the timing of neural stimulation pulse deliveries, such as to synchronize neural stimulation to cardiac cycles. 
       FIG.  5    is a block diagram illustrating an embodiment of an implantable system  530 . Implantable system  530  is an embodiment of implantable circuit  430  and includes an IMD  534  and an external system  536 . 
     IMD  534  is an embodiment of IMD  434  and includes an IMD circuit  539  and an implantable housing  538  encapsulating IMD circuit  539 . In one embodiment, IMD circuit  539  includes at least stimulation output circuit  111  and control circuit  212  or  312 . In another embodiment, IMD circuit  539  includes at least stimulation output circuit  111 , sensor processing circuit  215 , evoked response detection circuit  216  or  316 , control circuit  212  or  312 , and storage circuit  218 . In various embodiments, IMD circuit  539  includes various elements of system  200  or system  300 . 
     External system  536  is an embodiment of external system  436  and is communicatively coupled to IMD  534  via telemetry link  435 . External system  536  includes a user interface  540 . User interface  540  includes a presentation device  542  and a user input device  544 . Presentation device  542  includes a display screen  543  to display, for example, waveforms of the detected evoked responses, the one or more response signals, the measured one or more characteristics parameters, and/or the stimulation intensity. User input device  544  receives user commands from a user such as a physician or other caregiver. Examples of the user commands include a user command for starting a delivery of the neurostimulation pulses, a user command to initiate an adjustment of the one or more detection thresholds, a user command to initiate an adjustment of the stimulation intensity, and a user command to initiate automatic capture verification as discussed in this document. 
     In one embodiment, external system  536  includes a programmer including user interface  540 . In one embodiment, external system  536  includes a patient management system including an external device communicatively coupled to IMD  534  via telemetry link  435  and a remote device in a distant location and communicatively coupled to the external device via a communication network. The external device and/or the remote device include user interface  540 . 
       FIG.  6    is an illustration of evoked responses to a neurostimulation pulse delivered to the cervical vagus nerve as seen on a neural signal recorded from the vagus nerve and an electromyographic (EMG) signal recorded from a laryngeal muscle. The neural signal includes an evoked neural response that follows the delivery of the neurostimulation pulse. The time delay between the evoked neural response and the delivery of the neurostimulation pulse is a function of the distance between the neural signal sensing site and the stimulation site. The evoked neural response includes an evoked A-fiber response and an evoked B-fiber response. As seen in  FIG.  6   , the evoked A-fiber response precedes the evoked B-fiber response. The EMG signal includes an evoked muscular response that follows the delivery of the neurostimulation pulse. The time delay between the evoked muscular response and the delivery of the neurostimulation pulse is a function of the distance between the EMG signal sensing site and the stimulation site. In various embodiments, the time delays are estimated using the distance between the sensing site and the stimulation site to facilitate the detection of the evoked responses. 
       FIG.  7    is an illustration of evoked responses to neurostimulation pulses of various intensities. Neural and EMG signals including evoked neural and muscular responses to neurostimulation pulses with different pulse amplitudes are shown. The morphology changes in the neural and EMG signals indicate that more nerve fibers are captured as the pulse amplitude increases. The minimum pulse amplitude required to evoke the A-fiber and muscular responses is lower than the minimum pulse amplitude required to evoke the B-fiber responses. 
       FIG.  8    is an illustration showing recruitment curves indicative of various stimulation thresholds being the pulse amplitudes corresponding to percentage of fiber recruitment. The stimulation threshold curves show a trend consistent with what is observed from the neural and EMG signals of  FIG.  7   . The evoked A-fiber responses start to be detectable when the stimulation (current) amplitude is about 0.8 mA. The evoked B-fiber responses start to be detectable when the stimulation amplitude is between about 1.5 mA to 2 mA. It is believed that the A-fibers correspond to motor fibers of the vagus nerve that are primarily responsible for the activation of the laryngeal muscle, and the B-fibers correspond to part of the parasympathetic fibers of the vagus nerve that are primarily responsible for the modulation of physiologic functions including cardiovascular functions. 
       FIG.  9    is a block diagram illustrating an embodiment of a circuit for detecting evoked neural responses. In one embodiment, the circuit is part of IMD circuit  539 . The circuit includes stimulation output circuit  111 , a neural sensing circuit  915 , and an evoked neural response detection circuit  916 . Stimulation output circuit  111  delivers neurostimulation pulses to the vagus nerve. Neural sensing circuit  915  is an embodiment of sensor processing circuit  215  and senses a neural signal representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses. Evoked neural response detection circuit  916  is an embodiment of evoked response detection circuit  216  and detects the evoked neural responses using the neural signal. Evoked neural response detection circuit  916  includes a detection timer  926  and a comparator  927 . Detection timer  926  times a delay interval, a detection window A, and a detection window B in response to delivery of one of the neurostimulation pulses. As illustrated in  FIG.  10   , the delay interval starts upon the delivery of the one of the stimulation pulses. The detection window A, which is a time window within which an evoked A-fiber response is expected, starts upon expiration of the delay interval. The detection window B, which is a time window within which an evoked B-fiber response is expected, starts upon expiration of the detection interval A. Comparator  927  detects the evoked A-fiber response by comparing the sensed neural signal to a detection threshold A during the detection window A and detects the evoked B-fiber response by comparing the sensed neural signal to a detection threshold B during the detection window B. In another embodiment, evoked neural response detection circuit  916  includes comparator  927  but not detection timer  926 . Comparator  927  detects the evoked neural responses by comparing the sensed neural signal to one or more detection thresholds. 
       FIG.  11    is a block diagram illustrating an embodiment of a system for detecting evoked neural responses. The system includes stimulation electrodes  210 , stimulation output circuit  111 , neural sensing electrodes  1114 , neural sensing circuit  915 , and an evoked neural response detection circuit  1116 . 
     Neural sensing electrodes  1114  are an embodiment of evoked response sensor  214  and are configured to be placed in the patient&#39;s body in a location suitable for sensing the neural signal representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses. In one embodiment, neural sensing electrodes  1114  include implantable neural sensing electrodes being part of implantable system  432 . In one embodiment, neural sensing electrodes  1114  include one or more cuff electrodes to be placed on the vagus nerve. Neural sensing circuit  915  is an embodiment of sensor processing circuit  215  and senses a neural signal through neural sensing electrodes  1114 . 
     Evoked neural response detection circuit  1116  is an embodiment of evoked neural response detection circuit  916  and an embodiment of evoked response detection circuit  216  or  316  and detects the evoked neural responses using the sensed neural signal. Evoked neural response detection circuit  1116  includes at least comparator  927  and an evoked neural response measurement module  1128 , and includes detection timer  926  if at least one detection window is used. In various embodiments, evoked neural response detection circuit  1116  detects the evoked neural response according to a specified schedule, such as on a periodic basis, or in response to a user command. Evoked neural response measurement module  1128  is an embodiment of measurement module  328  and measures one or more characteristic parameters of the evoked neural responses. In one embodiment, evoked neural response measurement module  1128  measures and trends the one or more characteristic parameters. Examples of the one or more characteristic parameters include amplitude of the evoked A-fiber response being the peak amplitude of the sensed neural signal during the detection window A, width of the evoked A-fiber response being the time interval during which the amplitude of the sensed neural signal exceeds the detection threshold A during the detection window A, amplitude of the evoked B-fiber response being the peak amplitude of the sensed neural signal during the detection window B, and width of the evoked B-fiber response being the time interval during which the amplitude of the sensed neural signal exceeds the detection threshold B during the detection window B. 
     Experimental data from an animal study indicate that the amplitude of the evoked neural response in the detection window A (i.e., the evoked A-fiber response) is in a range of approximately 5 to 20.mu.V, and the evoked neural response in the detection window B (i.e., the evoked B-fiber response) is in a range of approximately 1 to 6.mu.V. During the study, the duration of the detection window A was set to 5 ms, and the duration of the detection window B was set to 5 ms. The delay interval, or timing for initiating each of the detection window A and the detection window B depended on the distance between the stimulation site and the sensing site. 
     If the stimulation site and the sensing site are close to each other, it may be difficult to set the delay interval and the detection window A accurately. Consequently, it may be difficult to detect the evoked A-fiber response. However, detection of the evoke B-fiber responses is of primary interest because the B-fibers are believed to be responsible for modulating target functions of vagus nerve stimulation such as cardiovascular functions. 
     In one embodiment, the distance between the stimulation site and the sensing site is received from the user by user input device  544 . In one embodiment, the stimulation site is where a neural stimulation electrode is placed on the vagus nerve, and the sensing site is where a neural sensing electrode is placed on the vagus nerve. Detection timer  926  determines the delay interval, the detection window A, and the detection window B each as a function of that distance. In one embodiment, the length of the delay interval, the detection window A, and the detection window B are each calibrated using the time between a non-capturing electrical neurostimulation pulse and a field effected by that pulse (not an evoked response). 
       FIG.  12    is a block diagram illustrating an embodiment of a circuit for detecting evoked muscular responses. In one embodiment, the circuit is part of IMD circuit  539 . The circuit includes stimulation output circuit  111 , a laryngeal activity sensing circuit  1215 , and an evoked muscular response detection circuit  1216 . Stimulation output circuit  111  delivers neurostimulation pulses to the vagus nerve. Laryngeal activity sensing circuit  1215  is an embodiment of sensor processing circuit  215  and senses a laryngeal signal representative of activities of the laryngeal muscle including evoked muscular responses each evoked by one of the neurostimulation pulses. Evoked muscular response detection circuit  1216  is an embodiment of evoked response detection circuit  216  and detects the evoked muscular responses using the laryngeal signal. In the illustrated embodiment, evoked neural response detection circuit  1216  includes a detection timer  1226  and a comparator  1227 . Detection timer  1226  times a detection window during which the detection of an evoked muscular response is anticipated. Comparator  1227  detects the evoked muscular responses by comparing the sensed laryngeal signal to one or more detection thresholds during the detection window. In another embodiment, evoked neural response detection circuit  1216  includes comparator  1227  but not detection timer  1226 . Comparator  1227  detects the evoked muscular responses by comparing the sensed laryngeal signal to one or more detection thresholds. 
       FIG.  13    is a block diagram illustrating an embodiment of a system for detecting evoked muscular responses. The system includes stimulation electrodes  210 , stimulation output circuit  111 , a laryngeal activity sensor  1314 , laryngeal activity sensing circuit  1215 , and an evoked muscular response detection circuit  1316 . 
     Laryngeal activity sensor  1314  is an embodiment of evoked response sensor  214  and is configured to be placed in or on the patient&#39;s body in a location suitable for sensing the laryngeal signal. In one embodiment, laryngeal activity sensor  1314  includes an implantable laryngeal activity sensor to be placed in the patient&#39;s body. In another embodiment, laryngeal activity sensor  1314  includes an external laryngeal activity sensor to be placed on the surface of the patient&#39;s body. Laryngeal activity sensing circuit  1215  is an embodiment of sensor processing circuit  215  and processes the signal sensed by laryngeal activity sensor  1314 . Examples of the laryngeal signal, laryngeal activity sensor  1314 , and laryngeal activity sensing circuit  1215  are discussed below with reference to  FIG.  14   . 
     Evoked muscular response detection circuit  1316  is an embodiment of evoked muscular response detection circuit  1216  and an embodiment of evoked response detection circuit  216  or  316 , and detects the evoked muscular responses using the sensed laryngeal signal. Evoked muscular response detection circuit  1316  includes at least comparator  1227  and an evoked muscular response measurement module  1328 , and includes detection timer  1226  if the detection window is used. In one embodiment, evoked muscular response detection circuit  1316  detects the evoked muscular response according to a specified schedule, such as on a periodic basis, or in response to a user command. Evoked muscular response measurement module  1328  is an embodiment of measurement module  328  and measures one or more characteristic parameters. In one embodiment, evoked muscular response measurement module  1328  measures and trends the one or more characteristic parameters. Examples of the one or more characteristic parameters include the amplitude of an evoked muscular response, the sum of multiple evoked muscular responses that follow multiple neurostimulation pulses, and the time between the delivery of a neurostimulation pulse and the detection of the evoked muscular response resulting from the delivery of that neurostimulation pulse. 
     Amplitude of the evoked muscular responses increases as more motor fibers (A-fibers) are captured by delivery of the neurostimulation pulses. More motor fibers are captured as the stimulation intensity increases. 
     It is believed that an approximately constant relationship can be identified between the stimulation threshold for capturing the A-fibers and the stimulation threshold for effectively modulating a target physiological function through capturing the B-fibers. The stimulation intensity is a minimum stimulation intensity required to evoke one or more specified physiological responses. Once an initial stimulation threshold providing for the initial evoked muscular response is determined, the stimulation intensity is set to a level that is determined by using the initial stimulation threshold and the identified approximately constant relationship. The initial evoked muscular response is the evoked muscular responses that start to become detectable as the stimulation intensity increases from a low initial level. The initial stimulation threshold is the stimulation intensity that produces the initial evoked muscular response. In one embodiment, the approximately constant relationship is quantitatively established using a patient population. The stimulation intensity for a vagus nerve stimulation therapy applied to the patient is then set using the initial stimulation threshold and the established approximately constant relationship. 
       FIG.  14    is a block diagram illustrating an embodiment of a circuit for sensing various laryngeal signals. The circuit includes a laryngeal activity sensor  1414  and a laryngeal activity sensing circuit  1415 . Laryngeal activity sensor  1414  is an embodiment of laryngeal activity sensor  1314 . Laryngeal activity sensing circuit  1415  is an embodiment of laryngeal activity sensing circuit  1215 . In the illustrated embodiment, laryngeal activity sensor  1414  includes EMG sensing electrodes  1414 A, an accelerometer  1414 B, and a voice sensor  1414 C, and laryngeal activity sensing circuit  1415  includes an EMG sensing circuit  1415 A, an activity sensing circuit  1415 B, and a voice sensing circuit  1415 C. This allows for selection of a laryngeal signal by the user or the system of  FIG.  13   , and also allows for use of multiple laryngeal signals for the detection of the evoked muscular responses. In various embodiments, laryngeal activity sensor  1414  includes any one or more of EMG sensing electrodes  1414 A, accelerometer  1414 B, and voice sensor  1414 C, and laryngeal activity sensing circuit  1415  includes the corresponding one or more of EMG sensing circuit  1415 A, activity sensing circuit  1415 B, and voice sensing circuit  1415 C, depending on the laryngeal signal(s) used. 
     EMG sensing electrodes  1414 A are configured to be placed in or on the patient&#39;s body in a location suitable for sensing an EMG signal as the laryngeal signal from laryngeal muscle  107 . The EMG signal is indicative of activities of laryngeal muscle  107  including the evoked muscular response. In one embodiment, EMG sensing electrodes  1414 A includes implantable EMG sensing electrodes such as intramuscular electrodes. EMG sensing circuit  1415 A senses the EMG signal through EMG sensing electrodes  1414 A. Evoked muscular response detection circuit  1216  or  1316  detects the evoked muscular responses using the sensed EMG signal. 
     Accelerometer  1414 B is configured to be placed in or on the patient&#39;s body in a location suitable for sensing an acceleration signal as the laryngeal signal. The acceleration signal is indicative of activities of laryngeal muscle  107  including the evoked muscular responses. In one embodiment, accelerometer  1414 B includes an implantable accelerometer. Activity sensing circuit  1415 B processes the acceleration signal sensed by accelerometer  1414 B. Evoked muscular response detection circuit  1216  or  1316  detects the evoked muscular responses using the processed acceleration signal. 
     Voice sensor  1414 C is configured to be placed in or on the patient&#39;s body in a location suitable for sensing a voice signal as the laryngeal signal. In one embodiment, voice sensor  1414 C includes a microphone. In one embodiment, voice sensor  1414 C includes an implantable voice sensor. Vagus nerve stimulation is known to cause change in a patent&#39;s voice, such as hoarseness, by activating the laryngeal muscle. Thus, certain changes in the voice signal are indicative of activities of laryngeal muscle  107  including the evoked neuromuscular responses. Voice sensing circuit  1415 C processes the voice signal sensed by voice sensor  1414 C. Evoked muscular response detection circuit  1216  or  1316  detects the evoked muscular responses using the processed voice signal, such as by detecting changes in frequency characteristics of the voice signal. 
       FIG.  15    is a flow chart illustrating an embodiment of a method  1500  for automatic threshold adjustment (also referred to as “Auto-Sense”) for evoked response detection during vagus nerve stimulation. In various embodiments, method  1500  is performed by using system  100 , including its various embodiments discussed in this document. The automatic threshold adjustment provides automatic adjustment of the one or more detection thresholds used by evoked response detection circuit  116 , including its various embodiments discussed in this document. In various embodiments, evoked response detection circuit  116  is configured to perform method  1500  according to a specified schedule. In one embodiment, evoked response detection circuit  116  is configured to perform method  1500  periodically, such as monthly, weekly, daily, hourly, once each burst of the neurostimulation pulses, or once each pulse of the neurostimulation pulses. 
     At  1502 , neurostimulation pulses are delivered to a vagus nerve. At  1504 , a physiological signal is sensed. The physiological signal is indicative of evoked responses each being a physiologic event evoked by one of the neurostimulation pulses. At  1506 , the evoked responses are detected by comparing the physiological signal to one or more detection thresholds. At  1508 , the one or more detection thresholds are adjusted, if necessary, using the detected evoked neural responses. 
     In one embodiment, the physiological signal is sensed using evoked response sensor  214  and sensor processing circuit  215  at  1504 . The evoked responses are detected by evoked response detection circuit  316  at  1506 . The one or more detection thresholds are adjusted by sensing parameter adjustor  321 . Sensing adjustment timer  322  times the performance of method  1500  according to the specified schedule or in response to a user command. 
     In one embodiment, the physiological signal includes a neural signal representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses, and the evoked responses include the evoked neural responses. At  1504 , the neural signal is sensed. At  1506 , the evoked neural responses are detected. In one embodiment, an evoked neural response waveform representative of the evoked neural responses is detected and stored. The waveform is of one detected evoked neural response or an average of several detected evoked neural responses. In one embodiment, one or more characteristic parameters of the evoked neural responses are measured. Examples of the one or more characteristic parameters include the amplitude of the evoked A-fiber response, the width of the evoked A-fiber response, the amplitude of the evoked B-fiber response, and the width of the evoked B-fiber response as discussed above. In one embodiment, the measured one or more characteristic parameters are trended and/or stored for presentation to the user as scheduled or needed. At  1506 , the one or more detection thresholds, such as the detection threshold A and the detection threshold B, are adjusted using the detected evoked neural responses. In one embodiment, the detected evoked neural responses are compared to a stored baseline response. This includes comparing the evoked neural response waveform to a stored baseline waveform and/or comparing the one or more characteristic parameters to stored one or more baseline characteristic parameters. The baseline waveform and/or the one or more baseline characteristic parameters are established for a patient during the initial system setup for the patient (such as implantation of implantable system  432 ), during a follow-up visit, or automatically by evoked neural response detection circuit  916  when certain criteria are met. The one or more detection thresholds are adjusted in response to the detected evoked neural responses substantially deviating from the stored baseline response. In one embodiment, the user is alerted in response to the detected evoked neural responses substantially deviating from the stored baseline response. 
     In one embodiment, the physiological signal includes a laryngeal signal representative of activities of the laryngeal muscle including evoked muscular responses each evoked by one of the neurostimulation pulses, and the evoked responses includes the evoked muscular responses. At  1504 , the laryngeal signal is sensed. At  1506 , the evoked muscular responses are detected. In one embodiment, an evoked muscular response waveform representative of the evoked muscular responses is detected and stored. The waveform is of one detected evoked muscular response or an average of several detected evoked muscular responses. In one embodiment, one or more characteristic parameters of the evoked muscular responses are measured. Examples of the one or more characteristic parameters include a maximum amplitude of the sensed laryngeal signal. In one embodiment, the measured one or more characteristic parameters are trended and/or stored for presentation to the user as scheduled or needed. At  1506 , the one or more detection thresholds are adjusted using the detected evoked muscular responses. In one embodiment, the detected evoked muscular responses are compared to a stored baseline response. This includes comparing the evoked response waveform to a stored baseline waveform and/or comparing the one or more characteristic parameters to stored one or more baseline characteristic parameters. The baseline waveform and/or the one or more baseline characteristic parameters are established for a patient during the initial system setup for the patient (such as implantation of implantable system  432 ), during a follow-up visit, or automatically by evoked muscular response detection circuit  916  when certain criteria are met. The one or more detection thresholds are adjusted in response to the detected evoked muscular responses substantially deviating from the stored baseline response. In one embodiment, the user is alerted in response to the detected evoked muscular responses substantially deviating from the stored baseline response. 
       FIG.  16    is a flow chart illustrating an embodiment of a method  1600  for adjusting stimulation intensity for vagus nerve stimulation. In various embodiments, method  1600  is performed by using system  100 , including its various embodiments discussed in this document. 
     At  1602 , neurostimulation pulses are delivered to a vagus nerve. At  1604 , the delivery of the neurostimulation pulses is controlled using a stimulation intensity. The stimulation intensity is adjustable by adjusting stimulation parameters including a pulse amplitude and a pulse width. At  1606 , the stimulation intensity is swept at specified increments. At  1608  a physiological signal is sensed. The physiological signal is indicative of evoked responses each being a physiologic event evoked by one of the neurostimulation pulses. At  1610 , the evoked responses are detected. At  1612 , a stimulation threshold is determined. The stimulation threshold is a minimum level of the stimulation intensity for providing one or more specified characteristics of the evoked responses. At  1614 , the stimulation intensity is adjusted for modulating a specified physiologic function using the stimulation threshold. In one embodiment, the physiologic function includes a cardiovascular function. In one embodiment, the stimulation threshold measured from each performance of method  1600  is trended. In various embodiments, the trend of the stimulation threshold is used to indicate pathological conditions and/or device problems. For example, a substantially increasing stimulation threshold may indicate device problems such as poor electrical connections or lead failure or pathological conditions such as nerve damages. When this happens, the user is alerted for examining the patient and the neurostimulation system. If the stimulation threshold is not determined after the stimulation intensity is swept through its maximum level, the user is also alerted because an abnormally high stimulation threshold is indicative of the device problems or pathological conditions. 
     In one embodiment, stimulation parameter adjustor  323  controls the sweeping of the stimulation intensity at  1606 . The physiological signal is sensed using evoked response sensor  214  and sensor processing circuit  215  at  1608 . The evoked responses are detected by evoked response detection circuit  316  at  1610 . The stimulation threshold is determined by stimulation parameter adjustor  323  at  1612 . The stimulation intensity is adjusted by stimulation parameter adjustor  323  at  1614 . Stimulation adjustment timer  324  initiates and/or times the performance of method  1600  according to the specified schedule or in response to a user command. 
     In one embodiment, the physiological signal includes a neural signal representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses, and the evoked responses include the evoked neural responses. At  1608 , the neural signal is detected. At  1610 , the evoked neural responses are detected. In one embodiment, an evoked neural response waveform representative of the evoked neural responses is detected and stored. The waveform is of one detected evoked neural response or an average of several detected evoked neural responses. At  1612 , the stimulation threshold for one or more specified effects in the evoked neural response is determined. Examples of the one or more specified effects include that the amplitude of the sensed neural signal during the detection window A reaches a threshold amplitude, that the width of the evoked response detected during the detection window A reaches a threshold width, that the evoked B-fiber response is detected during the detection window B, and that a correlation between the detected evoked neural response waveform and a stored baseline waveform reaches a threshold correlation. At  1614 , the stimulation intensity is adjusted for modulating a specified physiologic function, such as a cardiovascular function, using the stimulation threshold. 
     In one embodiment, the physiological signal includes a laryngeal signal representative of activities of the laryngeal muscle including evoked muscular responses each evoked by one of the neurostimulation pulses, and the evoked responses include the evoked muscular responses. At  1608 , the laryngeal signal is detected. At  1610 , the evoked muscular responses are detected. In one embodiment, an evoked muscular response waveform representative of the evoked muscular responses is detected and stored. The waveform is of one detected evoked muscular response or an average of several detected evoked muscular responses. At  1612 , the stimulation threshold for one or more specified effects in the evoked muscular response is determined. Examples of the one or more specified effects include that the amplitude of the sensed laryngeal signal during a detection window reaches a threshold amplitude, that an evoked muscular response is detected during the detection window, and a correlation between the detected evoked muscular response waveform and a stored baseline waveform reaches a threshold correlation. At  1614 , the stimulation intensity is adjusted for modulating a specified physiologic function, such as a cardiovascular function, using the stimulation threshold. 
     In one embodiment, in which the sensed physiological signal includes the laryngeal signal, the threshold amplitude is set to a minimum amplitude of the laryngeal signal that allows for the detection of the evoked muscular responses, and the corresponding stimulation threshold is recorded as an initial stimulation threshold. The stimulation intensity is adjusted to a level calculated by using a predetermined relationship between the initial stimulation threshold and a value of the stimulation intensity associated with the specified physiological function, such as the cardiovascular function. Because it is believed that the A-fibers correspond to the motor fibers of the vagus nerve that are primarily responsible for the activation of the laryngeal muscle, and that the B-fibers correspond to part of the parasympathetic fibers of the vagus nerve that are primarily responsible for the modulation of physiologic functions including cardiovascular functions, the predetermined relationship is a relationship between the stimulation threshold for activating the A-fibers (Threshold A) and the stimulation threshold for activating the B-fibers (Threshold B). Experimental data indicate that such a relationship can be approximated by a constant.  FIG.  17    is an illustration showing an example of such a relationship. The plot shows normalized dynamic ranges of a ratio of the difference between Threshold B and Threshold A to the sum of Threshold B and Threshold A, i.e., (Threshold B−Threshold A)/(Threshold B+Threshold A), for various pulse widths, where Threshold A is the stimulation threshold for recruiting about 50% of the A-fibers, and Threshold B is the stimulation threshold for recruiting about 50% of the B-fibers. Thus, the stimulation intensity is calculated by multiplying the initial stimulation threshold by the constant. 
     In one embodiment, method  1600  is performed by system  100  automatically. This automatic stimulation intensity adjustment (also referred to as “Auto-Threshold”) provides automatic adjustment of the stimulation intensity for modulating the specified physiological function. In various embodiments, control circuit  112  and evoked response detection circuit  116 , including their various embodiment as discussed in this document, are configured to perform method  1600  according to a specified schedule. In one embodiment, control circuit  112  and evoked response detection circuit  116  are configured to perform method  1600  periodically, such as monthly, weekly, daily, hourly, once each burst of the neurostimulation pulses, or once each pulse of the neurostimulation pulses. In other embodiments, adjustment of the stimulation intensity is performed by the user using system  100 , as discussed below with reference to  FIGS.  18  and  19   . 
       FIG.  18    is a flow chart illustrating an embodiment of a method  1800  for adjusting stimulation intensity for vagus nerve stimulation during implantation of an implantable medical device. In one embodiment, method  1800  is performed by the user using system  100 , including its various embodiments discussed in this document. 
     At  1802 , stimulation electrodes are placed on a vagus nerve of a patient. At  1804 , the stimulation electrodes are connected to a neurostimulator including stimulation output circuit  111  and control circuit  112  for delivering neurostimulation pulses to the vagus nerve. The neurostimulator may be an external device for use during the implantable procedure or the implantable medical device intended to be implanted into the patient. 
     At  1806 , an evoked response sensor is placed in the patient for sensing a physiological signal indicative of evoked responses each being a physiologic event evoked by one of the neurostimulation pulses. In various embodiments, this includes placing neural sensing electrodes on the vagus nerve for sensing a neural signal and/or placing a laryngeal activity sensor in a location suitable for sensing a laryngeal signal. The neural signal is representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses. The laryngeal signal is representative of activities of the laryngeal muscle including evoked muscular responses each evoked by one of the neurostimulation pulses. In various embodiments, the evoked response sensor may be for temporary use during the implantation procedure or intended to be implanted with the implantable medical device into the patient. 
     At  1808 , the neurostimulation pulses are delivered through the stimulation electrodes. The delivery of the neurostimulation pulses is controlled using a stimulation intensity that starts at a specified low level. The stimulation intensity is controlled by one or more stimulation parameters including the pulse amplitude and/or the pulse width. At  1810 , the physiological signal is sensed. 
     At  1812 , the evoked responses, including waveforms and measured information, are presented to the user on a display screen. When the physiological signal includes the neural signal, examples of the presented information include amplitude of the evoked neural responses, sum of a plurality of the evoked neural responses, time between the delivery of a neurostimulation pulse and the detection of the evoked neural response resulting from the delivery of that neurostimulation pulse, notation of response characteristics (e.g., “A-fiber” and “B-fiber” labels), and stimulation parameters including those controlling the stimulation intensity. When the physiological signal includes the laryngeal signal, examples of the presented information include amplitude of the evoked muscular responses, sum of a plurality of the evoked muscular responses, time between the delivery of a neurostimulation pulse and the detection of the evoked muscular response resulting from the delivery of that neurostimulation pulse, and stimulation parameters including those controlling the stimulation intensity. 
     At  1816 , if the user is not satisfied with the evoked neural responses at  1814 , the stimulation intensity is increased by increasing the pulse amplitude and/or the pulse width. If the stimulation intensity cannot be further increased, the user is alerted for examining the patient for possible pathological conditions preventing effectiveness of neurostimulation and/or the system for possible device and/or connection problems. At  1818 , if the user is satisfied with the evoked neural responses associated with a level of the stimulation intensity at  1814 , that level of the stimulation intensity (in terms of the pulse amplitude and the pulse width) is stored and used for the subsequent vagus nerve stimulation therapy delivered from the implantable medical device. The evoked response sensor is removed if it is for temporary use during the implantation procedure. 
       FIG.  19    is a flow chart illustrating an embodiment of a method  1900  for adjusting stimulation intensity for vagus nerve stimulation during follow-up visits by the patient using the implantable medical device. Method  1900  is performed subsequent to method  1800 . In one embodiment, method  1900  is performed by the user using system  100 , including its various embodiments discussed in this document. 
     At  1902 , an intensity adjustment feature of the implantable medical device is initiated by the user. In one embodiment, stimulation adjustment timer  423  initiates the adjustment of stimulation intensity in response to a user command entered by the user using an external system communicatively coupled to the implantable medical device. At  1904 , stimulation intensity levels are swept. This includes incrementally increasing the pulse amplitude and/or the pulse width from specified low values. 
     At  1906 , the physiological signal is sensed using the evoked response sensor that was implanted in the patient with the implantable medical device. This includes sensing of the neural signal and/or the laryngeal signal. At  1908 , the evoked responses, including the evoked neural responses and/or the evoked muscular responses, are detected. At  1910 , data representative of the detected evoked responses are telemetered to the external system. 
     At  1912 , the evoked responses, including waveforms and measured information, are presented to the user on a display screen of the external system using the telemetered data. When the physiological signal includes the neural signal, examples of the presented information include amplitude of the evoked neural responses, sum of a plurality of the evoked neural responses, time between the delivery of a neurostimulation pulse and the detection of the evoked neural response resulting from the delivery of that neurostimulation pulse, notation of response characteristics (e.g., “A-fiber” and “B-fiber” labels), and stimulation parameters including those controlling the stimulation intensity. When the physiological signal includes the laryngeal signal, examples of the presented information include amplitude of the evoked muscular responses, sum of a plurality of the evoked muscular responses, time between the delivery of a neurostimulation pulse and the detection of the evoked muscular response resulting from the delivery of that neurostimulation pulse, and stimulation parameters including those controlling the stimulation intensity. 
     At  1916 , if the user is not satisfied with the evoked neural responses at  1914 , the stimulation intensity is increased by increasing the pulse amplitude and/or the pulse width. If the stimulation intensity cannot be further increased, the user is alerted for examining the patient for possible pathological conditions preventing effectiveness of neurostimulation and/or the system for possible device and/or connection problems. At  1918 , if the user is satisfied with the evoked neural responses associated with a level of the stimulation intensity at  1914 , that level of the stimulation intensity (in terms of the pulse amplitude and the pulse width) is stored and used for the subsequent vagus nerve stimulation therapy delivered from the implantable medical device. 
       FIG.  20    is a flow chart illustrating an embodiment of a method  2000  for automatic capture verification (also referred to as “Auto-Capture”) for vagus nerve stimulation. In various embodiments, method  2000  is performed by using system  100 , including its various embodiments discussed in this document. The automatic capture verification provides automatic verification of capture of the vagus nerve by neurostimulation pulses and adjustment of the stimulation intensity. In various embodiments, control circuit  112  and evoked response detection circuit  116 , including their various embodiments as discussed in this document, are configured to perform method  1600  according to a specified schedule. In one embodiment, control circuit  112  and evoked response detection circuit  116  are configured to perform method  1600  periodically, such as monthly, weekly, daily, hourly, once each burst of the neurostimulation pulses, or once each pulse of the neurostimulation pulses. 
     At  2002 , neurostimulation pulses are delivered to a vagus nerve. At  2004 , the delivery of the neurostimulation pulses is controlled using a stimulation intensity. The stimulation intensity is adjusted by adjusting stimulation parameters including a pulse amplitude and a pulse width. At  2006 , a capture verification is performed. The capture verification includes sensing a physiological signal indicative of evoked responses each being a physiologic event evoked by one of the neurostimulation pulses at  2008 , detecting one of the evoked responses for each pulse of the neurostimulation pulses delivered at  2010 , and adjusting the stimulation intensity at  2012 . The stimulation intensity is adjusted in response to specified one or more of the evoked responses not detected (i.e. non-capture) for a specified number of the neurostimulation pulses delivered. This includes adjustment of the pulse amplitude and/or the pulse width of the neurostimulation pulses. In one embodiment, the stimulation intensity is adjusted in response to an evoked response not being detected for one of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to the evoked response not being detected for a specified first number of the neurostimulation pulses delivered out of a specified second number of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to an evoked response not being detected for a rolling average number of the neurostimulation pulses delivered. In one embodiment, method  2000  is performed with the stimulation intensity lowered to prevent unnecessary energy delivered with the neurostimulation pulses to promote device longevity. If an unacceptable degree of loss of capture occurs when the stimulation intensity is set to about the available maximum level, the user is alerted for examining the patient for possible pathological conditions preventing effectiveness of neurostimulation and/or the system for possible device and/or connection problems. 
     In one embodiment, stimulation parameter adjustor  323  controls the stimulation intensity at  2004 . The physiological signal is sensed using evoked response sensor  214  and sensor processing circuit  215  at  2008 . The evoked responses are detected by evoked response detection circuit  316  at  2010 . The stimulation intensity is adjusted by stimulation parameter adjustor  323  at  2012 . Stimulation adjustment timer  324  initiates and/or times the performance of method  2000  according to the specified schedule or in response to a user command. 
     In one embodiment, the physiological signal includes a neural signal representative of neural activities in the vagus nerve including evoked neural responses each evoked by one of the neurostimulation pulses, and the evoked responses include the evoked neural responses. At  2008 , the neural signal is sensed. At  2010 , one of the evoked neural responses for each pulse of the neurostimulation pulses delivered is detected. At  2012 , the stimulation intensity is adjusted in response to specified one or more of the evoked neural responses not detected (i.e. non-capture) for a specified number of the neurostimulation pulses delivered. In one embodiment, the stimulation intensity is adjusted in response to an evoked neural response not being detected for one of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to the evoked neural response not being detected for a specified first number of the neurostimulation pulses delivered out of a specified second number of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to an evoked neural response not being detected for a rolling average number of the neurostimulation pulses delivered. 
     In one embodiment, the physiological signal includes a laryngeal signal representative of activities of the laryngeal muscle including evoked muscular responses each evoked by one of the neurostimulation pulses, and the evoked responses include the evoked muscular responses. At  2008 , the laryngeal signal is sensed. At  2010 , one of the evoked muscular responses for each pulse of the neurostimulation pulses delivered is detected. At  2012 , the stimulation intensity is adjusted in response to specified one or more of the evoked muscular responses not detected (i.e. non-capture) for a specified number of the neurostimulation pulses delivered. In one embodiment, the stimulation intensity is adjusted in response to an evoked muscular response not being detected for one of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to the evoked muscular response not being detected for a specified first number of the neurostimulation pulses delivered out of a specified second number of the neurostimulation pulses delivered. In another embodiment, the stimulation intensity is adjusted in response to an evoked muscular response not being detected for a rolling average number of the neurostimulation pulses delivered. 
     In one embodiment, each of the automatic threshold adjustment (Auto-Sense), automatic stimulation intensity adjustment (Auto-Threshold), and automatic capture verification (Auto-Capture) is disabled or delayed if noise in the sensed physiological signal exceeds a specified threshold noise level, due to the patient&#39;s activities and speeches for example. In one embodiment, each of the automatic threshold adjustment (Auto-Sense), automatic stimulation intensity adjustment (Auto-Threshold), and automatic capture verification (Auto-Capture) is performed with various parameters such as the detection thresholds adjusted for the patient&#39;s posture and activity level. 
     It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. Other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.