Patent Publication Number: US-7711425-B2

Title: Defibrillation threshold prediction methods and systems

Description:
TECHNICAL FIELD 
   This patent document pertains generally to defibrillation threshold prediction, and more particularly, but not by way of limitation, to cardiac function management methods and systems. 
   BACKGROUND 
   When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body&#39;s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart. 
   Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electric stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. 
   Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electric stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart is not allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electric stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias. 
   One problem faced by cardiac rhythm management systems is the determination of the threshold energy required, for a particular defibrillation shock waveform, to reliably convert a tachyarrhythmia into a normal heart rhythm. Ventricular and atrial fibrillation are probabilistic phenomena that observe a dose-response relationship with respect to shock strength. The ventricular defibrillation threshold is the smallest amount of energy that can be delivered to the heart to reliably revert ventricular fibrillation to a normal rhythm. Similarly, the atrial defibrillation threshold is the threshold amount of energy that will terminate an atrial fibrillation. Such defibrillation thresholds vary from patient to patient, and may even vary within a patient depending on the placement of the electrodes used to deliver the therapy. In order to ensure the efficacy of such therapy and to maximize the longevity of the battery source of such therapy energy, the defibrillation thresholds must be determined so that the defibrillation energy can be safely set above the threshold value but not at so large of a value so as to waste energy and shorten the usable life of the implanted device. 
   One technique for determining the defibrillation threshold is to induce the targeted tachyarrhythmia (e.g., ventricular fibrillation), and then apply shocks of varying magnitude to determine the energy needed to convert the arrhythmia into a normal heart rhythm. However, this requires imposing the risks and discomfort associated with both the arrhythmia and the therapy. Electric energy delivered to the heart has the potential to both cause myocardial injury and subject the patient to pain. Moreover, if defibrillation thresholds are being obtained in order to assist the physician in determining optimal lead placement, these disadvantages are compounded as the procedure is repeated for different potential lead placements. 
   In another technique for determining the defibrillation threshold, referred to as the “upper limit of vulnerability” technique, a patient in a state of normal heart rhythm is shocked during the vulnerable (T-wave) period of the cardiac cycle during which time the heart tissue is undergoing repolarization. Shocks of varying magnitude are applied until fibrillation is induced. Of course, after such fibrillation is induced, the patient must be again shocked in order to interrupt the arrhythmia and reestablish a normal heart rhythm. In this technique, the corresponding fibrillation-inducing shock magnitude is then related to a defibrillation threshold energy using a theoretical model. The upper limit of vulnerability technique also suffers from imposing the risks and discomfort associated with both the arrhythmia and the shock therapy. Moreover, because of the discomfort associated with the fibrillation and countershocks, the patient is typically sedated under general anesthesia, which itself has some additional risk and increased health care cost. For these and other reasons, there is a need to estimate defibrillation thresholds without relying on a defibrillation shock to induce or terminate an actual arrhythmia. 
   SUMMARY 
   An example method includes delivering a first nondefibrillating and nonfibrillation-inducing energy at a first internal thoracic location and detecting a first resulting electric signal at a second internal thoracic location in or near a target region of a heart. The first resulting electric signal provides an indication of a first electric field strength in the target region. The method further includes estimating a defibrillation threshold using the first nondefibrillating and nonfibrillation-inducing energy, the first resulting electric signal, and a target electric field strength at the target region of the heart. 
   Another example method includes delivering a first nondefibrillating and nonfibrillation-inducing energy at a first internal thoracic location and detecting a first resulting electric signal at a second internal thoracic location in or near a target region of a heart. The first resulting electric signal provides an indication of a first electric field strength in the target region. The method further includes estimating a first defibrillation threshold using the first nondefibrillating and nonfibrillation-inducing energy, the first resulting electric signal, and a target electric field strength. The method also includes delivering a second nondefibrillating and nonfibrillation-inducing energy at the first internal thoracic location, detecting a second resulting electric signal at the second internal thoracic location, and determining a change in a defibrillation threshold using the first resulting electric signal and the second resulting electric signal. 
   Another example method includes delivering a first nondefibrillating and nonfibrillation-inducing energy to a thorax using a first defibrillation configuration, and detecting a first resulting electric signal at an internal thoracic location in or near a target region of a heart. The first resulting electric signal provides an indication of a first electric field strength in the target region. The method also includes delivering a second nondefibrillating and nonfibrillation-inducing energy to the thorax using a second defibrillation configuration and detecting a second resulting electric signal at the internal thoracic location. The second resulting electric signal provides an indication of a second electric field strength in the target region of the heart during delivery of the nondefibrillating and nonfibrillation-inducing energy using the second defibrillation configuration. The method further includes estimating at least one defibrillation threshold using at least one target electric field strength and at least one of the first and second resulting electric signals. In an example, delivering a first nondefibrillating and nonfibrillation-inducing energy to a thorax using a first defibrillation configuration includes delivering the energy through a plurality of electrodes, and delivering a second nondefibrillating and nonfibrillation-inducing energy to the thorax using a second defibrillation configuration includes changing a configuration of the plurality of electrodes and then delivering the energy through the plurality of electrodes. In an example, changing a configuration of the plurality of electrodes includes changing a location of at least one electrode or electrically connecting at least one electrode to at least one other electrode. In another example, delivering a first nondefibrillating and nonfibrillation-inducing energy includes delivering the first nondefibrillating and nonfibrillation-inducing energy using a first electrode configuration, delivering a second nondefibrillating and nonfibrillation-inducing energy includes delivering the second nondefibrillating and nonfibrillation-inducing energy using a second electrode configuration, and estimating at least one defibrillation threshold selecting an electrode configuration that produces a smaller defibrillation threshold for delivery of an antitachyarrhythmia therapy therefrom. 
   An example system includes an energy module adapted to deliver a nondefibrillating and nonfibrillation-inducing energy using at least a first electrode at a first internal thoracic location, a response signal module adapted to detect a resulting signal using at least a second electrode at a second internal thoracic location in or near a target region of a heart, the responsive signal resulting from the delivery of the energy and providing an indication of an electric field strength at the second internal thoracic location, and a controller communicatively coupled to the energy module and the response signal module, the controller adapted to estimate a defibrillation threshold using the nondefibrillating and nonfibrillation-inducing energy and the resulting signal. In an example, the controller is adapted to compare a detected resulting signal with a previous detected resulting signal to detect a change in the resulting signal indicative of a defibrillation threshold change, or to compare an estimated defibrillation threshold to a previously estimated defibrillation threshold to detect a change in the defibrillation threshold. In an example, the controller is adapted to deliver a notification if an changed defibrillation threshold is detected or to increase an energy level of an antitachyarrhythmia therapy if an increased defibrillation threshold is detected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic/block diagram illustrating portions of an example cardiac rhythm management system and portions of an environment of use. 
       FIG. 2A  is an illustration of example configuration of electrodes and a heart. 
       FIG. 2B  is an illustration of an example system including electrodes in and/or on a heart and a medical device including at least one electrode on the device. 
       FIG. 2C  is an illustration of the electrodes of  FIG. 2A , with some of the electrodes in a different position. 
       FIG. 2D  is an illustration of a lead having four electrodes. 
       FIG. 3  is a flow chart illustrating an example method that includes delivering an energy, detecting a resulting electric signal in or near a target region of a heart, and estimating a defibrillation threshold. 
       FIG. 4  is a flow chart illustrating an example method that includes delivering first and second energies, detecting respective first and second resulting electric signals, and determining a change in a defibrillation threshold. 
       FIG. 5  is a flow chart illustrating an example method that includes delivering first and second energies and detecting respective first and second resulting electric signals at different locations. 
       FIG. 6  is a flow chart illustrating an example method that includes delivering energy using different electrode configurations. 
       FIG. 7  is a schematic illustration of an electric field generated by defibrillation electrodes and a projection of an electric field vector on a vector defined by sensing electrodes. 
   

   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 electric changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. The term “and/or” refers to a nonexclusive “or” (i.e., “A and/or B” includes both “A and B” as well as “A or B”). 
   The present methods and apparatus will be described in applications involving implantable medical devices including, but not limited to, implantable cardiac function management systems such as pacemakers, cardioverter/defibrillators, pacer/defibrillators, biventricular or other multi-site coordination or resynchronization devices, and drug delivery systems for managing cardiac rhythm. However, it is understood that the present methods and apparatus may be employed in unimplanted devices, including, but not limited to, external pacemakers, cardioverter/defibrillators, pacer/defibrillators, biventricular or other multi-site coordination or resynchronization devices, monitors, programmers and recorders. 
   Overview 
   An estimated defibrillation threshold energy is determined based upon a desired electric field strength in a target region of the heart. In an example, an energy is delivered at a first internal thoracic location, such as a location in, on, or near the heart. The delivered energy is preferably, but not necessarily, a nondefibrillating and nonfibrillation-inducing energy. A resulting electric signal is detected at a second internal thoracic location in or near the target region of the heart. In an example, the energy is delivered to the right side of the heart, and the target region is in, on, or near the left apical region of the heart and/or a left free lateral wall. The resulting electric signal is used to estimate the defibrillation threshold energy required to create the desired electric field strength in the target region. In an example, the estimated defibrillation threshold energy is determined using a ratio of the desired (or “target”) electric field strength (e.g. 5 volts/cm) to the electric field strength produced by the delivered energy. In an example, a ventricular and/or atrial defibrillation threshold is determined. In an example, actual measurement of a resulting electric field in or near the target region avoids reliance on fluoroscopic distance measurement and electric field modeling computations. 
   In an example, defibrillation thresholds are determined for multiple shock electrode configurations, allowing for selection of a low-threshold configuration that can be used to conserve energy and prolong battery life. 
   An increase in defibrillation threshold may be indicative of hypertrophy, ventricle dilation, ischemia or myocardial infarction, scar tissue, or lead dislodgement. In an example, when an increased defibrillation threshold is detected, a defibrillation therapy energy is increased, or a notification such as a warning is delivered. 
   The present techniques typically reduce risks to a patient by permitting defibrillation threshold determination without inducing fibrillation or delivering a fibrillation-inducing shock energy. For example, using the present techniques, a very sick patient for whom a doctor hesitates to do a standard defibrillating test to determine a defibrillation threshold could be fitted with a defibrillator while avoiding delivery of a defibrillating shock to test the patient&#39;s defibrillation threshold. In another example, patients are screened using the present techniques to identify patients suitable for defibrillation. 
   EXAMPLE SYSTEMS 
     FIG. 1  is a schematic/block diagram illustrating generally, by way of example, and not by way of limitation, one embodiment of portions of a cardiac rhythm management system  100  and portions of an environment in which the present system  100  and associated techniques are used. In this example, the system  100  includes, among other things, cardiac rhythm management device  105  and leadwires (“leads”)  110 ,  180  for communicating signals between device  105  and a portion of a living organism, such as heart  115 . Embodiments of device  105  include, but are not limited to, bradycardia and antitachycardia pacemakers, cardioverters, defibrillators, combination pacemaker/defibrillators, drug delivery devices, and any other implantable or external cardiac rhythm management apparatus capable of diagnosing or providing therapy to heart  115 . 
     FIG. 1  illustrates, in an exploded view block diagram form, portions of an example device  105 . In an example, the illustrated device  105  is coupled to leads  110  and  180  that extend toward the heart; however, the illustrated connection lines associated with the exploded view are illustrative only. In  FIG. 1 , test energy module  150  generates an energy that is delivered by electrodes, such as two or more of electrodes  125 ,  130 , and/or  140 , at an internal thoracic location, such as a location in, on, or around the heart. In an example, electrodes  125 ,  130 , and  140  all act as separate electrodes. Alternatively, two or more electrodes are electrically commonly connected. In an example, SVC electrode  130  is electrically connected in common with housing electrode  140 . In an example, the test energy module  150  is adapted to deliver the energy at a specified time, such as at the end of systole when the intrinsic electric activity in the heart is relatively quiet. 
   A defibrillation threshold energy can be determined via response signal module  155 , which is connected for example to electrodes  185 ,  190  on lead  180 . In an example, electrodes  185 ,  190  are located at a second internal thoracic location, which is optionally in, or near, a target region of a heart. Electrodes  185 ,  190  detect a responsive electrical signal resulting from the delivery of the energy. The responsive electrical signal provides an indication of an electric field strength at the second internal thoracic location. In an example, the test energy module  150  is adapted to deliver a current and the response signal module  155  is adapted to detect a responsive voltage or a responsive current density. From the responsive detected signal(s), a defibrillation threshold is computed, for example, by a defibrillation threshold estimation module in controller  160 . Controller  160  is in device  105 , or in external programmer  170 , which is communicatively coupled to a transmitter or receiver in device  105 , such as transceiver  175 . The defibrillation threshold estimation module is typically implemented as a sequence of acts carried out on a microprocessor or other microsequencer, in analog, digital, or mixed-signal hardware, or in any suitable hardware and/or software configuration. 
   In an example, the controller  160  estimates a defibrillation threshold energy that when delivered to the first internal thoracic location creates an electric field strength in the target region of the heart that meets or exceeds a target electric field strength. In a further example, the controller  160  compares an estimated defibrillation threshold to a previously estimated defibrillation threshold to detect a change in the defibrillation threshold, if any. In an example, the controller  160  is adapted to increase an energy level of an antitachyarrhythmia therapy if an increased defibrillation threshold is detected, and/or deliver a notification (e.g., a warning) if an increased defibrillation threshold is detected. In an example, a notification is delivered if the defibrillation threshold increases by a specified amount in a specified amount of time. In an illustrative example, if the defibrillation threshold increases by at least 20% over 3 months, a notification is delivered. In another example, a notification is delivered if the defibrillation threshold increases by a specified amount, without regard to the amount of time over which the increase occurs. In an example, the controller is configured to switch to a (different) second electrode configuration, estimate a second defibrillation threshold for at least a second electrode configuration, compare estimated defibrillation thresholds for at least two electrode configurations, and optionally select a particular electrode configuration using the estimated defibrillation thresholds, so that the selected electrode configuration can be used to then deliver antiarrhythmia therapy. 
   In the example shown in  FIG. 1 , lead  110  includes multiple electrodes, and typically includes individual conductors for independently communicating an electrical signal from each electrode to device  105 . In one embodiment, these electrodes include a right ventricular (RV) tip-type electrode  120  at the distal end of lead  110 . In one example, electrode  120  has a macroscopic surface area that is approximately between 1 mm 2  and 20 mm 2 , inclusive. RV tip electrode  120  is sized and shaped to be positioned in the right ventricle, such as at or near its apex or at any other suitable location. RV shock electrode  125  is typically located on the lead at a known or estimated distance, d 1 , from RV tip electrode  120 , as measured from the edges of these electrodes. RV shock electrode  125  is typically located in the right ventricle or at any other suitable location. In one embodiment, RV shock electrode  125  is a coil-type electrode having a macroscopic surface area that is approximately between 2 cm 2  and 20 cm 2 , inclusive. Superior vena cava (SVC) electrode  130  is typically located in a portion of the superior vena cava, the right atrium, or both, or at any other suitable location. In one embodiment, SVC electrode  130  is a coil-type electrode having a macroscopic surface area that is approximately between 2 cm 2  and 20 cm 2 , inclusive. Although RV tip electrode  120 , RV shock electrode  125 , and SVC electrode  130  are particularly described above with respect to structural characteristics and locations for disposition, it is understood that these electrodes may take the form of any of the various cardiac or like electrodes known in the art (e.g., epicardial patch electrodes) and may be positioned elsewhere for association with heart  115  or other tissue. 
   The system  100  also typically includes second and third electrodes  185 ,  190  located in, on, or near the left side of the heart. In an example, the second electrode  185  is located at a portion of the heart that is expected to experience a relatively lesser effect of the defibrillation energy, such as at or close to the peripheral portion, apical region, and/or free lateral wall of the left ventricle, at which a target electric field magnitude (e.g., 5 volts/cm) is desired during defibrillation. In one embodiment, this electrode  185  is introduced into the left ventricular periphery (e.g., coronary sinus and/or great cardiac vein or the like) by a transvascular lead  180  through the right atrium and coronary sinus. 
   In an example, the lead  180  and/or electrodes  185 ,  190  are sized and shaped for implantation in the great cardiac vein or elsewhere in or on the left region of the heart, such as in or on a left apical region or left free lateral wall. In another embodiment, electrode  185  is a patch-type defibrillation electrode disposed on the exterior portion of the left ventricle. The lead  180  may also include additional electrodes. 
   Referring again to  FIG. 1 , in one embodiment, device  105  includes a hermetically sealed housing  135 , formed from a conductive metal, such as titanium, and implanted within a patient such as within the pectoral or abdominal regions. In one example, housing  135  (also referred to as a “case” or “can”) forms a “case” or “can” or “housing” electrode  140 . As understood by one of ordinary skill in the art, housing electrode  140 , although not located in the heart, is associated with the heart for providing what is sometimes referred to as “unipolar” sensing or pacing or defibrillation therapy. In one embodiment, a header  145  is mounted on housing  135 , such as for receiving lead  110 . Header  145  is formed of an insulative material, such as molded plastic. Header  145  also includes at least one receptacle, such as for receiving lead  110  and electrically coupling conductors of lead  110  to device  105 . Header  145  may also include one or more additional electrodes. 
   In an example, fibrillation is treated by delivering a shock between RV shock electrode  125  and SVC electrode  130 . In another example, ventricular fibrillation is treated by delivering a defibrillation shock between RV shock electrode  125  and the commonly connected combination of SVC electrode  130  and housing electrode  140 . It is understood that these electrodes could be differently configured, and that more or fewer electrodes could be used. 
   EXAMPLE METHODS AND SYSTEM CONFIGURATIONS 
   In various examples, a system of electrodes and at least one processor, such as the system shown in  FIG. 1 , is used to deliver a nondefibrillating and non-fibrillation inducing energy, detect a responsive electric signal, and determine a defibrillation threshold therefrom. 
   Referring now to  FIG. 2A , in an example, electrodes  185  and  190  are positioned a known (or estimated) distance (d) apart on lead  180  inserted into a vessel, such as the great cardiac vein. In an example, electrode  185  is a ring electrode and electrode  190  is a tip electrode. When a first voltage V 1  is delivered, for example using RV shock electrode  125  and SVC electrode  130 , a second voltage V 2  is detectable using electrodes  185  and  190 . The voltage V 2  provides an indication of the strength of the electric field in a target region around electrodes  185  and  190 . A field strength (potential gradient) can be determined from the voltage V 2  and the distance d between the electrodes  185 ,  190 . Dividing V 2  by the distance d provides a field strength, which is the projection of the potential gradient space vector on the vector defined by the electrodes. A defibrillation threshold energy is estimated using the voltages V 1  and V 2 . The estimated defibrillation threshold energy typically yields a somewhat conservative estimate (i.e., high), because the potential gradient V 2 /d is usually only a fraction of the actual potential gradient space vector. For example, in  FIG. 7 , when an energy is delivered using defibrillation electrodes  705 ,  710 , the voltage V 2  across sensing electrodes  715 ,  720  (which are space a distance d apart) is the projection of the total voltage gradient V T  on the vector defined by the sensing electrodes. 
   In an example, the estimated defibrillation threshold energy is selected to create an electric field strength in the target region of the heart that meets or exceeds a target electric field strength. In an example, the target electric field strength is 5 volts/cm. In another example, the target electric field strength is around 1-2 volts/cm. In an example, the target electric field strength is selected using a target voltage gradient that includes a safety margin. 
   In an example, a defibrillation threshold voltage is determined by multiplying V 1  by a ratio of a target potential gradient (e.g. 5 volts/cm) and a measured voltage gradient (ΔV L ): 
   
     
       
         
           
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   A defibrillation threshold energy EDFT can be determined from the defibrillation threshold voltage and a defibrillation energy storage capacitance (C) associated with the device that delivers the therapy: 
   
     
       
         
           
             E 
             DFT 
           
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   It is understood that the defibrillation threshold energy can be determined or expressed in terms of energy units, such as Joules, or alternatively in terms of the charged voltage of a capacitor, from which an energy therapy is delivered. 
   In an example, the target region is at or near a location where a relatively low electric field is expected for a given configuration of the electrodes that deliver the energy. For example, where the energy is delivered at the right side of the heart as shown in  FIG. 1 , the left apical region  187  of the heart or the left free lateral wall  188  is expected to experience a relatively low electric field, and in some instances the lowest electric field in the heart. Thus, it can be expected that an energy that generates a sufficient electric field in the target region to achieve defibrillation (e.g. 5 volts/cm) will generate an equal or greater electric field in most or all of the other locations in the heart. 
   Referring now to  FIGS. 2A and 2B , in an example, a nondefibrillating and nonfibrillation-inducing energy is delivered and/or sensed using two different electric configurations of the electrodes. In an example, the different electrode configurations provide different vectors for delivering such energy or sensing a responsive electric signal. For example,  FIG. 2B  shows an electric configuration different from the configuration shown in  FIG. 2A . In  FIG. 2B , electrode  130  is electrically connected to the electrode  140  on the housing  135 . In an example, a first nondefibrillating and nonfibrillation-inducing energy is delivered using the electrode configuration shown in  FIG. 2A , where electrode  135  is not electrically connected to electrode  140 , and a first resulting electric signal is detected. A second nondefibrillating and nonfibriulation-inducing energy is delivered using the electrode configuration shown in  FIG. 2B , and a second resulting electric signal is detected. The first and second resulting electric signals are each evaluated to determine a corresponding defibrillation threshold energy. In an example, an electrode configuration having a lower defibrillation threshold energy is selected. 
   Referring now to  FIGS. 2A and 2C , in an example, a defibrillation threshold is determined using two or more resulting electrical signals corresponding to two or more sensor electrode configurations.  FIG. 2C  shows the same components as  FIG. 2A  but in a different configuration. Lead  180  and electrodes  185  and  190  are positioned further from the apex  191  of the heart in  FIG. 2C  than in  FIG. 2A . In an example, a nondefibrillating and nonfibrillation-inducing energy is delivered with the electrodes in the configuration shown in  FIG. 2C , and then the lead  180  is inserted further into a blood vessel on the heart until the electrodes are positioned in the configuration shown in  FIG. 2A , at which time another nondefibrillating and nonfibrillation-inducing energy is delivered. Resulting electrical signals are detected using both lead configurations. A defibrillation threshold energy is estimated using one or both of the corresponding resulting electrical signals. In an example, more than two configurations of lead  180  are used. In another example, resulting electrical signals are detected occasionally, periodically, or continuously as lead  180  is inserted into a vessel or into the heart. Electrical signals at different locations can be used, for example, to assess electric fields at different locations of the left ventricle free wall or the apical region and find the minimum electric field in the target region. In another example, the resulting electric field in different directions is used to estimate an electric field strength projected in the strongest direction. In an example, lead  110  is maintained in a fixed position as lead  180  is moved. 
   Referring now to  FIGS. 2A and 2C , in an example, a defibrillation threshold is determined using two or more resulting electrical signals corresponding to two or more sensor electrode configurations.  FIG. 2C  shows the same components as  FIG. 2A  but in a different configuration: lead  180  and electrodes  185  and  190  are positioned further from the apex  191  of the heart in  FIG. 2C  than in  FIG. 2A . In an example, a nondefibrillating and nonfibrillation-inducing energy is delivered with the electrodes in the configuration shown in  FIG. 2C , and then the lead  180  is inserted further into a blood vessel on the heart until the electrodes are positioned in the configuration shown in  FIG. 2A , at which time another nondefibrillating and nonfibrillation-inducing energy is delivered. Resulting electrical signals are detected using both lead configurations. A defibrillation threshold energy is estimated using one or both of the corresponding resulting electrical signals. In an example, more than two configurations of lead  180  are used. In another example, resulting electrical signals are detected occasionally, periodically, or continuously as lead  180  is inserted into a vessel or into the heart. Electrical signals at different locations can be used, for example, to assess electric fields at different locations of the left ventricle free wall or the apical region and find the minimum electric field in the target region. In another example, the resulting electric field in different directions is used to estimate an electric field strength projected in the strongest direction. In an example, lead  110  is maintained in a fixed position as lead  180  is moved. 
   In another example, two or more configurations of lead  110  are evaluated using resulting electrical signals detected using electrodes on lead  180 . 
   Referring now to  FIG. 2D , in another example, a multi-polar lead is used to detect the first resulting electrical signal.  FIG. 2D  shows a lead  181  and electrodes  191 ,  192 ,  193 , and  194 . In an example, a portion of the lead including some or all of the electrodes  191 ,  192 ,  193 ,  194  has a nonlinear shape, such as a two-dimensional (2-D) or three-dimensional (3-D) shape, such as a serpentine or helix, respectively. In an example, the electrodes  191 ,  192 ,  193 ,  194  are all used to detect the first resulting electrical signal. In an example, the electrodes are located in the left apical region  187 , and/or in a left lateral free wall  188 . In an example, a two-part process is used. First, different directional potential gradients associated with the same particular region of the heart are measured. A representative potential gradient is selected for that region (e.g., an average gradient or, more typically, a maximum gradient). Second, after representative potential gradients have been determined for various regions of the heart, then that particular region having a minimum potential gradient is then used to estimate the defibrillation threshold energy. For example, a defibrillation threshold energy is selected to provide a specified electric field between electrodes at a region of the heart having a minimum detected voltage difference, on the assumption that the electrodes represent a region of the heart having a lowest electric field. 
   The electrodes used to detect a resulting electrical signal are not necessarily located on the same lead. In an example, an electrode at a distal end of lead  110  is in combination with an electrode on lead  180  to detect a resulting electrical signal. In an example, lead  110  is extended around to the left side of the heart and shock electrode  125  is located far enough from the distal end of the lead to allow for useful detection at the distal end. 
   Similarly, the electrodes used to deliver a first nondefibrillating and nonfibrillation-inducing energy are not necessarily on the same lead. For example, a defibrillation electrode could be placed on lead  180  or  181 , or on another separate lead, and used in combination with electrode  125  or  130  on lead  110  to deliver an energy. It should be noted that when the defibrillation electrode configuration is changed, the target region may also change. 
   It is also understood that the leads can extend in, on, or near the heart. In an example, lead  180  extends near the heart and the electrodes  185 ,  190  are located outside the target region of the heart. The lead  110  can also be positioned so that the electrodes  125 ,  130  are positioned in, on, or near the heart. In another example, the electrodes  125 ,  130  for delivering the nondefibrillating and nonfibrillation-inducing energy are implanted subcutaneously. In an example, far-field sensing is used to detect the resulting electrical signal. 
   It is also understood that, while the figures generally depict delivery of an energy to a right side of the heart and detection of a resulting electrical signal in, on, or near the left side of the heart, the methods and systems described herein can be used in other configurations. For example, in alternative configurations, energy is delivered to the left side of the heart, delivered by subcutaneous electrodes, or delivered to the coronary sinus, or delivered through epicardial electrodes. 
     FIG. 3  is a flowchart that schematically illustrates an example of a method. At  305 , a first energy is delivered at a first internal thoracic location. In an example, the first energy is nondefibrillating and nonfibrillation-inducing. In an example, the energy is delivered at a location in and/or near the heart. In an example, the energy is delivered at a subcutaneous location. In an example, the energy is non-stimulating. In an example, the energy is below a patient&#39;s pain threshold and/or below a patient&#39;s perception threshold. In an example, the energy is delivered during a specified portion of a cardiac cycle, such as at the end of systole. 
   At  310 , a first resulting electrical signal is detected at a second internal thoracic location in or near a target region of a heart. In an example, the second internal thoracic location is in the target region. In an example, the energy is delivered at a location in or near a right region of the heart, such as in the right ventricle or the superior vena cave, and the first resulting electrical signal is detected at a location in or near a left region of the heart, such as in or near a left apical region, and/or in or near a left free lateral wall. In an example, delivering the energy includes delivering a current and detecting the resulting signal includes detecting a voltage between two electrodes or a current density. In an example, the first resulting electrical signal is detected through a lead inserted into vasculature on or in the heart, such as vasculature on the left side of the heart. In an example, the lead is chronically implanted. In an example, the first resulting electric signal is detected through electrodes in a bipolar lead or multipolar lead, which is optionally located in the left side of the heart. In an example, the first resulting electrical signal is detected using a guide wire, for example by detecting a voltage difference between an electrode on or of the guide wire and an electrode on the lead. In an example, a distance between a distal end of the guide wire and an electrode on the lead is estimated using one or more features or markings at the proximal end of the guide wire. In an example, a position of a marking at the proximal end of the guide wire is recorded and correlated to a position of the distal end of the guide wire. When the guide is advanced, the displacement of the distal end of the guide wire is estimated using the marking at the proximal end. 
   In another example, a lead is temporarily inserted in or around the heart and used to detect resulting electrical heart signals. In an example, a temporary mapping catheter is inserted in or around the heart. In an example, the temporary mapping catheter has a multiplicity of electrodes (e.g. 16 or 24 electrodes) that detect resulting electrical signals at locations in or around the heart. In an example, each electrode is coupled to a separate conductor (e.g. the mapping lead has 16 conductors and 16 electrodes.) In another example, the lead assembly includes switching capacity (e.g. a multiplexor) so that a resulting electrical signal can be detected using a selected combination of selected electrodes. In an example, one or more of the resulting electrical signals are used to estimate a defibrillation threshold. 
   Returning to  FIG. 3 , at  315 , a defibrillation threshold energy is estimated. In an example, the defibrillation threshold includes an energy, a voltage, a current and/or a duration. The defibrillation threshold energy when delivered to the first internal thoracic location creates an electric field strength in the target region of the heart that meets or exceeds a target electric field strength. In an example, detecting the resulting signal includes detecting at least one voltage difference between (or among) electrodes, and the estimating includes determining the first electric field strength from the at least one voltage difference and a known configuration of the electrodes. In an example, the estimating further includes dividing the target defibrillating electric field strength by the first electric field strength to determine a defibrillation scaling factor; and multiplying the first nondefibrillating and nonfibrillation-inducing voltage or current by the defibrillation scaling factor to determine the defibrillation threshold voltage or current. In an example, estimating the defibrillation threshold energy includes combining fields from three or more electrodes. Optionally, at  325 , before a defibrillation threshold is estimated, at least one intrinsic electrical signal is separated from the first resulting electrical signal. In an example, a post-processing technique is used to subtract out electrical noise. In an example, a remainder of the detected resulting electrical signal is substantially attributable to the delivery of the energy at the first internal thoracic location. In an example, the remainder of the resulting electrical signal is used to estimate the defibrillation threshold. In an example, a blind source separation technique is used to separate at least one intrinsic electric signal from the detected resulting electric signal. Blind source separation techniques are described in copending U.S. patent application Ser. No. 10/876,008, filed Jun. 24, 2004, entitled “Automatic Orientation Determination for ECG Measurements Using Multiple Electrodes,” which is incorporated herein by reference in its entirety. 
   Referring again to  FIG. 3 , at  320  a notification is delivered if the estimated defibrillation threshold energy meets or exceeds a specified value or a specified change in value. In an example, a notification is received by a patient, for example by detecting an audible sound such as a beep. In another example, a notification is received by a physician, for example by using an external device that communicates with an implanted device wirelessly or otherwise. In yet another example, a notification is received by a physician via Internet or other communication over a communications network. In an example, a notification is delivered if the estimated defibrillation threshold energy meets or exceeds a prescribed defibrillation energy value by 20%. At  330 , the defibrillation threshold energy is communicated, for example to an external programmer. At  335 , a history of estimated defibrillation threshold energies is stored. Storing a history of estimated defibrillation threshold energies allows for monitoring of a physiological condition, for example. It also allows for monitoring of lead position change or dislodgement. A defibrillation threshold can change over time, for example due to myocardial infarction, cardiac remodeling, medication changes, hypertrophy, posture change, or lead dislodgement. At  340 , a defibrillation therapy is adjusted to meet or exceed the defibrillation threshold energy. 
   At  345 , a defibrillation therapy energy meeting or exceeding the defibrillation threshold is delivered. In an example, the antitachyarrhythmia therapy is delivered through an electrode that was also used to deliver the energy from which the defibrillation threshold was determined. In an example, the antitachyarrhythmia therapy includes a defibrillation shock delivered at an energy meeting or exceeding the estimated defibrillation threshold. In an example, an arrhythmia such as a tachyarrhythmia is detected or declared, a nondefibrillating and nonfibrillation-inducing energy is delivered, from which a defibrillation threshold is determined, and then an antitachyarrhythmia therapy having an energy meeting or exceeding the defibrillation threshold is delivered. 
   Turning now to the flow chart provided in  FIG. 4 , a first energy is delivered at  405  at a first internal thoracic location, a first resulting electrical signal is detected at  410 , and a first defibrillation threshold is estimated at  415 . At  420 , a second energy is delivered to the first internal thoracic location. At  425 , a second resulting electrical signal is detected. 
   At  430 , a change in defibrillation threshold is determined using the first resulting electrical signal and the second resulting electrical signal. In an example, a first electric field strength determined from the first resulting electrical signal is compared with a second electric field strength determined from the second resulting electrical signal. In an example, a second defibrillation threshold energy is estimated for the target region of the heart using the indication of a second electric field strength and the second energy, and the second estimated defibrillation threshold is compared to the first defibrillation threshold. 
   Returning to  FIG. 4 , at  445 , a notification is delivered if the second defibrillation threshold energy exceeds the first defibrillation threshold energy by a specified amount. In an example, a notification is delivered if the second defibrillation threshold exceeds the first defibrillation threshold by a specified amount over a specified amount of time. Optionally, at  435 , data from at least one other sensor is analyzed. In an example, the data from the other sensor is used to confirm a change in a defibrillation threshold or characterize a physiologic condition or change. In an example, such information is used to assess whether a notification is delivered or a defibrillation therapy should be altered. Factors such as posture, for example, can affect a defibrillation threshold, and defibrillation thresholds affected by such factors may not necessarily warrant delivery of a notification. At  440 , a notification is delivered or a defibrillation therapy is adjusted if the physiological parameter detected at  435  meets a specified criteria. 
   Referring now to  FIG. 5 , at  505  a first nondefibrillating and nonfibrillation-inducing energy is delivered at a first internal thoracic location. At  510 , a first resulting electrical signal is detected at a second internal thoracic location in or near a target region of a heart. In an example, the first resulting electrical signal is detected using one or more pacing/sensing electrodes. In another example, the first resulting electrical signal is detected using a guide wire and an intravascular lead. At  515 , a second nondefibrillating and nonfibrillation-inducing energy is detected at the first internal thoracic region. At  520 , a second resulting electrical signal is detected at a third internal thoracic location in or near the target region. In an example, the second internal thoracic location and third internal thoracic location are locations of electrodes on a lead. 
   In an example, detecting a first electrical signal includes detecting an electrical signal at a first position on a lead inserted into a coronary vein in the left side of the heart, and detecting a second resulting electrical signal includes detecting an electrical signal through the lead in a second position in a coronary vein in the left side of the heart.  FIGS. 2A and 2C , for example, show a lead at two positions in a coronary vein. In an example, detecting an electrical signal through a lead in a coronary vein in the left side of the heart includes detecting at least two voltage differences between electrodes on the lead. In an example, the lead includes a multi-polar lead, as shown in  FIG. 2D  for example. In an example, detecting an electrical signal through a lead in a coronary vein in the left side of the heart includes detecting an electrical signal through electrodes on a portion of the lead that extends has a two-dimension path or three-dimensional bias, such as a helical bias, for example. In an example, positioning electrodes on a portion of a lead that has 3-D bias promotes electric contact with tissue. In an example, positioning electrodes on a lead that follows a 3-D path allows for positioning of the electrodes at a desired contact location within a vessel, for example to avoid stimulating anatomy such as a nerve. 
   Returning to  FIG. 5 , at  525 , both the first resulting electrical signal and the second resulting electrical signal are combined and the combination is used to estimate a defibrillation threshold. In an example, an average of the first resulting electrical signal and the second resulting electrical signal is used to determine a defibrillation threshold. In an example, the average is a weighted average. In an example, multiple measurements (e.g. 10&#39;s or 100&#39;s of resulting electrical signals) are used to estimate a defibrillation threshold. In an example, a median or a specified standard deviation from a mean is used to estimate a defibrillation threshold. In an example, space vector synthesis is used to estimate a defibrillation threshold. Blind source separation may also be used to estimate a defibrillation threshold. Alternatively, at  530 , the smaller of the first and second resulting electrical signals is used to determine a defibrillation threshold. In an example, a resulting electrical signal is detected at various locations in, on, or near the heart to identify a region having a lowest value of a potential gradient. Determining a defibrillation threshold using the lowest potential gradient ensures that the location where the lowest potential gradient was observed will reach a sufficient defibrillating potential gradient when a therapeutic energy is delivered. 
   Referring now to  FIG. 6 , at  605 , a nondefibrillating and nonfibrillation-inducing energy is delivered to a thorax using a first electrode configuration. In an example, the energy is delivered in, on, or near a heart. At  610 , a first resulting electrical signal is detected at an internal thoracic location in or near a target region of a heart. At  615 , a second nondefibrillating and nonfibrillation-inducing energy is delivered to the thorax using a second electrode configuration. The second electrode is physically and/or electrically different from the first electrode configuration. In an example, the first and second energies are delivered using the same electrodes, but the electrode&#39;s position and/or orientation of the electrodes during delivery of the second energy differs from the position and/or orientation of the electrodes during delivery of the first energy. The first energy and second energy optionally have the same magnitude. At  620 , a second resulting electrical signal is detected at the internal thoracic location. At  625 , an electrode configuration that produces a smaller defibrillation threshold is selected for delivery of an antitachyarrhythmia therapy. In an example, selecting a configuration with a lower defibrillation threshold increases the longevity of a device, because less battery energy is used to deliver a therapy. In an example, three or more electrode configurations or combinations of electrodes are compared. In an example, a defibrillation threshold is estimated using at least one of the first and second resulting electrode signals. In an example, the first and second energy are the same magnitude (e.g. same charged capacitor voltage or same number of Joules), and the defibrillation threshold is estimated for the electrode configuration that yields a larger resulting signal. 
   While various operations in  FIGS. 3 ,  4 ,  5 , and  6  are shown as alternatives, it is understood that various operations can be performed in combination with other operations. 
   It is to be understood that the above description is intended to be illustrative, and not restrictive. Since many embodiments of the invention can be made without departing from the scope of the invention, the invention resides in the claims hereinafter appended.