# Patent

## Title
Hybrid autothreshold

## Abstract

An apparatus comprises a control circuit that initiates a normal pacing mode for delivery of electrostimulation energy to the heart chamber. In response to an indication to initiate a threshold test, the control circuit determines an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode, selects a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, wherein a cardiac activity signal is sensed using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes, and selects a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available for the heart chamber, wherein the cardiac activity signal is sensed using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.

## Background
CLAIM OF PRIORITY
This application claims the benefit of priority under 35 U.S.C. §119(e) of McCabe et al., U.S. Provisional Patent Application Ser. No. 61/564,684, filed on Nov. 29, 2011, the benefit of priority of which is claimed hereby, and is incorporated by reference herein in its entirety.


BACKGROUND
Medical devices include devices designed to be implanted into a patient. Some examples of these implantable medical devices (IMDs) include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of IMDs include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
Some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs can detect abnormally slow heart rate, or bradycardia. In response to an abnormally slow heart rate some CFM devices deliver electrical pacing stimulation energy to induce cardiac depolarization and contraction (sometimes called capture of the heart). The stimulation energy is delivered to provide a depolarization rate that improves hemodynamic function of the patient. It is desirable to optimize the pacing stimulation energy delivered when pacing the heart to ensure therapy delivery and yet avoid stressing the heart unnecessarily and compromising battery life.
Overview
This document relates generally to systems, devices, and methods that provide electrical pacing therapy to the heart of a patient or subject. In particular it relates to, systems, devices, and methods to automatically determine a pacing capture threshold of the heart of a patient or subject.
An apparatus example includes a therapy circuit configured to provide cardiac electrostimulation energy to a heart chamber of a subject using a set of implantable pacing electrodes, a cardiac signal sensing circuit configured to sense a cardiac activity signal, and a control circuit communicatively coupled to the cardiac signal sensing circuit and the therapy circuit. The control circuit is configured to initiate a normal pacing mode for delivery of electrostimulation energy to the heart chamber and receive an indication to initiate a threshold test to determine the optimum electrostimulation energy for capture of the heart chamber. In response to receiving the indication, the control circuit is configured to determine an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode, select a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, and select a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available for the heart chamber. In the first threshold test mode, the cardiac activity signal is sensed using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes, and in the second threshold test mode the cardiac activity signal is sensed using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.
This section is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application.



BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, the various examples discussed in the present document.
FIG. 1 is an illustration of portions of a system that uses an IMD.
FIG. 2 is an illustration of portions of another system that uses an IMD
FIG. 3 is a flow diagram of an example of a method of operating a medical device to perform an automatic capture threshold test.
FIGS. 4A and 4B show a graph of an example of performing a threshold test for the LV
FIG. 5 shows a block diagram of portions of an example of a medical device for performing a pacing threshold test.
FIG. 6 shows a flow diagram of another example of a method of operating a medical device to perform an automatic capture threshold test.



DETAILED DESCRIPTION
An implantable medical device (IMD) may include one or more of the features, structures, methods, or combinations thereof described herein. For example, a cardiac monitor or a cardiac stimulator may be implemented to include one or more of the advantageous features or processes described below. It is intended that such a monitor, stimulator, or other implantable or partially implantable device need not include all of the features described herein, but may be implemented to include selected features that provide for unique structures or functionality. Such a device may be implemented to provide a variety of therapeutic or diagnostic functions.
As explained above, pacing stimulation energy should be optimized for a patient. If the pacing energy is too high, the stimulation may cause stress on the heart and the battery life of an implanted device will be needlessly short. If the pacing stimulation energy is too low, the pacing energy will not evoke a response in the heart (i.e., will not induce capture of the heart, which is a cardiac depolarization that results in contraction). Tests can be run by IMDs to determine an optimum energy threshold for pacing therapy. The optimum threshold is the minimum level of stimulus energy that will induce capture in excitable cardiac tissue. In other words, the tests automatically try to find the minimum electrical stimulation required to consistently cause a cardiac depolarization. The optimum threshold may vary over time for a patient due to maturation of myocardial tissue around an implanted electrode, drug therapy prescribed to the patient, an episode of myocardial infarction, and defibrillation of the myocardial tissue. Therefore, the tests are run more than once by a device while the device is implanted.
FIG. 1 is an illustration of portions of a system that uses an IMD 110. Examples of IMD 110 include, without limitation, a pacer, a defibrillator, a cardiac resynchronization therapy (CRT) device, or a combination of such devices. The system 100 also typically includes an IMD programmer or other external device 170 that communicates wireless signals 190 with the IMD 110, such as by using radio frequency (RF) or other telemetry signals. The external device 170 may communicate with a remote system via a network, such as a computer network or cellular phone network. In some examples, the remote system provides patient management functions and may include one or more servers to perform the functions.
The IMD 110 is shown coupled by one or more leads 108A-C to heart 105. Cardiac leads 108A-C include a proximal end that is coupled to IMD 110 and a distal end, coupled by electrical contacts or “electrodes” to one or more portions of a heart 105. The electrodes typically deliver cardioversion, defibrillation, pacing, or resynchronization therapy, or combinations thereof to at least one chamber of the heart 105. The electrodes may be electrically coupled to sense amplifiers to sense electrical cardiac signals.
Heart 105 includes a right atrium 100A, a left atrium 100B, a right ventricle 105A, a left ventricle 105B, and a coronary sinus 120 extending from right atrium 100A. Right atrial (RA) lead 108A includes electrodes (electrical contacts, such as ring electrode 125 and tip electrode 130) disposed in an atrium 100A of heart 105 for sensing signals, or delivering pacing therapy, or both, to the atrium 100A.
Right ventricular (RV) lead 108B includes one or more electrodes, such as tip electrode 135 and ring electrode 140, for sensing signals, delivering pacing therapy, or both sensing signals and delivering pacing therapy. Lead 108B optionally also includes additional electrodes, such as for delivering atrial cardioversion, atrial defibrillation, ventricular cardioversion, ventricular defibrillation, or combinations thereof to heart 105. Such electrodes typically have larger surface areas than pacing electrodes in order to handle the larger energies involved in defibrillation. Lead 108B optionally provides resynchronization therapy to the heart 105. Resynchronization therapy is typically delivered to the ventricles in order to better synchronize the timing of depolarizations between ventricles.
The IMD 110 may include a third cardiac lead 108C attached to the IMD 110 through the header 155. The third cardiac lead 108C includes electrodes 160 and 165 placed in a coronary vein lying epicardially on the left ventricle (LV) 105B via the coronary vein. The third cardiac lead 108C may include a ring electrode 185 positioned near the coronary sinus (CS) 120.
Lead 108B may include a first defibrillation coil electrode 175 located proximal to tip and ring electrodes 135, 140 for placement in a right ventricle, and a second defibrillation coil electrode 180 located proximal to the first defibrillation coil 175, tip electrode 135, and ring electrode 140 for placement in the superior vena cava (SVC). In some examples, high-energy shock therapy is delivered from the first or RV coil 175 to the second or SVC coil 180. In some examples, the SVC coil 180 is electrically tied to an electrode formed on the hermetically-sealed IMD housing or can 150. This improves defibrillation by delivering current from the RV coil 175 more uniformly over the ventricular myocardium. In some examples, the therapy is delivered from the RV coil 175 only to the electrode formed on the IMD can 150. In some examples, the coil electrodes 175, 180 are used in combination with other electrodes for sensing signals.
Note that although a specific arrangement of leads and electrodes are shown the illustration, the present methods and systems will work in a variety of configurations and with a variety of electrodes. Other forms of electrodes include meshes and patches which may be applied to portions of heart 105 or which may be implanted in other areas of the body to help “steer” electrical currents produced by IMD 110. An IMD may be configured with a variety of electrode arrangements, including transvenous, endocardial, and epicardial electrodes (i.e., intrathoracic electrodes), and/or subcutaneous, non-intrathoracic electrodes, including can, header, and indifferent electrodes, and subcutaneous array or lead electrodes (i.e., non-intrathoracic electrodes).
FIG. 2 is an illustration of portions of another system 200 that uses an IMD 210 to provide a therapy to a patient 202. The system 200 typically includes an external device 270 that communicates with a remote system 296 via a network 294. The network 294 can be a communication network such as a phone network or a computer network (e.g., the internet). In some examples, the external device includes a repeater and communicated via the network using a link 292 that may be wired or wireless. In some examples, the remote system 296 provides patient management functions and may include one or more servers 298 to perform the functions.
Medical device based tests can be performed to automatically determine an optimum pacing threshold for the patient. This pacing threshold is ideally the minimum electrostimulation energy required to evoke a response (e.g., cause a cardiac depolarization) a chamber of the heart. This can be referred to as capturing the heart chamber. To determine appropriate pacing electrostimulation energy, the device delivers a sequence of electrostimulation pulses to the heart. The sequence may include a successive reduction of the energy of the electrostimulation pulses. A first electrostimulation pulse that will likely induce capture is delivered. The energy of subsequent electrostimulation pulses is reduced in steps until the device verifies that failure to induce capture has occurred. Alternatively, the sequence may include increasing the energy of the electrostimulation pulses. A first electrostimulation pulse that is below a threshold likely to induce capture is delivered. The energy of subsequent electrostimulation pulses is increased in steps until the device verifies that capture was induced.
The device uses information obtained from the threshold test or autothreshold test to recommend a pacing output setting, automatically adjust a pacing output setting, or merely store a pacing output setting for later access. The threshold test can be executed when a prompt is received by the device to begin a test or the device can be programmed to recurrently run the test according to a programmed schedule (e.g., daily, weekly, etc.). An approach for an automatic capture threshold test (or autothreshold test) can be found in U.S. Patent Pub. No. US 2008/0071319, by Sathaye et al., filed Sep. 14, 2006, and entitled “Capture Detection with Cross Chamber Backup Pacing,” which is incorporated herein by reference in its entirety.
The threshold test can be run on any heart chamber. Typically, two or more electrodes are used to provide the electrostimulation pulse, and two or more electrodes separate from the pacing electrodes are used to sense the resulting depolarization to detect capture of the heart chamber. For the example of FIG. 1, there are six pacing vectors available for the LV: i) LV Tip electrode to Can electrode (LVTip-Can), ii) LV Ring electrode to Can electrode (LVRing-Can), iii) LV Tip electrode to Right Ventricular electrode (LVTip-RV), iv) LV Ring electrode to Right Ventricular electrode (LVRing-RV), v) LVTip-LVRing, and vi) LVRing-LVTip.
The first four of the six listed pacing vectors include a sensing vector option available for the LV. For example, if the pacing vector is LVTip-Can, LVRing-Can is available as a sensing vector. In another example, if the pacing vector is LVRing-RV, LVTip-RV is available as a sensing vector. Note however, for the last two vectors that a sensing electrode independent of the pacing vector is not available. In these two cases, the electrodes of the bipolar lead at the LV are both dedicated to bipolar pacing, and a sensing electrode is unavailable for the LV chamber. It can be seen that the same would be true if the LV lead was a unipolar lead and unipolar pacing was used in the LV. No independent sensing electrode is available to detect and verify capture of the LV for a unipolar LV lead configuration. Thus, there is a need for a different approach to executing a threshold test when an independent sensing electrode is unavailable.
FIG. 3 is a flow diagram of an example of a method 300 of operating a medical device to perform an automatic capture threshold test. At block 305, cardiac electrostimulation energy is delivered to a heart chamber of a subject according to a normal pacing mode using a set of implantable pacing electrodes. The normal pacing mode can be used to treat bradycardia and the heart chamber can be any of the left atrium (LA), RA, RV, or LV.
At block 310, a device threshold test is initiated to determine the optimum electrostimulation energy for capture of the heart chamber. A prompt to begin the test may be received by the medical device from a second separate device, such as a programmer for the medical device or a remote server. The prompt may be generated by a user or automatically generated according to a schedule. In certain examples, the test is initiated by the medical device itself according to a specified (e.g., programmed) schedule.
At block 315, an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode is determined using the medical device. The medical device may make the determination by reading a specified configuration stored in the device, or may perform a discovery process to determine the configuration at the time of implant or at the time of the test. The test mode to perform the test is then selected based on the determined electrode configuration.
At block 320, a first threshold test mode is initiated when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber. The first threshold test mode includes sensing a cardiac activity signal from a subject using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes. For example, if a bipolar electrode pair is dedicated for pacing the LV in the normal pacing mode, one of the electrodes (LVTip or LVRing) can be included in a set of sensing electrodes that includes a RV electrode or the can electrode (e.g., LVRing-Can, or LVRing-RV).
At block 325, a second threshold test mode is initiated when a sensing channel independent from the set of pacing electrodes is available. The second threshold test mode includes sensing the cardiac activity signal from a subject using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes. For example, if pacing of the LV in the normal pacing mode includes pacing using LVRing-RV, the set of sensing electrodes can include LVTip-Can.
FIG. 4A shows a graph of an example of performing a threshold test for the LV when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber. The electrode configuration for the LV includes a lead having a bipolar electrode pair, and the bipolar electrode pair is dedicated to pacing the LV. To sense cardiac depolarizations to perform the threshold test, the cathode is shared between the set of pacing electrodes and the set of sensing electrodes (LVTip-Can) used in the test. The threshold test starts with a pacing voltage of 7.5 Volts (V) and decreases the pacing amplitude over fifteen test steps. Loss of capture of the LV occurs when the amplitude decreases to 1.3 V and can be detected based on the timing of the peaks of the depolarization. Based on the results of the test, the pacing amplitude can be set to the amplitude of the step immediately previous to the 1.3V step, or an amplitude safety margin can be added to the 1.3V step or to the amplitude of the step immediately previous to the 1.3V step. The graph shows the distinction between capture 405 and loss of capture 410. Note that the there is an initial peak variation for capture over the 5-7 beats that start the test. This can be addressed by adding a stabilization phase to the beginning of the threshold test before decreasing the pacing amplitude.
FIG. 4B shows a graph of another example of performing a threshold test for the LV when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber. The electrode configuration for the LV includes a lead configured for unipolar pacing of the LV (e.g., LVTip-RV). To sense cardiac depolarizations to perform the threshold test, the cathode is again shared between the set of pacing electrodes and the set of sensing electrodes (e.g., LVTip-Can) used in the test. The threshold test again starts with a pacing voltage of 7.5 Volts (V) and decreases the pacing amplitude over fifteen test steps. Loss of capture of the LV in this test example occurs when the amplitude decreases to 0.8 V. The distinction between capture 405 and loss of capture 410 is again evident in the graph.
FIG. 5 shows a block diagram of portions of an example of a medical device for performing a pacing threshold test. The device 500 includes a therapy circuit 505, a cardiac signal sensing circuit 510, and a control circuit 515. The therapy circuit 505 provides cardiac electrostimulation energy to a heart chamber of a subject using a set of implantable pacing electrodes, and the cardiac signal sensing circuit 510 senses a cardiac activity signal using a set of sensing electrodes.
The control circuit 515 can be a processor, a digital signal processor (DSP), application specific integrated circuit (ASIC), microprocessor, or other type of processor, interpreting or executing instructions in software modules or firmware modules. The control circuit 515 can include other circuits or sub-circuits to perform the functions described. These circuits may include software, hardware, firmware or any combination thereof. Multiple functions can be performed in one or more of the circuits as desired. The control circuit 515 is communicatively coupled to the therapy circuit 505 and the cardiac signal sensing circuit 510. The communicative coupling allows signals to be communicated among the control circuit 515, the therapy circuit 505, and the cardiac signal sensing circuit 510 even though there may intervening circuitry between the control circuit 515, the therapy circuit 505, and the cardiac signal sensing circuit 510.
The control circuit 515 initiates delivery of electrostimulation energy to the heart chamber according to a normal pacing mode, such as to treat bradycardia of the subject for example. The control circuit 515 also delivers electrostimulation pulses to the heart while in a threshold test mode. When in a threshold test mode, the control circuit 515 recurrently changes the electrostimulation energy delivered to the heart chamber using the set of pacing electrodes of the pacing vector. Both the first and second threshold tests determine the optimum pacing energy for evoked response. The control circuit 515 can include a capture detection circuit 520 configured to detect cardiac capture. The control circuit 515, as part of the first and second threshold test modes, delivers electrostimulation energy using a first energy level, and changes the electrostimulation energy level by at least one of: a) increasing the electrostimulation energy from the first energy level until detecting that the electrostimulation energy induces cardiac capture, or b) reducing the electrostimulation energy from the first energy level until detecting that the stimulation energy fails to induce cardiac capture. The control circuit 515 continues the changing of the stimulation energy level until confirming the inducement of stable capture or the failure to induce capture.
The control circuit 515 may then derive an electrostimulation energy value for the normal pacing mode using a determined minimum electrostimulation energy that induces stable capture. In some examples, this derived electrostimulation energy value may be the minimum amplitude that induced capture, or the minimum electrostimulation amplitude or electrostimulation energy plus a safety margin. Once the electrostimulation energy is determined the control circuit 515 may store the derived value in the memory circuit. In certain examples, the control circuit 515 sets the electrostimulation energy of the normal pacing mode using the derived pacing mode.
The control circuit 515 can receive an indication to initiate a threshold test to determine the optimum electrostimulation energy for capture of the heart chamber. The indication may be communicated from a second separate device and a communication circuit of the device 500 may receive the indication wirelessly from the second device.
In response to the indication, the control circuit 515 determines an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode and selects a threshold test mode based on the determined electrode configuration. In some examples, the control circuit 515 determines the electrode configuration by reading a memory circuit 525 communicatively coupled to or integral to the control circuit 515. The electrode configuration can be specified by writing the memory circuit at the time of implant of the device. In some examples, the device 500 includes an electrode detection circuit 530 communicatively coupled to the control circuit 515. In certain examples, the electrode detection circuit 530 can determine the electrode configuration by measuring impedance at device connectors, such as at lead connectors included in a header of the device 500. A very large measured impedance value would indicate that an electrode or lead is not present and a lower impedance value would indicate that an electrode or lead is present.
When the electrode configuration is determined, the control circuit 515 may select the test mode by referencing a table stored in the memory circuit 525, which may be indexed by electrode configuration. The control circuit 515 selects a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, and selects a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available for the heart chamber. In the first threshold test mode, the cardiac activity signal is sensed using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes, and in the second test mode, the cardiac activity signal is sensed using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.
In some examples, the control circuit 515 selects the first threshold test mode when determining that the electrode configuration for the heart chamber includes a unipolar lead. For instance, the electrode configuration may include a unipolar lead to pace the LV using an electrode of the unipolar lead and an electrode formed on the device can. The control circuit 515 selects the first threshold test mode and the set of sensing electrodes may include the electrode of the unipolar lead and an electrode in the RV.
In some examples, the control circuit 515 may determine that the electrode configuration for the heart chamber includes a lead having a bipolar electrode pair. The control circuit 515 selects the first threshold test mode when determining that the set of pacing electrodes includes both electrodes of the bipolar electrode pair, and selects the second threshold test mode otherwise. In some examples, the control circuit 515 may determine that the electrode configuration for the heart chamber includes a lead having at least three electrodes. The control circuit 515 selects the second threshold test mode when a sensing electrode independent of the set of pacing electrodes is available for the heart chamber, and selects the first threshold test mode when a sensing electrode independent of the pacing channel is unavailable for the heart chamber.
Executing a threshold a test when there is an electrode common to both the pacing vector and the sensing vector (e.g., the first threshold test mode) may make it difficult to sense the cardiac depolarization resulting from electrostimulation of the test. Typically, a blanking period is initiated for the cardiac signal sensing circuit 510 in association with delivery of an electrostimulation pulse. In some examples, sense amplifiers 540 included in the cardiac signal sensing circuits are “blanked” during the duration of the pulse by being electrically disconnected or otherwise electrically isolated from the sensing electrodes. After a specified period of time the sense amplifiers 540 are electrically reconnected to the electrodes. The blanking period prevents the sense amplifiers 540 from being swamped by the electrostimulation energy from the pace pulse.
The blanking period of the normal pacing mode can include a recharge time duration. The set of pacing electrodes are electrically coupled together during recharge. This helps dissipate any residual charge at the electrode-tissue interface after an electrostimulation pulse is delivered. To perform recharge, the device 500 may include a switching circuit 535 communicatively coupled to the control circuit 515 and one or more of the set of sensing electrodes and the set of pacing electrodes. The switching circuit 535 changes electrical coupling of at least one of the set of pacing electrodes and the set of sensing electrodes and can electrically couple a set of electrodes together.
When a threshold test is desired and there is an independent sensing electrode available (e.g., the second threshold test mode), there is no timing issue and the threshold test can be run with the normal pacing blanking period. The same recharge time can be used for the threshold test and the normal pacing mode. This is because the independent electrode will not have residual charge at the electrode-tissue interface. However, as shown by the timing in the examples of capture in FIGS. 4A, 4B, the capture of heart chamber may be missed while performing recharge when an independent sensing electrode is not available and an electrode used for pacing is also used for sensing. Capture may be missed because the pacing and sensing electrode is not electrically connected to the tissue interface at the time of capture.
Shortening the recharge time can resolve this signal sensing issue. The pace artifact appears sooner and allows for less time to dissipate the residual charge by the device 500. However, the recharge time can be shortened to a point where the difference in dissipation is not that significant and the pacing artifact occurs sooner; allowing the capture signal to be sensed. Thus, the control circuit 515 may initiate a first recharge time duration after delivery of an electrostimulation pulse during the normal pacing mode and the second threshold test mode, and initiate a second recharge time duration after delivery of an electrostimulation pulse that is shorter than the first recharge time duration during the first threshold test mode (when the set of sensing electrodes shares an electrode with the set of pacing electrodes). In certain examples, the recharge time is reduced to 5 milliseconds (ms) for the second recharge time duration; although the recharge time used may depend on the type of lead coupled to the device 500.
In addition to blanking the sense amplifiers 540, one or more sense amplifiers 540 may be clamped at the end of blanking period and the recharge time duration. Clamping attempts to zero-out any DC offset on the electrode-tissue interface of the sensing electrodes. The cardiac signal sensing circuit 510 can include a sense amplifier circuit 540 that is biased with a DC reference voltage (e.g., biased using a reference voltage circuit). As part of the blanking of the sense amplifiers, the control circuit 515 may apply the offset voltage present at the tissue-electrode interface as the DC reference voltage of the sense amplifier circuit 540 at the end of the recharge time duration.
However, if the electrostimulation pulse includes a negative pace artifact, clamping the DC reference voltage to the voltage at the electrode-tissue interface may result in the remaining part of the pace artifact appearing to be a positive voltage deflection associated with cardiac activation. If a threshold test is being executed with an electrode common to the set of pacing electrodes, the pacing artifact could be interpreted as the capture instead of correctly being identified as part of the pacing pulse energy. Thus, when the electrostimulation energy is delivered as part of the normal pacing or the second threshold test (where a sensing electrode independent of the set of pacing electrodes is available), the blanking period includes the first recharge time duration and clamping is performed at the end of the first recharge time duration. When the electrostimulation energy is delivered as part of the first threshold test (in which a sensing electrode independent of the set of pacing electrodes is not available), the blanking period includes the second recharge time and the clamping is not performed (e.g., disabled) at the end of the second recharge time.
In some examples, the cardiac signal sensing circuit 510 includes a filter circuit 545 that performs filtering on the sensed cardiac activity signal. This filtering may produce signal artifacts on the cardiac activity signal that are undesired during a threshold test where an electrode is shared between the pacing vector and the sensing vector. Thus, the control circuit 515 may configure the filter circuit 545 to filter the sensed cardiac activity signal using a first set of filter parameters during the normal pacing mode, and filter the sensed cardiac signal using a second set of filter parameters during a threshold test mode. The second set of filter parameters results in the filter circuit passing more of the original sensed cardiac activity signal than when using the first set of filtering parameters. For example, the second set of parameters may include less frequency poles and thus pass more signal components of different frequencies than the first set of filtering parameters.
According to some examples, the cardiac signal sensing circuit 510 includes a passive or analog input filter for filtering a sensed cardiac activity signal. In certain examples, the analog filter includes a bandpass filter having a high pass frequency pole (e.g., 0.50 Hertz (Hz)) and a low pass frequency pole (e.g., 100 Hz). The device filtering can also include a digital filter stage, such as implemented by a DSP, which can be configured (such as a bandpass filter) to filter signals resulting from defibrillation or cardioversion shock therapy. In certain examples, the poles of the digital filter are different from the poles of the analog filter (e.g., a high pass frequency of 3.45 Hz and a low pass frequency of 244 Hz).
Filtering can add artifacts a sensed signal. The high pass pole of the bandpass filter tries to eliminate the DC component of a sensed cardiac activity signal. If there is a broad peak in the sensed signal that starts to look like a DC signal, the filter will attenuate it. However, it may be desired to preserve as much of the unfiltered signal as possible to determine the evoked response in the signal. Obtaining a more direct and unaltered version of the evoked response signal by turning off the filtering can be useful in the first threshold test mode. In certain examples, the response of the analog filter may not be changeable, but the digital filter can be turned off to obtain a more direct and unaltered version of the evoked response signal.
When the digital filtering is left “on” there is a potential for the Evoked Response signal to have an artificial peak when using a shared electrode. This can happen due to an “overshoot” as the actual signal returns to baseline from a large signal artifact. This overshoot may not be an issue if there is moderate capture signal morphology, but it may be an issue at Loss of Capture (LOC) where there should be no significant capture signal morphology (i.e., the signal just returns to the baseline from the artifact). The overshoot may look like capture or a fusion beats, which is undesirable if the electrostimulation pace pulse is actually related to a LOC.
Different features of an autothreshold test may use different filtering settings. The settings can be one or more of low pass, high pass, and band pass. If there is more than one stage to the digital filtering, these can be used in combination. Additionally, the frequency poles or “corner frequencies” are selectable for each of the stages. Overall, with digital filtering turned on (and in the correct setting), the desired peaks may be more predominant and the less desired peaks may be less dominant, but when there is a sensing electrode in common with a pacing electrode the DSP may create a peak that may not really be there (e.g., the overshoot in the signal). Turning the digital filtering “off” during the first threshold test means that no digital filtering is used during a threshold test, but a more direct and unaltered version of the evoked response signal is available. Because different filtering options are available, different filtering stages may be turned off when performing a threshold test where there is independent pacing and sensing electrodes and when there is a common electrode between pacing and sensing.
FIG. 6 shows a flow diagram of an example of a method 600 of operating the medical device of FIG. 5 to perform an automatic capture threshold test. At block 605, the control circuit 515 identifies a lead type used to pace the heart chamber under test. The lead type may be read from memory by the device or determined by the device using an electrode detection circuit 530. If the control circuit 515 determines at block 610 that the lead is unipolar, the control circuit 515 initiates a threshold test mode at block 615 where cardiac activity is sensed using an electrode (e.g., the cathode) that is shared between the set of pacing electrodes and the set of sensing electrodes. The clamping of sensing circuits after an electrostimulation pulse and at least a portion of the filtering of sensed signals are turned off during the threshold test mode. In some examples, the recharge time is shortened.
If the control circuit determines at block 620 that the lead is a bipolar lead and determines at block 625 that the electrodes of the bipolar lead are dedicated to pacing of the heart chamber, the control circuit 515 initiates the threshold test mode at block 615 where cardiac activity is sensed using an electrode that is shared with the pacing electrodes. If the control circuit 515 determines at block 630 that the electrodes of the bipolar lead are not dedicated to pacing of the heart chamber, the control circuit 515 initiates a threshold test mode at block 635 where independent pacing and sensing (IPS) electrodes are available for the threshold test. The clamping remains enabled and the same or a different portion of the filtering stage is turned off. In some examples, the same recharge time is used for the IPS threshold test mode as for the normal pacing mode for the heart chamber.
According to some examples, other criteria can be used instead of, or in addition to, electrode configuration in selecting the threshold test mode. In some examples, the capture detection circuit 520 of FIG. 5 monitors a parameter related to cardiac capture using a sensed cardiac activity signal. The monitored parameter can include one or more of the time interval following an electrostimulation pulse that cardiac capture is detected, the morphology of the depolarization resulting from an electrostimulation pulse, the amplitude of the pulse that results in cardiac capture, and the width of the pulse that results in cardiac capture.
The control circuit 515 of FIG. 5 initiates one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, and changes the threshold test mode when the monitored parameter indicates that capture of the heart chamber is unstable during the initiated threshold test mode. For instance, the control circuit 515 may initiate the second threshold test mode (e.g., IPS test mode) based on the determined electrode configuration. However, the variability of the time interval between the electrostimulation pulse and detected capture may exceed a specified variability threshold value; indicating instability. The control circuit 515 may change to the first threshold test mode based on the instability even though a sensing electrode independent of the set of pacing electrodes is available.
In another example, the capture detection circuit 520 identifies a number of fusion beats in the detected cardiac capture. A fusion beat is a heartbeat that includes features of both an intrinsic depolarization and a paced depolarization, and may indicate there are both intrinsic and paced beats in the heart chamber. If the number of fusion beats exceeds a specified threshold number of fusion beats, this may indicate instability of cardiac capture and the control circuit 515 may change the threshold test mode.
In some examples, the device 500 includes an impedance measurement circuit communicatively coupled to the control circuit 515. The impedance measurement circuit may be included in the electrode detection circuit 530. A non-stimulating current is provided to the cardiac tissue and the resulting voltage is measured using the sensing electrodes. The measured voltage is divided by the non-stimulating current to determine the impedance. The control circuit 515 may initiate one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, and change the threshold test mode when the sensing electrodes used for the threshold test mode fails to provide a stable impedance measurement. Lack of stable impedance may indicate that the sensing electrodes are not electrically independent of the set of pacing electrodes.
In some examples, the capture detection circuit 520 may detect anodal capture. Electrostimulation can be either cathodal or anodal, or a combination of anodal and cathodal. In cathodal stimulation, a more negative electrode induces capture of the heart chamber. In anodal stimulation, a more positive electrode induces the capture. Various factors can influence whether anodal or cathodal stimulation occurs. For example, relative electrode size of an anode vs. a cathode can determine whether anodal, cathodal, or a combination of anodal and cathodal stimulation occurs. An electrode having a smaller surface area can have a greater current density through nearby tissue than a larger electrode. Greater current density can lower the threshold amount of energy needed to evoke a resulting cardiac depolarization and accompanying heart contraction. Anodal stimulation can be detected by the absence of a delay between a bipolar stimulation pulse and an evoked response sensed at an electrode functioning as the anode during stimulation. The control circuit 515 may initiate the second threshold test mode according to the determined electrode configuration and change to the first threshold test mode when the capture detection circuit 520 indicates anodal capture.
In some examples, the control circuit 515 selects the first threshold test mode according to a pulse width specified for the heart chamber capture. The control circuit 515 may select the test mode by referencing a table stored in memory circuit 525, which can be referenced by the pulse width. For example, the control circuit 515 may select the shared cathode threshold test mode regardless of the electrode configuration when the threshold test is to be run using a pulse width less than N milliseconds (ms), such as less than 1 ms for example.
The devices and methods described herein are often discussed in terms of the LV, but the methods can also be applied to the other heart chambers and to other pacing configurations. If it is desired to perform an auto threshold test for a heart chamber for which a sensing vector is present that is independent of the pacing vector for the heart chamber, sensing for the test can be performed without a significant change to the sensing parameters used for normal pacing. If an independent sensing vector is not present, sensing for the test can still be performed using any combination of the several methods described. Which combination is best may depend on the electrode configuration and the parameters of the sensing circuits.
ADDITIONAL NOTES AND EXAMPLES
Example 1 can include subject matter (such as an ambulatory medical device) comprising a therapy circuit configured to provide cardiac electrostimulation energy to a heart chamber of a subject using a set of implantable pacing electrodes, a cardiac signal sensing circuit configured to sense a cardiac activity signal, and a control circuit communicatively coupled to the cardiac signal sensing circuit and the therapy circuit. The control circuit is configured to initiate a normal pacing mode for delivery of electrostimulation energy to the heart chamber, and receive an indication to initiate a threshold test to determine the optimum electrostimulation energy for capture of the heart chamber. In response to the indication, the control circuit is configured to determine an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode, select a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, wherein the cardiac activity signal is sensed using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes, and select a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available for the heart chamber, wherein the cardiac activity signal is sensed using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.
In Example 2, the subject matter of Example 1 optionally includes a control circuit configured to initiate a first recharge time duration after delivery of an electrostimulation pulse during the normal pacing mode and the second threshold test mode, with the set of pacing electrodes are electrically coupled together during recharge, and initiate a second recharge time duration after delivery of an electrostimulation pulse that is shorter than the first recharge time duration during the first threshold test mode.
In Example 3, the subject matter of one or any combination of Examples 1 and 2 optionally includes a control circuit configured to select the first threshold test mode when determining the electrode configuration for the heart chamber includes a unipolar lead.
In Example 4, the subject matter of one or any combination of Examples 1-3 optionally includes a control circuit configured to, when determining that the electrode configuration for the heart chamber includes a lead having a bipolar electrode pair, select the first threshold test mode when determining that the set of pacing electrodes includes both electrodes of the bipolar electrode pair, and select the second threshold test mode otherwise.
In Example 5, the subject matter of one or any combination of Examples 1-4 optionally includes a control circuit configured to, when determining that the electrode configuration for the heart chamber includes a cardiac lead with at least three electrodes, select the second threshold test mode when a sensing electrode independent of the set of pacing electrodes is available for the heart chamber, and select the first threshold test mode when a sensing electrode independent of the set of pacing electrodes is unavailable for the heart chamber.
In Example 6, the subject matter of one or any combination of Examples 1-5 optionally includes a control circuit having a capture detection circuit configured to monitor a parameter related to cardiac capture using a sensed cardiac activity signal. The control circuit is configured to initiate one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, and change the threshold test mode when the monitored parameter indicates that capture of the heart chamber is unstable during the initiated threshold test mode.
In Example 7, the subject matter of one or any combination of Examples 1-6 optionally includes an impedance measurement circuit communicatively coupled to the control circuit. The control circuit can optionally be configured to initiate one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, and change the threshold test mode when a set of sensing electrodes used for the threshold test mode fails to provide a stable impedance measurement.
In Example 8, the subject matter of one or any combination of Examples 1-7 optionally includes a control circuit including a capture detection circuit communicatively coupled to the cardiac signal sensing circuit. The control circuit can optionally be configured to initiate the second threshold test mode according to the determined electrode configuration, and change to the first threshold test mode when the capture detection circuit indicates anodal capture.
In Example 9, the subject matter of one or any combination of Examples 1-8 optionally includes a control circuit configured to select the first threshold test mode according to a pulse width specified for the heart chamber capture.
In Example 10, the subject matter of one or any combination of Examples 1-9 optionally includes a cardiac signal sensing circuit that includes a sense amplifier circuit that is biased with a DC reference voltage. The control circuit is optionally configured to apply an offset voltage present at a tissue-electrode interface as the DC reference voltage at the end of the first recharge time duration, and to disable applying the offset voltage as the DC reference voltage at the end of the second recharge time duration.
In Example 11, the subject matter of one or any combination of Examples 1-10 optionally includes a cardiac signal sensing circuit that includes a filter circuit configured to filter the sensed cardiac activity signal. The filter circuit is configured to filter the sensed cardiac activity signal using a first set of filter parameters during the normal pacing mode, and filter the sensed cardiac signal using a second set of filter parameters during the first threshold test mode. The second set of filter parameters results in the filter circuit passing more of the original sensed cardiac activity signal than when the first set of filtering parameters is used.
Example 12 can include subject matter (such as a method, a means for performing acts, or a machine-readable medium including instructions that, when performed by the machine, cause the machine to perform acts), or can optionally be combined with the subject matter of one or any combination of Examples 1-11 to include such subject matter, comprising delivering cardiac electrostimulation energy to a heart chamber of a subject according to a normal pacing mode using a set of implantable pacing electrodes, initiating a device threshold test to determine an optimum electrostimulation energy for capture of the heart chamber, determining, with the medical device, an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode, initiating a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, and initiating a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available. The first threshold test mode includes sensing a cardiac activity signal from a subject using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes, and the second threshold test mode includes sensing the cardiac activity signal from a subject using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.
In Example 13, the subject matter of Example 12 can include initiating a first recharge time duration after delivery of an electrostimulation pulse, wherein the set of pacing electrodes are electrically coupled together during recharge, and initiating a second recharge time duration after delivery of an electrostimulation pulse that is shorter than the first recharge time duration.
In Example 14, the subject matter of one or any combination of Examples 12 and 13 optionally includes initiating a first threshold test mode that includes selecting the first threshold test mode when determining the electrode configuration for the heart chamber includes a unipolar lead.
In Example 15, the subject matter of one or any combination of Examples 12-14 optionally includes determining that the electrode configuration includes a lead having a bipolar electrode pair, initiating a first threshold test mode that includes selecting the first threshold test mode when determining that the set of pacing electrodes includes both electrodes of the bipolar electrode pair, and initiating a second threshold test mode that includes selecting the second threshold test mode when determining that the set of pacing electrodes includes less than both electrodes of the bipolar electrode pair.
In Example 16, the subject matter of one or any combination of Examples 12-15 optionally includes determining that the electrode configuration includes a cardiac lead with at least three electrodes, initiating a second threshold test mode that includes selecting the second threshold test mode when a sensing electrode independent of the set of pacing electrodes is available for the heart chamber, and initiating a first threshold test mode that includes selecting the first threshold test mode when a sensing electrode independent of the set of pacing electrodes is unavailable for the heart chamber.
In Example 17, the subject matter of one or any combination of Examples 12-16 optionally includes initiating one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, monitoring a parameter related to capture of the heart chamber using a cardiac activity signal sensed during the initiated test mode, and changing the threshold test mode when the monitored parameter indicates that capture of the heart chamber is unstable during the initiated threshold test mode.
In Example 18, the subject matter of one or any combination of Examples 12-17 optionally includes initiating one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration, and changing the threshold test mode when the set of sensing electrodes used for the threshold test mode fails to provide a stable impedance measurement.
In Example 19, the subject matter of one or any combination of Examples 12-18 optionally includes initiating the second threshold test mode according to the determined electrode configuration, and changing to the first threshold test mode when a cardiac activity signal sensed during the second threshold test mode indicates anodal capture.
In Example 20, the subject matter of one or any combination of Examples 12-19 optionally includes selecting the first threshold test mode according to a pulse width specified for the heart chamber capture.
Example 21 can include, or can optionally be combined with any portion or combination of any portions of any one or more of Examples 1-20 to include, subject matter that can include means for performing any one or more of the functions of Examples 1-20, or a machine-readable medium including instructions that, when performed by a machine, cause the machine to perform any one or more of the functions of Examples 1-20.
These non-limiting examples can be combined in any permutation of combination.
The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.
In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code can form portions of computer program products. Further, the code can be tangibly stored on one or more volatile or non-volatile computer-readable media during execution or at other times. These computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAM's), read only memories (ROM's), and the like.
The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

## Claims

1. An apparatus comprising:
a therapy circuit configured to provide cardiac electrostimulation energy to a heart chamber of a subject using a set of implantable pacing electrodes;
a cardiac signal sensing circuit configured to sense a cardiac activity signal; and
a control circuit communicatively coupled to the cardiac signal sensing circuit and the therapy circuit, wherein the control circuit is configured to:
initiate a normal pacing mode for delivery of electrostimulation energy to the heart chamber; and
receive an indication to initiate a threshold test to determine an optimum electrostimulation energy for capture of the heart chamber, and in response to the indication, the control circuit is configured to:
determine an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode;
select a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, wherein the cardiac activity signal is sensed using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes; and
select a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available for the heart chamber, wherein the cardiac activity signal is sensed using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.

2. The apparatus of claim 1, wherein the control circuit is configured to:
initiate a first recharge time duration after delivery of an electrostimulation pulse during the normal pacing mode and the second threshold test mode, wherein the set of pacing electrodes are electrically coupled together during recharge; and
initiate a second recharge time duration after delivery of an electrostimulation pulse that is shorter than the first recharge time duration during the first threshold test mode.

3. The apparatus of claim 1, wherein the control circuit is configured to select the first threshold test mode when determining the electrode configuration for the heart chamber includes a unipolar lead.

4. The apparatus of claim 1, wherein, when determining that the electrode configuration for the heart chamber includes a lead having a bipolar electrode pair, the control circuit is configured to:
select the first threshold test mode when determining that the set of pacing electrodes includes both electrodes of the bipolar electrode pair, and
select the second threshold test mode otherwise.

5. The apparatus of claim 1, wherein, when determining that the electrode configuration for the heart chamber includes a cardiac lead with at least three electrodes, the control circuit is configured to:
select the second threshold test mode when a sensing electrode independent of the set of pacing electrodes is available for the heart chamber, and
select the first threshold test mode when a sensing electrode independent of the set of pacing electrodes is unavailable for the heart chamber.

6. The apparatus of claim 1,
wherein the control circuit includes a capture detection circuit configured to monitor a parameter related to cardiac capture using a sensed cardiac activity signal, and
wherein the control circuit is configured to:
initiate one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration; and
change the threshold test mode when the monitored parameter indicates that capture of the heart chamber is unstable during the initiated threshold test mode.

7. The apparatus of claim 1, including an impedance measurement circuit communicatively coupled to the control circuit, wherein the control circuit is configured to:
initiate one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration; and
change the threshold test mode when a set of sensing electrodes used for the threshold test mode fails to provide a stable impedance measurement.

8. The apparatus of claim 1, wherein the control circuit includes a capture detection circuit communicatively coupled to the cardiac signal sensing circuit and wherein the control circuit is configured to:
initiate the second threshold test mode according to the determined electrode configuration; and
change to the first threshold test mode when the capture detection circuit indicates anodal capture.

9. The apparatus of claim 1, wherein the control circuit is configured to select the first threshold test mode according to a pulse width specified for the heart chamber capture.

10. The apparatus of claim 1,
wherein the cardiac signal sensing circuit includes a sense amplifier circuit that is biased with a DC reference voltage,
wherein the control circuit is configured to apply an offset voltage present at a tissue-electrode interface as the DC reference voltage at the end of the first recharge time duration, and
wherein the control circuit is configured to disable applying the offset voltage as the DC reference voltage at the end of the second recharge time duration.

11. The apparatus of claim 1,
wherein the cardiac signal sensing circuit includes a filter circuit configured to filter the sensed cardiac activity signal,
wherein the filter circuit is configured to:
filter the sensed cardiac activity signal using a first set of filter parameters during the normal pacing mode; and
filter the sensed cardiac signal using a second set of filter parameters during the first threshold test mode, wherein the second set of filter parameters results in the filter circuit passing more of the original sensed cardiac activity signal than when using the first set of filtering parameters.

12. A method of operating a medical device comprising:
delivering cardiac electrostimulation energy to a heart chamber of a subject according to a normal pacing mode using a set of implantable pacing electrodes;
initiating a device threshold test to determine an optimum electrostimulation energy for capture of the heart chamber;
determining, with the medical device, an electrode configuration used to deliver the electrostimulation energy in the normal pacing mode;
initiating a first threshold test mode when a sensing electrode independent from the set of pacing electrodes is unavailable for the heart chamber, wherein the first threshold test mode includes sensing a cardiac activity signal from a subject using a set of sensing electrodes that includes an electrode common to the set of pacing electrodes; and
initiating a second threshold test mode when a sensing electrode independent from the set of pacing electrodes is available, wherein the second threshold test mode includes sensing the cardiac activity signal from a subject using a set of sensing electrodes that excludes an electrode common to the set of pacing electrodes.

13. The method of claim 12,
wherein the normal pacing mode and the second threshold test mode include initiating a first recharge time duration after delivery of an electrostimulation pulse, wherein the set of pacing electrodes are electrically coupled together during recharge, and
wherein the first threshold test mode includes initiating a second recharge time duration after delivery of an electrostimulation pulse that is shorter than the first recharge time duration.

14. The method of claim 12,
wherein initiating a first threshold test mode includes selecting the first threshold test mode when determining the electrode configuration for the heart chamber includes a unipolar lead.

15. The method of claim 12,
wherein determining an electrode configuration includes determining that the electrode configuration includes a lead having a bipolar electrode pair,
wherein initiating a first threshold test mode includes selecting the first threshold test mode when determining that the set of pacing electrodes includes both electrodes of the bipolar electrode pair, and
wherein initiating a second threshold test mode includes selecting the second threshold test mode when determining that the set of pacing electrodes includes less than both electrodes of the bipolar electrode pair.

16. The method of claim 12,
wherein determining an electrode configuration includes determining that the electrode configuration includes a cardiac lead with at least three electrodes,
wherein initiating a second threshold test mode includes selecting the second threshold test mode when a sensing electrode independent of the set of pacing electrodes is available for the heart chamber, and
wherein initiating a first threshold test mode includes selecting the first threshold test mode when a sensing electrode independent of the set of pacing electrodes is unavailable for the heart chamber.

17. The method of claim 12, including:
initiating one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration;
monitoring a parameter related to capture of the heart chamber using a cardiac activity signal sensed during the initiated test mode; and
changing the threshold test mode when the monitored parameter indicates that capture of the heart chamber is unstable during the initiated threshold test mode.

18. The method of claim 12, including:
initiating one of the first threshold test mode and the second threshold test mode according to the determined electrode configuration; and
changing the threshold test mode when the set of sensing electrodes used for the threshold test mode fails to provide a stable impedance measurement.

19. The method of claim 12, including:
initiating the second threshold test mode according to the determined electrode configuration; and
changing to the first threshold test mode when a cardiac activity signal sensed during the second threshold test mode indicates anodal capture.

20. The method of claim 12, including selecting the first threshold test mode according to a pulse width specified for the heart chamber capture.