Heart rate based control of cardiac resynchronization therapy

In some examples, controlling delivery of cardiac resynchronization therapy (CRT) includes storing, in a memory of an implantable medical device system and in association with each of a plurality of heart rates, at least one respective value for an interval between an atrial event and a ventricular event. Processing circuitry of the implantable medical device system may determine a heart rate of a patient and select one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate. The processing circuitry may further determine whether to control therapy delivery circuitry of the implantable medical device system to deliver fusion pacing or biventricular pacing, based on the selected one of the stored values for the interval between the atrial event and the ventricular event.

TECHNICAL FIELD

The disclosure relates generally to medical device systems and, more particularly, cardiac therapy delivery by implantable medical devices.

BACKGROUND

Some types of implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic electrical stimulation to a heart of a patient via electrodes of one or more implantable leads. The therapeutic electrical stimulation may be delivered to the heart in the form of pulses or shocks for pacing, cardioversion or defibrillation. In some cases, an implantable medical device may sense intrinsic depolarizations of the heart, and control the delivery of therapeutic stimulation to the heart based on the sensing.

Cardiac resynchronization therapy (CRT) is one type of therapy delivered by an implantable medical device. Cardiac resynchronization therapy may help enhance cardiac output by resynchronizing the electromechanical activity of the ventricles of the heart. Ventricular desynchrony may occur in patients that suffer from congestive heart failure (CHF).

SUMMARY

In general, this disclosure is directed to techniques for controlling the delivery of CRT. Such techniques may include determining an interval between an atrial event and a ventricular event of a patient, and determining whether to deliver fusion pacing therapy, i.e., CRT pacing delivered to one of the ventricles, such as the left ventricle, or biventricular pacing therapy to the patient based on the determined interval. In some examples, the value of an interval between the atrial event and the ventricular event, e.g., an intrinsic ventricular event, may be obtained by periodically suspending the delivery of therapy and determining the current value of the interval.

However, the act of suspending the delivery of therapy and determining a current value of the interval may result in a loss of delivery of CRT pacing to the patient. Since increased CRT pacing may be associated with improved patient outcome, it therefore may be desirable to decrease the frequency at which the current value of the interval between the atrial event and the ventricular event is determined. Accordingly, techniques described herein may include determining a heart rate of a patient and selecting from a table, based on the heart rate, a respective value for the interval between the atrial event and the ventricular event. In this manner, the interval to be used for determining whether to apply fusion pacing therapy or biventricular pacing therapy may be obtained without suspending the delivery of therapy. In techniques described herein, the suspension of the delivery of therapy to determine the current value of the interval thus may be limited to relatively infrequent occasions, such periodic updates to the table.

In one example, a method for controlling delivery of cardiac resynchronization therapy (CRT) comprises storing, in a memory of an implantable medical device system and in association with each of a plurality of heart rates, at least one respective value for an interval between an atrial event and a ventricular event; and by processing circuitry of the implantable medical device system: determining a heart rate of a patient; selecting one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate; and determining whether to control therapy delivery circuitry of the implantable medical device system to deliver fusion pacing or biventricular pacing based on the selected one of the stored values for the interval between the atrial event and the ventricular event.

In another example, a system for controlling delivery of cardiac resynchronization therapy (CRT) comprises therapy delivery circuitry configured to deliver cardiac resynchronization pacing therapy to a heart of a patient; sensing circuitry configured to sense electrical activity of the heart; memory configured to store, in association with each of a plurality of heart rates, at least one respective value for an interval between the atrial event and the ventricular event; and processing circuitry configured to: determine the heart rate of a patient based on the electrical activity sensed by the sensing circuitry; select one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate; and determine whether to control the therapy delivery circuitry to deliver fusion pacing or biventricular pacing based on the selected one of the stored values for the interval between the atrial event and the ventricular event.

In another example, an implantable medical device (IMD) configured to deliver cardiac resynchronization therapy (CRT) to a patient comprises a housing configured for implantation within the patient; therapy delivery circuitry disposed within the housing and configured to deliver cardiac resynchronization pacing therapy to a heart of the patient; sensing circuitry disposed within the housing and configured to sense electrical activity of the heart; memory disposed within the housing and configured to store, in association with each of a plurality of heart rates, at least one respective value for the interval between the atrial event and the ventricular event; and processing circuitry disposed within the housing and configured to: determine a first heart rate of a patient for at least one preceding cardiac cycle based on the sensed electrical activity; select one of the stored values for the interval between the atrial event and the ventricular event associated with the determined first heart rate; and determine whether to control the therapy delivery circuitry to deliver fusion pacing or biventricular pacing for a current cardiac cycle based on the selected one of the stored values for the interval between the atrial event and the ventricular event, wherein the processing circuitry is further configured to periodically, the period being greater than a cardiac cycle: determine a second heart rate of a patient based on the sensed electrical activity select one of the stored values for the interval between the atrial event and the ventricular event associated with the determined second heart rate; suspend the delivery of CRT; while the delivery of CRT is suspended, measure a current value of the interval between the atrial event and the ventricular event; compare the current value of the interval to the selected value of the interval; and update at least the selected value of the interval associated with the determined heart rate in the memory based on the comparison.

In another example, a system for controlling delivery of cardiac resynchronization therapy (CRT) comprises means for storing, in association with each of a plurality of heart rates, at least one respective value for an interval between an atrial event and a ventricular event; means for determining a heart rate of a patient; means for selecting one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate; and means for determining whether to control therapy delivery circuitry of the implantable medical device system to deliver fusion pacing or biventricular pacing based on the selected one of the stored values for the interval between the atrial event and the ventricular event.

In another example, a non-transitory computer-readable medium storing instructions for causing processing circuitry of an implantable medical device system to perform a method for controlling delivery of cardiac resynchronization therapy (CRT), the method comprising storing, in a memory of the implantable medical device system and in association with each of a plurality of heart rates, at least one respective value for an interval between an atrial event and a ventricular event; determining a heart rate of a patient; selecting one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate; and determining whether to control therapy delivery circuitry of the implantable medical device system to deliver fusion pacing or biventricular pacing based on the selected one of the stored values for the interval between the atrial event and the ventricular event.

This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the methods and systems described in detail within the accompanying drawings and description below. The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below.

DETAILED DESCRIPTION

In general, this disclosure describes example techniques related to controlling the delivery of cardiac resynchronization therapy (CRT) to a patient. For each of a plurality of heart rates, a memory, e.g., of an implantable medical device or an external computing device, stores one or more associated values of an interval between an atrial event and a ventricular event, e.g., an A−V interval, determined for the patient. In some examples, the ventricular event is a sensed or intrinsic ventricular event, and the interval is an A−Vsinterval. In some examples, the memory stores values for both of an interval between a sensed atrial event and the ventricular event, e.g., an As−Vsinterval, and an interval between a paced atrial event and the ventricular event, e.g., an AP−Vsinterval. The values stored for the patient in the memory may be determined by monitoring the A−V conduction of the patient prior to, or during a suspension of, delivery of CRT.

To control the delivery of CRT by the implantable medical device, processing circuitry, e.g., of the implantable medical device or the external computing device, may determine the heart rate of the patient, and select the respective value for the A−V interval from the memory. The processing circuitry may control the delivery of CRT based on the selected value of the A−V interval. For example, the processing circuitry may determine whether to control therapy delivery circuitry of the implantable medical device to deliver fusion pacing, e.g., left-ventricular pacing, or biventricular pacing to the patient based on the selected A−V interval value. In examples in which the A−V interval value selected based on the heart rate is an A−Vsinterval value, the processing circuitry may further determine, based on the selected A−Vsvalue, an A−VPinterval value for timing the delivery of the fusion or biventricular pacing.

It is further contemplated that, in some examples, the A−V interval values stored in association with heart rates in the memory may be validated and, if necessary, updated periodically. In some examples, the processing circuitry may validate and/or update the stored A−V interval values by suspending the delivery of CRT, and measuring a current value of the A−V interval while delivery of CRT is suspended. Suspending the delivery of CRT may refer to, as examples, withholding ventricular pacing for one or more cardiac cycles, increasing an A−Vpdelay sufficiently so that intrinsic ventricular conduction may be observed, or pacing one ventricle at a sufficiently long A−VPdelay and measuring intrinsic conduction on the other ventricle. The processing circuitry may compare the currently measured value of the interval to the value associated with the current heart rate. If the values are not sufficiently similar, the processing circuitry may modify the stored value based on the currently measured value, e.g., by replacing the stored A−V interval value with the currently measured value.

In some medical devices configured to provide adaptive CRT, a pacing configuration, e.g., a fusion pacing configuration or a biventricular pacing configuration, and timing of the pacing stimuli based on periodic intrinsic conduction measurements may be periodically adjusted to achieve more efficient physiologic pacing and to improve hemodynamics of the patient. Fusion pacing and biventricular pacing are described in further detail below. While the pacing stimuli may be pacing pulses or continuous time signals, the pacing stimuli are primarily referred to herein as pacing pulses for ease of description.

Fusion-based CRT (also referred to herein as fusion pacing) may be useful for restoring a depolarization sequence of a heart of a patient, which may be irregular due to ventricular dysfunction, in patients with preserved intrinsic atrial-ventricular (AV) conduction. In a fusion pacing configuration, a medical device delivers one or more fusion pacing pulses to one of the ventricles, and not the other. In particular, the medical device delivers the one or more fusion pacing pulses to a later-contracting ventricle (V2) in order to pre-excite the V2 and synchronize the depolarization of the V2 with the depolarization of the earlier contracting ventricle (V1). The ventricular activation of the V2 may “fuse” (or “merge”) with the ventricular activation of the V1 that is attributable to intrinsic conduction of the heart. In this way, the intrinsic and pacing-induced excitation wave fronts may fuse together such that the depolarization of the V2 is resynchronized with the depolarization of the V1.

The medical device may be configured to deliver the fusion pacing pulse to the V2 according to a fusion pacing interval, which indicates the time relative to an atrial pace or sense event at which the fusion pacing pulse should be delivered to the V2. An atrial sense event may be, for example, a P wave of a sensed electrical cardiac signal and an atrial pacing event may be, for example, the time at which a stimulus is delivered to the atrium.

In some examples, the right ventricle (RV) may be the V1 and the left ventricle (LV) may be the V2. In other examples, the LV may be the V1 while the RV may be the V2. While the disclosure primarily refers to examples in which the first depolarizing ventricle V1 is the RV and the later depolarizing ventricle V2 is the LV, the devices, systems, techniques described herein for providing CRT may also apply to examples in which the first depolarizing ventricle V1 is the LV and the later depolarizing ventricle V2 is the RV.

In some fusion pacing techniques, a pacing pulse to the V2 (V2P) is delivered upon expiration of a fusion pacing interval that is determined based on the intrinsic depolarization of the V1, which may be indicated by a sensing of ventricular activation (V1S). Ventricular activation may be indicated by, for example, an R-wave of a sensed electrical cardiac signal. An example of a fusion pacing technique that times the delivery of the V2 pacing pulse (V2P) to the intrinsic depolarization of the V1 is described in U.S. Pat. No. 7,181,286 to Burnes et al., which is entitled, “APPARATUS AND METHODS OF ENERGY EFFICIENT, ATRIAL-BASED BI-VENTRICULAR FUSION-PACING,” and issued on Feb. 20, 2007. U.S. Pat. No. 7,181,286 to Burnes et al. is incorporated herein by reference in its entirety.

In one example disclosed by U.S. Pat. No. 7,181,286 to Burnes et al., a pacing pulse to the V2 (V2P) is delivered a predetermined period of time following an atrial pace or sense event (AP/S), where the predetermined period of time is substantially equal to the duration of time between the atrial pace or sense event (AP/S) and a V1 sensing event (V1S) of at least one prior cardiac cycle decremented by a duration of time referred to as the pre-excitation interval (PEI). Thus, one example equation that may be used to determine a fusion pacing interval (AP/S−V2P):
AP/S−V2P=(AP/S−V1S)−PEI  Equation (1)

A cardiac cycle may include, for example, the time between the beginning of one heart beat to the next heartbeat. The duration of time between the atrial pace or sense event (AP/S) and a V1 sensing event (V1S) may be, for example, a measurement of intrinsic AV conduction time from an atrium to the first contracting ventricle of the heart of the patient. The PEI may indicate the amount of time with which a V2 pacing pulse precedes a V1 sensing event in order to achieve the fusing of the electromechanical performance of the V1 and V2. That is, the PEI may indicate the amount of time from the delivery of the V2 pacing pulse that is required to pre-excite the V2, such that the electromechanical performance of V1 and V2 merge into a fusion event. In some examples, the PEI is automatically determined by a medical device delivering the pacing therapy, e.g., based on determined intrinsic conduction times, while in other examples, the PEI may be predetermined by a clinician. In some examples, the PEI is a programmed value (e.g., about one millisecond (ms) to about 250 ms or more, such as about 100 ms to about 200 ms, or about 10 ms to about 40 ms) or is an adaptive value, such as about 10% of a measured intrinsic A−V2 conduction interval or measured intrinsic A-A cycle length.

The magnitude of the PEI may differ based on various factors, such as the heart rate of the patient, a dynamic physiologic conduction status of the heart of the patient, which may change based upon the physiological condition of the patient (e.g., ischemia status, myocardial infarction status, and so forth), as well as factors related to the therapy system, such as the location of sensing electrodes of the leads of the therapy system, the location of the pacing electrodes of the therapy system, and internal circuitry processing delays of the medical device.

Another technique for determining the timing of the delivery of a pacing pulse to a later depolarizing ventricle (V2) (which is sometimes also referred to as a “fusion pacing interval”) is described in U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al., which is incorporated herein by reference in its entirety. In some examples described by U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al., the timing of the delivery of a pacing pulse is based on a depolarization of the V2 in at least one prior cardiac cycle. The depolarization of the V2 may be detected by sensing an event in the V2 (V2S), such as an R-wave of an electrical cardiac signal. The V2 pacing pulse (V2P) is timed such that an evoked depolarization of the V2 is effected in fusion with the intrinsic depolarization of the first depolarizing ventricle (V1), resulting in a ventricular resynchronization. In this way, the V2 pacing pulse (V2P) may pre-excite the conduction delayed V2 and help fuse the activation of the V2 with the activation of the V1 from intrinsic conduction. The interval of time between the V2 pacing pulse (V2P) and the V2 sensing event (V2S) of the same cardiac cycle may be referred to as the adjusted PEI.

In some examples disclosed by U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al., the predetermined period of time at which an IMD delivers the V2 pacing pulse (V2P) following an atrial pace or sense event (AP/S) is substantially equal to the duration of time between an atrial event (sensed or paced) (AP/S) and a V2 sensing event (V2S) of at least one prior cardiac cycle decremented by a duration of time referred to as an adjusted PEI. That is, in some examples, the adjusted PEI indicates the desired interval of time between the delivery of the V2 pacing pulse (V2P) and the V2 sensing event (V2S) of the same cardiac cycle. One example equation that may be used to determine the timing of a fusion pacing pulse using a technique described by U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al. is:
AP/S−V2P=(AP/S−V2S)−adjusted PEI  Equation (2):

The duration of time between an atrial pace or sense event (AP/S) and a V2 sensing event (V2S) may be referred to as an atrioventricular (AV) interval or delay. The adjusted PEI may indicate an interval of time prior to a V2 sensing event (V2S) at which it may be desirable to deliver the V2 pacing pulse (V2P) in order to pre-excite the V2 and merge the electromechanical performance of V2 and V1 into a fusion event. In some examples described by U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al., an adjusted PEI is a linear function that is based on V1 sensing event (V1S) and a V2 sensing event (V2S) of the same cardiac cycle, based on the time between an atrial pace or sense event (AP/S) and a V2 sensing event, or any combination thereof.

As an example, adjusted PEI may be determined as follows:
Adjusted PEI=a(V1S−V2S)+bEquation (3):
According to U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al., in Equation (3), the coefficients “a” and “b” may be fixed, empiric coefficients that are selected by a clinician or determined based on an adjusted PEI value selected by a clinician. In some examples, the coefficient “a” in Equation (3) may be about 1 and the coefficient “b” may be substantially equal to the PEI. In this case, the adjusted PEI is substantially equal to a time interval between a V1 sensing event (V1S) and a V2 sensing event (V2S) of the same cardiac cycle, incremented by the PEI. As a result, the AP/S−V2Pinterval for timing the delivery of a V2 pacing pulse may be determined as follows
AP/S−V2P=(AP/S−V2S)−[(V1S−V2S)+PEI)]  Equation (4):

Other values for the “a” and “b” coefficients in Equation (2) may be selected. In addition, other types of fusion pacing configurations may also be used in accordance with the techniques described herein. For example, other fusion pacing intervals described by U.S. Pat. No. 7,181,286 to Burnes et al. and U.S. Patent Application Publication No. 2010/0198291 by Sambelashvili et al. can also be used to control fusion pacing in accordance with techniques described herein. An example of CRT is described in U.S. Pat. No. 6,871,096 to Hill, which is entitled “SYSTEM AND METHOD FOR BI-VENTRICULAR FUSION PACING” and is incorporated herein by reference in its entirety.

In contrast to fusion pacing, in a biventricular pacing configuration, the medical device may deliver pacing pulses to both the LV and the RV in order to resynchronize the contraction of the LV and RV. In a biventricular pacing configuration, a medical device may deliver stimulation to coordinate contraction of the LV and the RV, even in the absence of intrinsic AV conduction of the heart.

In some proposed adaptive CRT pacing techniques, a pacing configuration (e.g., fusion pacing or biventricular pacing) and timing of the pacing pulses (e.g., a fusion pacing interval, such as a AP/S−V2Pinterval, or biventricular pacing intervals, such as an AP/S−V1Pand AP/S−V2Pintervals, or an AP/S−V1Pand V1P−V2Pintervals) are periodically adjusted based on periodic intrinsic conduction measurements in an attempt to achieve more efficient physiologic pacing and optimal hemodynamics. For example, some proposed cardiac rhythm management medical devices are configured to deliver adaptive CRT by delivering pacing to a heart of a patient in accordance with a fusion pacing configuration and, if loss of intrinsic AV conduction is detected (e.g., AV block), switching to a biventricular pacing configuration. Thus, a medical device configured for adaptive CRT may be configured to switch from a fusion pacing configuration to a biventricular pacing configuration in response to determining a heart of a patient is no longer intrinsically conducting. Biventricular pacing may consume more energy (compared to fusion pacing) due to the delivery of pacing to both the LV and the RV, and, accordingly, delivering fusion pacing until loss of intrinsic conduction may be a more efficient use of the power stored by a power source of a medical device compared to continuously delivering biventricular pacing.

In some existing proposed techniques for delivering adaptive CRT, a medical device switches from a fusion pacing configuration to a biventricular pacing configuration if the loss of intrinsic AV conduction is detected based on a measurement of intrinsic conduction time, which may be performed as part of the fusion-pacing interval determination. For example, loss of intrinsic AV conduction may be detected if a measured A−V1 conduction time (AP/S−V1S) is greater than (or greater than or equal to in some examples) a predetermined threshold value. In some examples, the predetermined threshold value is selected based on previous intrinsic conduction time intervals (e.g., may be a percentage of a mean or median of a certain number of prior intrinsic conduction time measurements). In other examples, the predetermined threshold value may be selected by a clinician to be, for example, a value that indicates the depolarization time of V1 that results maintenance of cardiac output at a desirable level.

In order to measure intrinsic conduction time, the CRT pacing delivered by the medical device to the heart may be suspended to allow the heart of the patient to conduct in the absence of cardiac rhythm management therapy and to avoid interference between the delivery of pacing pulses and sensing of ventricular activation. In some examples, if pacing is delivered to an atrium of the heart, such pacing may be maintained, while pacing to the ventricles may be suspended. The measurement of intrinsic conduction time may be determined, e.g., as the time between an atrial pace or sense event (AP/S) and a V1 sensing event (V1S), which may be referred to generally as an A−Vsinterval. In such examples, the determinations of the intrinsic conduction time measurements may take place, for example, once a minute, once an hour, or once every 24 hours, although other frequencies may also be used.

The determinations of intrinsic conduction time may involve the suspension of some or all pacing therapy to the heart of the patient for at least one cardiac cycle, which may reduce the amount of synchronization of the ventricles of the heart during at least that one cardiac cycle. However, as described herein, the devices, systems, and techniques for providing adaptive CRT are directed to providing adaptive CRT while lessening the frequency at which the delivery of electrical stimulation to the heart of the patient is suspended.

FIG. 1illustrates example medical device system10in conjunction with patient14. Medical device system10is an example of a medical device system that is configured to implement the example techniques described herein for controlling the delivery of CRT to heart12of patient14. In some examples, medical device system10includes an implantable medical device (IMD)16in communication with external device24. In the illustrated example, IMD16may be coupled to leads18,20, and22. IMD16may be, for example, an implantable pacemaker that provides electrical signals to heart12and senses electrical activity of heart12via electrodes coupled to one or more of leads18,20, and22. In some examples, IMD16may include cardioversion and/or defibrillation capabilities.

Leads18,20,22extend into heart12of patient14to sense electrical activity of heart12and to deliver electrical stimulation to heart12. In the example shown inFIG. 1, right ventricular (RV) lead18extends through one or more veins (not shown), the superior vena cava (not shown), and right atrium (RA)26, and into RV28. Left ventricular (LV) coronary sinus lead20extends through one or more veins, the vena cava, right atrium26, and into the coronary sinus30to a region adjacent to the free wall of LV32of heart12. Right atrial (RA) lead22extends through one or more veins and the vena cava, and into the RA26of heart12.

IMD16may sense electrical signals attendant to the depolarization and repolarization of heart12via electrodes (not shown inFIG. 1) coupled to at least one of the leads18,20,22. In some examples, IMD16may also sense electrical signals attendant to the depolarization and repolarization of heart12via extravascular electrodes (e.g., electrodes positioned outside the vasculature of patient14), such as epicardial electrodes, external surface electrodes, subcutaneous electrodes, and the like. The configurations of electrodes used by IMD16for sensing and pacing may be unipolar or bipolar.

IMD16is configured to provide adaptive CRT to heart12. In some examples, as part of the adaptive CRT, IMD16is configured to deliver at least one of fusion pacing to heart12and biventricular pacing to heart12. In some examples of fusion pacing, IMD16may deliver a pacing stimulus (e.g., a pacing pulse) to LV32via electrodes of lead20, where the pacing stimulus is timed such that an evoked depolarization of LV32is effected in fusion with the intrinsic depolarization of RV28, resulting in a ventricular resynchronization. In this way, the pacing pulse delivered to LV32(LVP) may pre-excite a conduction delayed LV32and help fuse the activation of LV32with the activation of RV28from intrinsic conduction. The fusion of the depolarization of LV32and RV28may result in synchronous activation and contraction of LV32with RV28. In the examples described herein, the fusion pacing configuration may be referred to as “left-ventricular” pacing. However, it should be understood that a fusion pacing configuration may also include right-ventricular pacing in any of the examples described.

In some examples, when IMD16is in a biventricular pacing configuration, IMD16may deliver a pacing stimulus (e.g., a pacing pulse) to RV28via electrodes of lead18and a pacing stimulus to LV32via electrodes of lead20in a manner that synchronizes activation and contraction of RV28and LV28.

As discussed in further detail below, IMD16may be configured to adjust one or more pacing parameters based on a cardiac status of heart12. In some examples, IMD16may be configured to adjust a pacing parameter by delivering electrical stimulation therapy to heart12according to either a fusion pacing configuration or a biventricular pacing configuration. In other examples, IMD16may be configured to adjust a pacing parameter by modifying an A−VPinterval that controls the timing of ventricular pacing. In still other examples, IMD16may be configured to adjust a pacing parameter by increasing or decreasing the pacing output (e.g., the frequency of pacing pulses or the intensity of the pacing pulses, such as the current or voltage amplitude). In such examples, the cardiac status of heart12may include a heart rate of heart12, and a respective anticipated value for an interval between an atrial event and a ventricular event, as determined or selected by one or more components of IMD16.

As described in some examples herein, an interval between an atrial event and a ventricular event may be referred to as an “A−V interval.” While IMD16may periodically measure an intrinsic A−Vsinterval of heart12, such as for the purpose of updating a respective anticipated value for an A−V interval, IMD16may more frequently determine whether adjust one or more pacing parameters based on a heart rate of heart12, and one or more respective anticipated values of an A−V interval of heart12. In this way, the one or more respective values of an anticipated A−V interval for a given heart rate may allow IMD16to determine pacing parameters that may be appropriate for the treatment needs of patient14at a given time.

An anticipated value of an A−V interval is not an actual current measurement of intrinsic A−V conduction time, but rather may be an inferred based on a heart rate of heart12and previous measurements of A−Vsconduction time. Asdiscussed below in greater detail with respect toFIG. 6, a memory of IMD16may store at least one respective value for an A−V interval for each of a plurality of heart rates, e.g., in the form of a look-up table. In this example, a respective anticipated value for an A−V interval for a given heart rate may be considered an anticipated value of the intrinsic A−Vsconduction time of heart12that is expected to occur at the given heart rate. Thus, by selecting pacing parameters based on a determined heart rate of heart12and one or more respective anticipated values for an A−V interval, IMD16may select pacing parameters, such as an A−Vpinterval or a pacing configuration, without necessarily suspending the delivery of CRT to determine an actual measurement of a current A−Vsinterval.

In some cases, it may be advantageous for IMD16to be configured to select pacing parameters without suspending CRT. For example, the clinical outcomes of some cardiac conditions for which CRT is indicated may be improved when biventricular pacing comprises a relatively greater proportion of the CRT delivered to heart12by IMD16. However, if an anticipated value for an A−V interval is not used to select pacing parameters, it may be necessary for an IMD to suspend CRT each time that pacing parameters are to be selected or confirmed, in order to allow for the determination of an actual measurement of a current A−Vsinterval. In some known examples, this may be done approximately once per minute, in order to adapt the pacing configuration of a CRT system to changing cardiac rhythm characteristics. In such known examples, the suspension of some or all pacing therapy to the heart of the patient for at least one cardiac cycle, during which a current A−Vsinterval is measured, may reduce the extent of synchronization of RV28and LV32of heart12during at least that one cardiac cycle. A reduced amount of synchronization may limit the frequency at which a suspension of pacing therapy may be tolerated. Thus, examples that rely on the measurement of a current A−Vsinterval for the adaptation of CRT may provide less effective therapy delivery than the examples described herein.

In some examples, the adaptive CRT provided by IMD16may be useful for maintaining the cardiac rhythm in patient14with a conduction dysfunction, which may result when the natural electrical activation system of heart12is disrupted. The natural electrical activation system of a human heart12involves several sequential conduction pathways starting with the sino-atrial (SA) node, and continuing through the atrial conduction pathways of Bachmann's bundle and internodal tracts at the atrial level, followed by the atrio-ventricular (AV) node, Common Bundle of His, right and left bundle branches, and a final distribution to the distal myocardial terminals via the Purkinje fiber network.

In a normal electrical activation sequence, the cardiac cycle commences with the generation of a depolarization wave at the SA Node in the wall of RA26. The depolarization wave is transmitted through the atrial conduction pathways of Bachmann's Bundle and the Internodal Tracts at the atrial level into the LA33septum. When the atrial depolarization wave has reached the AV node, the atrial septum, and the furthest walls of the right and left atria26,33, respectively, the atria26,33may contract as a result of the electrical activation. The aggregate right atrial and left atrial depolarization wave appears as the P-wave of the PQRST complex of an electrical cardiac signal, such as a cardiac electrogram (EGM) or electrocardiogram (ECG). When the amplitude of the atrial depolarization wave passing between a pair of unipolar or bipolar pace/sense electrodes located on or adjacent RA26and/or LA33exceeds a threshold, it is detected as a sensed P-wave. The sensed P-wave may also be referred to as an atrial sensing event, or an RA sensing event (RAs). Similarly, a P-wave sensed in the LA33may be referred to as an atrial sensing event or an LA sensing event (LAs).

During or after the atrial contractions, the AV node distributes the depolarization wave inferiorly down the Bundle of His in the intraventricular septum. The depolarization wave may travel to the apical region of heart12and then superiorly though the Purkinje Fiber network. The aggregate right ventricular and left ventricular depolarization wave and the subsequent T-wave accompanying re-polarization of the depolarized myocardium may appear as the QRST portion of the PQRST cardiac cycle complex. When the amplitude of the QRS ventricular depolarization wave passing between a bipolar or unipolar pace/sense electrode pair located on or adjacent RV28and/or LV32exceeds a threshold, it is detected as a sensed R-wave. The sensed R-wave may also be referred to as a ventricular sensing event, an RV sensing event (RVS), or an LV sensing event (LVS) depending upon the ventricle in which the electrodes of one or more of leads18,20,22are configured to sense in a particular case.

Some patients, such as patients with congestive heart failure or cardiomyopathies, may have left ventricular dysfunction, whereby the normal electrical activation sequence through heart12is compromised within LV32. In a patient with left ventricular dysfunction, the normal electrical activation sequence through the heart of the patient becomes disrupted. For example, patients may experience an intra-atrial conduction defect, such as intra-atrial block. Intra-atrial block is a condition in which the atrial activation is delayed because of conduction delays between RA26to LA33.

As another example, a patient with left ventricular dysfunction may experience an interventricular conduction defect, such as left bundle branch block (LBBB) and/or right bundle branch block (RBBB). In LBBB and RBBB, the activation signals are not conducted in a normal fashion along the right or left bundle branches respectively. Thus, in patients with bundle branch block, the activation of either RV28or LV32is delayed with respect to the other ventricle, causing asynchrony between the depolarization of the right and left ventricles. Ventricular asynchrony may be identified by a widened QRS complex due to the increased time for the activation to traverse the ventricular conduction paths. The asynchrony may result from conduction defects along the Bundle of His, the Right and Left Bundle Branches or at the more distal Purkinje Terminals. Typical intra-ventricular peak-to-peak asynchrony can range from about 80 milliseconds (ms) to about 200 ms or longer. However, in patients who are experiencing RBBB and LBBB, the QRS complex may be widened far beyond the normal range to a wider range, e.g., about 120 ms to about 250 ms or greater.

CRT delivered by IMD16may help alleviate heart failure conditions by restoring synchronous depolarization and contraction of one or more chambers of heart12. In some cases, the fusion pacing of heart12described herein enhances stroke volume of a patient by improving the synchrony with which RV28and LV32depolarize and contract.

The duration of a cardiac cycle of heart12, which includes the depolarization-repolarization sequence, may change depending on various physiological factors of patient14, such as a heart rate. As heart rate of patient14changes, the timing of the delivery of a pacing pulse to LV32(LVP) during fusion pacing therapy or the timing of the delivery of pacing pulses to RV28(RVP) and LV32(LVP) during biventricular pacing therapy may change. Accordingly, when IMD16is delivering fusion pacing, such as left-ventricular pacing, to heart12, it may be useful for IMD16to periodically adjust a fusion pacing interval (i.e., an A−VPinterval) in order to maintain the delivery of the LV32pacing pulse (LVP) at a time that results in a fusion of the depolarization of LV32and RV28. In addition, when IMD16is delivering biventricular pacing therapy to heart12, IMD16may periodically evaluate one or more biventricular pacing intervals (i.e., A−Vpintervals) in order to maintain the delivery of the LV32pacing pulse (LVP) at a time relative to the RV28pacing pulse (RVP) that results in a synchrony of contraction of LV32and RV28. As discussed in further detail below, in some examples, IMD16adjusts the A−Vpinterval for fusion pacing or for biventricular pacing based on a determined heart rate of a cardiac cycle and a respective anticipated A−V interval, where the respective anticipated A−V interval is selected from a table stored in a memory of IMD16based on a heart rate of heart12determined by IMD16. As such, with respect to an interval selected by processing circuitry80of IMD16(described in further detail below with respect toFIG. 3), an anticipated interval may be referred to as a “selected value of an A−V interval,” or, with respect to a given heart rate, a “respective selected value of an A−V interval.”

In some examples, IMD16delivers pacing pulses to LV32until IMD16determines that selected value for an A−V interval associated with a determined heart rate of heart12exceeds a threshold for fusion or left-ventricular pacing therapy. In some cases, upon determining that a selected value for an A−V interval exceeds such a threshold, IMD16switches to a different pacing configuration, such as a biventricular pacing configuration, after discontinuing fusion pacing therapy. Similarly, IMD16may deliver pacing pulses to LV32and RV28in a biventricular pacing configuration until IMD16determines that a respective selected value for an A−V interval associated with a determined heart rate of heart12does not exceed a threshold for left-ventricular pacing therapy, at which time IMD16may switch to a fusion pacing configuration, such as left-ventricular pacing.

In some examples, IMD16also provides defibrillation therapy and/or cardioversion therapy via electrodes located on at least one of the leads18,20,22. IMD16may detect arrhythmia of heart12, such as fibrillation of ventricles28and32, and deliver defibrillation therapy to heart12in the form of electrical shocks. In some examples, IMD16is programmed to deliver a progression of therapies, e.g., shocks with increasing energy levels, until a fibrillation of heart12is stopped. In examples in which IMD16provides defibrillation therapy and/or cardioversion therapy, IMD16may detect fibrillation by employing any one or more fibrillation detection techniques known in the art.

In some examples, external device24may be a handheld computing device or a computer workstation. External device24may include a user interface that receives input from a user. The user interface may include, for example, a keypad and a display, which may for example, be a cathode ray tube (CRT) display, a liquid crystal display (LCD) or light emitting diode (LED) display. The keypad may take the form of an alphanumeric keypad or a reduced set of keys associated with particular functions. External device24can additionally or alternatively include a peripheral pointing device, such as a mouse, via which a user may interact with the user interface. In some embodiments, a display of external device24may include a touch screen display, and a user may interact with external device24via the display.

A user, such as a physician, technician, or other clinician, may interact with external device24to communicate with IMD16. For example, the user may interact with external device24to retrieve physiological or diagnostic information from IMD16. A user may also interact with external device24to program IMD16, e.g., to select values for operational parameters of the IMD.

For example, the user may use external device24to retrieve information from IMD16regarding the rhythm of heart12, trends therein over time, or arrhythmia episodes. As another example, the user may use external device24to retrieve information from IMD16regarding other sensed physiological parameters of patient14, such as sensed electrical activity, activity, posture, respiration, or thoracic impedance. As another example, the user may use external device24to retrieve information from IMD16regarding the performance or integrity of IMD16or other components of system10, such as leads18,20, and22, or a power source of IMD16. In such examples, physiological parameters of patient14and data regarding IMD16may be stored in a memory of IMD16for retrieval by the user.

The user may use external device24to program a therapy progression, select electrodes used to deliver defibrillation pulses, select waveforms for the defibrillation pulse, or select or configure a fibrillation detection algorithm for IMD16. The user may also use external device24to program aspects of other therapies provided by IMD14, such as cardioversion or pacing therapies. In some examples, the user may activate certain features of IMD16by entering a single command via external device24, such as depression of a single key or combination of keys of a keypad or a single point-and-select action with a pointing device.

IMD16and external device24may communicate via wireless communication using any techniques known in the art. Examples of communication techniques may include, for example, include radiofrequency (RF) telemetry, which may be an RF link established via an antenna according to Bluetooth, WiFi, or medical implant communication service (MICS), though other techniques are also contemplated. In some examples, external device24may include a programming head that may be placed proximate to the patient's body near the IMD16implant site in order to improve the quality or security of communication between IMD16and external device24.

FIG. 2is a conceptual diagram illustrating IMD16and leads18,20,22of medical device system10in greater detail. Leads18,20,22may be electrically coupled to therapy delivery circuitry, sensing circuitry, or other circuitry of IMD16via connector block34. In some examples, proximal ends of leads18,20,22include electrical contacts that electrically couple to respective electrical contacts within connector block34. In addition, in some examples, leads18,20,22are mechanically coupled to connector block34with the aid of set screws, connection pins or another suitable mechanical coupling mechanism.

Each of the leads18,20,22includes an elongated insulative lead body, which may carry a number of conductors separated from one another by tubular insulative sheaths. In the illustrated example, bipolar electrodes40and42are located proximate to a distal end of lead18. In addition, bipolar electrodes44and46are located proximate to a distal end of lead20and bipolar electrodes48and50are located proximate to a distal end of lead22. Electrodes40,44, and48may take the form of ring electrodes, and electrodes42,46and50may take the form of extendable helix tip electrodes mounted retractably within insulative electrode heads52,54and56, respectively. Each of the electrodes40,42,44,46,48and50may be electrically coupled to a respective one of the conductors within the lead body of its associated lead18,20,22, and thereby coupled to respective ones of the electrical contacts on the proximal end of leads18,20and22.

Electrodes40,42,44,46,48and50may sense electrical signals attendant to the depolarization and repolarization of heart12. The electrical signals are conducted to IMD16via the respective leads18,20,22. In some examples, IMD16also delivers pacing pulses to LV32via electrodes44,46to cause depolarization of cardiac tissue of heart12. In some examples, as illustrated inFIG. 2, IMD16includes one or more housing electrodes, such as housing electrode58, which may be formed integrally with an outer surface of hermetically-sealed housing60of IMD16or otherwise coupled to housing60. In some examples, housing electrode58is defined by an uninsulated portion of an outward facing portion of housing60of IMD16. Other division between insulated and uninsulated portions of housing60may be employed to define two or more housing electrodes. In some examples, housing electrode58comprises substantially all of housing60. Any of the electrodes40,42,44,46,48, and50may be used for unipolar sensing or stimulation delivery in combination with housing electrode58. As described in further detail with reference toFIG. 3, housing60may enclose therapy delivery circuitry that generates cardiac pacing pulses and defibrillation or cardioversion shocks, as well as sensing circuitry for monitoring the patient's heart rhythm.

In some examples, leads18,20,22may also include elongated electrodes62,64,66, respectively, which may take the form of a coil. IMD16may deliver defibrillation pulses to heart12via any combination of elongated electrodes62,64,66, and housing electrode58. Electrodes58,62,64,66may also be used to deliver cardioversion pulses to heart12. Electrodes62,64,66may be fabricated from any suitable electrically conductive material, such as, but not limited to, platinum, platinum alloy or other materials known to be usable in implantable defibrillation electrodes.

The configuration of therapy system10illustrated inFIGS. 1 and 2is one example, and is not intended to be limiting. In other examples, a therapy system may include extravascular electrodes, such as subcutaneous electrodes, epicardial electrodes, and/or patch electrodes, instead of or in addition to the electrodes of transvenous leads18,20,22illustrated inFIG. 1. Further, IMD16need not be implanted within patient14. In examples in which IMD16is not implanted in patient14, IMD16may deliver defibrillation pulses, pacing pulses, and other therapies to heart12via percutaneous leads that extend through the skin of patient14to a variety of positions within or outside of heart12.

In other examples of medical device systems that provide electrical stimulation therapy to heart12, a therapy system may include any suitable number of leads coupled to IMD16, and each of the leads may extend to any location within or proximate to heart12. For example, a therapy system may include a dual chamber device rather than a three-chamber device as shown inFIG. 1. In one example of a dual chamber configuration, IMD16is electrically connected to a single lead that includes stimulation and sense electrodes within LV32as well as sense and/or stimulation electrodes within RA26, as shown inFIG. 3. In another example of a dual chamber configuration, IMD16is connected to two leads that extend into a respective one of RA28and LV32.

In some examples, a medical device system includes one or more intracardiac pacing devices instead of, or in addition to, an IMD coupled to leads that extend to heart12, like IMD16. The intracardiac pacing devices may include therapy delivery and processing circuitry within a housing configured for implantation within one of the chambers of heart12. In such systems, the plurality of pacing devices, which may include one or more intracardiac pacing devices and/or an IMD coupled to one or more leads, may communicate to coordinate sensing and pacing in various chambers of heart12to provide CRT according to the techniques described herein. Processing circuitry and memory of one or more of the pacing devices, and/or another implanted or external medical device, may provide the functionality for controlling delivery of CRT ascribed to processing circuitry and memory of IMD16herein.

FIG. 3is a functional block diagram of one example configuration of IMD16ofFIGS. 1 and 2. In the illustrated example, IMD16includes memory70, processing circuitry80, sensing circuitry82, one or more accelerometers84, therapy delivery circuitry86, telemetry circuitry88, and power source90, one or more of which may be disposed within housing60of IMD16. In some examples, memory70includes computer-readable instructions that, when executed by processing circuitry80, cause IMD16and processing circuitry80to perform various functions attributed to IMD16and processing circuitry80herein. Memory70may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital media. In addition to sensed physiological parameters of patient14(e.g., EGM or ECG signals), one or more time intervals for timing fusion pacing therapy and biventricular pacing therapy to heart12(e.g., PEI values, adjusted PEI values, biventricular pacing intervals, or any combination thereof) may be stored by memory70.

As illustrated in the example ofFIGS. 3 and 4, memory70may include one or more heart rate/A−V interval tables72. In some examples, heart rate/A−V interval tables72may include a plurality of heart rates and at least one respective selected value for an A−V interval for each of the plurality of heart rates. As discussed in further detail below with respect toFIG. 7, one or more of the selected values for an A−V interval stored within heart rate/A−V interval tables72may be updated according to update schedule92. For example, memory70may include computer-readable instructions that, when executed by processing circuitry80, cause processing circuitry80periodically to determine a heart rate of heart12, suspend the delivery of CRT by therapy delivery circuitry86, measure a current value of an A−V interval, and determine whether the measured current value for the A−V interval is not sufficiently similar to, e.g., falls outside of a range about, the respective selected value for an A−V interval stored in table72for the determined heart rate. If the measured current value for the A−V interval is not sufficiently similar to the respective selected value for an A−V interval associated with the heart rate, processing circuitry80may then update table72, e.g., by replacing the respective selected value for the A−V interval associated with the heart rate with the current value for the A−V interval.

Processing circuitry80may include one or more of a microprocessor, a controller, digital signal processing circuitry (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processing circuitry80may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processing circuitry80herein may be embodied as software, firmware, hardware or any combination thereof. Processing circuitry80may be configured to determine a heart rate of heart12based on electrical activity sensed by sensing circuitry82, select a respective A−V interval value associated with the heart rate from table72of memory70, and determine whether to control therapy delivery circuitry86to deliver CRT to heart12according to a fusion pacing configuration or a biventricular pacing configuration based on the selected A−V interval value. With further respect to this example, processing circuitry80may be configured to cause therapy delivery circuitry86to deliver electrical pulses in accordance with the selected A−V interval value by using the selected A−V interval value as an A−Vpinterval between an atrial sensing or pacing event and the delivery of a pacing pulse, or determining the A−Vpinterval from the selected A−V interval, e.g., by subtracting a PEI from the selected A−V interval. In some examples, the A−V intervals stored in table72within memory70are intervals between a paced or sensed atrial event and a sensed (i.e., intrinsic) ventricular event, i.e., are A−Vsintervals.

Sensing circuitry82is configured to monitor signals from at least one of electrodes40,42,44,46,48,50,58,62,64or66in order to monitor electrical activity of heart12, e.g., via EGM signals. For example, sensing circuitry82may sense atrial events (e.g., a P-wave) with electrodes48,50,66within RA26or sense an LV32event (e.g., an R-wave) with electrodes44,46,64within LV32. In some examples, sensing circuitry82includes switching circuitry to select which of the available electrodes are used to sense the electrical activity of heart12. For example, processing circuitry80may select the electrodes that function as sense electrodes via the switching circuitry within sensing circuitry82, e.g., by providing signals via a data/address bus. In some examples, sensing circuitry82includes one or more sensing channels, each of which may comprise an amplifier. In response to the signals from processing circuitry80, the switching circuitry of sensing circuitry82may couple the outputs from the selected electrodes to one of the sensing channels.

In some examples, one channel of sensing circuitry82may include an R-wave amplifier that receives signals from electrodes40and42, which are used for pacing and sensing in RV28of heart12. Another channel may include another R-wave amplifier that receives signals from electrodes44and46, which are used for pacing and sensing proximate to LV32of heart12. In some examples, the R-wave amplifiers may take the form of an automatic gain controlled amplifier that provides an adjustable sensing threshold as a function of the measured R-wave amplitude of the heart rhythm.

In addition, in some examples, one channel of sensing circuitry82may include a P-wave amplifier that receives signals from electrodes48and50, which are used for pacing and sensing in RA26of heart12. In some examples, the P-wave amplifier may take the form of an automatic gain controlled amplifier that provides an adjustable sensing threshold as a function of the measured P-wave amplitude of the heart rhythm. Examples of R-wave and P-wave amplifiers are described in U.S. Pat. No. 5,117,824 to Keimel et al., which issued on Jun. 2, 1992 and is entitled, “APPARATUS FOR MONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and is incorporated herein by reference in its entirety. Other amplifiers may also be used. Furthermore, in some examples, one or more of the sensing channels of sensing circuitry82may be selectively coupled to housing electrode58, or elongated electrodes62,64, or66, with or instead of one or more of electrodes40,42,44,46,48or50, e.g., for unipolar sensing of R-waves or P-waves in any of chambers26,28, or32of heart12.

In some examples, sensing circuitry82includes a channel that comprises an amplifier with a relatively wider pass band than the R-wave or P-wave amplifiers. Signals from the selected sensing electrodes that are selected for coupling to this wide-band amplifier may be provided to a multiplexer, and thereafter converted to multi-bit digital signals by an analog-to-digital converter for storage in memory70as an EGM. In some examples, the storage of such EGMs in memory70may be under the control of a direct memory access circuit. Processing circuitry80may employ digital signal analysis techniques to characterize the digitized signals stored in memory70to detect and classify the patient's heart rhythm from the electrical signals. Processing circuitry80may detect and classify the heart rhythm of patient14by employing any of the numerous signal processing methodologies known in the art.

Signals generated by sensing circuitry82may include, for example: an RA-event signal, which indicates a detection of a P-wave via electrodes implanted within RA26(FIG. 1); an LA-event signal, which indicates a detection of a P-wave via electrodes implanted within LA33(FIG. 1); an RV-event signal, which indicates a detection of an R-wave via electrodes implanted within RV28; or an LV-event signal, which indicates a detection of an R-wave via electrodes implanted within LV32. In the example of therapy system10shown inFIGS. 1 and 2, IMD16is not connected to electrodes that are implanted within LA33. However, in other example therapy systems, IMD16may be connected to electrodes that are implanted within LA33in order to sense electrical activity of LA33.

In some examples, IMD16may include one or more additional sensors, such as accelerometers84. In some examples, accelerometers84may comprise one or more three-axis accelerometers. Signals generated by accelerometers84may be indicative of, for example, gross body movement of patient14, such as a patient posture or activity level. Regardless of the configuration of accelerometers84, processing circuitry80may determine patient parameter values based on the signals obtained therefrom. Accelerometers84may produce and provide signals to processing circuitry80for a determination as to the posture and activity level of patient14at a given time. Processing circuitry80may then use the determined posture and activity level to further determine whether patient14is awake or asleep, and, if patient14is determined to be awake, to further determine whether patient14is at rest or exercising. As described below with respect toFIGS. 7-9, the wake/sleep and rest/exercise states of patient14determined by processing circuitry80may cause processing circuitry80to select a corresponding one of tables72, and then to select one or more values of an A−V interval from the selected one of tables72.

Therapy delivery circuitry86is electrically coupled to electrodes40,42,44,46,48,50,58,62,64, and66, e.g., via conductors of the respective lead18,20,22, or, in the case of housing electrode58, via an electrical conductor disposed within housing60of IMD16. Therapy delivery circuitry86is configured to generate and deliver electrical stimulation therapy. For example, therapy delivery circuitry86may deliver a pacing stimulus to LV32(FIG. 2) of heart12, in accordance with the fusion pacing techniques described herein, via at least two electrodes44,46(FIG. 2). As another example, therapy delivery circuitry86may deliver a pacing stimulus to RV28via at least two electrodes40,42(FIG. 2) and a pacing stimulus to LV32via at least two electrodes44,46(FIG. 2), e.g., in accordance with the biventricular pacing techniques described herein.

In some examples, therapy delivery circuitry86is configured to deliver cardioversion or defibrillation shocks to heart12. The pacing stimuli, cardioversion shocks, and defibrillation shocks may be in the form of stimulation pulses. In other examples, therapy delivery circuitry86may deliver one or more of these types of stimulation in the form of other signals, such as sine waves, square waves, or other substantially continuous time signals.

Therapy delivery circuitry86may include a switching circuitry, and processing circuitry80may use the switching circuitry to select, e.g., via a data/address bus, which of the available electrodes are used to deliver defibrillation pulses or pacing pulses. The switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple stimulation energy to selected electrodes. In other examples, processing circuitry80may select a subset of electrodes40,42,44,46,48,50,58,62,64, and66with which stimulation is delivered to heart12without a switching circuitry.

Processing circuitry80may select one or more respective A−V interval values from HR/A−V interval table72of memory70for timing the delivering of pacing pulses to heart12based on one or more signals sensed by sensing circuitry82. For example, processing circuitry80may determine a heart rate of heart12based on electrical activity of heart12sensed by sensing circuitry82, and then select one or more respective selected A−V interval values from HR/A−V interval table72based on the determined heart rate. These intervals may include, for example, an A−V interval indicative of the intrinsic conduction from the atria to the ventricles (e.g., AP/S−RVSinterval, also referred to more generally as an A−VSinterval). From such intervals, a pacing interval (e.g., an AP/S−RVPand/or AP/S−LVP, also referred to more generally as an A−Vpinterval) for fusion pacing or biventricular pacing may be determined, e.g., by subtracting a PEI, or otherwise as described above.

Processing circuitry80includes pacer timing and control circuitry96, which may be embodied as hardware, firmware, software, or any combination thereof. Pacer timing and control circuitry96may comprise a dedicated hardware circuit, such as an ASIC, separate from other processing circuitry80components, such as a microprocessor, or a software module executed by a component of processing circuitry80(e.g., a microprocessor or ASIC). Pacer timing and control circuitry96may help control the delivery of pacing pulses to heart12according to the one or more respective A−V interval values selected by processing circuitry80from HR/A−V interval table72.

In examples in which IMD16delivers a pacing pulse according to the one or more A−V interval values selected and/or determined by processing circuitry80, pacer timing and control circuitry96may include a timer for determining that a selected A−V interval has elapsed after processing circuitry80determines that an atrial pace or sense event (AP/S, or more generally A) has occurred. The timer of pacing timing and control circuitry96may be configured to begin upon the detection of the preceding atrial pace or sense event (AP/S) by processing circuitry80. Upon expiration of the particular timer, processing circuitry80may control therapy delivery circuitry86to deliver a pacing stimulus, according to a fusion or biventricular pacing configuration, to heart12. For example, pacing timing and control circuitry96may generate a trigger signal that triggers the output of a pacing pulse by therapy delivery circuitry86.

In examples in which IMD16is configured to deliver other types of cardiac rhythm therapy in addition to fusion pacing and biventricular pacing, pacer timing and control circuitry96may also include programmable counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamber pacing. In the aforementioned pacing modes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I” may indicate inhibited pacing (e.g., no pacing), and “A” may indicate an atrium. The first letter in the pacing mode may indicate the chamber that is paced, the second letter may indicate the chamber in which an electrical signal is sensed, and the third letter may indicate the chamber in which the response to sensing is provided.

In examples in which IMD16is configured to deliver other types of cardiac rhythm therapy in addition to CRT, intervals defined by pacer timing and control circuitry96within processing circuitry80may include atrial and ventricular pacing escape intervals, refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals, and the pulse widths of the pacing pulses. As another example, pacer timing and control circuitry96may define a blanking period, and provide signals from sensing circuitry82to blank one or more channels, e.g., amplifiers, for a period during and after delivery of electrical stimulation to heart12. The durations of these intervals may be determined by processing circuitry80in response to stored data in memory70. In some examples, the pacer timing and control circuitry96of processing circuitry80may also determine the amplitude of the cardiac pacing pulses.

During certain pacing modes, escape interval counters within pacer timing/control circuitry96of processing circuitry80may be reset upon sensing of R-waves and P-waves. Therapy delivery circuitry86may include pacer output circuits that are coupled, e.g., selectively by switching circuitry, to any combination of electrodes40,42,44,46,48,50,58,62, or66appropriate for delivery of a bipolar or unipolar pacing pulse to one of the chambers of heart12. Processing circuitry80may reset the escape interval counters upon the generation of pacing pulses by therapy delivery circuitry86, and thereby control the basic timing of cardiac pacing functions, including fusion cardiac resynchronization therapy.

The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used by processing circuitry80to measure the durations of R−R intervals, P−P intervals, P−R intervals and R−P intervals, which are measurements that may be stored in memory70. Processing circuitry80may use the count in the interval counters to detect a tachyarrhythmia event, such as ventricular fibrillation event or ventricular tachycardia event. Upon detecting a threshold number of tachyarrhythmia events, processing circuitry80may identify the presence of a tachyarrhythmia episode, such as a ventricular fibrillation episode, a ventricular tachycardia episode, or a non-sustained tachycardia (NST) episode. Examples of tachyarrhythmia episodes that may qualify for delivery of responsive therapy include a ventricular fibrillation episode or a ventricular tachyarrhythmia episode.

In some examples, processing circuitry80may operate as an interrupt driven device, and is responsive to interrupts from pacer timing and control circuitry96, where the interrupts may correspond to the occurrences of sensed P-waves and R-waves and the generation of cardiac pacing pulses. Any necessary mathematical calculations to be performed by processing circuitry80and any updating of the values or intervals controlled by the pacer timing and control circuitry96of processing circuitry80may take place following such interrupts. A portion of memory70may be configured as a plurality of recirculating buffers, capable of holding series of measured intervals, which may be analyzed by processing circuitry80in response to the occurrence of a pace or sense interrupt to determine whether the patient's heart12is presently exhibiting atrial or ventricular tachyarrhythmia.

In some examples, an arrhythmia detection method may include any suitable tachyarrhythmia detection algorithms. In one example, processing circuitry80may utilize all or a subset of the rule-based detection methods described in U.S. Pat. No. 5,545,182 to Olson et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issued on Aug. 13, 1996, or in U.S. Pat. No. 5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issued on May 26, 1998. U.S. Pat. No. 5,545,182 to Olson et al. and U.S. Pat. No. 5,755,736 to Gillberg et al. are incorporated herein by reference in their entireties. However, other arrhythmia detection methodologies may also be employed by processing circuitry80in other examples.

If IMD16is configured to generate and deliver defibrillation shocks to heart12, therapy delivery circuitry86may include a high voltage charge circuit and a high voltage output circuit. In the event that processing circuitry80determines that generation of a cardioversion or defibrillation shock is required, processing circuitry80may employ the escape interval counter to control timing of such cardioversion and defibrillation shocks, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, processing circuitry80may activate a cardioversion/defibrillation control circuitry (not shown), which may, like pacer timing and control circuitry96, be a hardware component of processing circuitry80and/or a firmware or software module executed by one or more hardware components of processing circuitry80. The cardioversion/defibrillation control circuitry may initiate charging of the high voltage capacitors of the high voltage charge circuit of therapy delivery circuitry86under control of a high voltage charging control line.

Processing circuitry80may monitor the voltage on the high voltage capacitor, e.g., via a voltage charging and potential (VCAP) line. In response to the voltage on the high voltage capacitor reaching a predetermined value set by processing circuitry80, processing circuitry80may generate a logic signal that terminates charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse by therapy delivery circuitry86is controlled by a cardioversion/defibrillation control circuitry (not shown) of processing circuitry80. Following delivery of the fibrillation or tachycardia therapy, processing circuitry80may return therapy delivery circuitry86to a cardiac pacing function and await the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization.

Therapy delivery circuitry86may deliver cardioversion or defibrillation shock with the aid of an output circuit that determines whether a monophasic or biphasic pulse is delivered, whether housing electrode58serves as cathode or anode, and which electrodes are involved in delivery of the cardioversion or defibrillation pulses. Such functionality may be provided by one or more switches or a switching circuitry of therapy delivery circuitry86.

Telemetry circuitry88includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as external device24(FIG. 1). Under the control of processing circuitry80, telemetry circuitry88may receive downlink telemetry from and send uplink telemetry to external device24with the aid of an antenna, which may be internal and/or external. Processing circuitry80may provide the data to be uplinked to external device24and the control signals for the telemetry circuit within telemetry circuitry88, e.g., via an address/data bus. In some examples, telemetry circuitry88may provide received data to processing circuitry80via a multiplexer.

In some examples, processing circuitry80may transmit atrial and ventricular heart signals (e.g., EGM signals) produced by atrial and ventricular sense amp circuits within sensing circuitry82to external device24. Other types of information may also be transmitted to external device24, such as the various intervals and delays used to deliver CRT. External device24may interrogate IMD16to receive the heart signals. Processing circuitry80may store heart signals within memory70, and retrieve stored heart signals from memory70. Processing circuitry80may also generate and store marker codes indicative of different cardiac episodes that sensing circuitry82detects, and transmit the marker codes to external device24. An example pacemaker with marker-channel capability is described in U.S. Pat. No. 4,374,382 to Markowitz, entitled, “MARKER CHANNEL TELEMETRY SYSTEM FOR A MEDICAL DEVICE,” which issued on Feb. 15, 1983 and is incorporated herein by reference in its entirety.

Telemetry circuitry88includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as external device24(FIG. 1). Under the control of processing circuitry80, telemetry circuitry88may receive downlink telemetry from and send uplink telemetry to external device24with the aid of an antenna, which may be internal and/or external. Processing circuitry80may provide the data to be uplinked to external device24and the control signals for the telemetry circuit within telemetry circuitry88, e.g., via an address/data bus. In some examples, telemetry circuitry88may provide received data to processing circuitry80via a multiplexer.

The various components of IMD16are coupled to power source90, which may include a rechargeable or non-rechargeable battery. A non-rechargeable battery may be selected to last for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis.

FIG. 4is a functional block diagram of memory70shown in greater detail. As illustrated inFIG. 4, HR/A−V table72may further include an HR/A−V awake-rest table74, HR/A−V awake-exercise table76, and HR/A−V sleep table78. As described above with respect toFIG. 3, signals generated by accelerometers84may be indicative of a posture or activity level of patient14. Processing circuitry80may then use the determined posture and activity level to further determine whether patient14is awake or asleep, and, if patient14is determined to be awake, to further determine whether patient14is at rest or exercising. In some examples, processing circuitry80may select a stored value for an A−V interval associated with a heart rate from one of tables74,76, or78based on the determined wake/sleep and rest/exercise states of patient14. For example, if processing circuitry80determines that patient14is awake and in a rest state, then processing circuitry80may select an A−V interval value stored in HR/A−V awake-rest table74upon determining a heart rate of heart12. Similarly, if processing circuitry80determines that patient14is awake and in an exercise state, then processing circuitry80may select an A−V interval value stored in HR/A−V awake-exercise table76upon determining a heart rate of heart12. If processing circuitry determines that patient14is in a sleep state, then processing circuitry80may select an A−V interval value stored in HR/A−V sleep table78upon determining a heart rate of heart12.

In some examples, the heart rates stored in tables74,76, and78may overlap to some extent. That is, the same value for a heart rate may be stored in more than one of tables74,76, and78. However, even if a given heart rate is represented in more than one of tables74,76, and78, the respective A−V interval values for the given heart rate may differ between tables74,76, and78. For example, a heart rate of 70 bpm may be present in both table74and table78, but the respective A−V interval values for the heart rate of 70 bpm may differ from table74to78. This feature may reflect differences in intrinsic A−V conduction that may occur between the waking and sleeping states, even at the same heart rate. Thus, 1 MB16may be configured to adapt the delivery of CRT to patient14based on wake/sleep states and rest/exercise states, in addition to adapting the delivery of CRT based on a heart rate of patient14.

Although not illustrated inFIG. 4, each of the one or more HR/A−V tables72may include two A−V interval values in association with each heart rate value. With respect to the A−V intervals described herein, atrial sensing events (As) and atrial pacing events (Ap) collectively may be referred to as atrial events (A). Thus, A−V interval designated as “A−Vp” may be understood as being either an As−V interval or an Ap−V interval. Similarly, ventricular sensing events (Vs) and ventricular pacing events (Vp) collectively may be referred to as ventricular events (V). In some examples, the first A−V interval value may be an As−V interval value indicating the time between a sensed or intrinsic atrial event and a ventricular event. The second A−V interval value may be an Ap−V interval value indicating the time between a paced atrial event and a ventricular event. Processing circuitry80may select between these two values for a determined heart rate based on whether the atrial event for a given cardiac cycle was intrinsic or paced.

As illustrated inFIG. 4, memory70of IMD16may further include update schedule92. One or more of the A−V interval values stored within tables72may be updated according to update schedule92. For example, as described above, memory70may include computer-readable instructions that, when executed by processing circuitry80, cause processing circuitry80periodically to determine a heart rate of heart12, suspend the delivery of CRT by therapy delivery circuitry86, measure a current value of an A−V interval, and determine whether the measured current value for the A−V interval is sufficiently similar to a selected value for the A−V interval associated with the determined heart rate that is stored in one of tables72. If the measured current value for the A−V interval is not sufficiently similar to the A−V interval value associated with the heart rate in the table72, processing circuitry may update the table, e.g., by replacing the selected A−V interval value stored in the table in association with the heart rate with the currently measured value for the A−V interval.

In some examples, update schedule92may include computer-readable instructions for processing circuitry80to conduct the update technique described above according to a predetermined period. For example, update schedule92may direct processing circuitry80to update the one or more respective selected values for an A−V interval associated with a determined heart rate approximately once per hour, although other periods for periodic updates such as every several minutes every six hours, or daily, are contemplated. In this example, one or more respective selected values for an A−V interval associated with the current heart rate of heart12, within the appropriate one of tables72, may be determined by processing circuitry80at the time the update technique is conducted. For example, if processing circuitry80determines that patient14is awake and in a rest state at the time of update, then any update made to one or more selected values for an A−V interval associated with the heart rate of heart12at the time of update may be made to HR/A−V awake-rest table74.

In some examples, memory70of IMD16further includes ranges and thresholds94, as shown inFIG. 4. As discussed below with respect toFIGS. 7-9, ranges and thresholds94may include values that indicate a threshold degree of similarity between currently-measured and stored A−V interval values for determining whether to update the stored value. The value may be expressed as a range about one of the values, and the determination may be whether the other value is within or outside of the range. The range may be defined by an absolute difference, or percentage difference, between the values. During the execution of an update procedure according to update schedule92, processing circuitry80may compare a currently measured value of an A−V interval to the selected value of the A−V interval stored in a corresponding one of tables72in order to determine whether to update the selected A−V interval value based on the one or more values stored as ranges and thresholds94in memory70.

In addition, ranges and thresholds94may include threshold values for the A−V intervals stored in tables72. During the execution of a technique to control the delivery of CRT to heart12, processing circuitry80may select an A−V interval value from one of tables72, and compare the selected A−V interval value to a threshold value94. Depending on whether the selected A−V interval value is greater or less than the threshold value, processing circuitry80may determine whether to control therapy delivery circuitry86to deliver fusion pacing or biventricular pacing.

FIG. 5is functional block diagram of an example external device24. Asshown inFIG. 5, external device24includes processing circuitry100, a memory102, a user interface104, telemetry circuitry106, and a power source108. External device24may be a dedicated hardware device with dedicated software for interacting with IMD16. Alternatively, external device24may be an off-the-shelf computing device running an application that enables external device24to interact with IMD16.

A user may use external device24to select programmable parameters that control the monitoring and delivery of therapy by IMD16, and to retrieve information collected by IMD regarding the condition of patient14or the performance of IMD16. For example, the user may program a period for update schedule92, values for ranges and thresholds94, PEI values or other values for determining A−Vpintervals, or any other programmable values described herein. The user may interact with external device24via user interface104, which may include display to present graphical user interface to a user, and a keypad or another mechanism for receiving input from a user.

Processing circuitry100can take the form one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processing circuitry102herein may be embodied as hardware, firmware, software or any combination thereof. Memory102may store instructions that cause processing circuitry100to provide the functionality ascribed to external device24herein, and information used by processing circuitry100to provide the functionality ascribed to external device24herein. Memory102may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory102may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow patient data to be easily transferred to another computing device, or to be removed before external device24is used to program therapy for another patient. Memory102may also store information that controls therapy delivery by IMD16, such as stimulation parameter values.

External device24may communicate wirelessly with IMD16, such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry circuitry106, which may be coupled to an internal antenna or an external antenna. An external antenna that is coupled to external device24may correspond to the programming head that may be placed over heart12, as described above with reference toFIG. 1.

Telemetry circuitry106may be similar to telemetry circuitry88of IMD16(FIG. 3). Telemetry circuitry106may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. Examples of local wireless communication techniques that may be employed to facilitate communication between external device24and another computing device include RF communication according to the 802.11 or Bluetooth specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. In this manner, other external devices may be capable of communicating with external device24without needing to establish a secure wireless connection.

Power source108is configured to deliver operating power to the components of external device24. Power source108may include a battery and a power generation circuit to produce the operating power. In some embodiments, the battery may be rechargeable to allow extended operation. Recharging may be accomplished by electrically coupling power source108to a cradle or plug that is connected to an alternating current (AC) outlet. In addition or alternatively, recharging may be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within external device24. In other embodiments, traditional batteries (e.g., nickel cadmium or lithium ion batteries) may be used. In addition, external device24may be directly coupled to an alternating current outlet to power external device24. Power source108may include circuitry to monitor power remaining within a battery. In this manner, user interface104may provide a current battery level indicator or low battery level indicator when the battery needs to be replaced or recharged. In some cases, power source108may be capable of estimating the remaining time of operation using the current battery.

In some examples, processing circuitry100and memory102of external device24may be configured to provide some or all of the functionality ascribed to processing circuitry80and memory70of IMD16. For example, memory102may be configured to store one or of HR/A−V interval tables72, update schedule92, or ranges and thresholds94. In some examples, processing circuitry100may be configured to determine heart rates, select A−V intervals, and/or control delivery of CRT by IMD16as described herein with respect to processing circuitry80of IMD16.

FIG. 6is a block diagram illustrating a system110that includes an external device112, such as a server, and one or more computing devices114A-114N that are coupled to IMD16and external device24shown inFIG. 1via a network120, according to one example. In this example, IMD16uses telemetry circuitry88(FIG. 3) to communicate with external device24via a first wireless connection, and to communicate with an access point122via a second wireless connection. In the example ofFIG. 6, access point122, external device24, external device112, and computing devices114A-114N are interconnected, and able to communicate with each other, through network120. In some cases, one or more of access point122, external device24, external device112, and computing devices114A-114N may be coupled to network120through one or more wireless connections. IMD16, external device24, external device112, and computing devices114A-114N may each comprise one or more processing circuitries, such as one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that may perform various functions and operations, such as those described herein.

Access point122may comprise a device that connects to network120via any of a variety of connections, such as telephone dial-up, digital subscriber line (DSL), or cable modem connections. In other examples, access point122may be coupled to network120through different forms of connections, including wired or wireless connections. In some examples, access point122may communicate with external device24and/or IMD16. Access point122may be co-located with patient14(e.g., within the same room or within the same site as patient14) or may be remotely located from patient14. For example, access point122may be a home monitor that is located in the patient's home or is portable for carrying with patient14.

During operation, IMD16may collect, measure, and store various forms of diagnostic data. For example, as described previously, IMD16may collect ECG and/or EGM signals, and determine different CRT configurations and A−V intervals. In certain cases, IMD16may directly analyze collected diagnostic data and generate any corresponding reports or alerts. In some cases, however, IMD16may send diagnostic data to external device24, access point122, and/or external device112, either wirelessly or via access point122and network110, for remote processing and analysis.

For example, IMD16may send external device24data that indicates whether a loss of intrinsic AV conduction was detected. External device24may generate reports or alerts after analyzing the data. As another example, IMD16may send a system integrity indication generated by processing circuitry80(FIG. 3) to external device24, which may take further steps to determine whether there may be a possible condition with one or more of leads18,20, and22. For example, external device24may initiate lead impedance tests or IMD16may provide lead impedance information, if such information is already available.

In another example, IMD16may provide external device112with collected EGM data, system integrity indications, and any other relevant physiological or system data via access point122and network120. External device112includes one or more processing circuitries118. In some cases, external device112may request such data, and in some cases, IMD16may automatically or periodically provide such data to external device112. Upon receipt of the diagnostic data via input/output device116, external device112is capable of analyzing the data and generating reports or alerts upon determination that there may be a possible condition with one or more of leads18,20, and22or with patient14.

In one example, external device112may comprise a secure storage site for information that has been collected from IMD16and/or external device24. In this example, network120may comprise an Internet network; and trained professionals, such as clinicians, may use computing devices114A-114N to securely access stored data on external device112. For example, the trained professionals may need to enter usernames and passwords to access the stored information on external device112. In one embodiment, external device112may be a CareLink server provided by Medtronic, Inc., of Minneapolis, Minn.

In some examples, processing circuitry and memory of one or more of access point122, server112, or computing devices114, e.g., processing circuitry118and memory of server112, may be configured to provide some or all of the functionality ascribed to processing circuitry80and memory70of IMD16. For example, server112may be configured to store one or more of HR/A−V interval tables72, update schedule92, or ranges and thresholds94. In some examples, processing circuitry118may be configured to determine heart rates, select A−V intervals, and/or control delivery of CRT by IMD16as described herein with respect to processing circuitry80of IMD16.

FIGS. 7-9are flow diagrams illustrating various techniques related to controlling delivery of adaptive CRT based on heart rate as a surrogate for more frequent measurements of intrinsic conduction in accordance with examples of this disclosure. As described herein, the techniques illustratedFIGS. 7-9may be employed using one or more components of system10, which have been described above with respect toFIGS. 1-6. Although described as being performed by IMD16, the techniques ofFIGS. 7-10may be performed, in whole or in part, by processing circuitry and memory of other devices of a medical device system, as described herein.

FIG. 7is a flow diagram illustrating an example technique130for creating a table (or other data structure) associating a plurality of heart rates and respective values of A−V intervals, delivering therapy according to one or more HR/A−V interval tables72, and updating a value of a selected interval within an HR/A−V interval table72. According to the example ofFIG. 7, IMD16may create one or more HR/A−V interval tables (132). In some examples, IMD16may create the HR/A−V interval tables72as part of a start-up phase of treatment following the implantation of IMD16within patient14.

In such examples, IMD16may deliver fusion pacing or biventricular pacing while processing circuitry80identifies a plurality of heart rates of heart12and determines one or more respective A−V interval values for each heart rate. For example, when processing circuitry80identifies a previously-unidentified heart rate of heart12during the start-up phase of treatment, processing circuitry80may then suspend CRT and measure a current A−V interval value associated with the determined heart rate. The measured A−V interval value may then be stored in one or more of tables72, as a respective value for an A−V interval associated with the identified heart rate.

In some examples, more than one respective value for an A−V interval may be stored for the determined heart rate, such as a first A−V interval value and a second A−V interval value for that heart rate. In such examples, the first A−V interval value for a given heart rate may be an interval between a sensed or intrinsic atrial event and a ventricular event, whereas the second A−V interval value for the given heart rate may be an interval between a paced atrial event and a ventricular event. In some examples, processing circuitry80may expedite population of the A−V values in tables72by controlling therapy delivery circuitry86to pace the atria at various rates, and/or expedite the population of A−V interval values in tables72by instructing patient14to undertake different activities.

This process may be repeated when processing circuitry80identifies another heart rate of heart12, and/or determines that patient14is in a different wake/sleep or rest/exercise state. In some examples, processing circuitry80may collect heart rates at 5-10 beat per minute (bpm) increments, and measure one or more respective A−V interval values for each 5-10 bpm increment, although other increments may be used. Although expressed in terms of bpm, the term “heart rate” as used herein encompasses any measure of the rate of depolarization or contraction of the heart, including electrical or mechanical cardiac cycle lengths, such as R−R or P−P intervals.

After the tables72are wholly or partially populated, control of CRT based on heart rate as a surrogate for A−V conduction delay may begin. For example, processing circuitry80of IMD16may determine a heart rate of heart12, and control therapy delivery circuitry86to deliver CRT according to a stored A−V interval value associated with the determined heart rate in one of tables72(134), as will be described in greater detail with respect toFIG. 8.

At (136) ofFIG. 7, processing circuitry80of IMD16may determine whether to update one or more of the A−V interval values stored in tables72of memory70. Asdescribed above with respect toFIG. 4, update schedule92of memory70may include computer-readable instructions for processing circuitry80to conduct the update technique described above according to a predetermined periodic schedule. For example, update schedule92may direct processing circuitry80to validate or update one or more respective selected values for an A−V interval associated with a determined heart rate approximately once per hour. In some examples, an A−V interval value associated with the heart rate of heart12, within the appropriate one of tables72as determined by processing circuitry80based on the activity of patient12at the time the update technique is conducted, may be updated. For example, if processing circuitry80determines that patient14is awake and in a rest state at the time of update, then any update made to one or more values for an A−V interval associated with the heart rate of heart12at the time of update will be made to HR/A−V awake-rest table74.

If processing circuitry80determines that one or more values for an A−V interval are to be updated according to update schedule92, then technique130may proceed to (138). If, however, processing circuitry80determines not to update one or more values for an AV interval, then technique130may proceed back to (134). At (138), processing circuitry80may suspend the delivery of CRT to heart12, and perform the validation/update techniques, e.g., as described in greater detail with respect toFIG. 9(140).

FIG. 8is a flow diagram illustrating an example technique150for providing CRT according to a heart rate as a surrogate for the current A−V conduction delay, e.g., as represented by a current or recently measured A−Vsinterval. The example technique150ofFIG. 8may be performed, but is not necessarily performed, on a beat-to-beat, or per-cardiac cycle, basis. At (154), processing circuitry80of IMD16determines a heart rate of heart12, as described above with respect toFIGS. 3 and 7. The heart rate may be a cardiac cycle length, as described above. Further, the heart rate may be a heart rate for one or more preceding cardiac cycles, such as the heart rate for the immediately preceding cardiac cycle, or a mean or median of heart rates of a plurality of preceding cycles.

Processing circuitry80further determines whether an atrial event for a current cardiac cycle of heart12was a sensed (or intrinsic) event or a paced event (156). If the atrial event for the current cardiac cycle was a sensed event (YES of156), processing circuitry80selects an As−V, e.g., As−RVs, value that corresponds with the determined heart rate from one of tables72(158). If the atrial event for the current cardiac cycle was a paced event (NO of156), processing circuitry80selects an Ap−V, e.g., Ap−RVs, value that corresponds with the determined heart rate from one of tables72(160). Asdiscussed above with respect toFIG. 7, more than one respective value for an A−V interval may be stored in HR/A−V tables74,76, and78, such that a first A−V interval value for a given heart rate may be an interval between a sensed or intrinsic atrial event and a ventricular event, whereas a second A−V interval value for the given heart rate may be an interval between a paced atrial event and a ventricular event.

According to the example ofFIG. 8, processing circuitry80compares the selected A−RVsinterval value to a threshold, which may be different depending on whether the atrial event was sensed or paced and, consequently, whether the interval is an As−V or Ap−V interval (162,164). If the selected A−Vsinterval value is not greater than the predetermined threshold value, then processing circuitry80proceeds to control therapy delivery circuitry86to deliver fusion, e.g., left-ventricular, pacing (166,170). If, however, the selected A−Vsinterval value is greater than the predetermined threshold value, then processing circuitry80proceeds to control therapy delivery circuitry86to deliver biventricular pacing (168,172).

Processing circuitry80may determine the A−Vpinterval(s) at which to deliver the fusion or biventricular pacing based on the selected A−Vsinterval, e.g., based on the application of a PEI to the A−Vsinterval. For example, processing circuitry80may determine an A−LVpinterval for delivery of left-ventricular fusion pacing based on the selected A−RVsinterval. Processing circuitry80may determine an A−RVpand A−LVpinterval based on the A−RVsinterval, or determine one of the A−RVpand A−LVpintervals based on the A−RVsinterval, and the other of the A−RVpand A−LVpintervals based on the determined interval and a programmed RVp−LVpdelay.

With further respect to technique150ofFIG. 8, once processing circuitry80has controlled therapy delivery circuitry86to control therapy delivery according to one of steps (166), (168), (170), or (172), then technique150proceeds back to (154), at which a new heart rate of heart12is determined by processing circuitry80.

FIG. 9is a flow diagram illustrating an example technique180for periodically validating or updating a value of a selected A−V interval within one of HR/A−V interval tables72. The example technique180ofFIG. 9may be performed periodically, with the period being greater than a cardiac cycle, such as hourly. According to the example technique180, processing circuitry80determines a heart rate of heart12, and selects a respective A−V interval value from one of HR/A−V tables72, as described above (182).

In order to validate or update a value of a selected AV interval value in one of HR/A−V tables72, processor80may suspend the delivery of CRT by IMD16to heart12(138ofFIG. 7), in order to allow heart12to conduct in the absence of cardiac rhythm management therapy. As discussed above, suspending the delivery of CRT may refer to, as examples, withholding ventricular pacing for one or more cardiac cycles, increasing an A−Vpdelay sufficiently so that intrinsic ventricular conduction may be observed, or pacing one ventricle at a sufficiently long A−Vpdelay and measuring intrinsic conduction on the other ventricle. While the delivery of CRT is suspended, processing circuitry80may measure a current value of an A−V interval without interference between the delivery of pacing pulses and sensing of ventricular activation (186). The measurement a current value of an A−V interval may represent, e.g., the time between an atrial pace or sense event (AP/S) and a V1 sensing event (V1S). In some examples, processing circuitry80may suspend CRT and determine a measured current value of an A−V interval, for example, approximately once per hour, although other frequencies also may be used.

Processing circuitry80compares the measured current value of the A−V interval of heart12to a value of the A−V interval stored in one or more of tables72that was selected based on the heart rate (188). At (190), if the measured current value of the A−V interval is within the predetermined range94of the selected AV interval value, then the stored A−V interval value is validated and technique180terminates (192).

If, however, the measured current value of the A−V interval is determined not to be within the predetermined range94of the selected A−V interval value (YES of190), then processing circuitry80may update the selected value of the A−V interval at (194) to reflect the measured current value of the A−V interval. In some examples, processor80may update the selected value of the A−V interval by replacing the selected value of the A−V interval with the measured current value of the A−V interval.

In some examples, the measured current value of the A−V interval is a first measured current value. In this example, while CRT is still suspended, processing circuitry80may then measure a second current value of the A−V interval, and determine whether the second measured current value of the A−V interval is within the predetermined range. If the second measured current value of the A−V interval is within the predetermined range, then processing circuitry80may replace the measured first measured current value of the A−V interval with the selected value of the A−V interval at (194), thereby reverting back to the value stored in the table prior to the measurement of the first current value. In still other examples, processing circuitry80may also update other values of the A−V interval associated with the determined heart rate at (194), based on the determination that the current value of the A−V interval is not within the predetermined range.

In addition, upon updating one or more selected values of the A−V interval associated with the determined heart rate, processing circuitry80may also update one or more values of A−V intervals associated with other heart rate values stored in one or more of HR/A−V tables72. For example, upon updating one or more values of the A−V interval associated with the determined heart rate in table74, processing circuitry80may update one or more values of one or more A−V intervals associated with other heart rate values in table74. The updated values may include all of the values of the A−V intervals stored in table74, or, in some examples, may include only values of the A−V intervals associated with heart rate values within a predetermined range of the determined heart rate. In the latter example, if the determined heart rate was 80 bpm, then processing circuitry80may update one or more A−V interval values associated with heart rate values in a range of, e.g., 80±10 bpm, although other ranges are contemplated. In other examples, processing circuitry80may update one or more values of one or more A−V intervals associated with other heart rate values stored in other tables72.

In some examples, processing circuitry80may update one or more values of A−V intervals associated with other heart rate values by adjusting the values of the A−V intervals associated with the other heart rate values by a predetermined value. In some examples, the predetermined value may be the difference between a measured current value of an A−V interval and the value of the selected A−V interval associated with the determined heart rate. In some examples, the predetermined value may be the same for each of the values of the A−V intervals associated with the other heart rate values. In other examples, the predetermined value may vary depending upon, e.g., the magnitude of the difference between the determined heart rate and a heart rate value associated with the values of the other A−V intervals to be updated. In examples in which processing circuitry80updates values associated with A−V intervals in more than one of tables72, the predetermined value by which processing circuitry80updates a given value of an A−V interval may depend at least partly upon the identity of the table72to be updated.

Various aspects of the techniques may be implemented within one or more processing circuitries, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in external devices, such as physician or patient external devices, electrical stimulators, or other devices. The term “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry or any other equivalent circuitry.

In one or more examples, the functions described in this disclosure may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media forming a tangible, non-transitory medium. Instructions may be executed by one or more processing circuitries, such as one or more DSPs, ASICs, FPGAs, general purpose microprocessors, or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processing circuitry,” as used herein may refer to one or more of any of the foregoing structure or any other structure suitable for implementation of the techniques described herein.

In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. Also, the techniques could be fully implemented in one or more circuits or logic elements. The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including an IMD, an external device, a combination of an 1 MB and external device, an integrated circuit (IC) or a set of ICs, and/or discrete electrical circuitry, residing in an IMD and/or external device.