Patent ID: 12194303

DETAILED DESCRIPTION

In general, this disclosure describes techniques for controlling the pacing mode of an implantable cardiac pacemaker, which may be an intracardiac ventricular pacemaker, to promote AV conduction while providing atrial synchronized ventricular pacing during periods of AV block. In the illustrative examples presented herein, an intracardiac ventricular pacemaker is configured to provide single chamber ventricular pacing and, at least during an atrial synchronous ventricular pacing mode, provide dual chamber (atrial and ventricular) sensing. Atrial sensing from an intraventricular signal produced by a sensor included in the pacemaker is performed for synchronizing the ventricular pacing pulses to the sensed atrial events during periods of AV block. As described below, the atrial systolic events may be sensed from an intraventricular motion signal produced by a motion sensor included in the pacemaker. The intraventricular motion signal includes an atrial systolic event signal corresponding to atrial mechanical contraction and the active filling phase of the ventricle, sometimes referred to as the “atrial kick.” In other examples, atrial event sensing may be performed using other techniques, such as sensing the far-field P-wave that is attendant to atrial depolarization, from a cardiac electrical signal sensed from within the ventricle.

The techniques disclosed herein promote AV conduction by controlling pacing mode switching in a manner that allows atrial depolarizations to conduct to the ventricles through the heart's normal conduction system when AV conduction is intact. When AV conduction block (or other conduction abnormalities) occurs, the pacemaker operates in an atrial synchronous pacing mode that relies on atrial event sensing from an intraventricular sensor signal for controlling the timing of ventricular pacing pulses, synchronized to the atrial events. As described below, the pacemaker may switch to an asynchronous ventricular pacing mode to determine if AV block is still present, based only on ventricular events without requiring atrial sensing in some examples. As used herein, an “asynchronous ventricular pacing mode” or “asynchronous pacing mode” refers to non-atrial synchronous ventricular pacing, which may be delivered in a non-atrial tracking, ventricular demand pacing mode, such as a VDI(R) or VVI(R) pacing mode. If AV block is still present, the pacemaker may switch back to the atrial synchronous ventricular pacing (with dual chamber sensing). When AV block is not detected, ventricular pacing may be controlled according to an asynchronous ventricular pacing mode with a relatively low base pacing rate so that AV conduction along the heart's intrinsic conduction system is promoted.

FIG.1is a conceptual diagram illustrating an implantable medical device (IMD) system10that may be used to sense cardiac signals and provide ventricular pacing to a patient's heart8in a manner that promotes AV conduction. IMD system10includes an intracardiac ventricular pacemaker14. Pacemaker14may be a transcatheter intracardiac pacemaker which is adapted for implantation wholly within a heart chamber, e.g., wholly within the right ventricle (RV) or wholly within the left ventricle (LV) of heart8for sensing cardiac signals and delivering ventricular pacing pulses in a single chamber pacing mode. Pacemaker14may be reduced in size compared to subcutaneously implanted pacemakers and may be generally cylindrical in shape to enable transvenous implantation in a heart chamber via a delivery catheter.

In the example shown, pacemaker14is positioned along an endocardial wall of the RV, e.g., near the RV apex. The techniques disclosed herein are not limited to the pacemaker location shown in the example ofFIG.1and other positions within heart8are possible. For example, an intracardiac ventricular pacemaker14may be positioned in the LV and configured to detect cardiac signals and deliver ventricular pacing to the LV using the techniques disclosed herein. Pacemaker14may be positioned within the right ventricle or left ventricle to provide respective right ventricular or left ventricular pacing and for sensing atrial signals from within the ventricular chamber for facilitating atrial synchronous ventricular pacing.

Pacemaker14is capable of producing electrical stimulation pulses, e.g., pacing pulses, delivered to heart8via one or more electrodes on the outer housing of the pacemaker. Pacemaker14is configured to generate and deliver ventricular pacing pulses and sense a cardiac electrical signal using housing based electrodes for producing a ventricular electrogram (EGM) signal. The cardiac electrical signals may be sensed using the housing based electrodes that are also used to deliver pacing pulses to the heart8.

Pacemaker14is configured to control the delivery of ventricular pacing pulses to the ventricle in a manner that promotes synchrony between atrial systole and ventricular systole, e.g., by maintaining a target atrioventricular (AV) interval between a sensed atrial systolic event and ventricular pacing pulses while operating in an atrial synchronous ventricular pacing mode. Pacemaker14senses atrial events from an intraventricular signal produced by a sensor included in or on the pacemaker and controls ventricular pacing pulse delivery to maintain a desired AV interval between atrial systolic events and ventricular pacing pulses delivered to cause ventricular depolarization and ventricular systole. The atrial synchronous ventricular pacing mode may be referred to as a “VDD” pacing mode since single chamber ventricular pacing is being delivered with dual chamber sensing and a dual response is provided to sensed events, either a pacing pulse is triggered in response to an atrial sensed event or inhibited in response to a ventricular sensed event, e.g., and R-wave.

The atrial synchronous ventricular pacing mode is provided to promote a more normal heart rhythm during periods of AV block. In patients that may have intermittent AV block (or other conduction abnormalities), pacemaker14operates to promote AV conduction along the normal conduction pathways of the heart by periodically switching to an asynchronous ventricular pacing mode, e.g., VVI or VDI pacing mode. If AV conduction is determined to be present, the pacemaker remains in the asynchronous pacing mode with a relatively low base pacing rate to promote conduction of atrial depolarizations to the ventricles via the heart's natural conduction system. If AV block is determined to be present, the pacemaker switches back to the atrial synchronous pacing mode, e.g., VDD pacing mode.

In some examples, pacemaker14includes a motion sensor, such as an accelerometer, that produces an intraventricular motion signal including atrial systolic event signals corresponding to the active filling phase of ventricular diastole. The motion signal produced by an accelerometer implanted within the RV, for example, includes motion signals caused by ventricular and atrial events. For instance, acceleration of blood flowing into the RV through the tricuspid valve16between the right atrium (RA) and RV caused by atrial systole may be detected by pacemaker14from the signal produced by an accelerometer included in pacemaker14. Other motion signals detected by pacemaker14, such as motion caused by ventricular contraction, motion caused by ventricular relaxation, and motion caused by passive filling of the ventricle are described below in conjunction withFIG.4. Pacemaker14may perform atrial event sensing to enable atrial synchronous ventricular pacing by sensing atrial events from an intraventricular motion signal.

In other examples, pacemaker14may sense atrial systolic events by sensing atrial P-waves that are attendant to atrial depolarizations. P-waves are relatively low amplitude signals in the near-field RV electrical signal received by pacemaker14(e.g., compared to the near-field R-waves) and therefore can be difficult to consistently detect from the cardiac electrical signal acquired by pacemaker14when implanted in a ventricular chamber. Atrial synchronous ventricular pacing by pacemaker14may not be reliable when based solely on a cardiac electrical signal received by pacemaker14. According to the techniques disclosed herein, the pacemaker14may therefore include a motion sensor, such as an accelerometer, and be configured to detect an atrial event corresponding to atrial mechanical activation or atrial mechanical systole using a signal from the motion sensor.

A target AV interval may be a programmed value selected by a clinician and is the time interval from the detection of the atrial event until delivery of the ventricular pacing pulse. The target AV interval may be identified as being hemodynamically optimal for a given patient based on clinical testing or assessments of the patient or based on clinical data from a population of patients. The target AV interval may be determined to be optimal based on relative timing of electrical and mechanical events as identified from the cardiac electrical signal received by pacemaker14and the motion sensor signal received by pacemaker14. The AV interval may be set to 10 to 50 ms, in some examples, to control pacemaker14to deliver a ventricular pacing pulse synchronized to the atrial event sensed from the motion signal.

Pacemaker14may be capable of bidirectional wireless communication with an external device20for programming the AV pacing interval and other pacing control parameters as well as both electrical and mechanical event sensing parameters utilized for detecting ventricular events (e.g., R-waves) from the cardiac electrical signal and atrial systolic events from the intraventricular motion sensor signal. Aspects of external device20may generally correspond to the external programming/monitoring unit disclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.), hereby incorporated herein by reference in its entirety. External device20is often referred to as a “programmer” because it is typically used by a physician, technician, nurse, clinician or other qualified user for programming operating parameters in pacemaker14. External device20may be located in a clinic, hospital or other medical facility. External device20may alternatively be embodied as a home monitor or a handheld device that may be used in a medical facility, in the patient's home, or another location. Operating parameters, including sensing and therapy delivery control parameters, may be programmed into pacemaker14using external device20.

External device20is configured for bidirectional communication with implantable telemetry circuitry included in pacemaker14. External device20establishes a wireless communication link24with pacemaker14. Communication link24may be established using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS) or other communication bandwidth. In some examples, external device20may include a programming head that is placed proximate pacemaker14to establish and maintain a communication link24, and in other examples external device20and pacemaker14may be configured to communicate using a distance telemetry algorithm and circuitry that does not require the use of a programming head and does not require user intervention to maintain a wireless communication link. An example RF telemetry communication system that may be implemented in system10is generally disclosed in U.S. Pat. No. 5,683,432 (Goedeke, et al.), hereby incorporated herein by reference in its entirety. External device20may display data and information relating to pacemaker functions to a user for reviewing pacemaker operation and programmed parameters as well as EGM signals transmitted from pacemaker14, motion sensor signals produced by pacemaker14, or other physiological data that is produced by and retrieved from pacemaker14during an interrogation session.

It is contemplated that external device20may be in wired or wireless connection to a communications network via a telemetry circuit that includes a transceiver and antenna or via a hardwired communication line for transferring data to a centralized database or computer to allow remote management of the patient. Remote patient management systems including a remote patient database may be configured to utilize the presently disclosed techniques to enable a clinician to review EGM, motion sensor, and marker channel data and authorize programming of sensing and therapy control parameters in pacemaker14, e.g., after viewing a visual representation of EGM, motion sensor signal and marker channel data (as show inFIG.7as an example).

FIG.2is a conceptual diagram of the intracardiac pacemaker14shown inFIG.1. Pacemaker14includes electrodes162and164spaced apart along the housing150of pacemaker14for sensing cardiac electrical signals and delivering pacing pulses. Electrode164is shown as a tip electrode extending from a distal end102of pacemaker14, and electrode162is shown as a ring electrode along a mid-portion of housing150, for example adjacent proximal end104. Distal end102is referred to as “distal” in that it is expected to be the leading end as pacemaker14is advanced through a delivery tool, such as a catheter, and placed against a targeted pacing site.

Electrodes162and164form an anode and cathode pair for bipolar cardiac pacing and sensing. In alternative embodiments, pacemaker14may include two or more ring electrodes, two tip electrodes, and/or other types of electrodes exposed along pacemaker housing150for delivering electrical stimulation to heart8and sensing cardiac electrical signals. Electrodes162and164may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black among others. Electrodes162and164may be positioned at locations along pacemaker14other than the locations shown.

Housing150is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing150may include an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide among others. The entirety of the housing150may be insulated, but only electrodes162and164uninsulated. Electrode164may serve as a cathode electrode and be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing150via an electrical feedthrough crossing housing150. Electrode162may be formed as a conductive portion of housing150defining a ring electrode that is electrically isolated from the other portions of the housing150as generally shown inFIG.2. In other examples, the entire periphery of the housing150may function as an electrode that is electrically isolated from tip electrode164, instead of providing a localized ring electrode such as anode electrode162. Electrode162formed along an electrically conductive portion of housing150serves as a return anode during pacing and sensing.

The housing150includes a control electronics subassembly152, which houses the electronics for sensing cardiac signals, producing pacing pulses and controlling therapy delivery and other functions of pacemaker14as described below in conjunction withFIG.3. A motion sensor may be implemented as an accelerometer included in control electronics subassembly152and enclosed within housing150in some examples. The accelerometer provides a signal to a processor included in control electronics subassembly152for signal processing and analysis for sensing atrial systolic events for timing ventricular pacing pulses during atrial synchronous ventricular pacing as described below.

Housing150further includes a battery subassembly160, which provides power to the control electronics subassembly152. Battery subassembly160may include features of the batteries disclosed in commonly-assigned U.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131 (Lund, et al.), both of which are hereby incorporated by reference herein in their entirety.

Pacemaker14may include a set of fixation tines166to secure pacemaker14to patient tissue, e.g., by actively engaging with the ventricular endocardium and/or interacting with the ventricular trabeculae. Fixation tines166are configured to anchor pacemaker14to position electrode164in operative proximity to a targeted tissue for delivering therapeutic electrical stimulation pulses. Numerous types of active and/or passive fixation members may be employed for anchoring or stabilizing pacemaker14in an implant position. Pacemaker14may include a set of fixation tines as disclosed in commonly-assigned U.S. Pat. No. 9,775,982 (Grubac, et al.), hereby incorporated herein by reference in its entirety.

Pacemaker14may optionally include a delivery tool interface158. Delivery tool interface158may be located at the proximal end104of pacemaker14and is configured to connect to a delivery device, such as a catheter, used to position pacemaker14at an implant location during an implantation procedure, for example within a ventricular heart chamber.

FIG.3is a schematic diagram of an example configuration of pacemaker14shown inFIG.1. Pacemaker14includes a pulse generator202, a cardiac electrical signal sensing circuit204(also referred to herein as “sensing circuit204”) a control circuit206, memory210, telemetry circuit208, motion sensor212and a power source214. The various circuits represented inFIG.3may be combined on one or more integrated circuit boards which include a specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine or other suitable components that provide the described functionality.

Motion sensor212is implemented as an accelerometer in the examples described herein and may also be referred to herein as “accelerometer212.” Motion sensor212is not limited to being an accelerometer, however, and other motion sensors may be utilized successfully in pacemaker14for detecting cardiac motion signals according to the techniques described herein. Examples of motion sensors that may be implemented in motion sensor212include piezoelectric sensors and MEMS devices.

Motion sensor212may be a single axis, one-dimensional sensor or a multi-axis sensor, e.g., a two-dimensional or three-dimensional sensor, with each axis providing a signal that may be analyzed individually or in combination for detecting cardiac mechanical events. Motion sensor212produces an electrical signal correlated to motion or vibration of sensor212(and pacemaker14), e.g., when subjected to flowing blood, cardiac motion and patient body motion due to physical activity such as exercise and activities of daily living or other motion imparted on the patient such as riding in a car. The motion sensor212may include filters, amplifiers, rectifiers, an analog-to-digital converter (ADC) and/or other components for producing a motion signal passed to control circuit206. For example, each vector signal corresponding to each individual axis of a multi-axis accelerometer may be filtered by a high pass filter, e.g., a 10 Hz high pass filter, and rectified for use by atrial event detector circuit240for sensing atrial systolic events. The high pass filter may be lowered (e.g., to 5 Hz) if needed to detect atrial event signals that have lower frequency content. In some examples, high pass filtering is performed with no low pass filtering. In other examples, each accelerometer axis signal is filtered by a low pass filter, e.g., a 30 Hz low pass filter, with or without high pass filtering. The motion sensor may include separate filtering of the accelerometer signal for passing a motion signal to control circuit206for use in detecting patient physical activity level to enable rate responsive ventricular pacing to meet the patient's metabolic demand.

One example of an accelerometer for use in implantable medical devices is generally disclosed in U.S. Pat. No. 5,885,471 (Ruben, et al.), incorporated herein by reference in its entirety. An implantable medical device arrangement including a piezoelectric accelerometer for detecting patient motion is disclosed, for example, in U.S. Pat. No. 4,485,813 (Anderson, et al.) and U.S. Pat. No. 5,052,388 (Sivula, et al.), both of which patents are hereby incorporated by reference herein in their entirety. Examples of three-dimensional accelerometers that may be implemented in pacemaker14and used for detecting cardiac mechanical events using the presently disclosed techniques are generally described in U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No. 6,044,297 (Sheldon), both of which are incorporated herein by reference in their entirety. Other accelerometer designs may be used for producing an electrical signal that is correlated to motion imparted on pacemaker14due to ventricular and atrial events and patient physical activity.

Sensing circuit204is a cardiac electrical signal sensing circuit configured to receive a cardiac electrical signal via electrodes162and164by a pre-filter and amplifier circuit220. Pre-filter and amplifier circuit may include a high pass filter to remove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a wideband filter having a passband of 2.5 Hz to 100 Hz to remove DC offset and high frequency noise. Pre-filter and amplifier circuit220may further include an amplifier to amplify the “raw” cardiac electrical signal passed to ADC226. ADC226may pass a multi-bit, digital electrogram (EGM) signal to control circuit206for use by atrial event detector circuit240in identifying ventricular electrical events (e.g., R-waves or T-waves) and/or atrial electrical events, e.g., P-waves. Identification of cardiac electrical events may be used for detecting atrial systolic events from the motion sensor signal, e.g., by setting atrial blanking and sensing windows relative to sensed R-waves. The digital signal from ADC226may be passed to rectifier and amplifier circuit222, which may include a rectifier, bandpass filter, and amplifier for passing the filtered and rectified cardiac electrical signal to cardiac event detector224.

Cardiac event detector224may include a sense amplifier or other detection circuitry that compares the incoming rectified, cardiac electrical signal to an R-wave detection threshold, which may be an auto-adjusting threshold. When the incoming signal crosses the R-wave detection threshold, the cardiac event detector224produces an R-wave sensed event signal that is passed to control circuit206. In other examples, cardiac event detector224may receive the digital output of ADC226for detecting R-waves by a comparator, morphological signal analysis of the digital EGM signal or other R-wave detection techniques. R-wave sensed event signals passed from cardiac event detector224to control circuit206may be used for scheduling ventricular pacing pulses by pace timing circuit242during asynchronous ventricular pacing, determining ventricular rate intervals or RR intervals, and for use in identifying the timing of ventricular electrical events by atrial event detector circuit240for facilitating detection of atrial systolic events from a signal received from motion sensor212.

In some examples, cardiac event detector224is configured to sense P-waves from the cardiac electrical signal received by electrodes162and164(and/or electrodes carried by a sensing extension extending away from housing150). Cardiac event detector224may compare the incoming signal to a P-wave sensing threshold and produce a P-wave sensed event signal passed to control circuit206in response to a threshold crossing. When pacemaker14is configured to sense R-waves and P-waves, sensing circuit204may include two different sensing channels, each including a pre-filter/amplifier, ADC, rectifier/amplifier and cardiac event detector configured to amplify and filter cardiac electrical signals received via one or two different sensing electrode pairs for separately sensing R-waves and P-waves from the cardiac electrical signals. P-wave sensing may be used for verifying atrial events sensed from a motion sensor signal or vice versa. In some examples, P-wave sensed event signals are used by control circuit206for starting an AV interval for controlling atrial synchronous ventricular pacing pulses delivered by pulse generator202.

Control circuit206includes an atrial event detector circuit240, pace timing circuit242, and processor244. Atrial event detector circuit240is configured to detect atrial mechanical events from a signal received from motion sensor212. In some examples, one or more ventricular mechanical events may be detected from the motion sensor signal in a given cardiac cycle to facilitate positive detection of the atrial systolic event from the motion sensor signal during the ventricular cycle.

Control circuit206may receive R-wave sensed event signals, P-wave sensed event signals, and/or digital cardiac electrical signals from sensing circuit204for use in detecting and confirming cardiac events and controlling ventricular pacing. For example, R-wave sensed event signals may be passed to pace timing circuit242for inhibiting scheduled ventricular pacing pulses during atrial synchronous ventricular pacing or scheduling ventricular pacing pulses when pacemaker14is operating in a non-atrial tracking (asynchronous) ventricular pacing mode. As described below, R-wave sensed event signals may be used by control circuit206for determining if AV conduction is intact during an asynchronous ventricular pacing mode.

R-wave sensed event signals may be passed to atrial event detector circuit240for use in setting atrial blanking periods and/or time windows used by control circuit206in sensing atrial systolic events from the motion sensor signal. Atrial event detector circuit240receives a motion signal from motion sensor212and may start an atrial blanking period in response to a ventricular electrical event, e.g., an R-wave sensed event signal from sensing circuit204or delivery of a pacing pulse by pulse generator202. The blanking period may correspond to a time period after the ventricular electrical event during which ventricular mechanical events, e.g., corresponding to ventricular contraction and isovolumic relaxation are expected to occur. Motion signal peaks that occur during the atrial blanking period are not sensed as atrial events to avoid falsely sensing a ventricular motion signal event as the atrial systolic event.

Atrial event detector circuit240determines if the motion sensor signal satisfies atrial mechanical event detection criteria outside of the atrial blanking period. The motion sensor signal during the atrial blanking period may be monitored by atrial event detector circuit240and/or processor244for the purposes of detecting ventricular mechanical events, which may be used for confirming or validating atrial systolic event detection or detecting ventricular event intervals in some examples. As such, ventricular mechanical event detection windows may be set during the atrial blanking period and may be set according to predetermined time intervals following identification of a ventricular electrical event.

Atrial event detector circuit240may set time windows corresponding to the passive ventricular filling phase and the active ventricular filling phase based on the timing of a preceding ventricular electrical event, either an R-wave sensed event signal or a ventricular pacing pulse. A motion sensor signal crossing of an atrial event sensing threshold during either of these windows may be detected as the atrial systolic event. As described below, two different atrial event sensing threshold values may be established for applying during the passive filling phase window and after the passive filling phase window (during an active filling phase window).

Atrial event detector circuit240passes an atrial event detection signal to processor244and/or pace timing circuit242in response to detecting an atrial event. Processor244may include one or more clocks for generating clock signals that are used by pace timing circuit242to time out an AV pacing interval that is started upon receipt of an atrial event detection signal from atrial event detector circuit240. Pace timing circuit242may include one or more pacing escape interval timers or counters that are used to time out the AV pacing interval, which may be a programmable interval stored in memory210and retrieved by processor244for use in setting the AV pacing interval used by pace timing circuit242. Other examples of atrial event sensing or detection for use in controlling atrial synchronized ventricular pacing by an intracardiac ventricular pacemaker are generally disclosed in commonly assigned U.S. Pat. No. 9,399,140 (Cho, et al.), U.S. Pat. No. 10,328,270 (Demmer, et al) and U.S. Pat. No. 10,350,317 (Cao, et al.), all of which are incorporated herein by reference in their entirety.

Pace timing circuit242(or processor244) may additionally receive R-wave sensed event signals from cardiac event detector224for use in controlling the timing of pacing pulses delivered by pulse generator202. Pace timing circuit242may include a lower pacing rate interval timer for controlling a lower ventricular pacing rate. For example, if an atrial systolic event is not detected from the motion sensor signal triggering a ventricular pacing pulse at the programmed AV pacing interval, a ventricular pacing pulse may be delivered by pulse generator202upon expiration of the lower pacing rate interval to prevent ventricular asystole and maintain a minimum ventricular rate. In order to avoid abrupt changes in ventricular rate, control circuit206may be configured to set the lower ventricular pacing rate interval to a rate smoothing interval during the atrial synchronous ventricular pacing mode and/or upon switching to the atrial synchronous ventricular pacing mode from an asynchronous ventricular pacing mode. The rate smoothing interval may be determined based on one or more preceding ventricular event intervals. For example, a ventricular pacing pulse delivered in the absence of a sensed atrial event during VDD pacing may be delivered at an interval that is within a predetermined interval of preceding Vpace-to-Vpace intervals or a median RR interval, e.g., within 150 ms or within 100 ms of the actual preceding ventricular rate interval(s).

At times, control circuit206may control pulse generator202in an asynchronous ventricular pacing mode, e.g., for checking for AV conduction and as long as AV block is not detected. During the asynchronous ventricular pacing mode, pace timing circuit242may set a VV pacing interval to a base pacing rate interval corresponding to a programmed minimum base rate, which may be 60 pulses per minute or less, e.g., 40 pulses per minute. As further described below, control circuit206may remain in the asynchronous pacing mode as long as AV block detection criteria remain unsatisfied. If AV block is detected, however, control circuit206may switch back to the atrial synchronous ventricular pacing mode to promote AV synchrony. At times, pacemaker14may adjust the VV pacing interval during asynchronous ventricular pacing to a temporary pacing interval set based on a patient physical activity metric to provide rate responsive ventricular pacing that supports the metabolic demand of the patient.

Control circuit206may determine the patient activity metric from the motion signal received from motion sensor212at a desired frequency for use in determining a sensor-indicated pacing rate (SIR). The SIR may vary between the programmed minimum base rate during periods of rest (minimal activity metric) and a maximum upper pacing rate during periods of maximum exertion. The SIR may be controlled according to a SIR transfer function as described below, which may include different rates of change of the SIR over different ranges of the activity metric.

In some examples, the activity metric is determined as an activity count. In these instances, control circuit206includes a counter to track the activity count as the number of times the signal from motion sensor212crosses a threshold during an activity count interval, for example a 2-second interval. The count at the end of each activity count interval is correlated to patient body motion during the activity count interval and is therefore correlated to patient metabolic demand. The threshold applied to the motion sensor signal, which when crossed by the motion sensor signal causes the activity count to be increased, may be a default or programmable threshold or may be an automatically adjusted threshold. Example methods for obtaining an activity count over an n-second interval and for adjusting the motions sensor signal threshold used for obtaining the activity count are generally disclosed in U.S. Pat. No. 5,720,769 (van Oort), incorporated herein by reference in its entirety.

In other examples, an activity metric may be obtained from the motion sensor signal by integrating or summing motion signal sample points over an activity count interval, e.g., a two-second interval though longer or shorter intervals of time may be used for determining an activity metric. The activity metric may be converted to a target heart rate to meet the patient's metabolic demand. The target heart rate may be converted to a SIR based on a SIR transfer function that includes a base pacing rate set point and an activities of daily living (ADL) range. As long as the activity metric is at or below the base pacing rate set point, the SIR remains at the base pacing rate.

As the activity count increases above the base pacing rate set point, the SIR may be determined according to the SIR transfer function slope or profile up to the ADL range. As long as the patient activity metric (and resulting target heart rate) remains between a lower and upper boundary of the ADL range, the SIR is set to an ADL rate, which is greater than the base pacing rate and is expected to provide adequate pacing support to the patient during normal daily activities, such as moving about the home, driving a car, light tasks, etc.

If the activity metric and resultant target heart rate rises to be greater than the ADL range, the SIR is increased according to a slope or profile of the SIR transfer function over the range from the upper boundary of the ADL range to reach the target heart rate, up to the maximum upper rate set point. The SIR is set to the maximum upper pacing rate for all activity metrics greater than the maximum upper rate set point. Each of the base pacing rate set point, the ADL range and the maximum upper rate set point may be tailored to a patient's particular needs based on activity metric history. In order to avoid abrupt changes in pacing rate, the target heart rate may be determined from the patient activity metric, and the SIR may be determined from the target rate according to the transfer function that controls how quickly the SIR accelerates or decelerates up to or down to the target rate as patient activity increases or decreases, respectively. Examples of methods for establishing a SIR transfer function applied to patient activity metrics determined from an intraventricular motion signal are generally disclosed in U.S. Pat. No. 9,724,518 (Sheldon, et al.), incorporated herein by reference in its entirety.

Other types of sensors that may produce a signal correlated to patient activity include sensors of respiratory activity, such as minute ventilation, blood or tissue oxygen saturation, as examples. Other types of patient physical activity sensors may be used for providing control circuit206with a signal correlated to metabolic demand for use in determining a SIR and enabling rate responsive pacing. Various examples of other types of implantable sensors that may be implemented with a rate responsive pacemaker for controlling pacing rate based on metabolic demand are generally described in U.S. Pat. No. 5,755,740 (Nappholz), U.S. Pat. No. 5,507,785 (Deno), and U.S. Pat. No. 5,312,454 (Roline). The techniques disclosed herein for controlling a rate responsive asynchronous ventricular pacing mode may be used in combination with any type of patient physical activity sensor that produces a signal that indicates patient activity level correlated to metabolic demand.

Processor244may retrieve programmable pacing control parameters, such as pacing pulse amplitude and pacing pulse width, which are passed to pulse generator202for controlling pacing pulse delivery from memory210. In addition to providing control signals to pace timing circuit242and pulse generator202for controlling pacing pulse delivery, processor244may provide sensing control signals to sensing circuit204(e.g., R-wave sensing threshold, P-wave sensing threshold, sensitivity, and/or various blanking and refractory intervals applied to the cardiac electrical signal) and to atrial event detector circuit240for sensing atrial events from the motion sensor signal as described below.

Pulse generator202generates electrical pacing pulses that are delivered to the RV of the patient's heart via cathode electrode164and return anode electrode162. Pulse generator202may include charging circuit230, switching circuit232and an output circuit234. Charging circuit230may include a holding capacitor that may be charged to a pacing pulse amplitude by a multiple of the battery voltage signal of power source214under the control of a voltage regulator. The pacing pulse amplitude may be set based on a control signal from control circuit206. Switching circuit232may control when the holding capacitor of charging circuit230is coupled to the output circuit234for delivering the pacing pulse. For example, switching circuit232may include a switch that is activated by a timing signal received from pace timing circuit242upon expiration of a pacing interval, e.g., an AV pacing interval, a VV rate smoothing interval, a SIR interval, or VV base pacing rate interval, and kept closed for a programmed pacing pulse width to enable discharging of the holding capacitor of charging circuit230. The holding capacitor, previously charged to the pacing pulse voltage amplitude, is discharged across electrodes162and164through the output capacitor of output circuit234for the programmed pacing pulse duration. Examples of pacing circuitry generally disclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.) and in commonly assigned U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both of which patents are incorporated herein by reference in their entirety, may be implemented in pacemaker14for charging a pacing capacitor to a predetermined pacing pulse amplitude under the control of control circuit206for generating and delivering a pacing pulse.

Memory210may include computer-readable instructions that, when executed by control circuit206, cause control circuit206to perform various functions attributed throughout this disclosure to pacemaker14. The computer-readable instructions may be encoded within memory210. Memory210may include any non-transitory, computer-readable storage media including 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 other digital media. Memory210may store timing intervals and other data used by control circuit206to control the delivery of pacing pulses by pulse generator202according to the techniques disclosed herein.

Power source214may correspond to battery subassembly160shown inFIG.2and provides power to each of the other circuits and components of pacemaker14as required. Power source214may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source214and other pacemaker circuits and components are not shown inFIG.3for the sake of clarity but are to be understood from the general block diagram ofFIG.3. For example power source214may provide power to charging circuit230for charging a holding capacitor to a pacing voltage amplitude, current to switch232and other circuitry included in pulse generator202as needed to generate and deliver pacing pulses. Power source214also provides power to telemetry circuit208, motion sensor212, and sensing circuit204as needed as well as memory210.

Telemetry circuit208includes a transceiver209and antenna211for transferring and receiving data, e.g., via a radio frequency (RF) communication link. Telemetry circuit208may be capable of bi-directional communication with external device20(FIG.1) as described above. Motion sensor signals and cardiac electrical signals, and/or data derived therefrom may be transmitted by telemetry circuit208to external device20. Programmable control parameters and programming commands for performing atrial event detection and ventricular pacing control according to the techniques disclosed herein may be received by telemetry circuit208and stored in memory210for access by control circuit206.

The functions attributed to pacemaker14herein may be embodied as one or more processors, controllers, hardware, firmware, software, or any combination thereof. Depiction of different features as specific circuitry is intended to highlight different functional aspects and does not necessarily imply that such functions must be realized by separate hardware, firmware or software components or by any particular circuit architecture. Rather, functionality associated with one or more circuits described herein may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, atrial systolic event detection from the motion sensor signal and ventricular pacing control operations performed by pacemaker14may be implemented in control circuit206executing instructions stored in memory210and relying on input from sensing circuit204and motion sensor212.

The operation of circuitry included in pacemaker14as disclosed herein should not be construed as reflective of a specific form of hardware, firmware and software necessary to practice the techniques described. It is believed that the particular form of software, hardware and/or firmware will be determined primarily by the particular system architecture employed in the pacemaker14and by the particular sensing and therapy delivery circuitry employed by the pacemaker14. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern pacemaker, given the disclosure herein, is within the abilities of one of skill in the art.

FIG.4is an example of a motion sensor signal250that may be produced by motion sensor212over a cardiac cycle. Vertical dashed lines252and262denote the timing of two consecutive ventricular events (an intrinsic ventricular depolarization or a ventricular pace), marking the respective beginning and end of the ventricular cycle251. The motion signal includes an A1 event254, an A2 event256, an A3 event258and an A4 event260. The A1 event254is an acceleration signal (in this example when motion sensor212is implemented as an accelerometer) that occurs during ventricular contraction and marks the approximate onset of ventricular mechanical systole. The A1 event is also referred to herein as a “ventricular contraction event.” The A2 event256is an acceleration signal that may occur during closure of the aortic and pulmonic valves and marks the approximate offset or end of ventricular mechanical systole. The A2 event may also mark the beginning of ventricular diastole and is generally an indication of the isovolumic relaxation phase of the ventricles that occurs with aortic and pulmonic valve closure. The A3 event258is an acceleration signal that occurs during passive ventricular filling and marks ventricular mechanical diastole. The A3 event is also referred to herein as the “ventricular passive filling event.”

The A4 event260is an acceleration signal that occurs during atrial contraction and active ventricular filling and marks atrial mechanical systole. The A4 event260may also referred to herein as the “atrial systolic event” or merely the “atrial event,” and is the atrial systolic event that is detected from motion sensor signal250by atrial event detector circuit240for controlling pace timing circuit242to trigger ventricular pacing pulse delivery by starting an AV pacing interval in response to detecting the A4 event260. In some examples, control circuit206may be configured to detect one or more of the A1, A2, and A3 events from motion sensor signal250, for at least some ventricular cardiac cycles, for use in positively detecting the A4 event260and setting atrial event detection control parameters. The A1, A2 and/or A3 events may be detected and characterized to avoid false detection of A4 events and promote reliable A4 event detection for proper timing of atrial-synchronized ventricular pacing pulses.

FIG.5depicts example motion sensor signals400and410acquired over two different cardiac cycles. A ventricular pacing pulse is delivered at time 0.0 seconds for both cardiac cycles. The top sensor signal400is received over one cardiac cycle and the bottom sensor signal401is received over a different cardiac cycle. The two signals400and410are aligned in time at 0.0 seconds, the time of the ventricular pacing pulse delivery. While motion signals400and410and motion signal250ofFIG.4are shown as raw accelerometer signals, it is recognized that control circuit80may receive a filtered, amplified and rectified signal from motion sensor212for detecting atrial events by atrial event detector circuit240.

The A1 events402and412of the respective motion sensor signals400and410, which occur during ventricular contraction, are observed to be well-aligned in time following the ventricular pacing pulse at time 0.0 seconds. Similarly, the A2 events404and414and the A3 events406and416(occurring during passive ventricular filling) are well-aligned in time. Since the A1, A2 and A3 events are ventricular events, occurring during ventricular contraction, ventricular isovolumic relaxation, and passive ventricular filling, respectively, these events are expected to occur at relatively consistent intervals following a ventricular electrical event, the ventricular pacing pulse in this example, and relative to each other. The time relationship of the A1, A2 and A3 events may be different following a ventricular pacing pulse compared to following a sensed intrinsic R-wave; however, during a stable paced or intrinsic ventricular rhythm, the relative timing of A1, A2 and A3 events to each other and the immediately preceding ventricular electrical event is expected to be consistent.

The A4 events408and418of the first and second motion sensor signals400and410respectively are not aligned in time. The A4 event occurs due to atrial systole and as such the time interval to the A4 event following the immediately preceding ventricular electrical event (sensed R-wave or ventricular pacing pulse) and the preceding A1 through A3 events may vary between cardiac cycles.

The consistency of the timing of the A1 through A3 events relative to each other and the immediately preceding ventricular electrical event may be used for determining an atrial blanking period436and increasing confidence in reliably detecting A4 events408and418. The atrial systolic event is not detected during the atrial blanking period436which extends from the ventricular electrical event (at time 0.0) to an estimated onset of ventricular diastole, for example. An A3 sensing window424may be set having a starting time420corresponding to the end of the atrial blanking period436and an ending time422. The atrial blanking interval436may be 600 ms, as one example with no limitation intended, and the A3 window424may extend 200 ms or other selected time interval after the atrial blanking interval436.

A4 events408and418may be detected based on a multi-level A4 detection threshold444. As seen by the lower motion sensor signal410, the A4 event418may occur earlier after the A3 window424due to changes in atrial rate. In some instances, as the atrial rate increases, the A4 event418may occur within the A3 window424. When this occurs, the A3 event416and the A4 event418may fuse as passive and active ventricular filling occur together. The fused A3/A4 event may have a high amplitude, even greater than the amplitude of either the A3 event416or the A4 event418when they occur separately. As such, in some examples the A4 detection threshold444includes a first, higher A4 threshold amplitude446established for detecting an early A4 event that is fused with the A3 event during the A3 window424. A second, lower A4 threshold amplitude448may be established for detecting relatively later A4 events, after the ending time422of the A3 window424. An A4 window445may extend from the end of the A3 window424until an atrial event is sensed or a ventricular event occurs, whichever occurs first. The earliest crossing of the A4 detection threshold444by the motion sensor signal after the starting time420of the A3 window (or after the expiration of the atrial blanking period436) may be sensed as the atrial systolic event. Various examples of an intracardiac pacemaker configured to detect atrial systolic events from a motion sensor signal for delivering atrial synchronous ventricular pacing are disclosed in commonly-assigned U.S. Publication No. 2018/0085589 (Splett et al.), U.S. Pat. No. 10,449,366 (Splett, et al.), U.S. Pat. No. 10,286,214 (Demmer, et al.), U.S. Pat. No. 10,207,116 (Sheldon, et al.), and U.S. Pat. No. 10,328,270 (Demmer, et al.), all of which are incorporated herein by reference in their entirety. The techniques disclosed herein for controlling the pacing mode for promoting AV conduction and minimize ventricular pacing may be implemented in any of the examples presented in the foregoing incorporated references.

FIG.6is a flow chart300of a method performed by pacemaker14for controlling pacing mode to promote AV conduction during periods of intact AV conduction and provide atrial synchronous ventricular pacing when AV conduction is blocked. At block302, control circuit206starts operation in an atrial synchronous ventricular pacing mode, e.g., a VDD pacing mode. The VDD pacing mode may be the programmed pacing mode of pacemaker14. During VDD pacing, ventricular pacing is synchronized to sensed atrial events promoting AV synchrony even in the presence of AV conduction block. Minimum ventricular pacing may be enabled in the VDD pacing mode to allow mode switching to an asynchronous ventricular pacing mode including a very low base ventricular pacing rate, e.g., 60 pulses per minute or less, 50 pulses per minute or less, 40 pulses per minute or less or even 30 pulses per minute, to allow AV conduction to occur along natural conduction pathways when the atrial rate is greater than the base ventricular pacing rate and AV conduction is intact, thereby inhibiting ventricular pacing.

In order to control when switching to the asynchronous ventricular pacing mode occurs, control circuit206starts a conduction check timer at block304. The conduction check timer may be set to a starting time period, e.g., one minute, five minutes or other selected starting time period, that is relatively short so that a conduction check is initially performed after a relatively short time period, e.g., 10 minutes or less, of atrial synchronous ventricular pacing. In other examples, the conduction check timer may be set to relatively longer starting time period, e.g., 15 minutes, 30 minutes, one hour, several hours, one day, or other selected time period.

While the conduction check time period is running, pacemaker14operates in the atrial synchronous pacing mode by sensing an atrial event at block306, setting an AV pacing interval at block308, and delivering a ventricular pacing pulse at block310upon the expiration of the AV interval at block310. As described above in conjunction withFIG.5, the atrial event may be sensed by control circuit206from the motion signal received from motion sensor212. Atrial synchronous ventricular pacing may continue in this manner until the conduction check timer expires at block312.

In response to the timer expiring at block312, control circuit206switches the pacing mode from the atrial synchronous pacing mode to an asynchronous pacing mode, e.g., VVI or VDI pacing mode, at block314. During the asynchronous pacing mode, the base ventricular pacing rate may be set to a relatively low rate, e.g., 40 pulses per minute, in order to promote conduction of atrial depolarizations to the ventricles before a ventricular pacing pulse is scheduled to be delivered at the base pacing rate interval (VV pacing interval). The base ventricular pacing rate may be a fixed value or a user-programmable value. If a ventricular event is not sensed, e.g., if control circuit206does not receive an R-wave sensed event signal from sensing circuit204before the VV pacing interval corresponding to the base pacing rate expires, control circuit206controls pulse generator202to deliver a ventricular pacing pulse. Ventricular pacing is delivered at the base rate in the absence of an R-wave sensed event signal to avoid asystole and provide ventricular pacing support at the base pacing rate if the patient is experiencing AV block.

At block316, control circuit206determines if AV block detection criteria are met based on ventricular events. AV block may be detected based on the frequency of ventricular pacing delivered after switching to the asynchronous pacing mode. During the asynchronous pacing mode, atrial event sensing may be disabled or ignored in some examples. Disabling atrial event sensing from the motion signal during the asynchronous pacing mode may extend the useful life of pacemaker14compared to the pacemaker longevity when atrial event sensing continues to be enabled during the asynchronous pacing mode. Pacemaker longevity may be extended by disabling atrial event sensing during the asynchronous pacing mode to conserve the power that would normally be required by motion sensor212for producing the motion signal for atrial event sensing. As such, the decision at block316as to whether AV block is being detected may be based solely on ventricular events, paced or sensed, without requiring atrial event sensing. Control circuit206may identify ventricular events, paced and/or sensed, and compare the number of paced and sensed ventricular events to AV block criteria without sensing or identifying atrial events. If ventricular pacing occurs at the base pacing rate, a conducted depolarization has not occurred so that the ventricular pacing pulse delivery is evidence of AV block. In one example, if at least X pacing pulses are delivered out of Y consecutive ventricular events, AV block detection criteria are met at block316. For instance, if two pacing pulses are delivered during four consecutive ventricular events, AV block is detected at block316. In other examples, if four paced events out of eight consecutive ventricular events, two out of two, or other X of Y (where X is less than or equal to Y) criteria are reached, AV block is detected at block316. The AV block detection criteria may require as few as one ventricular pacing pulse delivery at the base rate for detecting AV block and may require two or more ventricular pacing pulses in other examples. The AV block detection criteria may be programmable and may depend at least in part on an individual patient's conduction history.

AV block detection criteria are not met when R-wave sensed event signals are received by control circuit206at a rate greater than the asynchronous ventricular base pacing rate such that the frequency of ventricular pacing pulses required to detect AV block is not reached. As long as AV block detection criteria are not met at block316, control circuit206continues operating in the asynchronous pacing mode at block314. Control circuit206may remain in the asynchronous pacing mode until AV block detection criteria are met. Ventricular pacing pulses may be delivered occasionally at the asynchronous base pacing rate as needed. If the AV block detection criteria are satisfied, e.g., if at least 2 out of four consecutive ventricular events are ventricular pacing pulses, control circuit206switches back to the synchronous pacing mode at block302to provide atrial synchronous ventricular pacing. If previously disabled, atrial event sensing from the motion sensor signal may be re-enabled upon switching from the asynchronous pacing mode back to the atrial synchronous pacing mode.

FIG.7is a graph450of an example accelerometer signal452, electrocardiogram signal454with event markers and ventricular EGM signal456during an atrial synchronous pacing mode. The accelerometer signal452is an example of a motion signal that is received by the control circuit206from the motion sensor212(shown inFIG.3). Accelerometer signal452is shown as a non-rectified signal inFIG.7but may be rectified by motion sensor212or control circuit206for sensing atrial (A4) events. The A1, A2, A3 and A4 events, as described in conjunction withFIG.4, are denoted along the motion signal452.

The “VE” markers shown along ECG signal454indicate the end of the A3 window, e.g., corresponding to ending time422shown inFIG.5. The “AS” markers indicate the time of an atrial sensed event, e.g., when the A4 signals of accelerometer signal452cross the A4 sensing threshold amplitude. The “VP” markers indicate a ventricular pacing pulse delivered upon expiration of an AV interval set upon sensing the atrial events. Pacing-evoked R-waves (labeled “R”) are observed on EGM signal456. The signals452,454and456represent appropriate atrial synchronous ventricular pacing, e.g., during a VDD pacing mode, which may be used to control single chamber ventricular pacing during AV conduction block.

FIG.8is a flow chart500of a method for controlling single chamber ventricular pacing modes to promote AV conduction and to provide atrial synchronous ventricular pacing during periods of AV block according to another example. The control circuit206starts a conduction check timer at block502. Initially, the conduction check timer may be set to a relatively short time period, e.g., one minute. At block504, pacemaker14operates in the atrial synchronous pacing mode by sensing atrial events, e.g., from the signal received from motion sensor212as described above, and delivering ventricular pacing pulses at an AV pacing interval following each sensed atrial event.

Control circuit206may determine whether the conduction check timer has expired at block506. If conduction check criteria are not met at block506during the atrial synchronous pacing mode, e.g., the conduction check timer is still running and/or other criteria remain unmet as described below, control circuit206may determine if rate response criteria are met at block518. Control circuit206may determine the patient activity metric from the motion signal at block518for determining if the rate response criteria are met. Examples of rate response criteria that may be applied at block518are given below in conjunction with block521. If rate response criteria are met, control circuit206may switch to a rate responsive asynchronous ventricular pacing mode (e.g., VVIR or VDIR) at block522to provide asynchronous ventricular pacing at a SIR determined based on the patient activity metric to support the patient during increased physical activity. The VV pacing interval may initially be set to match the actual ventricular rate to avoid an abrupt change in ventricular rate upon switching to the rate responsive asynchronous pacing mode. For instance, control circuit206may determine an actual ventricular rate interval as the mean or median Vpace-to-Vpace interval (or RR interval) from a predetermined number of most recent ventricular cycles and set the initial VV pacing interval to the actual ventricular rate interval. The VV pacing interval may be adjusted from this initial VV pacing interval to a target heart rate interval according to the SIR to provide rate responsive pacing at block522.

The conduction check timer started at block502may expire during the atrial synchronous pacing mode or during the rate responsive, asynchronous pacing mode. The conduction check is initiated by switching to a non-rate response asynchronous ventricular pacing mode. Therefore, if the conduction check timer expires during the rate response pacing mode, the conduction check is delayed until rate responsive pacing is no longer needed and control circuit206has switched back to the atrial synchronous pacing mode. As such, during the rate response asynchronous ventricular pacing mode at block522, control circuit206determines if mode switch criteria are met at block524. Examples of the mode switch criteria that may be applied at block524are described below but generally require a decrease in the patient activity metric, target heart rate, and/or SIR to below a threshold level indicating that rate responsive pacing is no longer required to support the patient's metabolic demand. When the mode switch criteria are met at block524, control circuit206switches from the rate response asynchronous ventricular pacing mode back to the atrial synchronous pacing mode at block504.

When the conduction timer expires (or has already expired), indicating it is time for a conduction check as determined at block506, control circuit206may switch from the atrial synchronous pacing mode to the asynchronous ventricular pacing mode at block508. Atrial event sensing may be disabled or ignored during the asynchronous pacing mode. Only ventricular events may be used to determine if AV conduction is present or if AV block is detected.

In some examples, the switch to the asynchronous pacing mode at block508occurs only when the conduction check timer is expired and other conduction check criteria are satisfied. For instance, the patient activity metric, target heart rate or SIR may be required to be less than a threshold. The patient activity metric, target heart rate and/or SIR may be determined during the atrial synchronous pacing mode even though it is not used by pacemaker14for controlling the ventricular pacing rate. The SIR may be compared to the programmed lower pacing rate plus an increment rate during atrial synchronous pacing. The programmed lower pacing rate may range from 30 pulses per minute to 60 pulses per minute and the predetermined increment may be 5 to 10 pulses per minute, as examples. If the SIR is not less than or equal to the programmed base pacing rate plus the increment at the time that the conduction check timer expires, the control circuit206may wait for the SIR to fall to less than or equal to the threshold rate.

In other examples, conduction check criteria required at block506may include sensing at least one ventricular event following a sensed atrial event while in the atrial synchronous pacing mode. For instance, if the conduction check timer has expired (and the patient activity metric, target heart rate and/or SIR are less than a threshold), control circuit206may extend the AV pacing interval for one or more cardiac cycles to determine if an intrinsic R-wave sensed event signal is received from sensing circuit204before the extended AV pacing interval expires. The AV pacing interval may be extended from 10 ms to 100 ms, 150 ms, 200 ms or more to determine if an intrinsic R-wave follows the sensed atrial event within an expected AV conduction time, indicating AV conduction may be intact. In another example, the ventricular pacing pulse may be withheld for one cardiac cycle to determine if an intrinsic R-wave is sensed. The ventricular pacing pulse may be withheld by setting the AV pacing interval to a maximum time interval. If AV conduction evidence is detected based on one or more sensed R-waves during the atrial synchronous pacing mode, the switch to the asynchronous pacing mode is made at block508. If an R-wave is not sensed during the extended AV interval, the control circuit206may remain in the atrial synchronous pacing mode in some examples.

At block510, control circuit206determines if AV block detection criteria are satisfied during the asynchronous pacing mode. The base pacing rate during the asynchronous pacing mode may be set to a minimum or relatively low base pacing rate, e.g., 30, 40, or 50 pulses per minute. In other examples, the base pacing rate may be set relatively higher, e.g., 60 pulses per minute, and may be decreased at predetermined time intervals during the asynchronous pacing mode to determine if AV conduction block criteria are satisfied at block510. In one example if X out of Y ventricular events are ventricular pacing pulses during the asynchronous pacing mode, e.g., if at least two out of four ventricular events are ventricular pacing pulses, AV conduction block is detected, and control circuit206switches back to the atrial synchronous pacing mode (return to block502). In this example, if two consecutive ventricular pacing pulses or if two pacing pulses are delivered separated by one sensed R-wave, the X of Y criteria may be determined to be met without having to wait for a fourth ventricular event to occur.

When AV block is detected at block510relatively early after switching to the asynchronous pacing mode, control circuit206may increase the conduction check time period at block512. For instance, the conduction check time period may be doubled from the previous conduction check time period. If the timer is initially set to one minute at block502, the timer may be set to two minutes at block512and restarted at block502upon switching back to the atrial synchronous pacing mode at block504responsive to detecting AV block. Each time AV conduction block is detected relatively early after switching to the asynchronous pacing mode, resulting in control circuit206switching back to the atrial synchronous pacing mode, the conduction check time period may be increased, e.g., doubled, up to a maximum conduction check time period. In this way, frequent conduction checks and pacing mode switching is avoided if AV block is detected within a predetermined time period or number of ventricular events after switching to the asynchronous pacing mode, e.g., within one minute or less or within 30 ventricular events or less. In one example, the conduction check time period is doubled each time AV conduction block is detected within the first 20 seconds or the first 20 ventricular events after switching to the asynchronous pacing mode.

In other examples, the conduction check time period is increased, e.g., doubled, each time control circuit206switches to the asynchronous ventricular pacing mode from the atrial synchronous ventricular pacing mode in response to the conduction check timer expiring. When AV block is detected within a threshold number of ventricular cycles, e.g., 20 cycles, in the asynchronous ventricular pacing mode (or within a specified time interval), the conduction check timer remains at the increased setting upon switching back to the atrial synchronous ventricular pacing mode. When AV block is detected after the threshold number of ventricular cycles, the conduction check time period is reset to the minimum conduction check time period, e.g., 1 minute, upon switching back to the atrial synchronous ventricular pacing mode. The conduction check timer may be increased, e.g., doubled, each time control circuit206switches to the asynchronous pacing mode up to a maximum conduction check time period, after which the conduction check time period is no longer increased unless reset again to the minimum time period. The maximum conduction check time period may be 12 hours, 16 hours, 20 hours, or 24 hours or longer, as examples.

If the threshold number of X of Y ventricular events being ventricular pacing pulses is not reached at block510, and a predetermined time interval (e.g., 20 seconds) or a predetermined number of ventricular events (e.g., 20 ventricular events) has been reached (“yes” branch of block514), the conduction check time period may be reset back to the minimum time period, e.g., one minute, at block516. When the AV block detection criteria are not satisfied relatively early during the asynchronous pacing mode, e.g., during the first 20 seconds or first 20 ventricular events (or other predetermined interval), indicating AV conduction is occurring, more frequent AV conduction checks are warranted upon switching back to the atrial synchronous pacing mode to promote AV conduction along intrinsic conduction pathways and minimize ventricular pacing. When X ventricular paces out of Y ventricular events are detected after the conduction check time period is reset to a minimum (“yes” branch of block520), control circuit206switches back to atrial synchronous ventricular pacing by returning to block502. The conduction check timer is restarted at the minimum conduction check time period, and atrial synchronous pacing resumes at block504.

As long as the criteria of X ventricular paces out of Y ventricular events (or other AV block detection criteria) are not satisfied (“no” branch of block520), AV block is not detected, and control circuit206remains in the asynchronous pacing mode with a base pacing rate set to a relatively low rate, e.g., 40 pulses per minute, to promote AV conduction along intrinsic conduction pathways. During the asynchronous pacing mode, control circuit206may determine a patient activity metric from the motion signal received from motion sensor212. The patient activity metric is used to determine a target heart rate and SIR for determining if rate responsive pacing is needed due to an increase in patient activity. The patient activity metric, target heart rate, and/or SIR may be compared to rate response switching criteria at block521. In one example, the rate response switching criteria applied at block521(and at block518) requires that a pacing rate determined based on patient activity be greater than the ADL rate and greater than the actual ventricular rate plus a rate increment for a predetermined minimum time interval, e.g., at least 10 seconds. The pacing rate determined based on patient activity may be the target heart rate. In other examples, the SIR determined from the target heart rate, e.g., by applying a transfer function to the target heart rate, may be compared to the rate response switching criteria. The rate increment added to the actual ventricular rate may be 20 or 30 pulses per minute as examples. The actual ventricular rate may be determined as the rate corresponding to a median RR interval (RRI). Each RRI is determined by control circuit206as the time interval between two consecutive R-wave sensed event signals received from sensing circuit204. The median RRI is determined from a predetermined number of RRIs, e.g., from ten RRIs. In some examples, paced ventricular intervals may be included in the RRIs used for determining the median RRI at block521in cases where ventricular paces are delivered but less frequently than the X of Y criteria for detecting AV block.

If the patient activity metric, target heart rate or corresponding SIR does not satisfy rate response criteria at block521(“no” branch), control circuit206continues to operate in the asynchronous pacing mode and monitors for AV block based on the criteria of X of Y ventricular events being ventricular pacing pulses at block520. Control circuit206may return to block514to determine if Z seconds (or a predetermined number of ventricular cycles) has elapsed since entering the asynchronous pacing mode. It is recognized that once Z seconds (or a predetermined number of ventricular cycles) have elapsed after switching to the asynchronous pacing mode and the conduction time period has been reset back to the minimum time period at block516, control circuit206does not need to repeat the operations at blocks514and516again while still in the asynchronous pacing mode.

If the rate response criteria are satisfied at block521, control circuit206switches to a rate responsive asynchronous ventricular pacing mode, e.g., VVIR or VDIR pacing mode, at block522. In one example, if the target heart rate determined from the patient activity metric is more than the ADL rate and more than 20 pulses per minute (or other predetermined rate increment) greater than the actual ventricular rate (which may be determined as the median RRI), control circuit206switches to a rate responsive asynchronous ventricular pacing mode, e.g., VVIR or VDIR, at block522to provide ventricular rate support during patient activity. In order to avoid an abrupt change in ventricular rate, control circuit206may set the VV pacing interval to the actual ventricular rate, e.g., a most recently determined average or median RRI, upon switching to the rate responsive asynchronous pacing mode. The SIR may then be adjusted up to the target heart rate according to a transfer function to gradually adjust the pacing rate to the target heart rate based on patient physical activity.

During the rate responsive asynchronous pacing mode at block522, control circuit206continues to determine the SIR based on a patient activity metric and corresponding target heart rate determined from the motion signal. The pacing rate is controlled according to the SIR, and the patient activity metric, target heart rate and/or SIR may be compared to pacing mode switching criteria at block524. Control circuit206may monitor the patient activity metric, the target heart rate and/or SIR to determine when the patient's activity level has decreased to a point that rate responsive pacing is no longer needed. As long as the target heart rate and/or SIR remain greater than a rate threshold (“no” branch of block524), control circuit206remains in the asynchronous pacing mode (block522). In one example, the rate threshold is the ADL rate though other rate thresholds may be defined for comparison to the target heart rate and to the SIR. In an illustrative example, the mode switching criteria may be met at block524when the target heart rate is less than the ADL rate for at least ten seconds (or other predetermined time period or number of ventricular cycles) and the SIR is less than the ADL rate. When the mode switch criteria are met at block524, control circuit206switches back to the atrial synchronous pacing mode at block504, with atrial event sensing re-enabled (if previously disabled). In order to avoid abrupt changes in ventricular rate upon switching back to the atrial synchronous pacing mode from the rate responsive asynchronous pacing mode, the VV pacing interval may be gradually increased from the last SIR interval until atrial synchronous ventricular pacing takes over in the atrial synchronous pacing mode.

The conduction check timer may still be running upon switching back to the atrial synchronous pacing mode or may have expired while operating in the rate response pacing mode at block522. If the conduction check timer has expired, and other AV conduction check criteria are satisfied at block506(as described above), control circuit206may switch to the asynchronous pacing mode at block508to monitor for ventricular pacing frequency as evidence of AV block at block510. If the conduction check timer is still running, control circuit206continues to operate in the atrial synchronous pacing mode until the timer expires.

FIG.9is a flow chart600of a method for switching from an asynchronous pacing mode back to the atrial synchronous pacing mode according to some examples. Control circuit206may switch from the atrial synchronous pacing mode (block601) to an asynchronous pacing mode (block602) upon expiration of a conduction check timer as described above. At other times, control circuit206may switch from the atrial synchronous pacing mode (block601) to an asynchronous rate responsive pacing mode when ventricular rate support is needed based on rate response criteria being met as described in conjunction withFIG.8. Accordingly, the asynchronous pacing mode at block602may be a rate responsive mode, e.g., VVIR, or a non-rate responsive mode, e.g., VVI, with a lower base pacing rate of 40 pulses per minute (or other base pacing rate). When control circuit206switches from the atrial synchronous to the asynchronous pacing mode, one or more adjustable control parameters may be buffered in memory210. During the atrial synchronous pacing mode, one or more auto-adjusting control parameters may be used in controlling A4 event sensing from the motion sensor signal and/or scheduling ventricular pacing pulses.

For example, the A3 window ending time422, the first value446of the A4 threshold amplitude444during the A3 window and the second value448of the A4 threshold amplitude444after the A3 window ending time may be automatically adjusted by control circuit206during the atrial synchronous pacing mode. For instance, the A3 window ending time422may be set based on a percentage of the ventricular cycle length (or an average, median or other metric of multiple ventricular cycle lengths). The first value446of the A4 threshold amplitude444may be set based on the motion sensor signal peak amplitude determined during one or more the A3 windows424and/or the motion sensor peak amplitude after the end the A3 window. The second value448of the A4 threshold amplitude444may be adjustable based on the maximum peak amplitude of the motion sensor signal after the A3 window, which may be associated with one or more sensed A4 events. The latest values of the A3 window ending time422, the first value446and the second value448of the A4 threshold amplitude may be buffered in memory210upon switching from the atrial synchronous pacing mode to the asynchronous pacing mode at block602.

As described above, control circuit206may adjust a rate smoothing interval based on one or more actual ventricular cycle lengths. For example, the rate smoothing interval may be set to be within 100 to 150 ms of one or more of the most recent actual ventricular cycle lengths to avoid an abrupt change in ventricular rate when an A4 event is not sensed. For instance, the rate smoothing interval may be set to a running average, median or other metric of one or more recent ventricular cycle lengths plus an increment of 100 to 150 milliseconds. In the absence of a sensed A4 event, a ventricular pacing pulse is delivered at a VV pacing interval set to the rate smoothing interval. The latest value of the rate smoothing interval may be buffered in memory210upon switching to the asynchronous pacing mode at block602.

While control circuit206is operating in the asynchronous pacing mode, at block602, a counter or timer included in control circuit206may track the total number of ventricular cycles, the total number of ventricular pacing pulses delivered, and/or the total time since switching to the asynchronous pacing mode. The total number of ventricular cycles, including paced and sensed ventricular cycles (when an intrinsic R-wave is sensed before the VV pacing interval expires), may be counted during the asynchronous pacing mode.

At block604, control circuit206determines when mode switch criteria are met for switching back to the atrial synchronous pacing mode. The mode switch criteria may be met at block604when AV conduction block is detected as described above. For example, when two ventricular pacing pulses are delivered out of four consecutive ventricular cycles. Other criteria, also described above, may also be required at block604in order to switch back to the atrial synchronous pacing mode, such as the patient activity metric, SIR and/or target rate being less than respective thresholds for a predetermined time interval.

When the mode switch criteria are met, control circuit206switches back to the atrial synchronous pacing mode at block606. Control circuit206checks the ventricular cycle counter at block608to determine if the asynchronous pacing mode was in effect for more than a threshold number of ventricular cycles. The threshold number of ventricular cycles may be 10 cycles, 20 cycles, 30 cycles, or 40 cycles, as examples. In other examples, control circuit206may compare a total number of ventricular pacing pulses, total number of sensed R-waves, and/or a total time of operation in the asynchronous pacing mode to respective thresholds at block608. A threshold time interval, for instance, may be 20 seconds, 30 seconds, one minute, five minutes or other selected time interval.

When the asynchronous pacing mode is in effect less than or equal to a threshold number of ventricular cycles, (or threshold time interval) the latest values buffered in memory210for one or more adjustable control parameters used by control circuit206during the atrial synchronous pacing mode may be restored at block612upon switching back to the atrial synchronous pacing mode. For instance, when the asynchronous pacing mode is in effect for 20 ventricular cycles or less, a previous value or setting of at least one control parameter that was in effect at the time that the pacing mode switched from the atrial synchronous pacing mode to the asynchronous pacing mode may be restored and continue to be used upon switching back to the atrial synchronous pacing mode.

Among the control parameters that may be restored at block612may be the rate smoothing interval, the A3 window ending time, the first, higher value of the A4 threshold amplitude applied during the A3 window and/or the second, lower value of the A4 threshold amplitude applied after the A3 window. The most recently stored values of these control parameters, e.g., at the time of switching from the atrial synchronous pacing mode to the asynchronous pacing mode, may be restored at block612. The control parameter values may be retrieved from memory210by control circuit206.

When the asynchronous pacing mode is in effect for more than 20 ventricular cycles (or other selected cycle number threshold or time threshold) at block608, control circuit210may establish new starting values for one or more control parameters at block610upon switching back to the atrial synchronous pacing mode. In one example, the rate smoothing interval is set to the currently programmed base pacing rate interval at block610. At the end of the previous atrial synchronous ventricular pacing mode, the rate smoothing interval may be at a value based on one or more actual ventricular cycle lengths. This rate smoothing interval may be restored when switching back to the atrial synchronous pacing mode within a threshold number of ventricular cycle lengths (block612). After the threshold number of ventricular cycle lengths in the asynchronous pacing mode, however, the rate smoothing interval may be reset based on the programmed lower ventricular pacing rate, e.g., to an interval of 1000 milliseconds if the lower ventricular pacing rate is programmed to 60 pulses per minute during the atrial synchronous pacing mode. The rate smoothing interval may be adjusted from the lower ventricular pacing rate interval as needed based on actual ventricular cycle lengths determined by control circuit206during the atrial synchronized pacing mode. In other examples, the starting value of the rate smoothing interval set at block610may be based one or more of the most recent ventricular cycle lengths ending the asynchronous pacing mode. For example, the rate smoothing interval may be set to the last ventricular cycle of the asynchronous pacing mode or to the last ventricular cycle of the asynchronous pacing mode plus an increment. In still other examples, the rate smoothing interval may be set based on a patient physical activity level or metric or the SIR determined based on the motion signal or another signal correlated to patient physical activity level.

In another example, at block610, a starting value of the A3 window ending time may be established upon switching back to the atrial synchronous pacing mode after the ventricular cycle count exceeds the threshold at block608. The A3 window ending time may be set to a percentage of the last paced ventricular cycle that ended the asynchronous pacing mode, as an example. The A3 window ending time may be updated after switching to the atrial synchronous pacing mode based on actual ventricular cycle lengths determined by control circuit206, e.g., a percentage of a mean or median ventricular cycle length determined from a specified number of ventricular cycles. In other examples, the initial A3 window ending time may be set to a fixed interval from the ending asynchronous ventricular pacing pulse upon switching back to the atrial synchronous pacing mode.

In still other examples, the starting first and/or second values of the A4 threshold amplitude may be established at block610when the asynchronous pacing mode has been in effect for greater than the threshold number of ventricular cycles at block608. In one example, the first value446of the A4 threshold amplitude446(during the A3 window424) may be set initially to a maximum upper limit, e.g., to 255 ADC units or about 25.5 m/s2. The second lower A4 threshold amplitude value448(applied after the A3 window ending time422shown inFIG.5) may be set to the most recently buffered second value of the A4 threshold amplitude, at the end of the previous atrial synchronous pacing mode, plus an offset. The buffered second value of the A4 threshold amplitude may range from 0.5 m/s2to 5.0 m/s2, for instance. The offset added to the buffered second value may be 0.1 to 1.0 m/s2and is 0.3 m/s2in one example. To illustrate, if the buffered second value of the A4 threshold amplitude is 2.0 m/s2, the starting value may be established at block610as 2.3 m/s2. The starting, second A4 threshold amplitude value may be limited to a maximum possible value, e.g., corresponding to a maximum range of an ADC of motion sensor212. When adding the offset to the buffered value causes the second A4 threshold amplitude value to exceed the maximum limit, the starting second A4 threshold amplitude value may be set to the maximum limit at block610.

In some examples, the first A4 threshold amplitude value may be established at block610based on the established second A4 threshold amplitude value. For instance, the first A4 threshold amplitude value may be set to a multiple of the second A4 threshold amplitude value. In other examples, the first A4 threshold amplitude value may be established after switching back to the atrial synchronous pacing mode by determining a maximum peak amplitude (A3 event amplitude) during a predetermined number of A3 windows. For instance, the first A4 threshold amplitude value (during the A3 windows) may be held at its maximum upper limit, e.g., 255 ADC units, for a predetermined number of ventricular cycles after switching back to the atrial synchronous pacing mode, e.g., for the first 8 ventricular cycles after switching back to the atrial synchronous pacing mode. Control circuit206may determine the peak amplitude of the motion sensor signal as the A3 event amplitude during each of the A3 windows for each one of the predetermined number of ventricular cycles. The median of the predetermined number of A3 event amplitudes may be used to adjust the first A4 threshold amplitude value from the maximum upper limit after the predetermined number of ventricular cycles. For example, the first A4 threshold amplitude value may be set to a multiple of the median A3 event amplitude or to the median A3 event amplitude plus an offset.

In still other examples, the first value of the A4 sensing threshold amplitude may be set based on a combination of the second A4 threshold amplitude value established at block610and one or more A3 event amplitudes determined after switching back to the atrial synchronous pacing mode. For instance, the first A4 sensing threshold amplitude value may be set to the second A4 sensing threshold amplitude value established at block610plus the product of the median A3 event amplitude (determined from a specified number of A3 windows) multiplied by a predetermined factor, e.g., a factor of 1.5 to 2. In some examples, an offset, e.g., 0 to 0.5 m/s2, may be added to the sum of the second A4 threshold amplitude value and the product of the median A3 event amplitude and the predetermined factor.

It is recognized that numerous variations may be conceived for establishing a starting value of one or more auto-adjusting control parameters used for sensing cardiac events and/or scheduling ventricular pacing pulses upon switching back to the atrial synchronous pacing rate when the asynchronous pacing mode has been in effect for greater than a threshold number of ventricular cycles (or threshold time interval). Furthermore, it is to be understood that a different threshold number of ventricular cycles may be applied to different control parameters for determining when to restore a buffered value of a control parameter vs. establishing a new, starting value of a control parameter. For example, a new rate smoothing interval may be established when the ventricular cycle count is greater than a first threshold, and new A4 sensing control parameters may be established when the ventricular cycle count is greater than a second threshold, different than the first threshold. When the asynchronous pacing mode is in effect for less than (or equal to) the first threshold number of ventricular cycles, all buffered values of control parameters may be restored at block612. When the asynchronous pacing mode is in effect for more than the first threshold number of ventricular cycles, the buffered rate smoothing interval may be discarded, and a new rate smoothing interval may be established at block610. The A4 sensing control parameters, however, e.g., the A3 window ending time and A4 threshold amplitude values, may be restored to the most recently buffered values at block612when the ventricular cycle count is greater than a first threshold but less than or equal to a second threshold number of ventricular cycles. When the asynchronous pacing mode is in effect for more than the second threshold number of ventricular cycles, the buffered values of auto-adjusting control parameters, including the rate smoothing interval and the A4 sensing control parameters, may be discarded and new starting values may be established at block610.

It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device.

In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements.

Thus, a pacemaker has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.