Method and apparatus for monitoring myocardial conduction velocity for diagnostics of therapy optimization

A cardiac stimulation device and method to measure a myocardial conduction time and to store its long-term running average. A multipolar lead is used to deliver a stimulation pulse from a tip electrode and detect the evoked response after it has propagated to a ring electrode. The time between the stimulation pulse and a detected feature of the evoked response is determined as the myocardial conduction time. A long-term average myocardial conduction time is calculated by averaging on the order of five hundred stimulated cardiac cycles, and a running average is stored in memory. Shifts in the myocardial conduction time may be used for monitoring disease progression or the long-term response to a treatment.

FIELD OF THE INVENTION

The present invention relates generally to an implantable cardiac stimulation device and, more specifically, to a method for storing a long-term average myocardial conduction velocity as a diagnostic measure of the physiological condition of the heart for use in monitoring heart disease or optimizing a delivered therapy.

BACKGROUND OF THE INVENTION

In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.

The intrinsic heart rate is primarily controlled by the sympathetic and parasympathetic components of the autonomic nervous system. Both components have nerve fibers terminating on the sinus node. Increased sympathetic activation (increased sympathetic tone) increases the heart rate as well as the conduction velocity of action potentials through the heart. Increased parasympathetic tone, also referred to as “vagal tone” since the parasympathetic nerves enter the heart via the vagus nerve, decreases the heart rate and decreases conduction velocity through the heart. Other factors such as circulating hormones and heart wall stretch will also influence heart rate and conduction. Though not fully understood, cardiovascular diseases and other physiological states may alter sympathetic tone, parasympathetic tone, and circulating hormonal levels and thus alter the heart tissue conduction velocity.

Disruption of the natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing. Implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart at a desired energy and rate via electrodes implanted in contact with the heart tissue. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.

Cardiac stimulation devices have a great number of adjustable parameters that must be tailored to a particular patient's therapeutic needs. The process of selecting the optimal parameter settings can be lengthy and complicated. Recent clinical evidence supports the use of multichamber stimulation devices for improving hemodynamic efficiency in patients with congestive heart failure. With an increasing number of indications for cardiac pacing, the number of programmable parameters required for tailoring individual patient therapies further complicates the programming process.

Without feedback on the effect of programmed parameters, the physician faces a challenge in selecting the most effective pacing regimen. It would be advantageous, therefore, to provide the physician with physiological data that reflects the effect of an applied therapy, whether the therapy is an implanted stimulation device or a drug therapy.

Observation of changes related to sympathetic and vagal tone, which are known to occur with certain disease processes, may be one way to monitor the response to a therapy. For example, high, relatively constant sympathetic tone and low vagal tone are known chronic conditions in patients with heart failure. Unusual circadian changes in myocardial conduction velocity may be observed in patients with heart failure. Long-term monitoring of myocardial conduction velocity, therefore, would allow conduction changes to be detected that might be indicative of a change in heart condition. This monitoring would allow tracking of disease progression or the response to drug therapy or programmed pacing parameters.

A method for adjusting pacemaker parameters based on a measured myocardial conduction time has been proposed; however, the adjustments are made based on relatively short-term changes in conduction time, for example a change measured over three cardiac cycles. Since the physiological response to a change in drug therapy or programmed pacing parameters may not be instantaneous but may occur over an extended period of time, long-term monitoring of a physiological parameter reflective of the heart condition is desirable.

A device and method for long-term monitoring of myocardial conduction velocity, therefore, would improve the physician's ability to monitor a patient's disease state or therapy response. Such a method would preferably allow myocardial conduction data to be collected in an ongoing, day-to-day basis during a patient's normal activities. A physician may then examine the collected data for any shifts in long-term average myocardial conduction velocity and use this information in diagnosing the patient's heart condition and selecting treatments.

SUMMARY OF THE INVENTION

The present invention addresses these needs by providing an implantable cardiac stimulation device capable of measuring and storing myocardial conduction velocity. Myocardial conduction velocities may be stored over time in a histogram or log to provide long-term running average conduction velocity data for assessing the autonomic tone of the heart in patients having chronic heart conditions.

The foregoing and other features of the present invention are realized by providing an implantable cardiac stimulation device and associated electrodes for delivering stimulation pulses, sensing a propagated evoked response, and determining associated time intervals that quantify a myocardial conduction time. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device; a set of leads, which connect cardiac electrodes to the stimulation device for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; a data acquisition system, such as an A/D converter, for sampling cardiac signals such that the time that a propagated evoked response is sensed may be determined; and pulse generators for generating atrial and ventricular stimulation pulses. The stimulation device includes memory for storing operational parameters for the control system, such as stimulation parameter settings and timing intervals, as well as for storing myocardial conduction velocity data. The device also includes a telemetry circuit for communicating with an external programmer.

When operating according to a preferred embodiment, a stimulation pulse of sufficient energy to depolarize the heart is delivered in a unipolar fashion using a tip electrode of a bipolar lead. The depolarization, known as an “evoked response,” is sensed in a unipolar fashion using a ring electrode located on the same lead, a given distance from the tip electrode such that the propagating depolarization arrives at the ring electrode approximately 15 to 150 ms after delivery of the stimulation pulse. A characteristic feature of the sensed depolarization signal, such as a maximum negative slope, is detected as a waveform timing marker. The time interval between the delivered stimulation pulse and the detected waveform timing marker is determined as the myocardial conduction time. The myocardial conduction velocity is the known inter-electrode distance divided by the conduction time.

Preferably, a running average of the myocardial conduction velocity is determined. Such an average may be determined for a given number of heart beats, for example on the order of 500 to 5,000 heart beats. The average myocardial conduction velocity may be stored in memory on a periodic basis, along with the corresponding time of day, average pacing rate, activity level, or other desired parameters, in a data log or histogram format.

In one embodiment, the conduction velocity may be determined as a function of heart rate. Since the conduction time may be affected by the heart rate, multiple running averages of conduction velocity based on the heart rate may be determined. Alternatively, the running average conduction velocity may be computed as a ratio to the heart rate.

The stored conduction velocity data may be downloaded to an external device for analysis by a clinician. Shifts in the long-term average myocardial conduction velocity may indicate a change in disease state, a response to a drug therapy, or the response to a change in programmed stimulation parameters.

In another embodiment, a multi-electrode lead is used to measure myocardial conduction velocities between multiple sites of the heart tissue. Differences in conduction velocities associated with different segments of the myocardial tissue may be used in selecting a preferred stimulation site and may allow detection of local ischemia. Stimulation parameters or stimulation site may then be adjusted such that local ischemia is avoided or alleviated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is of a best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. The present invention is directed at providing a long-term running average of the myocardial conduction velocity of an evoked response for the purposes of monitoring disease progress or optimizing a medical or stimulation therapy. A general cardiac stimulation device will be described in conjunction withFIGS. 1 and 2, in which the features included in the present invention could be implemented. It is recognized, however, that numerous variations of such a device exist in which the methods included in the present invention could be implemented without deviating from the scope of the present invention.

FIG. 1illustrates a stimulation device10in electrical communication with a patient's heart12by way of three leads20,24and30suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the stimulation device10is coupled to an implantable right atrial lead20having at least an atrial tip electrode22, which typically is implanted in the patient's right atrial appendage. The right atrial lead20may also have an atrial ring electrode23to allow bipolar stimulation or sensing in combination with the atrial tip electrode22. In the present invention, right atrial stimulation is preferably delivered using the atrial tip electrode22, and the propagating evoked response is sensed using the atrial ring electrode23.

To sense the left atrial and ventricular cardiac signals and to provide left-chamber stimulation therapy, the stimulation device10is coupled to a “coronary sinus” lead24designed for placement in the “coronary sinus region” via the coronary sinus ostium so as to place a distal electrode adjacent to the left ventricle and additional electrode(s) adjacent to the left atrium. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.

Accordingly, the coronary sinus lead24is designed to: receive atrial and ventricular cardiac signals; deliver left ventricular pacing therapy using at least a left ventricular tip electrode26for unipolar configurations or in combination with left ventricular ring electrode25for bipolar configurations; deliver left atrial pacing therapy using at least a left atrial ring electrode27, and shocking therapy using at least a left atrial coil electrode28. In the present invention, the left ventricular tip electrode26is preferably used for stimulation, and the left ventricular ring electrode27is preferably used for sensing the evoked response.

The stimulation device10is also shown in electrical communication with the patient's heart12by way of an implantable right ventricular lead30having, in this embodiment, a right ventricular tip electrode32, a right ventricular ring electrode34, a right ventricular (RV) coil electrode36, and a superior vena cava (SVC) coil electrode38. Typically, the right ventricular lead30is transvenously inserted into the heart12so as to place the right ventricular tip electrode32in the right ventricular apex so that the RV coil electrode36will be positioned in the right ventricle and the SVC coil electrode38will be positioned in the right atrium and/or superior vena cava. Accordingly, the right ventricular lead30is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. In the present invention, the right ventricular tip electrode32is preferably used for stimulation, and the right ventricular ring electrode34is preferably used for sensing the propagating evoked response.

The stimulation device10includes a housing40which is often referred to as “can”, “case” or “case electrode”, and which may be programmably selected to act as the return electrode for all “unipolar” modes. The housing40may further be used as a return electrode alone or in combination with one or more of the coil electrodes28,36, or38, for defibrillation shocking purposes. The housing40further includes a connector having a plurality of terminals42,43,44,45,46,48,52,54,56, and58(shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the corresponding terminals). As such, to achieve right atrial sensing and stimulation, the connector includes at least a right atrial tip terminal (ARTIP)42adapted for connection to the atrial tip electrode22. The connector also includes a right atrial ring terminal (ARRING)43for connection to the right atrial ring electrode23.

To achieve left chamber sensing, pacing, and shocking, the connector includes at least a left ventricular tip terminal (VLTIP)44, a left ventricular ring terminal (VLRING)45, a left atrial ring terminal (ALRING)46, and a left atrial shocking coil terminal (ALCOIL)48, which are adapted for connection to the left ventricular tip electrode26, the left ventricular ring electrode25, the left atrial ring electrode27, and the left atrial coil electrode28, respectively.

To support right ventricular sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (VRTIP)52, a right ventricular ring terminal (VRRING)54, a right ventricular shocking coil terminal (RV COIL)56, and an SVC shocking coil terminal (SVC COIL)58, which are adapted for connection to the right ventricular tip electrode32, right ventricular ring electrode34, the RV coil electrode36, and the SVC coil electrode38, respectively.

At the core of the stimulation device10is a programmable microcontroller60that controls the various modes of stimulation therapy. The microcontroller60typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller60includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory. Any suitable microcontroller60may be used that carries out the functions described herein.

FIG. 2illustrates an atrial pulse generator70and a ventricular pulse generator72that generate stimulation pulses for delivery by the right atrial lead20, the right ventricular lead30, and/or the coronary sinus lead24via a switch74. It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial pulse generator70and the ventricular pulse generator72may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The atrial pulse generator70and the ventricular pulse generator72are controlled by the microcontroller60via appropriate control signals76and78, respectively, to trigger or inhibit the stimulation pulses.

The microcontroller60further includes timing control circuitry79which is used to control the timing of such stimulation pulses (e.g. pacing rate, atrio-ventricular (AV) delay, atrial interchamber (A—A) delay, or ventricular interchamber (V—V) delay, etc.), as well as to keep track of the timing of refractory periods, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc.

The switch74includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch74, in response to a control signal80from the microcontroller60, determines the polarity of the stimulation pulses (e.g. unipolar, bipolar, cross-chamber, etc.) by selectively closing the appropriate combination of switches. Atrial sensing circuits82and ventricular sensing circuits84may also be selectively coupled to the right atrial lead20, coronary sinus lead24, and the right ventricular lead30, through the switch74, for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial and ventricular sensing circuits82and84may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch74determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches. In this way, the clinician may program the sensing polarity independent of the stimulation polarity.

Each of the atrial sensing circuit82or the ventricular sensing circuit84preferably employs one or more low power, precision amplifiers with programmable gain and automatic gain or sensitivity control, bandpass filtering, and a threshold detection circuit, to selectively sense the cardiac signal of interest. The automatic sensitivity control enables the stimulation device10to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation.

The outputs of the atrial and ventricular sensing circuits82and84are connected to the microcontroller60for triggering or inhibiting the atrial and ventricular pulse generators70and72, respectively, in a demand fashion, in response to the absence or presence of cardiac activity, respectively, in the appropriate chambers of the heart. The atrial and ventricular sensing circuits82and84, in turn, receive control signals over signal lines86and88from the microcontroller60, for controlling the gain, threshold, polarization charge removal circuitry, and the timing of any blocking circuitry coupled to the inputs of the atrial and ventricular sensing circuits82and84.

For arrhythmia detection, the stimulation device10includes an arrhythmia detector77that utilizes the atrial and ventricular sensing circuits82and84to sense cardiac signals, for determining whether a rhythm is physiologic or pathologic. As used herein “sensing” refers to the process of noting an electrical signal. “Detection” refers to the step of confirming that the sensed electrical signal as the signal being sought by the detector. As an example, “detection” applies to the detection of both proper rhythms (i.e., “R wave” or “R wave”) as well as improper dysrhythmias including arrhythmia and bradycardia (e.g., detection of the absence of a proper rhythm.)

The timing intervals between sensed events (e.g. P-waves, R-waves, and depolarization signals associated with fibrillation which are sometimes referred to as “F-waves” or “Fib-waves”) are then classified by the arrhythmia detector77by comparing them to a predefined rate zone limit (e.g. bradycardia, normal, low rate ventricular tachycardia, high rate ventricular tachycardia, and fibrillation rate zones) and various other characteristics (e.g. sudden onset, stability, physiologic sensors, and morphology, etc.), in order to determine the type of remedial therapy that is needed (e.g. bradycardia pacing, anti-tachycardia stimulation, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”).

Cardiac signals are also applied to the inputs of a data acquisition system90, which is depicted as an analog-to-digital (A/D) converter for simplicity of illustration. The data acquisition system90is configured to acquire intracardiac electrogram (EGM) signals, convert the raw analog data into digital signals, and store the digital signals for later processing and/or telemetric transmission to an external device102. The data acquisition system90is coupled to the right atrial lead20, the coronary sinus lead24, and the right ventricular lead30through the switch74to sample cardiac signals across any pair of desired electrodes.

Advantageously, the data acquisition system90may be coupled to the microcontroller60or another detection circuitry, for detecting an evoked response from the heart12in response to an applied stimulus, thereby aiding in the detection of “capture”. In the embodiment shown inFIG. 2, the microcontroller60includes an automatic capture detector65that searches for an evoked response signal following a stimulation pulse during a “detection window” set by timing control circuitry79. The microcontroller60enables the data acquisition system90via control signal92to sample the cardiac signal that falls in the capture detection window. The sampled signal is evaluated by automatic capture detector65to determine if it is an evoked response signal based on its amplitude, peak slope, morphology or another signal feature or combination of features. The detection of an evoked response during the detection window indicates that capture has occurred.

Capture detection may occur on a beat-by-beat basis or on a sampled basis. When loss of capture is detected, a safety, back-up pulse is delivered shortly after the primary pulse in order to prevent asystole. Preferably, a capture threshold search is then performed in order to re-determine the threshold and appropriately adjust the stimulation pulse output. A capture threshold search may also be performed on a periodic basis, preferably once a day during at least the acute phase (e.g., the first 30 days) and less frequently thereafter. A capture threshold search would begin at a desired starting point (either a high output level or the level at which capture is currently occurring) and continue by decreasing the output level until capture is lost. The output level is then increased again until capture is regained. The lowest output level at which sustained capture is regained is known as the capture threshold. Thereafter, the stimulation output is adjusted to a level equal to the capture threshold plus a working margin.

In accordance with the present invention, capture detection algorithms and circuitry may also be used alone or in conjunction with special circuitry shown as a conduction velocity detector64for detecting an evoked response for the purposes of measuring a myocardial conduction time. Methods will be described herein for determining the time between a delivered stimulation pulse and the detection of the subsequent evoked response after it has propagated to a sensing electrode. This time interval is a measure of myocardial conduction time and is linearly proportional to the myocardial conduction velocity by a factor equal to the distance between the stimulating electrode and the sensing electrode.

The microcontroller60is further coupled to a memory94by a suitable data/address bus96, wherein the programmable operating parameters used by the microcontroller60are stored and modified, as required, in order to customize the operation of the stimulation device10to suit the needs of a particular patient. Such operating parameters define, for example, stimulation pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each stimulation pulse to be delivered to the patient's heart12within each respective tier of therapy.

The memory94may further be used for storing cardiac data. In accordance with the present invention, long-term myocardial conduction velocity data will be stored in memory94to allow later analysis by a physician. Advantageously, stored cardiac data may be non-invasively downloaded to an external device102through a telemetry circuit100. A log or histogram of long-term conduction velocity data may be downloaded to the external device102and displayed in a tabular or graphical format for analysis by a physician.

The operating parameters of the stimulation device10may be non-invasively programmed into the memory94through the telemetry circuit100in telemetric communication with the external device102, such as a programmer, transtelephonic transceiver, or a diagnostic system analyzer. The telemetry circuit100is activated by the microcontroller60by a control signal106. The telemetry circuit100advantageously allows intracardiac electrograms and status information relating to the operation of the stimulation device10(as contained in the microcontroller60or memory94) to be sent to the external device102through the established communication link104.

The stimulation device10may further include a physiologic sensor108, commonly referred to as a “rate-responsive” sensor because it is typically used to adjust stimulation rate according to the exercise state of the patient. However, the physiological sensor108may further be used to detect changes in cardiac output, changes in the physiological condition of the heart, or diurnal changes in activity (e.g. detecting sleep and wake states). Accordingly, the microcontroller60responds by adjusting the various stimulation parameters (such as rate, AV Delay, V—V Delay, etc.) at which the atrial and ventricular pulse generators70and72generate stimulation pulses.

While the physiologic sensor108is shown as being included within the stimulation device10, it is to be understood that the physiologic sensor108may alternatively be external to the stimulation device10, yet still be implanted within, or carried by the patient. A common type of rate responsive sensor is an activity sensor, such as an accelerometer or a piezoelectric crystal, which is mounted within the housing40of the stimulation device10. Other types of physiologic sensors are also known, for example, sensors of blood oxygen content, blood pH, respiration rate and/or minute ventilation, ventricular gradient, etc. Any sensor may be used which is capable of sensing a physiological parameter that corresponds to the exercise state of the patient.

The stimulation device10additionally includes a power source such as a battery110that provides operating power to all the circuits shown in FIG.2. For the stimulation device10, which employs shocking therapy, the battery110should be capable of operating at low current drains for long periods of time, preferably less than 10 μA, and also be capable of providing high-current pulses when the patient requires a shock pulse, preferably, in excess of 2 A, at voltages above 2 V, for periods of 10 seconds or more. The battery110preferably has a predictable discharge characteristic so that elective replacement time can be detected.

As further illustrated inFIG. 2, the stimulation device10is shown to include an impedance measuring circuit112which is enabled by the microcontroller60by control signal114. The known uses for an impedance measuring circuit112include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgment; detecting operable electrodes and automatically switching to an operable pair if dislodgment occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit112is advantageously coupled to the switch74so that any desired electrode may be used for impedance measurements.

If it is a function of the stimulation device10to operate as an implantable cardioverter/defibrillator (ICD) device, it must detect the occurrence of an arrhythmia, and automatically apply an appropriate electrical stimulation or shock therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller60further controls a shocking circuit116by way of a control signal118. The shocking circuit116generates shocking pulses of low (up to 0.5 joules), moderate (0.5-10 joules), or high (11 to 40 joules) energy, as controlled by the microcontroller60. Such shocking pulses are applied to the patient's heart through at least two shocking electrodes, and as shown in this embodiment, selected from the left atrial coil electrode28, the RV coil electrode36, and/or the SVC coil electrode38(FIG.1). As noted above, the housing40may act as an active electrode in combination with the RV electrode36, or as part of a split electrical vector using the SVC coil electrode38or the left atrial coil electrode28.

InFIG. 3, a flow chart is shown describing an overview of the operation and novel features implemented in one embodiment of the device10for measuring myocardial conduction time and storing a long-term running average of the conduction velocity. In this flow chart, and the other flow charts described herein, the various algorithmic steps are summarized in individual “blocks”. Such blocks describe specific actions or decisions that must be made or carried out as the algorithm proceeds. Where a microcontroller (or equivalent) is employed, the flow charts presented herein provide the basis for a “control program” that may be used by such a microcontroller (or equivalent) to effectuate the desired control of the stimulation device. Those skilled in the art may readily write such a control program based on the flow charts and other descriptions presented herein.

The method300begins at step305when a stimulation pacing pulse is delivered by either atrial pulse generator70or ventricular pulse generator72. The method300may be used in any heart chamber and may be used in more than one heart chamber in a given patient such that long-term average conduction velocities may be acquired for atrial and ventricular heart chambers. The stimulation pacing pulse is preferably delivered using a tip electrode so that, at step310, the evoked response may be sampled by data acquisition system90using a unipolar ring-to-case sensing configuration.

The stimulation pulse energy delivered at step305is sufficient to elicit an evoked response. An evoked response is a depolarization of the myocardial cells due to a stimulation pulse. The depolarization wavefront evoked by the stimulation pulse will take a certain amount of time to propagate through the myocardial tissue to the ring electrode. The time required for the depolarization wavefront to propagate from the tip electrode to the ring electrode is a function of the myocardial tissue conduction velocity and the inter-electrode distance:
conduction time=tissue conduction velocity×inter-electrode distance

The inter-electrode distance, which is the distance between the tip electrode and the ring electrode, is preferably a distance that allows the evoked response to be detected after the polarization signal associated with the stimulation pacing pulse has substantially decayed. For example, Model 1474T pacing lead available from St. Jude Medical, Inc. has a tip-to-ring spacing that allows the ring-to-case evoked response to be detected on the order of 15 to 80 ms after the stimulation pacing pulse.

At step315, conduction velocity detector64detects a predetermined depolarization waveform feature and determines the time of its occurrence relative to the time of the stimulation pulse. The waveform feature is preferably a peak negative slope, but may be any feature detectable from the sampled depolarization waveform such as a peak amplitude, a zero crossing, or an inflection point. The time of the waveform feature is used as a timing marker for determining the conduction time.

The myocardial conduction time is then determined at step320as the difference between the time of the stimulation pacing pulse delivery and the time of the detected depolarization waveform feature. At step320, additional arithmetic may also be performed to calculate the conduction velocity by dividing the conduction time by the known inter-electrode distance.

At step325, the long-term running average conduction velocity is updated. This long-term average is determined by averaging the conduction velocities determined from a large number of consecutive paced cardiac cycles from which a conduction time measurement has been made. Typically, a long-term average conduction velocity will be calculated from greater than 100 conduction time measurements, preferably from 500 to 5,000 conduction time measurements.

In one embodiment, each conduction time measurement is stored in memory, and, after a given number of conduction time measurements have been made, a new long-term average is calculated and stored. In another embodiment, a running average may be updated upon each new conduction time measurement using a given number of the most recent conduction time measurements. This running average may then be stored in memory94every time it is updated or on a periodic basis.

In a given patient the tip-to-ring spacing will remain fixed as long as the lead is not moved or replaced. Changes in the myocardial conduction velocity will therefore be linearly proportional to changes in conduction time. Thus, either the average conduction time or average conduction velocity may be used in the present invention; whichever is deemed most appropriate for the desired application or diagnostic. In most cases, either parameter will be appropriate since they are linearly proportional to each other. When using conduction time, the need to know the tip-to-ring distance and the related arithmetic required to perform the conversion from conduction time to conduction velocity made at step320are not necessary.

In another embodiment, the effect of heart rate on myocardial conduction time is taken into account. Rate-responsive pacing or dynamic overdrive pacing algorithms automatically adjust the pacing rate. During rate-responsive pacing, the rate is automatically increased or decreased to meet the metabolic needs of the patient based on signals from the physiological sensor108. During dynamic overdrive pacing, the pacing rate is adjusted to be greater than the sensed intrinsic rate. Changes in pacing rate due to such algorithms may influence the myocardial conduction velocity. Therefore, multiple average conduction velocities associated with a given pacing rate, or pacing rate range, may be stored simultaneously. Conduction time measurements may be stored in a histogram format with each histogram bin assigned to a given pacing rate or pacing rate range. The long-term average conduction velocity is then calculated for each pacing rate range.

This embodiment is outlined by the flow chart shown in FIG.4. During steps305through320, a conduction time is measured and a conduction velocity calculated, if desired, exactly as previously described for method300of FIG.3. At step405, a histogram bin assigned to the current pacing rate is enabled. The average conduction velocity stored in this histogram bin is updated at step410using the newly measured conduction velocity.

In another embodiment, the conduction time measurement is converted to a ratio, a product, or a function of the heart rate (i.e., conduction time/heart rate, heart rate*conduction time, or corrected conduction time=f(heart rate)*conduction time). The long term running average is then calculated from the conduction time to heart rate ratios.

In yet another embodiment, the effect of pacing pulse energy on conduction time is taken into account. Increasing stimulation pulse amplitudes can increase the size of the “virtual cathode”, defined to be the perimeter of the area of tissue that is depolarized with a pacing pulse. As the pacing pulse amplitude is increased, the reach of the electric field generated by the pulse increases, such that myocardial cells further from the electrode are depolarized. When this happens, increased stimulation pulse amplitudes may result in shorter measured conduction times because the depolarization wavefront will now start out closer to the ring electrode. These measurements would falsely indicate a physiologic change in the state of the myocardial tissue propagating the depolarization. Since long-term averages may incorporate a wide range of stimulation pulses, due to threshold changes or automatic capture algorithms, the effect of pacing pulse energy can be compensated for by associating each measured conduction time with the pacing pulse amplitude used at the time of measurement. This association or correction can be as for heart rate: a product or a function of the stimulating pacing pulse energy (pulse energy times conduction time, corrected conduction time=f(pulse energy)*conduction time).

A shift in the long-term average conduction velocity (or time) may indicate to a clinician a change in the patient's disease state, response to a drug therapy or a response to a pacing therapy. Thus, the long-term average conduction velocity may be used as a feedback parameter in optimizing drug or pacing therapy.

Long-term averages may be compared to one or more previous long-term averages in order to detect a change in myocardial conduction. Alternatively, a reference value may be obtained by computing a conduction velocity average over a much longer time period than the long-term average, for example an average of ten to fifty times more conduction velocity measurements than the long-term average. The long-term averages may then be compared to this reference value in order to detect shifts in the myocardial conduction velocity.

In another embodiment, a multipolar lead may be used having a pacing tip electrode and multiple sensing ring electrodes spaced along the length of the lead in contact with different myocardial tissue sites to allow detection of a propagating depolarization waveform at several locations. The time at which a waveform feature is detected at each sensing electrode may be determined such that the conduction time of multiple tissue segments may be determined. The methods used for this embodiment would be similar to the methods300or400used for measuring a single conduction time. The same method would be applied to multiple sensing electrodes simultaneously.

Identifying deviations in the conduction time associated with individual tissue segments may aid in the detection and diagnosis of local ischemia. Stimulation pacing parameters or electrode configuration selection may be adjusted to avoid or alleviate the locally ischemic region or to optimize the stimulation response in the presence of a slowly conducting region.

Thus, a system and method for monitoring the long-term average myocardial conduction velocity has been described. While detailed descriptions of specific embodiments of the present invention have been provided, it would be apparent to those reasonably skilled in the art that numerous variations of the methods described herein are possible in which the concepts of the present invention may readily be applied. The descriptions provided herein are for the sake of illustration and are not intended to be exclusive.