Adaptive windowing for cardiac waveform discrimination

Cardiac devices and methods provide adaptation of detection windows used to determine a cardiac response to pacing. Adapting a detection window involves sensing a cardiac signal indicative of a particular type of cardiac pacing response, and detecting a feature of the sensed cardiac signal. The cardiac response detection window associated with the type of cardiac pacing response is preferentially adjusted based on the location of the detected cardiac feature. Preferential adjustment of the detection window may involve determining a direction of change between the detection window and the detected feature. The detection window may be adapted more aggressively in a more preferred direction and less aggressively in a less preferred direction.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devices and, more particularly, to cardiac devices and methods used in cardiac pacing response determination.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions and is capable of pumping blood throughout the body. However, due to disease or injury, the heart rhythm may become irregular resulting in diminished pumping efficiency. Arrhythmia is a general term used to describe heart rhythm irregularities arising from a variety of physical conditions and disease processes. Cardiac rhythm management systems, such as implantable pacemakers and cardiac defibrillators, have been used as an effective treatment for patients with serious arrhythmias. These systems typically include circuitry to sense electrical signals from the heart and a pulse generator for delivering electrical stimulation pulses to the heart. Leads extending into the patient's heart are connected to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering stimulation pulses to the heart in accordance with various therapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissue adjacent to the electrodes to produce a contraction of the tissue. Pacemakers are cardiac rhythm management systems that deliver a series of low energy pace pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing one or more heart chambers.

A pace pulse must exceed a minimum energy value, or capture threshold, to produce a contraction. It is desirable for a pace pulse to have sufficient energy to stimulate capture of the heart without expending energy significantly in excess of the capture threshold. Thus, accurate determination of the capture threshold is required for efficient pace energy management. If the pace pulse energy is too low, the pace pulses may not reliably produce a contractile response in the heart and may result in ineffective pacing. If the pace pulse energy is too high, the patient may experience discomfort and the battery life of the device will be shorter.

Detecting if a pacing pulse “captures” the heart and produces a contraction allows the cardiac rhythm management system to adjust the energy level of pace pulses to correspond to the optimum energy expenditure that reliably produces capture. Further, capture detection allows the cardiac rhythm management system to initiate a back-up pulse at a higher energy level whenever a pace pulse does not produce a contraction.

When a pace pulse produces a contraction in the heart tissue, the electrical cardiac signal preceding the contraction is denoted the captured response. The captured response typically includes an electrical signal, denoted the evoked response signal, associated with the heart contraction, along with a superimposed signal associated with residual post pace polarization at the electrode-tissue interface. The magnitude of the residual post pace polarization signal, or pacing artifact, may be affected by a variety of factors including lead polarization, after-potential from the pace pulse, lead impedance, patient impedance, pace pulse width, and pace pulse amplitude, for example. The evoked response may be affected by interaction with intrinsic heart activity and resulting in a fusion or pseudofusion response.

A fusion beat is a cardiac contraction that occurs when two cardiac depolarizations of a particular chamber, but from separate initiation sites, merge. At times, a depolarization initiated by a pacing pulse may merge with an intrinsic beat, producing a fusion beat. Fusion beats, as seen on electrocardiographic recordings, exhibit various morphologies, since the merging depolarizations of a fusion beat do not contribute evenly to the total depolarization.

Pseudofusion occurs when a pacing stimulus is delivered on a spontaneous P wave during atrial pacing or on a spontaneous QRS complex during ventricular pacing. In pseudofusion, the pacing stimulus may be ineffective because the tissue around the electrode has already spontaneously depolarized and is in its refractory period.

Noise presents a problem in capture detection processes when the pacemaker mistakenly identifies noise as capture, fusion/pseudofusion, or intrinsic activity. Noise mistakenly identified as capture or fusion/pseudofusion may cause a pacemaker to erroneously withhold backup pacing under loss of capture conditions. Noise mistakenly identified as early intrinsic activity may lead to a premature loss of capture determination during threshold testing.

The present invention provides methods and systems used for enhancing the discrimination of types of cardiac pacing responses, such as those described above, and provides various advantages over the prior art.

SUMMARY OF THE INVENTION

The present invention involves cardiac devices and methods incorporating adaptive windows for cardiac waveform discrimination. An embodiment of the invention involves a method for adjusting detection windows used for detection of various types of cardiac response to pacing. The method includes sensing a cardiac signal associated with the type of cardiac response and detecting a feature of the sensed cardiac signal. The detection window is preferentially adapted based on the location of the detected cardiac signal feature. The detection window may be defined, for example, in terms of amplitude and time, or may be defined in terms of additional or alternative parameters. Adaptation of the detection window may involve adjusting the location, size, shape, area, or boundaries of the detection window, for example.

According to one aspect of the invention, the detection window may be preferentially adjusted based on a direction of change between the detection window and the location of the detected feature. The detection window may be adjusted more aggressively in a preferred direction of change and less aggressively in a less preferred direction of change. In another implementation, the detection window may be adapted based on a relationship between the location of the detected feature and a detection window limit. For example, features may be selectively used to update the detection window. If a feature is located close to or at a detection window limit, for example, it may not be used for detection window update.

A system for adapting detection windows used for cardiac response discrimination in accordance with embodiments of the present invention includes a sensing system configured to sense cardiac signals following pacing pulses delivered to a heart. A processor is coupled to the sensing system and is configured to detect a feature of the cardiac signals. The processor is configured to preferentially adjust the detection window based on the location of the cardiac signal feature.

For example, in one implementation, the processor may be configured to preferentially adjust the detection window based on a direction of change between the detected feature location and the cardiac response detection window position. Additionally, or alternatively, the processor may adjust the detection window based on the location of a cardiac signal feature with respect to a detection window limit.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Cardiac response classification may be implemented by a pacemaker or other cardiac rhythm management device to determine whether an applied electrical pacing stimulus captures the heart. The systems and methods described herein are related to the use of cardiac signal features to discriminate between various types of cardiac responses to pacing. Discrimination between cardiac pacing responses may involve discrimination between capture and fusion and/or non-capture with or without intrinsic activation. The approaches of the present invention provide for enhanced capture threshold testing and/or beat to beat automatic capture verification.

Several functions of cardiac devices rely on the heart response consistency. For example, automatic capture threshold testing and/or automatic capture verification algorithms may rely on templates of the heart's response as the basis for determining whether a future pacing stimulus produces a particular type of response. However, the cardiac responses may vary across patients and change over time.

Templates representative of various types of cardiac responses may comprise one or more detection windows. The detection windows are compared to a cardiac signal following delivery of a pacing pulse to determine the response to pacing. Devices and methods in accordance with the present invention adapt one or more detection windows as the patient's signal morphology changes over time. Cardiac response detection windows may be adapted based on prior and/or ongoing feature measurements that are consistent with a current template, for example.

In accordance with embodiments of the invention, a detection window may be preferentially adapted based on the location of a cardiac signal feature. In some implementations, the location of the cardiac signal feature and the location and boundaries of the capture detection windows may be defined in terms of amplitude and time, although additional or alternative parameters may be used. The preferential adaptation of a detection window may occur by modifying the location, shape, area or other characteristics or parameters of the detection window in accordance with embodiments of the invention. In one implementation, preferential adaptation of a detection window may be based, for example, on the direction of change between the detection window and the location of a measured cardiac signal feature. The detection window may be adapted more aggressively (higher rate of adaptation) in a more preferred direction and less aggressively (lower rate of adaptation) in a less preferred direction.

Allowing for different adaptation rates of a detection window may accommodate asymmetry in template dimensions, and preferred directional change/response to maintain operational limits, such as safety limits. Adapting detection windows within operational limits may insure proper classification through a feature's full measurement range, and help to avoid problems such as those associated with small amplitude captured response signals and sense amplifier saturation limits.

According to various aspects of the invention, features may be used to adapt the detection window and to compress the boundary of a detection window against an operational limit. In some implementations, features may not be used to update a detection window if the features fall too close to a detection window limit.

Consider the case where one or more peaks of the cardiac signal are cardiac signal features used to detect a captured response. One or more capture detection windows may be defined as a template associated with a captured response. The capture detection windows are regions, having coordinates of amplitude and time, bounding clustered signal peaks of multiple cardiac signals under conditions of captured response, as is illustrated inFIG. 1A. The graphs inFIG. 1Aillustrate cardiac waveform portions and their associated peak information, showing clustering and bounding of capture detection windows in accordance with embodiments of the present invention.

A graph100plots the positive and negative peaks of the cardiac waveforms graphed in an inset graph102. The inset graph102shows several cardiac waveforms104including positive peaks106and negative peaks108. The amplitudes and times associated with the peaks of the cardiac waveforms104in graph102provide the coordinate system measurements for the points plotted in the graph100.

The graph100includes a first capture detection window110associated with negative peaks of a captured response and a second capture detection window112associated with positive peaks of a captured response. The capture detection windows110,112are used to discriminate beats corresponding to capture from other types of pacing responses. If a waveform's negative peak falls within the first capture detection window110and a waveform's positive peak falls within the second capture detection window112, the beat is associated with a captured response. Non-captured beats, such as fusion beats or intrinsic beats, have peaks that fall outside one or both the windows110,112.

For example, a point114of the graph100corresponds to a peak value of a cardiac response signal that does not correspond to a captured beat. The measured coordinates of peak time and peak amplitude of the point114fall outside the second capture detection window112.

FIG. 1Billustrates a capture detection window120, having a reference point122, and associated coordinate information. The capture detection window120may have a height defined as2a, spanning an amplitude a above and below the reference point122. An amplitude a1corresponds to the reference point122y-axis coordinate minus a, and an amplitude a2corresponds to the reference point122y-axis coordinate plus a. The reference point122is illustrated inFIG. 1Bat the middle of the amplitude range of the capture detection window from a1to a2, but is not limited to this relationship. The reference point122may have any defined relationship to the upper and lower bounds of the capture detection window120, and the relationship may change over time.

For example, as the capture detection window120upper bound a2approaches a physical or desired limit of amplitude, the relationship between the reference point122and the upper bound a2may change, in order to clamp the capture detection window120upper bound a2to the desired amplitude limit. In another embodiment, after the capture detection window120upper bound a2reaches a limit, the capture detection window120may be restricted from moving higher. In another embodiment, after the capture detection window120upper bound a2reaches a limit, the upper bound a2of the capture detection window may be restricted from further modification, while modification of the lower amplitude bound and/or time boundaries t1and t2may continue. In a further embodiment, as the capture detection window120upper bound a2approaches a physical or desired limit of amplitude, the rate of change of the upper bound a2and/or the position of the reference point122may be altered to compress or limit the adaptation of the capture detection window120. In another embodiment, the sensitivity and/or rate at which the capture detection window120is adapted may be based on a preferred direction, amplitude, or other criteria. In yet another embodiment, current feature measurements may be selectively used to update the capture detection window. For example, if the feature location is within a predetermined distance of a detection window limit, then the feature may not be selected to adjust the capture detection window.

For example, the capture detection window120may be adapted using an algorithm that accounts for a preferred direction of movement and/or adaptation for the capture detection window120. One example involves using a filtering of the peak amplitude that is applied when criteria are met. A filtering equation may be used, such as:
New Amplitude=(1−α)·Old_Amplitude+α·Current_Peak_Amplitude

where the update coefficient, α, determines the significance of the newest measured peak amplitude contribution to the filtered value. The filtered value may then be used as the amplitude reference point for the capture detection window. In some embodiments, the system may determine whether to use a current peak amplitude for capture detection window update. For example, if the current peak amplitude is close to or equals the upper limit, then the current peak amplitude may not be used to update the capture detection window.

The capture detection window120may have a width defined as spanning from t1to t2, where t1and t2are respectively referenced to the left and right of the reference point122. A time t1corresponds to the reference point122x-axis coordinate minus t, and a time t2corresponds to the reference point122x-axis coordinate plus t. The reference point122is illustrated inFIG. 1Bat the middle of the time range from t1to t2, but is not limited to this relationship. The reference point122may have any defined relationship to the left and right bounds of the capture detection window120, and the relationship may change over time. The minimum, maximum, sensitivity, and/or rate at which t1and/or t2are adapted may also be based on a preferred direction, limits, or other criteria, similarly to amplitude bounds a1and a2.

Time values may be filtered using a differential update coefficient, which may be based on the relative position of the current time measurement relative to the capture detection window time reference. Using the differential update coefficient may account for detection window limits, where the time boundary for a detection window may have a tighter limit on one side and a looser limit on the other side, providing the ability of the window to move in a preferred direction. By using a preferred direction sensitivity, significant shifts in the window caused by a few large signals in an undesirable direction may be mitigated.

FIG. 1Cis a graph130of cardiac waveform peak information illustrating peak location drift. Signal drift may be accounted for by adapting detection windows in accordance with the present invention. The graph130plots the capture response negative peak time on the ordinate and successive cardiac cycle count in the abscissa. As is seen generally by the line approximation135through the negative capture peaks, the signal is drifting over time. If the capture detection window is adapted in accordance with embodiments of the present invention, the window may be shifted along with the creep in the signal peaks to provide improved discrimination capabilities.

For example, as with capture detection window amplitude, the capture detection window time reference may be adapted using a filtering of the peak time that is applied when criteria are met. A filtering equation may be used, such as:
New time=(1−β)·Old_Time+β·Current_Peak_time

where the update coefficient, β, determines the significance of the newest measured peak time contribution to the filtered value. The filtered value may then be used as the newly adapted time reference point for the capture detection window. Different β-values can be used to apply directional preference, determined by the location of the current peak time relative to the time reference of the appropriate capture detection window.

In various embodiments, multiple detection windows may be adapted, such as by using equations similar to those above. Consider the case where a template involves a first detection window associated with a first cardiac signal feature, e.g., a first peak, and a second detection window associated with a second cardiac signal feature, e.g., a second peak. The first and second detection windows may be adjusted based on the location of first and second cardiac signal features. As will be apparent to those skilled in the art, any number of parameters and any number of multi-variable detection windows in any combination may be used without departing from the scope of the present invention.

FIG. 2Ais a flowchart of a method200of adapting detection windows for cardiac response discrimination in accordance with embodiments of the invention. Cardiac signals from pacing pulses are sensed202. Cardiac signal features204are detected, and measurements are made of cardiac signal feature parameters, such as by sensing for a peak amplitude, zero crossing, inflection point, QRS width, or other measurements. Locations of one or more cardiac signal features are determined206. For example, a location of a signal feature may be defined by a current measurement of the Cartesian peak amplitude and peak time values of the cardiac signal. The amplitude and time values may be compared to boundaries associated with a detection window. If the feature location falls within the boundaries of a detection window, then the detection window may be preferentially adjusted208based on the current feature location in accordance with the present invention.

For example, the reference point of a detection window may be defined as a running average value of all signal peak amplitudes and peak times for the last sixty-four cardiac waveforms peaks that fell within the detection window. If the current waveform peak falls within the detection window, the current peak amplitude value and peak time may be averaged with the last sixty-three respective values, and the oldest value may be dropped from the running average. The use of sixty-four as the running average number is only an example of a useful average to improve the signal to noise ratio, and is not intended to be the only useful value or limiting in any way.

By utilizing a running average, the window will continuously adapt to the most recent patient responses. Limits of the detection window may be clamped and/or the reference point and/or boundary values may be compressed in accordance with embodiments of the present invention to account for operational limits of a patient implantable medical device (PIMD). The detection window parameters including, for example, boundary locations, size, shape, area, reference point location, and the like, may be constrained by desired or required maximum and/or minimum values, and/or preferential window movement directions to limit window movement magnitudes or rates in particular directions.

FIG. 2Bis a flowchart of a functional implementation of a method600for adapting capture detection windows for cardiac waveform discrimination in accordance with embodiments of the invention. AlthoughFIG. 2Billustrates processes as applied to capture detection windows, other types of detection windows, such as those used for the detection of fusion or early intrinsic beats, may be similarly adapted.

If a signal is ready to be classified, checks602and604are made to determine if the negative and positive peak features of the cardiac response signals reside in the respective capture detection windows, thereby determining a capture response. A capture response noise check606is then performed to eliminate the possibility that noise interfered with the capture determination. Once a capture response is determined, the current peak information can be considered for adaptation of the current capture detection window (CDW) reference location.

The checks for capture determination602and604also provide adaptation limits for the CDW against the peak feature's full measurement range, essentially allowing the CDW to compress against a designated measurement range, and thereby allowing proper classification throughout that measurement range.

The negative peak check602, may be made to determine if a negative peak, negPK, and a negative peak timing, negTPK, are within a first capture detection window, CDW1, that may be located against maximum or minimum measurement limits determined by the equations:
max{CDW1a1, −32 mV}<negPK<min{CDW1a2, ADT}
and
CDW1t1<negTPK<CDW1t2

where CDW1a1and CDW1a2are the values of a1and a2respectively for the CDW1, and CDW1t1and CDW1t2are the values of t1and t2respectively for CDW1, and ADT is an activity detection threshold designating a minimum expected response level. The maximum and minimum operations are used to asymmetrically compress CDW1against the maximum and minimum of a desired negative peak amplitude measurement range: a −32 mV amplifier saturation and a −2 mV ADT level respectively, in this example. If current negative peak values are outside the designated ranges, they may not be passed to the later CDW adaptation operations. The maximum and minimum operations also prevent later computed filtered peak and time values that are used to adapt the CDW1reference point, from exceeding the desired negative peak amplitude measurement range, thereby limiting CDW adaptation.

If the signal passes the check602, then a positive peak check604is made to determine if a positive peak amplitude, posPK, and a positive peak timing, posTPK, are within a second capture detection window, CDW2, that may also be located against maximum or minimum measurement limits determined by the equations:
max{CDW2a1, ADT}<posPK<min{CDW2a2, 32 mV}
and
CDW2t1<posTPK<CDW2t2

where CDW2a1and CDW2a2are the values of a1and a2respectively for the CDW2, and CDW2t1and CDW2t2are the values of t1and t2respectively for CDW2. Again, the maximum and minimum operations are used to asymmetrically compress CDW2against the maximum and minimum of a desired peak amplitude measurement range: a 32 mV amplifier saturation and a 2 mV ADT level respectively, in this example. Again, if current positive peak values are outside the designated ranges, they may not be passed to the later CDW adaptation operations. The maximum and minimum operations prevent later computed filtered peak and time values that are used to adapt the CDW2reference point, from exceeding the desired positive peak amplitude measurement range, thereby limiting CDW adaptation.

If the signal passes the checks602and604, an ER noise check606is performed to eliminate the possibility that noise interfered with the capture determination. For example, to avoid ER noise detection as capture, the additional peak information is used to check for multiple peaks in wrong places. Given capture is indicated, if other peaks are inconsistent with normal morphology, then ER noise is indicated rather than capture and adaptation of the capture detection window would be avoided.

Once capture is determined, then the signal may be used to adapt the capture detection window(s) in accordance with the present invention. As noted earlier, checks602and604limit CDW adaptation such that the CDW reference point will not move outside of designated measurement ranges. In this case, the entire upper or lower portions of the CDW are allowed to compress against the respective upper and lower limits of the measurement range. Additional checks608,610,622, and624may be put in place to control the amount of CDW compression allowed against designated measurement limits.

Considering the positive peak information first, a determination608is made using the equation:
|posPK|<|32 mV−a2+CDW_lim_buf|

where 32 mV is the maximum of the positive measurement range, a2is the more positive direction CDW amplitude offset from its reference, and CDW_lim_buf is a predetermined compression limit that may be less than a2. This check prevents any current posPK values within (a2−CDW_lim_buf) of the range maximum, from being used to adapt the CDW2reference point, thereby limiting CDW adaptation and the compression against the positive measurement range maximum to 32 mV−a2+CDW_lim_buf. If found in the affirmative, then the minimum of the positive measurement range must be considered next,610, otherwise adaptation for CDW2is bypassed. Thus, still considering the positive peak information, a decision610is made using the equation:
|posPK|>|ADT+a1−CDW_lim_buf|

where ADT is an Activity Detection Threshold designating a minimum expected response level, a1is the less positive direction CDW amplitude offset from its reference, and CDW_lim_buf is a predetermined compression limit that may be less than a1. This check prevents any current posPK values within (a1−CDW_lim_buf) of the range minimum, from being used to adapt the CDW2reference point, thereby limiting CDW adaptation and the compression against the positive measurement range minimum to −CDW_lim_buf.

In one example, a1and a2may be set equal to about 4 mV for CDW amplitude offsets, and CDW_lim_buf, which defines how much the CDW can be asymmetrically compressed against the minimum/maximum boundaries, may be set equal to about 1 mV.

Given checks608and610are found in the affirmative, then the CDW2amplitude reference point is adjusted,612, using the equation:
posPK_flt[n]=c3*posPK[n]+(1−c3)*posPK_flt[n-1]

where posPK[n] is the current peak amplitude measurement, PosPK_flt[n-1] is the previous amplitude reference for CDW2, posPK_flt[n] is the updated amplitude reference point for CDW2, and c3is the coefficient that determines how much the current peak amplitude measurement contributes to the update. Thus, update612performs a recursive low-pass filter value of the peak amplitude if the amplitude is in an acceptable range as determined by checks602,604,608, and610. If the update612is not performed, the CDW amplitude reference stays at a reasonable extreme. In some implementations, amplitude adjustments have limited value and c3may be set to zero or a minimal value.

Independent of CDW2reference amplitude adaptation, the CDW2reference time is adjusted next. In this case, a directional preference is established based on whether or not the current positive peak time information is before or after the CDW2time reference point, posTPK_flt. A check614is performed using the equation:
posTPK>posTPK_flt

Check614compares the current positive peak time to a CDW peak time reference point, posTPK_flt, and, depending on the outcome of the comparison, defines616,618the variable c4. Depending on the outcome of check614, variable c4may be defined616as c4=c41or may be defined618as c4=c42. Variable c4is the coefficient that determines how much the current peak time measurement contributes to the CDW time reference point update,620. The CDW time reference point update may be performed using the equation:
posTPK_flt[n]=c4*posTPK[n]+(1−c4)*posTPK_flt[n-1]

where posTPK[n] is the current peak time measurement, posTPK_flt[n-1] is the previous time reference for CDW2, posTPK_flt[n] is the updated time reference point for CDW2, and c4is as defined above. A larger c4value will establish preference toward the current peak time value and thereby a directional preference as established by the check614. For this example, the window is asymmetric in time about the time reference value: referencingFIG. 1B, t2>t1or there is a larger time delta when advancing in time versus retarding in time. Coefficient, c4, is adjusted,616and618, to advance the CDW reference location in time more slowly than retarding in time to prevent large movement of the window due to current time values at the time extreme of the CDW. For discussion purposes, c41may be set equal to about 0.15, and c42may be set equal to about 0.05, illustrative of reasonable numbers in this example. Thus, update620performs a directionally preferenced recursive low-pass filter value of the peak time if the time is in an acceptable range as determined by check604.

A similar check and adjustment process as described for positive peak amplitude and time,608through620, is now performed on the negative peak amplitude and time regarding CDW1. A check622is performed using the equation:
|negPK|<|32 mV−a1+CDW_lim_buf|

and a check624is performed using the equation:
|negPK|>|ADT+a2−CDW_lim_buf|

where 32 mV is the absolute value of the minimum of the negative measurement range, ADT is an Activity Detection Threshold designating the absolute value of the maximum of the negative measurement range, a2is the more positive direction CDW amplitude offset from its reference, a1is the less positive direction CDW amplitude offset from its reference, and CDW_lim_buf is a predetermined compression limit that may be less than a1and a2. These checks prevent any current negPK values within (a1−CDW_lim_buf) of the range minimum and (a2−CDW_lim_buf) of the range maximum, from being used to adapt the CDW1reference point, thereby limiting CDW adaptation and the compression against the negative measurement range extremes to CDW_lim_buf.

Again, in this example, a1and a2may be set equal to about 4 mV for CDW amplitude offsets, and CDW_lim_buf, which defines how much the CDW can be asymmetrically compressed against the minimum/maximum boundaries, may be set to about 1 mV, for example.

Given checks622and624are found in the affirmative, then the CDW1amplitude reference point is adjusted,626, using the equation:
negPK_flt[n]=c1*negPK[n]+(1−c1)*negPK_flt[n-1]
where negPK[n] is the current negative peak amplitude measurement, negPK_flt[n-1] is the previous amplitude reference for CDW1, negPK_flt[n] is the updated amplitude reference point for CDW1, and c1is the coefficient that determines how much the current peak amplitude measurement contributes to the update. Thus, update626performs a recursive low-pass filter value of the negative peak amplitude if the amplitude is in an acceptable range as determined by checks602,604,622, and624. If the update626is not performed, the CDW1amplitude reference stays at a reasonable extreme. In some implementations, amplitude adjustments have limited value. In these cases, c1may be set to zero or a minimal value.

Independent of CDW1reference amplitude adaptation, the CDW1reference time is adjusted next. In this case a directional preference is again established based on whether or not the current negative peak time information is before or after the CDW1time reference point, negTPK_flt. A check628is performed using the equation:
negTPK>negTPK_flt

to compare the current negative peak time to a CDW peak time reference point, negTPK_flt. Depending on the outcome of the comparison, the variable c2may be defined630as c2=c21or may be defined632as c2=c22. Variable c2is the coefficient that determines how much the current peak time measurement contributes to the CDW time reference point update,634. The CDW time reference point update may be performed using the equation:
negTPK_flt[n]=c2*negTPK[n]+(1−c2)*negTPK_flt[n-1]
where negTPK[n] is the current negative peak time measurement, negTPK_flt[n-1] is The previous time reference for CDW1, negTPK_flt[n] is the updated time reference point for CDW1, and c2is as defined above. As with c4, a larger c2value will establish preference toward the current peak time value and thereby a directional preference as established by the check628. For this example, the window is symmetric in time about the time reference value: referencingFIG. 1B, t2=t1. Coefficient, c2, is adjusted,630and632, to advance the CDW reference location in time more slowly than retarding in time. For discussion purposes, c21may be set equal to about 0.15, and c22may be set equal to about 0.05 here, illustrative of reasonable numbers in this example. Thus, update634performs a directionally preferenced recursive low-pass filter value of the negative peak time if the time is in an acceptable range as determined by check602.

To complete the illustrative method600, the updated values of the CDWs may be defined as:
CDW1a1=negPK_flt−a1CDW1a2=negPK_flt+a2
CDW1t1=negTPK_flt−t1CDW1t2=negTPK_flt+t2
CDW2a1=posPK_flt−a1CDW2a2=posPK_flt+a2
CDW2t1=posTPK_flt−t3CDW2t2=posTPK_flt+t4

The CDWs are adjusted relative to the filtered peak and peak time values, which are the CDW reference points, for every cycle where capture is determined. Some values may not actually need to be computed, as the earlier CDW boundary checks can be done against the offset filter values directly. These calculations may allow the CDW boundaries to extend beyond the maximum/minimum limits of the measurement ranges, but the earlier CDW boundary checks for capture detection and compression limits may be applied to define the maximum/minimum rules that asymmetrically compresses the CDW(s) against the maximum/minimum limits, such as are compressed per a definable compression limit: CDW_lim_buf. For example, dimensional constants may be set as follows:a1and a2may be about 4 mV;t1may be about 10 ms;t2may be about 15 ms, (with the additional constraint that t1+t2equal about 25 ms);t3may be about 10 ms; andt4may be about 50 ms.

In some implementations, pacing response classification may involve sensing cardiac signals associated with pacing pulses in one or more classification intervals before and/or after the pacing pulse.FIG. 3illustrates one example of classification intervals that may be implemented for cardiac response classification and detection window adaptation in accordance with embodiments of the invention. A pacing stimulation310is delivered to the heart, for example, to the right ventricle. The cardiac signal is blanked for a period of time320, typically about 0 milliseconds to about 40 milliseconds, following the delivery of the pacing stimulation310. After the blanking period320, a first time interval330is initiated. The duration of the first time interval330may be a programmable duration, for example, less than about 325 milliseconds.

The cardiac signal associated with the pacing pulse is sensed during the first time interval330. If the magnitude of the cardiac signal does not exceed a threshold in the first time interval330, then the cardiac response may be classified as a noncaptured response. If the cardiac signal exceeds a threshold value, then various features of the cardiac signal may be detected and used for detection window creation, matching, or adaptation. In some cases, sensing of the cardiac signal may be extended to additional time intervals, such as the second time interval340. The second time interval340may be programmable, and may have a duration less than about 325 milliseconds. The durations of the additional time intervals may be different or the same as the duration of the first time interval. Alternatively, the durations of the first and the second time intervals may be the same.

A delay period350may be established between the end of one time interval330and the beginning of another time interval340. The duration of the delay may be in a range of about 0 milliseconds (no delay) to about 40 milliseconds, for example. The cardiac response to the pacing stimulation310may be classified based on characteristics of the cardiac signal sensed in the first and/or the additional time intervals330,340.

FIG. 4illustrates cardiac response classification windows that may be utilized for cardiac devices and methods that discriminate between capture, noncapture, fusion, and noncapture with intrinsic activation, during evoked response detection, and that may be adapted in accordance with embodiments of the invention. Following delivery of a pace410, the sensing system is blanked, e.g., the sense electrodes are disconnected from sense amplifiers or the sense amplifiers are rendered inoperative, during a blanking period415. Following the blanking period, the cardiac signal is sensed in one or more time intervals. As illustrated inFIG. 4, sensing may occur in two time intervals420,450following the pacing pulse410.

In some scenarios, the second450and subsequent time intervals (not shown) may be triggered by events occurring in one or more previous intervals. In various implementations, sensing may be performed using the same electrode combination that was used to deliver the pacing stimulation. In other implementations, the pacing stimulation may be delivered using a first electrode configuration and sensing may use a second electrode configuration. Systems and methods for classifying a cardiac response to pacing using multiple time intervals and various sensing and pacing vectors are described in commonly owned U.S. patent applications: Ser. No. 10/733,869, filed Dec. 11, 2003, entitled “Cardiac Response Classification Using Multiple Classification Windows”; Ser. No. 10/734,599, filed Dec. 12, 2003, entitled “Cardiac Response Classification Using Retriggerable Classification Windows”; and Ser. No. 10/735,519, filed Dec. 12, 2003, entitled “Cardiac Response Classification Using Multisite Sensing And Pacing”; which are hereby incorporated herein by reference.

During the first time interval420, the system senses for a cardiac signal magnitude above a threshold level440. If the cardiac signal does not exceed the threshold440during the first time interval420, then the cardiac response is classified as noncapture and a backup pace470may be delivered. The backup pace470is typically a high energy pace that is delivered following a backup interval430. For example, the backup interval430may include an interval of about 100 ms timed from the delivery of the primary pace410.

The system may utilize one or more cardiac response classification windows455,456,460,495as illustrated inFIG. 4. An adaptive window method in accordance with embodiments of the invention involves determining if one or more peak values of the cardiac response signal falls, or does not fall, within one or more cardiac response classification windows455,456,460,495. The cardiac response detection windows455,456,460, and495are areas defined in terms of amplitude and time. In other embodiments, different or additional parameters may be used in addition to, or in place of the parameters of amplitude and time.

For this example, the system may classify a cardiac response as capture if a peak value of the cardiac signal is detected in the first capture detection window455and a peak value of the cardiac signal is detected in the second capture detection window456. If a cardiac signal peak is detected in the first non-captured intrinsic detection window460, or the second non-captured intrinsic detection window495, the cardiac response may be classified as noncapture with non-captured intrinsic activation. Otherwise, the beat is classified as a fusion/pseudofusion beat, or further discriminated. Depending on the cardiac response's classification, one or more windows associated with the classification may be adapted.

Devices and methods in accordance with embodiments of the present invention may involve the use of one or more noise detection windows435,436for further discrimination of cardiac waveforms. If signal peaks fall within the cardiac response classification windows455,456,460,495then the system may check for peaks opposite in polarity and comparable in magnitude to the cardiac response signal peaks.FIG. 4illustrates detection windows435,436in the first and the second time intervals420,450. The detection windows435,436may be any shape or size, and the shape and size may also be adaptable in accordance with the present invention. For example, the detection windows435,436may be the same size and/or shape as a corresponding capture detection window455,456in a particular time interval420,450, or may be a different size and/or shape. Formation of detection windows, aspects of which may be utilized in the approaches of the present invention are described in commonly owned U.S. patent application filed concurrently with this patent application, which is incorporated herein by reference and U.S. patent application Ser. No. 10/448,260, filed May 28, 2003, which is incorporated herein by reference.

FIG. 5illustrates three representative cardiac response waveform portions superimposed over the graph illustrated inFIG. 4. A non-captured intrinsic beat480, a PVC beat482, and a captured beat484are drawn, illustrating waveform parameters useful for adapting windows for cardiac waveform discrimination during cardiac response detection in accordance with the present invention. The waveform parameters of the PVC beat482illustrated in the graph ofFIG. 5include, but are not limited to, a negative peak amplitude481within the noise window436during the second time interval450, and a positive peak489within noise window435during the first time interval420. The waveform parameters of the non-captured intrinsic beat480illustrated in the graph ofFIG. 5include, but are not limited to, a positive peak amplitude491within the second time interval450and a negative peak amplitude493within the response classification window460. The waveform parameters of the captured beat484illustrated in the graph ofFIG. 5include, but are not limited to, a negative peak amplitude487within the response classification window455during the first time interval420, and a positive peak485within the response classification window456during the second time interval450.

As evident inFIG. 5, the non-captured intrinsic beat480and the captured beat484have morphologies similar enough that they may be confused if discrimination of non-captured intrinsic beats during evoked response detection and classification is not performed. Providing adaptive windowing in accordance with the present invention improves the discrimination capabilities of cardiac devices by allowing closer spacing of windows and smaller window sizes, and reduces or eliminates the inclusion of undesired response signals during capture threshold testing, capture verification, template initialization and/or updating, and/or for other purposes when cardiac response signal features vary over time.

The embodiments of the present system illustrated herein are generally described as being implemented in a patient implantable medical device (PIMD) such as a pacemaker/defibrillator (PD) that may operate in numerous pacing modes known in the art. Various types of single and multiple chamber implantable cardiac pacemaker/defibrillators are known in the art and may be used in connection with cardiac devices and methods that provide adaptive windowing during evoked response detection and classification in accordance with the present invention. The methods of the present invention may also be implemented in a variety of implantable or patient-external cardiac rhythm management devices, including single and multi chamber pacemakers, defibrillators, cardioverters, bi-ventricular pacemakers, cardiac resynchronizers, and cardiac monitoring systems, for example.

Although the present system is described in conjunction with an implantable cardiac pacemaker/defibrillator having a microprocessor-based architecture, it will be understood that the implantable pacemaker/defibrillator (or other device) may be implemented in any logic-based integrated circuit architecture, if desired.

Referring now toFIG. 6of the drawings, there is shown a cardiac rhythm management system that may be used to implement detection window adaptation in accordance with the present invention. The cardiac rhythm management system inFIG. 6includes a pacemaker/defibrillator800electrically and physically coupled to a lead system802. The housing and/or header of the pacemaker/defibrillator800may incorporate one or more electrodes908,909used to provide electrical stimulation energy to the heart and to sense cardiac electrical activity. The pacemaker/defibrillator800may utilize all or a portion of the pacemaker/defibrillator housing as a can electrode909. The pacemaker/defibrillator800may include an indifferent electrode908positioned, for example, on the header or the housing of the pacemaker/defibrillator800. If the pacemaker/defibrillator800includes both a can electrode909and an indifferent electrode908, the electrodes908,909typically are electrically isolated from each other.

The lead system802is used to detect electric cardiac signals produced by the heart801and to provide electrical energy to the heart801under certain predetermined conditions to treat cardiac arrhythmias. The lead system802may include one or more electrodes used for pacing, sensing, and/or defibrillation. In the embodiment shown inFIG. 6, the lead system802includes an intracardiac right ventricular (RV) lead system804, an intracardiac right atrial (RA) lead system805, an intracardiac left ventricular (LV) lead system806, and an extracardiac left atrial (LA) lead system808. The lead system802ofFIG. 6illustrates one embodiment that may be used in connection with the adaptive detection windowing methodologies described herein. Other leads and/or electrodes may additionally or alternatively be used.

The lead system802may include intracardiac leads804,805,806implanted in a human body with portions of the intracardiac leads804,805,806inserted into a heart801. The intracardiac leads804,805,806include various electrodes positionable within the heart for sensing electrical activity of the heart and for delivering electrical stimulation energy to the heart, for example, pacing pulses and/or defibrillation shocks to treat various arrhythmias of the heart.

As illustrated inFIG. 6, the lead system802may include one or more extracardiac leads808having electrodes, e.g., epicardial electrodes, positioned at locations outside the heart for sensing and pacing one or more heart chambers.

The right ventricular lead system804illustrated inFIG. 6includes an SVC-coil816, an RV-coil814, an RV-ring electrode811, and an RV-tip electrode812. The right ventricular lead system804extends through the right atrium820and into the right ventricle819. In particular, the RV-tip electrode812, RV-ring electrode811, and RV-coil electrode814are positioned at appropriate locations within the right ventricle819for sensing and delivering electrical stimulation pulses to the heart801. The SVC-coil816is positioned at an appropriate location within the right atrium chamber820of the heart801or a major vein leading to the right atrial chamber820of the heart801.

In one configuration, the RV-tip electrode812referenced to the can electrode909may be used to implement unipolar pacing and/or sensing in the right ventricle819. Bipolar pacing and/or sensing in the right ventricle may be implemented using the RV-tip812and RV-ring811electrodes. In yet another configuration, the RV-ring811electrode may optionally be omitted, and bipolar pacing and/or sensing may be accomplished using the RV-tip electrode812and the RV-coil814, for example. The RV-coil814and the SVC-coil816are defibrillation electrodes.

The left ventricular lead806includes an LV distal electrode813and an LV proximal electrode817located at appropriate locations in or about the left ventricle824for pacing and/or sensing the left ventricle824. The left ventricular lead806may be guided into the right atrium820of the heart via the superior vena cava. From the right atrium820, the left ventricular lead806may be deployed into the coronary sinus ostium, the opening of the coronary sinus850. The lead806may be guided through the coronary sinus850to a coronary vein of the left ventricle824. This vein is used as an access pathway for leads to reach the surfaces of the left ventricle824which are not directly accessible from the right side of the heart. Lead placement for the left ventricular lead806may be achieved via subclavian vein access and a preformed guiding catheter for insertion of the LV electrodes813,817adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented, for example, using the LV distal electrode referenced to the can electrode909. The LV distal electrode813and the LV proximal electrode817may be used together as bipolar sense and/or pace electrodes for the left ventricle. The left ventricular lead806and the right ventricular lead804, in conjunction with the pacemaker/defibrillator800, may be used to provide cardiac resynchronization therapy such that the ventricles of the heart are paced substantially simultaneously, or in phased sequence, to provide enhanced cardiac pumping efficiency for patients suffering from chronic heart failure.

The right atrial lead805includes a RA-tip electrode856and an RA-ring electrode854positioned at appropriate locations in the right atrium820for sensing and pacing the right atrium820. In one configuration, the RA-tip856referenced to the can electrode909, for example, may be used to provide unipolar pacing and/or sensing in the right atrium820. In another configuration, the RA-tip electrode856and the RA-ring electrode854may be used to provide bipolar pacing and/or sensing.

FIG. 6illustrates one embodiment of a left atrial lead system808. In this example, the left atrial lead808is implemented as an extracardiac lead with LA distal818and LA proximal815electrodes positioned at appropriate locations outside the heart801for sensing and pacing the left atrium822. Unipolar pacing and/or sensing of the left atrium may be accomplished, for example, using the LA distal electrode818to the can909pacing vector. The LA proximal815and LA distal818electrodes may be used together to implement bipolar pacing and/or sensing of the left atrium822.

Referring now toFIG. 7, there is shown an embodiment of a cardiac pacemaker/defibrillator900suitable for implementing detection window adaptation methods of the present invention.FIG. 7shows a cardiac pacemaker/defibrillator900divided into functional blocks. It is understood by those skilled in the art that there exist many possible configurations in which these functional blocks can be arranged. The example depicted inFIG. 7is one possible functional arrangement. Other arrangements are also possible. For example, more, fewer, or different functional blocks may be used to describe a cardiac pacemaker/defibrillator suitable for implementing the methodologies for detection window adaptation and cardiac response classification in accordance with the present invention. In addition, although the cardiac pacemaker/defibrillator900depicted inFIG. 7contemplates the use of a programmable microprocessor-based logic circuit, other circuit implementations may be utilized.

The cardiac pacemaker/defibrillator900depicted inFIG. 7includes circuitry for receiving cardiac signals from a heart and delivering electrical stimulation energy to the heart in the form of pacing pulses or defibrillation shocks. In one embodiment, the circuitry of the cardiac pacemaker/defibrillator900is encased and hermetically sealed in a housing901suitable for implanting in a human body. Power to the cardiac pacemaker/defibrillator900is supplied by an electrochemical battery980. A connector block (not shown) is attached to the housing901of the cardiac pacemaker/defibrillator900to allow for the physical and electrical attachment of the lead system conductors to the circuitry of the cardiac pacemaker/defibrillator900.

The cardiac pacemaker/defibrillator900may be a programmable microprocessor-based system, including a control system920and a memory970. The memory970may store parameters for various pacing, defibrillation, and sensing modes, along with other parameters. Further, the memory970may store data indicative of cardiac signals received by other components of the cardiac pacemaker/defibrillator900. The memory970may be used, for example, for storing historical EGM and therapy data. The historical data storage may include, for example, data obtained from long-term patient monitoring used for trending and/or other diagnostic purposes. Historical data, as well as other information, may be transmitted to an external programmer unit990as needed or desired.

The control system920and memory970may cooperate with other components of the cardiac pacemaker/defibrillator900to control the operations of the cardiac pacemaker/defibrillator900. The control system920depicted inFIG. 7incorporates detection window circuitry926configured to provide and adapt detection windows as previously described in accordance with embodiments of the invention.

The control system920further includes a cardiac response classification processor925for classifying cardiac responses to pacing stimulation. The cardiac response classification processor performs the function of analyzing the location of cardiac signal features with respect to one or more detection window boundaries to determine the cardiac response to pacing.

The control system920may include additional functional components including a pacemaker control circuit922, an arrhythmia detector921, along with other components for controlling the operations of the cardiac pacemaker/defibrillator900.

Telemetry circuitry960may be implemented to provide communications between the cardiac pacemaker/defibrillator900and an external programmer unit990. In one embodiment, the telemetry circuitry960and the programmer unit990communicate using a wire loop antenna and a radio frequency telemetric link, as is known in the art, to receive and transmit signals and data between the programmer unit990and the telemetry circuitry960. In this manner, programming commands and other information may be transferred to the control system920of the cardiac pacemaker/defibrillator900from the programmer unit990during and after implant. In addition, stored cardiac data pertaining to capture threshold, capture detection and/or cardiac response classification, for example, along with other data, may be transferred to the programmer unit990from the cardiac pacemaker/defibrillator900.

The telemetry circuitry960may provide for communication between the cardiac pacemaker/defibrillator900and an advanced patient management (APM) system. The advanced patient management system allows physicians or other personnel to remotely and automatically monitor cardiac and/or other patient conditions. In one example, a cardiac pacemaker/defibrillator, or other device, may be equipped with various telecommunications and information technologies that enable real-time data collection, diagnosis, and treatment of the patient. Various embodiments described herein may be used in connection with advanced patient management.

Methods, structures, and/or techniques described herein, which may be adapted to provide for remote patient/device monitoring, diagnosis, therapy, or other APM related methodologies, may incorporate features of one or more of the following references: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066, which are hereby incorporated herein by reference.

A right atrial sensing circuit931serves to detect and amplify electrical signals from the right atrium of the heart. Bipolar sensing in the right atrium may be implemented, for example, by sensing voltages developed between the RA-tip856and the RA-ring854. Unipolar sensing may be implemented, for example, by sensing voltages developed between the RA-tip856and the can electrode909. Outputs from the right atrial sensing circuit are coupled to the control system920.

A right ventricular sensing circuit932serves to detect and amplify electrical signals from the right ventricle of the heart. The right ventricular sensing circuit932may include, for example, a right ventricular rate channel933and a right ventricular shock channel934. Right ventricular cardiac signals sensed through use of the RV-tip812electrode are right ventricular near-field signals and are denoted RV rate channel signals. A bipolar RV rate channel signal may be sensed as a voltage developed between the RV-tip812and the RV-ring811. Alternatively, bipolar sensing in the right ventricle may be implemented using the RV-tip electrode812and the RV-coil814. Unipolar rate channel sensing in the right ventricle may be implemented, for example, by sensing voltages developed between the RV-tip812and the can electrode909.

Right ventricular cardiac signals sensed through use of the defibrillation electrodes are far-field signals, also referred to as RV morphology or RV shock channel signals. More particularly, a right ventricular shock channel signal may be detected as a voltage developed between the RV-coil814and the SVC-coil816. A right ventricular shock channel signal may also be detected as a voltage developed between the RV-coil814and the can electrode909. In another configuration the can electrode909and the SVC-coil electrode816may be electrically shorted and a RV shock channel signal may be detected as the voltage developed between the RV-coil814and the can electrode909/SVC-coil816combination.

Outputs from the right ventricular sensing circuit932are coupled to the control system920. In one embodiment of the invention, rate channel signals and shock channel signals may be used to develop morphology templates for analyzing cardiac signals. In this embodiment, rate channel signals and shock channel signals may be transferred from the right ventricular sensing circuit932to the control system920and analyzed for arrhythmia detection.

Left atrial cardiac signals may be sensed through the use of one or more left atrial electrodes815,818, which may be configured as epicardial electrodes. A left atrial sensing circuit935serves to detect and amplify electrical signals from the left atrium of the heart. Bipolar sensing and/or pacing in the left atrium may be implemented, for example, using the LA distal electrode818and the LA proximal electrode815. Unipolar sensing and/or pacing of the left atrium may be accomplished, for example, using the LA distal electrode818to can909vector or the LA proximal electrode815to can909vector.

A left ventricular sensing circuit936serves to detect and amplify electrical signals from the left ventricle of the heart. Bipolar sensing in the left ventricle may be implemented, for example, by sensing voltages developed between the LV distal electrode813and the LV proximal electrode817. Unipolar sensing may be implemented, for example, by sensing voltages developed between the LV distal electrode813or the LV proximal electrode817and the can electrode909.

Optionally, an LV coil electrode (not shown) may be inserted into the patient's cardiac vasculature, e.g., the coronary sinus, adjacent to the left heart. Signals detected using combinations of the LV electrodes813,817, LV coil electrode (not shown), and/or the can electrode909may be sensed and amplified by the left ventricular sensing circuitry936. The output of the left ventricular sensing circuit936is coupled to the control system920.

The outputs of the switching matrix910may be operated to couple selected combinations of electrodes811,812,813,814,815,816,817,818,856,854to an evoked response sensing circuit937. The evoked response sensing circuit937serves to sense and amplify voltages developed using various combinations of electrodes for discrimination of various cardiac responses to pacing in accordance with embodiments of the invention. The cardiac response classification processor925may cooperate with detection window circuitry926to analyze the output of the evoked response sensing circuit937for implementation of cardiac pacing response classification.

Various combinations of pacing and sensing electrodes may be utilized in connection with pacing and sensing the cardiac signal following the pace pulse to classify the cardiac response to the pacing pulse. For example, in some embodiments, a first electrode combination is used for pacing a heart chamber and a second electrode combination is used to sense the cardiac signal following pacing. In other embodiments, the same electrode combination is used for pacing and sensing. Use of different electrodes for pacing and sensing in connection with capture verification is described in commonly owned U.S. Pat. No. 6,128,535 which is incorporated herein by reference.

The pacemaker control circuit922, in combination with pacing circuitry for the left atrium, right atrium, left ventricle, and right ventricle941,942,943,944, may be implemented to selectively generate and deliver pacing pulses to the heart using various electrode combinations. The pacing electrode combinations may be used to effect bipolar or unipolar pacing pulses to a heart chamber using one of the pacing vectors as described above. In some implementations, the cardiac pacemaker/defibrillator900may include a sensor961that is used to sense the patient's hemodynamic need. In one implementation, the sensor may comprise, for example, an accelerometer configured to sense patient activity. In another implementation, the sensor may comprise an impedance sensor configured to sense patient respiration. The pacing output of the cardiac pacemaker/defibrillator may be adjusted based on the sensor961output.

The electrical signal following the delivery of the pacing pulses may be sensed through various sensing vectors coupled through the switch matrix910to the evoked response sensing circuit937and/or other sensing circuits and used to classify the cardiac response to pacing. The cardiac response may be classified as one of a captured response, a non-captured response, a non-captured response with intrinsic activation, and a fusion/pseudofusion beat, for example.

Subcutaneous electrodes may provide additional sensing vectors useable for cardiac response classification. In one implementation, a cardiac rhythm management system may involve a hybrid system including an intracardiac device configured to pace the heart and an extracardiac device, e.g., a subcutaneous defibrillator, configured to perform functions other than pacing. The extracardiac device may be employed to detect and classify the cardiac response to pacing based on signals sensed using subcutaneous electrode arrays. The extracardiac and intracardiac devices may operate cooperatively with communication between the devices occurring over a wireless link, for example. Examples of subcutaneous electrode systems and devices are described in commonly owned U.S. patent applications Ser. No. 10/462,001, filed Jun. 13, 2003 and Ser. No. 10/465,520, filed Jun. 19, 2003, which are hereby incorporated herein by reference in their respective entireties.

The components, functionality, and structural configurations depicted herein are intended to provide an understanding of various features and combination of features that may be incorporated in an implantable pacemaker/defibrillator. It is understood that a wide variety of cardiac monitoring and/or stimulation device configurations are contemplated, ranging from relatively sophisticated to relatively simple designs. As such, particular cardiac device configurations may include particular features as described herein, while other such device configurations may exclude particular features described herein.

Various modifications and additions can be made to the preferred embodiments discussed hereinabove without departing from the scope of the present invention. Accordingly, the scope of the present invention should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.