Cardiac signal template generation using waveform clustering

This is a divisional of U.S. patent application Ser. No. 11/116,544, filed on Apr. 28, 2005, to issue as U.S. Pat. No. 7,499,751 on Mar. 3, 2009, to which Applicant claims priority under 35 U.S.C. §120, and which is incorporated herein by reference.

The present invention relates generally to implantable medical devices and, more particularly, to cardiac devices and methods using cardiac waveform clustering for template generation.

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 comprise 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.

When a pace pulse produces a contraction in the heart tissue, the electrical cardiac signal preceding the contraction is denoted the captured response (CR). 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.

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.

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. 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.

Capture detection, for example, involves discriminating captured beats from fusion/pseudofusion beats, intrinsic beats, noise, and noncapture. Discriminating between various cardiac responses can be accomplished by comparing cardiac signals to templates representative of various response types. The present invention involves methods and systems for creating templates used in connection with recognizing various cardiac responses.

The present invention involves various cardiac devices and methods using cardiac waveform clustering for template generation. A method of characterizing a cardiac response in accordance with the present invention involves delivering pacing pulses to heart, the pacing pulses having an energy greater than a capture threshold. Cardiac signals are sensed following the pacing pulses. One or more initial templates may be selectively updated using the sensed cardiac signals. A cardiac response template may be formed using one or more selected templates. Selectively updating the initial templates may involve using selected ones of the sensed cardiac signals to update a particular template. Embodiments may further involve selectively updating the templates by comparing the sensed cardiac signals to each of the templates, and selectively updating the templates using the cardiac signals based on the comparison, such as by comparing cardiac signal features to template features and/or by determining a similarity between each template and each cardiac signal and selectively updating a particular template having a particular similarity to a particular cardiac signal.

Selectively updating the templates may involve associating a counter with each template and incrementing the counter associated with a particular template if the particular template is updated. Characterizing the cardiac response using the selected template may involve characterizing the cardiac response using a particular template correlated to a higher number of cardiac signals than other templates. The selected template may be used to classify a cardiac response to pacing beat by beat, during capture threshold testing, and/or for automatic capture verification.

Other methods of characterizing a cardiac response to pacing in accordance with the present invention involve delivering pacing pulses to a heart sufficient in energy to effect capture and sensing cardiac signals respectively following delivery of the pacing pulses. The cardiac signals are clustered, defining a plurality of clusters, so that cardiac response templates may be formed using a selected one or more of the plurality of clusters. One or more features of the cardiac signals may be used to define the plurality of clusters, such as peak times of the cardiac signals, peak times associated with negative polarity peaks and/or positive polarity peaks, peak-amplitudes of the cardiac signals, or other features or signal attributes. A cardiac response template may be formed using selected clusters by forming the cardiac response template using a cluster associated with a greatest number of cardiac signals having similar characteristics.

Further embodiments of the present invention are directed to devices for characterizing a cardiac response to pacing. The device may include a sensing system configured to sense cardiac signals following pacing pulses delivered to a heart, having a processor coupled to the sensing system. The processor may be configured to selectively update a plurality of templates using the cardiac signals, and to characterize the cardiac response to pacing using a selected template of the templates. The processor may further be configured to provide one or more initial templates and selectively update the initial templates. The processor may be configured to compare the cardiac signals from a paced response to each of the templates and to selectively update the templates using the cardiac signals based on the comparison.

Other embodiments provide for the processor to be configured to determine a similarity between each template and each cardiac signal, and to update a particular template having a particular similarity to a particular cardiac signal. The processor may be configured to characterize the cardiac response using a template selected by a criterion, such as by matching a predetermined number cardiac signals before other templates are matched to the predetermined number of cardiac signals.

Yet another embodiment involves system for characterizing a cardiac response to pacing. The system includes a pulse generator configured to deliver pacing pulses to a heart sufficient in energy to effect capture. A sensor system is configured to sense cardiac signals respectively following delivery of the pacing pulses. A processor is coupled to the sensor system and is configured to cluster the cardiac signals to define a plurality of clusters and to form a cardiac response template using a selected one or more of the plurality of clusters.

The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.