Patent Application
20100292596
The present invention is related to the detection of electrical signals originating from a human or animal heart. Such signals can be obtained from electrodes implanted in the heart tissue or from electrical contacts at the body of an implantable medical device. Typically, these electrical signals are processed to identify contractions of the heart tissue and are used to control electrical stimulation devices. Alternatively, the electrical signals are recorded within the implanted medical device and/or transmitted outside the body to external devices, remote databases, expert systems, or the like for further evaluation. The implantable medical device can be an implantable pacemaker, cardioverter-defibrillator, or a monitoring device.
In particular for monitoring devices it is desired to obtain electrical signals from a human or animal heart with electrical contacts at the body of an implantable medical device, hence without the need to implant electrical leads to the hearts. One drawback of such systems is to the decreased ability to correctly identify weak signals in the presence of increased noise.
The following abbreviations are used herein:
SEGM subcutaneous electrogram
QRS signal representing ventricular depolarization of the heart
ECG electrocardiogram
EMG electromyogram
Implantable monitoring devices that record and monitor electrical activity of the heart known in the art typically use two sensing electrodes. While they are smaller, signal quality is known to be poor. A single vector, whether on a short lead or integrated into the device, is highly susceptible to spurious noise and loss of sensitivity due to, for example, slight electrode orientation changes by movement of the implanted device within the body. Implantation of these devices is also time consuming since vector mapping must be done to ensure the device is implanted in an orientation to produce the best signal.
Presently, typical signal characteristics of the signals originating from heart activity, like QRS complexes, are detected by comparing the input signal amplitude to a dynamic sensing threshold. The threshold is set to a percentage of the previous detected maximum amplitude. Noise is detected when the signal crosses a target threshold and activates a noise counter. The noise counter can be re-activated every time the signal crosses the threshold while the counter is active. While the counter is active, no new QRS detections are allowed. In each case (sensing and noise detection), the absolute value of the signal is used to detect large positive or negative spikes, since the polarity of the QRS complex is not known. When the signal to noise ratio (in this case, amplitude of QRS complex to any other signal between those complexes) is low, the probability of oversensing (detecting noise as valid QRS events) or undersensing (missing QRS events) is greatly increased.
In U.S. Pat. No. 6,699,200 an implantable medical device with multi-vector sensing electrodes is described. It proposes monitoring different channels, each originating from a pair of electrodes, and capturing the maximum signal for appropriate sensing. In cases where the signal amplitude from one electrode pair drops dramatically, it is switched to another pair of electrodes.
The present invention is directed towards overcoming one or more of the above-identified problems.
The present invention is directed to a method to obtain electrical signals of a human or animal heart by an implantable device with sensing electrode contacts thereon, hence without the need for implanted electrodes at the heart tissue. In this case the amplitude of noise in relation to the signal amplitude is increased. This is the result of at least two factors, namely, the distance between electrodes and the proximity to muscle tissue. If the distance between electrodes is small electrocardiogram (ECG) amplitude is reduced, and the proximity to muscle tissue increases EMG (electromyogram) signals. Since ECG noise is overlapping the frequency range of the ECG, simple filtering does not always reduce it. The present invention is therefore further directed to a sensing algorithm that increases the signal-to-noise ratio and improves QRS detection from weak signals in the presence of increased noise.
To overcome the disadvantages of former solutions, the present invention proposes a method for detecting electrical signals originating from a human or animal heart where electrical signals of at least two sensing channels are evaluated. In a preferred embodiment, three electrodes are connected to three sensing channels. However, in other embodiments, more than three electrodes may be connected to more than three sensing channels.
According to the present invention, the electrical signals received by the at least two sensing channels are combined to a single channel, a so called combined channel. Preferably, the electrical signals pass through a filter sensing block before they are combined. In a preferred embodiment, the electrical signals are combined by summing the weighted signals of the at least two sensing channels.
According to the present invention, at least a template is extracted from the incoming electrical signals of the combined channel. The time window for the template depends from the event to be detected. In a preferred embodiment, a template of a QRS complex is extracted. For encompassing the entire QRS complex the width of the template is approximately 125 ms. For extracting the template from the incoming data stream of the combined channel, different suitable methods may be used, for example, pattern recognition or simple threshold methods. For extracting a QRS complex in a preferred embodiment, simple threshold methods are used. Extracted templates then are stored for further use.
At least a part of the extracted templates is then compared with the incoming electrical signals to detect signal patterns indicating an event such as, for example, abnormalities in the heart activities like contractions of the heart tissue. Templates are compared preferably with the electrical signals of the combined sensing channel. Prior to performing the comparison, in a preferred embodiment of the invention, detection thresholds are calculated. At least one noise threshold is also defined. Detection thresholds and/or noise thresholds can be derived, for example, based on the extracted template.
In a preferred embodiment, comparison of the incoming electrical signals with the at least one template includes performing a pseudo cross covariance between the template(s) and the incoming data stream, which can be an ECG stream, for example. From the calculated cross covariance signal and, if necessary, considering the thresholds, patterns are detected. In a preferred embodiment, QRS complexes are detected. In the case of QRS complexes in another preferred embodiment, detection is only performed on signals with positive values.
If patterns have been detected, electrical devices like a pacemaker, a cardioverter-defibrillator, a monitoring device or such may be controlled depending from the detected pattern, and/or the electrical signals are recorded within the implanted medical device and/or transmitted outside the body to external devices, remote databases, expert systems or the like for further evaluation.
In a further preferred embodiment, detected patterns are combined with the template for obtaining a new template which reflects, for example, changes in the ECG.
In another preferred embodiment, thresholds are adjusted, if within a predefined period of time no pattern has been detected. Preferably then the threshold is decreased.
A system for detecting electrical signals originating from a human or animal heart according to the invention includes at least one data processing device. It should be understood that the data processing device may include a memory coupled to the data processing device that has coding or software stored thereon for executing the method. The system is configured in such a way that the following steps may be performed:
In a preferred embodiment of the present invention, the device includes at least three sensing electrodes which are connected to at least two sensing channels. Preferably, the combination of the sensing electrodes with the sensing channels can be changed. In a preferred embodiment, a switch matrix is used for changing the assignment of the electrodes to the sensing channels.
In a preferred embodiment, the inventive system includes an implantable device, which is realized, for example, as pacemaker, cardioverter-defibrillator, or a monitoring device. The system can also include one or more external devices such as remote databases, expert systems or the like.
An implantable device has in a preferred embodiment an enclosure made from an electrically conductive and biocompatible material like titanium, for example. In another preferred embodiment, the implantable device is made of a conductive body covered with a non-conductive material. Preferably, the nonconductive material has one or more holes that allow the conductive body to contact the surrounding tissue.
It is further an object of the present invention to provide a computer-readable storage medium storing program code for causing a data processing device to perform a method for detecting electrical signals originating from a human or animal heart. It should be understood that the data processing device may include a memory coupled to the data processing device that has coding or software stored thereon for executing the method. Additionally, it should be appreciated that one example of a data processing device is a computer device for running a computer program. The method includes the steps of:
The implantable medical device described herein uses electrodes to sense subcutaneous electrograms (SEGM). In one embodiment, the enclosure of the implantable medical device is of an electrically conductive and biocompatible material like titanium and includes three or more isolated, electrically conductive electrode contacts forming electrodes. In another embodiment, electrode contacts are integrated in a part of the implantable medical device that is made of non-conductive material. In yet another embodiment, the conductive body is covered by a non-conductive material having one or more holes that allow the body to contact the tissue. It is to be understood that these embodiments can be combined in any suitable way.
The electrodes are connected to at least three sensing channels. In one embodiment with three sensing channels (A, B, C) and three electrodes (E1, E2, E3), sensing channel A is connected to electrodes E1 and E2, sensing channel B is connected to electrodes E1 and E3, and sensing channel C is connected to electrodes E2 and E3. It is to be understood that any combination of connecting electrodes to the sensing channels would be possible, and that also more electrodes, including the enclosure, and also more sensing channels can be implemented without departing from the spirit and scope of the present invention. In an alternative embodiment, the electrodes are connected to the sensing channels by a switch matrix that allows connecting any electrode contact to any sensing channel. In each sensing channel, the electrical signals pass through a filter sensing block for each channel. In one embodiment, the signals are digitized by an analog-to-digital converter, as known in the art. It is to be understood that the further processing of the digitized signals could be performed by a microprocessor or programmable microcontroller or the like, as known in the art.
The present invention seeks to first reduce noise levels by using an algorithm that relies on both the shape and amplitude of the QRS complex. Next the algorithm uses inherent properties to increase the detection SNR while not compromising noise detection.
Other objects, aspects and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims.
In a preferred embodiment, three electrodes are arranged in a triangle to form three sensing channels. Any one electrode is used in two sensing channels. In one exemplary embodiment, three sensing channels (A, B, C) and three electrodes (E1, E2, E3) are connected, such that sensing channel A is connected to electrodes E1 and E3, sensing channel B is connected to electrodes E2 and E3, and sensing channel C is connected to electrodes E1 and E2. In general, near field-noise is sensed on all channels but in different polarities, amplitudes and phases. EGMs from the heart are sensed as far field signals and are always coincident on all channels, but have varying polarities and amplitudes based on electrode spacing and orientation to the heart.
SNR is increased in the following manner:
In step 212 the detection threshold is calculated and the noise threshold is set based on the QRS template. The template is then cross-correlated in step 214 with the combined data channel to form a pseudo-cross-covariance (non-normalized). The resultant waveform reflects how similar the template and the combined data are. QRS complexes are detected in step 216 from this derived (cross-covariant) data stream. Cross-covariance significantly smoothes the combined data and increases SNR (by approximately 8 dB) while also transforming the signal from unipolar to bipolar. After each QRS detection, new complexes are extracted from the combined data. In step 222 the new complexes are then averaged into the current template, thus modifying it. This modification allows the template to maintain validity during changing signal conditions (i.e., device rotation due to postural changes). If, in step 218 it is recognized that no QRS structure is detected, the detection threshold is lowered in step 220 after enough time has elapsed. In step 224 noise detection is performed by a noise detection timer. If noise is detected, i.e., if the noise detection timer is active, no QRS detection can be performed.
In a preferred embodiment, the non-normalized cross-correlation is calculated according to the equation (1):
cross-covariance is calculated according to equation (2):
and the proposed pseudo-cross-covariance according to equation (3):
where x represents the template, y represents the incoming combined channel data stream, and μ represents the mean of the data set over indices i.
The calculations presented do not necessarily need to be done on all samples in the data stream and template. For example, every other point could be used. In an exemplary embodiment, the incoming signals are sampled at 512 Hz, yielding 64 data samples in the template for 125 ms. It is possible to use only 32 of those samples for calculation to reduce the needed calculations. Similarly, every 4th sample could be used to yield only 16 operations. However, improvement in SNR and detection accuracy degrades as fewer samples are used.
Because the combined data and the template are both positively oriented, the region of interest in the cross-covariance between them is also positive. Therefore, QRS detection need only be done on positive values. Ignoring negative values in the cross-covariant signal further increases the SNR (to approximately +10 dB). Noise detection can be performed on both positive and negative (or absolute value), or on positive only values.
In an alternative embodiment, the detection and noise thresholds can be dynamically altered based on the template. The invention uses the sum of the squares of the data points of the template to generate a value representing “signal quality.” Detection and noise thresholds are set based on this value. The resultant threshold is a measure of the fit between the template and the combined data and not simply the amplitude of the previously detected QRS peak.
It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teachings without departing from the spirit and scope of the present invention. The disclosed examples and embodiments are presented for purposes of illustration only and are not meant to limit the scope of the invention in any way. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.
This application claims the priority of U.S. Provisional Patent Application No. 61/177,671, filed on May 13, 2009 in the U.S. Patent Office, the disclosure of which is incorporated herein in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61177671 | May 2009 | US |
