The present invention relates generally to implantable pacemakers and more particularly to a subcutaneous electrode used to sense, record, and acquire electrocardiographic data and waveform tracings from an implanted pacemaker without the need for or use of surface (skin) electrodes. More particularly, the present invention relates to spiral electrodes placed into recesses incorporated within a compliant, insulative xe2x80x9csurround.xe2x80x9d Each spiral electrode becomes an integral element of a Subcutaneous Electrode Array or SEA that, in turn, detects cardiac depolarizations communicable and displayable by a portable device programmer.
The electrocardiogram (ECG) is commonly used in medicine to determine the status of the electrical conduction system of the human heart. As practiced, an ECG recording device is commonly attached to the patient via ECG leads connected to pads arrayed on the patient""s body so as to achieve a recording that displays the cardiac waveforms in any one of 12 possible vectors.
Since the implantation of the first cardiac pacemaker, implantable medical device technology has advanced with the development of sophisticated, programmable cardiac pacemakers, pacemaker-cardioverter-defibrillator arrhythmia control devices and drug administration devices designed to detect arrhythmias and apply appropriate therapies. The detection and discrimination between various arrhythmic episodes in order to trigger the delivery of an appropriate therapy is of considerable interest. Prescription for implantation and programming of the implanted device are based on the analysis of the PQRST electrocardiogram (ECG) and the electrogram (EGM). The waveforms are usually separated for such analysis into the P-wave and R-wave in systems that are designed to detect the depolarization of the atrium and ventricle respectively. Such systems employ detection of the occurrence of the P-wave and R-wave, analysis of the rate, regularity, and onset of variations in the rate of recurrence of the P-wave and R-wave, the morphology of the P-wave and R-wave and the direction of propagation of the depolarization represented by the P-wave and R-wave in the heart. The detection, analysis and storage of such EGM data within implanted medical devices are well known in the art. Acquisition and use of ECG tracing(s), on the other hand, has generally been limited to the use of an external ECG recording machine attached to the patient via surface electrodes of one sort or another.
The aforementioned ECG systems that utilize detection and analysis of the PQRST complex are all dependent upon the spatial orientation and number of electrodes available near or around the heart to pick up the depolarization wave front.
As the functional sophistication and complexity of implantable medical device systems increased over the years, it has become increasingly more important for such systems to include a system for facilitating communication between one implanted device and another implanted device and/or an external device, for example, a programming console, monitoring system, or the like. For diagnostic purposes, it is desirable that the implanted device be able to communicate information regarding the device""s operational status and the patient""s condition to the physician or clinician. State of the art implantable devices are available which can even transmit a digitized electrical signal to display electrical cardiac activity (e.g., an ECG, EGM, or the like) for storage and/or analysis by an external device. The surface ECG, however, has remained the standard diagnostic tool since the very beginning of pacing and remains so today.
To diagnose and measure cardiac events, the cardiologist has several tools from which to choose. Such tools include twelve-lead electrocardiograms, exercise stress electrocardiograms, Holter monitoring, radioisotope imaging, coronary angiography, myocardial biopsy, and blood serum enzyme tests. Of these, the twelve-lead electrocardiogram (ECG) is generally the first procedure used to determine cardiac status prior to implanting a pacing system; thereafter, the physician will normally use an ECG available through the programmer to check the pacemaker""s efficacy after implantation. Such ECG tracings are placed into the patient""s records and used for comparison to more recent tracings. It must be noted, however, that whenever an ECG recording is required (whether through a direct connection to an ECG recording device or to a pacemaker programmer), external electrodes and leads must be used.
Unfortunately, surface electrodes have some serious drawbacks. For example, electrocardiogram analysis performed using existing external or body surface ECG systems can be limited by mechanical problems and poor signal quality. Electrodes attached externally to the body are a major source of signal quality problems and analysis errors because of susceptibility to interference such as muscle noise, power line interference, high frequency communication equipment interference, and baseline shift from respiration. Signal degradation also occurs due to contact problems, ECG waveform artifacts, and patient discomfort. Externally attached electrodes are subject to motion artifacts from positional changes and the relative displacement between the skin and the electrodes. Furthermore, external electrodes require special skin preparation to ensure adequate electrical contact. Such preparation, along with positioning the electrode and attachment of the ECG lead to the electrode needlessly prolongs the pacemaker follow-up session. One possible approach is to equip the implanted pacemaker with the ability to detect cardiac signals and transform them into a tracing that is the same as or comparable to tracings obtainable via ECG leads attached to surface electrodes.
It is known in the art to monitor electrical activity of the human heart for diagnostic and related medical purposes. U.S. Pat. No. 4,023,565 issued to Ohlsson describes circuitry for recording ECG signals from multiple lead inputs. Similarly, U.S. Pat. No. 4,263,919 issued to Levin, U.S. Pat. No. 4,170,227 issued to Feldman, et al, and U.S. Pat. No. 4,593,702 issued to Kepski, et al, describe multiple electrode systems that combine surface EKG signals for artifact rejection.
The primary use for multiple electrode systems in the prior art appears to be vector cardiography from ECG signals taken from multiple chest and limb electrodes. This is a technique whereby the direction of depolarization of the heart is monitored, as well as the amplitude. U.S. Pat. No. 4,121,576 issued to Greensite discusses such a system.
Numerous body surface ECG monitoring electrode systems have been employed in the past in detecting the ECG and conducting vector cardiographic studies. For example, U.S. Pat. No. 4,082,086 issued to Page, et al., discloses a four electrode orthogonal array that may be applied to the patient""s skin both for convenience and to ensure the precise orientation of one electrode to the other. U.S. Pat. No. 3,983,867 issued to Case describes a vector cardiography system employing ECG electrodes disposed on the patient in normal locations and a hex axial reference system orthogonal display for displaying ECG signals of voltage versus time generated across sampled bipolar electrode pairs.
U.S. Pat. No. 4,310,000 to Lindemans and U.S. Pat. Nos. 4,729,376 and 4,674,508 to DeCote, incorporated herein by reference, disclose the use of a separate passive sensing reference electrode mounted on the pacemaker connector block or otherwise insulated from the pacemaker case in order to provide a sensing reference electrode that is not part of the stimulation reference electrode and thus does not have residual after-potentials at its surface following delivery of a stimulation pulse.
Moreover, in regard to subcutaneously implanted EGM electrodes, the aforementioned Lindemans U.S. Pat. No. 4,310,000 discloses one or more reference sensing electrode positioned on the surface of the pacemaker case as described above. U.S. Pat. No. 4,313,443 issued to Lund describes a subcutaneously implanted electrode or electrodes for use in monitoring the ECG.
U.S. Pat. No. 5,331,966 to Bennett, incorporated herein by reference, discloses a method and apparatus for providing an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes (located on the body of an implanted device).
Patent application Ser. No. 09/697,438, entitled, xe2x80x9cSurround Shroud Connector And Electrode Housings For A Subcutaneous Electrode Array And Leadless ECGs,xe2x80x9d by Ceballos et al., incorporated herein by reference, discloses an apparatus for providing a compliant surround circumferentially placed around the pacemaker""s perimeter and equipped with recesses for various types of electrodes used to detect cardiac depolarization waves. It is in conjunction with this compliant shroud that the present invention is practiced.
The present invention encompasses a Subcutaneous Electrode Spiral, Spiral Electrode, as a preferred embodiment, that is embedded individually into three or four recessed casings placed in a compliant surround that is attached to the perimeter of an implanted pacemaker. These electrodes are electrically connected to the circuitry of the implanted pacemaker and detect cardiac depolarization waveforms displayable as electrocardiographic tracings on the pacemaker Programmer screen when the programming head is positioned above an implanted pacemaker (or other implanted device) so equipped with a leadless Subcutaneous Electrode Array (SEA).
The recessed casing is composed of a polymer. This casing has walls into which body fluid is allowed to flow so that the body fluid is above and around the spiral, spiral coil, which is maintained within the casing by a small circumferential well constructed into the base of the recessed casing. The spiral coil may be constructed from any of the titanium group of metals, as well as platinum, platinum alloys, or any of the platinum group of metals with or without alloying components. This spiral coil constitutes the proximal end of a continuous wire that, upon egress from the recessed casing, is insulated by tubular, insulative material such as silicone. The insulated tubular wiring fits into channel(s) located on the inner arc of the aforementioned compliant surround. The distal end of this wire is welded to a connector feedthrough assembly in contact with amplifier(s) in the microprocessor circuitry.
The present invention is designed to solve several problems that may be encountered in the use of subcutaneous electrodes generally. The electrical potentials from the cardiac depolarizations are very small, in the order of tens of microvolts. As a result, the signal that reaches the electrodes may have a low slew rate. Solid, plate electrodes exhibit noise spikes and potential drifts that are large in comparison to the cardiac signals. These plate electrodes themselves may be the major contributors to such noise. Contamination of electrode surfaces at the molecular level may cause extraneous voltage excursions large enough to interfere with the sensing function. Motion artifacts from the perturbation caused by fluid or tissue motion next to the surface has been demonstrated in tank tests, and represents a commonly observed phenomenon.
The present invention, in its preferred embodiment, solves these issues in that it is a spiral, subcutaneous electrode that can be attached to a pulse generator via a compliant surround and provide a reliable response. The electrode has a large surface area, is manufactured from low cost materials, and can be assembled in an inexpensive way. The surface of the electrode has no direct contact with body tissue and moving body fluids, but it is treated with electrode coatings such as platinum black or titanium nitride to enhance its signal-conducting and depolarization properties. Such enhancing coatings are important since, as time passes, the pacemaker and, with it, the electrodes become encapsulated in scar tissue and thus come into indirect contact with the body tissue. Even indirect tissue contact may have a damping effect on the detection ability of the electrode if not treated with enhancing coatings.
The electrode, as designed, affords a continuous wire connection, without weld or bonds on the electrode surface. Moreover, the electrode is anchored in the device free of any chemical bonding that might cause potential excursions due to electro active species being released to the electrode surface. The portion of the electrode covered by the recessed casing is minimal thereby preventing any potential excursions caused by fluid creep or by trapped oxygen or hydrogen gradients, or Ph gradients. A second embodiment of the present invention employs a coiled electrode design that is manufactured in a rectangular configuration. This electrode has all the same electrical and signal detection properties as the spiral electrode and is constructed of the same materials and has the same enhancing coatings.
A third embodiment of the present invention uses a flat plated electrode embedded within the well of the recessed casing. To mechanically stabilize the electrode and reduce the possibility of electro kinetic potential disturbances on the electrode (due to tissue or fluid motion artifact), the bottom of the recess or well is patterned with a cross hatch of elevations and depressions with a minimum of contact area between the plate and the polymer used in the recessed casing. The purpose of the cross-hatch is to support the electrode and provide fluid contact to the underside of the electrode interface.
A protective cap, vented to allow fluid ingress to the active electrode element, may be placed over the well containing the flat electrode. This cap applies pressure on the electrode, pressing it against the textured bottom of the recess or well, thus preventing motion of the electrode. This cap also serves to dampen any external fluid motion that might otherwise reach the electrode and cause electro kinetic potential transients. The electrodes"" surfaces require protection during handling as well as to prevent contamination. A coating, such as may be provided by Dexamethazone Sodium Phosphate, provides such protection as well as enhancing the wetting of the electrode surface after implant. Conductive hydro gels, applied wet and allowed to dry, may also be applied to the electrode surfaces to protect them from damage during handling and prevent contamination.
The spacing of the electrodes in the present invention around the compliant surround provides maximal electrode spacing and, at the same time, appropriate insulation from the pacemaker casing due to the insulative properties of the surround and the cup recesses into which the electrodes are placed. The positioning maintains a maximum and equal distance between the electrode pairs. Such spacing with the four-electrode embodiment maintains the maximum average signal due to the fact that the spacing of the two vectors is equal and the angle between these vectors is 90xc2x0, as is shown in mathematical modeling. Such orthogonal spacing of the electrode pairs also minimizes signal variation. An alternate three-electrode embodiment has the electrodes arranged within the shroud in an equilateral triangle along the perimeter of the implanted pacemaker. Vectors in this embodiment can be combined to provide adequate sensing of cardiac signals (ECGs).
The present invention allows the physician or medical technician to perform leadless follow-up that, in turn, eliminates the time it takes to attach external leads to the patient. Such timesavings can reduce the cost of follow-up, as well as making it possible for the physician or medical technician to see more patients during each day. Though not limited to these, other uses include: Holter monitoring with event storage, arrhythmia detection and monitoring, capture detection, ischemia detection and monitoring (S-T elevation and suppression on the ECG), changes in QT interval, and transtelephonic monitoring.
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