Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1998-07-23
2002-04-09
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S005000, C607S009000, C607S121000, C607S120000
Reexamination Certificate
active
06370427
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable medical devices for treating cardiac dysrhythmias, and more particularly to a multi-mode device which is adapted to provide bi-ventricular therapy to the patient's heart in response to sensing applicable dysrhythmias.
Progress in medicine is based largely on progress in the technology of devices and apparatus for administering therapy. For example, significant advances in design techniques that have resulted in continuing reductions in the size of implantable defibrillators, including size of the function generator itself as well as in the heart leads associated therewith, have led to a capability to implant defibrillators at considerably lower risk to patients. During the first few years following the advent of implantable defibrillators, implant procedures required general anesthesia and thoracotomy, and the patient was faced with all of the risks associated with opening the chest cavity. The mortality rate of the procedure tended to limit widespread use of the device.
In recent years, with lower defibrillation thresholds (DFTs) and reduction in high voltage capacitor and battery sizes, smaller and more easily implantable devices have been developed, which have allowed this operation to be performed today under only local anesthesia. Smaller diameter and more easily inserted transvenous lead systems have overcome the need for a thoracotomy, and mortality associated with the procedure has been concomitantly reduced to less than one percent. The cosmetic aspects of such an implantation have also improved, with device size and weight allowing it to be implanted in the pectoral region that had previously been reserved for devices capable of only pacing functions, rather than the lower abdomen.
Nevertheless, at least two issues remain to be resolved with respect to present-day implantable defibrillators. For one thing, despite size reduction owing to the aforementioned advances in technology, the devices are still relatively large. At present, the limitations on size reduction are primarily attributable to the magnitude of energy required to achieve successful defibrillation with an adequate safety margin. A capacity for energy delivery of 25 to 32 joules (J), on average, currently remains the standard for implantable defibrillators. This minimum energy requirement mandates production and use of devices ranging from 40 to 50 cubic centimeters (cc) in volume and 80 to 100 grams (g) in weight.
Another issue that remains to be resolved is the provision of a continuously uniform, homogeneous electric field distribution during application of the relatively high energy defibrillating shocks to the heart. Studies performed on animals and humans indicate that to achieve a successful defibrillation with a lowered energy content shock requires a substantially uniform electric field distribution throughout the portion of the mass of cardiac tissue involved in the fibrillation. Lower energy requirement and fewer shocks to achieve a successful defibrillation are important not only from the standpoints of further size reduction and maintenance of an adequate reserve to increase the interval between defibrillator replacements, but also to avoid potential damage to the heart and skeletal frame of the patient that can occur with frequent or repeated application of high energy shocks.
Under typical defibrillator implant conditions, a coil is introduced into the right ventricle to serve as one electrode or pole, and the defibrillator case (or “can,” as it is often called in the art) that houses the batteries, capacitors, electronic components and circuitry is used as the second pole for the current path during the defibrillation shock. As noted above, the defibrillator case can now be implanted in the pectoral region, usually on the left side, to provide a more effective defibrillation pathway. This is desirable from the standpoint of the implant technique and the cosmetic aspect, but produces an energy and electric field distribution that is not equal, uniform or homogeneous throughout the region of the heart involved in the fibrillation. Measurements performed by the applicants have demonstrated that during application of a shock waveform using standard case, lead and defibrillation coil placements, a field of significantly lower energy (in volts (v) per centimeter (cm), i.e., v/cm) is present at the apex of the left ventricle compared to certain other regions of the heart such as the right ventricular outflow tract. The average electric field strength in the latter region is five to eight times greater than at the apex of the left ventricle.
In practice, then, because a relatively lower energy field is present at some regions that may be critical to defibrillation, the energy gradient sufficient to achieve successful defibrillation by application of the shock waveform mandates an adequate energy level in those regions and, by extension, a considerably higher electric field density in the normally higher energy field locations as well. The result is a further skewing of the inequality or inhomogeneity of the electric field distribution in the strategically important regions.
In one of its aspects, the present invention provides improvements in lead and electrode placements to assist in developing an equal, homogeneous field distribution during application of a defibrillation shock to the heart.
Another problem encountered with present day defibrillators, however, is that despite their capability to provide adequate therapy for sudden electrical instabilities of the cardiac function, they are not similarly capable of providing therapy for an underlying hemodynamically-compromised ventricular function. This means that the patient may suffer an ongoing deficiency in cardiac output, for example, even though the device is effective in correcting isolated events of fibrillation or pacing dysrhythmias.
Clinical investigation performed on patients who suffer from heart failure (i.e., inability of the heart to pump the required amount of blood) indicates that for a certain subset of these patients simultaneous stimulation of the left and right ventricles may be advantageous. In the cardiac cycle, a P wave of the subject's electrocardiogram (ECG) is produced by a depolarization of the atrial fibers just before they contract, and, when the cardiac impulse reaches the ventricular fibers to stimulate them into depolarization, a QRS complex is produced just before contraction of the ventricular walls. This is followed by a T wave which is indicative of the electrical activity occurring upon repolarization of the ventricular fibers. Simultaneous stimulation of the left and right ventricle would be beneficial therapy to patients whose ECG displays a marked desynchronization in contraction of the two ventricular chambers. In such cases, it is observed that after a right ventricular stimulation, considerable time may elapse for the cardiac impulse to travel from the apex of the right ventricle through the septum and to the free wall of the left ventricle, with the septum contracting earlier than the latter.
Consequently, the mechanical forces of the ventricular contraction are less favorable for an effective hemodynamic output in such patients. The duration or width of the QRS complex may increase because of an injury to the Purkinje fibers that inhabit and stimulate the ventricular septum and the lateral ventricular walls, and which could therefore increase the time for the impulse to spread throughout the ventricular walls. Patients who display a lack of ventricular synchronization primarily exhibit a wide QRS complex indicative of a bundle branch block—generally a left bundle branch block. Rather than the normal QRS complex width that ranges between 80 to 120 milliseconds (ms), the width of the QRS complex for these patients ranges between 140 and 200 ms.
It is a principal aim of the present invention to provide a method and apparatus for improved hemodynamic performance in patients with heart failure, utilizing an imp
Alt Eckhard
Sanders Richard
Stotts Lawrence J.
Blank Rome Comisky & McCauley LLP
Evanisko George R.
Intermedics Inc.
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