Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-02-08
2002-02-05
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C128S923000
Reexamination Certificate
active
06345200
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to implantable cardioverter defibrillators (ICD's) and in particular to techniques for determining and setting defibrillation thresholds for use with ICD's.
DESCRIPTION OF RELATED ART
An ICD is an implantable medical device capable of detecting the onset of ventricular fibrillation, or related arrhythmias, and for administering a defibrillation electrical pulse directly to the heart tissue to terminate the fibrillation. Preferably, the ICD is configured to discharge the minimum amount of electrical energy necessary to reliably defibrillate the heart. The amount of energy discharged in a single defibrillation pulse is the “defibrillation dosage”. By keeping the defibrillation dosage to a minimum: possible injury to the heart tissue caused by the electrical pulse is avoided; there is less discomfort to the patient after the patient has been revived; and the longevity of the power supply of the ICD is enhanced.
The defibrillation dosage must be sufficient to generate an “actual defibrillation voltage” in the heart of the patient sufficient to overcome a threshold “tissue defibrillation voltage” of the patient. The tissue defibrillation threshold is the minimum electrical voltage gradient that must be induced within the muscle tissue of the heart to reliably defibrillate the tissue. The tissue defibrillation threshold is typically about five volts per centimeter but can vary depending upon the characteristics of the heart, particularly the extent to which the tissue of the heart has been damaged by previous incidents of fibrillation or other arrhythmias.
The actual defibrillation voltage induced in the heart from a defibrillation pulse depends upon: the strength of the pulse generated by the ICD; the configuration of the ICD and its components, particularly the size, type and location of the leads of the ICD; and the physical characteristics of the heart and thorax of the patient, particularly the size and shape of the heart, the size and shape of the thorax, and the amount of fat present in the thorax. Accordingly, a particular defibrillation dosage administered by an ICD can result in significantly different actual defibrillation voltages within the heart tissue depending upon characteristics of the patient and upon the configuration of the ICD implanted in the patient.
Additional information regarding defibrillation dosages and thresholds may be found in chapters 4 and 5 of “Implantable Cardioverter Defibrillator Therapy—The Engineering—Clinical Interface,” edited by Mark W. Kroll and Michael H. Lehmann, Kluwer Academic Publishers (1996).
As noted, it is desirable to minimize the defibrillation dosage administered by the ICD. Hence, it is desirable to first determine an optimal implantation configuration for the ICD and its components which achieves the highest actual defibrillation voltage within the heart tissue using the lowest defibrillation dosage. The optimal configuration also minimizes overall costs and maximizes ICD longevity. The optimal configuration is one that not only specifies the components to be used, including the make and model of the ICD, the leads etc., but also specifies location in which each component is to implanted. The optimal configuration is, of course, preferably determined prior to implantation of the ICD and its components into the patient.
Heretofore, unfortunately, there has been no expedient, reliable and inexpensive noninvasive technique for determining the optimal ICD implantation configuration for a particular patient. As a result, the physician may erroneously select an implantation configuration which requires that the ICD discharge unnecessarily large amounts of electrical energy within each defibrillation pulse, thereby possibly damaging heart tissue, reducing the longevity of the ICD and increasing the amount of pain the patient experiences after being revived. A non-optimal implantation configuration may also result in greater costs and overall discomfort to the patient, particularly if a large and expensive ICD is implanted, when a smaller and less expensive one would be sufficient if implanted using an optimal configuration of leads, auxiliary wires, and the like.
Thus, it would be highly desirable to provide an improved technique for determining an optimal ICD implantation configuration—specifying the components to be used and the location in which each component is to implanted—based on the characteristics of a particular patient. It is to this end that aspects of the invention directed.
Once an ICD and its peripheral components have been selected and implanted, the defibrillation dosage of the ICD must be set to an amount sufficient to reliably defibrillate the heart, i.e. the defibrillation dosage must be set to an amount sufficient to achieve an actual defibrillation voltage gradient within the heart tissue exceeding the tissue defibrillation threshold. Herein, the minimum ICD defibrillation dosage sufficient to reliably defibrillate the heart is referred to as the “dosage defibrillation threshold” or DFT. Typically, the DFT is a programmable parameter of the ICD. In use, if ventricular fibrillation is detected, the ICD initially attempts to defibrillate the heart using a defibrillation dosage set to the DFT. If ventricular fibrillation is not terminated using pulses at the DFT, the ICD generates one or more additional pulses of higher dosage, typically using the maximum energy available.
As noted above, it is desirable to employ the lowest defibrillation dosage possible to reliably defibrillate the heart. Hence, the optimal setting for programming the DFT of the ICD is the lowest DFT value sufficient to achieve reliable defibrillation, which, as also noted, can depend greatly on the characteristics of the patient, such as the shape and size of the heart and thorax, and on the configuration of the ICD and its components. Thus, the optimal value may vary greatly from patient to patient and, for a particular patient, may vary greatly depending upon the ICD implantation configuration. Unfortunately, heretofore, there has been no reliable, inexpensive and expedient technique for accurately determining the optimal value for programming the DFT of an ICD based upon the characteristics of the patient and upon the ICD implantation configuration.
Conventionally, to program the DFT of an ICD, ventricular fibrillation is induced within the patient, then a series of defibrillation pulses of differing dosages are administered to the heart in an attempt to terminate the fibrillation. The DFT is then programmed to the lowest dosage level that terminated fibrillation, plus some safety margin. In one technique, fibrillation is repeatedly induced and pulses are output with decreasing dosage levels until the dosage level is insufficient to terminate the fibrillation. In another technique, a binary search pattern is employed whereby fibrillation is repeatedly induced, and then defibrillation pulses alternating between high and low output dosages are administered with a voltage differential between the high and low pulses incrementally reduced until an approximate DFT is identified.
Although these techniques have been found to be effective, considerable room for improvement remains. First, once fibrillation is induced, there is a risk that the physician will not be able to subsequently terminate the fibrillation, resulting in loss of life to the patient. Second, the sequence of defibrillation pulses may further injure the cardiac tissue of the patient and certainly can be painful to the patient. Moreover, the overall process typically takes several hours resulting in significant costs. Typically, the ICD itself is employed to administer the electrical defibrillation shocks and, if a relatively large number of shocks are required to determine the DFT, the power supply of the ICD can be depleted considerably during the determination process. If relatively few test defibrillation pulses are used, then the technique can only approximate the correct DFT thereby requiring the physician to
Adinolfi David W.
Kroll Mark W.
Mouchawar Gabriel A.
Bird John
Evanisko George R.
Pacesetter Inc.
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