Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-09-25
2004-05-18
Schaetzle, Kennedy (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S072000
Reexamination Certificate
active
06738664
ABSTRACT:
BACKGROUND OF THE INVENTION
When the first animal and human defibrillations were reported with both internal and external electrodes, the electrical waveform utilized was a 60 cycles/second (Hz) sinusoidal waveform. This electrical shock was obtained by modifying the available voltage, typically 110 V(rms), such as by stepping up or stepping down the voltage. Durations were approximately in the range of 100 to 150 milliseconds (msec). Disadvantages to this methodology included: the defibrillator was very large and not easily portable; the defibrillator had to be plugged into the wall and, therefore, the patients had to be in the hospital; and there was a large current draw during the shock, which blew fuses and dimmed other lights on the circuit.
In the 1960's Edmark and Lown independently developed new waveforms for defibrillation which are called RLC waveforms. These waveforms are generated with a circuit containing a capacitor (C), an inductor (L), and a resistance (R). Advantages to the use of these waveforms included: the defibrillator was small and portable; it could be powered by a battery and used out of the hospital; and it did not draw huge amounts of current. These waveforms quickly became the standard for transthoracic defibrillation, and are still the industry standard for transthoracic defibrillation today.
When the implantable cardioverter/defibrillator (ICD) was developed in the 1970s [Schuder et al, Trans ASAIO, 16:207-12], the waveform of choice was the monophasic truncated exponential (MTE) as this waveform could be generated without an inductor (which could not be miniaturized for implantable devices). The MTE waveform was pioneered by the laboratory at University of Missouri [Gold et al, Circ 56:745-50, 1977] and was incorporated into most ICDs for clinical use for the first decade of their use.
In a series of papers in the early 1980s, the laboratory at the University of Missouri (“MU Lab”) pioneered a new class of waveforms for electrical ventricular defibrillation, called bidirectional or biphasic waveforms. The MU Lab demonstrated that if one were to reverse the polarity of an MTE waveform for the second half of the duration to yield a biphasic truncated exponential waveform (BTE), that one could dramatically improve the efficacy of defibrillation. The MU Lab studies covered the cases where the second phase amplitude was equal to the first phase amplitude and constant; where the second phase amplitude was smaller than the first phase amplitude and constant; and where the first and second phase amplitudes were allowed to decay exponentially and the second phase amplitude was either smaller than or equal to the first phase amplitude. Some of these waveforms studied were the first use of single capacitor waveforms (waveforms that could be generated by switching the polarity of a single capacitor) for defibrillation. These early studies utilized both internal and external electrodes.
The early studies from the MU lab arbitrarily set the first phase duration equal to the second phase duration. In 1987, Dixon et al published a paper, which found that if the first phase was longer than the second phase, that one could improve the efficacy of defibrillation over the case where the second phase duration was longer than the first phase duration [Dixon et al, Circ 76:1176-84]. The company that sponsored this research (Intermedics, Inc.) subsequently received U.S. Pat. No. 4,821,723 relating to this variation of the biphasic waveform. Biphasic truncated exponential (BTE) waveforms are now the industry standard for ICDs and also for implantable atrial defibrillators (IADs).
Several theories have been put forward, in an effort to understand why biphasic waveforms are generally more effective for electrical defibrillation than are monophasic waveforms. Understanding the mechanism of biphasic waveform superiority will possibly allow the design of even better waveforms for the next generation of defibrillators. The dominant theory in the field is currently a group of theories which can collectively be called RC circuit model theories. These theories have the common feature of modeling the response of the heart to a defibrillation shock, as the response of a resistor-capacitor (RC) circuit to that same shock. These theories also share the view that defibrillation efficacy is determined by the maximal capacitor voltage (model response) and the final capacitor voltage (model response). Taken as a group, these theories have led investigators to postulate optimal BTE waveforms for both internal and external defibrillation. As an example, a 1997 PCT publication “External defibrillator having low capacitance and small time constant” [WO 97/38754] relates to a BTE according to one version of the RC circuit model theory. Other theories of defibrillation have similarly led to different optimal waveform designs.
There are three different phenomena where electrotherapy shocks such as these are useful. The three phenomena are ventricular defibrillation, atrial defibrillation, and cardioversion; which are the treatment by electrical shock of ventricular fibrillation, atrial fibrillation, and atrial and/or ventricular tachycardia. Each of these three phenomena can be accomplished with electrodes that are external to the body, or with electrodes that are implanted either permanently or temporarily in the body. The state of the art treatment for all six combinations of these conditions and electrodes is presently some variation of the biphasic waveform. Currently, the same device is typically used for both ventricular defibrillation and cardioversion. For example, CPI Guidant calls their internal defibrillator an Automatic Implantable Cardioverter Defibrillator, which implies a single device with two functions.
The efficacy of these cardioverter and/or defibrillator devices in practice, is determined by the electrical waveform generated, and by the way the device compensates for variations in the patients to which it is applied. Specifically, the electrical impedance varies from patient to patient, and over time within a patient. This variation is much larger in magnitude when external electrodes are used, than when internal electrodes are used. Consequently, compensation for this variation in impedance is more critical in external defibrillators than in implantable defibrillators. Some devices use a passive impedance compensation strategy, whereby changes in impedance cause waveform changes without active intervention. Other devices actively compensate for impedance variation by measuring electrical parameters before or during the discharge such as capacitor voltage, patient impedance, or current flow; and modifying the electrical waveform based on these measurements.
For external ventricular defibrillation and cardioversion, the biphasic waveform of the Heartstream Inc.'s FORERUNNER® device is representative, and this device uses an active impedance compensation strategy. For an average impedance patient, it delivers a single capacitor BTE waveform with a 7 msec first phase and a 5 msec second phase, and uses a 100 microfarad capacitor. In response to variations in patient impedance, this device changes the durations of the two phases, the overall duration of the waveform, and the ratio of the durations of the two phases. The FIRSTSAVE® AED device made by SurVivaLink Corporation also delivers a biphasic waveform, and optimizes the waveform in terms of a charge-burping theory of defibrillation with active compensation for variations in impedance. Another alternative by Zoll Medical Corporation is the defibrillator waveform which has a saw-tooth (roughly constant current for all impedances) first phase, followed by a decaying exponential second phase. The biphasic waveform of the LIFEPAK® device by Physio-Control Corporation differs by using a longer time constant, and therefore a larger capacitance (about 300 microfarads). This device also uses an active impedance compensation strategy. External ventricular defibrillators on the marke
Droesch Kristen
Schaetzle Kennedy
Senniger Powers Leavitt & Roedel
The Curators of the University of Missouri
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