Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-06-12
2002-11-12
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06480738
ABSTRACT:
The invention relates to a cardiac defibrillator, particularly an implantable one, of the kind set forth in claim
1
and a corresponding method.
BACKGROUND OF THE ART
Defibrillators of this kind are known from sources such as European Pat. application No. 0 515 059. Defibrillators are generally implanted in increasing numbers in the case of patients who repeatedly suffer from fibrillation which requires electrotherapeutic help. To provide assistance without the presence of a doctor—more specifically because in many cases the doctor would not be on hand sufficiently quickly—nowadays defibrillators of that kind are already being implanted in relatively large numbers and are thus available to the patient at any time. To keep down the size of those units while still providing sufficient energy even for repeated defibrillation procedures, optimum utilization of energy during an individual defibrillation procedure is a particularly important consideration.
The previously known defibrillator admittedly includes a plurality of capacitors which can be switched in different configurations. A disadvantage in that respect however that no indications whatsoever in regard to the sequence and the times of switching over the capacitors for optimization of the energy demand in a defibrillator of that kind are known. In this connection attention is directed to the following literature which however also does not provide any more detailed indications in this respect. It represents a summary of the previous endeavors to provide information about the energy demand in connection with defibrillators:
1. Schudder J C, Stoeckle H, West J A, et al: Transthoracic ventricular defibrillation in the dog with truncated exponential stimuli. IEEE Trans Biomed Eng BME 1971; 18: 410-415
2. Hamzei A, Mouchavar G, Badelt St et al: Three-capacitor multistep waveform lowers defibrillation threshold. PACE 1999; 22(5, II): abstract #87
3. Irnich W: The fundamental law of electrostimulation and its application to defibrillation. PACE 1990; 13: 1433-1447
4. Irnich W: Optimal truncation of defibrillation pulses. PACE 1995; 18: 673-688
5. Natale A, Sra J, Krum D et al: Relative efficacy of different tilts with biphasic defibrillation in humans. PACE 1996; 19: 197-206
6. Hahn St J, Heil J E, Lin Y et al: Optimization of 90 &mgr;F biphasic defibrillation waveform for ICDs using a theoretical model and central composite design of experiments. PACE
7. Schauerte P, Schöndube F A, Grossmann M et al: Optimized pulse duration minimizes the effect of polarity reversal on defibrillation efficacy with biphasic shocks. PACE 1999; 22: 790-797
8. Cleland B G: A conceptual basis for defibrillation waveforms. PACE 1996; 19: 1186-1195
9. Kroll M W: A minimal model of the monophasic defibrillation pulse. PACE 1993; 16: 769-777.
These references will be referred to hereinafter by these numbers.
Even these publications do not give any indications in regard to the stated problem, as will also be apparent from the systematic presentation hereinafter of the problems involved and the concept of the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a defibrillator of the above-indicated kind or a corresponding defibrillation method, in which there are provided automatic control means which optimize the defibrillation effect with a plurality of capacitors.
The object is attained by the features recited in claim
1
.
The object of the invention is attained by realizing that the degree of efficiency &eegr; (also referred to as “eta”) of the defibrillator in the various discharging procedures must be optimized in each case in such a way that the overall effect is also an optimum. The term &eegr; is used in electrical engineering to mean the “efficiency” which normally defines the ratio of useful to applied energy. That consideration is based on an “input”- to “output”-comparison which in the case of the defibrillator would have to be such that the energy taken from the battery is compared to the energy delivered to the heart. Here however the notion of efficiency is additionally expanded in such a way that it embraces more than a straightforward input-output calculation, but also includes the question of the biological effectiveness of different pulse shapes.
The problem involved can be illustrated by reference to two examples: A favorable input-output ratio would be achieved in defibrillation if the output capacitor or capacitors was or were completely discharged. The efficiency would be 1. However Schudder et al (Ref. 1) already found in 1970 that the effectiveness is increased if the capacitor or capacitors is or are not completely discharged, but the discharge procedure is prematurely interrupted (truncated or curtailed). Now, it is worth noting that the optimum “tilt” (this English expression to denote slope or gradient is an established part of defibrillator terminology, and it should better be referred to as “degree of utilization”) has not hitherto been systematically investigated. On the contrary, the electrophysiological problem was made more complicated by the fact it was postulated as being self-evident that the optimum “tilt”, once found, enjoyed general applicability. However the engineers of the defibrillator manufacturer which for a long time was the only one prejudiced the discussion about optimum tilt by virtue of the fact that they implemented the idea which from the point of view of electrical engineering appears a reasonable one of providing for discharge of the output capacitor or capacitors to 20% of the initial voltage (corresponding thereto is a degree of utilization or tilt of 80%), to which there corresponds an efficiency of 96% (as the residual voltage is involved in quadratic terms in the energy calculation). That assumption was not derived from any defibrillation experiments but was based on purely electrical engineering considerations.
As a second example mention may be made of a poster (Ref. 2) which was displayed in Toronto, Canada, in May 1999, on the occasion of the NASPE-Conference. The authors reported that, by virtue of serial connection of three output capacitors previously discharged to about 85%, they had required a lesser amount of stored energy in comparison to just one capacitor which was discharged to 45%. They explained that increased efficiency on the basis that a pulse with a rising pulse shape is more desirable, on the basis of the “membrane-response-model” hypothesis thereof. That interpretation cannot be reconciled with the basic law of electrostimulation which is also applicable in regard to defibrillation (Refs. 3, 4). The fact that nonetheless a first parallel and then a serial discharge can be advantageous involves reasons related to electrical engineering, which will be discussed in greater detail hereinafter.
According to the present invention, however, a superior defibrillator is provided compared to the known defibrillators. When ascertaining the tilt or the normalized residual voltage in the discharge procedure in the prior art, measurements were made directly on the patient and which thus best correspond to the prevailing factors by virtue of the fact that the residual voltage is adapted to the defibrillation impedance upon discharge or in terms of tilt.
In this respect consideration was given inter alia to the fact that stimulation and defibrillation obey the law which was already published in 1909 by Lapicque and which can be formulated as follows:
U(mean)=U
rheobase
(1+T
chronaxie
/T) (1)
wherein:
U(mean)=mean voltage during a stimulation pulse,
U
rheobase
=the voltage which just still stimulates with an infinitely long pulse duration (a more theoretical value),
T
chronaxie
=pulse duration at double the rheobase value.
The present invention is based upon the realization that two rules apply in regard to the effect of defibrillation as a function of the pulse duration:
the voltage-time integral which increases linearly with the pulse duration is decisive, and
the pulse worsens the defibrillation
Getzow Scott M.
Grant Stephen L.
Hahn Loeser + Parks LLP
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