Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-01-18
2003-11-11
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S008000
Reexamination Certificate
active
06647290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a defibrillator and, more particularly, to defibrillators that provide variable waveforms.
2. Related Art
An external defibrillator is a device used to administer a high intensity electrical shock through two or more electrodes, commonly referred to as “paddles” or “pads,” to the chest of a patient in cardiac arrest. Energy typically is stored in a charge-storage device (e.g., a capacitor) and is then electrically discharged into the patient through the electrode circuit.
If an initial attempt at defibrillation is not successful, one or more additional attempts typically are made. However, repeated defibrillation attempts, particularly if they are made at increasing levels of intensity, are increasingly likely to cause damage to the heart or other body tissue. Although the threshold levels for damage are not well quantified, it appears that there is not a great deal of margin between an effective defibrillation level and a damaging defibrillation level. Also, the delay associated with repeating the defibrillation procedure may allow the patient's condition to deteriorate. For example, metabolic imbalance and hypoxia may develop in response to prior attempted resuscitations. Moreover, the development of these conditions typically makes it more difficult to defibrillate the patient and, even if defibrillation is achieved, reduces the prospect of successful recovery. Thus, early and optimal selection of various waveform parameters is crucial to improving the chances of a successful outcome.
One set of waveform parameters thought to be important in determining the safety and success of the defibrillation procedure are those that define the shape of the defibrillation waveform. Waveforms having a variety of shapes have conventionally been used. Some defibrillators employ monophasic (single polarity) voltage pulses. Others employ biphasic (both positive and negative polarity) pulses. Monophasic or biphasic pulses may be damped-sinusoidal, truncated-exponential, constant “tilt” (a measure of the difference between the start and end voltage, often expressed as the difference between the initial and final voltages, divided by the initial voltage), combinations of such forms, and so on. Many other forms, such as rectilinear pulses, are possible. In addition, the shape of a waveform may be adjusted by varying its amplitude or duration, or the amplitudes or durations of one or more of its constituent parts. Some conventional approaches for determining what are considered to be optimal shapes for defibrillation waveforms, delivered by both implanted and external defibrillators, are described in U.S. Pat. No. 5,431,686 to Kroll et al., U.S. Pat. No. 4,953,551 to Mehra et al., and U.S. Pat. No. 4,800,883 to Winstrom.
The choice of waveform shape also may depend on whether the defibrillator is implanted or is external. If the defibrillator is implanted, the patient's unique electrical characteristics and overall physiology may be investigated and the waveform tailored to that particular patient's needs. External defibrillators, in contrast, are intended to be applied to numbers of patients that have generally varying physiological characteristics. Moreover, a patient may require different waveforms for optimal operation depending, for example, on the contact that is achieved between the electrode and the patient. Thus, external defibrillators may be designed for optimal use on an average patient. Alternatively, they may be designed so that they are capable of providing a variety of waveforms depending on an evaluation of the patient's physiology, the electrical connection achieved between the electrode and the patient, new knowledge about the operation and affect of electrotherapeutic discharges, or other factors.
Several factors have been used to determine the defibrillation waveform parameters. In particular, many defibrillators presently in use are designed to deliver one or more specific quantities of energy, typically measured in joules, to the patient's heart. With respect to external defibrillators, practical considerations have contributed to an emphasis on energy-based defibrillation methods. In particular, energy is a relatively easy quantity to control at the power levels and pulse width's required for transthoracic defibrillation.
Guidelines of the American Heart Association applicable to external defibrillation suggest that a first discharge be administered to deliver a total energy of 200 joules to the patient, a second discharge be administered to deliver 200 to 300 joules, and a third discharge be administered to deliver 360 joules. In conformance with these guidelines, many conventional external defibrillators are designed to deliver these quantities of energy to a patient assuming a typical transthoracic impedance (e.g., 50 ohms). Other defibrillators take into account the variability of transthoracic impedance from one patient to another. In general, these defibrillators measure the transthoracic impedance of the patient and adjust the amount of energy stored in a discharge capacitor or other energy storage device in order to achieve a desired amount of energy applied to the patient's heart. Some of these conventional defibrillators also vary the shape of the defibrillation waveform as a function of transthoracic impedance and the quantity of energy to be delivered. The rationales for these and other conventional energy-based approaches are described in numerous sources such as U.S. Pat. No. 4,771,781 to Lerman, U.S. Pat. No. 5,620,470 to Gliner, et al., U.S. Pat. No. 5,607,454 to Cameron, et al., and International Application PCT/US98/07669 (PCT International Publication No. WO 98/47563).
The Lerman patent also describes another type of conventional design in which the defibrillation discharge is determined based on current delivered to the patient. In particular, Lerman describes a method for calculating a level of energy necessary to deliver to the patient an amount of peak current pre-selected by an operator. A measured transthoracic resistance of the patient, together with the selected peak defibrillation current, are used to control the charge that is applied to a discharge capacitor of the defibrillator. Upon discharge, the selected level of peak current is applied to the patient. U.S. Pat. No. 4,840,177 to Charbonnier, et al., also describes a method for determining a charge level for an energy storage device such that, when the device is discharged, a desired current flows into the patient. These and other conventional current-based designs seek, among other things, to limit or avoid the damage that may be inflicted by the delivery of an excessive amount of energy. For example, in situations in which the transthoracic resistance is low, a particular selection of energy for discharge into the patient will result in a larger applied current than would be realized if the transthoracic resistance had been high. On the theory that it is the application of current, rather than energy per se, that achieves the desired defibrillation, the energy discharged into a low-resistance patient therefore may be selected to be less than it would be for a high-resistance patient. Thus, the supposed therapeutic benefit is achieved while exposing the patient to a level of energy that is thought to be less likely to cause damage. Various other conventional techniques for determining defibrillation discharge parameters based on operational parameters such as desired energy, current, and/or shape are noted and discussed in the above noted PCT Publication No. 98/47563.
SUMMARY OF THE INVENTION
Although current-based defibrillators are feasible, they typically must operate over a wide range of energy and power in order to deliver a specified current over a wide range of possible transthoracic impedances. These requirements often complicate the design of conventional current-based defibrillators. Moreover, it is not clear that the delivery of current, per se, i
Bradford Roderick
Evanisko George R.
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