Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-04-22
2002-06-11
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S007000
Reexamination Certificate
active
06405081
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electrotherapy circuits and in particular to a defibrillator which is capable of applying damped biphasic defibrillation pulses to a patient.
Electro-chemical activity within a human heart normally causes the heart muscle fibers to contract and relax in a synchronized manner that results in the effective pumping of blood from the ventricles to the body's vital organs. Sudden cardiac death is often caused by ventricular fibrillation (VF) in which abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. The only effective treatment for VF is electrical defibrillation in which an electrical shock is applied to the heart to allow the heart's electrochemical system to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of cardiac rhythm.
FIG. 1
is an illustration of a defibrillator
10
being applied by a user
12
to resuscitate a patient
14
suffering from cardiac arrest. In cardiac arrest, otherwise known as sudden cardiac arrest, the patient is stricken with a life threatening interruption to their normal heart rhythm, typically in the form of ventricular fibrillation (VF) or ventricular tachycardia (VT) that is not accompanied by a palpable pulse (shockable VT). In VF, the normal rhythmic ventricular contractions are replaced by rapid, irregular twitching that results in ineffective and severely reduced pumping by the heart. If normal rhythm is not restored within a time frame commonly understood to be approximately 8 to 10 minutes, the patient
14
will die. Conversely, the quicker defibrillation can be applied after the onset of VF, the better the chances that the patient
14
will survive the event. The defibrillator
10
may be in the form of an automatic external defibrillator (AED) capable of being used by a first responder. The defibrillator
10
may also be in the form of a manual defibrillator for use by paramedics or other highly trained medical personnel.
A pair of electrodes
16
are applied across the chest of the patient
14
by the user
12
in order to acquire an ECG signal from the patient's heart. The defibrillator
10
then analyzes the ECG signal to detect ventricular fibrillation (VF). If VF is detected, the defibrillator
10
signals the user
12
that a shock is advised. After detecting VF or other shockable rhythm, the user
12
then presses a shock button on the defibrillator
10
to deliver defibrillation pulse to resuscitate the patient
14
.
The patient
14
has a transthoracic impedance (“patient impedance”) that spans a range commonly understood to be 20 to 200 ohms. It is desirable that the defibrillator
10
provide an impedance-compensated defibrillation pulse that delivers a desired amount of energy to any patient across the range of patient impedances and with a peak current limited to safe levels substantially less than a maximum value.
The minimum amount of patient current and energy delivered that is required for effective defibrillation depends upon the particular shape of the defibrillation waveform, including its amplitude, duration, shape (such as sine, damped sine, square, exponential decay). The minimum amount of energy further depends on whether the current waveform has a single polarity (monophasic), both negative and positive polarities (biphasic) or multiple negative and positive polarities (multiphasic).
If the peak current of the defibrillation pulse that is delivered to the patient
14
exceeds the maximum value, damage to tissue and decreased efficacy of the defibrillation pulse will likely result. Peak current is the highest level of current that occurs during delivery of the defibrillation pulse. Limiting peak currents to less than the maximum value in the defibrillation pulse is desirable for both efficacy and patient safety.
Most external defibrillators use a single energy storage capacitor charged to a fixed voltage level resulting in a broad range of possible discharge times and tilt values of the defibrillation pulse across the range of patient impedances. A method of shaping the waveform of the defibrillation pulse in terms of duration and tilt is discussed in U.S. Pat. No. 5,607,454, “Electrotherapy Method and Apparatus”, issued Mar. 4, 1997 to Gliner et al. Using a single capacitor to provide the defibrillation pulse at adequate energy levels across the entire range of patient impedances can result in higher than necessary peak currents being delivered to patients with low patient impedances. At the same time, the charge voltage of the energy storage capacitor must be adequate to deliver a defibrillation pulse with the desired amount of energy to patients with high patient impedances.
Various prior art solutions to the problem of high peak currents exist using resistance placed in series with the patient
14
to compensate for variations in patient impedance. In U.S. Pat. No. 5,514,160, “Implantable Defibrillator For Producing A Rectangular-Shaped Defibrillation Waveform”, issued May 7, 1996, to Kroll et al., an implantable defibrillator having a rectilinear-shaped first phase uses a MOSFET operating as a variable resistor in series with the energy storage capacitor to limit the peak current. In U.S. Pat. No. 5,733,310, “Electrotherapy Circuit and Method For Producing Therapeutic Discharge Waveform Immediately Following Sensing Pulse”, issued Mar. 31, 1998, to Lopin et al., an electrotherapy circuit senses patient impedance and selects among a set of series resistors in series with the energy storage capacitor to create a sawtooth approximation to a rectilinear shape in the defibrillation pulse. Using current limiting resistors to limit peak current as taught by the prior art results in substantial amounts of power being dissipated in the resistors which increases the energy requirements on the defibrillator battery. Furthermore, such prior art circuits require complex, active control systems to regulate the current during the delivery of the defibrillation pulse.
The use of inductors in the energy storage circuit along with the energy storage capacitor to shape the defibrillation pulse is well known in the art. The basic RLC defibrillator topology is explained in U.S. Pat. No. 4,168,711, “Reversal Protection for RLC Defibrillator”, issued Sep. 25, 1979 to Cannon, III et al. RLC defibrillators utilize an inductor in series with the energy storage capacitor to deliver a damped sine wave defibrillation pulse. Such waveforms are typically not truncated and the discharge time is on the order of 50-60 milliseconds (ms). RLC defibrillator designs according to the prior art do not address the problem of limiting peak currents or otherwise compensating for the range of patient impedances.
More recent biphasic defibrillator designs such as the Heartstream Forerunner® automatic external defibrillator (AED) utilize solid state switches such as silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs) connected in an H-bridge to produce a biphasic truncated exponential (BTE) defibrillation pulse. Such solid state switches require snubber circuits in series with the energy storage capacitor to control the rate of change of voltage or current through the switches to prevent switch damage as well as to prevent false triggering from transient energy. The snubber circuit in the Forerunner AED employs a 150 microHenry (uH) inductor. Similarly, in U.S. Pat. No. 5,824,017, “H-Bridge Circuit For Generating A High-Energy Biphasic Waveform In An External Defibrillator”, issued Oct. 20, 1998, to Sullivan et al., a protective element having resistive and inductive properties is interposed between the energy storage capacitor and the H bridge. Sullivan et al teach that the protective element
27
is used to the limit the rate of change of voltage across, and current flow to, the SCR switches of the H bridge. However, snubber circuits, while designed to protect the switch compone
Gliner Bradford E.
Lyster Thomas D.
Powers Daniel J.
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