Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1998-10-13
2001-03-27
Schaetzle, Kennedy (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06208895
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to apparatus for generating stimulation waveforms, and more particularly to a circuit for generating both pacing and defibrillation waveforms in an external unit.
BACKGROUND
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient's chest. The switching mechanism used in most contemporary external defibrillators is a high-energy transfer relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
The relay used in contemporary external defibrillators has traditionally allowed a monophasic waveform to be applied to the patient. It has recently been discovered, however, that there may be certain advantages to applying a biphasic rather than a monophasic waveform to the patient. For example, preliminary research indicates that biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
The American Heart Association has recommended a range of energy levels for the first three defibrillation pulses applied by an external defibrillator. The recommended energy levels are: 200 joules for a first defibrillation pulse; 200 or 300 joules for a second defibrillation pulse; and 360 joules for a third defibrillation pulse, all within a recommended variance range of no more than plus or minus
15
percent according to standards promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). These high energy defibrillation pulses are required to ensure that a sufficient amount of the defibrillation pulse energy reaches the heart of the patient and is not dissipated in the chest wall of the patient.
On the other hand, pacers are typically used to administer a series of relatively small electrical pulses to a patient experiencing an irregular heart rhythm. For example, each pacing pulse typically has an energy of about 0.05 J to 1.2 J. Because of the small energies used for pacing pulses, the circuitry used to generate the pacing pulses cannot typically be used for generating defibrillation pulses.
There are some systems that combine both a pacer and a defibrillator in a single unit for providing pacing pulses and defibrillation pulses as required. These conventional systems typically use separate defibrillation and pacing generation circuits. For example,
FIG. 1
shows a combined pacing defibrillation unit
5
having a defibrillation circuit
6
and a pacing circuit
7
. Unit
5
selectively delivers defibrillation or pacing pulses to the patient. Implantable systems generally use separate electrodes for pacing and defibrillation. An example of an implantable combined defibrillator/pacer is found in U.S. Pat. No. 5,048,521. Of course, having separate defibrillation and pacing circuits tends to increase the cost and size of the unit. In addition, because implantable defibrillators and pacers typically apply relatively low energy pulses, the output circuitry for such implantable units is generally not adaptable for use in an external unit.
The present invention is directed to an apparatus that overcomes the foregoing and other disadvantages in an external pacing/defibrillation unit. More specifically, the present invention is directed to a single output circuit for an external pacer/defibrillator that is capable of applying both high-energy biphasic defibrillation pulses and low-energy pacing pulses to a patient.
SUMMARY
In accordance with the present invention, an external defibrillator/pacer having an output circuit that is used in generating both a defibrillation pulse and a pacing pulse is provided. The output circuit includes four legs arrayed in the form of an “H” (hereinafter the “H-bridge output circuit”). Each leg of the output circuit contains a solid-state switch. By selectively switching on pairs of switches in the H-bridge output circuit, biphasic or monophasic defibrillation and pacing pulses may be applied to a patient.
In accordance with one aspect of the invention, the switches in three of the legs of the H-bridge output circuit are silicon controlled rectifiers (SCRs). A single SCR is used in each of these three legs. The switch in the fourth leg is an insulated gate bipolar transistor (IGBT). The bypass circuit is capable of conducting the relatively small pacing currents, which are generally too small to trigger the SCRs required to conduct the relatively large defibrillation currents. The addition of the bypass circuit eliminates the need for separate defibrillation and pacing output circuits.
In accordance with another aspect of the invention, the H-bridge output circuit has two IGBT legs and two SCR legs. The second IGBT leg allows the polarity of the defibrillation and pacing pulses to be opposite. In one embodiment, the pacing current is adjusted by adjusting the voltage on an energy storage capacitor.
In accordance with yet another aspect of the invention, instead of a bypass circuit, an adjustable current source is used to provide the pacing current. This current source is coupled to the energy storage capacitor. In one embodiment, the current source is an IGBT operated in the linear region.
In accordance with still another aspect of the invention, all of the H-bridge legs are implemented with IGBTs. This aspect allows for generation of biphasic pacing pulses. Further, by biasing the IGBTs in the linear region, the IGBTs can be used as current sources to control the pacing current. This would eliminate the need for a bypass circuit or separate current source.
REFERENCES:
patent: 4402322 (1983-09-01), Duggan
patent: 4693253 (1987-09-01), Adams
Borschowa Lawrence A.
Nova Richard C.
Sullivan Joseph L.
Christensen O'Connor Johnson & Kindness PLLC
Physio-Control Manufacturing Corporation
Schaetzle Kennedy
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