Solid-state multiphasic defibrillation circuit

Details Solid-state multiphasic defibrillation circuit Solid-state multiphasic defibrillation circuit

2000-03-01

2002-07-16

Schaetzle, Kennedy (Department: 3762)

Surgery: light, thermal, and electrical application

Light, thermal, and electrical application

Electrical therapeutic systems

C428S908000

Reexamination Certificate

active

06421563

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to apparatus for generating defibrillation waveforms, and more particularly to a solid-state circuit for generating a multiphasic defibrillation waveform in an external defibrillator.
BACKGROUND OF THE INVENTION
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 mechanical high-energy transfer relay. A discharge control signal causes the mechanical 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 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, after accounting for energy dissipated in the chest wall of the patient.
The mechanical 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 a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
One prior art circuit for generating a biphasic waveform of the energy levels recommended by the American Heart Association is illustrated in U.S. Pat. No. 5,824,017, which is hereby incorporated by reference. FIG. 1 of U.S. Pat. No. 5,824,017 has been reproduced as
FIG. 1
herein. The circuit of
FIG. 1
shows a defibrillator
8
which includes a mechanical relay
35
. As will be described in more detail below, in order to make the defibrillator
8
into an entirely solid-state defibrillator, the mechanical relay
35
would have to be replaced with a solid-state relay or else eliminated. However, certain problems such as leakage currents would be associated with an entirely solid-state defibrillator. In order to provide a better understanding of the problems associated with an entirely solid-state defibrillator, the structure and operation of the circuit of
FIG. 1
will now be described in detail.
FIG. 1
includes a block diagram of an external defibrillator
8
that is connected to a patient
16
. The defibrillator includes a microprocessor
20
that is connected to an energy storage capacitor
24
via a charging circuit
18
. During the operation of the defibrillator
8
, the microprocessor
20
controls the charging circuit
18
by a signal on a control line
25
to charge the energy storage capacitor
24
to a desired voltage level. In order to generate the necessary defibrillation pulse for external application to a patient, the energy storage capacitor
24
is charged to between 100 volts and 2,200 volts. To monitor the charging process, the microprocessor
20
is connected to a scaling circuit
22
by a pair of measurement lines
47
and
48
, and by a control line
49
. The scaling circuit
22
is connected to the energy storage capacitor
24
by a bridge line
28
, which connects to the negative lead of the capacitor
24
, and by a line
30
, which connects to the positive lead of the capacitor
24
. A clock
21
is also connected to the microprocessor
20
.
After charging to a desired level, the energy stored in the energy storage capacitor
24
may be delivered to the patient
16
in the form of a defibrillation pulse. An output circuit
14
is provided to allow e controlled transfer of energy from the energy storage capacitor
24
to the patient
16
. The output circuit
14
includes four switches
31
,
32
,
33
, and
34
, each switch on a leg of the output circuit
14
arrayed in the form of an “H” (hereinafter the “H-bridge” output circuit). Switches
31
and
33
are coupled through a protective component
27
to the positive lead of the energy storage capacitor
24
by a bridge line
26
. The protective component
27
has both inductive and resistive properties, and thereby limits the current and voltage changes from the energy storage capacitor
24
.
Switches
32
and
34
are coupled to the energy storage capacitor
24
by a bridge line
28
. The patient
16
is connected to the left side of the H-bridge by an apex line
17
, and to the right side of the H-bridge by a sternum line
19
. As depicted in
FIG. 1
, the apex line
17
and the sternum line
19
are connected to electrodes
15
A and
15
B, respectively, by a patient isolation relay
35
. The microprocessor
20
is connected to the switches
31
,
32
,
33
, and
34
by control lines
42
A,
42
B,
42
C, and
42
D, respectively, and to the patient isolation relay
35
by control line
36
. Application of appropriate control signals by the microprocessor
20
over the control lines causes the switches of the output circuit
14
to be appropriately opened and closed (described in more detail below), whereby the output circuit
14
conducts energy from the energy storage capacitor
24
to the patient
16
.
In order to conduct a first phase of a biphasic pulse from the energy storage capacitor
24
to the patient
16
, switches
31
and
32
are closed along with relay
35
. Thus, during the first phase, energy travels from the positive terminal of the capacitor
24
down through switch
31
, out lines
17
and
15
A to the patient
16
, and then back from the patient
16
through lines
15
B and
19
, down through switch
32
to the negative terminal of the capacitor
24
. The first phase is ended by opening switches
31
and
32
before the capacitor
24
is completely discharged. Then the second phase of the biphasic defibrillation pulse is begun by closing switches
33
and
34
with relay
35
also closed. Thus, during the second phase, energy travels from the positive terminal of the storage capacitor
24
down through switch
33
, out lines
19
and
15
B to the patient
16
, and then back from the patient
16
through lines
15
A and
17
, and down through switch
34
to the negative terminal of the capacitor
24
. It can be seen with reference to
FIG. 1
that the travel of energy through the patient
16
during the first phase of the biphasic defibrillation pulse is opposite in direction to the travel of energy through the patient
16
during the second phase of the biphasic defibrillation pulse.
The mechanical relay
35
is a large, expensive, and relatively finicky component. It would be desirable to eliminate the mechanical relay if possible and replace it with solid-state switches, or else eliminate it altogether. However, there are at least two problems with either of these solutions. The first problem has

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