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Claims  |
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What we claim as our invention is:
1. A defibrillator comprising:
a pair of electrodes for coupling to a patient;
an HV switch coupled to said pair of electrodes; and
a configurable energy storage capacitor network for delivering an
impedance-compensated defibrillation pulse through said HV switch to said
patient
a controller for obtaining a patient impedance and coupled to said energy
storage capacitor network to select one of a plurality of configurations
based on said patient impedance.
2. A defibrillator according to claim 1 wherein said energy storage
capacitor network comprises a plurality of capacitors that are arranged
according to a plurality of configurations.
3. A defibrillator according to claim 2 further comprising a high voltage
charger coupled to said energy storage capacitor network for charging each
of said capacitors.
4. A defibrillator according to claim 3 further comprising a set of
charging switches interposed between said high voltage charger and each of
said capacitors.
5. A defibrillator according to claim 2 further comprising:
a front end coupled to said pair of electrodes to provide a patient
impedance; and
wherein said controller is coupled to said front end to obtain said patient
impedance and to said energy storage capacitor network to select one of
said plurality of configurations based on said patient impedance and a
selected energy level.
6. A defibrillator according to claim 5 wherein said controller is coupled
to said HV switch to control a duration and polarity of said
impedance-compensated defibrillation pulse.
7. A defibrillator according to claim 6 wherein said impedance-compensated
defibrillation pulse comprises one of monophasic, biphasic, and
multiphasic.
8. A defibrillator according to claim 5 wherein said controller selects one
of said configurations based on a fixed selected energy level.
9. A defibrillator according to claim 5 wherein said controller determines
said selected energy level according to a protocol.
10. A defibrillator according to claim 5 wherein said controller selects
one of said configurations based on an energy level manually selected by a
user.
11. A defibrillator according to claim 5 wherein said selected energy level
is determined as a function of said patient impedance.
12. A defibrillator according to claim 2 wherein said energy storage
capacitor network further comprises:
a plurality of capacitors coupled in series and in parallel to said HV
switch; and
a plurality of switches coupled between each of said capacitors and ground,
wherein said energy storage capacitor network is configured to deliver an
impedance-compensated defibrillation pulse by setting said plurality of
switches according to one of said configurations.
13. A defibrillator according to claim 1 wherein said impedance-compensated
defibrillation pulse has a peak current less than a maximum value.
14. A defibrillator according to claim 1 wherein said energy storage
capacitor network comprises:
a plurality of sections, each of said sections comprising a capacitor, a
resistor, and a diode coupled in series and each of said sections coupled
in parallel to said HV switch;
wherein each of said capacitors is charged to a charge voltage according to
a rank order and each of said resistors is has a resistance chosen
according to said rank order wherein said energy storage capacitor network
is configured to deliver an impedance-compensated defibrillation pulse by
sequentially discharging each of said sections.
15. A method for delivering an impedance-compensated defibrillation pulse
to a patient, comprising:
measuring a patient impedance of said patient;
selecting from a set of configurations in an energy storage capacitor
network to deliver an impedance-compensated defibrillation pulse to said
patient responsive to said patient impedance; and
delivering said impedance-compensated defibrillation pulse to said patient.
16. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 15 further comprising selecting from said
set of configurations responsive to a selected energy level.
17. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 16 wherein said selected energy level is
fixed at one level.
18. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 16 wherein said selected energy level is
determined according to a protocol.
19. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 16 wherein said selected energy level is
manually selected by a user.
20. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 15 further comprising charging said energy
storage capacitor network using a high voltage charger.
21. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 15 further comprising coupling said
patient to said energy storage capacitor network via a pair of electrodes.
22. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 15 further comprising delivering said
impedance-compensated defibrillation pulse to said patient with a peak
current less than a maximum value.
23. A defibrillator comprising
a pair of electrodes for coupling to a patient;
a front end circuit coupled to said pair of electrodes to provide a patient
impedance and an ECG signal;
an HV switch coupled to said pair of electrodes;
an energy storage capacitor network having a plurality of configurations;
and
a controller coupled to said front end, to said HV switch, and to said
energy storage capacitor network;
wherein said controller selects one of said configurations based on said
patient impedance and a selected energy level and delivers an
impedance-compensated defibrillation pulse through said HV switch to said
patient responsive to detecting a shockable rhythm in said ECG signal.
24. A defibrillator according to claim 23 wherein said controller selects
one of said configurations based on a fixed selected energy level.
25. A defibrillator according to claim 23 wherein said controller
determines said selected energy level according to a protocol.
26. A defibrillator according to claim 23 wherein said controller selects
one of said configurations based on an energy level manually selected by a
user.
27. A defibrillator according to claim 23 wherein said selected energy
level is determined as a function of said patient impedance.
28. A defibrillator according to claim 23 further comprising a high voltage
charger coupled to said energy storage capacitor network for charging said
energy storage capacitor network.
29. A defibrillator according to claim 23 wherein said controller
determines a duration and a polarity of said impedance-compensated
defibrillation pulse.
30. A defibrillator according to claim 23 wherein said
impedance-compensated defibrillation pulse comprises one of monophasic,
biphasic, and multiphasic.
31. A defibrillator according to claim 23 wherein said energy storage
capacitor network provides for said selected energy level above 200
joules.
32. A defibrillator according to claim 23 wherein said energy storage
capacitor network comprises:
a plurality of capacitors coupled in series and parallel to said HV switch;
and
a plurality of switches coupled between each of said capacitors and ground
wherein said energy storage capacitor network is configured to deliver an
impedance-compensated defibrillation pulse by setting said plurality of
switches according to said one of said configurations.
33. A defibrillator according to claim 23 wherein said
impedance-compensated defibrillation pulse has a peak current less than a
maximum value.
34. A method for delivering an impedance-compensated defibrillation pulse
to a patient, comprising:
providing a pair of electrodes for coupling to said patient;
providing an HV switch coupled to said electrodes;
providing a plurality of sections coupled in parallel in an energy storage
capacitor network coupled to said HV switch, each of said sections
comprising a capacitor, a resistor, and a diode coupled in series;
charging each capacitor to a charge voltage in said rank order with said
other sections; and
delivering an impedance matched defibrillation pulse through said HV switch
and said pair of electrodes to said patient by sequentially discharging
each of said sections.
35. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 34 further comprising providing a
controller coupled to said HV switch.
36. A method for delivering an impedance-compensated defibrillation pulse
to a patient according to claim 35 wherein said controller determines a
duration and a polarity of said impedance-compensated defibrillation
pulse.
37. A defibrillator according to claim 36 wherein said
impedance-compensated defibrillation pulse comprises one of monophasic,
biphasic, and multiphasic.
38. In an energy storage capacitor network having a plurality of capacitors
coupled in series and charged using a high voltage charger, a fault
detection resistor network comprising:
a first resistor network coupled in shunt across each of said plurality of
capacitors to develop a first test voltage;
a second resistor network coupled across said high voltage charger to
develop a second test voltage; and
a comparison circuit coupled to said first and second test voltages to
generate a fault signal if said first test voltage differs from said
second test voltage by greater than a predetennined limit. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to electrotherapy circuits and in particular to a
defibrillator using multiple capacitors to provide for an
impedance-compensated delivery of defibrillation pulses to the 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
electro-chemical system to re-synchronize itself. Once organized
electrical activity is restored, synchronized muscle contractions usually
follow, leading to the restoration of cardiac rhythm.
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), and whether the current
waveform has a single polarity (monophasic), both negative and positive
polarities (biphasic) or multiple negative and positive polarities
(multiphasic). At the same time, there exists a maximum value of current
in the defibrillation pulse delivered to the patient above which will
result in damage to tissue and decreased efficacy of the defibrillation
pulse.
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. Because the transthoracic impedance ("patient impedance")
of the human population may vary across a range spanning 20 to 200 ohms,
it is desirable that an external defibrillator provide an
impedance-compensated defibrillation pulse that delivers a desired amount
of energy to any patient with the range of patient impedances and with
peak currents limited to safe levels substantially less than the maximum
value.
Most external defibrillators employ a single energy storage capacitor or a
fixed bank of energy storage capacitors charged to a single voltage level.
Controlling the amount of energy delivered to any given patient across the
range of patient impedances is a problem commonly solved by controlling
the "tilt" or difference between initial and final voltages of the energy
storage capacitor as well as the discharge time of the defibrillation
pulse. 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 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 high peak
currents being delivered to patients with relatively low 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 impedances.
Various prior art solutions to the problem of high peak currents exist. One
method involves placing resistors in series with the energy storage
capacitor to prevent excessive peak currents to low impedance patients. 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 have 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 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.
Another approach to limiting peak currents involves using multiple
truncated decaying exponential waveforms from multiple capacitors to form
a sawtooth approximation of a rectilinear shape of the discharge waveform
in an implantable defibrillator. In U.S. Pat. No. 5,199,429, "Implantable
Defibrillation System Employing Capacitor Switching Networks", issued Apr.
6, 1993, to Kroll et al., a set of energy storage capacitors are charged
and then successively discharged during the first phase to create the
sawtooth pattern. Kroll et al. teach that multiple capacitors may be
arbitrarily arranged in series, parallel, or series-parallel arrangements
during the delivery of the defibrillation pulse in order to tailor the
shape of the defibrillation waveform with a high degree of flexibility.
In U.S. Pat. No. 5,836,972, "Parallel Charging of Mixed Capacitors", issued
Nov. 17, 1998, to Stendahl et al., a method for charging banks of energy
storage capacitors in parallel is taught. The banks of energy storage
capacitors may then be coupled in series in order to deliver a
defibrillation pulse.
However, neither Kroll et al. nor Stendahl et al. address the issue of
obtaining impedance-compensated defibrillation pulses which have peak
currents less than the maximum value and with less variation of discharge
times across the range of patient impedances. It would therefore be
desirable to provide a defibrillator that selects among configurations of
energy storage capacitors to deliver an impedance-compensated
defibrillation pulse to the patient.
SUMMARY OF THE INVENTION
A defibrillator having an energy storage capacitor network with a set of
configurations selected according to patient impedance and desired energy
level for delivery of an impedance-compensated defibrillation pulse is
provided. Impedance-compensation according to the present invention means
providing an energy storage capacitor network with an overall capacitance
and charge voltage that are tailored to the patient impedance and the
desired energy level. The peak current is limited to values less than the
maximum value for low patient impedances while the variation of discharge
times of the defibrillation pulse is reduced for high impedance patients.
The set of configurations of the energy storage capacitor network may
include various series, parallel, and series/parallel combinations of
energy storage capacitors within the energy storage capacitor network that
are selected as a function of patient impedance to provide a variety of
overall capacitances and charge voltages. The impedance-compensated
defibrillation pulse may be delivered over an expanded range of energy
levels while limiting the peak current to levels that are safe for the
patient using configurations tailored for lower impedance patients. At the
same time, adequate current levels are delivered using selected
configurations tailored for high impedance patients. Other configurations
may be readily added to the energy storage capacitor network to extend the
range of available energy levels well above 200 joules.
The defibrillator according to the present invention is constructed using
an energy storage capacitor network using at least two capacitors that
store energy for delivery of the defibrillation pulse to the patient. The
defibrillator is typically portable and operates using a conventional
battery as an energy source. A high voltage charger operates to charge the
capacitors in the energy storage capacitor network to desired voltage
levels. An HV switch couples the capacitors across the patient according
to a desired pulse duration and polarity. In the preferred embodiment, the
HV switch comprises an "H bridge" consisting of four commutating switches
for applying a biphasic defibrillation pulse to the patient through a pair
of electrodes.
A controller controls the process of charging the energy storage capacitor
network. Responsive to a press of a shock button, the controller delivers
the impedance-controlled defibrillation pulse to the patient by selecting
the configuration of the energy storage capacitor network and controlling
the HV switch to obtain the desired duration and polarity of the
impedance-compensated defibrillation pulse.
Measuring the patient impedance may be done immediately before delivery of
the defibrillation pulse. Based on the patient impedance, an appropriate
configuration of capacitors may be selected to deliver the
impedance-compensated defibrillation pulse at the desired energy level
while limiting the peak current to levels that are safe for the patient.
The energy level of the impedance-compensated defibrillation pulse may be
readily selected according to the present invention. The energy storage
capacitor network has a set of configurations tailored to the patient
impedance and the desired energy level. The controller selects the
appropriate configuration after determining the patient impedance and the
desired energy level. Defibrillator applications involving selectable
energy levels in excess of 200 joules (j) can benefit from using
impedance-compensated defibrillation pulses because the peak currents can
be limited to less than the maximum value across a wider range of patient
impedances and energy levels.
An alternative embodiment of the present invention provides for the energy
storage capacitor network that employs parallel combinations of capacitors
and resistors that deliver energy for the defibrillation pulse using
blocking diodes in place of switches. In this way, an impedance-matched
defibrillation pulse may be delivered without the active intervention of
the controller to measure the patient impedance and select the various
configurations of capacitors. Component count would be substantially
reduced over that of the first embodiment but at the expense of
flexibility and the ability to select energy levels.
One feature of the present invention is to provide a defibrillator that
delivers impedance-compensated defibrillation pulses with a selected
amount of energy.
A further feature of the present invention is to provide a defibrillator
that delivers impedance-compensated defibrillation pulses using multiple
capacitors.
Another feature of the present invention is to provide a method of
delivering impedance-compensated defibrillation pulses by selecting among
a set of configurations of the energy storage capacitor network.
A further feature of the present invention is to provide an energy storage
capacitor network for a defibrillator that is capable of delivering
impedance-compensated defibrillation pulses with energy levels above 200
joules.
Another feature of the present invention is to provide an energy storage
capacitor network using diode switching to deliver impedance-compensated
defibrillation pulses.
Other features, attainments, and advantages will become apparent to those
skilled in the art upon a reading of the following description when taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block diagram of a defibrillator with an energy
storage capacitor network according to the present invention;
FIG. 2 is a schematic diagram of the energy storage capacitor network
according to the present invention;
FIG. 3 is a graph of initial current versus patient impedance using the
energy storage capacitor network according to the present invention;
FIGS. 4A-C are a set of graphs of patient current over time for patient
impedances of 20, 50, and 120 ohms respectively using the energy storage
capacitor network according to the present invention;
FIG. 5 is a graph of energy delivered versus patient impedances using the
energy storage capacitor network according to the present invention;
FIG. 6 is a schematic diagram of a fault detection circuit as applied in
the energy storage capacitor network;
FIG. 7 is an illustration of a set of configurations of the energy storage
capacitor network which may be selected according to the patient impedance
and desired energy level according to the present invention;
FIG. 8 is a flow diagram of the process of delivering an
impedance-compensated defibrillation pulse based on the method according
to the present invention;
FIG. 9 is schematic diagram of the energy storage capacitor network
according to an alternative embodiment of the present invention; and
FIGS. 10A and 10B are graphs of patient current over time for low impedance
and high impedance patients using the energy storage capacitor network
according to the alternative embodiment of the present invention shown in
FIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a simplified block diagram of a defibrillator 10 according to the
present invention. A pair of electrodes 12 for coupling to a patient (not
shown) are connected to a front end 14 and further connected to an HV
switch 16. The front end 14 provides for detection, filtering, and
digitizing of the ECG signal from the patient. The ECG signal is in turn
provided to a controller 18 which runs a shock advisory algorithm that is
capable of detecting ventricular fibrillation (VF) or other shockable
rhythm that is susceptible to treatment by electrotherapy.
The front end 14 is preferably capable of measuring the patient impedance
across the electrodes 12 using a low level test signal. The patient
impedance may be measured and digitized in the front end 14 using an
analog to digital converter (not shown) in order to provide the patient
impedance data to the controller 18. The patient impedance may also be
measured using a variety of other methods such as by delivering a
low-level non-therapeutic pulse to the patient prior to delivery of the
defibrillation pulse and measuring the voltage drop across the electrodes
12.
A shock button 20, typically part of a user interface of the defibrillator
10 allows the user to initiate the delivery of a defibrillation pulse
through the electrodes 12 after the controller 18 has detected VF or other
shockable rhythm. A battery 22 provides power for the defibrillator 10 in
general and in particular for a high voltage charger 24 which charges the
capacitors in an energy storage capacitor network 26. Typical battery
voltages are 12 volts or less, while the capacitors in the energy storage
capacitor network 26 may be charged to 1500 volts or more. A charge
voltage control signal from the controller 18 determines the charge
voltage on each capacitor in an energy storage capacitor network 26.
The energy storage capacitor network 26 according the present invention
contains multiple capacitors which may be arranged in series, parallel, or
a combination of series and parallel arrangements responsive to a
configuration control signal from the controller 18. The energy storage
capacitor network 26 has an effective capacitance and effective charge
voltage that depend on the selected configuration. For example, a
configuration that consists of three series capacitors with a capacitance
value C and charge voltage V will have an effective capacitance of 1/3 C
and effective voltage of 3 V.
The controller 18 uses the patient impedance and the selected energy level
to select a configuration of the energy storage capacitor network 26 from
the set of configurations in order to deliver the impedance-compensated
defibrillation pulse to the patient. The operation of the energy storage
capacitor network 26 in delivering the impedance-compensated
defibrillation pulse is described in more detail below.
The energy storage capacitor network 26 is connected to the HV switch 16
which operates to deliver the defibrillation pulse across the pair of
electrodes 12 to the patient in the desired polarity and duration response
to the polarity/duration control signal from the controller 18. The HV
switch 16 is constructed using an H bridge to deliver biphasic
defibrillation pulses in the preferred embodiment but could readily be
adapted to deliver monophasic or multiphasic defibrillation pulses and
still realize the benefits of the present invention.
In FIG. 2, there is shown a simplified schematic of the energy storage
capacitor network 26. The high voltage charger 24 is selectively connected
to each of a set of capacitors 60-68 via a set of charging switches 50-56
to facilitate charging the capacitors 60-68 to a desired voltage level.
Charging each of the capacitors 60-68 be either done sequentially or
simultaneously in parallel as needed and with each of the capacitors 60-68
charged to either the same voltage level or different voltage levels
according to the requirements of the application. The set of capacitors
60-68 may have the same capacitance value or have different capacitance
values depending on the application. In the preferred embodiment, each of
the capacitors 60-68 has the same capacitance value and is charged to the
same initial voltage. The set of charging switches 50-56 are controlled by
the controller 18 to facilitate the charging process. A set of blocking
diodes may be substituted for the set of charging switches 50-56 to
facilitate the charging of the capacitors 60-68. Each of the switches
50-56 and 70-78 is preferably controlled by the controller 18 via a set of
control lines to each of the switches 50-56 and 70-78 (not shown).
A set of switches 70-78 coupled between the switches 60-68 and ground
provide for creating the desired series, parallel, or series-parallel
circuits. The capacitors 60-64 are shown coupled in series, with the
series capacitors numbering as many as needed or as few as one. Similarly,
the capacitors 66-68 are shown coupled in parallel. The number of parallel
capacitors can be scaled up to as many as needed to obtain the desired
effective capacitance necessary to deliver the desired energy level in the
impedance-compensated defibrillation pulse.
Obtaining higher energy levels in the defibrillation pulse without
increasing the charge voltage or encountering current levels that exceed
the maximum level may be done by adding parallel capacitors to selected
series or parallel capacitor combinations in a manner that increase the
overall effective capacitance without increasing the charge voltage. For
example, if a configuration for the series arrangement of the capacitors
62 and 64 is called for to obtain a desired voltage level for a given
patient impedance but a higher level of capacitance is needed to obtain
the desired energy level, additional capacitors (not shown) can be placed
in parallel with each of the capacitors 62 and 64 using additional
switches.
Obtaining energy levels above 200 joules (j) may be achieved in this manner
using 100 uF capacitors without increasing the charge voltage level above
2,000 volts. Such higher energy level options could be available as
additional configurations in the set of configurations of the energy
storage capacitor network 26. The versatility of selecting among the
configurations allows higher levels of energy to be delivered by the
impedance-compensated defibrillation pulse while avoiding current levels
that exceed the maximum value.
The capacitors 60-68 are coupled in one of a selected configuration from a
set of series, parallel, or series-parallel configurations to the HV
switch 16 under the control of the controller 18 which determines the
polarity and duration of the impedance-compensated defibrillation pulse to
the patient. In the preferred embodiment, the selected configuration of
the energy storage capacitor network 26 remains constant throughout each
phase of the defibrillation pulse, such as the first and second phases of
a biphasic defibrillation pulse. Alternatively, the selected configuration
may be changed between phases, such as to obtain additional energy
transfer during the second phase.
In FIGS. 3, 4, and 5 that follow, the operation of the energy storage
capacitor network 26 using a set of two configurations is illustrated for
purposes of example. A series configuration employs two 100 microFarad
(uF) capacitors coupled in series and is selected for patient impedances
above 72 ohms. A parallel configuration employs two 100 uF capacitors
coupled in parallel and is selected for patient impedances below 72 ohms.
The value of 72 ohms was arbitrarily chosen as the delineation between
high impedance and low impedance patients. The energy level remains fixed
at 150 joules in this example, leaving just the two configurations of the
energy storage capacitor network 26 in the set that are selected by the
controller 18 based on impedance.
The same two 100 uF capacitors may be used for both the series and parallel
configurations according to this example or different capacitors may be
may be selected within the energy storage capacitor network 26. Additional
series, parallel, and series-parallel configurations of capacitors may be
readily added to allow for closer compensation of the defibrillation pulse
for the patient impedance as explained above. The energy level can be
increased by adding configurations that provide for parallel capacitors
that are addd to the existing configuration to increase its equivalent
capacitance without increasing the total voltage or peak current delivered
to the patient in the defibrillation pulse.
FIG. 3 is a graph of initial current versus patient impedance. Initial
current is equivalent to peak current since the peak current occurs at the
initial application of the defibrillation pulse. As shown in the graph, a
discontinuity appears at 72 ohms where the changeover is made by the
controller 18 between the series and parallel configurations based on the
patient impedance measured by the front end 14. In the region below 72
ohms, the parallel configuration is selected in the energy storage
capacitor network 26 in which the 100 uF capacitors, each charged to 1300
volts, are coupled in parallel. This parallel configuration is equivalent
to a single 200 uF capacitor charged to 1300 volts. In the region above 72
ohms, the series configuration is selected in the energy storage capacitor
network 26 in which the 100 uF capacitors are coupled in series. This
series configuration is equivalent to a single 50 uF capacitor charged to
2600 volts.
The use of the series and parallel configurations corresponding to the
patient impedances below and above the cutoff resistance of 72 ohms
respectively allows for peak currents to remain below a maximum value of
60 amperes for low impedance patients and above 15 amperes for high
impedance patients. In this way, an impedance-compensated defibrillation
pulse is delivered to the patient by the defibrillator 10.
FIGS. 4A-C are a set of graphs showing patient current versus time to form
the defibrillation pulses for the patient impedances of 20 ohms, 50 ohms,
and 120 ohms respectively. Each of the defibrillation pulses in this
example is a biphasic truncated exponential (BTE) type pulse. The energy
storage capacitor network 26 according to the present invention may be
applied equally well for other types of defibrillation pulses including
monophasic and multiphasic pulses. In this example, tilt, which is the
percentage decrease in capacitor voltage, and pulse duration are
controlled to regulate the amount of energy delivered to the patient by
the defibrillation pulse. The peak current for each defibrillation pulse
is the initial current at time 0 when the defibrillation pulse is first
applied.
In comparing FIGS. 4A-C, the times t1, t2, and t3 are the duration times of
the defibrillation pulses for the defibrillation pulse delivered to
patients having impedances of 20 ohms, 50 ohms, and 120 ohms respectively.
Time t2 for the 50 ohm impedance is greater than time t1 for the 20 ohm
impedance since the parallel configuration is selected for patient
impedances below 72 ohms and a longer duration is needed to deliver the
required amount of energy. In FIG. 4C, the series configuration is
selected for the 120 ohm impedance which requires a shorter duration of
time t3 relative to time t2 to deliver the required amount of energy to
the patient. In this way, the impedance-compensated defibrillation pulse
is delivered over the 20 to 200 range of patient impedances with a smaller
range of pulse duration times using the energy storage capacitor network
26 according to the present invention than by using a single energy
storage capacitor.
More configurations of the energy storage capacitor network 26 may be
readily added to deliver an impedance-compensated defibrillation pulse
that is more finely tailored to the patient impedance and increase the
range of energy levels that may be delivered to the patient. The spread of
duration times of the defibrillation pulse | | |
