U.S. patent application number 10/124062 was filed with the patent office on 2002-10-31 for damped biphasic energy delivery circuit for a defibrillator.
This patent application is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Gliner, Bradford E., Lyster, Thomas D., Powers, Daniel J..
Application Number | 20020161408 10/124062 |
Document ID | / |
Family ID | 23154871 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020161408 |
Kind Code |
A1 |
Lyster, Thomas D. ; et
al. |
October 31, 2002 |
Damped biphasic energy delivery circuit for a defibrillator
Abstract
A defibrillator capable of delivering a damped biphasic
truncated (DBT) defibrillation pulse is provided. An energy storage
circuit is coupled across a high voltage switch such as an H-bridge
for delivering a defibrillation pulse to the patient through a pair
of electrodes. A controller operates to control the entire
defibrillation process and detects shockable rhythms from the
patient via an ECG front end. The energy storage circuit consists
of an energy storage capacitor, a series inductor, a shunt diode,
and optionally a resistor in series with the inductor. The
controller measures as the patient dependent parameter the time
interval between the initial delivery of the defibrillation pulse
and the occurrence of the peak current or voltage to determine the
first and second phases of the defibrillation pulse to provide for
compensation for patient impedance. Other types of patient
dependent parameters, measured either before or during delivery of
the DBT defibrillation pulse, could be alternatively employed to
achieve the impedance compensation.
Inventors: |
Lyster, Thomas D.; (Bothell,
WA) ; Gliner, Bradford E.; (Issaquah, WA) ;
Powers, Daniel J.; (Issaquah, WA) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
Koninklijke Philips Electronics
N.V.
|
Family ID: |
23154871 |
Appl. No.: |
10/124062 |
Filed: |
April 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10124062 |
Apr 17, 2002 |
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09299456 |
Apr 22, 1999 |
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6405081 |
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Current U.S.
Class: |
607/5 |
Current CPC
Class: |
A61N 1/3912 20130101;
A61N 1/3937 20130101; A61N 1/3904 20170801 |
Class at
Publication: |
607/5 |
International
Class: |
A61N 001/39 |
Claims
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 an
energy storage circuit for delivering a damped biphasic truncated
defibrillation pulse through said HV switch to said patient.
2. A defibrillator according to claim 1 wherein said energy storage
circuit comprises: an energy storage capacitor coupled across said
HV switch; a series inductor and a series resistor coupled in
series with said energy storage capacitor; and a shunt diode
coupled across said series inductor and series resistor.
3. A defibrillator according to claim 2 wherein said series
inductor has an inductance value between 5 and 100
milliHenries.
4. A defibrillator according to claim 1 further comprising: an ECG
front end coupled to said pair of electrodes to provide ECG
information; and a controller coupled to said front end to receive
said ECG information, to said HV switch to control a first and
second phase duration of said damped biphasic truncated
defibrillation pulse, and to said energy storage circuit to receive
at least one of a current signal and a voltage signal.
5. A defibrillator according to claim 4 wherein said controller
measures a patient dependent parameter according to said one of
said current signal and said voltage signal to determine said first
and second phase durations.
6. A defibrillator according to claim 5 wherein said patient
dependent parameter comprises a time duration between an initiation
of said damped biphasic truncated defibrillation pulse and a peak
current.
7. A defibrillator according to claim 5 wherein said patient
dependent parameter comprises a time duration between an initiation
of said damped biphasic truncated defibrillation pulse and a peak
voltage.
8. A defibrillator according to claim 4 wherein said first and
second phase durations are determined according to a look up
table.
9. A defibrillator according to claim 1 wherein said damped
biphasic truncated defibrillation pulse has a peak current limited
to less than a maximum value.
10. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient, comprising: coupling a
defibrillator via pair of electrodes to said patient; initiating
delivery of a first phase of said damped biphasic truncated
defibrillation pulse to said patient; measuring a patient dependent
parameter during said delivery of said first phase; and determining
a first and second phase duration of said damped biphasic truncated
defibrillation pulse based on said patient dependent parameter.
11. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient according to claim 10, said
measuring step comprising measuring a time interval to a peak
current in said damped biphasic truncated defibrillation pulse.
12. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient according to claim 10, said
measuring step comprising measuring a time interval to a peak
voltage in said damped biphasic truncated defibrillation pulse.
13. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient according to claim 10 further
comprising: truncating said first phase according to said first
phase duration; and delivering a second phase of said damped
biphasic truncated defibrillation based on said second phase
duration.
14. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient, comprising: coupling a
defibrillator via pair of electrodes to said patient; measuring a
patient dependent parameter of said patient; determining first and
second phase durations of said damped biphasic truncated
defibrillation pulse based on said patient parameter; and
delivering said damped biphasic truncated defibrillation pulse to
said patient according to said first and second phase
durations.
15. A method for delivering a damped biphasic truncated
defibrillation pulse to a patient according to claim 14, said
measuring step comprising measuring a patient impedance as said
patient dependent parameter.
16. A defibrillator comprising a pair of electrodes for coupling to
a patient; an HV switch coupled to said pair of electrodes; an
energy storage circuit for generating a damped biphasic truncated
defibrillation pulse; and a controller coupled to said HV switch
and to said energy storage circuit; wherein said controller
initiates delivery of said damped biphasic truncated
defibrillation, measures a patient dependent parameter, and
determines first and second phase durations of said damped biphasic
truncated defibrillation pulse based on said patient dependent
parameter.
17. A defibrillator according to claim 16 wherein said energy
storage circuit comprises: an energy storage capacitor coupled
across said HV switch; a series inductor and a series resistor
coupled in series with said energy storage capacitor; and a shunt
diode coupled across said series inductor and series resistor.
18. A defibrillator according to claim 17 wherein said series
inductor has an inductance value between 5 and 100
milliHenries.
19. A defibrillator according to claim 16 further comprising an ECG
front end coupled to said pair of electrodes to provide ECG
information to said controller to detect a shockable rhythm.
20. A defibrillator according to claim 16 wherein said controller
measures said patient dependent parameter according to a current
signal from said energy storage circuit.
21. A defibrillator according to claim 20 wherein said patient
dependent parameter comprises a time duration between an initiation
of said damped biphasic truncated defibrillation pulse and a peak
current measured from said current signal.
22. A defibrillator according to claim 16 wherein said controller
measures said patient dependent parameter according to a voltage
signal from said energy storage circuit.
23. A defibrillator according to claim 22 wherein said patient
dependent parameter comprises a time duration between an initiation
of said damped biphasic truncated defibrillation pulse and a peak
voltage measured from said voltage signal.
24. A defibrillator according to claim 16 wherein said first and
second phase durations are determined according to a look up table
based on said patient dependent parameter.
25. A defibrillator according to claim 16 wherein said damped
biphasic truncated defibrillation pulse has a peak current limited
to less than a maximum value.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to electrotherapy circuits and in
particular to a defibrillator which is capable of applying damped
biphasic defibrillation pulses to a patient.
[0002] 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.
[0003] 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 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.
[0004] 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 the
deliver the defibrillation pulse to resuscitate the patient 14.
[0005] 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.
[0006] 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).
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] More recent biphasic defibrillator designs such as the
Heartstream Forerunner.RTM. 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 components of the H-bridge, do not address the problem of
limiting peak current to the patient across the range of patient
impedances.
[0012] It would therefore be desirable to provide a defibrillator
that delivers an impedance-compensated defibrillation pulse to the
patient with limited peak currents.
SUMMARY OF THE INVENTION
[0013] A defibrillator capable of delivering a current-limited
defibrillation pulse is provided. An energy storage circuit charged
to a high voltage by a high voltage charger circuit which receives
its energy from a battery. The energy storage circuit is coupled
across a high voltage switch such as an H-bridge for delivering a
defibrillation pulse to the patient through a pair of electrodes. A
controller operates to control the entire defibrillation process
and detects shockable rhythms from the patient via an ECG front
end.
[0014] The energy storage circuit consists of an energy storage
capacitor, a series inductor, a shunt diode, and optionally a
resistor in series with the inductor. The series inductor has an
inductance value chosen to limit the peak current of the
defibrillation pulse delivered to the patient for the lowest
expected value of patient impedance. The inductance value is chosen
as a function of the capacitance value and charge voltage of the
energy storage capacitor. The series resistor may be added
depending on the internal resistance of the series inductor.
Alternatively, the series inductor may be modeled as an ideal
inductor and the series resistor represents the effective series
resistance (ESR) of the series inductor. The shunt diode is
necessary to clamp the voltage generated by the energy stored in
the series inductor because the defibrillation pulse is truncated
by the high voltage switch. The waveform developed according to the
present invention is a damped biphasic truncated (DBT) waveform
which is distinct from the biphasic truncated exponential (BTE)
waveform of the prior art.
[0015] The controller uses the current and voltage information
supplied by the energy storage circuit to determine a patient
dependent parameter. In the preferred embodiment, the patient
dependent parameter is measured during the delivery of the
defibrillation pulse. Alternatively, the patient dependent
parameter may be measured before delivery of the defibrillation
such by the use of a low level signal or the delivery of a
non-therapeutic pulse to the patient.
[0016] A patient dependent parameter is a measurement of time,
voltage or current that is directly related to the patient
impedance for the given combination of capacitance, inductance, and
series resistance. This patient dependent parameter can be used by
the controller to set the time of the first and second phases of
the defibrillation pulse to control the amount of energy delivered
to the patient. One such patient dependent parameter that can be
determined in the case of the DBT defibrillation pulse is the
measured time interval between the initial delivery of the
defibrillation pulse to the time that the current or the voltage
peaks. Other patient dependent parameters, such as the percentage
voltage drop across the patient, percentage voltage drop across the
energy storage capacitor, or measured time to reach a circuit
charge delivery using a current integrator, may also be effectively
used.
[0017] One feature of the present invention is to provide a
defibrillator that delivers current limited defibrillation
pulses.
[0018] A further feature of the present invention is to provide a
defibrillator that delivers damped biphasic truncated (DBT)
defibrillation pulses.
[0019] Another feature of the present invention is to provide an
energy storage circuit capable of delivering current limited
defibrillation pulses.
[0020] A further feature of the present invention is to provide a
method of delivering damped biphasic truncated defibrillation
pulses.
[0021] 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
[0022] FIG. 1 is an illustration of a defibrillator being applied
by a user to resuscitate a patient suffering from cardiac
arrest;
[0023] FIG. 2 is a simplified block diagram of a defibrillator
according to the present invention;
[0024] FIG. 3 is a schematic diagram of the energy storage circuit
of the defibrillator of FIG. 2;
[0025] FIG. 4A-C are graphs comparing the typical waveshapes of a
biphasic truncated exponential (BTE) defibrillation pulse generated
according to the prior art with the damped biphasic truncated (DBT)
defibrillation pulse generated according to the present
invention;
[0026] FIG. 5 is a graph of the DBT defibrillation pulse generated
according the present invention; and
[0027] FIG. 6 is a flow diagram of the method of generating a DBT
defibrillation pulse according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] FIG. 2 is a simplified block diagram of a defibrillator 10
according to the present invention. The pair of electrodes 16 for
coupling to the patient 14 are connected to an ECG front end 18 and
further connected to an HV switch 28. The ECG front end 18 provides
for detection, filtering, and digitizing of the ECG signal from the
patient 14. The ECG signal is in turn provided to a controller 26
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.
[0029] A shock button 30, typically part of a user interface of the
defibrillator 10 allows the user to initiate the delivery of a
defibrillation pulse through the pair of electrodes 16 after the
controller 26 has detected VF or other shockable rhythm. A battery
24 provides power for the defibrillator 10 in general and in
particular for a high voltage charger 22 which charges the
capacitors in an energy storage circuit 20. Typical battery
voltages are 12 volts or less, while the energy storage circuit 20
may be charged to 1500 volts or more. A charge voltage control
signal from the controller 26 determines the charge voltage in the
energy storage circuit 20.
[0030] The energy storage circuit 20 is connected to the RV switch
28 which operates to deliver the defibrillation pulse across the
pair of electrodes 16 to the patient 14 in the desired polarity and
duration response to the switch control signal from the controller
26. The HV switch 28 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.
[0031] In FIG. 3, there is shown a simplified schematic of the
energy storage circuit 20. An energy storage capacitor 50 stores
the energy for the defibrillation pulse and is typically charged to
over 2,000 volts in the preferred embodiment. The energy storage
capacitor 50 is connected to the HV switch 28 through a current
sense resistor 52 on one lead and through a series inductor 54 and
series resistor 56 on the other lead. A shunt diode 58 is coupled
in shunt across the series inductor 54 and series resistor 56. The
HV switch 28 connects the energy storage circuit 20 to the pair of
electrodes 16 in a selected polarity and for a selected duration
responsive to the switch control signal from the controller 26. The
shunt diode 58 is necessary to include in the energy storage
circuit 20 because the defibrillation pulse is truncated, leaving a
sudden interruption in the current path. The shunt diode 58 becomes
forward biased upon truncation in order to shunt the voltage
generated by the energy stored in the series inductor 54.
[0032] A voltage measurement circuit 62 is connected across the
energy storage capacitor 50 to measure the charge voltage and
provide a voltage signal back to the controller 26. The voltage
measurement circuit 62 could be implemented using a voltage divider
network to scale the voltage down to lower level signal that is
then supplied to an analog to digital converter (ADC) which
provides the voltage signal to the controller 26 in the form of
digital measurement values.
[0033] A current measurement circuit 60 is connected across the
current sense resistor 52 to measure the current flowing from the
energy storage capacitor 50. The current sense resistor 52 has a
relatively small resistance value that does not interfere with
operation of the energy storage circuit 20. The current measurement
circuit 60 may include an ADC to provide the current signal to the
controller 26 in the form of digital measurement values. The
current measurement circuit 60 may also include an integrator
circuit to determine the amount of charge that has been discharged
from the energy storage capacitor 50 if required for measuring a
patient dependent parameter. Such an integrator could be an analog
integrator constructed using well known techniques using
operational amplifiers and capacitors. Depending on the method of
measuring a patient dependent parameter used for controlling the
first and second phase durations of the defibrillation pulse,
either the voltage measurement circuit 62 or the current
measurement circuit 60 could be omitted from the energy storage
circuit 20. The use of the DBT defibrillation pulse according to
the present invention allows for a selected energy level, such as
150 joules, to be delivered with an acceptable level of accuracy,
to a patient of unknown impedance within the range of 20 to 200
ohms, and with both pulse duration and peak current within
predetermined limits.
[0034] The choice of capacitance value and charge voltage for the
energy storage capacitor 50, inductance value for the series
inductor 54, and resistance value for the series resistor 56 to
achieve the DBT defibrillation pulse with the desired
characteristics requires a reasonable amount of experimentation.
The DBT defibrillation pulse must have the desired limit on peak
current for low patient impedances. At the same time, the DBT
defibrillation pulse should provide an adequate time duration
between the initialization of the defibrillation pulse and the peak
current or peak voltage to allow for accurate measurement of the
patient dependent parameter if that patient dependent parameter is
chosen for the control method.
[0035] The following tables provide the results of various
combinations of component values and charge voltages that could be
chosen to achieve a DBT defibrillation pulse.
1TABLE 1 Combinations of Component Values R series L series
Capacitance Charge Stored Combination (ohms) (mH) (uF) Voltage (V)
Energy (J) 1 10 10 100 1800 160 2 10 20 100 1800 160 3 10 20 70
2150 160 4 20 20 70 2300 185
[0036]
2TABLE 2 Peak current and energy delivered for each combination
from Table 1. Patient Combination Combination Combination
Combination Impedance 1 2 3 4 25 ohms Peak 44 41 46 41 Current (A)
Energy 108 108 109 96.7 Delivered (J) 50 ohms Peak 28 27 31 29
Current (A) Energy 127 127 127 124.3 Delivered (J) 100 ohms Peak 16
15 18 18 Current (A) Energy 138 138 138 145 Delivered (J) 175 ohms
Peak 9.8 9.6 11.5 11.5 Current (A) Energy 144 144 144 156.2
Delivered (J)
[0037] All delivered energy calculations assume that approximately
95% of the energy stored in the energy storage capacitor 50 is
delivered to the patient 14. Taking combination 4 as an example,
omitting the series inductor 54 to obtain a BTE defibrillation
pulse would result in substantially higher peak currents of 51 A
for the 25 ohm patient impedance and 33 A for the 50 ohm patient
impedance compared with 41 A and 29 A respectively according to the
DBT defibrillation pulse of the present invention.
[0038] The data from Table 2 show that inductance values in the 10
to 20 mH range for the combinations 1-4 deliver acceptable results
in limiting peak current for low patient impedances while providing
adequate energy delivery for high patient impedances. The
inductance value of the series inductor 54 may be in the range of 5
to 100 mH in conjunction with the energy storage capacitor 50 with
a capacitance of 50 to 100 uF to achieve the DBT defibrillation
pulse according to the present invention. Inductance values than 5
mH tend to provide insufficient limiting of the peak current to the
patient 14. Inductance values greater than 100 mH require inductors
that are physically too large for portable defibrillator
applications.
[0039] FIG. 4A-C are graphs (not to scale) of patient current
versus time from initial delivery of the defibrillation pulse
comparing the typical waveshapes of a biphasic truncated
exponential (BTE) defibrillation pulse generated according to the
prior art with the damped biphasic truncated (DBT) defibrillation
pulse generated according to the present invention.
[0040] In FIG. 4A, there is shown a graph of a typical BTE waveform
drawn as trace 70. In the first phase spanning time T0 to T2, the
peak current occurs at or very close to time T0, with an
exponential decay in the patient current that is truncated at time
T2. Truncation means that the defibrillation pulse is switched off
while a substantial amount of current is continuing to flow to the
patient 14. The second phase spans from time T3 to T5 with a peak
current at or very close to time T3 followed by exponential decay
to time T5 where the second phase is truncated.
[0041] In FIG. 4B, there is shown a graph of a typical DBT waveform
according to the present invention drawn as trace 72. The
truncation times T2 and T5 are aligned for reasons of comparison to
show how the BTE and DBT waveforms are roughly equivalent in the
amount of energy delivered. In the first phase, the peak current
now occurs at time T1 which is substantially later than time T0 and
at a value substantially less than peak current of the BTE waveform
of FIG. 4A. The decay is no longer exponential and substantially
flatter than the BTE waveform. Similarly in the second phase, the
peak current occurs at time T4 which is substantially later than
time T3 and at a value substantially less than the peak current at
time T3 of the BTE waveform.
[0042] In FIG. 4C, there is shown a graph of the traces 70 and 72
superimposed on each other to visually compare the BTE and DBT
waveshapes. It has been discovered that the area 74 between the
traces 70 and 72 which represents the energy delivered in the peak
current has relatively little therapeutic value to the patient 14.
This energy is therefore wasted and a potentially harmful to the
patient 14 if the peak current of the BTE waveform exceeds the
maximum value. Limiting the peak current according to the present
invention may have the additional advantage of allowing the use of
higher charge voltages on the energy storage capacitor 50 while
decreasing its capacitance value creates advantages in terms of
reduced physical size and weight.
[0043] In FIG. 5, there is shown a graph of a typical DBT waveshape
illustrating a method of controlling the durations of the first and
second phases. It is well known that the amount of energy delivered
by a defibrillation pulse varies by patient impedance and can be
controlled by appropriate setting of the duration time of the first
and second phases. A patient dependent parameter which correlates
with patient impedance can be measured, either before or during the
delivery of the defibrillation pulse, and then used to determine
the first and second phase durations.
[0044] As shown in FIG. 5, a method of controlling the first and
second phase durations of the DBT defibrillation can be based on a
measurement of the time interval between time T0, the initial
delivery of the defibrillation pulse, and time T1, the time that
the peak current occurs. This value of this time interval directly
depends on the patient impedance which can then be used as a
patient dependent parameter to determine the first phase duration
spanning time T0 to T2 and the second phase duration spanning time
T3 to T5. It will be noted that this method will work equally well
by measuring the voltage across the pair of electrodes 16 and
measuring the time interval to the peak voltage.
[0045] In this way, the patient dependent parameter is determined
during the delivery of the defibrillation pulse. This method has
the advantage of eliminating the separate step of determining the
patient dependent parameter prior to delivery of the defibrillation
pulse. Measuring the patient dependent parameter prior to delivery
of the defibrillation pulse may be also be used using existing
circuitry which detects patient contact across the pair of
electrodes 16 and still gain the advantages of the DBT
defibrillation pulse according to the present invention.
[0046] In FIG. 6 there is shown a flow diagram of a process of
delivering an impedance-compensated defibrillation pulse by the
defibrillator 10 based on one embodiment of the present invention
that addresses the time interval to peak current. Another
embodiment which is substantially similar is the time interval to
peak voltage.
[0047] In step 200 labeled BEGIN DELIVERY OF FIRST PHASE OF
DEFIBRILLATION PULSE TO PATIENT, the delivery of the defibrillation
pulse is initiated by the controller 26 which controls the HV
switch 28 using the switch control signal. In the typical scenario
such as in an AED, the pair electrodes 16 have been placed on the
patient 14, the controller 26 has detected a shockable rhythm such
as VF, and the user 12 has pressed the shock button 30 to deliver
the defibrillation pulse. The controller 26 then begins the process
of delivering the defibrillation pulse and functions as a control
system to provide for the proper sequence of first and second
phases and with the phase durations appropriate for the patient
impedance of the patient 14.
[0048] In an alternative embodiment, the patient dependent
parameter would have already been measured prior to the step 200
using, for example, with the ECG front end 18 equipped with
circuitry capable of measuring patient impedance either directly or
indirectly. A low level test signal or a non-therapeutic pulse
could be generated and then measured to obtain the patient
dependent parameter before delivery of the DBT defibrillation
pulse.
[0049] In step 202 labeled MEASURE TIME TO PEAK CURRENT, the
controller 26 monitors the current signal during the delivery of
the defibrillation pulse and determines the time interval from the
initial delivery of the defibrillation pulse to the time of the
peak current. On the graph of FIG. 5, these are times T0 and T1
respectively.
[0050] In step 204 labeled DETERMINE FIRST AND SECOND PHASE
DURATIONS, the time interval from step 202 is used to determine the
first and second phase durations. Such a determination can be made
using a mathematical formula or more simply by using a look up
table containing the phase duration values. The phase duration
values would be predetermined for the desired energy level such as
150 joules. As much resolution as needed could be added to the look
up table to achieve delivery of the defibrillation pulse according
to the desired energy level within specification limits.
[0051] In step 206 labeled TRUNCATE THE FIRST PHASE ACCORDING TO
THE FIRST PHASE DURATION, the first phase is allowed to continue
for the first phase duration that was determined in step 204. It is
assumed that the first phase duration is sufficiently long to allow
step 204 to complete in order to properly truncate the first phase
according to the first phase duration.
[0052] In step 208 labeled DELIVER THE SECOND PHASE ACCORDING TO
THE SECOND PHASE DURATION, the second phase is delivered over the
second phase duration which was determined in step 204.
Alternatively, a second time duration measurement which measures
the time interval between times T3 and T4 could be made to
determine the second time interval in a manner similar to the first
time interval.
[0053] Other methods of determining the patient dependent parameter
during the delivery of the DBT defibrillation pulse may be used.
For example, the current measurement circuit 60 could contain the
integrator circuit which provides a measurement of integrated
current which is a function of total charge delivered to the
patient. A time interval measurement could be made of the time to
reach a predetermined level of charge which is then used to
determine the first and second phase durations. This method is used
in the Heartstream Forerunner.RTM. defibrillator to control the BTE
biphasic waveform and could also be used to control the DBT
defibrillation pulse of the present invention.
[0054] Another example of determining a patient dependent parameter
is to measure the voltage across the energy storage capacitor 50
and truncate the first and second phases when the voltage has
dropped to predetermined percentages of the fill charge voltage. A
further example is to measure the voltage across the pair of
electrodes 16 and truncate each phase when the charge voltage
across the energy storage capacitor 50 has dropped to a
predetermined percentage of a maximum level.
[0055] It will be obvious to those having ordinary skill in the art
that many changes may be made in the details of the above-described
preferred embodiments of the invention without departing from the
spirit of the invention in its broader aspects. For example, a wide
range of inductance values for the series inductor, resistance
values for the series resistor, and capacitance values for the
energy storage capacitor may be chosen as long as the DBT
defibrillation pulse has a peak current that is sufficiently
limited and the time interval from initiation to peak current is
sufficiently long to enable an accurate measurement for the control
method explained above. Therefore, the scope of the present
invention should be determined by the following claims.
* * * * *