U.S. patent application number 11/569366 was filed with the patent office on 2007-12-27 for electrotherapy apparatus.
This patent application is currently assigned to Bioinduction Ltd.. Invention is credited to Timothy William Coates, Ivor Stephen Gillbe, Roger Kenneth Littlewood, Alexander John Ranald Macdonald.
Application Number | 20070299482 11/569366 |
Document ID | / |
Family ID | 32670986 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070299482 |
Kind Code |
A1 |
Littlewood; Roger Kenneth ;
et al. |
December 27, 2007 |
Electrotherapy Apparatus
Abstract
An apparatus for producing analgesia in a patient through
electrical signals applied through electrodes to a patient's body,
and methods of treating patients using the apparatus. The apparatus
comprises a signal generator arranged to generate a biphasic
waveform comprising successive cycles each containing a positive
and negative pulse.
Inventors: |
Littlewood; Roger Kenneth;
(Somerset, GB) ; Macdonald; Alexander John Ranald;
(Bristol, GB) ; Coates; Timothy William;
(Somerset, GB) ; Gillbe; Ivor Stephen; (Bristol,
GB) |
Correspondence
Address: |
DARBY & DARBY P.C.
P.O. BOX 770
Church Street Station
New York
NY
10008-0770
US
|
Assignee: |
Bioinduction Ltd.
178-180 Hotwell Road
Bristol Avon
GB
BS8 4RP
|
Family ID: |
32670986 |
Appl. No.: |
11/569366 |
Filed: |
May 24, 2005 |
PCT Filed: |
May 24, 2005 |
PCT NO: |
PCT/GB05/02075 |
371 Date: |
December 20, 2006 |
Current U.S.
Class: |
607/46 ;
606/42 |
Current CPC
Class: |
A61N 1/36021 20130101;
A61N 1/36071 20130101 |
Class at
Publication: |
607/046 ;
606/042 |
International
Class: |
A61N 1/34 20060101
A61N001/34 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2004 |
GB |
0411610.9 |
Claims
1. An apparatus for producing analgesia in a patient through
electrical signals applied by electrodes to the patient's body, the
apparatus comprising electrodes for application to the patient's
body and a signal generator which is connectable to the electrodes,
wherein the signal generator is arranged to generate a biphasic
waveform comprising successive cycles each containing a positive
and negative pulse, wherein the mean pulse width P.sub.w is 10
.mu.s or less, and wherein V.sub.p.sup.2.Pw.F.sub.p is at least
200, where V.sub.p is the mean pulse voltage, and F.sub.p is the
number of forward and reverse pulses per second.
2. The apparatus of claim 1, wherein the mean pulse width is 4
.mu.s or less.
3. The apparatus of claim 1, wherein the mean pulse width is 1.5
.mu.s or less.
4. The apparatus of claim 1, wherein the mean pulse width is 0.75
.mu.s or less.
5. The apparatus of claim 1, wherein the mean pulse voltage is at
least 100V.
6. The apparatus of claim 1, wherein the mean pulse voltage is at
least 150V.
7. The apparatus of claim 1, wherein the voltage is at least 100V
and the pulse width is 6 .mu.s or less.
8. The apparatus of claim 1, wherein the voltage is at least 200V
and the pulse width is 1.5 .mu.s or less.
9. The apparatus of claim 1, wherein the pulse frequency is at
least 1000 Hz.
10. The apparatus of claim 1, wherein the pulse frequency is at
least 5 KHz.
11. The apparatus of claim 1, wherein V.sub.p.sup.2.Pw.F.sub.p is
at least 250.
12. The apparatus of claim 1, wherein V.sub.p.sup.2.Pw.F.sub.p is
at least 340.
13. The apparatus of claim 1, wherein the biphasic wave is
continuous.
14. The apparatus of claim 1, wherein each pulse in the biphasic
wave has an edge rate exceeding 250V/.mu.s.
15. The apparatus of claim 1, wherein each pulse in the biphasic
wave has an edge rate exceeding 500V/.mu.s.
16. The apparatus of claim 1, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 3 mA.
17. The apparatus of claim 1, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 6 mA.
18. The apparatus of claim 1, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 10 mA.
19. The apparatus of claim 1, wherein the interpulse spacing is
less than 4 .mu.s.
20. The apparatus of claim 1, wherein the interpulse spacing is
less than 1 .mu.s.
21. The apparatus of claim 1, wherein the interpulse spacing is 0
.mu.s.
22. A method of inducing analgesia in a patient, the method
comprising providing an apparatus according to claim 1, applying
the electrodes to the patient's body, and providing a waveform as
described.
23. A method of inducing analgesia in a patient's body through
electrical signals applied by electrodes to the patient's body,
using an apparatus comprising electrodes for application to the
patient's body and a signal generator which is connectable to said
electrodes, wherein the signal generator is arranged to generate a
biphasic waveform comprising successive cycles each containing a
positive and negative pulse, the method comprising: applying the
electrodes to two or more locations on the patient's body such that
at least one of said locations on the patient's body does not
overlie the central nervous system; providing said biphasic
electrical waveform so as to induce analgesia at said site.
24. The method of claim 23, wherein the locations span the area of
pain or discomfort, and/or peripheral nerves conducting signals
from said area of pain or discomfort.
25. The method of claim 23, which relieves pain or discomfort
associated with physical injury.
26. The method of claim 23, which relieves pain or discomfort
associated with inflammatory conditions.
27. The method of claim 23, wherein the mean pulse width is 10
.mu.s or less.
28. The method of claim 23, wherein the mean pulse width is 4 .mu.s
or less.
29. The method of claim 23, wherein the mean pulse width is 1.5
.mu.s or less.
30. The method of claim 23, wherein the mean pulse width is 0.75
.mu.s or less.
31. The method of claim 23, wherein the mean pulse voltage is at
least 100V.
32. The method of claim 23, wherein the mean pulse voltage is at
least 150V.
33. The method of claim 23, wherein the pulse voltage is at least
100V and the pulse width is 6 .mu.s or less.
34. The method of claim 23, wherein the pulse voltage is at least
200V and the pulse width is 1.5 .mu.s or less.
35. The method of claim 23, wherein the pulse frequency is at least
1200 Hz.
36. The method of claim 23, wherein the pulse frequency is at least
5 kHz.
37. The method of claim 23, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 3 mA.
38. The method of claim 23, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 6 mA.
39. The method of claim 23, wherein the signal generator is
arranged to provide a biphasic waveform providing a mean modulus
current flowing through the patient of at least 10 mA.
40. The method of claim 23, wherein the interpulse spacing is less
than 4 .mu.s.
41. The method of claim 23, wherein the interpulse spacing is less
1 .mu.s.
42. The method of claim 23, wherein the interpulse spacing is 0
.mu.s.
43. The method of claim 23, wherein the apparatus is an apparatus
according claim 1.
44. An apparatus for producing analgesia in a patient through
electrical signals applied by electrodes to the patient's body, the
apparatus comprising electrodes for application to the patient's
body and a signal generator which is connectable to the electrodes,
wherein the signal generator is arranged to generate a biphasic
waveform comprising successive cycles each containing a positive
and negative pulse, and wherein the apparatus comprises a control
element for varying the spacing between the positive and negative
pulse.
45. The apparatus of claim 44, wherein the signal generator is
arranged to generate a biphasic waveform having a mean pulse width
of less than 4 .mu.s.
46. The apparatus of claim 44, wherein the signal generator is
arranged to generate a biphasic waveform having a mean pulse width
of at least 0.5 .mu.s.
47. The apparatus of claim 44, wherein the signal generator is
arranged to generate a biphasic waveform having a voltage of at
least 100V.
48. The apparatus of claim 44, wherein the control element is
operable by an operator.
49. The apparatus of claim 44, wherein the control element provides
automated variation of interpulse spacing.
50. The apparatus of claim 49, wherein the control element provides
automated variation of interpulse spacing in a series of modulation
cycles.
51. The apparatus of claim 50, wherein modulation rates are between
1200 Hz and 0.25 Hz.
52. A method of inducing analgesia through electrical signals
applied by electrodes to the patient's body, wherein the induction
of analgesia comprises providing an apparatus comprising electrodes
for application to the patient's body and a signal generator
connected to said electrodes, wherein the signal generator is
arranged to provide a biphasic electrical waveform comprising
successive cycles each having a positive and negative pulse,
applying the electrodes to two or more locations on the patient's
body; and providing said biphasic electrical waveform; wherein the
spacing between the positive and negative pulse is modulated during
the treatment.
53. The method of claim 52, wherein the spacing between the
positive and negative pulse is modulated so as to alter the level
of sensation experienced by the patient during the treatment.
54. The method of claim 52, wherein the biphasic waveform has a
mean pulse width of less than 4 .mu.s.
55. The method of claim 52, wherein the biphasic waveform has a
mean pulse width of at least 0.5 .mu.s.
56. The method of claim 52, wherein the signal generator is
arranged to generate a biphasic waveform having a voltage of at
least 100V.
57. The method of claim 52, wherein the modulation is repeated
during the treatment in a series of modulation cycles.
58. The method of claim 57, wherein modulation rates are between
1200 Hz and 0.25 Hz.
59. A method for pre-setting the operating parameters of an
apparatus for producing analgesia in a patient through electrical
signals applied by electrodes to the patient's body, the apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to said electrodes, wherein
the signal generator is arranged to provide a biphasic waveform
comprising successive cycles each having a positive and negative
pulse, the method comprising selecting the voltage and pulse width,
and selecting the interpulse spacing with reference to a
predetermined relationship between sensation and interpulse spacing
at said voltage and pulse width.
60. A method of selecting treatment parameters during the induction
of analgesia through electrical signals applied by electrodes to
the patient's body, wherein said therapy comprises: providing an
apparatus comprising electrodes for application to the patient's
body and a signal generator connectable to said electrodes, wherein
the signal generator is arranged to generate a biphasic waveform
comprising a successive cycles each containing a forward and
reverse pulse, applying the electrodes to two or more locations on
the patient's body; and providing said waveform at a starting
voltage and pulse width; the method of selecting treatment
parameters comprising varying the voltage and/or pulse width until
the patient experiences a comfortable level of sensation, and
selecting the voltage and pulse width at which comfortable
sensation is felt.
61. An apparatus for producing analgesia in a patient through
electrical signals applied by electrodes to the patient's body, the
apparatus comprising electrodes for application to the patient's
body and a signal generator which is connectable to the electrodes,
wherein the signal generator is arranged to generate a biphasic
waveform comprising successive cycles each containing a positive
and negative pulse, and comprises: a converter for producing a
desired voltage from a power supply; a first capacitor which is in
electrical connection with said converter and which is arranged to
provide the positive pulse to one of said electrodes; a second
capacitor which is in electrical connection with said converter and
which is arranged to provide the negative pulse to one of said
electrodes; wherein said first and second capacitors are
respectively connected to an output by a corresponding pathway, the
pathways including respective switches, and wherein the signal
generator includes a controller arranged to control the operation
of the switches to cause alternate discharge of the first and
second capacitors through the corresponding pathway to generate
said positive and negative pulses.
62. The apparatus of claim 61, wherein the capacitance of the
capacitor decreases with voltage.
63. The apparatus of claim 61, wherein the capacitors are ceramic
type capacitors.
64. The apparatus of claim 61, wherein the first and second
capacitors are connected via respective diodes to a common supply
generated by the converter, and wherein the pathways corresponding
to the first and second capacitors are provided respectively by two
arms of an H-bridge.
65. The apparatus of claim 61, wherein the output has a peak pulse
current of greater than 1 A.
66. The apparatus of claim 65, wherein the output has a peak pulse
current of greater than 2 A.
67. The apparatus of claim 61, wherein the apparatus comprises a
control to control the operation of switches, such that during the
pulse off time, the switches in the pathway connecting the first
and second capacitor to the respective output are switched off, and
one or more additional switches are operated to short the patient
outputs.
68. The apparatus of claim 61, wherein the signal generator is
arranged to generate a biphasic waveform wherein each pulse of the
biphasic waveform has an edge rate is greater than 250 V/.mu.s.
69. The apparatus of claim 61, wherein the signal generator is
arranged to generate a biphasic waveform wherein each pulse of the
biphasic waveform has an edge rate is greater than 500 V/.mu.s.
70. The apparatus according to claim 61, wherein the apparatus
further comprises means to limit the pulse width to less than a
predetermined period.
71. The apparatus according to claim 61, wherein the apparatus
further comprises means for detecting when said positive or
negative pulse is of over-prolonged duration, and for limiting said
duration.
72. The apparatus of claim 70, wherein the pulse "on" time is
limited by circuitry in the switch drive logic or level translation
circuitry.
73. The apparatus of claim 70, wherein the pulse length is limited
by one or both of a logic circuit and output transistors, which
cannot remain in the "on" state for more than a fixed period
without edge transitions on the gate drives.
74. The apparatus of claim 61, wherein the therapeutic pulse charge
does not exceed a safe value in the event of software failure or
single component failure.
75. The apparatus of claim 74, wherein said safe value is 75
.mu.C.
76. The apparatus of claim 61, having means for earthing said
electrical connection of each of said first and second capacitors,
thereby to halt the provision of said pulses.
77. The apparatus of claim 76, wherein said earthing means
comprises a switch controlled by said controller.
78. The apparatus of claim 61, wherein the apparatus further
comprises one or more independent means for monitoring the signal
produced by the apparatus.
79. The apparatus of claim 61, wherein the signal generator is
arranged to generate a biphasic waveform comprising successive
cycles each containing a positive and negative pulse, wherein the
mean pulse width P.sub.w is 10 .mu.s or less, and wherein
V.sub.p.sup.2.Pw.F.sub.p is at least 200, where V.sub.p is the mean
pulse voltage, and F.sub.p is the number of forward and reverse
pulses per second.
80. (canceled)
Description
[0001] The present invention relates to electrotherapy, and
provides apparatus and methods for application of such therapy.
[0002] There are a number of electrotherapy techniques that induce
action potentials. A.beta. fibres have low thresholds that enable
them to respond to such innocuous events as movement, vibration and
light touch. Melzack and Wall 1965 and Wall 1986 described how
analgesia could be produced when A.beta. fibres are stimulated at
100 Hz, a frequency that none of the other sensory nerves can
follow faithfully. Wall 1986 produced these effects by applying the
current through needles inserted into the patient's nerves. To
avoid the inconvenience and possible complications of inserting
needles into nerves, he soon employed surface electrodes, leading
to the term Transcutaneous Electrical Nerve Stimulation (TENS).
[0003] Woolf 1989 reviewed the use of these devices, and described
their electrical parameters. The usual TENS machine develops a
pulse, whose width can be varied from 50-500 .mu.s, employing a
current whose amplitude can be increased from 0-50 mA, and whose
frequency is generally 100 Hz. The TENS pulse width (50-500 .mu.s)
is sufficiently long in duration to excite A.beta. nerves at low
voltage causing a painless tingling and stimulation of
interneurones releasing GABA (Duggan et al 1985) that inhibits the
release of SP within the spinal cord by C-fibres. Johnson et al
1991 showed that `high intensity stimulation`; where the amplitude
is increased sufficiently to recruit A.delta. fibres, invokes
release of met-enkephalin in the spinal cord which produces a more
prolonged analgesic effect than that provided by the release of
GABA produced by the more usual `low intensity stimulation` of
A.beta. fibres. Salar et al 1981 observed opioids were released
slowly into the cerebrospinal fluid when TENS is performed at
frequencies of 40-60 Hz and at amplitudes of 40-80 mA: signals that
readily recruit A.delta. fibres, whose firing is associated with
sharp pain.
[0004] To stimulate A.beta. fibres, frequencies below 1 kHz are
employed. The typical TENS device runs at 100 Hz. However the TENS
frequency may be reduced still further to below 80 Hz.
[0005] As tissue impedance is capacitive, it tends to fall as
frequency is increased. In order to increase tissue penetration,
signals may be provided at a frequency where the intervals between
each electric signal are less than the refractory periods of axons
that require stimulation. In order to produce action potentials,
such signals are modulated to provide low frequency stimulation
either by interference or interruption.
[0006] The interference method of applying medium frequency
currents is exemplified by Nemec U.S. Pat. No. 2,622,601, Griffith
U.S. Pat. No. 3,096,768 and many others. Two signal sources are
each connected to a pair of electrodes. They can produce an
amplitude modulated medium frequency signal in the tissues called
interferential current as follows. The first signal source uses a
medium frequency carrier wave (typically 4.0 kHz) while the other
operates at a slightly different frequency (typically 4.1 kHz).
Their respective pairs of surface electrodes are arranged on the
body in a manner that allows the two oscillating currents to meet
in deep tissues where modulation produces interference or a beat
frequency in the low frequency range typically at 100 Hz. This in
turn is said to stimulate deeply placed A.beta. fibres to produce
analgesia.
[0007] As an example of the interrupted form of modulation, in
order to avoid the surgical complications of implanting electrodes
in direct contact with the brain, Limoge U.S. Pat. No. 3,835,833
and Stinus et al 1990 describe Transcutaneous Cranial Stimulation
(TCES), an application of intermittent 4 ms trains of medium
frequency current (typically 166 kHz) arranged so each pulse
provided is 100 mA peak-to peak to a patient's head via one frontal
cathode and two posterior anodes (one placed over each mastoid
process). The positive going element of each pulse (67 mA) lasts 2
.mu.s; while the negative going portion has a lower amplitude of 33
mA but lasts 4 .mu.s. Each 4 ms train is repeated typically at 77
or 100 times per second. The medium frequency is employed to
penetrate the tissues of the brain from surface electrodes. Stinus
(1990) has observed that such stimulation only raised pain
thresholds when opiates have been administered.
[0008] Macdonald and Coates GB 2290033, U.S. Pat. No. 5,776,170
explored the effects of applying signals whose pulse width is so
brief, typically 4 .mu.s, that the voltage gated channels lying in
excitable membranes of peripheral nerve axons that lie in the path
of the current do not have time to respond to these electric
signals sufficiently to reach membrane threshold and produce action
potentials. This form of electrotherapy produces analgesic and mood
altering effects provided that surface electrodes are placed over
the spinal cord. Macdonald and Coates 1995 called this method TSE
(Transcutaneous Spinal Electroanalgesia). Towell et al 1997
performed a controlled trial that demonstrated TSE produces
beneficial changes in mood provided the electrodes are placed over
the spinal cord.
[0009] GB2290033 states that as frequency is increased the voltage
has to be reduced. The document quotes 150V as being a sufficient
voltage at 5 kHz and 25V as being sufficient at 150 kHz.
[0010] The present invention provides methods and apparatus related
to electrotherapy, which address certain limitations or
disadvantages of the prior art.
[0011] In a first aspect, the present invention provides an
apparatus for producing analgesia in a patient through electrical
signals applied by electrodes to the patient's body, the apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to the electrodes, wherein
the signal generator is arranged to generate a biphasic waveform
comprising successive cycles each containing a positive and
negative pulse, wherein the mean pulse width P.sub.w is less than
10 .mu.s, and wherein V.sub.p.sup.2.Pw.F.sub.p is at least 200,
where V.sub.p is the mean pulse voltage, and F.sub.p is the number
of forward and reverse pulses per second.
[0012] In this application, references to waveform are to an
electrical waveform. The term "pulse" refers to either the positive
or negative (forward or reverse) element of the biphasic waveform.
Hence the pulse voltage is the amplitude (V) of the positive or
negative pulse and the pulse frequency is the number of forward and
reverse pulses per second, counting both. The pulse width Pw is
measured in seconds.
[0013] A cycle consists of both a forward and reverse pulse.
Therefore, the number of forward and reverse pulses per second (the
pulse frequency) will be equal to twice the cycle frequency. The
leading edge of the cycle can be either positive or negative.
Normally, the leading pulse in each cycle will be of the same
polarity, but this is not essential.
[0014] It has been found that the apparatus as described above can
be used to deliver a high power treatment to a patient. This may be
done without generating action potentials in sensory nerves at a
level that might be uncomfortable (as would be the case for example
with a TENS pulse).
[0015] It has also been found that embodiments of the present
invention are capable of treating surprisingly large volumes of
tissue, and/or treating deep structures more effectively than the
prior art. Moreover, unlike conventional TSE treatment, it has been
found that treatment is possible by applying the electrodes to the
body at points which provide current flow in the region of the
affected area, and not necessarily over the central nervous system.
Among other things, this has been found to be helpful in reducing
inflammation, which can for example be used to treat inflammatory
arthritic and visceral disease.
[0016] The present waveform is referred to herein as a "HPSP" (high
power short pulse) waveform. The relationship
V.sub.p.sup.2.Pw.F.sub.p>=200 is derived from consideration of
the power which can be applied to the patient. The mean power
dissipation can be approximated to
W.sub.m=Pw.F.sub.p.V.sub.p.sup.2/Z where W.sub.m is the mean power
dissipation and Z is the impedance of the tissues and connecting
means to the device (electrodes and leads). This aspect of the
invention provides a greater power level for a given load impedance
than was employed or suggested in the prior art. For example, at
with signals applied that contain most energy in harmonics at
frequencies of at least 20 kHz, the load impedance of the human
body can be approximated at a constant 150 .OMEGA.. Based on this,
the maximum power taught by GB2290033 is 1.125 W. The present
invention operates significantly above this level.
[0017] The relationship V.sub.p.sup.2.Pw.F.sub.p>=200 is
equivalent to and can also be expressed as V.sub.RMS>= 200.
[0018] The high RMS value of the applied voltage when compared with
other electrotherapy techniques produces similarly high RMS
currents in the patient. For example, at V.sub.RMS= 200, the RMS
current with a 150 .OMEGA. load (typical of a patient) would be 94
mA, which is six times higher than what is generally accepted to be
a strong TENS signal of 15 mA.
[0019] V.sub.p.sup.2.Pw.F.sub.p may be at least 220, preferably it
is at least 250, 340 or 500.
[0020] Due to heating effects at higher powers, it may be preferred
that V.sub.p.sup.2.Pw.F.sub.p is below 1800, more preferably below
1200.
[0021] Preferably, the biphasic waveform is continuous. By a
"continuous" biphasic waveform is meant a series of cycles in which
the leading pulses are equally spaced.
[0022] The mean pulse width for the forward and reverse pulses, and
preferably the pulse width of each of the forward and reverse
pulses, may be 6 .mu.s or less, 4 .mu.s or less, e.g., 3 .mu.s, 2
.mu.s, 1.5 .mu.s, 1 .mu.s, 0.75 .mu.s or less, and optionally at
least 0.01 .mu.s, 0.05 .mu.s, or 0.5 .mu.s. In some embodiments,
short pulse widths are preferred so as to increase the rate of
change of the electrical field, e.g., for a given mean modulus or
RMS current in the tissues.
[0023] Without wishing to be bound by theory, the inventors believe
increasing the rate of change of the electrical field may increase
coupling to cellular structures involved in transmission of pain.
The signal penetrates deep tissues, and it is believed that it may
produce beneficial effects by producing changes that affect one or
more processes that occur in the central and or peripheral nervous
systems, for example the behaviour of microtubules, the rate of
release of certain ligands and or the responses to them by various
ligand gated receptors. The signal may also have effects on the
mobility of ions associated with the transmission of action
potentials and act directly on other cell structures such as
voltage gated channels in both the peripheral and central nervous
system.
[0024] In some embodiments of the invention, it is preferred that
the mean pulse voltage, and preferably the voltage of each positive
and negative pulse, is at least 100V, preferably 150V and more
preferably 200V. Optionally, the mean pulse voltage and/or voltage
of each pulse has an upper limit of 500V, 400V, 300V or 250V, e.g.,
to meet safety requirements.
[0025] While all combinations of preferred voltages and pulse
widths are specifically included, optionally, when the voltage
(mean value or voltage of each pulse) is at least 100V, the pulse
width (mean value or width of each pulse) is 6 .mu.s or 4 .mu.s or
less; when the voltage is at least 150V the pulse width is 3 .mu.s
or less; and when the voltage is at least 200V the pulse width is
1.5 .mu.s or less.
[0026] The pulse frequency (i.e. the number of forward and reverse
pulses per second) may be at least 1000 Hz or 1200 Hz, more
preferably at least 5 KHz, and still more preferably 10 KHz, 20 KHz
or above.
[0027] The pulse frequency may be less than 2 MHz, more preferably
less than 1 MHz or 500 kHz, still more preferably less than 250 kHz
or 100 kHz.
[0028] It is preferred that the duty cycle (the ratio of "on time"
to "off time") through one complete cycle should be less than 10%
or 5%, preferably less than 2% or 1%, and greater than 0.1%,
particularly where the biphasic wave is continuous.
[0029] Each pulse in the biphasic wave preferably has a rapid rise
and fall phase, e.g., is substantially rectangular, subject to
capacitor droop. Preferably the edge rate exceeds 250V/.mu.s, more
preferably 500V/.mu.s or 1000V/.mu.s.
[0030] Preferred embodiments of invention have a high pulse current
during the pulse "on" time. For example the waveform may have a
pulse current of at least 0.3 A throughout the pulse period. The
current may vary over the pulse period due to capacitor droop, and
may for example be 0.7 A-3 A at the start of the pulse, falling to
0.5 A to 2 A at the end of the pulse. The mean modulus current
flowing through the patient is preferably at least 3 mA, preferably
at least 6 mA and more preferably at least 10 mA. When measured at
150 ohm load, V.sub.RMS= 200 gives an RMS current of 94 mA.
[0031] The present inventors have found that sensation can be
obtained with the wave form used in embodiments of the present
invention. The inventors measured the threshold of sensation
produced by a biphasic square wave, that is, the point at which
sensation is first felt. While the voltage threshold of sensation
was affected by the pulse width, it was substantially unaffected by
the frequency. Unexpectedly, therefore the threshold of sensation
is not affected by the RMS or mean modulus current. Moreover,
sensation can be provided at pulse frequencies well above the
physiological limits, those frequencies greater than that which
A.beta. fibres can faithfully follow, for example 800 Hz to 1200
Hz.
[0032] It has furthermore been found that the threshold of
sensation can be varied by varying the interpulse spacing. As the
interpulse spacing is reduced, the voltage at which sensation is
first perceived for a given pulse width increases, particularly for
pulse widths below 4 .mu.s. This allows the level of sensation to
be varied independently of the power level or current supplied by
the device or the associated rate of change of the electrical field
in the tissues. In some embodiments it may be desirable to reduce
the level of sensation that is felt, so that there is low or no
sensation, e.g., allowing the patient to sleep when the device is
operating. Alternatively, it may be desirable to provide a mild
sensation, as this can be comforting to the user, and can help to
distract from aches and pains. It is also believed that some mild
tingling might play a role in expression of neurotransmitters.
[0033] It may be preferred that the mean pulse width, and more
preferably the pulse width of each forward and reverse pulse, is
less than 4 .mu.s, to improve the ability of the sensation level to
be varied with the interpulse spacing.
[0034] The interpulse spacing may be less than 4 .mu.s, preferably
less than 3, 2 or 1 .mu.s and most preferably 0 .mu.s. In some
embodiments, a low interpulse spacing may be used together with a
low pulse width (mean value or value per pulse), e.g., a pulse
width of 2 .mu.s or less, more preferably a pulse width of 1.5
.mu.s or 1 .mu.s or less, to provide mild or no sensation even at
high voltages (i.e., peak pulse voltages), such as greater than
150V, 200V or 250V. Without wishing to be bound by theory, it is
believed that a low interpulse space and preferably an interpulse
space of 0 .mu.s may also be beneficial because it provides a high
rate of change in the electrical field per pulse.
[0035] In other embodiments, e.g., where some sensation may be
desirable, the interpulse spacing may be at least 5 .mu.s,
preferably at least 6, 7, 8, 9 or 10 .mu.s.
[0036] The interpulse spacing may also be varied to alter the wave
harmonics. Without wishing to be bound by theory, the present
inventors believe that the harmonic content of the wave is
important in determining its treatment efficacy. For instance, the
continuous wave form which is preferred in this aspect of the
invention produces a series of harmonics at frequencies spread
widely over the spectrum, when compared to a burst wave form in
which most of the wave energy is concentrated around a narrow
peak.
[0037] The inventors have compared the harmonic components of a
square wave of 2 .mu.S pulse width with equal positive and negative
pulses and zero inter pulse space, with a square wave of the same
pulse width with 2 .mu.S inter pulse space, both at a cycle
frequency of 20 kHz. The spacing between components in both cases
was 20 kHz, but the waveform with 2 .mu.S inter pulse space has its
peak at approximately half the frequency of the waveform with zero
space.
[0038] The inventors also examined a square wave of 2 .mu.S pulse
width with cycle frequency of 5 kHz and inter pulse spacing of 100
.mu.S; in this case the return pulses are equally spaced between
the forward pulses. This provides an interesting result with two
distinct curves made up of harmonic components of the signal. One
could be said to represent the harmonic components of the cycle
frequency and the other the components of the pulses themselves.
This dual curve structure may be desirable in increasing the chance
of exciting resonance in the cellular structures, regardless of
natural variation between them, e.g., in orientation. Hence, in
some embodiments, it may be desirable for the interpulse spacing to
be at least 20, 40, 60, 80 or 100 .mu.s, up to half the distance
between cycles.
[0039] Preferred HPSP pulses range from 4 .mu.S width with the main
harmonic components of this centred around approximately 125 kHz,
through to 0.05 .mu.S or less, equivalent to 10 MHz, and
incorporate pulse frequencies varying in the range 1200 Hz through
to 2 MHz.
[0040] Selection of inter pulse space and pulse width therefore
plays a primary role in determining the range of frequencies
provided by the therapy.
[0041] HPSP therapy can be considered to be a form of radio
frequency stimulation of the tissues with a much wider spread of
harmonics than that associated with the known types of pulsed and
continuous radio frequency electrotherapy in use today. The
waveform parameters can be selected to deliver oscillating electric
fields in the ranges of 1 kHz to 20 MHz or more. The region
covering 50 kHz to 1 MHz (harmonics in fields) is thought to be
that at which effects on cell structures and ion mobility are
maximised and therefore this is the preferred range of frequencies
that are employed.
[0042] Kotnik et al 2000 explored the degree of amplification of an
external electric within the cell at various harmonic frequencies
and showed the cell membrane amplifies externally applied
alternating current electric fields by a factor of several
thousands provided the harmonic frequency is 100 kHz or less. As
harmonic frequency is increased above 100 kHz, this amplified
effect greatly decreases, and the capacitive properties of
structures contained within the cell and extracellular fluid become
increasingly important. Liu et al 1990, studied the effects of
oscillating electric fields on the activation of Na+ and K+ pumping
modes of (Na,K)-ATPase. This is an active transport system
(energised by the hydrolysis of ATP) in the membrane for regulating
the extrusion of Na+ and influx of K+ ions into the cell (the
sodium pump) either to maintain cellular electrolyte balance or
provide action potential transmission. At a voltage of 20V/cm,
various frequencies were tested and uncoupled transport modes were
shown at two frequencies; increased influx only was observed at 1
kHz; while increased efflux only was observed at higher frequencies
particularly 1 MHz.
[0043] In another aspect, the apparatus as described above can be
used in a method of inducing analgesia in a patient, the method
comprising applying the electrodes to the patient's body, and
providing a waveform as described.
[0044] In another aspect of the invention, there is provided an
apparatus for producing analgesia in a patient through electrical
signals applied by electrodes to the patient's body, the apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to the electrodes, wherein
the signal generator is arranged to generate a biphasic waveform
comprising successive cycles each containing a positive and
negative pulse, and wherein the apparatus comprises a control
element for varying the spacing between the positive and negative
pulse.
[0045] The mean pulse width, and preferably the width of each
positive and negative pulse of the biphasic waveform, is preferably
less than 4 .mu.s, to maximise the ability to vary sensation by
varying the interpulse spacing, and optionally at least 0.5 .mu.s
or 2 .mu.s. Preferably, the control element for varying the spacing
between the positive and negative pulse is able to vary the
interpulse spacing between 0 .mu.s and half of the cycle time, more
preferably 0 .mu.s and 20 .mu.s or 0 .mu.s is and 10 .mu.s.
[0046] This apparatus may be particularly beneficial at high mean
pulse voltages, e.g., at least 100V, 150V or 200V where sensation
is more likely to be experienced. More preferably, these voltages
refer to the voltage of each pulse. Optionally,
V.sub.p.sup.2.Pw.F.sub.p is at least 200 and/or less than 1200 or
1800. It may also or alternatively be preferred that pulse
frequency is at least 1000 Hz or 1200 Hz, more preferably at least
5 kHz, and still more preferably 10 KHz, 20 KHz or above. The pulse
frequency may be less than 2 MHz, more preferably less than 1 MHz
or 500 kHz, still more preferably less than 250 kHz or 100 kHz. In
some embodiments, the wave form may be a HPSP waveform, as
above.
[0047] The control element may be operable by an operator, e.g.,
the user, for example so that the user can set the interpulse
spacing to a level which provides a comfortable level of
sensation.
[0048] Alternatively, the control element may provide automated
variation of interpulse spacing, for example rhythmical modulation,
or automated random modulation, e.g., in a series of modulation
cycles. This may be of particular benefit at modulation rates below
1200 Hz, preferably modulation rates below 100 Hz or 50 Hz, and/or
greater than 0.25 Hz, so as to modulate the sensory nerves within
the physiological range. Moreover, since the carrier signal can be
applied at well above the physiological range and with high peak
voltages it may be of particular benefit when the power levels are
fairly high so as to penetrate large volumes of tissues.
[0049] The control element may be for example a suitably programmed
processor, a manual control or circuit adapted to provide a
rhythmic or automated random modulation.
[0050] In some embodiments, the apparatus (preferably an apparatus
having automated variation in the interpulse spacing) also
comprises a control for varying the pulse voltage or pulse width or
both, for example a control which would allow the base level of
sensation to be set by the user.
[0051] In another aspect, the present invention provides a method
of inducing analgesia through electrical signals applied by
electrodes to the patient's body, wherein the induction of
analgesia comprises [0052] providing an apparatus comprising
electrodes for application to the patient's body and a signal
generator connected to said electrodes, wherein the signal
generator is arranged to provide a biphasic electrical waveform
comprising successive cycles each having a positive and negative
pulse, [0053] applying the electrodes to two or more locations on
the patient's body; [0054] and providing said biphasic electrical
waveform;
[0055] and wherein the spacing between the positive and negative
pulse is modulated during the treatment.
[0056] The spacing between the positive and negative pulse can be
modulated so as to alter the level of sensation experienced by the
patient during the treatment.
[0057] In some embodiments, it may be preferred that the method
comprises adjusting the interpulse spacing such that no sensation
is felt, except optionally for a transient sensation which quickly
fades. In other embodiments, it may be preferred that the
interpulse spacing is adjusted to provide the user with mild
sensation. For example, this can be reassuring to the user.
[0058] The modulation may take place one or more times during the
treatment to achieve a desired sensation level, or may be ongoing
or repeated, e.g., in a series of modulation cycles.
[0059] The modulation of interpulse spacing may also have
beneficial treatment effects, for example, it may aid the
expression of neurotransmitters or aid relaxation.
[0060] In preferred embodiments, the modulation is repeated during
the treatment, e.g., randomly or rhythmically. The modulation rate
of the interpulse spacing may preferably be below 1200 Hz, more
preferably below 100 Hz or 50 Hz, and/or greater than 0.25 Hz, so
as to modulate the sensory nerves within the physiological range.
Preferably, the user experiences at least some sensation.
[0061] This method preferably allows use of a high voltage, high
current signal which may penetrate deep tissues more effectively
than a typical TENS pulse and then modulating the signal by varying
the inter pulse spacing in such a way that action potentials are
generated in deep nerves at interpulse spacing modulation rates
well within the physiological range, preferably 0.25 Hz to 0.5 Hz
to 50 Hz or 120 Hz.
[0062] In each of the above methods, the spacing can be modulated
automatically, e.g., rhythmically (i.e. with an identifiable
pattern even a complex one) or randomly, e.g., under control of a
microprocessor, or via a manual control. When the method provides a
particular, desired level of sensation, it may be preferred that
the spacing is modulated by a manual control and/or a control which
is operable by a user. In other embodiments, an automated control
may be preferred. A random modulation may help in reducing adaption
to the signal.
[0063] Each of the above methods may also comprise selecting the
base level of sensation by adjusting the voltage or pulse width or
both. For example, the base level may be selected such that the
user experiences a transient sensation, e.g., at an interpulse
spacing of 0 .mu.s.
[0064] Optionally, in setting the base level of sensation, the
pulse width may be varied, and the pulse voltage maintained at a
value of greater than 150, 180V or 200V. Modulation of the
interpulse spacing may then give a good correlation between
sensation and interpulse space.
[0065] In another aspect, the present invention provides a method
for pre-setting the operating parameters of an apparatus for
producing analgesia in a patient through electrical signals applied
by electrodes to the patient's body, the apparatus comprising
electrodes for application to the patient's body and a signal
generator which is connectable to said electrodes, wherein the
signal generator is arranged to provide a biphasic waveform
comprising successive cycles each having a positive and negative
pulse, the method comprising [0066] selecting the voltage and pulse
width, and [0067] selecting the interpulse spacing with reference
to a predetermined relationship between sensation and interpulse
spacing at said voltage and pulse width.
[0068] In each of the above methods, preferred operating conditions
and parameters of the waveform are as described for the above
apparatus, comprising a control element for varying the spacing
between the positive and negative pulse.
[0069] In another aspect, the present invention provides an
apparatus for producing analgesia in a patient through electrical
signals applied by electrodes to the patient's body, the apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to said electrodes, wherein
the signal generator is arranged to provide a biphasic waveform
comprising successive cycles each containing a positive and
negative pulse, wherein the mean pulse width P.sub.w is less than
1.5 .mu.s and the spacing between the positive and negative pulse
is between 0 .mu.s and 2 .mu.s.
[0070] Preferably, the mean pulse voltage (more preferably the
voltage of each pulse) is at least 130V, more preferably at least
150V or 170V.
[0071] It may be preferred that the interpulse spacing is 1.5 .mu.s
or less, in which case the voltage (mean or voltage of each
positive and negative pulse) may be at least 140V, preferably at
least 160V or 180V. Still more preferably, the interpulse spacing
may be 1 .mu.s or less, in which case the voltage may be at least
180V, preferably at least 200V, more preferably at least 220V. An
interpulse spacing of 0.5 .mu.s or less, or 0 .mu.s may be
preferred in some embodiments. For example, a zero interpulse
spacing may be advantageous in providing a high rate of change of
the electrical field per pulse.
[0072] Preferably, the mean pulse width and preferably the width
each pulse is less than 1.5 .mu.s, more preferably less than 1.25,
1 .mu.s or 0.75 .mu.s.
[0073] In some embodiments, the pulse frequency may be at least
1000 or 1200 Hz, more preferably at least 5 kHz, 10 kHz or 20 kHz,
and/or less that 2 MHz, 1 MHz, 500 kHz or 250 kHz. Optionally,
V.sub.p.sup.2.Pw.F.sub.p is at least 200 and/or less than 1200 or
1800.
[0074] In another aspect the present invention provides a method of
inducing analgesia in a patient's body through electrical signals
applied by electrodes to the patient's body, using an apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to said electrodes, wherein
the signal generator is arranged to generate a biphasic waveform
comprising successive cycles each containing a positive and
negative pulse, the method comprising: [0075] applying the
electrodes to two or more locations on the patient's body such that
at least one of said locations on the patient's body does not
overlie the central nervous system; [0076] providing said biphasic
electrical waveform so as to induce analgesia at said site.
[0077] Preferably, said two or more locations do not overlie the
central nervous system, i.e., brain or spinal cord. More
preferably, at least one location is not in the immediate vicinity
of the spinal cord, for example is not on the neck, and/or is
located more than 10 or 15 cm laterally from the spine and/or on
the ventral side of the body. The locations preferably span, i.e.,
are above and below or on either side of, the area of pain or
discomfort, and/or peripheral nerves conducting signals from said
area of pain or discomfort. They preferably provide current through
an area which includes the area of pain or discomfort, and/or
peripheral nerves conducting signals from said area of pain or
discomfort. For example, at least one location may be on or in the
region of a limb (arm or leg including the hand or foot), the
abdomen or the face.
[0078] The method of inducing analgesia may be a method of
relieving pain or discomfort associated with chronic or acute
conditions. For example, it may be a method of relieving pain or
discomfort associated with physical injury (including
post-operative pain, fracture, bruising, muscle strain and the
like), or with chronic or acute diseases or disorders.
[0079] Said method may preferably be a method of relieving pain or
discomfort associated with inflammatory conditions, particularly
rheumatoid arthritis, and other inflammatory arthritic and visceral
conditions. The method may also be a method of treating said
conditions.
[0080] Preferably, the mean pulse width, and more preferably the
width of each pulse, is less than 10 .mu.s, more preferably 6 .mu.s
or less, 4 .mu.s or less, more preferably 3 .mu.s, 2 .mu.s, 1.5
.mu.s, 1 .mu.s, 0.75 .mu.s or less. It may be preferred that the
mean voltage and preferably the voltage of each pulse is at least
100V, preferably at least 150V or 200V.
[0081] While all combinations of preferred voltages and pulse
widths are specifically included, optionally, in certain
embodiments, when the voltage (mean value or voltage of each pulse)
is at least 100V, the pulse width (mean value or width of each
pulse) is 6 .mu.s or 4 .mu.s or less; when the voltage is at least
150V the pulse width 3 .mu.s or less; and when the voltage is at
least 200V the pulse width is 1.5 .mu.s or less.
[0082] The pulse frequency may be at least 1200 Hz, more preferably
at least 5 kHz, and still more preferably 10 kHz, 20 KHz or above,
and/or less than 2 MHz, 1 MHz, 500 KHz, 250 kHz or 100 KHz.
[0083] Optionally, V.sub.p.sup.2.Pw.F.sub.p is at least 200 and/or
less than 1200 or 1800. In some embodiments, the waveform is a HPSP
waveform, as described above.
[0084] In another aspect, the present invention comprises a method
of selecting treatment parameters during the induction of analgesia
through electrical signals applied by electrodes to the patient's
body, wherein said therapy comprises: [0085] providing an apparatus
comprising electrodes for application to the patient's body and a
signal generator connectable to said electrodes, wherein the signal
generator is arranged to generate a biphasic waveform comprising a
successive cycles each containing a forward and reverse pulse,
[0086] applying the electrodes to two or more locations on the
patient's body; [0087] and providing said waveform at a starting
voltage and pulse width; [0088] the method of selecting treatment
parameters comprising varying the voltage and/or pulse width until
the patient experiences a comfortable level of sensation, and
selecting the voltage and pulse width at which comfortable
sensation is felt.
[0089] Preferably, the method comprises varying the pulse voltage,
e.g., so that the mean pulse voltage and preferably the voltage of
each pulse is varied within the range of 0 -500V, preferably 0
-250V, 200V or 150V.
[0090] The starting pulse width (mean value and preferably also
value for each pulse width), may be 10 .mu.s or below, most
preferably 4 .mu.s or below. It is preferably varied in the range
of 0.01 .mu.s, 0.05 .mu.s or 0.5 .mu.s to 4 .mu.s or 10 .mu.s.
[0091] Optionally, the method also comprises providing the waveform
at a starting interpulse spacing, and varying the interpulse
spacing, e.g., in the range of 0 .mu.s-20 .mu.s, preferably in the
range of 0 .mu.s-10 .mu.s. In this embodiment, the starting pulse
width (mean value and preferably also value for each pulse width)
may be 4 .mu.s or less.
[0092] The pulse frequency may be greater than 1200 Hz, more
preferably at least 5 kHz, and still more preferably 10 kHz or 20
KHz or above.
[0093] In a preferred embodiment, the method comprises a further
step of, after selecting the voltage and pulse width at which
comfortable sensation is felt, increasing the number of pulses per
second F.sub.p such that F.sub.p is at least 200/(V.sub.p.sup.2.Pw)
but preferably less than 1800/(V.sub.p.sup.2 .Pw) or
1200/(V.sub.p.sup.2.Pw). Preferably, F.sub.p is at least
300/(V.sub.p.sup.2.Pw) or 400/(V.sub.p.sup.2.Pw).
[0094] In some or all of the aspects of the invention described
above, it may be preferred each pulse of the biphasic waveform has
fast rise and fall, e.g., is substantially rectangular, subject to
capacitor droop. It may have an edge rate of 250V/.mu.s or above,
more preferably 500V/.mu.s or 1000V/.mu.s or above.
[0095] In some or all of these aspects, the biphasic waveform may
be a "burst" waveform comprising a train a multiple pulses followed
by a quiet period. However, it may be preferred that it is a
continuous wave form, in which case it is preferred that the duty
cycle (the ratio of "on time" to "off time") through one complete
cycle is preferably be less than 10% or 5%, more preferably less
than 2% or 1%, and preferably greater than 0.1%.
[0096] It may also be preferred that the amount of electrical
charge in the forward and reverse pulse is equal. This gives a mean
current of zero, and helps to minimise ionic transport.
Conveniently, this can be achieved by having a forward and reverse
pulse of equal voltage and pulse width. If the amplitude (voltage)
or pulse width is not equal for the forward and reverse pulse, then
the mean value for the two pulses is calculated. In the case where
it is desired the amplitude or pulse width of the second pulse is
not the same as the first pulse, then the parameters of the second
pulse are preferably adjusted so the mean value of the voltage
applied to the patient is zero.
[0097] It may further be preferred that the apparatus for providing
analgesia in a patient comprises a single signal generator arranged
to produce one biphasic waveform. Similarly, it is preferred that
some or all of the methods comprise the provision of a single
biphasic waveform from a single signal generator. If more than one
waveform is provided by the apparatus/method, then it is preferred
that these waveforms are of the same cycle frequency, or that one
is an integer multiple of the other to reduce interference
effects.
[0098] A difficulty with treatment methods which involve imparting
relatively high power to the patient is that safety mechanisms must
be put in place which prevent a dangerous level of charge from
being applied in the event of malfunction. This is also a concern
in an apparatus in which variable frequency, pulse width and
voltage can be applied.
[0099] There are two international safety standards of particular
relevance, these are IEC 60601-2-10, "Particular requirements for
the safety of nerve and muscle stimulators" and the US standard
AAMI NS4-1985 (Transcutaneous Electrical Nerve Stimulation).
[0100] Key safety requirements of 60601-2-10 are: [0101] Maximum
limits on output current (rms) are 80 mA at DC, 50 mA at 400 Hz, 80
mA at 1500 Hz and 100 mA above 1500 Hz (with a 500 ohm resistive
load). [0102] The maximum pulse energy should not exceed 300 mJ.
[0103] The peak output voltage should not exceed 500V.
[0104] AAMI NS4 is being revised and its American National Standard
status has been withdrawn because it is more than 10 years old. It
however remains an AAMI (Association for the Advancement of Medical
Instrumentation) standard and is the most directly relevant
published document for the design of TENS devices. The key
requirements of NS4 are: [0105] Resistive loads of 200 .OMEGA., 500
.OMEGA. and 1 k.OMEGA. are defined as the test loads. 500 .OMEGA.
resistive is considered as the reference waveform for safety
purposes. [0106] The minimum output for efficacy (with the controls
at maximum) is either 7 .mu.C per pulse or a complex waveform whose
average stimulating component amplitude is at least 0.5 mA into a
load of 500 .OMEGA.. [0107] The maximum charge per pulse should
under no circumstances exceed 75 .mu.C into a 500 .OMEGA. load.
[0108] Maximum average current shall not exceed 10 mA, the limit
for DC currents to reduce burns due to ionic transport.
[0109] In GB 2 290 033, it is suggested that a capacitor could be
placed in series with one of the electrodes to isolate the patient
from the possibility of direct current stimulation. However, such
an arrangement is not suitable for producing square wave shaped
pulses.
[0110] In a further aspect, the present invention provides an
apparatus for producing analgesia in a patient through electrical
signals applied by electrodes to the patient's body, the apparatus
comprising electrodes for application to the patient's body and a
signal generator which is connectable to the electrodes, wherein
the signal generator is arranged to generate a biphasic waveform
comprising successive cycles each containing a positive and
negative pulse, and comprises: [0111] a converter for producing a
desired voltage from a power supply; [0112] a first capacitor which
is in electrical connection with said converter and which is
arranged to provide the positive pulse to one of said electrodes;
[0113] a second capacitor which is in electrical connection with
said converter and which is arranged to provide the negative pulse
to one of said electrodes; [0114] wherein said first and second
capacitors are respectively connected to an output by a
corresponding pathway, the pathways including respective switches,
and wherein the signal generator includes a controller arranged to
control the operation of the switches to cause alternate discharge
of the first and second capacitors through the corresponding
pathway to generate said positive and negative pulses.
[0115] This apparatus may thus be operable so that it cannot
generate a sustained harmful average current, particularly where
the converter produces a desired voltage but is limited in current
output, so that it cannot generate a dangerous current during the
pulse time either by itself or in combination with the capacitors,
nor deliver a pulse of more than the safety value in the even of
either software failure or single component failure.
[0116] An advantage of this apparatus is that a key safety measure
limiting the amount of charge which would be applied to the patient
in the event of malfunction is present in the hardware.
[0117] The inventors have realised that in order to limit the
current which can be supplied in the event of malfunction to an
acceptable level, it is desirable that sum total of the charge
transferred in the positive and negative cycles plus the current
which can be supplied by the converter over the pulse period should
not exceed a dangerous level. Preferably, it does not exceed a
maximum of 75 .mu.C, e.g., even under fault conditions.
[0118] As a result of these considerations, there is a limit on the
rate that charge can be delivered from the converter and/or on the
amount of charge which can be stored in the capacitors. As a
result, there may be a significant droop in voltage over the pulse
period, particularly for pulses of longer duration.
[0119] In the present apparatus, the wave is produced by
discharging two independent capacitors fed from a common converter,
one providing the positive pulse and one providing the negative
pulse. This produces a wave which is substantially symmetrical in
nature, since any voltage drop over the course of the capacitor
discharge affects the positive and negative pulse to the same
extent.
[0120] The converter is preferably adapted to produce the desired
voltage from a battery, though it may also be adapted to produce
the desired voltage from the mains supply as an additional or
alternative source. The apparatus may optionally include a suitable
battery, e.g., a rechargeable battery.
[0121] In a preferred embodiment, the signal generator produces an
output which has a fast rise and fall phase, e.g., is substantially
rectangular.
[0122] The output may have a pulse current (i.e., a peak pulse
current) of greater than 1 A, 1.5 A or 2 A or 3 A. It may
optionally be preferred that the output
V.sub.p.sup.2.Pw.F.sub.p>200.
[0123] In some embodiments, the output will be a HPSP wave, as
described above.
[0124] It is preferred that the capacitance of the capacitor
decreases with voltage. This improves the ability of the device to
be with a wide voltage range, storing only a safe level of charge
at a high voltage but also storing enough charge at a low voltage
to deliver effective stimulation with longer pulses, which are
necessary at lower voltages. Ceramic type capacitors are
particularly preferred.
[0125] In a preferred embodiment, when no output from the capacitor
is desired, i.e. during the pulse off time, switches in the pathway
connecting each capacitor to the output are turned off and an
additional switch or switches are operated to short the patient
outputs together either directly or via device 0V. Preferably, the
apparatus includes a control to control the operation of said
switches. This helps to provide a rapid return of the trailing edge
of the pulse to zero. Preferably, the edge rate is greater than 250
V/.mu.s, more preferably greater than 500V/.mu.s or
1000V/.mu.s.
[0126] In preferred embodiments, further control and safety systems
are provided. For example, pulse width may be controlled and/or
limited by one, two or more independent systems. For example, pulse
width can be limited by a logic circuit and/or by output
transistors which cannot remain in the "on" state for more than a
fixed period without edge transitions on the gate drives. The
latter also protects against a microprocessor failure since the
microprocessor may be expected to fail with its outputs in a frozen
state.
[0127] By careful selection of the capacitor rating and the use of
pulse "on time" limiting controls (such as circuitry in the switch
drive logic, and level translation circuitry) the apparatus can be
designed with a wide range of pulse and output voltage where the
therapeutic pulse charge does not exceed 25 .mu.C per pulse under
normal operating conditions, and 75 .mu.C per pulse under single or
double fault conditions.
[0128] The signal generator may also include one, two or more
independent means of monitoring the signal, for example monitoring
the current and/or voltage produced by the apparatus. These may
include means for measuring the output current from the converter
(wherein the measured current may be fed back into hardware
circuitry in the converter to provide either a current limit or a
current control loop and/or fed into a microprocessor for
monitoring processes), means for measuring the current from one or
both capacitors, and/or a means for measuring the voltage and
current applied to the patient.
[0129] Preferably, said first and second capacitors are
respectively connected by a corresponding pathway to an output path
to the patient. Means for monitoring this signal may preferably be
located in the output path. Alternatively, if electronically more
convenient, it may be located in the in the return pathway from the
output since this is at device 0V potential and therefore does not
require translation of the signal from the output sensor.
[0130] Preferably, the device includes a safety device which
operates to discharge the capacitors to device 0V in the event that
the voltage in either pathway and/or the output current exceeds a
predetermined limit, e.g., as detected by monitoring circuits
implemented in hardware. Most preferably, this device is a Silicon
Controlled Rectifier (SCR). In preferred embodiments, the device
should also be operable by a microprocessor in the event of an
error or shutdown identified by the microprocessor.
[0131] Preferably, the apparatus comprises at least two independent
circuits for monitoring the voltage and/or current produced by the
apparatus and also comprises means for comparing the measured
values, thus enabling an error in either circuit to be detected,
and optionally causing shutdown of the device.
[0132] Means for monitoring and/or comparing the signal and/or
controlling discharge of a pathway should preferably be implemented
in hardware and conveniently may also be implemented using a
microprocessor as backup.
[0133] While aspects of the invention have been discussed
independently, they may also be used together in any
combination.
[0134] In the above aspects of the invention, the electrodes of the
device may be either surface electrodes or implanted electrodes.
References to the application of an electrode to a location on the
patient's body are to be construed accordingly to include electrode
implantation. Normally, implanted electrodes are covered with
insulating material except at their tip, but this is not
essential.
[0135] Specific embodiments of certain aspects of the present
invention will now be described in more detail with reference to
the figures. These are provided by way of explanation and example,
and are not to be construed as limiting.
[0136] FIG. 1 illustrates a basic continuous bi-phasic wave form,
with a large mark space ratio (time axis not shown to scale).
[0137] FIG. 2 illustrates a burst waveform having multiple biphasic
cycles separated by quiet periods.
[0138] FIG. 3 shows a body impedance measurement taken over the
median nerve.
[0139] FIG. 4 shows a body equivalent circuit including lead
capacitance and electrodes.
[0140] FIG. 5 shows voltage and current of a waveform which is an
embodiment of the invention, over a cycle.
[0141] FIG. 6 illustrates a pulse shape according to certain
embodiments of the invention, showing an exaggeration of the
voltage droop.
[0142] FIG. 7 shows the power dissipated assuming pulse width is
varied linearly with cycle frequency, allowing for the droop in the
current due to series load capacitance.
[0143] FIG. 8 shows a comparison of continuous and burst waveforms,
in the frequency domain.
[0144] FIG. 9 shows a comparison between a waveform having an
interpulse space of 0 .mu.s and an interpulse space of 2 .mu.s, in
the frequency domain.
[0145] FIG. 10 shows the harmonic components of a 5 kHz, 2 .mu.s
biphasic pulse, with a 100 .mu.s interpulse spacing.
[0146] FIG. 11 shows the relationship between the threshold of
sensation and voltage at different pulse widths for four cycle
frequencies with a fixed interpulse spacing of 1 .mu.s.
[0147] FIG. 12 shows the relationship between the threshold of
sensation and voltage at different pulse widths for four cycle
frequencies, on a log-log scale.
[0148] FIG. 13 shows the voltages required to produce sensation at
various interpulse spacings at a 5,000 Hz cycle frequency,
employing a symmetrical biphasic waveform of 1.5 .mu.s pulse
width.
[0149] FIG. 14 shows a block diagram of an embodiment of the
apparatus according to one aspect of the invention.
[0150] Body Impedance and Power Dissipation
[0151] Using stainless steel mesh electrodes of size 50.times.50
mm, the inventors made various measurements of the impedance
between electrodes across the following positions: a) on the
forearm over the median nerve with the centre of electrodes 170 mm
apart; b) on the posterior aspect of the thorax with electrodes
over the spine at T1 and T12 and, c) across the neck with the
electrodes positioned just below the mastoid processes.
[0152] These measurements are subject to error since the measured
impedance can move up to +/-6 dB by changes in adhesion of the
electrodes for example. The measurements are also changed by using
different electrode types. However, with good quality electrodes
using a stainless steel mesh substrate and careful application
techniques the impedance varies a surprisingly small amount with
different electrode locations on the body, provided the
measurements are made on the subject over a short period of
time.
[0153] FIG. 3 shows a graph of impedance of the tissues measured
over the median nerve as described above. As a useful
approximation, the impedance looks like a series R-C combination,
as shown in FIG. 4. R1 and C1 represent the tissues and C2
represents the capacitance of the leads and electrodes. C2 causes
the slight fall off at frequencies approaching 10 MHz, this is seen
by the apparatus but can be neglected for practical considerations.
A parallel resistor of high value could be added to the equivalent
circuit to simulate the DC body DC resistance, although this is
several M.OMEGA. and can therefore be neglected. By inspection of
the graph, the tissues can be considered to be primarily resistive
above 21 kHz in the example shown. Typical values for resistance
are 150 .OMEGA. along the forearm and between T1 and T12 on the
spine and 120 .OMEGA. across the neck, measured with stainless
steel mesh conductor electrodes.
[0154] The effect of the C and R terms can be seen if one examines
the current and voltage in a typical HPSP cycle of pulse time 1
.mu.S and the inter-pulse spacing 1 .mu.S, as shown in FIG. 5. The
current trace shows the current falling off during the pulse
on-time due to the effect of the series capacitance. During the
inter pulse space, the apparatus shorts the electrodes together
allowing the capacitance to discharge, which gives rise to the
small reverse pulse in the inter pulse space of the same magnitude
as the slope on the top of the waveform.
[0155] As previously mentioned, for HPSP pulses of short duration
with dominant harmonics in the 20 kHz to 2 MHz region, the tissues
can be approximated by a resistive load. Consequently, the power
dissipation in the load can be estimated by considering only the
amplitude of the pulse. The mean power dissipation, W.sub.m, can be
therefore be approximated to: W.sub.m=.delta..V.sub.p.sup.2/R.
Where, V.sub.p is the voltage of the pulse and R the measured load
resistance and .delta. is the duty cycle. This neglects droop on
the output current due to the capacitance of the tissues, which
means that the power formula is an over estimate for longer
pulses.
[0156] A better approximation, that can be easily calculated, is
given by allowing for the series capacitance in the load equivalent
circuit and assuming the cycle time to be long relative to the load
time constant, approximating a pulse cycle to the sum of four step
inputs as represented by the following expression: V 2 R 1 T [
.intg. 0 t 1 .times. e - 2 .times. t .tau. .times. d t + .intg. t 1
t 2 .times. [ e - t .tau. - e - ( t - t 1 ) .tau. ] 2 .times. d t +
.intg. t 2 t 3 .times. [ e - t .tau. - e - ( t - t 1 ) .tau. - e -
( t - t 2 ) .tau. ] 2 .times. d t + .intg. t 3 T .times. [ e - t
.tau. - e - ( t - t 1 ) .tau. - e - ( t - t 2 ) .tau. + e - ( t - t
3 ) .tau. ] 2 .times. d t ] ##EQU1##
[0157] Where V is the applied voltage, R is the load resistance,
.tau.=C.R where C is the series capacitance,
t.sub.1=t.sub.2=t.sub.3=t is the pulse width and inter-pulse space
which are assumed the same and T=the cycle time.
[0158] The result of this approximation is shown in FIG. 7 where
the pulse width is reduced linearly as frequency is increased in an
attempt to keep power dissipation constant, according the to ratio:
Pulse Width=T/100.
[0159] Over the range T=50 to 500 .mu.S this simulates pulse widths
varied linearly from 0.5 to 5 .mu.S as the cycle frequency changes
from 20 to 2 kHz, with V=200 V, R=150 .OMEGA. and C=50 nF. For
these short pulses, the power dissipation increases from 4.4 to 5.3
W over the range, a difference of 20%. This error may be accounted
for by approximating a correction factor to a straight line if
required in a practical apparatus, but normally it is acceptable to
neglect it as postulated in the preceding paragraph.
[0160] At high power levels the tissues start to become warm under
the electrodes, which puts a limit on the amount of power that can
be delivered to the patient. The typical limitation on tissue
heating (with 50.times.50 mm electrodes, but extendable using
larger electrodes) found experimentally is in the range 8-12 W.
[0161] Neglecting the droop in current due to series capacitance of
the tissues as discussed above, the heating limit may be
approximated to: V.sub.p= (Z.P/.delta.), where V.sub.p=pulse
voltage (V), Z=patient impedance (.OMEGA.) which is assumed
constant at 150 .OMEGA., P is a power limit derived experimentally
and .delta. is the duty cycle. .delta.=Pw.F.sub.p, where Pw is the
pulse width and F.sub.p=number of pulses per second (counting both
forward and reverse pulses).
[0162] Macdonald and Coates GB2290033 state that heating is a
limitation that means that as frequency is increased the voltage
has to be reduced; Macdonald quotes 150 V as being the limiting
voltage with a mono polar wave at 5 kHz and 25 V at 150 kHz.
Fitting a curve of constant power through these points gives a
power limit which approximates to V.sub.p= (168/.delta.). The power
limit assuming a patient impedance of 150 .OMEGA. as defined above
is 1.125 W. At the frequencies specified by Macdonald and Coates,
the apparatus according to aspects of this invention has limits of
better than 400V and 73V respectively, well over twice those quoted
in GB2290033.
[0163] Waveform Harmonics
[0164] An idealised pulse shape for the therapeutic waveform is
illustrated in FIG. 1. The pulse train may be either a continuous
stream of pulses with a small mark-space ratio, as illustrated in
FIG. 1, or in bursts of multiple pulses followed by a gap during
which there is no activity, as illustrated in FIG. 2.
[0165] The frequency content of a waveform is commonly expressed in
terms of its components by means of Fourier analysis.
[0166] FIG. 8 shows the components of two simplified waveforms,
similar to FIGS. 1 and 2 but with zero inter pulse space. The
single cycle curve represents one cycle of a 500 kHz square wave
repeated at 20 kHz. The burst curve is a 500 kHz square wave in
bursts of ten cycles repeated at 2 kHz. Consequently the energy of
both waveforms is identical. The curves are representative of a
biphasic pulse of 1 .mu.S pulse width with zero inter pulse
space.
[0167] It can be seen from the figure that the single-cycle wave
has a series of harmonics at frequencies spaced by 20 kHz spread
widely over the spectrum, while the burst wave has harmonics spaced
at 2 kHz (i.e. the burst repetition frequency) and has most energy
concentrated around a narrow peak at 500 kHz, with little energy
outside the range 400 kHz to 600 kHz. This peak becomes more
pronounced as the number of cycles in the burst is increased. The
single-cycle form may be preferred in certain aspects and
embodiments of this invention because it maximizes the possibility
of cellular coupling by distributing the energy in the waveform
across the spectrum.
[0168] FIG. 9 shows the components of a square wave of 2 .mu.S
pulse width with equal positive and negative pulses and zero inter
pulse space, compared with a square wave of the same pulse width
with 2 .mu.S inter pulse space, both at a cycle frequency of 20
kHz. The graph shows the familiar spacing between components of 20
kHz, but the waveform with 2 .mu.S inter pulse space has its peak
at approximately half the frequency of the waveform with zero
space. This can be explained by the fact that the second square
wave is representative of the peaks of a sine wave of similar
period.
[0169] FIG. 10 shows a square wave of 2 .mu.S pulse width with
cycle frequency of 5 kHz and inter pulse spacing of 100 .mu.S; in
this case the return pulses are equally spaced between the forward
pulses. This provides an interesting result with two distinct
curves made up of harmonic components of the signal. One could be
said to represent the harmonic components of the cycle frequency
and the other the components of the pulses themselves.
[0170] Threshold of Sensation
[0171] According to the mechanisms of action postulated, there are
two independent modes of action, one related to the generation of
action potentials to provide a therapeutic effect as in a TENS
machine, and one independent of this based on oscillating electric
fields. A gentle tingling sensation is often reassuring for the
patient and as previously mentioned this may also play an important
role in the expression of neurotransmitters.
[0172] Table 1 shows threshold of sensation as a function of pulse
width and pulse amplitude using HPSP stimulation. Electrodes were
placed on the forearm over the median nerve, spaced 170 mm apart,
with the leading pulse nearest to the elbow. The waveform used was
a symmetrical biphasic waveform with a fixed 1 .mu.S inter-pulse
space as illustrated in FIG. 5. The pulse width, t, was varied as
specified in Table 1. The pulse amplitude is the minimum voltage,
V.sub.s, zero to peak recorded across the electrodes that produces
sensation. In the biphasic waveform used the peak to peak voltage
value is twice that expressed in Table 1. TABLE-US-00001 TABLE 1
Voltages required to produce sensation at various pulse widths and
frequencies employing a symmetrical biphasic waveform as shown in
FIG. 5 with a fixed 1 .mu.S inter-pulse space. Voltage (zero to
peak) at threshold of sensation at various cycle repetition Pulse
Width, frequencies .mu.S 5000 Hz 2500 Hz 1000 Hz 100 Hz 1.5 189 185
171 188 2 146 136 127 132 3 84 80 79 78 4 63 60 59 57 5 48 49 50 48
6 39 40 38 38 8 28 29 29 29 10 23 25 23 25 20 12 12 12 13
[0173] The results in Table 1 are illustrated graphically in FIG.
11. This curve is a straight line when plotted on a log-log scale
as shown in FIG. 12. By inspection of this curve, the relationship
between the minimum voltage, V.sub.s, that produces sensation and
pulse width, p, for a fixed 1 .mu.S inter pulse space, may be
approximated by the following relationship: V.sub.s, =k p.sup.m,
where k and m are constants and for this case k=270 and
m=-1.0265.
[0174] Although derived quite differently, the curve in FIG. 11
looks remarkably similar in shape to the `strength-duration curves`
observed by Li et al 1976. Here they studied the amplitude
required, for any given duration of a single pulse applied to a
dissected nerve, to produce an action potential recorded from that
nerve. Their observations and the generally accepted view today, is
that the strength duration curve indicates that the stimulus
current and duration can be mutually traded off over a certain
range.
[0175] However, this view is not consistent with our results. Table
1 and FIGS. 7 and 8 reveal the effects of the minimum amplitudes
required to produce sensation from 50.times.50 mm surface
electrodes placed on the forearm at a variety of pulse durations
from 0.5 to 20 .mu.S: at each given pulse duration, the effects of
four different cycle frequencies (ranging from 100 Hz to 5 kHz)
were compared when we employed the type of short high power pulses
that we can now generate. It can be seen that sensation occurs at
stimulation rates well above the accepted physiological limits, and
also that at each pulse duration the threshold of sensation does
not vary greatly with cycle frequency, despite the fact that the
amount of current at any given amplitude that flows over a period
of time is directly proportional to cycle frequency. The mean
modulus current (proportional to rate of charge transferred)
flowing in the patient at 5 kHz is 50 times that at 100 Hz, but
from this observation at any given pulse duration the threshold of
sensation is almost the same. For example, with a pulse of 2 .mu.S
duration, the experimentally measured mean modulus current at 100
Hz was 0.27 mA (approx 15 mA RMS), whereas at 5,000 Hz it was 13.5
mA (approx 95 mA RMS). With a typical TENS pulse of 50 .mu.S or
more, the former would represent a painful level of stimulation and
the latter an intolerable one.
[0176] We next investigated the effect of varying inter pulse
spacing. The data reported below was also obtained with electrodes
over the median nerve as described previously. Table 2 shows the
relationship between threshold of sensation and inter pulse spacing
for a 5 kHz cycle frequency, 1.5 .mu.S biphasic pulse.
TABLE-US-00002 TABLE 2 Voltages required to produce sensation at
various pulse inter pulse spacing at 5,000 Hz cycle frequency
employing a symmetrical biphasic waveform of 1.5 .mu.S pulse width.
Inter pulse Onset of space, .mu.S sensation, V 0.5 222 1 202 1.5
162 2 150 3 124 4 114 5 100 6 94 8 90 10 86 20 83 50 79 100 76
[0177] These results are presented graphically in FIG. 13. It can
be seen that there is a rapid reduction in the voltage at which
first sensation is felt as inter pulse spacing is increased from
0.5 to approximately 10 .mu.S, after which sensation is virtually
independent of inter pulse spacing.
[0178] If one considers a 2,500 Hz HPSP waveform of 0 .mu.S inter
pulse space, compared with an identical one of 1 .mu.S inter pulse
space, the threshold of sensation is 200V compared to 140V for 2
.mu.S pulse width, but only 50V compared to 48V for 5 .mu.S pulse
width. With pulses of the order of 2 to 4 .mu.S and voltages of the
order of 200V, it is possible to vary the level of sensation
between an intense tingling and little or no sensation by varying
only the inter pulse spacing between 0 and 10 .mu.S. This provides
a new approach to controlling the perceived level of treatment in
an electrotherapy apparatus, while delivering a constant treatment
current to the patient. It also represents a new way of modulation
of the sensory nerves at rates within the physiological range for
signals that are delivered at high continuous power levels so as to
penetrate deep tissues. This may be also combined with control of
the overall amplitude of the signal.
[0179] The threshold of sensation reported here is the minimum
level of stimulation at which the subject reports the first
sensation. After a few seconds, the nerves habituate and this
sensation disappears. A typical level of stimulation employed would
be somewhat higher even for very mild sensation. These sensation
limits therefore provide useful benchmark at which the minimum
level of treatment may be set, thereby delivering maximum charge to
the patent for a given waveform without discomfort. It should be
noted that the level of sensation varies a little on other parts of
the body, for instance it is slightly higher with electrodes on the
spine over T1 and T12.
[0180] These results demonstrate that it is possible to
independently control the edge rate and amount of charge that can
be delivered safely and comfortably to the patient, and the level
of sensation that the patient feels, by variation of cycle
frequency, inter pulse space and pulse width. The ability to
deliver high average currents at high voltages interpedently of
sensation levels allows those modes of action based on electric
field effects to be maximised and penetration of large volumes of
deep tissues to be achieved.
[0181] Medical Observations
[0182] In certain aspects and embodiments, this invention allows
pulses to be applied that can affect function within deep tissues
in the periphery or centrally without distress, while producing
beneficial changes particularly within inflamed regions.
[0183] When applied to painful musculoskeletal regions, whether
associated with acute or chronic conditions, a temporary reduction
in tenderness and suffering tends to be produced within 60 mins.
There have been no reported side effects and no limitations have
been found for long term stimulation.
[0184] To give an example of the surprising amount of tissue that
can be treated without producing sensation: one 50.times.50 mm
surface electrode can be placed on the sole of one foot and the
other on the other foot, allowing signals to traverse one leg to
the other. In this manner aches and pains in any region (e.g. knee
or hip) in both lower limbs can be relieved simultaneously.
[0185] The parameters used for the study were biphasic waveforms of
0.5 .mu.S pulse width at 20 kHz cycle frequency with a interpulse
space that varied from 0 -4 .mu.S at mean pulse voltages of
typically 220V.
[0186] Table 3 gives details of a pilot study where electrodes were
placed in 17 patients suffering acute and chronic musculoskeletal
pain while being treated with HPSP at 20 kHz for 60 mins. All had
50% or more relief of pain immediately after treatment was
discontinued. TABLE-US-00003 TABLE 3 Sites of electrodes of the
other polarity Dura- (Key: if two or more tion Site of tenderness
electrodes are attached of and associated to this polarity they
pain cause(Key: {hacek over (c)} = Sites of electrode are
differentiated by No Sex Age (mth) associated with) of one polarity
FIGS. 1, 2, 3 etc) 1 F 48 240 Both legs from Mid-line of back at 1:
dorsum of right foot knee to ankle the level of L4 2: dorsum of
left foot following road traffic accident 2 M 43 12 Right neck and
arm Palm of right hand Back of neck, at the pain {hacek over (c)}
ankylosing level of C5 spondylitis 3 F 26 0.10 Region overlying
Dorsum of left foot 1: origin of the long fractured cuboid
extensors of the left foot bone of left foot 2: origin of the long
flexors of the left foot 4 F 53 0.16 Left scapula pain Front of
neck at the 1: back of neck at the associated with level of C5
level of C3 {hacek over (c)} cervical 2: back of neck at the
spondylosis level of C7 5 M 75 3 Right shoulder and Anterior aspect
of the 1: in the shoulder upper arm pain right elbow region over
acromioclavicular joint 2: in the upper chest region overlying
pectoralis major 6 F 48 168 Back pain {hacek over (c)} tender
Mid-line of the back 1: over the left erector spinae and at the
level of T12 anterior superior iliac abdominal oblique spine
muscles on both 2: over the right sides from T12 anterior superior
iliac downwards spine 7 F 42 4 Post-operative On the abdomen just
1: on the abdomen just pain in the left above the tender below the
tender region hypogastric and region to the left side of it
inguinal region {hacek over (c)} 2: on the abdomen just
hysterectomy below the tender region to the right side of it 8 F 74
60 Pain in cervical On the mid-line of the 1: on the palm of the
region and vertigo back of the neck at left hand {hacek over (c)}
cervical the level of C2 2: on the palm of the spondylosis right
hand 9 M 42 48 a tender neck and Right cheek Right side of the neck
jaw region on the overlying the right side {hacek over (c)}
supraclavicular region migrainous neuralgia 10 F 38 192 Back pain
{hacek over (c)} Mid-line of the back 1: on the anterior herniation
of at the level of L5/S1 aspect of the abdomen at L5/S1 the level
of L5/S1 a intervertebral hand's breadth to the disc left of the
mid line 2: on the anterior aspect of the abdomen at the level of
L5/S1 a hand's breadth to the left of the mid line 11 M 52 0.4 Pain
and Mid-line of the back right inguinal region incapacity {hacek
over (c)} a at the level of T12 tender right loin and degenerative
changes and osteophyte formation at the level of L5/S1 12 M 66 1.35
Pain in the right Right buttock 1: on the back overlying
sacro-iliac region overlying gluteal the iliolumbar ligament region
2: on the back just above the iliac crest a hand's breadth to the
right of B.sup.1 13 M 63 2 Pain in the right At the base of the
over the anterior aspect thumb {hacek over (c)} swollen, neck
overlying the of the right thumb tender first right supraclavicular
metacarpophalangeal fossa joint 14 F 36 36 Left shoulder pain On
the left side of 1: on the back overlying the neck over left middle
trapezius transverse process of 2: on the back overlying C3 left
serratus anterior 15 M 60 1 Right buttock and In the mid-line of
the 1: on the back overlying thigh pain {hacek over (c)} back at
the level of 2: sacro-iliac joint tender quadratus T12 overlying
the right loin lumborum muscle 16 M 67 18 Low back pain In the
mid-line of the 1: on the buttock back at the level of overlying
the right L5 gluteal region 2: on the buttock overlying the right
gluteal region 17 M 77 5 Tender ankle {hacek over (c)} Sole of foot
1: above the tender paratendinitis of region to the left Achilles
tendon 2: above tender region to the right
[0187] Detailed Description of an Electrotherapy Apparatus
[0188] With reference to FIG. 14, the apparatus comprises a boost
converter, 03, which produces a variable high voltage typically in
the range of 0-500V from either the batteries, 01, or from an
external mains power supply, 02.
[0189] The level of the output voltage is commanded by a
microprocessor, 04, via a digital to analogue converter or other
means, 05. The output current of the power supply is sensed by a
circuit, 06, and the measured current, I.sub.0, is fed back into
hardware circuitry in the boost converter to provide either a
current limit or current control loop and also fed into the
microprocessor for monitoring purposes. The boost converter is
designed such that the maximum voltage and current that it can
produce continuously is limited to a safe maximum in hardware and
the microprocessor only sets the output within these limits.
[0190] The output stage of the device consists of an H-bridge
arrangement, 09, that allows the forward and reverse pulses of the
output waveform to be synthesized by switching sequences generated
by the logic circuit, 12, under control of the microprocessor. The
logic circuit includes a circuit which limits the pulse width
independently of the microprocessor; this is important for limiting
the charge delivered to the patient under fault conditions as
discussed later.
[0191] Level translation circuitry, 13, is also provided. This is
the electronics that translates the logic level signals from the
logic circuit, 12, to signals referenced to the high voltage DC
supply to provide switching signals for the output transistors.
This level translation circuit is designed such that the output
transistors cannot remain in the on state for longer than a fixed
period (typically 200 .mu.S). This provides two levels of
protection. Firstly it limits the maximum period that a pulse can
be applied to the patient in normal operation which acts as a
back-up to the pulse width limiting function implemented in the
logic block and described in the previous paragraph. Secondly, it
provides a further level of protection against microprocessor
failure, since the microprocessor may be expected to fail with its
outputs in a frozen state (additional protection against
microprocessor failure is provided by a watchdog circuit that is
periodically reset by the system, not shown on the figure).
[0192] The two arms of the H-bridge, 09, are fed by two bus
capacitors, 08, through two diodes, 07. Each bus capacitor is sized
identically and provides the energy for the forward and reverse
pulses respectively. The typical shape of a single forward or
reverse pulse is illustrated in FIG. 10 with the slope of the top
of the pulse exaggerated for clarity. The waveform exhibits high
rates of a change on its forward and trailing edges, and is
substantially of square wave form except for the droop in the bus,
02, which is the result of partial discharge of the bus capacitor
supplying the energy for the pulse. It should be noted that the
trailing edge, 03, has a rapid descent to zero volts. This is
achieved by turning on both bottom devices in the H-bridge
arrangement during the off period.
[0193] As previously discussed, limitation of charge delivered to
the patient is a key consideration in the safety of the apparatus,
it should not exceed the limit of 75 .mu.C which is the value at
which charge may be hazardous through the chest (AAMI NS4 ) and it
should also not exceed 300 mJ per pulse (IEC 60601-2-10). The bus
capacitors are designed such that the total charge delivered to the
patient can never reach dangerous levels even in the event of
multiple component failure causing the entire stored charge to be
delivered. The charge delivered to the patient is calculated by
adding the charge transferred in positive and negative cycles and
the continuous output of the boost converter during the pulse time.
For this reason, the boost converter cannot be sized to maintain
the voltage on load during the pulse output. The arrangement of
separate forward and reverse bus capacitors permits the forward and
reverse pulses to deliver essentially identical charge to the
patient despite the droop in the bus, thereby ensuring that there
is no net DC current which prevents adverse reactions caused by
ionic transport to one or more electrodes. With HPSP waveforms,
pulse currents of 1-4 A maximum are preferred (higher pulse
currents may be possible).
[0194] Here, the output stage uses two ceramic type capacitors,
each providing the forward and reverse pulses so that a balanced,
approximately square wave biphasic pulse of the type shown in FIG.
5 is produced.
[0195] Referring back to FIG. 14, as secondary protection an
independent safety circuit is provided. This consists of an output
current sensor, 11, and Silicon Controlled Rectifier (SCR), 10, and
other sensing and reference circuitry not shown in the figure that
independently measures the bus voltage and the current applied to
the patient. The circuit provides two main functions:
a) it discharges both buses in the event that either the bus
voltages V.sub.0 and/or V.sub.1 or the output current I.sub.1
exceeds limits set by the standards.
b) it provides a second means of measuring the currents and
voltages generated by the primary circuit which are reported to the
microprocessor.
[0196] The SCR can also be operated externally by the
microprocessor, thereby providing a means of discharging the DC
buses in the event of shutdown or an error identified by the
microprocessor. The output current sensor is shown in the output
circuit in the figure.
[0197] The independent circuit allows the microprocessor to
determine if there is a failure in the voltage control part of the
boost converter, by comparing the voltage set point with the
voltage reported by the independent circuit. In addition the system
voltage reference is continually checked by the microprocessor
against a further secondary reference. Further the microprocessor
makes another safety check by comparing the average output current
of the boost converter with the average patient current.
[0198] Consequently, the apparatus comprises three sub-systems:
[0199] a) The power supply and output stage which is the means of
generating and controlling the output waveform and also has limits
for output parameters such as voltage and current implemented in
hardware and a means of reporting the values of key parameters.
b) The independent safety circuit which provides a secondary means
of limiting the output parameters to safe values, and reporting
measured values.
[0200] c) A means of controlling the output level by reducing the
output of the first circuit from its maximum safe value and a means
of comparing the voltages and currents sensed by the two
independent circuits, thereby identifying if there is an error in
either circuit and causing shut-down of the device.
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