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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to implantable defibrillator systems, and
particularly to the electrodes and pulse generators used with such
systems.
2. Description of the Prior Art
The departure of the heart from normal action to uncoordinated and
ineffectual contractions "fibrillation" can lead to death within minutes
unless corrected. One method of treatment to restore the normal heart
action involves passing electrical current through the heart muscle. The
effectiveness of such treatment is dependent on a number of factors,
including the location of the electrodes used to apply the electrical
current, the shape of the electrodes, and the magnitude, timing, and
waveform of the current. While all these factors are significant, a
fundamental problem of all such electrical treatments arises from the fact
that they all require large currents, several amperes to accomplish
defibrillation. And, because the heart muscle typically presents an
electrical impedance in the range of 40 to 100 ohms, signal amplitudes of
several hundred volts are required to obtain the necessary current. The
requirements for relatively high voltage and several-ampere currents
combine to place great importance on efficient, low-resistance electrode
arrangements for delivering the defibrillation signal to the heart.
Ideally the electrode would have no resistance itself and would be placed
directly against the heart muscle to avoid the voltage drop across the
tissue that surrounds the heart.
Various approaches to the optimal electrode have been attempted. For
example, the epicardial-patch electrodes comprise conductive and
relatively large-area elements stitched directly onto the exterior of the
heart itself. While this approach is satisfactory from the electrical
standpoint, the attachment of the electrodes requires a major surgical
procedure, such as opening the chest cavity and exposing the heart, as
depicted schematically in FIG. 1. Aside from the danger that such surgery
presents to all patients, many patients who require this treatment are in
such poor condition that this procedure presents an unacceptable risk. In
situations where such radical surgery is inappropriate, other, less
effective, electrode configurations have been used. For example, the
transvenous technique utilizes a conducting filament threaded through an
opening in a vein, and into the heart interior. When the filament coils up
in a heart chamber, ideally against the chamber wall, a relatively
large-area contact to the cardiac muscle can be made. This approach
requires that two such electrodes be used, one in the right-atrium (RA)
position or in the nearby superior vena cava (SVC) position, and the other
placed at the right-ventricular-apex (RVA) position. Despite the fact that
transvenous electrodes can be inserted with a relatively simple surgical
procedure, they have a serious shortcoming. Because of the design
constraints that permit them to be threaded through the blood vessels,
they cannot be depended upon to make adequate contact with the interior
wall of the heart, and therefore they sometimes do not direct adequate
current through a sufficient portion of the heart-muscle volume to achieve
defibrillation.
Another option is to combine a transvenous electrode with a subcutaneous
patch (SUB) in the fashion described in U.S. Pat. No. 4,817,608 to
Shapland, and in U.S. Pat. No. 4,953,551 to Mehra. This approach implants
a shallow, just-under-the-skin conductive element of appreciable area on
the patient's left side to serve as an electrode, as illustrated in FIG.
2. Since the patch is not directly on the heart, current must pass through
the intervening body tissue and fluid to reach the heart. The resistance
of the intervening tissue and fluid requires the application of a higher
voltage to achieve the desired current through the heart muscle, and the
passage of the current through the intervening material may lead to
patient discomfort. Additionally, while the surgical procedure for
implanting the subcutaneous patch is relatively minor compared to that
required for implantation of electrodes directly against the heart muscle,
it still presents some risk to the patient. Although the
subcutaneous-patch approach provides the advantage of simpler and less
risky surgery, the proximity of a subcutaneous patch to the body's surface
leaves the electrode relatively unprotected, and as a result, such
electrodes have been subject to flexure and breakage from mishaps, and
even from normal body motions.
Many patients have experienced ventricular fibrillation, or are likely to
experience it. These patients are best treated by a defibrillator that is
implanted in the body. Because of the relatively high voltage and
substantial currents involved in treatment, the size and weight of the
implanted pulse generator (PG) is an important factor. The term PG is used
to identify the single package or module that contains the entire
implanted defibrillator system, excluding only its electrodes and
associated electrical leads. The package is usually a sealed housing made
of titanium, selected for its relatively light weight and corrosion
resistance. The weight of the PG is normally in excess of 200 grams, or
roughly half a pound. While electrical efficiency would be better served
with pectoral implantation, the size and weight of the PG usually
precludes this location for cosmetic and comfort considerations, and the
more spacious abdominal cavity is normally the chosen implantation site.
This, of course, is in spite of the fact that PG implantation nearer the
heart would result in a more compact system, with shorter leads.
Implantation of the PG nearer the heart provides the advantage of a more
efficient system which in turn allows the size of the PG to be reduced. PG
implantation near the heart also permits various new electrode
arrangements, which are the subject of the present invention. In
particular, it permits use of the metallic PG housing as an electrode
(hereinafter abbreviated as "CAN". This is, in a sense, a "free" electrode
since the housing is required in any case. While use of the PG enclosure
as an electrode is suggested in U.S. Pat. No. 4,727,877 to Kallok, the
resulting consequences were not addressed.
It is anticipated here that electrode use of the pectorally implanted PG
housing will be primarily an augmentation of present systems that employ a
catheter for one or more purposes. Implanting the PG involves surgery
little more invasive than that required to implant a subcutaneous patch.
Furthermore, it eliminates the troublesome requirement for tunneling wires
under the skin that accompanies the subcutaneous patch, and the PG is also
not subject to crumbling and breakage.
It is possible to use the PG enclosure as an electrode in combination with
electrodes of the prior art, such as the RVA, SVC and subcutaneous-patch
(SUB) electrodes. This facilitates the use of sequential defibrillation
pulses having differing spatial axes, demonstrated in the prior art to
reduce the amount of energy needed for defibrillation. Energy consumption
is a vital concern since it is directly related to size and therefore also
implantability. This is discussed in more detail by D. L. Jones, et al.,
"Internal Cardiac Defibrillation in Man: Pronounced Improvement with
Sequential Pulse Delivery to Two Different Lead Orientations",
Circulation, Volume 73, pages 484-491, March, 1986, and in their U.S. Pat.
No. 4,548,203. See also, Saksena, U.S. Pat. No. 4,944,300, and Kallok,
U.S. Pat. No. 4,727,877, as well as Tacker, European Patent Application
0,095,726.
SUMMARY OF THE INVENTION
The invention provides system and electrode designs, configurations, and
current-application patterns that are simpler, less troublesome, more
reliable, more efficient, and also are less risky to the patient, and less
costly than those of the prior art.
One objective of the present invention is to accommodate pectoral
implantation of a PG, with the PG housing used as an electrode in
combination with prior art electrode arrangements. One such arrangement is
shown schematically in FIG. 3. This arrangement is more compact than
before, and has substantially shorter leads. Eliminating even a portion of
the parasitic resistance in the leads (by shortening them) is significant
here because of the high peak currents required for effective
defibrillation. A platinum or other polarization-decreasing coating for
the titanium case has been found to be advantageous.
It will be appreciated that the use of the PG case as an electrode is not
possible for abdominal implantations. Use of the PG CAN electrode in an
abdominal implant would cause severe and painful shock to the patient.
Aside from this, such an arrangement would cause an intolerable energy
waste because of the need to push large currents through the diaphragm and
portions of the abdominal organs in order to reach the heart.
Another aspect of the present invention is to use the PG-housing electrode
both in lieu of the subcutaneous-patch electrode and as an augmentation to
it, providing either two, three, or four electrodes. Either case opens
wide, new opportunities for a variety of pulse-sequence and pulse-axis
combinations, with the second term referring to the spatial direction of
the discharge, fixed by polarity and electrode choices.
In pectoral implantation of a PG, the entire PG exterior may be employed as
an electrode. This provides a large electrode area, and hence a small
parasitic contact resistance. While the low contact resistance is a
desirable goal, the system could pose a serious shock hazard to medical
personnel handling it before and during implantation. Also, this
arrangement would not allow steering the current in a desired direction.
The application of an insulating layer to portions of the PG's external
surface largely eliminates the shock hazard and provides the beneficial
result of allowing the current to be steered in a direction most
advantageous for defibrillation. The PG housing desirably approximates a
somewhat flattened rectangular parallelpiped. This geometry allows most of
one major face of the housing to serve as the electrode, with the balance
being insulated, as is illustrated in FIGS. 4, 5 and 6. Because the four
smallest faces, or edges, as well as one major face, of the PG are
insulation-covered, safe handling of the PG is comparatively
straightforward and can be accomplished without risk to the surgeon during
implantation. A further benefit of this arrangement is that the electrical
discharge can be aimed in a chosen direction. For example, aiming the
discharge toward the interior of the body causes primary current
conduction to avoid the skin, which largely avoids the additional
discomfort normally accompanying an electrode not in direct contact with
the heart. On the other hand, aiming the discharge away from the interior
of the body causes the path length, and hence parasitic resistance, to
increase, but causes less skeletal muscle "jerk". By this is meant a
reflexive contraction of skeletal muscles in the path of the electrical
discharge, and stimulated by it, with uncomfortable, and possibly
injurious results.
As a further option, another portion of the PG housing could be covered
with an insulating coating, as shown in FIGS. 7 and 8. The ease and safety
of handling of this configuration approximates that for the preceding
option, but additionally provides a wider range of aiming options due to
the increased number of surfaces which are not insulated.
While the conductive PG housing will be most advantageously used in the
pectoral implant, it can also be used in conventional abdominal
implantation by adding a single-pole, single-throw selector switch to the
system, as shown in FIG. 9. When selector switch 94 is open, as in FIG. 9,
the metal housing of the PG is isolated from all circuitry, and the PG may
be conventionally implanted in the abdominal cavity. But when selector
switch 94 is closed, the PG housing is activated as the CAN electrode. By
the simple act of plugging in the lead from a SUB electrode, and (or) an
RA electrode, the surgeon can realize various electrode-pattern options to
accompany the pectorally implanted CAN electrode.
In the event that further protection against shock is desired, this
invention provides a circuit, shown in FIG. 10, for sensing that the
implantation procedure has not yet been performed and develops a disabling
signal to prevent inadvertent generation of the defibrillation signal.
This feature totally eliminates the shock hazard to medical personnel. It
can be viewed as a safety element that augments the exterior insulation
described above.
It is evident that combining the PG-housing or CAN electrode with the
well-established defibrillation electrodes SVC and RVA, that are often
associated with a cardiac catheter, makes possible a number of polarity
patterns for applying defibrillation pulses. Beyond this, is the choice of
the monophasic pulse pictured in FIG. 11, the biphasic pulse in FIG. 12,
and the sequential pulses in FIG. 13. Let it be said that the two pulses
in the biphasic waveform, as well as in the sequential waveform are of
comparable amplitude and duration, thus avoiding the infinity of possible
waveform variations. Let "comparable" be taken to mean "within a factor of
four".
Consider first the monophasic pulse. Taking the three electrodes in the
sequence RVA, SVC, and CAN, FIG. 14 identifies four polarity patterns that
are useful. The number in the left-hand column identifies the pattern. The
plus and minus symbols indicate relative polarities of the respective
electrodes during discharge, and the zero symbol means that the circuit to
the corresponding electrode is open, or else that the electrode is
otherwise omitted from the systems. It will be seen that options assigning
a zero to the RVA electrode are omitted, because the RVA electrode plays a
dominant role in directing current through the bulk of the
left-ventricular muscle. Furthermore, it has been found that assigning the
same polarity to the RVA and SVC electrodes, that is, making them
electrically common, is an ineffective option. Note that simple polarity
reversal has been treated as a separate pattern. That is, pattern 3 is the
reverse of 1, and 4 is the reverse of 2. Finally, the case with the CAN
electrode open or removed is omitted because it reverts to the prior art.
Next, the four patterns in FIG. 14 may be interpreted as a description of
the first pulse in the biphasic waveform of FIG. 12. Thus, FIG. 14 deals
fully with both the monophasic and biphasic cases. The case of two pulses
in sequence involves additional considerations. First, identify a given
sequential-pulse option by using the pattern identification numbers. Thus,
"12" would mean that the first pulse is of pattern 1, and the second,
pattern 2. It has been found that two same-pattern (and otherwise similar)
pulses inca sequence are not beneficial. In the sequential-pulse
representation of FIG. 13, different polarity patterns are assumed for the
two pulses. Therefore, the sequence options 11, 22, 33 and 44 are dropped
from consideration. Next, a sequence involving simple polarity inversion
on all electrodes in going from the first pulse to the second is also
omitted because this simply constitutes one of the biphasic options. This
removes 13, 31, 24 and 42. Next, consider that a pattern eliminating the
RVA electrode may be useful as one of the two sequential pulses, even
though it is not useful in the monophasic case. There are two such
patterns given in FIG. 15, and numbered 5 and 6. Thus, it is possible to
list exhaustively all useful pattern combinations in the sequential case,
as has been done in FIG. 16.
When a subcutaneous-patch or SUB electrode is present in addition to the
RVA, SVC, and CAN electrodes, the list of patterns must be reconsidered.
Once again, a pattern with RVA and SVC common is rejected for the same
reason as before. Further, a pattern with CAN and SUB having opposite
polarities is rejected because the current from one to the other would be
remote from the heart and wasted. In addition, a pattern with CAN open is
avoided because it constitutes prior art, and a pattern with SUB open is
also avoided because such cases have already been treated in FIGS. 14, 15
and 16. Thus, there are four patterns again this time, as given in FIG.
17. Again, there are two additional patterns that are potentially useful
in the sequential case, as given in FIG. 18. Because the symmetries in
FIGS. 17 and 18 are identical to those in FIGS. 14 and 15, it follows that
FIG. 16 gives the useful pattern combinations for the case of four
electrodes, as well as for the case of three electrodes.
One significant aspect and feature of the present invention is an
implantable pulse generator for defibrillation that is lighter in weight,
as well as being smaller in size, than those of the prior art, and hence
lends itself to pectoral implantation.
Another significant aspect and feature of the present invention is a
compact defibrillation system having leads shorter than those of the prior
art.
A further significant aspect and feature of the present invention is using
the PG's metal housing as an electrode without creating a hazard to
medical personnel during implantation, nor undue discomfort to the patient
during the defibrillation process.
A further significant aspect and feature of the present invention is a PG
metal housing designed to serve as an electrode, but which is partly
covered by an insulating layer that has the combined function of providing
protection from discharges for medical personnel who handle the system
before and during implantation, and of "steering" the electrical current
within the body.
Yet another significant aspect and feature of the present invention is-the
addition of a selector switch to the PG of the invention that will permit
its use in a conventional abdominal implantation with conventional
electrodes, as well as in the pectoral-implantation option wherein the
housing is an electrode.
An even further significant aspect and feature of the present invention is
a safety feature involving a comparator circuit that senses
metal-housing-to-circuit-common resistance, and disables the PG unless
that resistance is low enough to signify system implantation has been
completed, further protecting medical personnel before and during surgical
implantation.
Yet a further aspect and feature of the present invention is the use of the
PG-metallic-housing (CAN) electrode, in lieu of or in combination with a
subcutaneous-patch (SUB) electrode, and in combination with the RVA and
SVC electrodes, to provide a wide range of polarity-pattern and
discharge-axis choices for monophasic and biphasic waveforms, as well as a
larger number of pulse-pair combinations for use of the sequential
technique.
Having thus described the embodiments of the present invention, it is a
principal object hereof to provide a pulse generator for defibrillation
that is small enough in size and weight to be suitable for pectoral
implantation.
Another object of the present invention is to create an implantable
defibrillation system that is compact and comprises leads much shorter
than those of the prior art.
A further object of the present invention is to provide a defibrillation
electrode that is efficient, well-positioned, and "free" in the sense that
it is the CAN or housing of the pulse generator that must in any case be
present.
Yet another object of the present invention is to protect medical personnel
from hazardous shocks during implantation by partly covering the housing
electrode by an insulating layer.
Yet a further object of the present invention is an ability to steer the
defibrillating current within the body by insulating selected portions of
the housing electrode.
Another object of the present invention is versatility in pulse-generator
design, with a selector switch being able to convert it from a module
suitable for pectoral implantation with the housing as an electrode to a
module suitable for conventional abdominal implantations.
A related object of the present invention is to provide a wide range of
polarity pattern and discharge-axis options for monophasic and biphasic
defibrillation waveforms, as well as a larger number of pulse-pair
combinations for use of the sequential technique.
A further object of the present invention is a safety provision for medical
personnel before and during implantation in the form of a comparator
circuit that senses output resistance and disables the pulse generator
unless that resistance is low enough to signify that implantation has been
completed.
Yet another object of the present invention is to provide several
polarity-pattern and discharge-axis options by combining the PG-housing
electrode with more conventional electrodes such as the RVA and SVC coils
on a standard catheter.
Yet a further object of the present invention is to replace the
subcutaneous-patch electrode by a PG-housing electrode that eliminates the
need for tunneling wires under the skin and eliminates the hazards of
breakage and crumbling that accompany the subcutaneous-patch option.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects of the present invention and many of the attendant advantages
of the present invention will be readily appreciated as the same becomes
better understood by reference to the following detailed description when
considered in connection with the accompanying drawings, in which like
reference numerals designate like parts throughout the figures thereof and
wherein:
FIG. 1 illustrates a schematic representation of a defibrillating system of
the prior art implanted in the abdominal cavity, and having
epicardial-patch electrodes attached directly to the heart;
FIG. 2 illustrates a schematic representation of a defibrillating system of
the prior art having one transvenous electrode and one subcutaneous-patch
electrode;
FIG. 3 illustrates a schematic representation of a defibrillating system of
the present invention having a SVC electrode, an RVA electrode and one
subcutaneous-patch electrode;
FIGS. 4, 5 and 6 illustrate schematic representations of a defibrillating
system of the present invention having a PG housing with one major
metallic face exposed to serve as an electrode, and the balance of the PG
surface area covered by an insulating layer;
FIGS. 7 and 8 illustrate schematic representations of a defibrillating
system of the present invention having a PG housing with approximately
half its surface area exposed to serve as an electrode, and the balance of
the PG surface area covered by an insulating layer;
FIG. 9 illustrates a schematic representation of a defibrillating system of
the present invention incorporating a selector switch that permits the PG
to serve either in the PG-housing-as-electrode mode or in other
conventional modes;
FIG. 10 illustrates a schematic representation of a defibrillating system
of the present invention incorporating one possible safety circuit that
disables the pulse generator when the housing-to-circuit-common resistance
is higher than that encountered by the system after implantation, thus
protecting medical personnel who must handle the system before and during
implantation;
FIGS. 11 illustrates a monophasic waveform that in the present invention is
applied to a novel set of electrodes in novel patterns;
FIG. 12 illustrates a biphasic waveform that in the present invention is
applied to a novel set of electrodes in novel patterns;
FIG. 13 illustrates a sequential-pulse waveform that in the present
invention is applied to a novel set of electrodes in novel patterns;
FIG. 14 illustrates a chart of useful polarity patterns for three
electrodes, RVA, SVC, and CAN, describing the cases of monophasic and
first-biphasic-pulse waveforms;
FIG. 15 illustrates a chart of additional polarity patterns for use in
sequential-pulse waveforms in the three-electrode case;
FIG. 16 illustrates a chart of three- and four-electrode pattern
combinations useful in sequential-pulse defibrillation;
FIG. 17 illustrates a chart of useful polarity patterns for four
electrodes, RVA, SVC, CAN, and SUB, for the cases of monophasic and
first-biphasic-pulse waveforms; and,
FIG. 18 illustrates a chart of additional polarity patterns for use in
sequential-pulse waveforms in the four-electrode case.
DESCRIPTION OF THE PRIOR ART
FIG. 1 illustrates a schematic drawing of a patient 10 fitted with a
defibrillating system of the prior art consisting of a PG 12 implanted in
the abdominal cavity and connected to epicardial-patch electrodes 14 and
16 by electrical-lead harness 18.
FIG. 2 illustrates a schematic drawing of a patient 20 fitted with a
defibrillating system of the prior art consisting of a PG 22 implanted in
the abdominal cavity and connected to transvenous RVA electrode 24 and
subcutaneous-patch electrode 26 by means of electrical-lead harness 28
where all numerals correspond to those elements previously described.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 illustrates a schematic drawing of a patient 30 fitted with a
defibrillating system of the present invention comprising a pectorally
implanted PG 32, a subcutaneous-patch electrode 34, and transvenous
catheter 36, carrying an SVC electrode 38, and an RVA electrode 39 where
all numerals correspond to those elements previously described.
FIG. 4 illustrates the top face 40 of the PG 42 having an insulating layer
44 that covers the entire top surface of the PG exterior where all
numerals correspond to those elements previously described.
FIG. 5 illustrates an elevation of a PG 42 having an insulating layer 52
that covers the entire surface of the face 50 depicted, and also covers
the remaining three "edge" faces where all numerals correspond to those
elements previously described.
FIG. 6 illustrates the bottom face 60 of the PG 42 having an insulating
layer 62 that covers only the periphery of the bottom major face 60,
leaving the balance 64 of the bottom face 60 within the periphery of the
insulating layer 62 to serve as an exposed-metal electrode.
FIG. 7 illustrates a side view of a PG 72, including a plurality of faces
70a-70n, having an insulating layer 74 that covers a significant fraction
of the exterior surface of the PG 72, leaving the balance 76 consisting of
faces 70a-70n of the exterior surface of the PG 72 in the form of exposed
metal to serve as an electrode.
FIG. 8 illustrates a top view of the PG 72 and the insulating layer 74 that
covers a significant fraction of the faces 70a-70n, leaving the balance 84
consisting of faces 70a-70n in the form of exposed metal to serve as an
electrode.
FIG. 9 illustrates a PG module 90 and represents schematically certain of
its internal elements that permit flexible application of the system where
all numerals correspond to those elements previously described. The
pulse-generator circuit 92 has a first output lead 93 connected through
the externally controlled SPST selector switch 94 to the PG housing 95 at
the connection point 96. When the switch 94 is open, the PG module 92 can
be abdominally implanted in conventional fashion; when the switch 94 is
closed, the PG housing 95 can be employed as a defibrillation electrode in
the case of pectoral implantation. The first output lead 93 is also
connected to a first self-sealing output jack 98 into which an SVC
electrode lead can be plugged when desired, as well as to a second
self-sealing output jack 100 into which a SUB electrode can be plugged
when desired. A second output lead 101 from the pulse-generator circuit 92
is permanently connected inside a lead 102 that is intravenously installed
to place an electrode in the RVA position. Activation of an SVC electrode
is accomplished by plugging its lead into jack 98, and activation of a SUB
electrode is accomplished by plugging its lead into jack 100. With these
options, in addition to that provided by selector switch 94, it is evident
that the flexibility of the present invention offers the choice of three
single-electrode options, of three common-double-electrode options, and
one common-triple-electrode option, for a total of seven options for an
electrode pattern to deliver a shock directed at the opposing RVA
electrode that is connected to the pulse-generator circuit 92 through the
lead 102.
FIG. 10 illustrates a PG module 110 that incorporates a safety circuit for
disabling the pulse generator until the system has been implanted where
all numerals correspond to those elements previously described. The safety
circuit senses when the system has been implanted by monitoring the
resistance between the implanted RVA electrode 134 and the metal housing
130 of the system. When the resistance drops to a low level, the system
develops a signal that allows defibrillation pulses to be passed to the
CAN or PG-housing electrode.
When the pulse generator 140 is prepared to deliver its pulse or other
waveform it closes SPST switch 112 by conventional circuit means. Closing
SPST switch 112 causes current from low-voltage power supply 114 to flow
through a center-tapped 1-megohm resistor, that is through resistors 116a
and 116b. This creates a reference voltage, having a value one half that
of the output from the low-voltage supply 114, to be developed across
resistor 116a, and causes the centertap 118 to become a reference
terminal.
The reference voltage at the centertap 118 is fed to a first, positive,
input terminal 120 of comparator 122. A "test" voltage, responsive to the
resistance between the CAN electrode metal housing 130 and the RVA
electrode 134 is applied to a second, negative, input terminal 124 of
comparator 122. This voltage is derived from a voltage divider consisting
of a 500-ohm resistor 126 as the "upper" element, and as the "lower"
element, the resistance 128 existing at that time from the metal housing
of the PG or CAN electrode 130 to the common terminal 132 of the high- and
low-voltage circuits, which is also common to the RVA electrode 134. It
will be appreciated that, while FIG. 10 illustrates the resistance between
the CAN electrode 130 and the RVA electrode 134 as a resistor 128 shown in
dotted lines, in actuality, the resistance is not a discrete resistor, but
rather the resistance of the path that exists at the time between these
electrodes. Before the device is implanted, the path will be largely air
and have a very high resistance. However, after implantation, the path
will be through relatively highly conductive body tissue, and therefore,
have a relatively low resistance.
Even when a person is handling the system, and holding the metal housing of
the system in one hand and the RVA electrode in the other, the resistance
between circuit points 130 and 134 (from hand to hand) is typically
several kilohms, so that the test voltage at negative input terminal 124
is much more positive than the reference voltage at positive input
terminal 120, so that the comparator delivers a logical "low" or zero
voltage at output terminal 136. This output signal controls the switch
138, and zero voltage to that switch, which is preferably an FET, meaning
that the switch is inactivated and hence open. With switch 138 open, the
defibrillation pulses from pulse generator 140 are blocked and do not
reach the CAN or housing electrode 130.
When the PG module 110 is properly implanted, the electrical path
represented by the resistor 128 from the housing electrode 130 to the RVA
electrode 134 will lie through body tissue and have a resistance value
well below 500 ohms, causing the reference voltage at positive input
terminal 120 to be more positive than the test voltage at negative input
terminal 124, causing the comparator to switch to the logical "high"
condition at output terminal 136. The high signal at comparator output
terminal 136 causes switch 138 to close, thereby permitting the normal
delivery of the defibrillation pulses from pulse generator 140 to the
metal housing 130.
The safety circuit operates for all CAN electrode 130 configurations
without modification and functions to prevent accidental shock regardless
of the selected pulse polarity. Thus, the medical team is protected in all
situations where the shock hazard is | | |
