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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to implantable cardioverter defibrillator (ICD)
systems, and particularly to the electrodes and pulse generators thereof.
The invention provides optimal materials for constructing pulse generator
housings for use as an electrode.
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 tile
fact that they all require large currents to accomplish defibrillation.
And, because the heart muscle typically presents an electrical impedance
in tile 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 tile 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, implanted in the body, have
been attempted. For example, tile epicardial-patch electrodes comprise
conductive and relatively large- surface area elements stitched directly
onto the exterior of the heart. While this approach is satisfactory from
an electrical standpoint, the attachment of the electrodes requires a
major surgical procedure.
Another approach, 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 tile 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). 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 tile 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.
A final option is to utilize the pulse generator itself as an electrode.
Because of the relatively high voltage and substantial currents involved
in treatment, the size and weight of an implanted pulse generator (PG) is
an important factor in defibrillation. The package or outer shell of the
PG 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 approximately one half pound. The
patient abdominal cavity is normally the chosen implantation site for
space and comfort reasons. However, implantation of tile PG nearer the
heart, for example in the pectoral region, provides the advantage of a
more efficient system which in mm allows the size of the PG to be reduced.
PG implantation near the heart also permits use of the metallic PG housing
as an electrode, also called a "Can". This is, in a sense, a "free"
electrode in that the housing is required in any case. Implanting the PG
pectorally 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 different spatial axes,
demonstrated in the prior art to reduce the amount of energy needed for
defibrillation (i.e. lower defibrillation threshold). Energy consumption
is a vital concern since it is directly related to size and therefore also
implantability.
Known "active can" electrode designs have been found to be less than
optimal due to oxidation of the can material. Insofar as is known, no
device has been made or proposed which solves this problem as applicant
has.
SUMMARY OF THE INVENTION
The invention provides system and electrode design that is more reliable
and more efficient than those of the prior art. The present invention
involves optimizing the material used in tile pulse generator housing to
improve its function as an electrode in the defibrillator system.
BRIEF DESCRIPTION OF THE DRAWINGS
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 tile 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;
FIG. 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 PREFERRED EMBODIMENTS
Recent developments in defibrillator, electrode and lead designs have
demonstrated that lower defibrillation thresholds are achievable with
transvenous leads where the ICD can is implanted in the left pectoral
region and used as an electrode. This position, along with an electrode in
the right ventricle, yields a current vector that transverses the critical
areas of the heart.
The ICD can makes a suitable electrode because of its large surface area.
However, known designs still do not provide optimal electrode function.
Known cans are constructed of titanium or stainless steel. The
unidirectional current flow from these cans causes the titanium to
oxidize, thereby increasing impedance and allowing plating of metal frown
one electrode to the other. This permits an unacceptably large potential
for change of original electrode characteristics.
Future devices will undoubtedly be smaller and deliver less energy. It is
therefore desirable to optimize the performance of the can as an
electrode. The can material cannot easily be changed to a different
material with better electrode properties because the characteristics that
make the can suitable as an electrode may make it unsuitable for use as a
structural element, for example its function as a hermetic sealing element
and as an EMI barrier.
The present invention involves coating the entire ICD can, or in the
alternative, predetermined portions of the can, with a coating to reduce
the effects of oxidation of the can material over a time period with
multiple shocks. Oxidation of the can may cause changes in the impedance,
polarization and appearance of the can. The coating comprises a noble
metal based substance. The coating preferably comprises platinum Platinum
coating has been demonstrated to give superior electrode performance
compared to prior an titanium and stainless steel. The coating may be
accomplished by plating, vapor deposition, cladding, or welding. The can
may be completely coated. Alternatively, selected areas may be coated,
either in a pattern, such as a grid, or continuously. Selective coating is
desirable in a can design where two can parts are welded together so that
the coating doesn't interfere with the welding process. Such interference
may cause poor welds or create new alloys at the weld seam that may
promote corrosion.
A primary 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. Both
cases permit 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 tile 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 parallelepiped. 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 and 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 tile 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 tile 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 tile 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, tile 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 tile 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 infinite possible
waveform variations. Let "comparable" be taken to mean "within a factor of
four".
Consider first the monophasic pulse. Taking the three electrodes in
sequence P-VA, 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 the 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 tile pattern identification numbers.
Thus, "12" would mean that tile first pulse is of pattern 1, and the
second, pattern 2. It has been found that two same-pattern (and otherwise
similar) pulses in a sequence are not beneficial. In tile sequential-pulse
representation of FIG. 13, different polarity patterns are assumed for the
two pulses. Therefore, tile sequence options 11, 22, 33 and 44 are dropped
from consideration. Next, a sequence involving simple polarity inversion
on all electrodes in going form 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 i 5, it follows
that FIG. 16 give the useful pattern combinations for the case of four
electrodes, as well as for the case of three electrodes.
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.
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 a 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 10 1 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 pulse 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-megaohm 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. This 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 present and the safety feature
imposes no limitations on the electrode selection, the choice of pulse
polarity, or other options such as the pulse sequence or waveform.
Further, it is evident that the PG module 110 and its circuitry of FIG. 10
can be combined with the PG module 90 and its circuitry of FIG. 9 by
combining the switches 138 and 94 into one switch operable by either of
two means.
FIG. 11 illustrates a defibrillation waveform 150 known in the prior art as
monophasic that hi tile present invention is applied to a novel set of
electrodes in novel patterns.
FIG. 12 illustrates a defibrillation waveform 160 known in the prior art as
biphasic that in the present invention is applied to a novel set of
electrodes in novel patterns.
FIG. 13 illustrates a defibrillation waveform 170 comprising a pair of
sequential pulses that in the present invention is applied to a novel set
of electrodes in novel patterns.
FIG. 14 illustrates a chart set 180 of useful polarity patterns for
defibrillation using three electrodes: right-ventricular apex (RVA);
superior vena cava (SVC); and PG housing (CAN). The set 180 omits patterns
that have been found ineffective. 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 that the corresponding electrode is otherwise removed form the system.
The set 180 is applicable to a monophasic waveform, and to the initial
pulse of a biphasic waveform.
FIG. 15 illustrates a chart set 190 of additional polarity patterns for
defibrillation using the RVA, SVC and CAN electrode patterns that are for
use in one of the pulses in a two-pulse sequential waveform.
FIG. 16 illustrates a chart set 200 of twenty-four pattern combinations for
use in sequential-pulse defibrillation. Each digit in the chart refers to
the corresponding polarity pattern defined in FIGS. 14 and 15, and each
pair of digits represents a sequential-pulse option for two pulses in the
case of three electrodes as in FIGS. 14 and 15, and for the case of four
electrodes as in FIGS. 17 and 18 which follow.
FIG. 17 illustrates a chart set 210 of useful polarity patterns for
defibrillation using the RVA, SVC, CAN and SUB (subcutaneous-patch)
electrodes. The set 210 omits patterns that are know to be ineffective,
and is applicable to a monophasic waveform, and to the initial pulse of a
biphasic waveform.
FIG. 18 illustrates a chart set 220 of additional polarity patterns for
defibrillation using the RVA, SVC, CAN and SUB electrodes, patterns that
are for use in one of the pulses in the two-pulse sequential waveform.
As many changes are possible to the embodiments of this invention utilizing
the teachings thereof, the descriptions above, and the accompanying
drawings should be interpreted in the illustrative and not the limited
sense.
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