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CROSS-REFERENCE TO RELATED APPLICATIONS
Reference is hereby made to commonly assigned, co-pending U.S. patent
application Ser. No. 07/834,446, filed Feb. 12, 1992, in the name of Gust
Bardy, M.D.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to medical electrical leads generally, and
more particularly to the emulation of defibrillation electrodes in the
surgical work up of a patient for the implantation of a pulse generator
having an active case electrode.
DESCRIPTION OF THE BACKGROUND ART
By way of definition, in the field of automatic implantable arrhythmia
control devices, the term "cardioversion" or "cardioverter" refers to the
process of and device for discharging relatively high energy electrical
pulses into or across cardiac tissue to arrest a life threatening
tachyarrhythmia. Cardioversion pulses may or may not be synchronized with
a cardiac depolarization or rhythm and may be applied to arrest a
ventricular tachycardia with a lower range energy pulse of around 1-15
joules or ventricular fibrillation with a medium to high energy pulse of
7-40 joules, nominally. The arrest of ventricular fibrillation by such
pulses is referred to as "defibrillation", a form of cardioversion, and
"defibrillators" have been characterized as a form of cardioverter. In the
following description and claims, it is to be assumed that these terms are
interchangeable, and that use of one term is inclusive of the other device
or operation, unless specific distinctions are drawn between them in the
context of the use.
Efforts to enhance efficacy and decrease cardioversion/defibrillation
efficiency have led to the suggestion of lead systems employing
endocardial electrodes and a subcutaneous electrode taking the form of
some or all of the housing of the implantable cardioverter. U.S. Pat. No.
4,922,927, issued to Fine et al., proposes the use of an electrode system
using a right ventricular lead and a subcutaneous electrode, which may
correspond to prior art subcutaneous electrodes or may be the metal
enclosure of the defibrillator. The right ventricular lead carries an
elongated coil electrode. The Bardy '446 application first referenced
above also discloses an implantable defibrillator having a conductive
housing used as an electrode. In U.S. Pat. No. 5,133,353, issued to
Hauser, a "mesh" electrode formed on a portion of the pulse generator case
or housing is proposed to be employed as one electrode in a two or three
electrode system.
As described briefly in the Bardy '446 application, acute human clinical
testing has been conducted using a right ventricular defibrillation lead,
carrying a single, platinum coil defibrillation electrode, and one half of
a titanium housing of a Medtronic Model 7217 implantable defibrillator to
gather threshold data on the potential efficacy of an active can electrode
system. The "can" half has a total exposed surface area of approximately
100 square centimeters with the planar, major surface having an area of
approximately 70 square centimeters. The can half was located in the left
infraclavicular pectoral region and was employed as the cathode electrode
during the initial phase of a biphasic defibrillation pulse. In such a
configuration, it was determined that effective defibrillation at pulse
energies of approximately eight joules could be achieved.
As a result of these improvements and findings in relation to the use of
the pulse generator case as a subcutaneous electrode in certain
implantation locations and with certain wave forms, implantable systems
capable of being implanted and used in this fashion are being developed. A
need exists for a simplified and inexpensive system for conducting the
clinical tests for screening patients for the implantation of these
systems when they become available for general clinical use.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a simple and
largely re-usable active can electrode emulator and method of use thereof
for screening patients for suitability of permanent implantation with an
electrode system employing an active can electrode.
It is a further object of the invention to form such an active can emulator
having a major re-usable component and a minor disposable component.
It is a still further object of the present invention to provide a
simplified method for conducting cardioversion efficacy patient testing
with an emulation of an implantable defibrillator without surgically
implanting all of the components of the actual implantable system.
The present invention is thus directed toward the method of testing a
patient for suitability of implantation of a defibrillator pulse generator
having an active housing surface electrode on its housing for use with at
least a first lead bearing a remote electrode, wherein the testing is
conducted by: implanting the first lead with the remote electrode in an
intended position for permanent implantation; implanting a reusable, dummy
housing shaped electrode, with a detachable and disposable connector and
second lead attached thereto, the dummy housing shaped electrode having
dimensional characteristics corresponding to those of the defibrillator
pulse generator housing, in an expected position for permanent
implantation of the pulse generator housing; coupling the first and second
lead to an external cardioverter; inducing a tachyarrythmia in the
patient's heart; operating the external cardioverter to deliver one or
more cardioverting pulses to the patient's heart through the dummy housing
electrode and the remote electrode to determine the pulse characteristics
sufficient to effect cardioversion; removing the dummy housing electrode
and second lead; detaching and disposing of the second lead and connector;
and retaining the dummy housing for sterilization and reuse.
The method preferably is repeated with respect to the steps of: implanting
the dummy housing and lead at further intended implantation locations of
the pulse generator housing; inducing a tachyarrhythmia of the patient's
heart at each such location; and delivering cardioversion pulses and
determining the threshold energy necessary for successful cardioversion at
each such position; and wherein the steps of removing, detaching and
retaining are conducted after a suitable position is found for permanent
implantation of the implantable defibrillator pulse generator.
In a second aspect of the invention, an active can emulator for emulating
the electrical and dimensional characteristics of an implantable pulse
generator of the type having a hermetically sealed housing configured to
operate as an active electrode and a connector block for receiving and
connecting with a lead comprises: a reusable, sterilizable, conductive can
conforming to the dimensional characteristics of the hermetically sealed
housing and having a first electrical and mechanical attachment mechanism;
and a disposable lead having a second electrical and mechanical attachment
mechanism for mating attachment with said first electrical and mechanical
attachment mechanism and means configured to conform to the dimensional
characteristics of said connector block upon attachment of said first and
second electrical and mechanical attachment mechanisms.
In respect to this aspect of the invention, an active can emulator for
testing the locations of implantation of an electrical pulse generator
enclosed in a housing having predetermined electrode dimensions and
connector dimensions comprises: an emulator housing having an electrode
shaped to conform to the electrode dimensions of said housing of the
implantable pulse generator; a first connector element attached to said
emulator housing for electrical connection therewith; and a detachable
lead having a second connector element adapted to make electrical and
mechanical contact with said first connector element, said second
connector element of said detachable lead shaped to conform to said
connector dimensions of said connector of said pulse generator housing.
The active can emulator including the reusable can and disposable lead is
preferably fabricated with dimensions, weight, balance and "feel" with the
lead attached, and with the same active surface electrode size and shape,
that corresponds to the actual defibrillator pulse generator intended to
be implanted in the patient.
In a preferred embodiment of the invention, the active can emulator is
employed in testing of patients which are receiving an endocardial lead
bearing an active electrode located in the right ventricle and a second
active electrode formed on the housing or can of a defibrillator pulse
generator that is adapted to be coupled to the endocardial lead through a
connector block. The defibrillator pulse generator is subcutaneously
located in the left, pectoral region of the chest, rather than at the
level of the ventricles. The pulse generator preferably delivers a
monophasic or a symmetrical or asymmetrical, biphasic capacitive discharge
pulse between the two electrodes, e.g., of the type illustrated in U.S.
Pat. No. 4,953,551, issued to Mehra et al., wherein the initial phase of
the pulse is delivered using the subcutaneous electrode as the cathode
(coupled to the negatively charged terminal of the output capacitor during
the initial phase).
The active can emulator of the present invention has the important
advantages of simplicity of construction and of use in the
pre-implantation screening of patient's ability to benefit from the above
described cardioversion system. The active can emulator is advantageously
fabricated so that the can is reusable through standard autoclave or
ethylene oxide sterilization, and only the more porous lead and connector
need be disposed of.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages, objects and features of the invention will be
further understood when reference is made to the following description,
taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates the implantable defibrillator and lead system in which
the invention may be employed prior to implantation of the system;
FIG. 2 is a perspective view of the dummy can employed as the reusable
portion of the active can emulator according to an embodiment of the
present invention;
FIG. 3 is an exploded perspective view of the components of the dummy can
of FIG. 2;
FIG. 4 is a perspective view of the disposable lead and connector block
adapted to be attached to the dummy can of FIGS. 2 and 3;
FIG. 5 is a bottom view of the disposable lead and connector block adapted
to be attached to the dummy can of FIGS. 2 and 3; and
FIG. 6 is a side sectional view of the disposable lead and connector block
adapted to be attached to the dummy can of FIGS. 2 and 3.
The drawings are not necessarily to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a combined defibrillator pulse generator housing and
active electrode 10, which would be implanted, according to the present
invention, in conjunction with a transvenous lead 11 extending into the
right ventricle of a heart 17. Lead 11 is exemplary of the type of lead
that may be employed in conjunction with the present invention, but any of
the various transvenous defibrillation leads presently on sale or in
clinical evaluation may be employed as well. The lead 11 includes an
elongated insulated lead body 16, carrying three concentric coiled
conductors, separated from one another by tubular insulative sheaths.
Located adjacent the distal end of the lead 11 are a ring electrode 24, an
extendable helix electrode 26, mounted for retractable movement within an
insulative electrode head 28, and an elongated coil electrode 20. The
defibrillation electrode 20 may be fabricated from platinum, platinum
alloy or other materials known to be usable in implantable defibrillation
electrodes. Electrodes 24 and 26 are employed for cardiac pacing and for
sensing ventricular depolarizations.
Each of the electrodes 20, 24 and 26 is coupled to one of the coiled wire
conductors within the lead body 16. At the proximal end of the lead is a
bifurcated connector 14 which carries three electrical connectors (not
shown), each coupled to one of the coiled wire conductors. The two
connectors coupled by coiled wire conductors to the pace/sense electrode
pair 24, 26 are arranged on a common connecter pin, and the other
connector coupled by a further coiled wire conductor to the defibriilation
electrode 20 forms the second connector pin of the bifurcated connector
14.
The implantable defibrillator housing 10 comprises an active electrode
formed by attaching it to a defibrillation circuit output terminal. The
housing or can 10 may have engraved information on one major exposed
surface that is intended to be implanted facing outward. The defibrillator
housing 10 is formed with a connector block 12, into which the bifurcated
connector 14 is inserted and mechanically attached. The housing 10 and
connector block 12 is typically referred to as a defibrillator pulse
generator and is referenced collectively as 13.
In the preferred embodiment, the pulse generator 13 is a multi-programmable
device that provides bradycardia and anti-tachycardia pacing,
cardioversion and defibrillation, depending on the nature of the detected
heart rhythm and programmed-in operating modes. A specific example of a
pacing/cardioversion/defibrillation pulse generator 13, which may be used
in conjunction with the illustrated lead system to deliver biphasic
pulses, is disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al.,
incorporated herein by reference in its entirety. The size and shape of
the pulse generator 13 may differ from that illustrated in FIG. 1.
The connector block 12 of the pulse generator 13 is molded of a
biocompatible, electrically insulating epoxy compound that has been
employed for many years in the fabrication of such devices to support and
insulate the connector components from one another and from body fluids.
Such epoxy compounds are, however, difficult to sterilize once they are
exposed to blood and body fluids. Consequently, re-use of a functioning
pulse generator 13 on another patient is not advised even if the pulse
generator is exposed only briefly in the attempt to implant it in a
patient. Inasmuch as implantable defibrillators of the type specified are
rather expensive, it is undesirable to expose them to the patient during
the pre-implantation work-up that includes testing the defibrillation
thresholds of the electrode and pulse generator system at one or more
implantation systems.
Similarly, implantable leads are typically constructed to high standards
and with relatively expensive components. The insulation employed in such
leads is also contaminated by blood and body fluids, and such leads are
not recommended for sterilization and reuse. Temporary leads used in acute
patient care or testing are constructed of less expensive materials and
are typically disposed of after use on a single patient.
Pre-implantation threshold testing is conducted to obtain a reasonable
assurance that the pulse energy available exceeds the threshold necessary
to cardiovert the patient's heart (in which fibrillation or a high rate
tachycardia is induced) by an acceptable safety margin. The tests may be
conducted using an external device having all the capabilities of the
implanted defibrillator that is connected to the electrodes expected to be
used in the implanted system. All of the functions of the implanted system
may thus be emulated without exposing the implanted pulse generator.
In addition, use of the implanted device in the testing procedure is
awkward and may pose dangers to operating room staff as fibrillation
episodes are induced and responded to by the device. It may be difficult
to determine just when a shock will be delivered by the device in response
to an induced tachyarrhythmia, risking a shock being delivered while the
device is being handled.
As described above, can halves have been used for experimental testing of
the concept of using an active can electrode as one of the electrodes of
the electrode system. However, the can halves have been awkward to use and
to attach leads to. Consequently, the use of such can halves for clinical
use in conducting the pre-implant threshold tests is not considered
acceptable. "Dummy" cans mimicking the size and shape of the implanted
pulse generator 13 having suitable connector blocks attached thereto for
connection by a temporary lead to the external cardioverter/defibrillator
would not be reusable in conducting such tests in other patients.
Moreover, they would add further acquisition and disposal costs to the
procedure for each patient.
Turning to FIGS. 2 and 3, one embodiment of the reusable can component 60
of the active can emulator 50 is depicted in greater detail in perspective
and exploded views. In FIGS. 4-6, one embodiment of the disposable
component 90 of the active can emulator 50 is depicted in several views.
The active can emulator 50 is completed by the attachment together of the
components 60 and 90 in a manner to be described. When so attached, the
combination of the can component 60 and the distal end 92, specifically
the connector block emulator header 94, is of a size, weight and shape
that it emulates the pulse generator housing 10 and connector block 12 of
the implantable defibrillator depicted in FIG. 1.
In particular, the un-insulated and electrically active surface electrode
62 is shaped to conform to that of the pulse generator housing 10, so that
a cardioversion pulse may be applied through the surface electrode 62 in
the same fashion that it would if the implantable defibrillator were
itself implanted in the same location. The entire exterior metallic
surface of the can component 60 is expected to be exposed, so that it
matches the fully exposed electrode configuration of the actual
defibrillator pulse generator housing 10.
Turning to the can component 60 depicted in FIGS. 2 and 3, it is formed by
enclosing parts described below, welding two mirror image can halves 64
and 66 together at welding seam 68, testing the integrity of the weld
seam, and polishing and engraving the outer surface to a finish mimicking
the emulated pulse generator housing leaving the active electrode 62
exposed. Each can half has a male connector element 70 or 72 protruding
from a respective end 74 or 76. The male connector elements 70 and 72 are
electrically and mechanically attached to the can halves 64 and 66,
respectively, by welding or the like, so that a secure and redundant
electrical and mechanical attachment may be made to the mating connector
elements in the connector block emulator portion 94 described hereafter.
The male connector elements 70 and 72 may be simple nine volt battery,
snap connector male connector elements that snap into and out of mating
female connector elements.
In order to emulate the weight and balance of the actual pulse generator
housing, a weight 80 formed of aluminum is placed within the can halves 64
and 66.
The weight 80 may be dimensioned so that it is tightly contacted when the
can halves 64 and 66 are welded together. A pair of double sided tape
strips 82 and 84 can be adhered to the interior surface of can half 66 and
to one surface of the weight 80 to also stabilize the weight 80 prior to
seam welding the can halves 64 and 66. Further strips of tape may be
employed between the interior surface of the can half 64 and the other
surface of the weight 80. Silicone adhesive may also be employed to
reinforce the attachment.
Other weight shapes and attachment mechanisms may be substituted for that
shown and described above. The effect sought is the emulation of the
weight, balance and feel of the actual pulse generator housing without
having the internal weight move around and change the balance. In testing
the fit of the surgical pocket made to accommodate the defibrillator pulse
generator, the surgeon considers whether the implanted device will be
retained in place or have a tendency to migrate and change the electrode
pathways, which could render the system ineffectual.
The can component 60 thus forms one part of the active can emulator 50
which is completed on attachment of the lead component 90 and particularly
the connector block emulating header 94 thereto. The lead component 92 is
constructed of a single wire conductor 96 insulated by a tubular sheath 98
extending between the header 94 at the distal end 92 and to a connector
pin 100 at the proximal connector end 102. An identification tag 104 is
attached to the sheath 98 which bears an identification and instruction
"NOT FOR RE-USE".
The header 94 is formed of a compound of the type used to form the
connector block of the emulated defibrillator pulse generator and is sized
and shaped to mimic it. The distal end of the conductor extends through an
end opening 106 and into the interior space 108 as shown in FIG. 6 and
into abutment with a tab 110 of a conductive plate 112 which fits into the
interior space or recess 108. The conductive plate 112 has a pair of
female nine volt battery snap connector elements attached to it and spaced
to receive the male connector elements 70 and 72. The plate 112 is
positioned within recess 108 by its peripheral shape and a pair of holes
120 and 122 which receive positioning pins 124 and 126 formed as part of
the header 94. Attachment of the distal end of the conductor 96 is
effected by a spot weld or solder joint of the conductor to the tab 110
effected through opening 130. Conductive adhesives may also be used in the
opening 130, and the entire assembly may be secured with adhesives applied
at contacting edges of the components, as long as the adhesive does not
interfere with attachment of the male and female connector elements.
When the components 60 and 90 are attached, the edge 132 of the header 94
bears against the flat end surfaces 74 and 76 of the can halves so that
the active can emulator 50 mimics the weight balance, feel and overall
dimensions of the implantable defibrillator pulse generator.
In such use, the testing of defibrillation thresholds and system
performance is conducted by implanting a first lead having a remote
electrode (that is the transvenous right ventricular electrode in the
preferred two electrode system shown in FIG. 1) in the intended position
for permanent implantation, creating the surgical pocket to receive the
pulse generator and implanting the active can electrode emulator in the
surgical pocket, coupling the first lead and the lead of the active can
emulator to an external cardioverter/defibrillator, inducing a
tachyarrythmia in the patient's heart, and operating the external
cardioverter to deliver one or more cardioverting pulses to the patient's
heart through the active can electrode emulator and the remote electrode
to determine whether or not the patient's heart can be cardioverted at a
satisfactory threshold. The testing may be repeated with variations in the
positioning of the active can emulator to locate a position providing a
satisfactory safety factor. After testing is completed, the lead component
90 may be disposed of, but the can component 60 may be cleaned and
re-sterilized by conventional autoclaving or gas sterilization and reused.
The active can electrode emulator components 60 and 90 are also useful in
preparing the surgical pocket in which the permanent implantable pulse
generator may be positioned. In the course of conducting the threshold
tests, a surgical pocket may be made subcutaneously in a pectoral or
abdominal position relative to the heart and the indwelling electrode
using the component 60 in enlarging the opening and insuring that it is
properly sized. This avoids using the pulse generator itself, which could
be damaged in such handling, and avoids contamination if for any reason it
is later found that the placement achieved is not sufficient.
Although a variety of embodiments of the invention have been depicted and
described, it will be understood that further variations and alternatives
will suggest themselves to those of skill in the art. For example,
although the construction of the active can emulator described above is
preferred, it will be understood that other construction and fabrication
techniques and materials may be employed to emulate the size, shape,
weight and materials employed in the fabrication of the mimicked pulse
generator.
Although it is contemplated that the active can electrode emulator of the
invention will mimic pulse generator cases which will have the capability
of being used as active can electrodes and will have polished exterior
surfaces with engraving on one major surface, the invention may be
employed in emulators mimicking pulse generators having mesh electrodes of
the type referred to above or other electrodes attached thereto. Moreover,
while it is anticipated that the entire case or can surface of the pulse
generator is expected to be exposed to act as an electrode, it will be
understood that the emulators and their methods of use of the present
invention will be applicable to pulse generators having partially
insulated surfaces. In such a situation, a further removable and
replaceable "boot" may be supplied to fit over the insulated areas of the
case or can of the pulse generator being mimicked. The replaceable boot of
silicon rubber or the like may be removed and discarded after use and
prior to cleaning and sterilization of the emulator component 60.
Accordingly, it will be understood that various changes and modifications
may be made without departing from the broader aspects of this invention.
It is therefore the intention in the appended claims to cover all such
changes and modifications as fall within the true spirit and scope of this
invention.
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