Implanted medical system including a self-powered sensing system

Self-contained, self-powered, flexible electrical control signal generating means are located in the right ventricle of a heart and generate electrical control signals solely by mechanical movements caused by actions of and conditions in the heart without any electrical connection to and supply of electrical energy from any other power source which electric control signals are transmitted to a control circuit system of an implanted control unit which control unit transmits an electrical signal to a stimulation electrode implanted in tissue at the apex of the right ventricle of the heart which stimulation electrode uses the electrical signal to stimulate the heart.

FIELD OF THE INVENTION 
This invention relates generally to an implanted medical system including a 
self-powered sensing system for use in obtaining information relative to 
the operation of a portion of a human body and to correlate such 
information with the operation of the medical system implanted in a 
portion of a human body, such as a heart pacemaker, and more particularly 
to a self-powered sensing system which can readily be used in close 
relationship with a stimulation electrode of a heart pacemaker. 
BACKGROUND OF THE INVENTION 
Heart pacemaker systems have been operating on the basis of detecting the 
electrical activity of the heart and adjusting the pulse generator output 
accordingly. More sophisticated pacemaker systems require that more 
information is obtained about the status of the heart, particularly in 
relation to the mechanical/hemodynamic activity, to more accurately 
simulate a natural, non-diseased heart. Sophisticated pacemaker systems 
are required to pace at different rates, determine the minimum amount of 
energy required to stimulate the heart and to provide back-up physiologic 
information to supplement the sensed electrical signal. 
One way to minimize energy consumption of a pacing system is to use pulse 
energies that are as close as possible to actual tissue stimulation 
threshold requirements. To make a pacemaker system that can automatically 
adjust the pulse energy output necessitates the determination of the 
automatic stimulation threshold. This has been difficult to achieve as the 
measurement of the heart evoked potential following a pacemaker stimulus 
cannot be detected reliably due to electrical charge build up around the 
stimulating electrode immediately after pulse delivery. An alternate 
method to detect the heart capture is by a sensor that detects the 
mechanical/hemodynamic changes in the heart associated with a heart 
contraction. 
Artificial Implantable Defibrillators (AID) also require the sensing of the 
hemodynamic state of the heart as electrical sensing alone may not be 
sufficiently reliable to distinguish between fibrillation and tachycardia. 
Transducers for measuring various physical or chemical parameters from the 
heart, or other parts of the body, have long been a goal of pacemaker 
system designers. Up until now, these transducers have been affected by 
poor reliability and a great deal of complexity. Transducers to measure 
pressure within the body are known to the art from U.S. Pat. No. 4,023,562 
Hynecek et al, which is incorporated herein by reference, but are 
typically only used acutely, such as for temporary diagnostic purposes. 
A successful sensor should have the following requirements: 
sufficient stability to give useful measurements over the life-span of the 
heart pacemaker system; 
a size small enough to be compatible with the heart pacemaker system and 
also have minimal power consumption; 
packaging of the sensor in such a way that toxicity or other undesirable 
effects such as thrombosis or mechanical limitation are avoided; 
compatibility with the rest of the hardware system; 
an output signal which can be readily processed by the pacemaker's software 
system (where fitted); and 
an output signal which provides useful physiological data. 
One attempt which has been made at producing a sensor which produces 
hemodynamic data in the form of pressure information and which meets some 
of these requirements is disclosed in U.S. Pat. No. 4,485,813 to Anderson 
et al., the disclosure of which is incorporated herein by reference. In 
this design a piezoelectric ceramic is used to convert pressure and motion 
into electrical signals. This design has the disadvantage that it required 
power from the pulse generator to make it function. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention provides a sensing system which converts a force produced by 
a portion of a human body into an electrical signal which is used to 
provide information relating to the operation of that portion of the human 
body to an implanted medical device. The sensing system of this invention 
requires no outside electrical power and functions solely in response to 
the force applied by the human body to produce the electrical signal. When 
the sensing system has been implanted into the heart, the force can be 
from a heart motion or an altered hemodynamic state. 
In one embodiment of the invention, a quantity of material, which is either 
piezoelectric or piezoresistive, is used to generate a voltage signal that 
is related to heart motion or an altered hemodynamic state. The quantity 
of material comprises a poly(vinylidene fluoride) (PVDF) film which has 
been specially processed to provide it with the desired piezoelectric 
characteristics. This quantity of material is very efficient at detecting 
the dynamic behavior of the heart, such as the stroke volume, pressure, 
contractility, valve closure, force of contraction and other hemodynamic 
indicators of heart activity. Also, this quantity of material does not 
require any power from an implanted medical system or any other device to 
accomplish its function. While the invention is particularly described in 
a functional relationship with a heart pacemaker system, it is understood 
that the invention may be used to generate an electric signal for use with 
any implanted medical system. 
A sensing system used in conjunction with an implantable heart pacemaker 
system for control of the operation of the heart would include a control 
unit mounted in the body in a location remote from the heart. An 
electronic control circuit is associated with the control unit for 
controlling the operation thereof. A battery is included for supplying 
electrical energy to the control unit and to the electronic control 
circuit. A flexible lead mounted in the body extends between and is 
connected to the control unit and a stimulation electrode in the right 
ventricle of the heart for transmitting an electric signal from the 
control unit through the flexible lead to the stimulation electrode to be 
applied to the heart. The sensing system includes a sensing means 
comprising self-contained, self-powered, flexible, electrical signal 
generating means mounted in a portion of the flexible lead and located in 
the heart in spaced relationship to the stimulation electrode. The sensing 
means generates electrical control signals solely by mechanical movements 
induced solely by flexible movements of the flexible lead in the heart 
caused by action of and conditions in the heart and functions without any 
electrical connection to and supply of electrical energy from any other 
power source. The electrical control signals generated by the 
self-contained, self-powered, flexible, electrical signal generating 
sensing means are transmitted by suitable electrical transmitting means to 
the electronic control circuit means. 
It is an object of this invention to provide a sensing system for use with 
an implanted medical system which sensing system is self-powered and 
generates electric control signals in response to a force applied thereto 
by the operation of a part of the human body and transmits the electrical 
control signals to the implanted medical system for use in controlling the 
operation thereof and which sensing system requires no electrical power 
from any other source to accomplish its functions. 
It is another object of the invention to utilize a quantity of material, 
which is either piezoelectric or piezoresistive, to generate a voltage 
signal that is related to heart motion or an altered hemodynamic state. 
Additional objects, advantages, and novel features of the invention are set 
forth in part in the description which follows which will be understood by 
those skilled in the art upon examination of the following or may be 
learned by practice of the invention. The objects and advantages of the 
invention may be realized and obtained by means of the instrumentalities 
and combinations particularly pointed out in the appended claims.

DESCRIPTION OF THE INVENTION 
In FIG. 1, there is illustrated a sensing system of this invention in use 
with a heart pacemaker system HPS having a heart pacemaker control unit 
HPC containing a power supply B, an electronic control circuit ECC and a 
bipolar electrical connector EK. A bipolar electrical plug ES of a 
flexible lead FL is attached to the electrical connector EK. The flexible 
lead extends via the superior vena cava SVA into the right auricle RA and 
then into the right ventricle RV and is attached to a stimulation 
electrode SE which is secured in a conventional manner to the heart tissue 
at the apex of the heart AH. The signal generating sensing means SGSM of 
this invention is associated with the flexible lead FL within the right 
ventricle RV in spaced relationship to the stimulation electrode SE. 
A view of one embodiment of a sensing system SS, presenting its principal 
parts, is shown in FIG. 2. The first means comprises a conventional 
multipolar electrical connection 10 to the heart pacemaker system. The 
electrical connections may be either coaxial or in some other arrangement. 
The connections may be made out of any biocompatible electrically 
conducting material with biocompatible insulating medium used to separate 
the various poles. Conventional sealing mechanisms 12 maintain a barrier 
between the electrical connections 10 and the conductive fluids of the 
body. A self-contained, self-powered, flexible, electrical signal 
generating sensing means 14 is mounted for cantilever action on a fixed 
support 16 secured to the wall of a continuous flexible tube 18 which 
comprises a portion of the flexible lead FL in this embodiment. Details of 
the sensing means are described below. Electrically conducting lead lines 
20 and 22 are connected to the sensing means 14 so as to conduct electric 
signals generated by the sensing means 14 to a heart pacemaker system HPS. 
The stimulation electrode 24 of the heart pacemaker system is positioned 
using a plurality of tines 26 which are attached in a conventional manner 
to heart tissue so as to fix the stimulation electrode 24 in position. The 
continuous flexible tube 18 is formed from a biocompatible, fatigue 
resistant, insulation material such as a silicon rubber, teflon or 
polyurethane. 
In FIG. 3, there is schematically illustrated the electrical functioning of 
the sensing means 14 of one embodiment of this invention. A sensing means 
14 is mounted on a fixed support 16 in a cantilever mode so that it may be 
delfected as illustrated by the end 28 thereof. Electrically conducting 
lead lines 20 and 22 are connected by suitable means to surfaces 30 and 32 
of the sensing means 14. As illustrated in FIG. 3, the end 28 has been 
deflected from its original location, the dashed lines of FIG. 3, in an 
amount illustrated by the two headed arrow 34. The deflection of the end 
28 generates an electric voltage Vo which is transmitted, in a preferred 
embodiment of the invention, through an electrical circuit to the heart 
pacemaker system. 
The preferred embodiments of this invention use flexible piezoelectric 
polymer films. Flexible piezoelectric polymers are now readily available 
from vendors such as the Penwalt Corporation in the United States of 
America and sold under the trade name "Kynar". The polymers are available 
in a variety of shapes, thicknesses, metallizations and factory supplied 
wire bonding pads. The most common type of commercially available 
piezoelectric polymers consist of poly (vinylidene fluoride) (PVDF), 
processed by a combination of heat, pressure (drawing out of material) and 
exposure to strong electric fields. Composite type PVDF polymers are also 
being developed which incorporate powdered lead--zirconate--titanate (PZT) 
and also PZT powder incorporated in some other polymer matrix such as 
epoxy resin. In this invention PVDF has been utilized in the sensing 
system as the sensing means to detect displacement within the heart caused 
by hemodynamic changes. 
The theory to calculate the electrical output available from piezoelectric 
devices is well established and is available from a number of sources. In 
terms of the proposed device a number of relevant factors are discussed 
below. 
The output available from a PVDF sensor increases according to a number of 
factors. As the thickness of a PVDF film decreases, the output increases, 
however, as the film becomes very thin, it becomes difficult to fabricate. 
The problem of maximizing the output can be solved by constructing a 
"bimorph", or if required, a multimorph sandwiched construction. The 
properties of such a construction are such that as the thickness of the 
sandwich increases, so does the electrical output. Also the electrical 
output increases as a function of the inverse of the length of the 
sandwich, for a cantilever mode of bending. 
A piezo film laminate as proposed in one embodiment of the sensing means 
can be modelled for a layout, such as that illustrated in FIG. 3, as 
follows: 
EQU V=3/4(g.sub.31 Yt.sup.2 /L.sup.2)x 
where: 
V=Voltage output 
g.sub.31 =piezoelectric voltage or strain constant 
Y=Young's modulus 
t=thickness of bimorph 
L=length of bimorph 
x=amount of deflection 
Further information related to this model can be found in the "Kynar Piezo 
Film Technical Manual" by Penwalt Corporation, the disclosure of which is 
incorporated herein by reference. 
The embodiment of the invention illustrated in FIG. 4 comprises a flexible 
outer tube 36 and a flexible inner tube 38 joined together through a 
member 40 by suitable means, such as welds or adhesive 42, a portion of 
which which also joins the inner flexible tube 38 to another flexible tube 
44. The end of the flexible tube 44 (not shown) may be secured to an 
implanted medical device. The interior 46 of the flexible tubes 38 and 44 
may be used as desired. A sensing means 14 is located between the outer 
flexible tube 36 and the inner flexible tube 38. A rigid member 48 is 
secured to the outer flexible tube 36 so as to provide a relatively fixed 
support for the fixed support 16 which is secured thereto. A displacement 
of the flexible tubes 36 and 38 will cause a deflection of the sensing 
means 14 so as to generate a voltage which is transmitted as desired by 
the electrically conducting lead lines 20 and 22. 
The construction of one embodiment of a sensing means 14 is illustrated in 
FIG. 5 and comprises a thin film 50, usually in micron sizes, of poly 
(vinylidene fluoride) which has been specially processed so as to have 
electrical energy generating surfaces 52 and 54 when subjected to forces 
bending the film 50. The film 50 can be in strip form of any desired 
length and width but preferably has a longitudinal extent substantially 
greater than its lateral extent. Electrically conducting lead lines 20 and 
22 are electrically connected to the surfaces 52 and 54 and are used to 
transmit the electrical energy generated by the surfaces 52 and 54. 
In FIG. 6, there is illustrated another embodiment of a sensing means 14 
which comprises a lamination of two of the sensing means 14, illustrated 
in FIG. 5, laminated together. An adhesive 56 is used to bond the adjacent 
surfaces of the sensing elements 14 together. The adhesive 56 provides an 
electrical connection between the adjacent surfaces of the sensing means 
14. The adhesive used must be tough, fatigue resistant, mositure 
resistant, biocompatible and resistant to peel and shear forces. Adhesives 
which have been found to be suitable are cyanoacrylates and acrylics, 
particularly the toughened varieties. The sensing means 14, as illustrated 
in FIG. 6, is known as a bimorph. The connection illustrated in FIG. 6 is 
a series connection. Two lead lines (not shown) similar to electrically 
conducting lead lines 20 and 22 are used to transmit the generated 
electrical signal. 
In the embodiment illustrated in FIG. 7, the sensing means 14 comprises a 
laminate of a plurality of sensing means 58, each comprising a ring shaped 
film 50 having electrical energy generating surfaces 52 and 54. The 
sensing means 58 are laminated together by adhesive 56. 
The sensing means 14 in the embodiment illustrated in FIG. 8 comprises an 
elongated, flexible tube 60, constructed as described above using a film 
50 in tubular form, FIG. 8a, with electrical energy generating inner and 
outer surfaces 52 and 54. The inner and outer surfaces 52 and 54 of the 
flexible tube 60 are coated with an electrically conducting compound 62. 
In FIG. 8, the electrically conducting lead line corresponding to 
electrically conducting lead line 20 of FIG. 5 comprises an electrically 
conducting helix 64 that is electrically secured to an electrical 
connector 66 by a conventional swaging device 68. The electrical connector 
66 has a plurality of barbs 70 that provide the electrical connection 
between the inner surface 52 of the flexible tube 60 and the helix 64. In 
FIG. 8, the electrically conducting lead line corresponding to 
electrically conducting lead line 22 of FIG. 5 comprises an electrically 
conducting helix 72 that is electrically secured to an electrical 
connector 74 by a conventional swaging device 76. The electrical connector 
74 has an annular inwardly directed projection 78 that provides the 
electrical connection between the outer surface 54 of the flexible tube 60 
and the helix 72. An inslating tube 79 electrically separates the helix 64 
from the helix 72. The flexible tube 60 should not be of totally uniform 
thickness. This can be accomplished by running one or more grooves (not 
shown) along the length of each of the outer and inner surfaces of the 
flexible tube 60. 
The sensing system in the embodiment in FIGS. 9 and 9a is similar in many 
respects to the embodiment of FIG. 4. The sensing means 14 is located 
between the outer flexible tube 36 and the inner flexible tube 38. The 
ends of the flexible tubes 36 and 38 are joined to a housing 80 comprising 
a portion of a heart pacemaker implant. The stimulation electrode 24 and 
the tines 26 are the same as in FIG. 2. The coil 82 transmits a signal 
from the heart pacemaker system to cause the mechanism 84 to function to 
cause the stimulation electrode 24 to stimulate the heart. The sensing 
means 14, as illustrated in FIG. 9a, is a bimorph, as described above in 
relation to FIG. 6, comprising two sensing means 14 joined together by the 
adhesive 56. If desired, an electrical insulator 85 is positioned between 
the upper portions of the sensing means 14. A pair of fixed supports 16 
are used to provide the cantilever mounting for the sensing means 14. The 
fixed supports are in frictional engagement with the adjacent surfaces of 
the outer tube 36 and the inner tube 38. 
The sensing system in the embodiment illustrated in FIG. 10 is similar in 
many respects to the embodiment illustrated in FIG. 8. The sensing means 
comprises the flexible tube 60 and is associated with a heart pacemaker 
implant system similar to that in FIG. 9. The coil 82 is located between 
an outer flexible tube 86 and an inner flexible tube 88. The fixed support 
16 is in frictional engagement with the inner surface of the flexible tube 
88 and provides the cantilever mounting for the flexible tube 60. The 
electrical connector 66 has barbs 70 in electrical contact with the inner 
surface of the flexible tube 60 and transmits a generated electric signal 
over electrically conducting the lead line 20. The lead electrically 
conducting lead line 22 is used to transmit an electric signal generated 
by the outer surface of the flexible tube 60. An insulating tube 79 
electrically separates the lead line 20 from the lead line 22. 
Electrical connections may be made to the piezoelectric polymer by means of 
thin metal pads to which are attached the electric wires and the polymer. 
Such an arrangement is presented in FIG. 11 where an insulated, braided 
wire 90 is bonded by welding 92, or by means of a conductive adhesive, to 
a conductive thin metal pad 94 which is in turn bonded by conductive 
adhesive 96 to a conducting coating 98 of a piezoelectric polymer 
substrate 50. 
Other methods for making electrical connections to the polymer include (but 
are not limited to) the use of a polymeric conductor 100 attached directly 
to the piezolectric substrate 102 as illustrated in FIG. 12. 
The embodiment illustrated in FIGS. 13 and 13a is very similar to that in 
FIGS. 9 and 9a, but includes an electronics package 104 connected to the 
electrically conducting lead lines 20 and 22 to pre-process the signal 
from the sensing means 14 and produce a new electric control signal. 
Electrically conducting lead lines 106 and 108 transmit the new electrical 
control signal to the electronic control circuit ECC. 
It may be possible to drive information on pressure from the sensing system 
by a mathematical analysis of the output of the sensing means. However, it 
is also possible to include a pressure transducer or a transducer to 
measure other physical or chemical parameters in the sensing system SS as 
shown in FIG. 14. The opening 110 for a sensor window for detecting 
pressure is incorporated as part of the sensing system. 
A hemodynamic sensing system constructed according to the present invention 
(as shown in FIG. 9) was implanted in the right ventricle of the heart of 
a sheep. A typical output of the device in shown in FIG. 15. The ECG 
waveform (X) is represented by its mechanical equivalent and (Y) is the 
output of the hemodynamic sensing system. It can be seen that the output 
of a sensing system gives an indication of the hemodynamic state of the 
heart showing a good correspondence with the ECG waveform. This data may 
be digitized in a number of ways, one of which is presented in (Z). Such 
variables as pulse width, height and rate can all be analyzed and 
translated into data such as stroke volume, type of heart activity, heart 
rate, capture verification for threshold tracking etc. The hemodynamic 
sensing system may be inserted by a transvenous insertion as described in 
an article on pages 62-64 of the Journal of the American Medical 
Association, Jan. 4, 1980, Volume 243, which is incorporated herein by 
reference. 
FIG. 16 shows a charge amplifier configuration which maintains the input 
terminals at an extremely small voltage so that a negligible current flows 
in any leakage path. This is used to reduce voltage excursion across the 
piezoelectric sensing means 14. This reduces the effect of leakage paths 
and stray capacitance across the piezoelectric sensing means. The 
operation of this circuit is to transfer the charge generated on the 
piezoelectric sensing means to the feedback capacitor of the charge 
amplifier, while maintaining the input terminal voltage at zero. The 
resulting electrical signal is then fed through an electrically conducting 
lead system to the implanted medical device. Within the implanted medical 
device, the signal is recovered and used by the implanted medical device. 
Any material in which some electrically measurable property reversibly 
changes as a function of strain on the material is suitable in a sensing 
means of this kind. Another possible material is a piezoresistive material 
such as some types of metallic glass which change their resistance as a 
function of the degree of deformation. Alternatively piezoresistive films 
can be constructed by metal deposition on suitable polymeric or metallic 
substrate by chemical vapour deposition or ion beam etching to result in 
the desired properties. These are regarded to fall within the scope of the 
present invention. 
It is contemplated that the inventive concepts herein described may be 
variously otherwise embodied and it is intended that the appended claims 
be construed to include alternative embodiments of the invention except 
insofar as limited by the prior art.