 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The invention relates to a cardiac pacemaker system of the type including a
stimulation electrode adapted for being arranged in the heart; an output
capacitor coupled to the stimulation electrode; a first circuit, coupled
to the output capacitor, for generating a pulse following each stimulation
pulse for at least one of reducing a residual charge of the output
capacitor and eliminating an afterpotential following a stimulation pulse
by the stimulation electrode; and a third circuit, for acquiring an evoked
heart action from an electrical signal picked up by an electrode arranged
in the heart.
For a long time, it has been a goal in the development of artificial
cardiac pacemakers to verify the success of a heart stimulation through
the measurement and evaluation of signals, which can be picked up in the
heart on the basis of the evoked heart action via an electrode which is
installed in the heart--and preferably the stimulation electrode itself.
The pickup of the electrical "response signal" of the heart after a
stimulation is disturbed by the aftereffects of the stimulation pulse,
which are caused by the polarization of the stimulated tissue, which can
be reduced only when the recharging of the output (coupling) capacitor
connected to the stimulation pulse is also eliminated.
This follows from the fact that the evoked potential, which indicates the
success of the stimulation and is present at the heart approximately up to
300 ms after the stimulation, is superposed by an afterpotential in the
order of magnitude of more than 10 mV. The afterpotential, which disturbs
the effectiveness for recognition of the evoked potential, is caused by
the effect of the stimulation electrode as an electrochemical electrode,
from which results a saturation of the detection amplifier.
Apart from various circuits, with which attempts are made to eliminate the
consequences of the afterpotential, an apparatus for the stimulation of
the heart is known from EP-B1-0 000 989, wherein the disturbing
afterpotential of the stimulation electrode is intended to be reduced in
an accelerated manner by means of an additional, transistor-controlled
resistor branch, which essentially short-circuits the Helmholtz capacity.
The total time needed for the reduction of the disturbing afterpotential,
however, is too long with customary electrodes to make possible an
effective effectiveness recognition under all circumstances.
SUMMARY OF THE INVENTION
Starting from the drawbacks of the prior art, it is the object of the
invention to provide a cardiac pacemaker system of the generic type
mentioned in the introduction, in which an effective detection of evoked
heart signals is possible in an effective manner, also under the different
changing operating conditions of a cardiac pacemaker, which occur, for
example, during the settling in of the electrode.
The above and other objects are accomplished in the context of a cardiac
pacemaker system of the type first described above, wherein according to
the invention the stimulation electrode includes a porous surface coating
made of an inert material and having an active surface that is
substantially larger than a surface of the basic geometric form of the
stimulation electrode; and the pacemaker system includes circuit means for
changing the time duration of the activation of the second circuit as a
function of the acquisition of the evoked heart action, with the time
duration of the activation of the second circuit being limited to no more
than 70 ms.
The invention includes the finding that when an electrical voltage is
applied to a pacemaker electrode, which is anchored in the heart, two
layers of different charge carriers are formed, which, however, are
separated by a monolayer of hydrogen molecules based on hydration effects.
In its structure and electrical behavior, this so-called Helmholtz double
layer corresponds to a plate capacitor. If, during the stimulation of the
heart, a current flows via this Helmholtz capacity, a voltage is generated
there which forms the afterpotential, with the voltage increasing as the
Helmholtz capacity decreases. The afterpotential is additionally increased
through further electrochemical reactions with charged reaction products
taking place at the phase boundary. Apart from the increase of the
Helmholtz capacity, a reduction of the stimulation pulse amplitude, above
all, is important for reducing the afterpotential so that a definitive
effectiveness recognition can be carried out with the same electrode. In
addition, a reduction of the amplitude of the stimulation pulse
contributes in an advantageous manner to increasing the service life of
the pacemaker's current supply source.
The selection of the measures according to the invention can thus, on the
one hand, reduce the stimulation pulse amplitude so that the
afterpotential as a whole becomes lower. Moreover, the reduction of the
afterpotential is accelerated so that the afterpotential is reducible in a
defined manner within a predetermined period of time.
This reduction can take place by means of an active counterpulse or also
through passive means at the output of the stimulation circuit, as is
known from the prior art that was mentioned (autoshort).
According to advantageous modifications of the invention, it becomes
possible through automatically operating circuit means to automatically
determine the time duration of the blocking of the input amplifier for the
evoked pulses and to adapt it to the implantation conditions or their
temporal change. In this process, an increased stimulus energy is used,
which, with certainty, leads to a stimulation.
Additionally, in an advantageous modification, a cardiac pacemaker system
can be provided, which overall only has a low energy requirement because
of the automatic adjustment of the stimulation amplitude.
It was recognized here that
while a stimulation pulse having an excessive amplitude leads with
certainty to a stimulation of the myocardium, the service life of the
pacemaker's current supply source, however, is considerably reduced
because of the increased energy consumption so that an early
reimplantation must be carried out,
a sufficiently reliable detection of evoked signals, based on a myocardium
stimulation that has taken place, is possible only after a sufficient
reduction of the afterpotential, which occurs due to the stimulation
pulse, if stimulation and detection are carried out with the same
electrode,
the afterpotential may, at most, reach such a value which can be reduced to
a negligible level within a period of approximately 30 to 80 ms
(autoshort), before an evoked potential has decayed and
the materials of the known electrodes and, in particular, titanium,
vanadium, zircon and niobium tend to, at times, show extreme oxidation and
that, in case of contact with aqueous electrolytes, this high oxidation
tendency leads to the formation of a thin, insulating or semiconductive
oxide layer at the electrode surface, with the oxide layer representing a
capacity C.sub.ox connected in series with the Helmholtz capacity C.sub.H
and thus leading to a slow reduction of the total capacity and therewith
to the corresponding increase of the respectively required stimulation
energy.
The pulse control of the control system according to the invention is
configured both for the automatic determination of the width of the
autoshort pulses, which is necessary for the detection of evoked
potentials, and for maintaining a minimum amplitude of the stimulation
pulses, which exceeds the stimulus threshold of the myocardium at the
determined necessary width of the autoshort pulses, and it is provided
with the electrical means necessary for this purpose. These essentially
comprise a controllable autoshort pulse generator, a generator for the
generation of amplitude-controlled stimulation pulses controlled by a gate
circuit at a predetermined pulse repetition frequency and devices for the
detection of the potentials evoked by the stimulation pulses as a function
of the width of the autoshort pulses. The automatic setting of the
autoshort time is among the most essential advantages of the pulse
control.
The operation of the pulse control circuit represented here takes place in
two different operating conditions, "alignment" and "continuous
operation". According to the preferred embodiment of the invention, a
pulse generator is provided for the generation of the autoshort pulses, in
which a variation of the pulse width in the "alignment" operating
condition is carried out in a scanning manner by a ramp generator. In this
process, the stimulation pulses are kept constant with regard to their
amplitude through a corresponding setting in the pulse amplitude control
of the stimulation pulse generator, at a level which is above the stimulus
threshold of the myocardium and at which an evoked potential is released
with certainty. The correspondingly detected, pulse-shaped signals are fed
to a memory after sufficient amplification.
The memory is configured in a matrix fashion or array fashion and is
addressed by the above-mentioned ramp generator such that an allocation of
the memory locations to the evoked signals takes place as a function of
the respective autoshort pulses of a certain width.
An evaluation unit downstream of the matrix memory determines the most
effective detection of the evoked potentials with respect to the pulse
width of the autoshort pulses. This autoshort time is fixed in the
generator for the autoshort pulses and sets as a self-adjusted value the
width of the autoshort pulses for the "continuous operation" operating
condition of the pulse control following the "alignment" operating
condition. For the change-over of the operating conditions, a cyclical
timer switch is provided by means of which the ramp generator, the
generator for the autoshort pulses and the amplitude control stage of the
stimulation pulses can be correspondingly switched on or off. During the
"continuous operation" of the pulse control, a gate circuit, which is
provided at the input of the amplitude control stage for the stimulation
pulses, is activated by the timer switch and the detection pulses of the
evoked potentials.
Each pulse that corresponds to a detected potential leads to a reduction of
the amplitude of the stimulation pulses by a certain amount. If, after a
number of stimulations, the stimulation pulse remains below the stimulus
threshold, an evoked potential can no longer be picked up. A corresponding
output signal at a gate circuit leads to an increase of the amplitude of
the stimulation pulses in the downstream amplitude control stage to the
value that was last applied successfully. This accomplishes that the
stimulation pulse, which follows the missing detection of an evoked
potential, leads with certainty to a renewed stimulation of the myocardium
and that a "falling-out-of-step" of the synchronization of the total
system is prevented.
It is evident that, instead of the stimulation amplitude, also the pulse
width or another value that determines the stimulus energy can be changed.
It is also particularly advantageous if the afterpotential is compensated
through an active counterpulse, because the electrode used in the cardiac
pacemaker system according to the invention can also be operated
anodically, without an oxide layer impairing the stimulation threshold.
According to an advantageous modification of the invention, the amplitude
increase in case of a missing detection of an evoked potential is a
multiple of the value of the amplitude decrease when a detection took
place. This is accomplished in a simple manner by means of a divider
circuit, which provides the output signals of the gate circuit for the
amplitude reduction with this factor.
According to another advantageous embodiment of the invention, a change of
the switching conditions of the timer switch takes place in time intervals
of equal length, which cyclically repeat themselves, in order to regularly
carry out a control of the selected autoshort time. It has proven
advantageous to again carry out an "alignment" after a predetermined
number of lowering cycles of the amplitude of the stimulation pulses until
a potential detection first fails to appear so as to adjust the autoshort
time, if necessary, to a possible change of the ability of the myocardium
to be stimulated.
The function of the pulse control according to the invention is only
guaranteed for autoshort times in the range of 50 ms if the constructive
configuration of the stimulation electrode accomplishes that only a
relatively low afterpotential is built up following the stimulation pulse.
According to the preferred embodiment of the invention, the stimulation
electrode is provided with a porous surface coating made of an inert
material, with the active surface of the coating being considerably larger
than the surface that results from the geometric shape of the electrode.
Because of the fractal spatial geometry, the active surface is so large
that the energy required for the stimulation can be set to a minimum
value. Thus, because of the electrodes' large relative surface, a
successful stimulation with low energy is possible, in principle, for the
conventional coated, porous electrodes. It was now recognized that the
Helmholtz capacity is reduced due to the oxidation tendency, which leads
to an increase in the electrode impedance. The reason why the influence,
which is thus generated, on the electrode properties in the course of the
implantation time is so serious is that the deterioration of the electrode
properties has consequences which, in turn, contribute to the fact that
the stimulation properties are also influenced adversely.
Thus, for a deteriorating electrode, a greater pulse energy is necessary so
that, for the effectiveness recognition, a counterpulse with a greater
energy requirement is also necessary which, in turn, again contributes to
the deterioration of the electrode properties. Since the pulse energy and
the counterpulses necessary for the effectiveness recognition are set to
values that have to have validity over the total implantation time of the
pacemaker, the deterioration of the operating conditions ultimately is
essentially based on measures, which are actually intended to counteract
the deteriorated operating conditions.
The long-term-stable, biocompatible surface coating of the stimulation
electrode according to the invention is made of a material whose oxidation
tendency is very low, with the coating being applied on the electrode
using vacuum technology, preferably by using an inert material, namely a
nitride, carbide, carbonitride or a pure element or certain alloys from
the group gold, silver, platinum, iridium, titanium or carbon. Owing to
the fractal spatial geometry of a surface layer applied in this manner,
its active surface is very large so that the amount of energy needed for
the stimulation can be kept extremely low.
The afterpotential of a stimulation electrode made of titanium, which is
provided with a sputtered iridium nitride layer or titanium nitride layer
by means of the reactive cathode sputtering, is smaller by up to six times
(from approximately 600 mV to approximately 100 mV) than the
afterpotential of a bare stimulation electrode made of titanium. Owing to
this significant reduction of the afterpotential, the recognition of the
intracardiac ECG is possible not only in the conventional manner by means
of an amplifier and a triggering device, but an operative effectiveness
recognition can be applied, which can do without counterpulse and
autoshort times for the reduction of the afterpotential in the magnitude
of 50 ms.
Advantageous modifications of the invention are described below in greater
detail in conjunction with the accompanying Figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the preferred embodiment of the invention.
FIG. 2 is an overview diagram containing the most important influencing
quantities for the pulse control in a schematic representation.
FIG. 3 is an amplitude-time diagram, shown schematically, for the generated
stimulation pulses and the detected evocation potentials.
FIG. 4 is an embodiment of a stimulation electrode represented
schematically in side view.
FIG. 5 is an enlarged representation of detail A in FIG. 4 in a sectional
view.
FIG. 6 is a diagram to compare the impedance of the embodiment of the
electrode with the impedance of corresponding electrodes known from prior
art having the same geometric dimensions.
FIG. 7 is a representation of the afterpotential as a function of the
autoshort time in dependence of the surface configuration of the electrode
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The means for the pulse generation of the control system according to the
invention are schematically illustrated in FIG. 1. The stimulation
electrode, which is part of the control system, is connected to the output
capacitor 1 and is not shown in the drawing.
The electrode is provided with a pulse generator 4 for stimulation pulses
that are released in the direction 2 onto the stimulation electrode.
A time control stage 6 determines the point in time of the release of
stimulation pulses and, in this case, corresponds to a fixed frequency
pacemaker. The schematic circuit diagram is also usable for other
pacemaker circuits, where merely additional control lines must be
provided, through which, for example, in a demand pacemaker, stimulation
is prevented through the release of stimulation pulses in case of signals
stemming from heart actions that come in before the end of the so-called
escape interval. With an amplitude control stage 5, the amplitude (or the
energy) of the stimulation pulses can be raised ("+") or lowered ("-") via
additional inputs.
In addition, a pulse generator 15 is provided for the generation of
autoshort pulses via the final pulse generator stage 4. Via a galvanic
connection or an active counterpulse, the potential of the inner
connection of the capacitor 1 is returned in this process to the initial
state prior to the last stimulation pulse so that, by way of the charge
shift generated, the afterpotential at the electrode is counteracted. The
time duration of the pulse for eliminating the aftereffects of the
stimulation pulse can be set via a corresponding input of the pulse
generator 15.
Via an amplifier 11, signals that are generated by the heart are picked up,
with the amplifier being switched so as to be insensitive by switching
means, that are not shown, when a stimulation pulse occurs. An evoked
event is retained in a memory 12. In order to be able to optimize the time
duration of the autoshort pulse, a signal indicating an evoked event is
retained in allocation to the duration of the corresponding autoshort
pulse.
The "alignment" operating mode is set by means of a control switch 17
while, otherwise, the circuit is in the "continuous operation" operating
mode.
During the "alignment" operating condition, the optimum autoshort time is
determined, which is then maintained in the "continuous operation"
position. For this purpose, a pulse amplitude is predetermined by the
timer switch 17 via the control line 24 in the amplitude control 5 at a
constant frequency (time control 6), at which pulse amplitude an evoked
potential at the myocardium is generated with certainty. Simultaneously,
the timer switch 17 activates, via the control line 26, a ramp generator
16, which is connected to the pulse generator 15 via a change-over switch
14 to vary the width of the autoshort pulses in in a scanning manner. The
AND gate 9 is blocked, also controlled by way of the timer switch 17 via
the line 27 and a negator 10.
A picked up evoked potential or a corresponding signal 3 indicating this
condition is fed to an amplifier 11 via the connecting line 32 and
acquired in a matrix memory 12. The allocation of the individual memory
locations takes place in dependence of the time function of the ramp
generator 16 so that to each pulse width a signal can be allocated, which
indicates the pickup of an evoked potential. An evaluation circuit 13
determines the most favorable autoshort time for the detection of the
evoked potentials 3. In this process, a mean value of all pulse durations
of the autoshort pulse, at which an evoked potential could be picked up,
is selected so as to have a certain amount of certainty with respect to
the change of the signal pickup conditions in the course of the operating
time of the pacemaker.
Subsequently, the switch 17 is reset to the "continuous operation"
operating condition, during which process the mean value of the autoshort
time, at which an evoked potential could be picked up, is retained in the
pulse generator 15 via the change-over switch 14 and the line 25, and the
AND gate 9 is released via the negator 10. Afterwards, the stimulation
amplitude is again lowered to its normal value.
It is now possible with the evoked potentials, which can be recognized
reliably because of the alignment that was carried out, to set the
stimulation energy (stimulation amplitude) during the operation with
threshold control via an effectiveness recognition in such a way that the
stimulation threshold is reliably exceeded without a premature exhaustion
of the energy source occurring because of an excessive stimulation energy.
Each detection of an evoked potential 3 generates a pulse via the AND
element 9 at the divider 7, which pulse decreases the amplitude of the
next stimulation pulse 2 by a certain amount. This step-by-step amplitude
reduction takes place until no evoked potential is detected at the
predetermined autoshort time. The level change at the output of the AND
gate 9 switches the negator 8 and then effects a raising of the
stimulation amplitude up to a preceding value at which a stimulation took
place reliably. Via the divider 7, an amplitude decrease only takes place
at every nth (here 20. This value only represents an example, because, in
practice, the stimulus threshold will stabilize in the long term so that
divider ratios of several thousand will be practicable.) successful
stimulation pulse--but a raising immediately following every failed
stimulation. Thus, the stimulation pulses are always provided with a
stimulus energy, in particular, amplitude, which is only slightly above
the stimulus threshold, respectively leading to a heart stimulation with
great certainty.
In order to acquire possible changes in the transmission ratios of the
myocardium, it is of particular advantage after a "continuous operation"
phase of the pulse control to again determine the autoshort time, which is
optimal for the stimulation and detection of the heart activity, in a
repetition of the "alignment." It has proven advantageous to carry out a
further "alignment" for the amplitude of the stimulation pulses after a
"continuous operation" with, for example, m-cycles. In addition, it is
possible to adjust the change-over cycle of the switch 17 to the
patient-specific conditions.
The schematically illustrated diagram of FIG. 2 shows, on the time axis,
the possibility of picking up evoked potentials in the heart as a function
of the variation of the autoshort time and of the stimulation amplitude.
Evoked signals can only be picked up if a stimulation pulse is effective,
which means that the pulse has exceeded a predetermined threshold energy,
as it is indicated by the horizontal line 21. In addition, the possibility
of the pickup of evoked signals is further limited by the decay of the
evoked potential, which is indicated by line 19 as limit for the decay of
the stimulation effect (afterpotential). The line has a slight gradient,
because, with a higher stimulation amplitude, the (disturbing)
afterpotential also increases or the duration of its decay becomes longer.
The point in time 20 forms that time mark after which an evoked potential
has decayed to such a low level that its detection is no longer possible
or the event of interest has passed. With the measures according to the
invention, a time range for the measures to eliminate the afterpotential
is set during an automatic adjustment of the duration of the autoshort
time, this time range being within the effective range. Between the limit
values generated by the lines 19 and 20, in particular, a mean value is
set. The coating of the stimulation electrode according to the invention
makes possible a lowering of the afterpotential, which disturbs the
detection of the evoked potentials, at an autoshort time of 50 ms to a
value of almost 0 mV (compare FIG. 7).
FIG. 3 shows the amplitude-time-diagram of the stimulation pulses 24 in
relation to detectable evoked potentials 25 during the "continuous
operation" operating condition of the pulse control. After each
stimulation pulse 24, for which an evoked potential 25 is detected after
the autoshort time T=t.sub.E -t.sub.S, a step-by-step amplitude reduction
takes place via the pulse amplitude control (compare position 5 in FIG.
1). If the detection limit with the stimulus threshold 21 is reached or if
a slight shortfall occurs, the resulting change in potential at the output
of the gate circuit (comprising elements 7, 8, 9, 10 in FIG. 1) effects a
renewed increase of the amplitude of the subsequent stimulation pulse 24.
The amplitude jump occurs, in particular, to the amplitude value at which
a successful stimulation has last taken place.
In order to keep the number of shortfalls of the stimulus threshold, at
which effective stimulation does not occur, as low as possible, a lowering
is only carried out at every nth stimulation pulse in advantageous
embodiments of the invention, with a raising immediately following every
threshold shortfall.
The stimulation electrode 100, illustrated in FIG. 4 in a schematic side
view, is a unipolar nap electrode having a head that is provided with a
cylinder-shaped basic body 126 made of titanium. The cylinder-shaped basic
body 126 is provided with a surface coating 127 consisting of an inert
material iridium nitride (IrN), which is applied to the cylinder-shaped
basic body 126 of the titanium electrode by means of cathode sputtering | | |
