Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1998-10-30
2001-10-16
Evanisko, George R. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S008000
Reexamination Certificate
active
06304781
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electrostimulator having an output connection to a working electrode and a test generator having an output connected to the output connection for generating either a pulse-shaped or periodically changeable test signal and feeding the signal to the output connection.
Implantable pacemakers have been used for some time to treat cardiac disfunctions, as is known, and in particular bradycardia conditions. These pacemakers transmit electrical stimulation pulses to the heart via an endocardially arranged stimulation electrode if the heart stops beating or does not beat sufficiently fast.
Owing to the fact that each stimulation pulse leads to a partial discharge of the pacemaker battery, efforts are made to lower the amplitude of the stimulation pulses as much as possible to increase the battery service life, wherein it should be taken into consideration that the heart will no longer react with a contraction to a stimulation with an amplitude below a specified threshold value, also referred to as stimulus threshold value.
It is therefore also known to conduct a so-called stimulus threshold value test to determine the stimulus threshold value of the heart individually for each pacemaker carrier and to be able to program the stimulation pulse amplitude accordingly. For this, the pacemaker emits successive stimulation pulses with a decreasing amplitude, wherein it is respectively determined whether the heart reacts with a contraction to the preceding stimulation pulse by evaluating an extracorporeal recorded electrocardiogram (ECG). The stimulus threshold value for the heart in that case approximates the amplitude at which the heart is barely stimulated by the stimulation pulse.
However, one problem with this is that a change in the stimulus threshold value, e.g. due to changes in the chronic stimulus threshold, is not detected during the normal pacemaker operation, which can lead either to a stimulation with unnecessarily high amplitudes or—considerably worse—to an unsuccessful stimulation.
That is why in recent years pacemakers have become known which determine automatically whether the heart is successfully stimulated by a stimulation pulse and which accordingly optimize the amplitude for the stimulation pulses. For this, the pacemaker measures the so-called evoked potential by means of the pacemaker electrode, in each case immediately following a stimulation pulse, which evoked potential causes the cardiac muscle contraction and represents the response to the preceding stimulation pulse. The problem is that the electrode system which encloses two metal electrolytic boundary surfaces is electrically charged with a stimulation pulse, owing to its capacitive properties, so that the evoked potentials can be concealed by the electrical after-effects of a stimulation pulse (artifacts on both boundary layer capacities). For that reason, this concept is only used in connection with high-capacity electrodes which, owing to their high capacity, are charged only to a relatively low voltage by a stimulation pulse, which does not interfere with the detection of the evoked potential.
Until now, suitable electrodes were selected on the basis of an extracorporeal measurement of the electrode capacity by means of separate measuring instruments, resulting in higher implantation expenditure and the disadvantage that a post-operative change in the electrode capacity is not detected by the pacemaker. Problems furthermore had to be expected with the new implantation of a pacemaker and continued use of the previously implanted electrode.
SUMMARY OF THE INVENTION
Thus, it is the object of an invention to create an electrostimulator, which permits measuring the electrode capacity even in the implanted state, without using separate instruments.
Starting with an electrostimulator as defined in the preamble to claim
1
, this object is solved by its characterizing features.
The above and other objects are accomplished according to the invention by the provision of an electrostimulator comprising: an output connection for connection to a working electrode; a test generator having an output connected to the output connection for generating one of a pulse-shaped and periodically changeable test signal and feeding the test signal to the output connection; a first measuring device having an input connected to the output connection for measuring at least one of an electrical voltage present at the output connection and a current flowing over the output connection; and an evaluation device having an input connected at least indirectly to the first measuring device for generating an output signal that reflects the working electrode capacity in dependence on at least one of the current and voltage present at the output connection.
One variant of the invention provides that the pulse generator generates a pulse with a specified electrical charge Q for determining the electrode capacity, e.g. a constant-current pulse with specified amplitude and duration. Subsequently, the voltage U to which the stimulation electrode was charged by the pulse is measured by the measuring instrument at the output connection between pacemaker and stimulation electrode, and this measured value is transmitted to a subsequently connected arithmetic unit, which uses the following formula:
1
C
EL
+
1
C
CASE
=
U
Q
,
to compute the electrode capacity C
EL
while the housing capacity C
CASE
is known. However, the invention is not limited to a constant-current pulse for this variant. The important thing is that the electrical charge Q that is discharged with the pulse or the current flowing during the pulse duration is known. It is optionally possible to generate a pulse with specified charge for this, or to measure the time during which a pulse with known current course is discharged. A constant-current pulse is preferably used.
In order to improve the accuracy of the capacity measurement, resulting from a reduction in polarization effects at the electrode system, the constant-current pulse can be a double pulse, with mutually inverse current direction of the two partial pulses.
Another variant of the invention provides that a pulse with specified voltage course, preferably a constant voltage pulse, be transmitted to determine the electrode capacity.
If the stimulation electrode is viewed electrically as a series connection, consisting of a capacity C
E1
and an ohmic resistor R
E1
, then the voltage over the electrode capacity increases exponentially during the pulse duration for a constant voltage pulse and approaches asymptotically the voltage amplitude U
Stim
of the pulse. In accordance with the formula:
C
El
=
-
T
R
·
ln
(
1
-
U
EL
U
Stim
)
the electrode capacity C
E1
is then computed from the pulse duration T, the voltage amplitude U
Stim
of the pulse, the electrode voltage U
E1
measured at the output connection following the end of the pulse, as well as the ohmic charging resistance R, consisting of the ohmic resistance R
E1
for the electrode and additional ohmic resistances in the charging circuit, which are presumed to be known.
According to another variant of the invention, the pacemaker electrode is a component of an oscillating circuit, wherein the electrode capacity can be determined based on the effect it has on the oscillatory response of the oscillating circuit. The pacemaker for this variant has an internal inductance that is connected to the output connection or can be connected to it via a switching element. In this case, the inductance can be either connected in series or parallel with the electrode capacity. The oscillating circuit set up in this way is stimulated by an oscillator, which is also optionally connected to the interface or can be connected to it via a switching element.
The oscillator for one embodiment of this variant generates a preferably sinusoidal oscillating signal with constant frequency and voltage amplitude, so that the current flowing through the oscillating circuit depends on the frequency tuning between the oscillator on the
Bartels Klaus
Busch Ulrich
Biotronik Mess- und Therapiegeräte GmbH & Co. Ingenieurbüero Ber
Evanisko George R.
Kinberg Robert
Spencer George H.
Venable
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