Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Patent
1995-03-17
1997-02-18
Kamm, William E.
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
A61N 1362
Patent
active
056037252
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an arrangement for for determining the impedance in the heart by way of the pacemaker electrode disposed in the ventricle chamber.
2. Background Information
In some cases it is favorable to determine the electrical impedance in the heart region, using measuring technology, during operation of pacemakers, particularly in connection with an adaptation of the stimulation rate to the instantaneous stress on the patient.
It is known, for example, to determine the cardiac stroke volume in the manner of impedance plethysmography, i.e., by means of a measurement of the impedance in the heart. To do this, in the simplest case two electrodes are inserted into the heart. If a current of a known magnitude flows through the cardiac volume via these electrodes, the impedance in the heart can be determined by means of a voltage measurement. A corresponding apparatus for measuring the ventricular volume is described in DE-36 29 587-A1. This type of arrangement is not suitable for use in pacemakers.
From EP-A-0 140 472, a pacemaker is known in which the impedance in the heart is measured by way of further electrodes disposed at the pacemaker electrode, and the stimulation rate is controlled with the stroke volume determined from this. In this case, a series of measurements is provided from whose average value the stroke volume is calculated, which however, has disadvantages.
SUMMARY OF THE INVENTION
The object of the invention is to disclose an arrangement of the generic type mentioned at the outset, which is particularly suited for use in pacemakers.
This object is accomplished with the characterizing features of the invention, as will be described below.
The invention is based on the recognition that, in impedance measurements in the heart which ultimately always amount to combined current/voltage measurements, it is important to eliminate, if possible, the interference voltages occurring there, of which, among other things, a polarization voltage stemming from the stimulation pulse can be a part. This must take place with the smallest possible energy expenditure because of the limited battery charge.
In the circuit according to the invention, the pacemaker electrode is connected to a current source that can be switched in its current direction and be controlled with respect to its current intensity. The voltage applied to the pacemaker electrode is measured by at least one voltage measurement apparatus.
Measurement is particularly effected outside of the times of the stimulation pulses or the times that must be available for detection of the signals emitted by the heart.
If a voltage not stemming from the current flow is present at the pacemaker electrode, this voltage makes a corresponding contribution to the measured voltage, i.e., this voltage measurement is erroneous. This measuring error is eliminated by the reversal of the current direction by means of corresponding switching means, and the remeasurement of the voltage applied to the pacemaker electrode. In this measurement, the voltage not stemming from the current flow makes the same contribution as in the first measurement. In the subtraction of the two measured voltages, these contributions cancel each other out, while the non-erroneous components are added, because their operational signs are reversed by the current reversal.
The currents necessary for measurement need not be made so large in this way that the voltage dropoff caused by them, which takes place at the impedance, is significantly greater than the polarization voltage. This leads to an increased service life of the pacemaker battery.
In an advantageous modification of the invention, the magnitude of the current delivered by the current source and flowing by way of the pacemaker electrode is controlled as a function of the difference between the voltages measured consecutively in the currentless state. In this way the intensity of the current can be optimally adapted to the interference level, which cannot be compen
REFERENCES:
patent: 3871359 (1975-03-01), Pacela
patent: 3874368 (1975-04-01), Asrican
patent: 4702253 (1987-10-01), Nappholz et al.
G. Boheim et al.: "Intrakardiale Impedanzmessung zur Regelung frequenzadaver Schrittmachersystems", In: Biomedizinische Technik, vol. 32, Sep. 1987, Berlin, Germany, pp. 41-42.
Biotronik Mess- und Therapiegeraete GmbH Ingenieurbuero Berlin
Getzow Scott M.
Kamm William E.
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