Apparatus for measuring impedance of body tissue

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BACKGROUND OF THE INVENTION

1. Related Application

The present application is related to a co-pending application of the same inventors entitled "Apparatus For Measuring Impedance Of Body Tissue" filed simultaneously herewith, Ser. No. 061,547.

2. Field of the Invention

The present invention is directed to an apparatus for making an impedance measurement of body tissue, and in particular to such an apparatus having a signal source which impresses a signal on the tissue to be measured, and means for acquiring an impedance signal from the tissue dependent on the impresses signal.

3. Description of the Prior Art

Body tissue impedance measurement devices are known wherein an impedance signal is obtained from body tissue to which a current or voltage signal has been supplied. Such known devices include an evaluation means which separates higher frequency signal components from the impedance signal so acquired. Such a unit is described, for example, in combination with frequency-controlled heart pacemakers in U.S. Pat. No. 4,303,705.

Other devices of this type are known wherein the evaluation means separates both lower frequency and higher frequency signal components from the impedance signal to identify the degree of blood loss occurring during an operation, as in U.S. Pat. No. 3,532,086. The low-frequency component is a measure of the blood volume.

Heretofore, impedance measurements in body tissue (including blood) were for the purpose of identifying mechanical volume changes of the body, for example, the volume of a beating heart or the thorax volume during respiration movement. The change in the impedance can then be employed for controlling the stimulation pulse frequency of a heart pacemaker. In simplified terms, the basis of making such an impedance measurement is the following physical relationship:

R=l/,(.sigma..sub.R .multidot.F)

wherein R is the impedance, .sigma..sub.R is the conductance (1/.OMEGA..multidot.cm), l is the effective electrode spacing (cm), and F is the effective line cross-section (cm.sup.2) between the electrodes.

Thus by measuring the periodic impedance fluctuation, the changes in l or F of the line path are monitored.

Thus in such known devices, a direct measurement of the metabolism of interest, which in turn is a direct measure of certain types of body stress or body changes, is not possible.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus for making an impedance measurement of body tissue which enables a signal directly corresponding to the metabolism of interest to be generated.

The above object is achieved in accordance with the principles of the present invention in an apparatus for impedance measurement having an evaluation means which filters out the low frequency signal components of the impedance signal, those low frequency components corresponding to the conductance, and the components which are filtered out are supplied to a signal output.

The invention is based on the perception that metabolic components are associated with the conductance .sigma..sub.R in the aforementioned relationship. These components specify a direct measure for the metabolism, and thus constitute a direct relationship to the body stress. The invention also proceeds based on the perception that higher-frequency signal components are contained within the required impedance signal, which reflect changes in l or F, and that a lower frequency component is also present which is a direct measure for the conductance .sigma..sub.R. This is in contrast to the known methods of, for example, U.S. Pat. No. 3,532,086, which uses the low-frequency components as indicators for blood volume. As stated above, known devices for impedance measurement in heart pacemaker technology such as, for example, the impedance measuring device in U.S. Pat. No. 4,303,075, filter the higher frequency signal components out of the acquired impedance signal as a measure for the respiration or the beat volume of the heart and thus control, for example, the frequency of the pacemaker. In the apparatus disclosed and claimed herein, however, the lower frequency signal components of the impedance signal are filtered out instead of the higher frequency signal components. A signal which is directly dependent on the conductance is thus obtained, permitting a direct statement (signal) identifying the patient's metabolism to be made, which in turn is a direct indicator of the momentary stress of the patient. In the context of a frequency-controlled heart pacemaker, this signal is a considerably more precise control signal for the stimulation frequency than the conventionally used higher-frequency component of the impedance signal.

In one embodiment of the invention, the low-frequency components of the measured impedance signal can be supplied as a control signal to the frequency control unit of a heart pacemaker for controlling the stimulation frequency. This is accomplished by modifying the stimulation frequency in accord with the changing conductivity, as measured by the device.

In another embodiment of the invention, the limit frequency of the low-pass filter, which is used to filter out the lower frequency components, may be variable dependent on the stimulation frequency. The limit frequency in this embodiment is shifted upwardly (toward higher frequency values) given an increasing stimulation frequency, and is correspondingly shifted downwardly (toward lower frequency values) given decreasing stimulation frequency. Such variation in the limit frequency has the advantage of maintaining the limit frequency of the filter optimally high dependent on the respiratory frequency, which changes dependent on the stress to the patient (analogously to the changing stimulation frequency), but nonetheless will be maintained below the respiratory frequency. A disturbing influence of the respiration on the measured result thus cannot occur.

In a further embodiment of the invention, a means for compensating for the influence of temperature is additionally divided, so that the measured conductance is rid of temperature influences.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a portion of an impedance measuring device constructed in accordance with the principles of the present invention.

FIG. 2 is a schematic block diagram of an impedance measuring device constructed in accordance with the principles of the present invention.

FIG. 3 is a schematic block diagram showing the embodiment of an impedance measuring device of the type shown in either FIG. 1 or FIG. 2 in a frequency-controlled heart pacemaker.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An impedance measuring unit 1 constructed in accordance with the principles of the present invention is shown in FIG. 1. The impedance measuring unit includes a signal source for impressing an electrical signal on the tissue of a patient. In the embodiment of FIG. 1, the signal source is an alternating voltage source 2 (for example, a 1 kHz ac voltage generator) which impresses an ac voltage V.about. of unchanging amplitude (for example, 1 kHz ac voltage) on body tissue (not shown) via electrode lines 3 and 4, with associated electrodes 5 and 6. The apparatus shown in FIG. 1 is preferably for intracorporeal measurement. At least the electrodes 5 and 6 are thus implanted in the body tissue, preferably the entire measuring instrument is implanted. Dependent on the impressed ac voltage V.about., the voltage drop caused by the current in the electrode lines 3 and 4 is acquired via a low-impedance series resistor 7 (for example, 100 ohms), by means of a voltmeter 8. The output signal of the voltmeter 8 is supplied to a low-pass filter 9 having an upper limit frequency in the range from about 0.1 through about 0.4 Hz (the upper limit frequency being preferably variable in this frequency range, as described in detail below with reference to FIG. 3). The low-pass filter 9 filters only the low-frequency signal components S.sub.NF out of the output signal S.sub.I (impedance signal) from the voltmeter 8. The low-frequency signal components S.sub.NF correspond to the conductance .sigma..sub.R in the body tissue. The low-frequency signal components S.sub.NF filtered out are supplied to a signal output 10 of the low-pass filter 9.

In the embodiment of FIG. 2, another embodiment of an impedance measuring unit 11 is shown which includes an alternating current source 12 (for example, a 1 kHz alternating current source) which impresses an alternating current I.about. of unchanging amplitude (for example, 1 kHz alternating current) on the body tissue via the electrode lines 3 and 4 and the associated electrodes 5 and 6. The entire measuring system is again preferably designed as an intracorporeal measuring system. In this embodiment, the ac voltage between the electrodes 5 and 6 is measured by a parallel voltmeter 13 which contains a divider for forming the value 1/V.about.. The output signal S.sub.I (impedance signal) of the voltmeter 13 is then evaluated in the low-pass filter 9 in the same manner as set forth above in connection with FIG. 1.

A heart pacemaker 14 embodying either of the embodiments of FIG. 1 or FIG. 2 is shown in FIG. 3. The pacemaker 14 is a frequency-controlled heart pacemaker. Identical components identified above are provided with the same reference numerals in FIG. 3. The electrode 5 in the embodiment of FIG. 3 serves simultaneously as the stimulation electrode for the heart pacemaker 14, whereas the electrode 6 is formed by the conductive (for example, metallic) housing of the heart pacemaker 14. The electrode line 3 corresponds to the stimulation catheter of the heart pacemaker 14.

The frequency-controlled heart pacemaker 14 includes a pulse generator 15 for generating stimulation pulses schematically identified at 16. The repetition rate of the stimulation pulses 16 (stimulation frequency) is controllable at the pulse generator 15 by a frequency control unit 17. The stimulation frequency is controllable dependent on the filtered-out, low-frequency signal component S.sub.NF at the output 10 of the low-pass filter 9. Control is undertaken by modifying the stimulation frequency in proportion to the changing conductance .sigma..sub.R. The stimulation frequency thus increases when the low-frequency signal component S.sub.NF filtered out (and thus, the conductance .sigma..sub.R) increases. Conversely, the stimulation frequency is lowered when the signal component S.sub.NF (and, thus, the conductance .sigma..sub.R) decreases.

As shown in FIG. 3, the limit frequency of the low-pass filter 9 is adjustable via a control line 19, dependent on the output signal of the frequency control unit 17. Control of the limit frequency is undertaken such that the limit frequency is shifted toward higher frequency values in the range of about 0.1 through about 0.4 Hz given an increasing stimulation frequency, and is correspondingly shifted toward lower frequency values in that range given a decreasing stimulation frequency. As discussed above, this results in the advantage that the measured result remains uninfluenced by the respiration of the patient, while maintaining the limit frequency of the low-pass filter 9 at the desired highest possible value.

As also shown in FIG. 3, the stimulation frequency can be controlled by the frequency control unit 17 through a correction element 20 which compensates for temperature influences, rather than being controlled directly by the signal S.sub.NF. The conductance .sigma..sub.R increases with increasing temperature. The correction unit 20 corrects the signal S.sub.NF for the conductance inverse fashion. A temperature sensor 21 acquires the temperature value, the temperature sensor 21 being disposed, for example, in the housing of the heart pacemaker or outside of the housing. The impedance measuring unit 1 or 11 and the low-pass filter 9 are preferably accommodated inside the pacemaker housing.

Instead of the temperature sensor 21 and the correction element 20, the stimulation catheter 3 may itself consist of a material (for example, NTC resistor material) which compensates for the temperature response of the conductivity. Another alternative is to integrate a resistor constructed of NTC material within the stimulation catheter 3.

The frequency control unit 17 may be connected to a subtraction stage 22 which subtracts a programmable fixed value (for example, between 80 and 90 percent of the mean value of the low-frequency signal) from the temperature-compensated low-frequency signal S.sub.NF. An offset signal component which is not needed for frequency control may be thereby eliminated.

Although modifications and changes may be suggested by those skilled in the art it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

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