Title: Device for detecting tachycardiac rhythm disturbances
Abstract: A device for detecting tachycardiac rhythm disturbances, has a measuring unit for picking up and reproducing a measurement signal dependent on intracardial impedance at its output and an evaluation unit connected at the input to the measuring unit. The evaluation unit is adapted to output both a first signal which corresponds to the difference of a maximum and a minimum measurement signal within at least one predeterminable period of time and also a second signal which is dependent on the integral of the measurement signal over at least one predeterminable period of time.
Patent Number: 6,961,614 Issued on 11/01/2005 to Kaye
| Inventors:
|
Kaye; Gerry C. (55 West Ella Rd., Kirk Ella, Hull HU10 7QL, GB)
|
| Appl. No.:
|
203976 |
| Filed:
|
February 19, 2001 |
| PCT Filed:
|
February 19, 2001
|
| PCT NO:
|
PCT/EP01/01827
|
| 371 Date:
|
October 24, 2002
|
| 102(e) Date:
|
October 24, 2002
|
| PCT PUB.NO.:
|
WO01/60451 |
| PCT PUB. Date:
|
August 23, 2001 |
Foreign Application Priority Data
| Feb 17, 2000[DE] | 100 08 324 |
| Current U.S. Class: |
607/14; 600/547 |
| Intern'l Class: |
A61N 001/18 |
| Field of Search: |
607/14,17,20,24
600/547
|
References Cited [Referenced By]
U.S. Patent Documents
| 4303075 | Dec., 1981 | Heilman.
| |
| 5179946 | Jan., 1993 | Weiss.
| |
| 5184615 | Feb., 1993 | Nappholz et al.
| |
| 5247939 | Sep., 1993 | Sjoquist.
| |
| 5385576 | Jan., 1995 | Noren.
| |
| 5427112 | Jun., 1995 | Noren.
| |
| 5645575 | Jul., 1997 | Stangl.
| |
| 5713366 | Feb., 1998 | Armstrong et al.
| |
| 5755742 | May., 1998 | Schuelke et al.
| |
| 6064907 | May., 2000 | Thong.
| |
| 6154674 | Nov., 2000 | Meier.
| |
| 6263243 | Jul., 2001 | Lang.
| |
| 6317633 | Nov., 2001 | Jorgenson et al.
| |
| Foreign Patent Documents |
| 44 47 447 | Jul., 1996 | DE.
| |
| 196 09 362 | Jun., 1997 | DE.
| |
| 198 04 843 | Aug., 1999 | DE.
| |
| 0 009 255 | Apr., 1980 | EP.
| |
| 0 793 976 | Mar., 1997 | EP.
| |
| WO 98/1424/0 | Apr., 1998 | WO.
| |
Primary Examiner: Manuel; George
Assistant Examiner: Faulcon, Jr.; Lenwood
Attorney, Agent or Firm: Hahn Loeser & Parks LLP
Claims
1. A device for detecting tachycardiac rhythm disturbances, comprising:
a measuring unit for picking up and reproducing a measurement signal dependent
on intracardial impedance at an output thereof; and
an evaluation unit for detecting tachycardiac rhythm disturbances which is connected
at an input side thereof to the measuring unit and which is adapted to determine
an output a first signal which corresponds to the difference of a maximum and a
minimum measurement signal within at least one defined period of time,
wherein the evaluation unit is additionally adapted to determine and output a
second signal which is dependent on the integral of the measurement signal over
at least one defined second period of time, and the first signal associated with
the second period of time, wherein the first-mentioned period of time is before
the second period of time or is identical to the second period of time.
2. The device of claim 1, further comprising:
means for measuring unipolar impedance.
3. The device of claim 1, further comprising:
means for measuring bipolar impedance.
4. The device of claim 1, wherein the measuring unit comprises at least two electrodes,
of which at least one can be introduced into a chamber of the heart.
5. The device of claim 4, further comprising:
a current source connected to the electrodes in such a way that it produces a
predetermined measuring current between the electrodes; and
voltage measuring means connected to the electrodes for measuring an electrical
voltage therebetween.
6. The device of claim 4, further comprising:
a voltage source connected to the electrodes in such a way that it produces a
predetermined measuring voltage between the electrodes, and
current measuring means connected to the electrodes for measuring an electrical
current therebetween.
7. The device of claim 5, wherein the current source is adapted to output an
alternating current with a predetermined time dependency.
8. The device of claim 7, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the voltage measuring means, which is modulated by the alternating
current produced by the current source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
9. The device of claim 1, further comprising:
a filter stage for the removal of measurement signal components of a frequency
of up to a maximum of 1 Hz.
10. The device of claim 1, further comprising:
control means adapted to cause the evaluation unit to start or stop integration
of a signal at the input of the evaluation unit.
11. The device of claim 10, wherein the control means is so designed to cause
the evaluation unit to effect integration respectively over the period of time
of one or some cardiac periods.
12. The device of claim 10, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
13. The device of claim 10, wherein the evaluation unit has a memory.
14. The device of claim 13, wherein the evaluation unit is so designed that within
the predetermined period of time the hitherto maximum and minimum signals at the
input of the evaluation unit are continuously determined and stored in the memory,
that at the end of the period of time the difference between the currently stored
maximum and minimum signals is calculated and that at the beginning of a respective
subsequent period of time the signals last stored in the preceding period of time
are erased from the memory.
15. The device of claim 14, wherein the control means is connected to the evaluation
unit and designed to signal to the evaluation unit the beginning and the end of
the predetermined period of time.
16. The device of claim 15, wherein the control means is designed to cause the
evaluation unit to effect integration and to determine the difference between the
maximum and the minimum respectively within the same period of time.
17. The device of claim 16, wherein the evaluation unit is so designed that in
operation of the device at the end of the respective period of time the evaluation
unit outputs a signal which corresponds to the integral of the measurement signal
over the period of time less the product of the duration of the period of time
and the measurement signal minimum of the period of time.
18. The device of claim 17, further comprising:
a respective comparator connected to a reference value memory containing reference
values and/or reference value ranges and which is so designed that in operation
of the device the comparator compares the difference of the extreme values or the
time integral of the measurement signal to a respective reference value or reference
value range contained in the reference value memory and produces an output signal
which indicates whether the respective comparison result corresponds to a deviation
from the reference value or reference value range or not and possibly what deviation
is involved.
19. The device of claim 17, further comprising:
a respective comparator connected to a reference value memory containing fluctuation
reference values and/or fluctuation reference value ranges and which is so designed
that in operation of the device the comparator compares the change in the difference
of the extreme values or the time integral of the measurement signal in relation
to the respectively precedingly determined value to a respective fluctuation reference
value or fluctuation reference value range contained in the reference value memory
and produces an output signal which indicates whether the respective comparison
result corresponds to a deviation from the fluctuation reference value or fluctuation
reference value range or not and possibly what deviation is involved.
20. An implantable electrostimulation device for the treatment of tachycardiac
rhythm disturbances comprising:
a detection unit;
a control unit;
a therapy unit for the treatment of tachycardiac rhythm disturbances adapted
to produce a cardioversion or defibrillation electrotherapy to be applied to the
heart,
wherein the detection unit comprises:
a measuring unit for picking up and reproducing a measurement signal corresponding
to an intracardial impedance at an output thereof; and
an evaluation unit connected at an input to the measuring unit, and
wherein the control unit receives output signals from the evaluation unit and
controls the activity of the measuring unit, the evaluation unit and the therapy
unit, and
wherein the evaluation unit is adapted to determine and output a first signal
which corresponds to the difference of a maximum and a minimum measured signal
within at least one defined period of time, and
wherein the evaluation unit is additionally adapted to determine and output a
second signal which is dependent on the integral of the measurement signal over
at least one defined second period of time, and the first signal associated with
the second period of time, wherein the first-mentioned period of time is before
the second period of time or is identical to the second period of time.
21. The electrostimulation device of claim 20, wherein the therapy unit is so
designed that it can also selectively apply a pacemaker electrostimulation therapy
to the heart.
22. The electrostimulation device of claim 21, further comprising:
a signal pattern memory connected to the control unit and in which one or more
control signals for the measuring unit, the evaluation unit and/or the therapy
unit are associated with signal patterns, that is to say, output signals or combinations
of output signals from the evaluation unit, and that the control unit is so designed
that it compares output signals received from the evaluation unit to the signal
patterns of the signal pattern memory and produces the control signals associated
with the respectively correct signal pattern and transmits same to the corresponding
unit.
23. The electrostimulation device of claim 22, wherein, for determining the control
signals, the control unit additionally or exclusively accesses an assessment algorithm
which is contained in a program memory and by means of which the control signals
to be produced are computed in a computing unit on the basis of the output signals
of the evaluation unit.
24. The electrostimulation device of claim 23, further comprising:
a housing which can be used as an electrode in the measurement of intracardial
impedance.
25. The device of claim 2, wherein the measuring unit comprises at least two
electrodes, of which at least one can be introduced into a chamber of the heart.
26. The device of claim 3, wherein the measuring unit comprises at least two
electrodes, of which at least one can be introduced into a chamber of the heart.
27. The device of claim 25, further comprising:
a current source connected to the electrodes in such a way that it produces a
predetermined measuring current between the electrodes; and
voltage measuring means connected to the electrodes for measuring an electrical
voltage therebetween.
28. The device of claim 26, further comprising:
a current source connected to the electrodes in such a way that it produces a
predetermined measuring current between the electrodes; and
voltage measuring means connected to the electrodes for measuring an electrical
voltage therebetween.
29. The device of claim 25, further comprising:
a voltage source connected to the electrodes in such a way that it produces a
predetermined measuring voltage between the electrodes, and
current measuring means connected to the electrodes for measuring an electrical
current therebetween.
30. The device of claim 26, further comprising:
a voltage source connected to the electrodes in such a way that it produces a
predetermined measuring voltage between the electrodes, and
current measuring means connected to the electrodes for measuring an electrical
current therebetween.
31. The device of claim 6, wherein the voltage source is adapted to output an
ac voltage with a predetermined time dependency.
32. The device of claim 27, wherein the current source is adapted to output an
alternating current with a predetermined time dependency.
33. The device of claim 28, wherein the current source is adapted to output an
alternating current with a predetermined time dependency.
34. The device of claim 29, wherein the voltage source is adapted to output an
ac voltage with a predetermined time dependency.
35. The device of claim 30, wherein the voltage source is adapted to output an
ac voltage with a predetermined time dependency.
36. The device of claim 31, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the current measuring means, which is modulated by the ac
voltage produced by the voltage source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
37. The device of claim 32, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the voltage measuring means, which is modulated by the alternating
current produced by the current source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
38. The device of claim 33, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the voltage measuring means, which is modulated by the alternating
current produced by the current source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
39. The device of claim 34, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the current measuring means, which is modulated by the ac
voltage produced by the voltage source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
40. The device of claim 35, further comprising:
a demodulation device, at an input of which in operation of the device is the
measurement signal of the current measuring means, which is modulated by the ac
voltage produced by the voltage source, the demodulation device being so designed
that a signal can be taken off at an output, which corresponds in its configuration
in respect of time to the envelope of the measurement signal or the positive or
negative half-period of the measurement signal.
41. The device of claim 37, further comprising:
a filter stage for the removal of measurement signal components of a frequency
of up to a maximum of 1 Hz.
42. The device of claim 39, further comprising:
a filter stage for the removal of measurement signal components of a frequency
of up to a maximum of 1 Hz.
43. The device of claim 41, further comprising:
control means adapted to cause the evaluation unit to start or stop integration
of a signal at the input of the evaluation unit.
44. The device of claim 42, further comprising:
control means adapted to cause the evaluation unit to start or stop integration
of a signal at the input of the evaluation unit.
45. The device of claim 43, wherein the control means is so designed to cause
the evaluation unit to effect integration respectively over the period of time
of one or some cardiac periods.
46. The device of claim 44, wherein the control means is so designed to cause
the evaluation unit to effect integration respectively over the period of time
of one or some cardiac periods.
47. The device of claim 11, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
48. The device of claim 43, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
49. The device of claim 44, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
50. The device of claim 45, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
51. The device of claim 46, wherein the control means is designed to cause the
evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the cardiac period duration.
52. The device of claim 50, wherein the evaluation unit has a memory.
53. The device of claim 51, wherein the evaluation unit has a memory.
54. The device of claim 52, wherein the evaluation unit is so designed that within
the predetermined period of time the hitherto maximum and minimum signals at the
input of the evaluation unit are continuously determined and stored in the memory,
that at the end of the period of time the difference between the currently stored
maximum and minimum signals is calculated and that at the beginning of a respective
subsequent period of time the signals last stored in the preceding period of time
are erased from the memory.
55. The device of claim 53, wherein the evaluation unit is so designed that within
the predetermined period of time the hitherto maximum and minimum signals at the
input of the evaluation unit are continuously determined and stored in the memory,
that at the end of the period of time the difference between the currently stored
maximum and minimum signals is calculated and that at the beginning of a respective
subsequent period of time the signals last stored in the preceding period of time
are erased from the memory.
56. The device of claim 54, wherein the control means is connected to the evaluation
unit and designed to signal to the evaluation unit the beginning and the end of
the predetermined period of time.
57. The device of claim 55, wherein the control means is connected to the evaluation
unit and designed to signal to the evaluation unit the beginning and the end of
the predetermined period of time.
58. The device of claim 56, wherein the control means is designed to cause the
evaluation unit to effect integration and to determine the difference between the
maximum and the minimum respectively within the same period of time.
59. The device of claim 57, wherein the control means is designed to cause the
evaluation unit to effect integration and to determine the difference between the
maximum and the minimum respectively within the same period of time.
60. The device of claim 58, wherein the evaluation unit is so designed that in
operation of the device at the end of the respective period of time the evaluation
unit outputs a signal which corresponds to the integral of the measurement signal
over the period of time less the product of the duration of the period of time
and the measurement signal minimum of the period of time.
61. The device of claim 59, wherein the evaluation unit is so designed that in
operation of the device at the end of the respective period of time the evaluation
unit outputs a signal which corresponds to the integral of the measurement signal
over the period of time less the product of the duration of the period of time
and the measurement signal minimum of the period of time.
62. The device of claim 60, further comprising:
a respective comparator connected to a reference value memory containing reference
values and/or reference value ranges and which is so designed that in operation
of the device the comparator compares the difference of the extreme values or the
time integral of the measurement signal to a respective reference value or reference
value range contained in the reference value memory and produces an output signal
which indicates whether the respective comparison result corresponds to a deviation
from the reference value or reference value range or not and possibly what deviation
is involved.
63. The device of claim 61, further comprising:
a respective comparator connected to a reference value memory containing reference
values and/or reference value ranges and which is so designed that in operation
of the device the comparator compares the difference of the extreme values or the
time integral of the measurement signal to a respective reference value or reference
value range contained in the reference value memory and produces an output signal
which indicates whether the respective comparison result corresponds to a deviation
from the reference value or reference value range or not and possibly what deviation
is involved.
64. The device of claim 60, further comprising:
a respective comparator connected to a reference value memory containing fluctuation
reference values and/or fluctuation reference value ranges and which is so designed
that in operation of the device the comparator compares the change in the difference
of the extreme values or the time integral of the measurement signal in relation
to the respectively precedingly determined value to a respective fluctuation reference
value or fluctuation reference value range contained in the reference value memory
and produces an output signal which indicates whether the respective comparison
result corresponds to a deviation from the fluctuation reference value or fluctuation
reference value range or not and possibly what deviation is involved.
65. The device of claim 61, further comprising:
a respective comparator connected to a reference value memory containing fluctuation
reference values and/or fluctuation reference value ranges and which is so designed
that in operation of the device the comparator compares the change in the difference
of the extreme values or the time integral of the measurement signal in relation
to the respectively precedingly determined value to a respective fluctuation reference
value or fluctuation reference value range contained in the reference value memory
and produces an output signal which indicates whether the respective comparison
result corresponds to a deviation from the fluctuation reference value or fluctuation
reference value range or not and possibly what deviation is involved.
66. The electrostimulation device of claim 20, further comprising:
a signal pattern memory connected to the control unit and in which one or more
control signals for the measuring unit, the evaluation unit and/or the therapy
unit are associated with signal patterns, that is to say, output signals or combinations
of output signals from the evaluation unit, and that the control unit is so designed
that it compares output signals received from the evaluation unit to the signal
patterns of the signal pattern memory and produces the control signals associated
with the respectively correct signal pattern and transmits same to the corresponding
unit.
67. The electrostimulation device of claim 22, wherein, for determining the control
signals, the control unit additionally or exclusively accesses an assessment algorithm
which is contained in a program memory and by means of which the control signals
to be produced are computed in a computing unit on the basis of the output signals
of the evaluation unit.
68. The electrostimulation device of claim 23, further comprising:
a housing which can be used as an electrode in the measurement of intracardial
impedance.
69. The electrostimulation device of claim 20, further comprising:
a housing which can be used as an electrode in the measurement of intracardial
impedance.
Description
The invention concerns a device for detecting tachycardiac rhythm disturbances,
comprising a measuring unit for picking up and reproducing a measurement signal
dependent on intracardial impedance at its output and an evaluation unit connected
at the input to the measuring unit.
BACKGROUND OF THE ART
The invention further concerns an implantable electrostimulation device for the
treatment of tachycardiac rhythm disturbances, comprising a detection unit, a control
unit and a therapy unit which is adapted to produce a cardioversion or defibrillation
electrotherapy to be transmitted to the heart, wherein the detection unit has a
measuring unit for picking up and reproducing a measurement signal corresponding
to intracardial impedance at its output and an evaluation unit connected at the
input to the measuring unit and the control unit receives output signals from the
evaluation unit and controls the activity of the measuring unit, the evaluation
unit and the therapy unit.
By virtue of the lower specific resistance of blood in comparison with the myocardium
tissue intracardial impedance varies with the volume of blood in the chambers of
the heart, which changes in the course of the cardiac cycle. Thus, the pump activity
of the heart can be monitored by evaluation of an intracardial impedance measurement
and the existence of cardiac rhythm disturbances such as tachycardia or fibrillation
can be inferred from variations in the amplitude pattern or the frequency of the
periodic impedance signal.
Such a monitoring device is known from European patent application No. 0 009
255 A1, to Geddes, published 2 Apr. 1980. That publication discloses an implantable
defibrillator having two measuring electrodes arranged at an axial spacing from
each other at the distal end of a catheter which is introduced into the right ventricle.
A control logic unit starts an intracardial impedance measurement procedure when
automatic evaluation of an ECG signal indicates the possible existence of fibrillation.
Impedance is determined by means of voltage measurement between the electrodes
with an alternating current flow with a constant modulation amplitude and subsequent
demodulation of the voltage signal. If the amplitude of the impedance signal indicates
excessively low pump activity on the part of the heart defibrillation therapy is initiated.
A disadvantage with that device is that the signals from the measuring electrodes
react sensitively to interference effects by virtue of their arrangement in a ventricle.
Thus, body movement can already give rise to contact between the electrodes and
the myocardium tissue. That means however that the measurement voltage and thus
impedance measurement are falsified. Unnecessary defibrillation shocks which are
painful to the patient can be the consequence of defective impedance measurement.
It is known from U.S. Pat. No. 5,427,112, to Noren, issued 27 Jun. 1995, for
increasing
the reliability of detection of cardiac rhythm disturbances, to record two signals
and to monitor the in-phase periodic recurrence thereof, which is coupled to the
heartbeat. In addition to measurement of the intracardial impedance signal, that
known device also provides for determining the derivative thereof in respect of
time. The impedance signal is recorded in a parameter representation as a function
of its derivative in respect of time. The curve recorded in that way is compared
to stored pattern curves, whereupon a decision is made about the necessity for
and possibly the nature of a therapy. That procedure suffers from the disadvantage
that it is highly costly in terms of computation and memory; it is firstly necessary
to determine the derivative in respect of time of the measurement signal. The measurement
value together with the derivative in respect of time have to be stored over at
least the duration of a cardiac period. Then the phase position of the stored data
has to be determined, in comparison with a previously stored pattern curve, and
that requires extensive computations. Finally it is then necessary to form the
difference of the measured pairs of values and corresponding pairs of values of
the pattern curve, and ultimately evaluate same on the basis of a mathematical criterion.
U.S. Pat. No. 5,179,946, to Weiss, issued 19 Jan. 1993, discloses an implantable
defibrillator in which, to increase the level of reliability of detection of cardiac
rhythm disturbances, the intracardial impedance between two defibrillation electrodes
fixed to the outside of the heart is measured. Adequate pump activity on the part
of the heart is monitored on the basis of the impedance signal, by means of an
amplitude discriminator. As an alternative to amplitude discrimination that known
defibrillator provides for integration of the measured impedance signal. That device
is complicated and expensive in circuitry terms because impedance measurement and
tachycardia therapy are effected by way of the same electrodes. Therefore, avoiding
destruction of the measuring unit by the high electrical voltages which are produced
in a cardioversion or defibrillation procedure requires a protective circuit which
is to be connected between the defibrillation electrodes and the measuring unit
prior to application of the therapy. A disadvantage with that known defibrillator
is also major operative involvement which stresses the patient when implanting
the epicardial electrodes.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a reliable but less complicated
and expensive device for detecting tachycardiac rhythm disturbances and a less
complicated and expensive implantable electrostimulation device for the treatment
of tachycardiac rhythm disturbances.
In regard to an device for detecting tachycardiac rhythm disturbances that object
is attained by the features of claim 1 and in regard to an implantable electrostimulation
device for treating tachycardiac rhythm disturbances that object is attained by
an device having the features of claim 20.
The basic idea of the invention is to increase the reliability of detection of
cardiac rhythm disturbances by determining two signals which supplement each other
in regard to their information content about cardiac activity and in that respect
however at the same time use only such signals, for the determination of which
the computing and memory demands are particularly low.
In the device according to the invention for detecting tachycardiac disturbances,
based on the features of the classifying portion of claim 1, that is attained
in that the evaluation unit is adapted to output both a first signal which corresponds
to the difference of a maximum and a minimum measurement signal within at least
one defined period of time and also a second signal which is dependent on the integral
of the measurement signal over at least one defined period of time.
The defined period of time can extend over a predetermined period of time, for
example 5 seconds, and/or over at least one cardiac cycle. The duration of a cardiac
cycle can be determined for example in an electrocardiogram (ECG) as the period
of time between two identical characteristic points of successive cardiac cycles,
for example the Q-points.
The difference in the extreme values of the measurement signal within a cardiac
cycle, that is to say the peak-to-peak amplitude PPA of the measurement signal
indicates the difference between the intracardial impedance of the systolic and
diastolic heart. During the systole there is a minimum amount of blood in the heart
whereby the measured impedance assumes a maximum value within the cardiac period.
During the diastole the heart contains a maximum amount of blood which causes the
measured impedance to assume a minimum. The PPA is therefore a measurement in respect
of the pump activity of the heart. When a tachycardia occurs the PPA of the intracardial
impedance is significantly reduced by virtue of the reduced pump efficiency of
the heart. In the case of a fibrillating heart the PPA of the intracardial impedance
falls to approximately zero because the volume of blood in the heart is scarcely
variable because of the uncoordinated movement of the heart muscles. In that respect
the fall in the PPA below a predeterminable (patient-dependent) threshold value
is a clear indication of the existence of a tachycardiac rhythm disturbance.
The PPA can be determined in a manner known to the man skilled in the art in
a simple fashion and without complicated computations and without the storage of
relatively large amounts of data.
Information about the pump activity of the heart is also collected by
virtue of additionally determining, in accordance with the invention, a second
signal which is dependent on the integral of the measurement signal over at least
one defined period of time. When a tachycardia or defibrillation exists that second
signal, by virtue of the worsened pump activity of the heart, assumes different
values from the case involving a heart which is beating normally.
Integration of the measurement signal can also be implemented in per
se known manner in a simple fashion and without involving memory complication and
expenditure by virtue of the evaluation unit of the device according to the invention.
In addition it is advantageous that short-term deflections of the measurement
signal have a substantially lesser effect on the second signal than on the PPA.
Such deflections occur for example upon a variation in the position of measuring
electrodes relative to the vessel walls of the heart. They can falsify the PPA
by virtue of the fact that such a random, unusually high or low measurement signal
forms the maximum or minimum during the measurement time interval, under some circumstances
even a plurality of measurement time intervals in succession. Thus the first signal
can incorrectly indicate the existence of a cardiac arrhythmia which does not exist.
However, because of the relatively long period of time over which integration is
effected such a deflection has an only slight influence on the integral of the
measurement signal. In that case therefore the second signal will correctly indicate
normal cardiac activity. That avoids unnecessary administration of a cardioversion
or defibrillation therapy.
In that respect the items of information which the first and second signals furnish
about cardiac activity supplement each other and provide for a high level of reliability
in the detection of cardiac rhythm disturbances by the device according to the invention.
The first and the second signal can be determined by evaluation of the measurement
signal within one or more periods of time. For example it is possible to briefly
interrupt and then continue evaluation of the measurement signal. It is also possible
to average the first or second signal which is determined over a plurality of periods
of time.
The first and second signals however do not always have to be determined at the
same time. For example it can also be provided that it is only when the first signal
provides an indication of the existence of an arrhythmia that the additional determination
of the second signal is implemented in a subsequent measurement step.
The duration of one or more cardiac cycles is desirable as the evaluation period
of time. In that case, the operation of determining the start and stop times of
the evaluation procedure relative to the cardiac cycle can be effected with per
se known means. It is important that the device can alternatively evaluate over
predetermined periods of time which are independent of the cardiac cycle as for
example in a fibrillation situation triggering relative to the cardiac cycle cannot function.
The measures in accordance with the invention ensure on the one hand that electrostimulation
therapy is also administered when there is an acute requirement. At the same time
the invention avoids therapy which is unpleasant for the patient being unnecessarily
administered solely by virtue of fluctuations in the measurement signal. The evaluation
of two measurement signals which supplement each other in terms of their significance
therefore affords a substantial increase in the level of reliability in the detection
of tachycardiac rhythm disturbances. On the other hand both measurement signals
can be easily determined; the evaluation thereof does not require any major computing
expenditure. Simple comparison with reference values or reference value ranges
already indicates the current condition of the heart.
In a preferred embodiment of the invention impedance measurement can be carried
out in a unipolar mode. In unipolar impedance measurement, besides the measuring
electrode which is introduced for example into the right ventricle, the implanted
housing of the device is used at the same time as the second electrode. By virtue
of the relatively large spacing between those electrodes, the unipolar impedance
signal involves not just the information about the volume of blood in the heart.
Rather for example information about the respiration rate is also included in the
unipolar impedance signal. That can be used separately for ascertaining physiological
parameters for example in the context of a pacemaker therapy. If it is only information
about the pump activity of the heart that is to be processed, the low measurement
signal frequencies occurring due to respiration (max. 1 Hz) can be easily separated
from the higher-frequency component of the cardiac activity by suitable filtering
of the signal.
Alternatively impedance measurement in the device according to the
invention can also be implemented in a bipolar mode. In this case a measuring probe
which is introduced into the heart will have two electrically mutually insulated
electrodes. The flow of current induced in the measurement procedure is effected
through the blood in the ventricle. The housing of the device does not play any
part here.
In both cases the measuring unit has at least two electrodes of which at least
one can be introduced into a chamber of the heart. In the case of unipolar measurement
the second electrode is formed by an implantable housing of the device. In the
case of bipolar measurement both electrodes are arranged directly in the region
of the heart. In that case the second electrode can be arranged outside the heart,
for example at the outside of the heart, or it can also be introduced into a chamber
of the heart, but not necessarily into the same chamber as the first electrode.
The essential point is that the flow of current in impedance measurement goes through
a volume of blood within one or more chambers of the heart, that volume varying
in the course of the cardiac period.
Preferably intracardial impedance can be determined in unipolar impedance
measurement and also in bipolar impedance measurement by measurement of an electrical
voltage between the electrodes when they are subjected to a preadjustable electrical
current. Therefore the operation of determining impedance is effected on the basis
of the preset current value and the measured voltage value in accordance with Ohm's
law. The measuring unit accordingly has a current source. It is connected to the
electrodes of the measuring unit in such a way that it produces a predetermined
measuring current between them. The measuring current is, preferably of a pulsed
nature, as will be described in greater detail hereinafter. In this embodiment
the device also has voltage measuring means which are connected to the electrodes
to measure an electrical voltage between the electrodes. In this embodiment impedance
can be determined by division of the measurement signal by the current strength.
The amount of energy E(T) applied for impedance measurement during a predetermined
period of time T, with a constant current I, can be calculated easily from the
time integral, which is determined in any case, of the voltage U:
##EQU1##
The energy determined in that way can be evaluated as an additional information source.
Alternatively, in the device according to the invention, intracardial
impedance can be determined by measurement of a flow of current between the electrodes
when subjected to a preadjustable electrical voltage. For that purpose the measuring
unit has a voltage source which is connected to the electrodes of the measuring
unit in such a way that it produces a predetermined measuring voltage between them,
and current measuring means which are connected to the electrodes to measure an
electrical current between the electrodes.
In an embodiment of the invention, to avoid polarization of the electrode, the
applied constant current strength or voltage can be modulated with a predeterminable
time dependency in such a way that a period alternating current flows or a periodic
ac voltage is applied, the maximum current strength of which or the maximum voltage
amplitude of which is of the same value in each half-period. The pulse shape and
frequency can be predetermined. Preferably an alternating current involving a square-wave
pulse shape is used. In an embodiment, bipolar square-wave pulses with a time spacing
of about 50 milliseconds are used. The positive and the immediately adjoining negative
half-periods of the square-wave pulses each involve a duration of about 1 microsecond.
By virtue of the differing frequency dependency of the impedance of the blood
and the myocardium tissue, the impedance contrast between systole and diastole
can also be increased by a suitable choice of frequency in order to make evaluation
of the PPA still more reliable, in terms of detecting cardiac arrhythmia. At a
frequency of 4096 Hz approximately the specific resistance of blood is only a third
of the specific resistance of the myocardium tissue.
In this embodiment the modulation of the measurement signal, which is caused
by
the frequency impressed on the measurement current or the measurement voltage respectively
is preferably removed by a demodulation stage connected upstream of the time integration
step. In operation of the device applied to the input of the demodulation stage
is the measurement signal of the voltage measuring means (or current measuring
means), which is modulated by the alternating current (or ac voltage) produced
by the current source (or voltage source). The demodulation stage is designed in
such a way that it is possible to take off at the output thereof a signal which
corresponds in its configuration in respect of time to the envelope of the measurement
signal or the positive or negative half-period of the measurement signal.
A further embodiment of the invention has control means which are adapted to
cause
the evaluation unit to start or stop integration of a signal at the input of the
evaluation unit. The control means are preferably so designed that they cause the
evaluation unit to effect integration over the respective period of time of one
or some cardiac periods. For that purpose it is possible to have recourse to known
trigger methods. In the case of a normally beating heart integration is started
at a given phase point in the cardiac period so that normal, non-pathological frequency
changes are not crucial in determining the integral of the measurement signal.
The integration duration, that is to say the predetermined period of time T, is
therefore adapted variably within certain limits to the cardiac frequency.
The control means however is preferably additionally so designed that it can
cause the evaluation unit to effect integration over at least one respective predeterminable
period of time which is independent of the duration of the cardiac period. That
is required in the case of a fibrillating heart as triggering here is not possible.
If triggering fails therefore, that is to be assessed as a first indication of
arrhythmia. The device reacts to that situation by switching over to a predetermined
fixed period of time for the integration operation. A new impedance measurement
procedure is then started in order to check the condition of the heart.
In a further embodiment of the invention the evaluation unit has a memory. The
evaluation unit is further so designed that, within the predetermined period of
time, the hitherto maximum and minimum signals applied to the input of the evaluation
unit are continuously determined and stored in the memory, that at the end of the
period of time the difference between the currently stored maximum and minimum
signals is calculated and that at the beginning of a respective subsequent period
of time the signals last stored in the preceding period of time are erased from
the memory. The memory therefore serves here only for temporarily receiving the
values which are currently determined as the maximum and the minimum. They are
overwritten as soon as a fresh maximum or minimum of the measurement signal is
established within the measurement time interval. The operation of determining
the extreme values can be re-started with each measurement period.
The period of time for which the operation of determining the PPA is executed
is preferably also established by control means which are connected to the evaluation
unit and which are so designed that they signal the evaluation unit the beginning
and the end of the predetermined period of time.
In a particularly advantageous embodiment time control in respect of determining
the first and second signals is effected centrally. The control means are correspondingly
so designed that they cause the evaluation unit to effect integration and to determine
the difference between the maximum and the minimum in each case within the same
period of time. In that way, with the first and second signals, there are two partially
complementary items of information about cardiac activity within the same period
of time. Arrhythmia can be reliably and quickly diagnosed.
In a preferred embodiment the evaluation unit is so designed that, in operation
of the device, at the end of the respective period of time, it outputs such a second
signal which corresponds to the integral of the measurement signal over the period
of time less the product of the duration of the period of time and the measurement
signal minimum of the period of time.
In that way the significance of the second signal can be increased. That will
be immediately apparent if it is considered that intracardial impedance of a fibrillating
heart is approximately constant, because of the volume of blood which is little
variable, and it assumes a value which is lower than the systolic impedance value
but markedly higher than the diastolic impedance value of the heart. The time integral
ZI of the intracardial impedance of the fibrillating heart will therefore differ
only slightly from that of the normally beating heart over the same period of time T.
In order to increase significance therefore integration is effected over the
difference
between the current impedance value and a minimum impedance value Z
0(T)
which is individual for each integration period T. That value Z
0(T)
is in any case constantly fixed in the context of determining the PPA. That minimum
impedance value Z
0 corresponds in the normal situation to the diastolic
impedance signal. If therefore the intracardial impedance is denoted by Z, the
integration interval by T and the second signal by ZI(T), then ZI(T) is given by:
##EQU2##
For the sake of simplicity, without any loss in terms of mathematical accuracy,
the differencing operation is effected downstream of integration and uses the minimum
extreme value Z
0(T) of intracardial impedance Z, which is fixed in the
integration period T for determining the PPA.
In the case of a fibrillating heart the difference between Z
o(T) and
Z(t) is so slight that ZI(T) assumes a markedly lower value than in the case of
a normally beating heart. In the condition of tachycardia it is admittedly possible
to detect greater modulation of intracardial impedance than in the case of fibrillation,
but the value of ZI(T) is still always markedly lower than in the case of a normally
beating heart.
A further embodiment of the invention, for evaluation of the PPA and ZI signals,
has a respective comparator which is connected to a first memory for reference
values and/or reference value ranges and which is so designed that it compares
the difference between successive extreme values or the time integral of the measurement
signal to a respective reference value or reference value range contained in the
first memory and produces an output signal which indicates whether the respective
comparison result corresponds to a deviation from the reference value or reference
value range or not, and possibly what deviation is involved. The two output signals
of the comparators therefore indicate to a downstream-connected processing unit
whether the respective signal is in the standard range or whether it deviates from
the standard range. In the event of a deviation the amount by which and the direction
in which the measurement signal deviates from the respective standard range can
also be found from those output signals.
The reference values or reference value ranges which are stored in the first
memory can be adaptive, that is to say variable, for example in accordance with
the current physical activity, for further enhancing the reliability of detection
of tachycardiac rhythm disturbances. For that purpose the memory content is influenced
by a processing unit which analyzes the impedance signal by means of an algorithm,
for example a "Regional Effective Slope Quality" (RQ-) algorithm (see Max Schaldach,
Electrotherapy of the Heart, Springer Verdag, Berlin, Heidelberg, New York, 1992,
pages 114 ff). For that purpose however the processing unit can also be adapted
for processing further or other known signals which reflect physical activity.
The above-described advantages and features of the detection device according
to the invention are of great benefit in an implantable electrostimulation device
for the treatment of tachycardiac rhythm disturbances.
Particularly desirable is the integration of pacemaker functions into
the therapy unit of the electrostimulation device, in addition to cardioversion/defibrillation.
For that purpose the therapy unit of the electrostimulation device is so designed
that it can optionally also apply a pacemaker electrostimulation therapy to the
heart. The pacemaker therapy can be applied by way of the same electrode with which
impedance is also measured. In that case no impedance measurement is effected during
the duration of the stimulation pulse. The control unit implements the time control
of the measuring unit and the therapy unit, which is required for that purpose.
A preferred embodiment of the electrostimulation device according to the invention
additionally has a signal pattern memory which is connected to the control unit
and in which one or more control signals for the measuring unit, the evaluation
unit and/or the therapy unit are associated with signal patterns, that is to say
output signals or combinations of output signals from the evaluation unit. The
control unit is so designed that it compares output signals received from the evaluation
unit to the signal patterns of the signal pattern memory and produces control signals
associated with the respectively correct signal pattern and transmits them to the
corresponding unit.
For example the control unit receives output signals from the evaluation unit,
which indicate that the PPA value is 10% below the associated reference value range
while the ZI value does not deviate from the reference value. The control unit
compares that signal pattern to those stored in the signal pattern memory and recognizes
from the entry which is correct in that case that a first control signal is to
be sent to the measuring unit for again executing the operation of determining
the PPA and ZI values, and a second control signal is to be sent to the therapy
unit for continuing normal pacemaker therapy with the parameters applied hitherto.
The control unit executes those steps.
Alternatively or supplemental to control of the electrostimulation
device on the basis of the output signals of the evaluation unit by means of the
signal pattern memory, the control unit can also access one or more assessment
algorithms contained in a program memory in order to assess the output signals
of the evaluation unit and to determine and produce the necessary control signals.
A computing unit is provided for executing the assessment algorithm.
BRIEF DESCRIPTION OF THE FIGURE
Further advantages of the invention will be apparent from the description
hereinafter of an embodiment with reference to the drawing.
FIG. 1 shows a block diagram of an electrostimulation device according to the
invention for treating tachycardiac rhythm disturbances with integrated pacemaker functions.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The stimulation device
10 shown in FIG. 1 involves an implantable integrated
pacemaker and cardioverter/defibrillator which is used for the stimulation of a
human heart
12.
For impedance measurement a measuring electrode
14 is introduced through
the right atrium
16 into the right ventricle
18 of the heart
12.
In the region of its distal end the measuring electrode
14 has an electrode
20 at which, in the impedance measurement procedure, an electrical measuring
current issues into the right ventricle
18 or the blood contained therein.
The measuring current is an electrical alternating current of square-wave pulse
shape and with the same value of current strength in both half-periods. The measuring
current is produced within an assembly
22 in a measuring unit
23
by a current source
24 using a modulator
26. The function of the
current source
24 is controlled by a central control
28 and synchronized
with other functional procedures. For example the measuring electrode
20
can be used at the same time for stimulation of the right ventricle in the context
of pacemaker therapy. Impedance measurement is then stopped by the control
28,
for the duration of the stimulation pulse.
Impedance measurement is effected in a unipolar mode. Therefore the electrical
voltage between the electrode
20 and the housing
36 of the assembly
22
