A cardiac pacemaker, whose stimulation rate is controlled dependent on an impedance signal acquired between two electrodes, has one of the two electrodes disposed in an atrial electrode catheter and the other electrode being disposed in a separate, ventricular electrode catheter. Catheters carrying only a single electrode can thus be used, thereby avoiding the use of a bipolar electrode catheter. The two electrodes are respectively connected to two different stimulation pulse generators within the pacemaker housing, constructed as a dual chamber pacemaker.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a rate-adaptive cardiac pacemaker for 
generating stimulation pulses at a rate correlated to the physical 
activity of the person in whom the pacemaker is implanted, of the type 
having an electrode arrangement with at least two electrodes disposed in 
the heart, and impedance measuring means for acquiring the electrical 
impedance between the two electrodes and a control means for controlling 
the pulse rate dependent on the acquired impedance signal. 
2. Description of the Prior Art 
A rate-adaptive cardiac pacemaker, wherein the stimulation pulse rate is 
controlled on the basis of an impedance measurement made between two 
electrodes disposed in the heart, is disclosed in European Application 0 
327 292. The stimulation pulse generator and the impedance measuring means 
are both connected, through controllable switches, to two electrodes which 
are spaced from each other within an electrode catheter which is 
introduced into the heart of the patient. The impedance between the two 
electrodes, which changes dependent both on cardiac activity and on the 
respiration of the patient, is acquired by the impedance measuring means 
in the pauses between the stimulation pulses. The frequency components 
correlated with respiration are filtered out of the impedance signal, and 
the breadth-per-minute volume is identified therefrom, which is used for 
controlling the stimulation rate of the pulse generator. 
In a similar cardiac pacemaker disclosed in U.S. Pat. No. 4,702,253, three 
electrodes are arranged within a single electrode catheter, with the 
stimulation pulses being generated between a first of these three 
electrodes and a reference electrode formed by the housing of the 
pacemaker, a constant test current for measuring impedance is generated 
between a second of the three electrodes and the reference electrode, and 
the voltage drop produced in the heart by the test current is measured as 
impedance signal between the third electrode and the reference electrode. 
Both of these known cardiac pacemakers necessarily employ electrode 
catheters having two (bipolar) or more electrodes. In contrast to unipolar 
electrode catheters having only one electrode, such multiple electrode 
catheters are comparatively thick and inflexible. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a rate-adaptive cardiac 
pacemaker which undertakes an impedance measurement as the basis for 
controlling the stimulation pulse rate in a dual chamber cardiac pacemaker 
without the use of electrode catheters having multiple poles. 
The above object is achieved in accordance with the principles of the 
present invention in a rate-adaptive pacemaker having impedance measuring 
means connected to two electrodes, one electrode being disposed in a 
first, atrial electrode catheter and the other electrode being disposed in 
a second, ventricular electrode catheter. The two electrodes are connected 
to two different stimulation pulse generators within cardiac pacemaker 
housing, which is constructed as a dual chamber pacemaker. Measurement of 
the impedance ensues using the two electrode catheters, which are already 
present in a conventional dual chamber pacemaker, so that no special or 
extra electrode catheter having two or more poles is necessary for the 
impedance measurement. 
The aforementioned two known rate adaptive pacemakers are both single 
chamber pacemakers, are thus constructed only for the connection of a 
single electrode catheter. By contrast, in a dual chamber pacemaker, 
terminals for two electrode catheters are already provided, so that 
modification of the electrode connection configuration of a conventional 
dual chamber cardiac pacemaker is not required for achieving the impedance 
measurement of the invention. In accordance with the principles of the 
invention, means are provided within the pacemaker housing (which are not 
present in a conventional dual chamber pacemaker), for undertaking an 
impedance measurement employing the two-separately catheterized 
electrodes. 
In one embodiment of the invention, the impedance measuring means includes 
a current or voltage source for generating a test current or a test 
voltage between an output terminal and a reference potential terminal 
formed by the housing of the pacemaker, and the atrial electrode, through 
a first, controllable switch arrangement, and the ventricular electrode, 
through a second, controllable switch arrangement, are selectively 
connectable to the output terminal or to the reference potential terminal. 
This permits the test current or the test voltage to be optionally 
generated between both electrodes or between one of the two electrodes and 
the pacemaker housing. 
In another embodiment of the invention, the impedance measuring means 
includes a measuring amplifier having two input terminals, one input 
terminal being selectively connectable through a third, (i.e., different 
from the aforementioned first and second) controllable switch arrangement 
to the atrial electrode or to the ventricular electrode, and the other 
input terminal is selectively connectable through a fourth, controllable 
switch arrangement to one of the two electrodes or to the reference 
potential terminal. This permits the impedance signal to be selectively 
acquired between the two electrodes or between one of the two electrodes 
and the reference potential terminal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A cardiac pacemaker 1 in the form of a dual chamber cardiac pacemaker is 
shown in FIG. 1. The pacemaker 1 includes a first, atrial electrode 
catheter 2, which terminates in a first electrode 5 at its distal end, 
which is placed in the right atrium 3 of the heart 4 of a patient in whom 
the pacemaker 1 is implanted. The pacemaker 1 also includes a second, 
ventricular electrode catheter which terminates in a second electrode 7 at 
its distal end, which is introduced into the right ventricle 6 of the 
heart 4. 
The details of the pacemaker constructed in accordance with the principles 
of the present invention are shown in block circuit form in FIG. 2. The 
atrial electrode 5 is connected to a first output terminal 10 of a 
stimulation pulse generator 11 through a controllable switch 9. The 
stimulation pulse generator 11 has a second output terminal 12 which is 
electrically connected to the housing 13 of the pacemaker 1, so that the 
second output terminal 12 and the housing 13 are at the same electrical 
potential. A detector 14 for detecting atrial events has a first input 
terminal 15 connected to the output terminal 10 of the stimulation pulse 
generator 11, and has a second input terminal 16 connected to the 
pacemaker housing 13. 
The ventricular electrode 7 is connected through a second controllable 
switch 17 to an output terminal 18 of a second stimulation pulse generator 
19. The second stimulation pulse generator 19 has a second output terminal 
20 connected to the pacemaker housing 13. A further detector 21 for 
detecting ventricular events has a first input terminal 22 connected to 
the output terminal 18 of the second stimulation pulse generator 19, and 
has a second input terminal 23 connected to the pacemaker housing 13. The 
two stimulation pulse generators 11 and 19 and the two detectors 14 and 21 
are connected to pacemaker control circuitry 24, which starts a base time 
interval after each stimulated event or detected natural event in the 
atrium 3, and initiates the output of a stimulation pulse in the atrium by 
the atrial stimulation pulse generator 11 if the base time interval 
expires without a natural atrial event having been detected by the atrial 
detector 11. After each stimulated or detected natural event in the 
atrium, an atrial-ventricular time interval is started, and the output of 
a stimulation pulse to the ventricle 8 is triggered if the 
atrial-ventricular time interval expires without a natural event in the 
ventricle 8 having been detected by the detector 21. 
An impedance measuring circuit 27, for measuring the impedance of the body 
tissue between the electrodes 5 and 7, and possibly between one of those 
electrodes and the pacemaker housing 13, is also connected to the atrial 
electrode 5 and to the ventricular electrode 7 through two further 
switches 25 and 26. The measured impedance represents a physiological 
function parameter which changes dependent both on the respiration of the 
patient and on the cardiac activity of the patient. The impedance signal 
from the impedance measuring circuit 27 is supplied through an output line 
28 to the pacemaker control circuitry 24, wherein the base time interval 
is modified, after filtering of the impedance signal, dependent on the 
respiration of the patient, for example the breadth-per-minute volume, or 
dependent on the cardiac activity, for example changes in the cardiac 
volume per unit of time. 
An exemplary embodiment of the impedance measuring circuit 27 is shown in 
FIG. 3, which includes a current source 29 and a measuring amplifier 30. 
The current source 29 is supplied by the supply voltage of a battery (not 
shown) of the pacemaker 1 tapped between a battery potential terminal 
V.sub.DD and a reference potential terminal 31 connected to the pacemaker 
housing 13. The current source 29 supplies an output in the form of 
current having a constant amplitude at its output terminal 32. As used 
herein, "constant amplitude" encompasses a pulsed current wherein the 
pulses are all of the same amplitude, and in fact the output of the 
current source 29 is preferably pulsed. 
The atrial electrode 5 is connected to the output terminal 32 of the 
current source 29 and to the reference potential terminal 31 through a 
first, controllable switch arrangement composed of a switch 33 and a 
switch 34. Correspondingly, the ventricular electrodes 7 is connected to 
the output terminal 32 of the current source 29 and to the reference 
potential terminal 31 through a second, controllable switch arrangement 
composed of switches 35 and 36. Dependent on which of the switches 33 
through 36 is closed, the current output from the current source 29 flows 
through the body impedance between the two electrodes 5 and 7, or between 
one of the two electrodes 5 or 7 and the pacemaker housing 13. 
The measuring amplifier 30 has two input terminals 37 and 38; the input 
terminal 37 being selectively connectable to the atrial electrode 5 or to 
the ventricular electrode 7 through a third, controllable switch 
arrangement composed of the switches 39 and 40. The other input terminal 
38 of the measuring amplifier 30 is selectively connectable to the 
ventricular electrode 7 or to the reference potential terminal 31 through 
a fourth, controllable switch arrangement composed of switches 41 and 42. 
As an alternative to the specifically shown exemplary embodiment of FIG. 3, 
it is possible that the second input terminal 38 be connectable through 
the switch 41 to the atrial electrode 5, instead of to the ventricular 
electrode 7. 
Dependent upon the respective states of the switches 39 through 42, the 
input side of the measuring amplifier 30 will be connected to the 
electrodes 5 and 7, or to one of the electrodes 5 and 7 and to the 
reference potential terminal 31. If all of the switches 33 through 36 and 
39 through 42 are open, the current source 29 and the measuring amplifier 
30 are uncoupled from the electrodes 5 and 7 and from the reference 
potential terminal 31, so that the stimulation pulse generators 11 and 19 
and the detectors 14 and 21, and the measuring impedance circuit 27 cannot 
influence one another. 
In particular, it should be noted that although the invention is 
illustrated in the exemplary embodiments as comprising an atrial electrode 
and a ventricular electrode carried on an atrial and a ventricular 
electrode catheter, respectively, it is also contemplated that the atrial 
and ventricular electrodes could be arranged on a common single catheter 
for inserting the atrial electrode in an atrial position and the 
ventricular electrode in a ventricular position. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.