A self-calibrating adaptive-rate cardiac pacemaker having a measuring and processing device for measuring the course over time of a physiological variable over a predetermined portion of a heart cycle and for obtaining a course parameter from the course over time, a stimulation parameter determining device downstream of the measuring and processing device and controlled by a sequence controller for determining a stimulation parameter value, in particular the adaptive stimulation rate, a body sensor connected to one input of the stimulation parameter determining device, for detecting an exertion variable, whose signal is used jointly with the course parameter to determine the stimulation parameter value, and a stimulator unit for generating and outputting stimulation pulses at the determined stimulation parameter value. The measuring and processing device is arranged to obtain a plurality of different values of the course parameter as a function of at least one setting point in a calibration phase, and the stimulation parameter determining device has a stimulation parameter calculating device connected downstream of the measuring and processing device for calculating a plurality of stimulation parameter selection values, each as a function of one value for the course parameter, a reference parameter calculating device connected downstream of the body sensor, for calculating an activity -determined reference parameter value, a comparator unit connected on the input side to the stimulation parameter calculating device and to the reference parameter calculating device, for comparing the parameter selection values with the reference parameter value, and a decision-making device connected on the input side to the output of the comparator unit, for selecting one of the setting point values of the measuring and processing device, associated with the parameter selection values, as current setting point value for further control of the pacemaker, substantially on the basis of the course parameter as the outcome of the comparison.

CROSS-REFERENCE TO RELATED APPLICATION 
This application claims the priority of German Patent Application No. 198 
04 843.2 filed Jan. 29, 1998, the subject matter of which is incorporated 
herein by reference. 
FIELD OF THE INVENTION 
The present invention relates to a self-calibrating adaptive-rate cardiac 
pacemaker having a measuring and processing device for measuring the 
course over time of a physiological variable over a predetermined portion 
of a heart cycle and for obtaining a course parameter from the course over 
time, a stimulation parameter determining device arranged downstream of 
the measuring and processing device and controlled by a sequence 
controller for determining a stimulation parameter value, in particular 
the adaptive stimulation rate, a body sensor connected to one input of the 
stimulation parameter determining device, for detecting an exertion 
variable, whose signal is used jointly with the course parameter to 
determining the stimulation parameter value, and a stimulator unit for 
generating and outputting stimulation pulses at the determined stimulation 
parameter value. 
BACKGROUND OF THE INVENTION 
Adaptive-rate cardiac pacemakers in which the stimulation frequency or rate 
is set as a function of signals picked up in the body of the patient that 
reflect the physiological requirements of the patient with regard to 
cardiac activity have been known and been in clinical use for years in 
many versions. A survey of the goals pursued in the development of rate 
adaptation in pacemaker technology and the relevant paths taken is given 
in K. Stangl et al, Frequenzadaptive Herzschrittmacher [Adaptive-Frequency 
Cardiac Pacemakers], Darmstadt, 1990. 
Many arrangements for measuring impedance in the area of the chest or the 
heart to obtain an impedance signal for adaptive-rate cardiac pacemakers 
are also known, and thus the technology of intracardial impedance 
measurement per se is familiar to one skilled in the art. Most of these 
arrangements are designed to obtain a signal that represents the tidal 
volume or the cardiac output as an expression of the patient's physical 
exertion level and as an actual rate control parameter; for this aspect, 
see for instance European Patent Disclosures EP 0 151 689 B1 and EP 0 249 
818, or German Patent Disclosure DE 42 31 601 A1. 
The so-called ResQ method (for Regional Effective Slope Quality) is also 
known (Max Schaldach, Electrotherapy of the Heart, First Edition, 
Springer-Verlag, pp. 114 ff.), in which the course over time of 
intracardial impedance is used to determine the physiologically 
appropriate adaptive heart rate. 
This method is based on the recognition that the intracardial impedance has 
an especially significant dependency on the exertion level of the organism 
within a certain time slot after a QRS complex--the so-called "region of 
interest" or ROI. 
The slope of the impedance curve in the ROI is therefore determined, and 
the difference between the slope of a resting or reference curve and the 
slope of the currently measured impedance curve (exertion curve) is 
calculated. The adaptive heart rate is set as a function of this 
difference. The association of the calculated slope difference with the 
heart rate to be set is done here as well by means of a characteristic 
curve. Since this curve differs for different people and is dependent on 
the physical condition, the cardiac pacemaker must be calibrated 
individually for each patient, and the calibration must be repeated if the 
patient's health and capacity for exertion changes, or if his life 
circumstances change, and in that case then the location of the ROI must 
be checked as well. 
Various suggestions for self-adaptation (autocalibration) of adaptive-rate 
cardiac pacemakers are found in European Patent Disclosure EP 0 325 851 
A2, U.S. Pat. No. 5,074,302, or European Patent Disclosure EP 0 654 285 
A2. U.S. Pat. No. 5,303,702 shows a trend calculation for the 
self-adaptation of a pacemaker controlled on the basis of the cardiac 
output. 
International Patent Disclosure WO 93/20889 shows a two-sensor arrangement 
with one circuit for detecting the cardiac output and with an additional 
activity sensor, in which the stimulation rate is determined as a function 
of the target rates that can be derived for the individual sensors. 
European Patent Application EP 97 250 057.3 shows an impedance-controlled 
pacemaker is proposed, which is capable of making do without a 
patient-specific calibration and adapts automatically to altered 
peripheral conditions. In this pacemaker, the time integral of the 
impedance over a predetermined portion of the heart cycle is determined as 
a primary impedance variable and used for rate adaptation. 
OBJECT AND SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a cardiac pacemaker of 
the above type that functions reliably in self-calibrating fashion, at 
acceptable expense for calculation and acceptable current consumption. 
This object is achieved by a cardiac pacemaker of the above type whereas 
the measuring and processing device is arranged to obtain a plurality of 
different values of the course parameter as a function of at least one 
setting point in a calibration phase and wherein the stimulation parameter 
determining device has a stimulation parameter calculating device 
connected downstream of the measuring and processing device, for 
calculating a plurality of stimulation parameter selection values each as 
a function of one value for the course parameter, a reference parameter 
calculating device connected downstream of the body sensor, for 
calculating an activity determined reference parameter value, a comparator 
unit connected on the input side to the stimulation parameter calculating 
device and to the reference parameter calculating device, for comparing 
the parameter selection values with the reference parameter value, and a 
decision-making device connected on the input side to the output of the 
comparator unit, for selecting one of the setting point values of the 
measuring and processing device, associated with the parameter selection 
values, as a current setting point valued for further control of the 
pacemaker, substantially on the basis of the course parameter as the 
outcome of the comparison. 
The invention encompasses the concept of utilizing the course over time of 
the relevant physiological variable--especially the intracardial 
impedance--during a calibration phase to derive a family of model 
stimulation parameter curves, from which, by comparison with a stimulation 
parameter curve obtained independently of the detection of the 
aforementioned parameter value (impedance) the most suitable is chosen for 
further pacemaker control. As a rule, this will be a curve based on the 
aforementioned various whose course correlates most strongly with the 
curve ascertained independently of this variable. 
The evaluation of impedance measurements is then done in accordance with 
the ResQ or slope method sketched above, in such a way that at every 
instant, n stimulation rate values are calculated on the basis of n 
different pairs of scanning times along the impedance curve. The time 
dependency of the rate values pertaining to a fixed pair of scanning times 
in each case forms a model stimulation rate curve from which a selection 
can be made. 
In a preferred version, the cardiac pacemaker of the invention evaluates 
the intracardial impedance, in particular the right-ventricular impedance 
measured in unipolar fashion, over a wide range, which typically includes 
the ROI regions set for individual patients. The pairs of scanning times 
are also preprogrammed in such a way that they will be certain to contain 
the "optimal" pair for the applicable patient. Defining them, however, 
does not require patient-specific programming after implantation; instead, 
they are preferably stored in memory in a read only memory when the 
pacemaker is manufactured. 
The characteristic curve that determines the dependency of the stimulation 
rate on the impedance variable and that is an essential operating 
parameter of the rate determining device is not necessarily static but can 
instead be optimized continuously or at certain time intervals, in order 
in particular to attain an adaptation in the range of variation of the 
impedance variable to the allowable variation range of the heart rate. 
In this version, the variation range is not known at the onset of operation 
but instead is ascertained during operation by ongoing measurement of the 
impedance and then optimized after each measurement. At the onset of 
operation, an estimated value for the lower and upper limit values of the 
variation range is specified as a starting value. 
In the setting, two cases can then be distinguished: First, a measured 
value for the impedance can exceed the previously ascertained variation 
range either at the top or the bottom. In that case, the variation range 
is widened accordingly and thus updated. The time constant of this 
adaptation operation is preferably on the order of magnitude of a few 
second, to achieve fast adaptation and to prevent a heart rate 
that--however briefly--is excessively high. Second, the case can arise 
where the impedance, over a relatively long period of time, no longer 
fully exhausts the previously determined variation range. In that case, 
the variation range can be reduced again slowly (with a time constant 
preferably on the order of magnitude of weeks). The stimulation 
rate/impedance characteristic curve assigns the base rate to the lower 
limit value for the variation range of the activity variable and 
corresponding assigns the maximum stimulation rate to the upper limit 
value of the activity variable. By varying these limit values during the 
optimization, the characteristic curve itself consequently changes as 
well. 
Advantageous refinements of the invention are also defined by the dependent 
claims and will be described in further detail below, along with the 
description of a preferred embodiment of the invention, in conjunction 
with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As the exemplary embodiment of the invention, FIG. 1 shows an adaptive-rate 
cardiac pacemaker 100 in the form of a fragmentary block circuit function 
diagram, in which only the components important to describing the 
invention are shown. The layout otherwise is like the known cardiac 
pacemaker. 
The cardiac pacemaker 100 has a unipolar pacemaker and measuring electrode 
101a in the right ventricle of the heart H for detecting heart signals, 
measuring the right-ventricular intracardial impedance Z, and as a 
stimulation electrode. A housing electrode 101b acts as a 
counterelectrode. 
The impedance measurement is effected in a manner known per se in clocked 
form, by imposing a measurement voltage on the electrode 101a via a 
measurement voltage generator 102 and by measuring the current I.sub.z 
(t.sub.m) between the measurement electrode 101a and the counterelectrode 
101b at m equidistant preprogrammed times t.sub.m via a current measuring 
circuit 103. The values Z(t.sub.m) characterizing the course over time of 
the impedance result, at the output of an impedance calculation stage 104 
downstream of the current measuring circuit 103, as quotients of the 
(fixedly preset) measurement voltage and the measured current values. 
In an RQ calculation stage 105, which is connected via a control input to a 
scanning time memory 106, a selection quantity of n rate control parameter 
values RQ.sub.i for each detected impedance course over time Z(t.sub.m) is 
calculated, by dividing the difference (Z(t.sub.s1)-Z(t.sub.s2)) of the 
impedance values for each of n scanning time value pairs (t.sub.s1, 
t.sub.s2).sub.i by the associated time interval (t.sub.s1 
-t.sub.s.sub.2).sub.i. 
In FIG. 2, the conductivity .sigma. measured (in relative units) at a 
unipolar electrode is plotted for two different exertion states of one 
patient (solid line=high exertion; dot-dashed line=resting) over the time 
after an excitation pulse. In the region between the times t.sub.s1, 
t.sub.s2, the curve course differs the most strongly, so that the rises 
RQ.sub.1, RQ.sub.2 of the curves in this time region can provide a usable 
rate control parameter. The rise is approximated by means of the quotient 
of the difference in the conductivity measured values or the impedance 
measured values reciprocal to them at the scanning times and their 
spacing. 
In a reprocessing stage 107 downstream of the RQ calculating stage, 
respiration-dictated abrupt changes from one heartbeat to another in the 
rate control parameter values causes by corresponding impedance 
fluctuations are filtered out with the aid of a smoothing algorithm (known 
per se), and a quantity of smoothed RQ values &lt;RQ.sub.i &gt; is available at 
the output of stage 107. The reprocessing stage 107, for performing the 
reprocessing, in particular has an internal FIFO working memory 107a, in 
which in each case in sliding fashion for a predetermined sequence of 
successive heartbeats, the associated RQ.sub.i values for performing the 
reprocessing are stored in memory. For stage 107, further functions can be 
programmed selectively, such as an algorithm for reducing the influence of 
the so-called orthostasis effect, that is, to partly eliminate irregular 
impedance fluctuations that are dictated solely by changes of position of 
the patient and have no significance for rate control. 
The components 102-107 described above form an impedance processing device 
100A of the pacemaker 100. 
Connected downstream of the post processing stage 107 is a rate calculating 
device 108, in which, on the basis of the smoothed values of the rate 
control parameters &lt;RQ.sub.i &gt;, in accordance with a fixedly programmed 
algorithm, one impedance-based value R.sub.zi of the stimulation rate is 
calculated at a time. Connected to the output of the rate calculating 
device 108 is a rate time course memory 109 with n memory regions, in 
which the time dependencies of the individual impedance-based rate values 
R.sub.zi (t), calculated on the basis of different scanning times, are 
stored in memory over a predetermined calibration period (see FIG. 3). 
The pacemaker 100 is assigned an activity sensor 110, whose signals are 
subjected in a manner are subjected in a manner known per se to 
amplification and filtration in an activity signal preprocessing stage 
111. 
A rate determining sequencer 112 controls both the impedance detection and 
evaluation, described above, in stage 100A and the activity signal 
detection and evaluation. The impedance measurements and activity 
detections are tripped by the sequence controller 112 at times 
preprogrammed in an internal timing program memory within a 24-hour 
calibration period, and the impedance measurements are moreover tripped in 
synchronism with stimulated or spontaneous cardiac events. The repetition 
period of the calibration cycles is also programmed internally in the 
sequencer 112. 
The preprocessed activity signal S.sub.A is delivered to a reference rate 
calculating stage 113, where in accordance with an algorithm known per se 
an activity-based value R.sub.A for the stimulation rate is calculated. In 
a reference rate time course memory 114, connected downstream of the stage 
113 and also having n memory regions, the time dependency of the 
activity-based calculated rate values R.sub.A (t) over the calibration 
period (see again FIG. 3) is stored in memory. 
The rate time course memory 109 and the reference rate time course memory 
114 are connected to the two signal inputs of a correlator stage 115, in 
which the n correlation coefficients of the individual impedance-based 
rate time dependencies R.sub.zi (t) are ascertained with regard to the 
reference rate time course R.sub.A (t). The output of the correlator stage 
115 is connected to a decision-making device 116, in which the maximum 
value of the transient fluctuation is determined, and the scanning time 
value pair (t.sub.s1, t.sub.s2).sub.j for which the time dependency of the 
rate correlates most strongly with the time dependency of the rate 
calculated on the basis of the activity signal is defined as the optimal 
pair for the impedance-based rate calculation. 
In FIG. 3, two time courses by way of example of stimulation rates 
R.sub.Z1, R.sub.Z2 calculated on the basis of impedance are compared with 
the time course of the rate R.sub.A calculated on the basis of activity. 
FIG. 3 also shows an example (in the form of a curve drawn in a fine solid 
line) for the course over time of a rate control parameter RQ; this time 
dependency, however, is not ascertained cohesively per se in the 
arrangement described here. Of the two impedance-based curves shown, that 
for R.sub.Z1 clear has the higher transient fluctuation, at R.sub.a. 
The stages 108, 109 and 113-116 together form the rate determining device 
100B of the pacemaker. 
The decision-making device 116 outputs a corresponding control signal to 
the memory access controller of the scanning time memory 106, on the basis 
of which, for subsequent pacemaker operation (until the next calibration 
procedure), only this j.sup.th pair of values is made the basis for 
ascertaining the rate control parameter RQ.sub.j. Furthermore, a control 
signal is output to a rate control switchover unit 117, at whose two 
signal inputs the output signal R.sub.zj of the (impedance-based) rate 
calculating device 108 and that of the (activity-based) reference rate 
calculating device 113 are applied. In the rate control switchover unit 
117, once the result of calibration is available, a switchover is made 
from the activity-based rate control of current pacemaker operation, which 
is established during the calibration phase, to the impedance-based rate 
control. 
The current rate value HR present at the output of the switchover unit 117 
is delivered to a stimulation pulse generator 118, which together with an 
ECG input stage 119, a pacemaker operation controller 120, and an output 
stage 121, as well as other components known per se, forms the actual 
pacemaker block 100C. The function of these components is assumed here to 
be known per se. 
It should be noted that the signal connection shown in FIG. 1 of the 
pacemaker controller 120 to the measuring electrode 101a, with which the 
heart actions or intracardial ECGs are also detected, makes it possible to 
distinguish between spontaneous and evoked cardiac actions and thus to 
take into account the type of event in the impedance-based rate 
calculation. In particular, the calculated stimulation rate can have a 
rate offset added to it each time the event type changes; the amount of 
which the is selected, as a function of the former rate value and the 
current rate value, such that the rate jump does not exceed a 
predetermined amount, and in subsequent cardiac events this offset is 
gradually reduced to zero again. The specific circuitry means for 
realizing this additional function are available to one skilled in the art 
from known arrangements for rate smoothing or rate adaptation. 
The invention is not limited in its embodiment to the preferred exemplary 
embodiments described above. On the contrary, a number of variants that 
make use of the claimed embodiment even in another kind of version are 
also conceivable. 
For instance, instead of the activity sensor, some other sensor can be used 
for calibration that furnishes a physiological variable representing the 
exertion of the patient. 
For taking the orthostasis effect into account in an improved way, a 
position sensor can additionally be provided, whose signals, via a 
suitable algorithm, affect the processing of the impedance signal. As an 
alternative to this last refinement, the correlation determination can be 
done with reduced weighting of the impedance values picked up in resting 
phases compared with the curve components in exertion phases, because the 
orthostasis effect is most problematic in resting phases. 
A predetermined period of time need not necessarily be adhered to in the 
calibration; instead, the calibration can also be ended once a 
predetermined threshold value for the correlation variable is reached. 
Besides saving electrical energy, this variant can have the particular 
advantage that normal pacemaker operation can be resumed sooner. To 
realize this, suitable means should be provided for storing the threshold 
correlation variable value in memory and for performing a repeated 
comparison of the correlation variable values calculated in a calibration 
state with this memorized threshold value. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.