Medical device for generating a therapeutic parameter

A device for generating a therapeutic value for a patient as a function of t least one variable parameter picked up within the body and constituting a first input value, with a change in the first parameter being a function of a second parameter which also constitutes an input value. The device includes circuitry for varying the generation of the therapeutic value by varying the second parameter so that the difference of the values of the first parameter, at selected limits of a variation range of the first parameter, constitutes a maximum in an intended treatment range of the patient. A memory retains a value of the second parameter for which the variation range of the first parameter constitutes a maximum. Control circuitry changes the therapeutic value as a function of the first parameter while maintaining the previously stored second parameter.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The invention relates to a medical device of the type for generating a 
therapeutic value for a patient as a function of at least one variable 
parameter picked up within the body and constituting a first input value, 
with a change in the first parameter being a function of a second 
parameter which also constitutes an input value. 
2. Background Information 
In such devices which generate a therapeutic value for a patient from a 
value (parameter) picked up within the patient's body, there often exists 
the problem that this parameter, with the aid of which the patient is 
treated, is a function of another parameter about which no reliable 
knowledge is available in connection with the intended treatment to enable 
it to be used as a further variable in the course of the treatment or 
which cannot be considered for other reasons. Since the further parameter 
thus takes on a relatively random value in each case, success of the 
treatment cannot be ensured with the necessary reliability. 
Thus, it has not yet been possible in connection with cardiac pacemakers to 
provide a reliable rule for influencing the stimulation rate with maximum 
efficiency as a function of conductivity values that were picked up in the 
heart within the cardiac cycle at certain points in time during the 
pre-ejection period. 
The obtained values differed greatly from patient to patient so that the 
transfer of settings found for one patient to another patient was 
increasingly connected with difficulties. 
Usually it was the increase in conductivity that was evaluated, determined 
by electrodes installed in the right ventricle which preferably may 
simultaneously constitute electrodes of the stimulation system. 
SUMMARY OF THE INVENTION 
It is the object of the invention to make it possible, in a medical device 
of the above-mentioned type, to influence in such a way the acquisition 
conditions for the parameter that controls a therapeutic value and is 
picked up within the patient's body that the sensitivity of the control or 
regulation becomes a maximum and, in particular, the regulation algorithms 
to be employed for various patients can be transferred to other patients. 
This is accomplished by means for varying the generation of the therapeutic 
value by varying the second parameter so that the difference of the values 
of the first parameter, at selected limits of a variation range of the 
first parameter, constitutes a maximum in an intended treatment range of 
the patient, a memory in which a value of the second parameter for which 
the variation range of the parameter constitutes a maximum is retained; 
and a control means for changing the therapeutic value as a function of 
the first parameter while maintaining the previously stored second 
parameter. 
The invention is based on the realization that often there are one or 
several further parameters which influence the acquisition of the 
parameter to be evaluated in an undetected manner. If it is now possible 
to set this parameter for the measurement in such a way that the change in 
the parameter to be evaluated becomes a maximum in the intended treatment 
range for the patient, the information available for a change in the value 
influencing the treatment of the patient can in many cases be 
significantly improved or discovered at all. 
Thus, a further parameter in the form of another value that influences the 
patient is changed by means of suitable measures in such a way that the 
influencing of physical processes within the patient is maximized as a 
function of a first value derived from the body so that maximum response 
to the treatment is ensured. This maximization of the influenceability of 
the patient is determined during a "learning cycle" of the device and the 
determined result is then used for the further treatment. 
In the medical device of this type for generating a therapeutic value for a 
patient as a function of at least one first parameter picked up within the 
body it is initially assumed that the first parameter is a function of a 
further (second) parameter. This may be, for example, a (possibly 
temporary) masking of the signal responsible for a change in the 
therapeutic value to be picked up within the patient's body (first 
parameter). 
If the first parameter can be positively changed by means of external 
measures so that it passes through the variation range which is 
significant for the patient's treatment, if thus, all conditions, for 
example, all conditions in the patient's daily life under which treatment 
is to take place--particularly by means of portable or implantable medical 
devices--can be simulated or set, it is possible to observe the 
corresponding change in the first parameter. 
The measures according to the invention now generate the therapeutic value 
by varying the second monitorable parameter in such a way that the 
difference of the values of the first parameter at the limits of the 
variation range of the first parameter constitutes a maximum. 
A preferable further parameter to be changed during signal pickup within 
the patient's body is a time window during which the signals are picked up 
within a particular cyclic sequence, such as the cardiac rhythm or another 
biorhythmic cycle. Another variable parameter is possibly given by the 
location where the signal is obtained--for example, if it is possible to 
switch between several sensors distributed in space, such as electrodes in 
a conductivity value determination. 
This setting of the second controllable parameter is now stored in a memory 
and used as a basis for further influencing the therapeutic value for the 
patient as a function of the first parameter so that the control of the 
therapeutic value within the variation range of the first parameter takes 
place while maintaining the thus obtained second parameter. 
As another advantageous feature of the invention, different second 
parameters are determined for different variation ranges of the first 
parameter, each associated with such a variation range, and are stored in 
the memory. Now--even if the influence of the second parameter on the 
dependency of the value influencing the patient upon the first parameter 
does not remain unchanged over the variation range of the first parameter, 
appropriate switching of the selection of the second parameter from 
section to section permits an optimization of the control or regulation of 
the value that influences the patient. The switching of the influence of 
the second parameter may here be effected by way of additional means; it 
may be changed by a third parameter which changes in its tendency in the 
same manner as the first parameter and can also be derived from within the 
patient's body or from his environment. 
In another preferred embodiment, the switching of the selection of the 
second parameter may also be effected by the first parameter itself if 
care is taken by way of a switching hysteresis or other suitable measures 
that the control has the necessary stability and hunting cannot occur.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The invention is advantageously employed with implantable cardiac 
pacemakers that are provided with a circuit for the requirement-dependent 
change of the stimulation rate. The electrical conductivity K measured in 
the heart by means of a stimulation electrode E.sub.1 is shown in FIG. 1 
as a cardiac cycle with time window. 
Particularly the change in electrical conductivity in the heart, as the 
first parameter, constitutes a measure for physical stress that requires a 
corresponding pumping output (volume per minute) of the heart. Regardless 
of whether the circuit involved is a closed regulating circuit or a 
control circuit, this value is an input value for the 
requirement-dependent circuit for changing stimulation rate 10. The 
stimulation rate of the cardiac pacemaker (for on-demand pacemakers, the 
basic rate) is varied by a control and/or regulating device 3 as a 
function of the value representing physical stress. 
This representative value is the increase S in electrical conductivity 
.kappa. preferably in the right ventricle, that is, the difference between 
the electrical conductivity .DELTA..kappa. at the beginning and at the end 
of a certain predeterminable time interval .DELTA.t.sub.ij. 
To this end, a time window Z.sub.ij =f(.DELTA.t.sub.i, .DELTA.t.sub.j) is 
provided by a memory 6 and constitutes the second parameter which 
influences the measuring signal. In an input circuit 1, a fixed time 
reference point t.sub.O is derived from the input value. Thus, 
.DELTA.t.sub.ij is the preselectable time interval that begins at time 
t.sub.i, that is, at a distance .DELTA.t.sub.i from reference point 
t.sub.0, and ends at a distance .DELTA.t.sub.j from reference point 
t.sub.O. In the past, many unsuccessful attempts have been made to find a 
respectively suitable time window which yields reproducible results with 
sufficient accuracy and, in particular, also permits a transfer to other 
patients. 
The change (increase) in the short-term integral of the electrical 
conductivity within a certain time window within the cardiac cycle thus 
constitutes at least indirectly an input value for the circuit for the 
requirement-dependent change of the stimulation rate 10. 
According to an advantageous feature of the invention, that time interval 
(time window) is now determined and utilized for the later difference 
formation (determination of increase) of the measured conductivity values 
in which a difference for two different stress states of the patient 
constitutes a maximum. Another variation is that a switch is made between 
electrodes arranged differently in space until a signal is found that has 
the greatest significance for the respective parameter as a function of 
which a value within the patient's body is to be influenced. 
For the first embodiment of an implantable cardiac pacemaker, FIG. 2 shows 
a circuit for the requirement-dependent change of the stimulation rate 10. 
The electrical conductivity .kappa. measured within the heart by means of 
the stimulation electrode E.sub.1 is processed in a first input circuit 1 
and in a second input circuit 2, and constitutes an input value for a 
control and/or regulating device 3 in the circuit for 
requirement-dependent changes in the stimulation rate 10. 
The amplified conductivity signal determined in the right ventricle by way 
of suitable pickup points at the stimulation electrode E1, and transmitted 
from the first input circuit, to the second input circuit 2, is integrated 
in the time interval in the second input circuit 2 in order to eliminate 
short-term fluctuations that might falsify the measurement signal. 
By way of an analog-digital converter 7 and input circuit 1, input circuit 
2 is connected with control and/or regulating device 3, particularly with 
its input/output channel 14. In addition to input/output channel 15, the 
control and/or regulating device includes a logic unit, preferably a 
processor 11, and selection circuits 4, 5, 9 suitable for connection of a 
memory, as well as a data input/output circuit 15 for the increase limit 
values, a rate selection circuit 8 for controlling the variation ranges, 
and a clock pulse generator 16. 
The control and/or regulating circuit 3 is connected, on by way of its two 
selection circuits 4 and 9 for selecting the time intervals with the 
second input circuit 2, with a memory region selection circuit 5 with 
memory 6. As a further feature, the data input/output circuit 15 is also 
connected with memory 6 in order to store limit values for the first 
parameter. The input/output, the input/output circuit 14 of control and/or 
regulating circuit 3 is further connected with a stimulation signal output 
circuit 17. 
The circuit for the requirement-dependent change of the stimulation rate 10 
further includes a communication unit 18 that is coupled with the control 
and/or regulation circuit 3. In addition to programming the implanted 
cardiac pacemaker, the communication unit 18 serves to transmit data to 
external programming and monitoring units. 
Controlled by processor 11, a memory region is selected by way of the 
memory region selection circuit 5 which is connected with memory 6. Now 
the particular time interval (time window) at which the difference for two 
different stress states of the patient constitutes a maximum. Controlled 
by processor 11, the data in memory 6 are selected by way of memory 
selection circuit 5. 
For this purpose it is initially necessary to pick up a series of 
measurements for different physical stresses during which the respective 
time window is varied. To this end, control and/or regulating device 3 is 
provided with a selection circuit 9 for selecting the time window distance 
.DELTA.t.sub.i from reference point t.sub.0. Selection circuits 4, 5 and 9 
include either independent forward/backward counters that are actuated by 
the control and regulating device 3, or they are alternately 
advantageously realized as software in processor 11 which, in particular, 
evaluates the measured values and automatically finds the regions of 
maximum increase .DELTA..kappa..sub.max for a fixed setting (storage in 
memory 6). 
First, during the lowest stress stage B.sub.0, the associated first 
parameter .DELTA..kappa..sub.0ij is successively determined for different 
time windows in the individual cardiac cycles, with the length of time 
intervals .DELTA.t.sub.i and .DELTA.t.sub.j being varied. In stress stage 
B.sub.1, a first time window with associated first parameter 
.DELTA..kappa..sub.1ij is stored in memory 6. If during this stress stage 
a time window occurs that includes time intervals .DELTA.t.sub.m and 
.DELTA.t.sub.1 as well as an associated first parameter to which the 
inequality .DELTA..kappa..sub.1ml -.DELTA..kappa..sub.0ml 
&gt;.DELTA..kappa..sub.1ij -.DELTA..kappa..sub.oij applies, this time window 
Z.sub.ij is overwritten by the new time window Z.sub.m1 which has the new 
highest absolute value of the differences .DELTA..kappa..sub.1ml 
-.DELTA..kappa..sub.0ml of the first parameters. At the same time, the 
value .DELTA..kappa..sub.1ml associated with stress stage B.sub.1 is 
stored in memory 6. 
A stress stage B.sub.0 has thus precisely one time window Z.sub.ij with 
associated time intervals .DELTA.t.sub.i +.DELTA.t.sub.j. Thus, 
particularly with the use of a processor 11 that is coupled with memory 
region selection circuit 5, the time window at which the difference in 
increase of the integral of the conductivity values constitute a maximum 
is retained in its position relative to a fixed reference point in the 
cardiac cycle (limits of the pre-ejection period) for at least one further 
stress stage B.sub.1 of the stress range. 
The absolute values of the increases are here of importance which, in the 
range involved, may of course also change their sign. It is also advantage 
that, in the curve shown in FIG. 1, only those regions of are considered 
where the increase is monotonous. 
A further embodiment of the invention will be described in greater detail 
with reference to FIG. 3. FIG. 3 depicts an embodiment of the circuit for 
the requirement-dependent change of the stimulation rate 10 in which the 
determination of the reference time t.sub.0 and the formation of the 
short-term integral are performed by logic unit/processor 11 itself. 
Processor 11 is connected with memory 6 by way of an interface circuit 19. 
The electrical conductivity .kappa. measured in the heart by means of 
stimulation electrode E.sub.1 is amplified and fed by way of an input 
circuit 1 to analog/digital converter 7 which is connected with 
input/output channel 14. An integrator 2 (as in the FIG. 2) and the 
associated connections are not required since the integration is effected 
by software and processer 11. The determined increase S in the electrical 
conductivity .kappa., preferably in the right ventricle, that is, the 
difference between electrical conductivity .DELTA..kappa..sub.ij at the 
beginning and end of a certain predeterminable time interval .DELTA.t 
.sub.ij causes rate selection circuit 8 to vary the stimulation rate of 
the cardiac pacemaker. 
In order to better approximate the integral by way of parabolas, processor 
11 samples an odd number of measuring points at uniform time intervals 
within time window Z.sub.ij =f(.DELTA.t.sub.i, .DELTA.t.sub.j) and inputs 
the measured values through analog/digital converter 7 and input/output 
channel 14. 
The different stress states constituting the reference values are, on the 
one hand, preferably a first state of low physical stress B.sub.0 (rest 
state) and, on the other hand, a second state in the range of a relatively 
great physical stress B.sub.1 (working state). The mathematical evaluation 
of the differences of the integrals is preferably effected according to 
the Simpson theorem so that the calculating efforts of a processor 
provided in the pacemaker circuit are greatly reduced. 
The entire range traversed during changes of stress may also be subdivided 
into a number of successive stress zone ranges which have associated 
stress stages. For the n.sup.th stress stage B.sub.n, precisely one time 
window Z.sub.ij with time intervals .DELTA.t.sub.i and .DELTA.t.sub.j is 
then determined and care is taken that switchable intermediate ranges are 
available for different stress zone ranges. 
Switching the influence of the second parameter on the first parameter is 
here done, for example, by means of an additionally provided activity 
sensor 12 which detects the intensity of the movements or other physical 
activity. Activity sensor 12 is connected with processor 11 by way of a 
third input circuit 13 of control and/or regulating device 3. By way of 
data input/output circuit 15 (FIG. 2), for examples, processor 11 controls 
the storage for the respective stress stage and the call-up of either a 
value .DELTA..kappa..sub.max if the lower conductivity limit .kappa..sub.i 
is fixed, or storage of the limit values .kappa..sub.i and .kappa..sub.j 
into or from memory 6, respectively. According to the limit values, rate 
selection circuit 8 determines the stimulation rate. Using processor 11, 
the function of rate selection circuit 8 may also be realized as software. 
Such an activity sensor 12 may then also serve as self-calibration or 
monitor in that it determines which changes in conductivity are to be 
associated with certain stimulation rates. 
In another embodiment which does not include the third input circuit, the 
switching of the influence of the second parameter on the first parameter 
is effected by the output signal of the circuit for the 
requirement-dependent change of stimulation rate 10 itself. If necessary a 
switch is made with hysteresis whenever the processor determines an 
increase (.DELTA..kappa..sub.max in the time window) to which another 
stress stage B.sub.n+l can already be associated. The prerequisite for 
this is that the system must be self-adapting, that is, an attempt is made 
during the stimulation to find regions with a greater change in increase. 
The circuit technology for a rate controlled pacemaker can be stipulated to 
be known, with it being important in this modification of the present 
invention that the signal pickup of the parameter determining the 
stimulation rate is improved. In this connection, the use of the invention 
is not dependent on whether the control is an open loop control or a 
closed loop control. 
The present invention is not limited in its embodiments to the 
above-described preferred embodiments. Rather, a number of variations are 
conceivable which take advantage of the described solution even for 
basically different configurations.