Rate-responsive pacemaker with circuitry for processing multiple sensor inputs

An implantable rate-responsive pacemaker is disclosed wherein two or more sensors indicative of physiological demand are utilized in a fashion designed to realize the advantages of both sensors in a circuit producing a rate command signal which is used to operate the pacemaker at an optimum pacing rate which will closely match physiological need of the patient. A rate matrix is used to produce a specific selected rate which is unique to the particular combination of sensor inputs which are being measured at the particular time. In the preferred embodiment, a plurality of rate matrices are provided, with the appropriate rate matrix to be used being selected by a switch matrix which monitors the logic-processed and time-processed signals from the sensors, the outputs of the processing circuitry being used to select a cell in the switch matrix which corresponds to the exact set of conditions currently being encountered by the sensors.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to rate responsive cardiac 
pacemakers, and more particularly to an implantable rate-responsive 
pacemaker wherein two or more sensors indicative of physiological demand 
are utilized in a fashion designed to realize the advantages of both 
sensors in a circuit producing a rate command signal which is used to 
operate the pacemaker at an optimum pacing rate which will closely match 
physiological need of the patient. 
A pacemaker is an implantable medical device which delivers electrical 
stimulation pulses to a patient's heart in order to keep the heart beating 
at a desired rate. Early pacemakers provided stimulation pulses at a fixed 
rate or frequency, such as 70 pulses per minute (ppm), thereby maintaining 
the heart beat at that fixed rate. Subsequently, pacemakers were designed 
to not only stimulate the heart, but also to monitor the heart. If a 
natural heart beat was detected within a prescribed time period (usually 
referred to as the "escape interval"), no stimulation pulse was delivered, 
thereby allowing the heart to beat on its own without consuming the 
limited power of the pacemaker or interfering with the normal operation of 
the heart. Such pacemakers are referred to as "demand pacemakers" because 
stimulation pulses are provided only as demanded by the heart. 
Early demand pacemakers had a fixed base rate associated with them. In 
later versions, the base rate was programmably selectable, and thereafter 
became commonly known as the "programmed rate." If the heart was able to 
beat on its own at a rate exceeding the base (or programmed) rate, then no 
stimulation pulses were provided. However, if the heart was not able to 
beat on its own at a rate exceeding the base rate, then stimulation pulses 
were provided to ensure that the heart would always beat at least at the 
base (or programmed) rate. Such operation was achieved by simply 
monitoring the heart for a natural beat during the escape interval. If 
natural activity was sensed, the timer which defined the escape interval 
was reset. If no natural activity was sensed, a stimulation pulse was 
provided as soon as the escape interval had timed out. Changing the base 
(or programmed) rate was accomplished by simply changing the duration of 
the escape interval. 
In recent years, rate-responsive pacemakers have been developed which 
automatically change the rate at which the pacemaker provides stimulation 
pulses as a function of a sensed physiological parameter. The 
physiological parameter provides some indication of whether the heart 
should beat faster or slower, depending upon the physiological needs of 
the pacemaker user. Thus, for example, if a patient is at rest, there is 
generally no need for a faster-than-normal heart rate, so the 
rate-responsive pacemaker maintains the "base rate" at a normal value, 
such as 60 pulses per minute (ppm). 
However, if the patient is exercising, or otherwise physiologically active, 
there is a need for the heart to beat much faster, such as, for example, 
100 beats per minute. For some patients, the heart is not able to beat 
faster on its own, so the pacemaker must assist. In order to do this 
effectively, the physiological need for the heart to beat faster must 
first be sensed, and the "base rate" of the rate-responsive pacer must be 
adjusted accordingly. Hence, rate-responsive pacemakers are known in the 
art which increase and decrease the "base rate" as a function of sensed 
physiological need. 
Numerous types of sensors are taught in the art for use with a 
rate-responsive pacer. In each, an increase or decrease in the parameter 
being monitored signals a need to increase or decrease the rate at which 
pacing pulses are provided. Note, as used herein, the term "pacing rate" 
refers to the rate at which the pacer provides stimulation pulses, or in 
the case of demand pacers, the rate at which the pacer would provide 
stimulation pulses in the absence of naturally occurring heart beats. 
One common type of sensor is an activity sensor which senses the physical 
activity level of the patient. See, for example, U.S. Pat. No. 4,140,132, 
to Dahl, and U.S. Pat. No. 4,485,813, to Anderson et al. In accordance 
with the teachings of Dahl or Anderson et al., a piezoelectric crystal is 
used as an activity sensor. Such a crystal generates an electrical signal 
when subjected to physical movement and stress according to well known 
principles. The electrical signal generated by the crystal may be 
processed and used to vary the pacing rate. 
Other types of sensors used in prior art rate-responsive pacers include 
sensors that sense respiration rate, respiratory minute volume, blood 
oxygen level, blood and/or body temperature, blood pressure, the length of 
the Q-T interval, the length of the P-R interval, etc. All of the sensors 
which may be used in rate-responsive pacers have particular advantages and 
disadvantages. 
The next generation of rate-responsive pacemakers will use two or more 
sensors simultaneously to control the pacing rate. It will be appreciated 
by those skilled in the art that the combination of signals from two or 
more sensors to be used to control pacing rate is a difficult and complex 
task. 
The goal of a system using two or more sensors should be to utilize the 
best properties of each of the sensors, while eliminating or minimizing 
their drawbacks. For example, an activity sensor will react very quickly 
to the onset of exercise, closely mimicking the response of the sinus node 
in a healthy heart. However, an activity sensor does not measure any true 
physiological variable of the body, and as such may be a poor predictor of 
work level and of the optimum heart rate. 
Alternately, a sensor measuring respiratory minute volume or venous blood 
temperature will provide a very good correlation to the level of exercise 
at higher levels of exercise. However, the sensor response of a 
respiratory minute volume sensor or venous blood temperature sensor is 
much slower than the response of the SA node, typically of the order of 
sixty to ninety seconds. Thus, it may be seen that all single sensor 
systems will have both significant advantages and disadvantages. 
Theoretically, a combination of an activity sensor and a respiratory minute 
volume sensor or venous blood temperature sensor could be used to control 
pacing rate in a manner which is more physiologic than either of the 
sensors separately. The combination technique may, however, prove quite 
complex in its implementation. For example, a summation and averaging of 
the two signals would not be optimum for the following reasons. At the 
onset of exercise, the activity sensor would deliver a signal, while the 
other sensor would not yet have reacted. Thus, the onset of heart activity 
would be slower than in the case of using activity alone. 
During prolonged exercise, the good sensor response of the respiratory 
minute volume or blood temperature sensor would be averaged with the 
poorer response of the activity sensor. In this case, the result would not 
be as accurate as using the respiratory minute volume or blood temperature 
sensor alone. During prolonged exercise at a low level, the activity 
sensor may be as good as the respiratory minute volume or blood 
temperature sensor because the latter two are inaccurate at low levels of 
exercise. In the case of a false positive indication of activity of the 
activity sensor (caused, for example, by riding in a car on a bumpy road), 
the poor response of the activity sensor would be averaged with the good 
response of the respiratory minute volume or blood temperature sensor. 
Thus, the result again would not be as accurate as using the respiratory 
minute volume or blood temperature sensor alone. 
Another possible technique which may be used to combine the inputs from two 
sensors would be to take the highest value of the two sensors. This would 
in at least some cases yield a better result than the averaging technique 
discussed above. This combination would not, however, eliminate the 
erroneous increase in pacing rate resulting from external vibration picked 
up by the activity sensor. 
Thus, while this technique could give a better response to exercise in some 
situations, it would not eliminate problems occurring due to erroneous 
responses of the sensors. In addition to the problem of external vibration 
mentioned above, if the other sensor used was a blood temperature sensor, 
the shortcomings of this sensor would be propagated. For example, heavy 
clothing or external temperature change would result in erroneous changes 
to the pacing rate. In short, it will be perceived by those skilled in the 
art that it is difficult and complex to utilize inputs from more than one 
sensor in an intelligent fashion which will enhance the advantages of each 
sensor without proliferating the drawbacks of the sensors. 
It is accordingly the objective of the present invention that it provide a 
system which will utilize inputs from two or more sensors to provide a 
sensor-indicated rate signal, which will control the pacing rate of the 
pacemaker. It is the primary objective of the system of the present 
invention to utilize the best properties of each of the sensors, while 
minimizing or eliminating their drawbacks. The control strategy must be of 
a complexity sufficient to provide as an output a highly flexible 
sensor-indicated rate signal which will accurately follow a control 
strategy paralleling the physiological response of a healthy heart. 
It is a further objective of the present invention that its implementation 
be relatively simple and easy to accomplish in a pacemaker, which is 
necessarily limited in size since it is an implanted device. The system of 
the present invention should be useable with at least two sensors, but 
should also be capable of working with more than two sensors. The system 
should also be economic of power, not requiring more power to operate than 
do other rate response systems. Finally, it is also an objective that all 
of the aforesaid advantages and objectives be achieved without incurring 
any substantial relative disadvantage. 
SUMMARY OF THE INVENTION 
The disadvantages and limitations of the background art discussed above are 
overcome by the present invention. With this invention, two or more 
sensors are used to supply inputs to circuitry which will extract relevant 
information from each sensor and combine these signals in an optimum way 
so as to produce a signal which controls the rate of the pulse generator 
output. This circuitry is hereafter called a rate response processor. In 
the preferred embodiment, two sensors are used, with one of them being an 
activity sensor (which responds quickly) and the other being a more 
physiologically responsive (but slower responding) sensor such as a blood 
temperature sensor or a respiratory minute volume sensor. Other sensors 
which could be used include a blood oxygen sensor (either blood oxygen 
saturation or blood oxygen partial pressure), a pH sensor, a pCO.sub.2 
sensor, a QT interval sensor, a respiratory rate sensor, a stroke volume 
sensor, a QRS morphology change sensor, etc. Alternately, the system may 
be used to combine signals from two sensors, one of which has a good 
specific response for low levels of exercise and the other of which has a 
good specific response for high levels of exercise. In either case, 
processing circuitry is used to condition the raw signals from the sensors 
into processed sensor signals. 
In the simplest embodiment of the present invention, the concept of a 
two-dimensional rate matrix is used. The coordinates of the rate matrix 
are the processed signals from sensors, with the value of one of the 
processed signals being used to select one axis (the columns, for example) 
of the rate matrix, and the value of the other of the processed signals 
being used to select the other axis (the rows, for example) of the rate 
matrix. 
The processed sensor signals are digitized into a desired number of 
increments corresponding to the size of the rate matrix. For example, a 10 
by 10 rate matrix could be used. In this case, the processed signal from 
the activity sensor would be digitized so that any processed signal from 
the activity sensor would be in one of 10 ranges. Similarly, the processed 
signal from the more physiologically responsive sensor would be digitized 
so that any processed signal from the more physiologically responsive 
sensor would be in one of 10 ranges. 
The rate matrix has in its cells values which correspond to selected values 
of pulse generator output rate. Thus, the rate matrix is essentially a 
look-up table, with the value of the selected rate being unique and 
depending on the value of the signals from the sensors. This selected rate 
is then supplied to a reaction and recovery time circuit, as is 
conventional in the art. The reaction and recovery time circuit is a 
device which will limit how quickly the pacing rate can increase or 
decrease. The output from the reaction and recovery time circuit is 
supplied as the sensor-indicated rate signal to the other circuitry of the 
device. An alternate approach to performing the reaction and recovery time 
functions would be to include these functions on the input side of the 
system rather than on the output side. Since the reaction and recovery 
time circuit tends to mask the sensor output, it is normally advantageous 
to place the reaction and recovery time circuit after the rate matrix. 
In the preferred embodiment, the values inscribed in the rate matrix are 
programmable. An external programmer is used to program the values of the 
rate matrix. The rate matrix may thus be tailored for individual 
conditions and lifestyles for each patient, with factors such as age, 
activity level, and physical condition being used to select the values 
inscribed in the rate matrix. The external programmer may embody an expert 
system to guide the physician to the proper matrix values. 
Alternately, multiple rate matrices may be provided in the implanted device 
with the specific rate matrix to be used being selectable by programming. 
The external programmer would be used to program which of the rate 
matrices is to be selected. The different rate matrices may be tailored 
for different lifestyles, ages, activity levels, or physical conditions. 
In the preferred embodiment of the present invention, the versatility and 
intelligence of the system is enhanced through the use of a switch matrix 
and processing circuitry for driving the switch matrix. In this 
embodiment, a switch matrix is provided which is used to select which one 
of a plurality of rate matrices will be used. Thus, there are as many rate 
matrices in the implanted device as there are cells in the switch matrix. 
The processing circuitry consists of logic circuits and timing circuits 
which monitor the processed signals from the sensors and supply inputs 
enabling the selection of a row and a column in the switch matrix. The 
timing circuits are used to deal with time-dependent rate variation, and 
the logic circuits are used to implement various transfer functions 
requiring the occurrence of specific conditions. 
The outputs of the processing circuitry are used to select a cell in the 
switch matrix which corresponds to the exact set of conditions currently 
being encountered by the sensors. Thus, for example, if the switch matrix 
is five by seven, there may be seven different conditions which the 
processed signal from the first sensor may be evidencing. Similarly, there 
may be five different conditions which the processed signal from the 
second sensor may be evidencing. Thus, there are 35 different sets of 
conditions which the two sensors may be encountering, and there are 35 
corresponding rate matrices which may be used by the system. 
The particular rate matrix which is selected by the switch matrix has as 
inputs the processed signals from the sensors. Thus, the selected rate 
matrix will output a particular selected rate which is used to control the 
paced rate of the pacemaker. The switch matrix is also preferably 
programmable. It is apparent that in the example given above, there are 35 
different rate matrices, each of which would have 100 different values 
contained therein. These values would all be contained in memory in the 
form of lookup tables. 
In this example, there would be 3,500 memory cells, each containing one 
rate value. Semiconductor memories are the most densely packaged devices 
available today. A system of this complexity would thus be feasible with 
present technology. With a smaller switch matrix, the amount of memory 
space required will be reduced substantially. 
It will thus be appreciated by those skilled in the art that the switch 
matrix is used to analyze the particular circumstances indicated by the 
signals from the sensors. A rate matrix specifically designed for use 
under the particular circumstances is selected, and the processed signals 
from the sensors are supplied to the selected rate matrix. These sensor 
signals would point to a specific cell in the selected rate matrix, which 
cell contains the desired pacing rate used to operate the pacemaker under 
the particular circumstances indicated by the sensors. 
In an alternate embodiment, the processed signals from only one of the 
sensors are supplied to a switch matrix via processing circuitry. Based on 
the particular circumstances indicated by the signal from the one sensor, 
a particular rate matrix is indicated. The processed signals from both 
sensors are supplied to the selected rate matrix, where a selected rate is 
indicated by the values of the processed signals from the sensors. That 
selected rate is then used to control the pacing rate of the pacemaker. 
This situation is equivalent to having a switch matrix of size 1.times.N; 
in other words, the switch matrix has degenerated into a vector. 
Consider the following example using an activity sensor and a respiratory 
minute volume sensor. Following a period of rest with no signal from 
either sensor, the activity sensor provides an output indicating moderate 
exercise by the patient. The respiratory minute volume sensor has, as 
expected, not had time to react and indicates no exercise. During a 
predetermined time, dependent on the indicated level of exercise from the 
activity sensor, the switch matrix will select a rate matrix which will 
allow the activity sensor to control the paced rate to the full extent 
indicated by the activity sensor. 
After the predetermined time, the switch matrix would switch to another 
rate matrix which would require inputs from both the activity sensor and 
the respiratory minute volume sensor to achieve the same rate as before. 
For prolonged exercise at a high level of exercise, the switch matrix 
would select yet another rate matrix which would further reduce the 
influence of the activity signal and make the pacemaker rate more closely 
follow the respiratory signal. For prolonged low-level exercise, a 
different combination of sensor signals may be selected to compensate for 
the lesser accuracy of the minute volume sensor at low levels of exercise. 
It may therefore be seen that the present invention teaches a system which 
will utilize inputs from two or more sensors to provide a sensor-indicated 
rate signal, which will control the pacing rate of the pacemaker. The 
system of the present invention can utilize the best properties of each of 
the sensors, while minimizing or eliminating their drawbacks. The control 
strategy is of a complexity sufficient to provide as an output a highly 
flexible sensor-indicated rate signal which will accurately follow a 
control strategy paralleling the physiological response of a healthy 
heart. In addition to being highly flexible, the control strategy of the 
preferred embodiment of the present invention is also fully programmable 
by using the external programmer. 
The implementation of the present invention is relatively simple and easy 
to accomplish in a pacemaker, and will not significantly increase the size 
of the pacemaker. The system of the present invention is useable with two 
sensors, and is capable of working with more than two sensors. The system 
is also economic of power, and does not require more power to operate than 
do other rate response processors. Finally, all of the aforesaid 
advantages and objectives are achieved without incurring any substantial 
relative disadvantage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment is illustrated embodied in a rate-responsive 
pacemaker 20 shown in FIG. 1. The pacemaker 20 utilizes a first 
rate-responsive sensor 22 and a second rate-responsive sensor 24. The 
example illustrated as the preferred embodiment of the present invention 
utilizes two sensors, although more than two sensors could be utilized, as 
illustrated by an Nth rate responsive sensor 26 illustrated in phantom 
lines in FIG. 1. The output of the first rate-responsive sensor 22 is 
identified as raw signal 1, and the output of the second rate-responsive 
sensor 24 is identified as raw signal 2. 
The outputs of the first rate-responsive sensor 22 and the second 
rate-responsive sensor 24, the raw signal 1 and the raw signal 2, 
respectively, are supplied to a rate responsive processor 28. The rate 
responsive processor 28 is the heart of the present invention, and its 
operation will be described in detail below. The rate responsive processor 
28 accesses a memory circuit 30, and produces as an output a 
sensor-indicated rate signal. The sensor-indicated rate signal is a signal 
which indicates the rate which the rate responsive processor 28 has 
calculated the pacemaker 20 should use as the pacing rate, based on the 
inputs from the first rate-responsive sensor 22 and the second 
rate-responsive sensor 24. 
The pacemaker 20 also includes a conventional pacemaker circuit 32 which 
typically includes at least a pulse generator 34, a timing and control 
circuit 36, and a telemetry circuit 38. The pulse generator 34 provides 
electrical pulses to two leads 40 and 42, which provide electrical contact 
with a patient's heart 44. The leads 40 and 42 may be either unipolar 
leads, bipolar leads, or other multi-pole leads, all of which are known in 
the art. It should be noted that while the system shown in FIG. 1 is a 
dual chamber pacemaker, the principles of the present invention could just 
as easily be applicable to a single chamber pacemaker. 
An external programmer 46 is also used to send programming signals to the 
telemetry circuit 38. These programming signals are depicted symbolically 
as a wavy line in FIG. 1. It should be noted that signals may be sent 
either from the external programmer 46 to the pacemaker 20, or from the 
pacemaker 20 to the external programmer 46. 
Functionally, the pulse generator 34 generates stimulation pulses supplied 
to the leads 40 and 42 at a rate determined by a rate control signal, 
which is an input to the pulse generator 34. These stimulation pulses, in 
turn, are delivered to the heart 44 through the leads 40 and 42 in a 
conventional manner. It should also be understood that although the first 
rate-responsive sensor 22 and the second rate-responsive sensor 24 are 
shown in FIG. as being included within the pacemaker 20, one or both of 
the first rate-responsive sensor 22 and the second rate-responsive sensor 
24 could also be included within, or coupled to, one of the leads 40 and 
42. In addition, the first rate-responsive sensor 22 and/or the second 
rate-responsive sensor 24 cloud also be placed externally of the pacemaker 
20. 
The leads 40 and 42 also present electrical signals occurring within the 
heart 44, such as intracardiac P-waves and R-waves (evidencing natural 
cardiac activity of the atria and ventricles, respectively), to the timing 
and control circuit 36. Hence, for example, when programmed in a demand 
mode of operation, the pacemaker 20 is able to inhibit the generation of a 
pacing pulse when natural cardiac activity is sensed within a designated 
time period, in a conventional manner. 
A more complete description of the pacemaker circuit 32 and its operation 
may be found in several patents. For example, note U.S. Pat. No. 
4,232,679, entitled "Programmable Human Tissue Stimulator," U.S. Pat. No. 
4,686,988, entitled "Pacemaker System and Method for Measuring and 
Monitoring Cardiac Activity and for Determining and Maintaining Capture," 
and U.S. Pat. No. 4,712,555, entitled "Physiologically Responsive 
Pacemaker and Method of Adjusting the Pacing Interval Thereof." While not 
disclosing the exact same pacemaker circuit 32 or circuits which are used 
in the preferred embodiment of the present invention, these patents 
nonetheless disclose the primary components of a conventional pacing 
system and teach the basic operation thereof. U.S. Pat. No. 4,232,679, 
U.S. Pat. No. 4,686,988 and U.S. Pat. No. 4,712,555 are hereby 
incorporated herein by reference. 
In operation, the rate-responsive pacemaker 20 may operate in either a 
SENSOR ON mode or a SENSOR OFF mode. The selection of a desired mode of 
operation is controlled by a sensor on/off selector 48, shown functionally 
in FIG. 1 as a switch. The sensor on/off selector 48 connects either a 
base rate signal supplied from the timing and control circuit 36 or the 
sensor-indicated rate signal supplied from the rate responsive processor 
28 as the rate control signal input to the pulse generator 34. Control of 
the sensor on/off selector 48 is obtained from the timing and control 
circuit 36, which control may be selected by an appropriate programming 
signal received from the external programmer 46. 
When the SENSOR OFF mode is selected, the sensor on/off selector 48 directs 
the base rate signal, generated by the timing and control circuit 36, to 
be the rate control signal input to the pulse generator 34. This base rate 
signal thus controls the pacing rate of the pacemaker 20 in a conventional 
manner. 
When the SENSOR ON mode is selected, the rate control signal input to the 
pulse generator 34 is connected by way of the sensor on/off selector 48 to 
the sensor-indicated rate signal output from the rate responsive processor 
28. (It will of course be appreciated by those skilled in the art that 
there are other ways to accomplish the sensor on/off function.) The 
sensor-indicated rate signal, as mentioned above, is derived from the raw 
signal 1 and the raw signal 2 from the first rate-responsive sensor 22 and 
the second rate-responsive sensor 24, respectively. 
Typically, the rate control signal may be thought of as simply a signal 
responsible for generating a trigger pulse at the timing out of an escape 
interval (also generated by the timing and control circuit 36). However, 
if natural cardiac activity is sensed during the escape interval, no 
trigger pulse is generated by the pulse generator 34 and the portion of 
the timing and control circuit 36 responsible for generating the escape 
interval is reset, thereby starting a new escape interval. Hence, 
regardless of the source of the rate control signal (either the base rate 
signal from the timing and control circuit 36 or the sensor-indicated rate 
signal from the rate responsive processor 28), such signal may be 
overridden (if the pacemaker 20 is so programmed) by the sensing of 
natural cardiac activity. 
Referring next to FIG. 2, one possible construction of the rate responsive 
processor 28 of FIG. 1 is illustrated. In the preferred embodiment, signal 
processing circuitry is used to process raw signal 1 from the first 
rate-responsive sensor 22 and raw signal 2 from the second rate-responsive 
sensor 24. Such processing circuitry is known in the art and is not 
necessarily an integral part of the present invention. The amplification, 
filtering, and linearization may be modified by reprogramming by the 
external programming unit. This processing circuitry may vary widely, so 
the following description of processing circuitry is merely exemplary. 
A first amplifier 50 is used to amplify raw signal from the first 
rate-responsive sensor 22. The amplified signal from the first amplifier 
50 is supplied to a first filter 52 used to minimize the effects of noise 
on the signal from the first rate-responsive sensor 22. The output of the 
first filter 52 is supplied to a first linearization circuitry 54, which 
serves to linearize the signal from the first rate-responsive sensor 22 if 
it varies in a non-linear fashion. Such linearization circuitry is 
typically a non-linear amplifier designed to correct the non-linear 
response characterizing the sensor. The output of the first linearization 
circuitry 54 is a processed sensor 1 signal. 
Similarly, a second amplifier 56 is used to amplify raw signal 2 from the 
second rate-responsive sensor 24. The amplified signal from the second 
amplifier 56 is supplied to a second filter 58 used to minimize the 
effects of noise on the signal from the second rate-responsive sensor 24. 
The output of the second filter 58 is supplied to a second linearization 
circuitry 60, which serves to linearize the signal from the second 
rate-responsive sensor 24 if it varies in a non-linear fashion. The output 
of the second linearization circuitry 60 is a processed sensor 2 signal. 
The outputs of the first linearization circuitry 54 and the second 
linearization circuitry 60 are supplied as inputs to a rate matrix logic 
62. The rate matrix logic 62 accesses the memory circuit 30. The operation 
of the rate matrix logic 62 will become apparent in conjunction with the 
discussion of FIG. 3 below. At this point, suffice it to say that the rate 
matrix logic 62 takes the processed sensor signal and the processed sensor 
2 signal and from them produces a selected rate. The selected rate is 
indicated by the values of the inputs from the processed sensor 1 signal 
and the processed sensor 2 signal. Each matrix value, R.sub.nm, denotes 
one particular rate. 
The selected rate is supplied as an input to a reaction and recovery time 
circuit 64. The reaction and recovery time circuit 64 serves both to limit 
the rate at which the pacing rate will be allowed to rise and the rate at 
which the pacing rate will be allowed to fall. Thus, the reaction and 
recovery time circuit 64 will mimic the natural operation of a healthy 
heart, which is limited in how fast it will change rate. The reaction and 
recovery time circuit 64 is thus used to rate limit the selected rate 
output from the rate matrix logic 62 in the case when the sensor signal 
reacts faster than the normal heart, as is the case when an activity 
sensor is used. The output of the reaction and recovery time circuit 64 is 
the sensor-indicated rate signal. 
An alternate approach to performing the reaction and recovery time 
functions would be to include these functions on the input side of the 
system rather than on the output side as shown herein. Another way of 
implementing a reaction and recovery time function is by the use of a 
switch matrix. In a simpler system, the reaction and recovery time 
functions may be part of the initial signal processing, instead of being 
done after the rate matrix processing. In such a system, the slope of the 
input signals over time would be limited. 
The operation of the rate matrix logic 62 may be explained in conjunction 
with FIG. 3, which illustrates an example of the rate matrix logic 62. The 
rate matrix illustrated is a 10 by 10 rate matrix, with a total of 100 
cells. The cells each represent a particular selected rate which will be 
outputted from the rate matrix logic 62 for a set of specific values for 
the processed sensor signal and the processed sensor 2 signal. 
The value of the processed sensor 1 signal will be used to select which row 
the selected rate will be in, and the value of the processed sensor 2 
signal will be used to select which column the selected rate will be in. 
For example, if the processed sensor 1 signal is 32%, the fourth row will 
be indicated. Similarly, if the processed sensor 2 signal is 78%, the 
eighth column will be indicated. Thus, for these conditions, the selected 
rate would be R.sub.37. 
In the example illustrated, the processed sensor 1 signal and the processed 
sensor 2 signal are both analog signals. This example is made in analog 
terms for clarity. In the preferred embodiment, the processed sensor 1 
signal and the processed sensor 2 signal would be digital signals used to 
select the row and column, respectively, of the matrix shown in FIG. 3. 
The implementation of the control circuitry of the present invention could 
be either analog or digital circuitry. 
The rate matrix logic 62 is a look-up memory device which accesses the 
memory circuit 30 (typically a RAM) to find the selected rate indicated by 
the processed sensor 1 signal and the processed sensor 2 signal. It will 
thus be appreciated by those skilled in the art that the system of the 
present invention has a unique selected rate for each individual 
combination of the processed sensor 1 signal and the processed sensor 2 
signal. This represents a degree of versatility previously unknown in the 
art in utilizing inputs from two sensors, and is a tremendous improvement 
over the art. 
The processed sensor 1 signal and the processed sensor 2 signal values will 
be sampled at regular intervals, followed by a look-up of the proper 
selected rate value in the rate matrix. It is apparent that the overall 
transfer function may be linear or non-linear in any manner desired. If 
desired, a threshold value for one or both of the sensors may be built in, 
requiring the sensor signal to exceed a predetermined level before a rate 
change is implemented. 
When using an activity sensor together with a slow metabolic sensor, it is 
easy to allow the activity sensor to have greater influence over the rate 
in the absence of sensed metabolic activity until the patient has 
exercised long enough for the metabolic sensor to respond. Similarly, when 
metabolic activity is sensed, the metabolic sensor may be programmed to 
have greater influence over the rate than does the activity sensor. The 
versatility of this system thus retains the advantages of each sensor in a 
system which is truly better than the sum of its parts. 
Similarly, the system may be used to combine signals from two sensors, one 
of which has a good specific response for low levels of exercise (low work 
levels) and the other of which has a good specific response for high 
levels of exercise (high work levels). The sensor which has a good 
response for low levels of exercise will be primarily in control during 
situations when the exercise level is low, and the sensor which has a good 
response for high levels of exercise will be primarily in control when the 
exercise level is high. Again, the advantages of each of the sensors are 
retained while the disadvantages are discarded by the system of the 
present invention. 
It should be noted at this point that the utility of the present invention 
may be extended to systems using more than two sensors by using a rate 
matrix having more than two dimensions. Thus, for a system having three 
sensors, a three dimensional rate matrix could be used. For each unique 
combination of processed sensor values, there will be a corresponding 
unique selected rate value. 
In the preferred embodiment, the values inscribed in the rate matrix are 
programmable. An external programmer may be used to selectively program 
each of the rate values stored in the rate matrix. The rate matrix may 
thereby be tailored for different lifestyles, with factors such as age, 
activity level, and physical condition being used to select the values 
inscribed in the rate matrix. The external programmer may embody an expert 
system in software to guide the physician to the proper matrix values for 
various different patient qualities. 
As a variation on this first embodiment, multiple rate matrices may instead 
be stored in the memory circuit 30 with the rate matrix to be used being 
programmably selectable. The external programmer 46 (FIG. 1) would be used 
to select a desired rate matrix from the plurality of rate matrices stored 
in the memory circuit 30. Thus, the rate matrices thus may be tailored for 
different lifestyles, with factors such as age, activity level, and 
physical condition being used to select a desired rate matrix. 
While this system is quite flexible, the preferred embodiment of the 
present invention is even more sophisticated and versatile. In the 
preferred embodiment. a plurality of rate matrices are provided, with the 
particular rate matrix to be used being selected by a switch matrix. The 
criteria used to select which particular rate matrix is to be used may be 
various timing characteristics of the sensor response, or the satisfaction 
of various logic conditions. In the preferred embodiment a combination of 
these criteria is used. 
In this preferred embodiment, the rate matrix logic 62 and the memory 
circuit 30 of FIG. 2 may be replaced by the apparatus shown in FIG. 4. The 
example used in FIG. 4 has an activity sensor utilized as the first 
rate-responsive sensor 22 (FIG. 1) and a temperature sensor utilized as 
the second rate-responsive sensor 24 (FIG. 1). It will, of course, be 
realized by those skilled in the art that any combination of sensors could 
utilize the principles of operation of the present invention. 
Referring now to FIG. 4, it is readily apparent that a number of processing 
circuits receive as inputs the processed sensor 1 signal and the processed 
sensor 2 signal. These processing circuits provide outputs which are 
supplied to either a first priority logic circuit 63 or a second priority 
logic circuit 65. The priority logic circuits 63 and 65 each provide an 
input on one of a plurality of lines to a switch matrix 66. These inputs 
enable the selection by the switch matrix 66 of a single rate matrix. The 
output from the switch matrix 66 is a switch matrix row number i and a 
switch matrix column number j, which together identify a particular rate 
matrix contained in a memory containing rate matrices 68. 
The function of the priority logic circuits 63 and 65 is to receive a 
number of inputs, more than one of which may be digital ones. The priority 
logic circuits 63 and 65 output signals on a plurality of lines, only one 
of which from each of the priority logic circuits 63 and 65 may be a 
digital one at any given time. Therefore, the priority logic circuits 63 
and 65 each function to determine which one of a plurality of digital ones 
received as an input which should be provided as an output to the switch 
matrix 66. 
The various processing circuitry supplying inputs to the switch matrix 66 
may now be discussed. A first comparator 70 has as its input the processed 
sensor 1 signal, and is used to determine whether the processed sensor 
signal indicated that no activity is being sensed. In FIG. 4 the other 
input is grounded, but it could be a small threshold signal as well. If no 
activity (or a level of activity below the threshold) is being sensed, 
then the first comparator 70 outputs a digital one signal to the first 
priority logic circuit 63. 
A first timer 72 has as its input the processed sensor 1 signal, and is 
used to determine whether activity has been sensed for less than A 
seconds. If activity has in fact been sensed for less than A seconds, then 
the first timer 72 outputs a digital one signal to the first priority 
logic circuit 63. A second timer 74 has as its input the processed sensor 
1 signal, and is used to determine whether activity has been sensed for 
more than A seconds but less than B seconds, where B seconds is longer 
than A seconds. If activity has in fact been sensed for more than A 
seconds but less than B seconds, then the second timer 74 outputs a 
digital one signal to the first priority logic circuit 63. 
A third timer 76 has as its input the processed sensor 1 signal, and is 
used to determine whether activity has been sensed for more than C 
seconds, where C seconds is a considerable length of time indicating 
prolonged exercise, or a false positive. If activity has in fact been 
sensed for more than C seconds, then the third timer 76 outputs a digital 
one signal to the first priority logic circuit 63. 
A high positive slope detector 78 has as its input the processed sensor 1 
signal, and is used to detect the existence of a high positive rate of 
change in the processed sensor signal. If this high positive rate of 
change in the processed sensor 1 signal is detected by the high positive 
slope detector 78, and if activity has been sensed for more than B seconds 
but less than C seconds, then the high positive slope detector 78 outputs 
a digital one signal to the first priority logic circuit 63. 
A low positive slope detector 80 has as its input the processed sensor 1 
signal, and is used to detect the existence of a positive rate of change 
in the processed sensor 1 signal which is lower than that detected by the 
high positive slope detector 78. If this lower positive rate of change in 
the processed sensor 1 signal is detected by the low positive slope 
detector 80, and if activity has been sensed for more than B seconds but 
less than C seconds, then the low positive slope detector 80 outputs a 
digital one signal to the first priority logic circuit 63. The high 
positive slope detector 78 and the low positive slope detector 80 have 
mutually exclusive digital one outputs. 
A negative slope detector 82 has as its input the processed sensor 1 
signal, and is used to detect the existence of a negative rate of change 
in the processed sensor signal. If a negative rate of change in the 
processed sensor 1 signal is detected by the negative slope detector 82, 
and if activity has been sensed for more than B seconds but less than C 
seconds, then the negative slope detector 82 outputs a digital one signal 
to the first priority logic circuit 63. The priority logic circuits 63 and 
65 will determine the relative priority order of the input signals when 
several of the conditions have been met at the same time. They may also 
contain sequential circuitry which makes the priority order dependent on 
preceding inputs. The priority logic may be incorporated wholly or in part 
as part of the processing circuitry. 
If there is a digital one signal from the first comparator 70, the first 
priority logic circuit 63 will pass this signal on to the switch matrix 
66. If there is a digital one from the first timer 72, the first priority 
logic circuit 63 will pass this signal on to the switch matrix 66. If 
there is a digital one signal from the second timer 74, the first priority 
logic circuit 63 will pass this signal on to the switch matrix 66. If 
there is a digital one from the third timer 76, the first priority logic 
circuit 63 will pass this signal on to the switch matrix 66. Signals from 
the first comparator 70, the first timer 72, the second timer 74, and the 
third timer 76 are mutually exclusive. 
If there is a digital one signal from the high positive slope detector 78, 
and there are no digital one signals from the first comparator 70, the 
first timer 72, the second timer 74, or the third timer 76 (indicating 
that activity has been sensed for more than B seconds but less than C 
seconds), the first priority logic circuit 63 will pass the signal from 
the high positive slope detector 78 on to the switch matrix 66. 
If there is a digital one signal from the low positive slope detector 80, 
and there are no digital one signals from the first comparator 70, the 
first timer 72, the second timer 74, or the third timer 76 (indicating 
that activity has been sensed for more than B seconds but less than C 
seconds), the first priority logic circuit 63 will pass the signal from 
the low positive slope detector 80 on to the switch matrix 66. 
If there is a digital one signal from the negative slope detector 82, and 
there are no digital one signals from the first comparator 70, the first 
timer 72, the second timer 74, or the third timer 76 (indicating that 
activity has been sensed for more than B seconds but less than C seconds), 
the first priority logic circuit 63 will pass the signal from the negative 
slope detector 82 on to the switch matrix 66. The comparator 70, the first 
timer 72, the second timer 74, the third timer 76, the high positive slope 
detector 78, the low positive slope detector 80, and 82 are all used to 
select the row of the switch matrix 66, as will become apparent below in 
the discussion in conjunction with FIG. 5. The remaining processing 
circuitry to be discussed is used to select the column of the switch 
matrix 66. 
A second comparator 84 has as its input the processed sensor 2 signal, and 
is used to determine whether the processed sensor 2 signal indicated that 
no sensor 2 activity is being sensed. In FIG. 4 the other input is 
grounded, but it could be a small threshold signal as well. If no sensor 2 
activity (or a level of activity below the threshold) is being sensed, 
then the second comparator 84 outputs a digital one signal to the second 
priority logic circuit 65. 
A third comparator 86 has as inputs the processed sensor 1 signal and the 
processed sensor 2 signal. If the processed sensor 2 signal is greater 
than the processed sensor 1 signal, then the third comparator 86 outputs a 
digital one signal to the second priority logic circuit 65. (This of 
course would only happen when at least some activity was being sensed by 
sensor 2, indicating either a high body temperature in the absence of 
physical activity, or that the second sensor indicates a higher level of 
exercise than the first sensor. Thus, the second comparator 84 and the 
third comparator 86 have mutually exclusive digital one outputs.) 
An initial dip detector 88 has as its input the processed sensor 2 signal, 
and is used to detect an initial dip in temperature characterized by a 
drop in the processed sensor 2 signal following a long period of 
inactivity of the processed sensor 2 signal. If this initial dip in the 
processed sensor 2 is detected by the initial dip detector 88, then the 
initial dip detector 88 outputs a digital one signal to the second 
priority logic circuit 65. 
A positive slope detector 90 has as its input the processed sensor 2 
signal, and is used to detect the existence of a positive rate of change 
in the processed sensor 2 signal. If a positive rate of change in the 
processed sensor 2 signal is detected by the positive slope detector 90, 
then the positive slope detector 90 outputs a digital one signal to the 
second priority logic circuit 65. 
A negative slope detector 92 has as its input the processed sensor 2 
signal, and is used to detect the existence of a negative rate of change 
in the processed sensor 2 signal. If a negative rate of change in the 
processed sensor 2 signal is detected by the negative slope detector 92, 
then the negative slope detector 92 outputs a digital one signal to the 
second priority logic circuit 65. If there is a digital one signal from 
the second comparator 84, the second priority logic circuit 65 will pass 
this signal on to the switch matrix 66. If there is a digital one signal 
from the third comparator 86, the second priority logic circuit 65 will 
pass this signal on to the switch matrix 66. If there is a digital one 
signal from the initial dip detector 88, and if the output of the fourth 
comparator 86 is not a digital one signal, the second priority logic 
circuit 65 will pass the signal from the initial dip detector 88 on to the 
switch matrix 66. 
If there is a digital one signal from the positive slope detector 90, and 
if the output of the fourth comparator 86 is not a digital one signal, the 
second priority logic circuit 65 will pass the signal from the positive 
slope detector 90 on to the switch matrix 66. If there is a digital one 
signal from the negative slope detector 92, and if the output of the 
fourth comparator 86 is not a digital one signal, and if the output of the 
initial dip detector 88 is not a digital one signal, the second priority 
logic circuit 65 will pass the signal from the positive slope detector 90 
on to the switch matrix 66. 
Referring now to FIG. 5 in addition to FIG. 4, the operation of the switch 
matrix 66 may be explained. The switch matrix shown in FIG. 5 is a 7 by 5 
switch matrix used to select a specific rate matrix contained in the 
memory containing rate matrices 68. The selection of a cell in the rate 
matrix of FIG. 5 thus corresponds to the selection of a specific rate 
matrix intended to optimize the pacemaker rate for the specific conditions 
identified by the switch matrix. 
The process used to select a specific cell in the switch matrix of FIG. 5 
will now be explained. If the first comparator 70 outputs a signal which 
is provided by the first priority logic circuit 63 to the switch matrix 
66, then the first row of the switch matrix of FIG. 5 will be chosen. If 
the first timer 72 outputs a signal which is provided by the first 
priority logic circuit 63 to the switch matrix 66, then the second row of 
the switch matrix of FIG. 5 will be chosen. If the second timer 74 outputs 
a signal which is provided by the first priority logic circuit 63 to the 
switch matrix 66, then the third row of the switch matrix of FIG. 5 will 
be chosen. 
If the high positive slope detector 78 outputs a signal which is provided 
by the first priority logic circuit 63 to the switch matrix 66, then the 
fourth row of the switch matrix of FIG. 5 will be chosen. If the low 
positive slope detector 80 outputs a signal which is provided by the first 
priority logic circuit 63 to the switch matrix 66, then the fifth row of 
the switch matrix of FIG. 5 will be chosen. If the negative slope detector 
82 outputs a signal which is provided by the first priority logic circuit 
63 to the switch matrix 66, then the sixth row of the switch matrix of 
FIG. 5 will be chosen. If the third timer 76 outputs a signal which is 
provided by the first priority logic circuit 63 to the switch matrix 66, 
then the seventh row of the switch matrix of FIG. 5 will be chosen. 
If the second comparator 84 outputs a signal which is provided by the 
second priority logic circuit 65 to the switch matrix 66, then the first 
column of the switch matrix of FIG. 5 will be chosen. If the initial dip 
detector 88 outputs a signal which is provided by the second priority 
logic circuit 65 to the switch matrix 66, then the second column of the 
switch matrix of FIG. 5 will be chosen. If the positive slope detector 90 
outputs a signal which is provided by the second priority logic circuit 65 
to the switch matrix 66, then the third column of the switch matrix of 
FIG. 5 will be chosen. 
If the negative slope detector 92 outputs a signal which is provided by the 
second priority logic circuit 65 to the switch matrix 66, then the fourth 
column of the switch matrix of FIG. 5 will be chosen. If the fourth 
comparator 86 outputs a signal which is provided by the second priority 
logic circuit 65 to the switch matrix 66, then the fifth column of the 
switch matrix of FIG. 5 will be chosen. This completes the selection of a 
cell, since both a row and a column will have been selected. 
This description is made for explanatory purposes only. In a realistic 
system, the outputs from the individual sensor processing circuits 
defining certain conditions of each sensor signal would be interconnected 
to the switch matrix by a (programmable) logic circuit network which would 
establish the priority order of the detected conditions. For instance, 
following a time period with zero signal from the first sensor, the first 
timer 72 may be given priority over slope detectors 78, 80, and 82. 
For example, if the second timer 74 outputs a signal through the first 
priority logic circuit 63 to the switch matrix 66 (indicating that 
activity has in fact been sensed for more than A seconds but less than B 
seconds), the third row in the switch matrix of FIG. 5 will be selected. 
If the third comparator 86 outputs a signal through the second priority 
logic circuit 65 to the switch matrix 66 (indicating that the processed 
sensor 2 signal is greater than the processed sensor signal), the fifth 
column in the switch matrix of FIG. 5 will be selected. Thus rate matrix 
RM.sub.35 will have been selected. 
The rate matrix RM.sub.35, like all the other rate matrices referenced in 
the switch matrix of FIG. 5, is contained in the memory containing rate 
matrices 68 of FIG. 4. Thus, the value of i supplied from the switch 
matrix 66 to the memory containing rate matrices 68 would be 3, and the 
value of j supplied from the switch matrix 66 to the memory containing 
rate matrices 68 would be 5. It will be noted that the processed sensor 1 
signal and the processed sensor 2 signal are also supplied to the memory 
containing rate matrices 68, and hence to the selected rate matrix 
RM.sub.35. 
Thus, following selection of the appropriate rate matrix, the inputs of the 
processed sensor 1 signal and the processed sensor 2 signal will enable 
the selected rate matrix to find the particular cell therein which 
corresponds to the processed sensor signal and the processed sensor 2 
signal. That cell will contain the selected rate, which is output from the 
memory containing rate matrices 68. The balance of the operation is as 
explained with reference to FIGS. 1 and 2. 
It may thus be appreciated by those skilled in the art that the switch 
matrix 66 is used to analyze the particular circumstances indicated by the 
signals from the sensors. A rate matrix specifically designed for use 
under the particular circumstances is indicated, and the processed signals 
from the sensors are supplied to the selected rate matrix, which is 
contained in the memory containing rate matrices 68. A selected rate 
indicated in the selected rate matrix by the values of the processed 
signals from the sensors is selected, and used to control the pacing rate 
of the pacemaker 20. 
In the preferred embodiment, the switch matrix would also be programmable 
using an external programmer. This would even further allow the operation 
of the pacemaker to be individually tailored by the physician. Again, an 
expert program would be used to assist the physician in programming the 
switch matrix. In addition, the logic circuitry used to drive the switch 
matrix may also be programmable, at least to some extent. Time values may 
be programmable in the timers, for example. It may thus be appreciated 
that the system may be virtually fully programmable, if desired. 
As an example, when a signal is first present from the activity sensor it 
may be allowed to drive the rate to a higher value in the absence of a 
change in the temperature signal. For example, in the first 45 seconds of 
action in the activity signal, a high signal from the activity sensor with 
no activity indicated by a temperature sensor may be allowed to drive the 
rate of the pacemaker to a high rate. 
Then, after such a time when one would anticipate a change in the 
temperature signal if activity were in fact occurring, if there is no 
change in the temperature signal indicating activity, a different rate 
matrix will be accessed causing the selected rate to diminish. For 
example, in the interval between 45 and 90 seconds after the onset of the 
activity signal, a high signal from the activity sensor with no activity 
indicated by the temperature sensor would be allowed to drive the rate of 
the pacemaker to a moderate rate only. 
Then, after a time interval when there would definitely be a change in the 
temperature signal if activity were in fact occurring, if there is no 
change in the temperature signal indicating activity, still another rate 
matrix will be accessed causing the selected rate to further diminish. For 
example, after 90 seconds from the onset of the activity signal, a high 
signal from the activity sensor with no activity indicated by the 
temperature sensor would be almost totally discarded, driving the 
pacemaker at or only slightly above the rest rate. 
In an alternate embodiment differing from the embodiment in FIG. 4, a 
switch matrix may be used in conjunction with only one of the two sensor 
inputs to the system. Referring now to FIG. 6, the system of FIG. 4 is 
shown modified so that only the first sensor is used to operate the switch 
matrix. This configuration will work well when one sensor has a nearly 
ideal response with virtually no artifacts. One example of such a sensor 
is an oxygen saturation sensor. The example used in FIG. 6 again has an 
activity sensor utilized as the first rate-responsive sensor 22 (FIG. 1) 
and a temperature sensor utilized as the second rate-responsive sensor 24 
(FIG. 1). It will, of course, be realized by those skilled in the art that 
any combination of sensors could utilize the principles of operation of 
the present invention. 
It is readily apparent that a number of processing circuits receive as 
inputs the processed sensor signal. These processing circuits provide 
outputs which are supplied through the first priority logic circuit 63 as 
inputs to a switch matrix 96. These inputs enable the selection by the 
switch matrix 96 of a single rate matrix. The output from the switch 
matrix 96 is a switch matrix row number i which identifies a particular 
rate matrix contained in a memory containing rate matrices 98. 
The various processing circuitry supplying inputs to the first priority 
logic circuit 63 are similar to those used in FIG. 4 for the first sensor 
input. The first comparator 70, the first timer 72, the second timer 74, 
the third timer 76, the high positive slope detector 78, the low positive 
slope detector 80, and the negative slope detector 82 are used to process 
the processed sensor 1 signal, and to supply inputs to the memory 
containing rate matrices 98. The operation of these components is the same 
as explained above in conjunction with FIG. 4. The operation of the first 
priority logic circuit 63 is also the same as explained above in 
conjunction with FIG. 4. 
Referring now to FIG. 7 in addition to FIG. 6, the operation of the switch 
matrix 96 will be apparent. The switch matrix shown in FIG. 7 is a 7 by 1 
switch matrix used to select a specific rate matrix contained in the 
memory containing rate matrices 98. The selection of a cell in the rate 
matrix of FIG. 7 thus corresponds to the selection of a specific rate 
matrix. 
The process used to select a specific cell in the switch matrix of FIG. 7 
is analogous to the process used to select a specific row in the system of 
FIG. 4. If the first comparator 70 outputs a signal through the first 
priority logic circuit 63 to the switch matrix 96, then the first row of 
the switch matrix of FIG. 7 will be chosen. If the first timer 72 outputs 
a signal through the first priority logic circuit 63 to the switch matrix 
96, then the second row of the switch matrix of FIG. 7 will be chosen. If 
the second timer 74 outputs a signal through the first priority logic 
circuit 63 to the switch matrix 96, then the third row of the switch 
matrix of FIG. 7 will be chosen. 
If the high positive slope detector 78 outputs a signal through the first 
priority logic circuit 63 to the switch matrix 96, then the fourth row of 
the switch matrix of FIG. 7 will be chosen. If the low positive slope 
detector 80 outputs a signal through the first priority logic circuit 63 
to the switch matrix 96, then the fifth row of the switch matrix of FIG. 7 
will be chosen. If the negative slope detector 82 outputs a signal through 
the first priority logic circuit 63 to the switch matrix 96, then the 
sixth row of the switch matrix of FIG. 7 will be chosen. If the third 
timer 76 outputs a signal through the first priority logic circuit 63 to 
the switch matrix 96, then the seventh row of the switch matrix of FIG. 7 
will be chosen. This completes the selection of a cell, since for the 
switch matrix of FIG. 7 only a row need be selected. 
For example, if the second timer 74 outputs a signal through the first 
priority logic circuit 63 to the switch matrix 96 (indicating that 
activity has in fact been sensed for more than A seconds but less than B 
seconds), the third row in the switch matrix of FIG. 7 will be selected. 
Thus rate matrix RM.sub.3 will have been selected. The rate matrix 
RM.sub.3, like all the other rate matrices referenced in the switch matrix 
of FIG. 7, is contained in the memory containing rate matrices 98 of FIG. 
6. Thus, the value of i supplied from the switch matrix 96 to the memory 
containing rate matrices 98 would be 3. It will again be noted that the 
processed sensor 1 signal and the processed sensor 2 signal are both 
supplied to the memory containing rate matrices 98, and hence to the 
selected rate matrix RM.sub.3. 
Thus, following selection of the appropriate rate matrix, the inputs of the 
processed sensor signal and the processed sensor 2 signal will enable the 
selected rate matrix to find the particular cell therein which corresponds 
to the processed sensor signal and the processed sensor 2 signal. That 
cell will contain the selected rate, which is output from the memory 
containing rate matrices 98. The balance of the operation is again as 
explained with reference to FIGS. 1 and 2. 
It may therefore be appreciated from the above detailed description of the 
preferred embodiment of the present invention that it teaches a system 
which will utilize inputs from two or more sensors to provide a 
sensor-indicated rate signal, which will control the pacing rate of the 
pacemaker. The system of the present invention utilizes the best 
properties of each of the sensors, while minimizing or eliminating their 
drawbacks. The control strategy is of a complexity sufficient to provide 
as an output a highly flexible sensor-indicated rate signal which will 
accurately follow a control strategy paralleling the physiological 
response of a healthy heart. In addition to being highly flexible, the 
control strategy of the preferred embodiment of the present invention is 
also fully programmable by using an external programmer. 
The implementation of the present invention is relatively simple and easy 
to accomplish in a pacemaker, and will not increase the size of the 
pacemaker. The system of the present invention is useable with two 
sensors, and is capable of working with more than two sensors. The system 
is also economic of power, and does not require more power to operate than 
do other rate response processors. Finally, all of the aforesaid 
advantages and objectives are achieved without incurring any substantial 
relative disadvantage. 
Although an exemplary embodiment of the present invention has been shown 
and described, it will be apparent to those having ordinary skill in the 
art that a number of changes, modifications, or alterations to the 
invention as described herein may be made, none of which depart from the 
spirit of the present invention. All such changes, modifications, and 
alterations should therefore be seen as within the scope of the present 
invention.