Rate-responsive pacemaker having automatic sensor threshold with programmable offset

A programmable offset is added to an automatically generated baseline reference value to provide a Threshold value used by the rate-responsive sensor processing circuits of an implantable rate-responsive pacemaker to determine the significance of a sensor input signal. The rate-responsive pacemaker provides stimulation pulses on demand at a pacing rate determined by a sensed physiological parameter. The physiological parameter is sensed by a physiological sensor included within, or coupled to, the rate-responsive pacemaker. The physiological sensor generates a sensor input signal having a magnitude that varies as a function of the sensed physiological parameter. The invention provides a way for the rate-responsive pacemaker, when operating in an autothreshold mode, to automatically determine when the magnitude of the sensor input signal is sufficiently large to justify an increase in the pacing rate. A long-term running average of the sensor input signal is continuously maintained, and is used as a baseline threshold value. A programmable offset is added to the baseline reference value. Any sensor input signal that exceeds the baseline reference value plus the programmable offset is considered to be sufficiently large to effect an increase in the pacing rate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to implantable medical devices and methods, 
and more particularly to a rate-responsive pacemaker having an automatic 
sensor threshold with a programmable offset. 
In Applicants' earlier U.S. Pat. No. 4,940,052, there is disclosed a 
microprocessor-controlled, rate-responsive pacemaker having automatic rate 
response threshold adjustment. The present invention relates to an 
improvement over the invention disclosed in the '052 patent. The '052 
patent is incorporated herein by reference. 
As taught in the '052 patent, the rate-responsive pacemaker therein 
disclosed includes a physiological sensor that generates a raw signal, or 
"raw sensor signal," as a function of a sensed physiological parameter, 
which physiological parameter provides an indication of what the pacing 
rate of the pacemaker should be. The raw sensor signal is converted to one 
of a plurality of discrete sensor level index signals. The particular 
sensor level index signal derived from the raw sensor is then used, in 
conjunction with a selected Slope parameter, to point to a particular 
sensor-indicated rate (SIR) signal. The SIR signal may then be used by the 
pacemaker to define a pacing rate. 
In order to prevent inappropriate increases in pacing rate while a patient 
is at rest or at low levels of activity, the '052 patent offers a 
plurality of programmable sensor rate response threshold values. The 
selected sensor response threshold value represents a minimum level of 
patient activity that must occur before the raw sensor signal is 
considered sufficiently large to represent meaningful physiological 
activity that should increase the SIR signal. One embodiment disclosed in 
the '052 patent provides for the automatic setting of the sensor rate 
response threshold by averaging the sensor index signal over a prescribed 
period to time and by adding thereto a fixed threshold offset value. In a 
preferred embodiment, the '052 patent teaches that the sensor level index 
signals be averaged over a preceding 18 hour period, and that a fixed 
offset of two (2) be added. (Note, that as used herein the sensor "offset" 
is measured in sensor units, which are relative units related to the 
maximum raw sensor output signal. For example, the full scale sensor 
output may be represented by 13 sensor units.) 
Unfortunately, while the "autothreshold" feature of the invention disclosed 
in the '052 patent represents a valuable tool for a physician to use in 
programming the parameters of a rate-responsive pacemaker, the fixed 
sensor offset that is added to the 18 hour average as taught in the '052 
patent makes such autothreshold feature impractical and undesirable for 
most patients. This is because a fixed offset, even when added to an 
automatically adjustable threshold, does not invoke the same pacemaker 
behavior in different patients. A sensor threshold and offset that 
provides ideal performance in one patient may be over response or 
under-responsive in another patient, or even for the same patient at a 
future time. Thus, each patient is unique, and requires a highly 
customized and versatile setting of the sensor rate response threshold and 
offset. What is needed, therefore, is a more versatile way of conditioning 
the raw sensor signal to fit the needs of a particular patient over time, 
and a way of programmably and/or automatically setting the "offset" that 
is used in conjunction with determining a sensor threshold. 
The present invention advantageously addresses the above and other needs. 
SUMMARY OF THE INVENTION 
The present invention provides a way for a rate-responsive pacemaker to 
automatically determine when the magnitude of a sensor input signal, 
generated by a physiological sensor as a result of sensing a prescribed 
physiological parameter, is sufficiently large to evidence significant 
physiological activity to justify an increase in the pacing rate of the 
pacemaker. In accordance with the invention, the raw sensor signal is 
preliminary processed to produce a sensor input signal during each pacing 
cycle. In the autothreshold mode, a long-term running average of the 
sensor input signal is continuously maintained. The long-term running 
average of the sensor input signal is used as a baseline threshold value. 
A programmable offset is then added to the baseline threshold value. Any 
sensor input signal that exceeds the baseline threshold plus the 
programmable offset is considered to be sufficiently large to effect an 
increase in the pacing rate. 
The use of the programmable offset advantageously makes the rate-responsive 
pacemaker non-responsive to small changes in the sensor input signal, yet 
still allows such changes to be included in the overall determination of 
an appropriate long-term average of the sensor input signal. 
In operation, a suitable long-term average of the sensor input signal is 
computed as an average of the last n short-term averages of such sensor 
input signal, where n is a large integer, e.g., greater than 30. 
(Alternately, instead of using short-term averages, the raw or processed 
signal could also be used.) A short-term average of the sensor input 
signal may comprise, for example, the sensor input signal as filtered, or 
otherwise processed, over the previous pacing cycle. Alternatively, a 
short-term average may comprise m consecutive samples of the sensor input 
signal, where m is a small integer, e.g., 3-10. The short-term average 
thus provides a means of smoothing or filtering the raw sensor signal, and 
the sensor input signal may thus generally be considered as the smoothed 
or filtered raw sensor signal. 
The long term average provides a baseline threshold ("T") reference value. 
In accordance with the present invention, a programmable offset ("O") 
value is added to the baseline threshold reference value to produce a 
threshold plus offset ("T+O") reference value. A given short-term average 
of the sensor input signal, i.e., a given sensor reading, must be greater 
than the T+O reference value before the sensor input signal is considered 
sufficiently large to evidence significant physiological activity. In 
other words, unless a given short-term average of the sensor input signal 
is greater than the threshold plus offset reference value, the sensor 
input signal, including the minor variations and fluctuations therein, are 
not considered by the rate-responsive sensor processing circuits, just as 
if no physiological activity had been sensed. 
The value of the offset may be determined in various ways. In one 
embodiment of the invention, for example, the offset is programmable, and 
may be programmed to assume one of a plurality of values, e.g., 0, 0.5, 
1.0, or 1.5 sensor units, where a full scale (maximum value of the raw 
sensor signal) is roughly 13 sensor units. In another embodiment of the 
invention, the offset is automatically determined as a function of the 
peak values of the short term raw sensor signal averaged over the last n 
short term averages, i.e., as a function of the peak values of the sensor 
readings over the last n sensor readings, where n is a large integer. 
One embodiment of the present invention may be characterized as an 
implantable rate-responsive pacemaker. Such rate-responsive pacemaker 
includes: (1) physiological sensing means for sensing a physiological 
parameter and generating a raw sensor signal as a function of the sensed 
physiological parameter; (2) pre-processing means for processing the raw 
sensor signal and converting it to a sensor input signal; (3) means for 
computing a long-term average of the sensor input signal; (4) means for 
adding a programmably selectable offset to the long-term average so as to 
provide a Threshold value; (5) threshold means for determining when the 
sensor input signal exceeds the Threshold value; and (6) means responsive 
to the threshold means for providing stimulation pulses on demand at a 
pacing rate determined by a programmably selectable Slope value and the 
amount by which the sensor input signal exceeds the Threshold value. 
Another embodiment of the invention may similarly be characterized as an 
implantable rate-responsive pacemaker. In this embodiment, the 
rate-responsive pacemaker is characterized as including: (1) a 
physiological sensor that senses a physiological parameter and generates a 
sensor input signal as a function of the sensed physiological parameter; 
(2) an averaging circuit that computes a long-term average of the sensor 
input signal; (3) a threshold circuit that determines when the sensor 
input signal exceeds the long-term average of the sensor input signal plus 
a selectable offset value; and (4) a pulse generator responsive to the 
threshold circuit that provides stimulation pulses on demand at a pacing 
rate determined by a programmably selectable Slope value and the amount by 
which the sensor input signal exceeds the long-term average of the sensor 
input signal plus the selectable offset value. 
Yet another embodiment of the invention may be characterized as a method of 
automatically setting the Threshold value of a rate-responsive pacemaker. 
The rate-responsive pacemaker used with such method has a physiological 
sensor that senses a physiological parameter and generates a sensor input 
signal as a function thereof. The Threshold value is used by the 
rate-responsive pacemaker to provide a reference value that the sensor 
input signal must exceed before such sensor input signal is considered as 
evidence of significant physiological activity to which the 
rate-responsive pacemaker should respond. The method in accordance with 
this embodiment includes the steps of: (a) processing the sensor input 
signal over a prescribed time period to provide a reference sensor input 
signal that is representative of the sensor input signals during the 
prescribed time period; (b) programmably selecting one of a plurality of 
offset values; and (c) adding the offset value selected in step (b) to the 
baseline reference sensor input signal determined in step (a) to obtain 
the Threshold value. 
Thus, it is a feature of the present invention, in accordance with one 
embodiment thereof, to provide a rate-responsive pacemaker wherein a 
programmable offset is added to an automatically generated baseline 
threshold value in order to define a Threshold level above which the 
sensor input signal of the rate-responsive pacemaker must go before such 
sensor input signal is considered as evidence of significant physiological 
activity. 
It is another feature of the invention, in accordance with another 
embodiment thereof, to automatically determine an offset that is added to 
an automatically generated baseline threshold value in order to define a 
Threshold level above which the sensor input signal of the rate-responsive 
pacemaker must go before such sensor input signal is considered as 
evidence of significant physiological activity. 
It is an additional feature of the invention to provide a programmable 
rate-responsive pacemaker that may be selectively programmed to provide 
either: (1) an automatically generated baseline threshold, to which a 
selected one of a plurality of offset values may be added, or (2) a 
programmed threshold value selected from a plurality of fixed baseline 
threshold values, to which no offset is added, in order to define the 
sensor Threshold level of the pacemaker. The sensor Threshold level of the 
pacemaker, as above, is that threshold reference level which must be 
exceed by the sensor input signal before it is considered as an indication 
of significant physiological activity.

DETAILED DESCRIPTION OF THE INVENTION 
The following description is of the best mode presently contemplated for 
carrying out the invention. This description is not to be taken in a 
limiting sense, but is made merely for the purpose of describing the 
general principles of the invention. The scope of the invention should be 
determined with reference to the claims. 
The present invention relates to a rate-responsive pacemaker having an 
automatic sensor threshold with a programmable offset. Before describing 
the invention, and in order to better understand the description of the 
invention that follows, it will first be helpful to have a basic 
understanding of how a rate-responsive pacemaker operates, as well as an 
understanding of how such a pacemaker is programmed. Accordingly, an 
overview of the operation of a rate-responsive pacemaker will first be 
presented, including a description of the programmable sensor parameters 
that are used with such a rate-responsive pacemaker. More complete details 
associated with the rate-responsive pacemaker used with the present 
invention, as well as the preferred programmer used to program such 
pacemaker may be found in U.S. Pat. Nos. 4,809,697 and 4,940,052. The '697 
patent is incorporated herein by reference. (The '052 patent has already 
been incorporated herein by reference.) Further, additional details 
associated with some related features of the present invention may be 
found in the following copending and commonly owned U.S. patent 
applications: (1) Ser. No. 07/846,461, filed concurrently herewith, 
entitled METHOD AND SYSTEM FOR RECORDING AND REPORTING THE DISTRIBUTION OF 
ING EVENTS OVER TIME; (2) Ser. No. 07/846,460, filed concurrently 
herewith, entitled METHOD AND SYSTEM FOR RECORDING AND REPORTING A 
SEQUENTIAL SERIES OF ING EVENTS; and (3) Ser. No. 07/844,818, also 
filed concurrently herewith, entitled METHOD AND SYSTEM FOR AUTOMATICALLY 
ADJUSTING THE SENSOR AMETERS OF A RATE-RESPONSIVE EMAKER. Each of 
the above-identified U.S. Patent Applications are also incorporated herein 
by reference. 
Referring first to FIG. 1, there is shown a functional block diagram of a 
rate-responsive pacemaker 16 that illustrates the manner in which the 
pacemaker operates. The pacemaker 16 includes pacemaker state logic 42, 
also referred to as the pacemaker state machine. Coupled to the state 
machine 42 are pacemaker timer circuits 50, also referred to as the 
pacemaker timer block. The pacemaker 16 receives as inputs, i.e., signals 
sensed by the pacemaker that are not programmed, signals from an atrial 
sensor 102 and a ventricular sensor 104. The atrial sensor 102 and 
ventricular sensor 104 sense P-waves and R-waves, evidencing the natural 
contraction of the atria or ventricles, respectively. The atrial sensor 
102, for example, may include an atrial tip electrode, an atrial lead 31 
(not shown) and atrial channel amplifier 48 (not shown). Similarly, the 
ventricular sensor 104 may comprise a ventricular tip electrode, a 
ventricular lead and a ventricular amplifier. 
The inputs to the rate-responsive pacemaker 16 also include a raw sensor 
signal 27 obtained from a physiological sensor 26. (It is noted that while 
only a single physiological sensor 26 is shown in FIG. 1, more than one 
such sensor may be used, each providing its own sensor input.) The raw 
sensor signal 26 is input to a rate-responsive sensor processing circuit 
124. After appropriate processing, as described more fully below, the 
sensor processing circuit 124 provides a sensor indicated rate (SIR) 
signal 126 to the pacemaker state machine 42. 
In addition to the above-described pacemaker inputs, there are several 
pacemaker control parameters that are input to the pacemaker state machine 
42 in order to control its operation in a desired fashion. Such control 
parameters are normally programmed into the pacemaker 16 using an external 
programmer 20, such as the APS-II/MTM external programmer manufactured by 
Siemens Pacesetter, Inc. of Sylmar, Calif., that establishes a telemetry 
link 70 with a telemetry circuit 40 included within the pacemaker 16. 
The parameters programmed into the pacemaker are typically stored in a 
memory 62 of the pacemaker 16. (The memory 62 is not shown as a separate 
block in FIG. 1, but it is to be understood that the programmed parameters 
may be held in such memory, as may output data generated by the pacemaker 
that is to be telemetered to the programmer 20.) Such control parameters 
include, e.g., the programmed rate at the which the stimulation pulses are 
to be generated by the pacemaker (used to define various time periods 
within the timer block 50), the particular mode of operation of the 
pacemaker, a set of sensor control parameters 130 (described below), and 
the like. 
The rate-responsive pacemaker outputs, i.e., signals generated by the 
pacemaker state machine 42 in response to the pacemaker inputs and/or 
pacemaker control parameters include an atrial output 106 and a 
ventricular output 108. The atrial output 106 provides an atrial 
stimulation pulse ("A-pulse") for delivery to the atrium at an appropriate 
time, e.g., on demand as needed to maintain a programmed or 
sensor-indicated heart rate. The ventricular output 108 similarly provides 
a ventricular stimulation pulse ("V-pulse") for delivery to the ventricle 
at an appropriate time, e.g., on demand as needed to maintain a programmed 
or sensor-indicated heart rate. 
The pacemaker timer circuits 50 include at least five separate timers. A 
rate timer 110 determines or measures the pacing cycle duration. An AV 
Delay Timer 112 defines the time period between an A-pulse and a V-pulse. 
A Max Track Timer 114 defines the time period of the maximum rate at which 
the pacemaker is allowed to provide stimulation pulses, i.e., it defines 
the maximum paced rate. An Atrial Refractory Timer 116 defines the atrial 
refractory period (i.e., that time period during which the atrial channel 
is refractory). Similarly, a Ventricular Refractory Timer 118 defines the 
ventricular refractory period, or that time during which the ventricular 
channel is refractory. 
Note from the symbols used in FIG. 1 that two kinds of data are passed to 
and from the pacemaker state machine 42. Such data may take the form of a 
trigger signal or a parameter signal. A trigger signal, represented by an 
input line with an arrow pointing the direction of flow of the trigger 
data, is a signal that operates substantially immediately, much like an 
interrupt signal, to bring about a desired result. That is, for example, 
immediately upon sensing atrial activity through the atrial sensor (or 
within a few clock cycles thereafter, where a clock cycle is typically on 
the order of a few microseconds), the state of the state machine 42 
changes appropriately to bring about a desired result. In contrast, a 
parameter signal, represented by an input line passing through a circle 
with an arrow pointing the direction of flow of the parameter data, is a 
signal that is made available to the state machine 42 for use at the 
appropriate time during the normal timing cycle of the state machine. 
In accordance with the present invention, the rate-responsive sensor 
processing circuits 124 receive the raw signal 27 from the sensor 26 and 
derive a SIR signal 126 therefrom based on a set of sensor control 
parameters 130. The SIR signal 126 may then be used by the state machine 
42, if so programmed, to control the rate at which stimulation pulses are 
provided to the heart on demand through the atrial or ventricular outputs 
106 and 108. The SIR signal 126 is sampled at a fixed, but programmable 
rate (which may be, e.g., every event, every 1.6 seconds, or every 26 
seconds). The SIR signal as sampled may be classified by rate and stored 
in an SIR Table 120. (The SIR Table 120, which effectively accumulates SIR 
Histogram and Event Record data, is described more fully in one of the 
above referenced copending patent applications, and is also described in 
the '052 patent cited above.) 
The SIR Histogram Table 120 and the beneficial use of the data stored 
therein is not the subject of the present invention. Rather, the present 
invention relates to the manner in which the SIR signal is derived from 
the raw sensor signal 27. More particularly, the present invention relates 
to the manner in which a determination is made as to whether the raw 
sensor signal is of sufficient magnitude to evidence significant 
physiological activity. As explained below, such determination is made as 
controlled by one or more programmed sensor control parameters. 
The sensor control parameters that may be programmed in a rate-responsive 
pacemaker 16 are functionally illustrated in FIG. 2. FIG. 2 shows a 
functional block diagram of the rate-responsive processing subsystem 124 
of the rate-responsive pacemaker 16 of FIG. 1. FIG. 2 shows the set of 
sensor control parameters 130 that are used in deriving the sensor 
indicated rate (SIR) signal 126. FIG. 2 further diagrammatically 
illustrates how such derivation is accomplished as a function of the raw 
sensor signal 27. 
As seen in FIG. 2, there may be up to six sensor control parameters that 
are programmably selected. Such programmable sensor control parameters may 
thus be considered as inputs to the rate-responsive processing subsystem 
124. These six sensor control parameters are: a Threshold parameter 131 
(which, as explained below, may be selected to be an autothreshold value 
with a programmable offset); a Recovery Time parameter 134; a Reaction 
Time parameter 136; a Base Rate parameter 138; Maximum Sensor Rate 
parameter 142; and Slope parameter 144. As also seen in FIG. 2, there is 
one output parameter: a Sensor Indicated Rate (SIR) parameter or signal 
126. These parameters are explained more fully below. 
The sensor 26 (which is illustrated in FIG. 1 and FIG. 2 as an "activity 
sensor"; but which may be another type of physiological sensor, or 
combination of physiological sensors) generates a raw signal 27 in 
response to detected physiological stress in the patient. The raw signal 
27 is processed in an appropriate manner, e.g., to determine the energy 
content thereof as taught in U.S. Pat. No. 4,940,053, in order to provide 
a suitable sensor input signal 133 that may be processed by the 
rate-responsive sensor processing subsystem 124. (The '053 patent is also 
incorporated herein by reference.) The processed sensor input signal 133 
will thus vary as a function of time, as suggested by the graph 135, as 
the physiological stress of the patient varies as detected by the sensor 
26. (Note that this signal 133 is termed a "sensor input signal" because 
it is input into the rate-responsive sensor processing subsystem.) 
The Threshold parameter 131 defines the level above which the sensed 
physiological stress, e.g., activity, must rise before it is considered 
significant. Once the stress level has risen above this level, referred to 
as the Threshold 132 (and represented in FIG. 2 as a dotted line 132), 
then the amount that it exceeds the Threshold 132 is used as an input to 
the rate-responsive sensor processing subsystem 124. This signal amount 
(the signal amount above the Threshold 132) is schematically represented 
in FIG. 2 as the bracketed area 137 for the time corresponding to the 
right edge of the graph 135. As described more fully below, the present 
invention relates to the specific manner in which the signal amount 137 is 
determined at any given time. More particularly, as will be evident from 
the description presented below in conjunction with FIGS. 3-6, the present 
invention provides a means whereby the Threshold 132 may be automatically 
set based on a long-term average of the sensor input signal 133 and a 
programmable offset value that is added to such long-term average. 
Still referring to FIG. 2, the Slope parameter 144 defines the relationship 
between the amount the sensor input signal is above the Threshold 132, if 
any, and the increase or decrease in pacing rate. That is, the Slope 
parameter may be considered, as its name implies, as a curve or transfer 
function that converts that portion 137 of the sensor input signal above 
the Threshold level to the sensor indicated rate (SIR) signal 126. It 
should also be pointed out that if the sensor input signal is not above 
the Threshold level, then that fact too may influence the determination of 
the SIR signal 126. That is, if the sensor input signal is not above the 
Threshold 132, then that evidences, in effect, a zero sensor input signal 
(i.e., the lack of significant physiological activity). The lack of sensed 
physiological activity, as explained below, can cause the SIR signal to 
decrease to the base rate. 
There are a multiplicity of possible Slope parameters or curves 144 that 
may be programmably selected, each one providing a different rate increase 
in response to sensed physiological stress above the Threshold. Such 
multiplicity of Slope parameters 144 are schematically represented in FIG. 
2 as the family of Slope curves 141. 
The Maximum Sensor Rate parameter 142 defines the upper limit of the rate 
range of the rate-responsive pacemaker 16. The pacemaker will not pace 
above this rate, regardless of the amount by which the sensor input signal 
exceeds the Threshold 132. Such upper limit is schematically represented 
in FIG. 2 as the dotted line 139. 
The Base Rate parameter 138 defines the lower limit of the rate range of 
the rate-responsive pacemaker 16. The pacemaker will not pace below this 
rate, even if the sensor input signal is below the Threshold 132. When the 
pacemaker is pacing at the base rate, the patient is at rest or undergoing 
physiological stress at a level below the Threshold 132. Such lower limit 
is schematically represented in FIG. 2 as the solid line 143. 
Still referring to FIG. 2, the Reaction Time parameter 136 determines the 
minimum time to be allowed for an increase in pacing rate from the Base 
Rate to the programmed Maximum Rate. The Reaction Time controls the amount 
of time the pacemaker spends at a given pacing rate by requiring a minimum 
number of stimulation pulses at that rate. Once these pulses occur, the 
rate can be increased. A short Reaction Time allows the pacing rate to 
accelerate rapidly in response to sensed physiological activity above the 
Threshold; a long Reaction Time forces a slow increase in the pacing rate. 
The Recovery time parameter 134 determines the minimum time allowed for a 
decrease in pacing rate from the programmed Maximum Rate 142 to the Base 
Rate 138. It uses the same principle as the Reaction Time. That is, it 
controls the amount of time the pacemaker spends at a given pacing rate by 
requiring a minimum number of stimulation pulses before the rate can be 
decreased. A short Recovery Time allows a rapid deceleration of the pacing 
rate; a long Recovery Time forces a slower decrease in pacing rate. 
In operation, the sensor 26 senses physiological activity and generates a 
raw signal 27 in response thereto. The raw signal 27 is processed in an 
appropriate manner in order to produce the sensor input signal 133. The 
amount 137 by which the sensor input signal 133 exceeds the Threshold 132, 
in conjunction with the Reaction Time 136, provides a sensor index signal 
140 that points to a specific entry point on one axis of the selected 
Slope curve 144. The Reaction Time 136 determines how rapidly the sensor 
index signal moves along the selected Slope curve 144 towards the Maximum 
Rate 139. If the number of pulses at the current SIR signal 126 has 
reached the amount required by the Reaction Time, the SIR may be increased 
to its next value, as defined by the current value of the SIR rate 126, 
and limited by the Maximum Sensor Rate 139. If the Reaction Time pulse 
count has not been reached, the SIR signal will not change. 
If no sensor input signal is detected as being above the Threshold 132, and 
if this lack of activity has occurred for the number of pulses specified 
by the Recovery Time parameter, the SIR signal may be decreased to its 
next value as defined by the selected Slope curve and as limited by the 
Base Rate parameter 138. If the Recovery Time pulse count has not been 
reached, the SIR signal will not change. 
Note, as described above, that the Reaction Time 136 controls the rate of 
increase of the SIR signal 126, and hence the rate of increase of the 
pacing rate. The Recovery Time 134 controls the rate of decrease of the 
SIR signal 126, and hence the rate of decrease of the pacing rate. The 
Reaction Time 136 is schematically illustrated in FIG. 2 as a roller 146 
that controls how fast the sensor index signal 140 is allowed to move 
left-to-right along the horizontal axis of the selected Slope curve 144. 
Similarly, the Recovery Time 134 is schematically illustrated in FIG. 2 as 
a roller 148 that controls how fast the sensor index signal 140 is allowed 
to move right-to-left along the horizontal axis of the selected Slope 
curve 144. 
In accordance with the present invention, the Threshold 132, i.e., that 
level above which the sensor input signal must reach before such sensor 
input signal is considered as representing significant physiological 
activity, may be programmably set in one of two ways. First, the Threshold 
132 may be set to one of a plurality of fixed values. Second, the 
Threshold 132 may be programmed to be set automatically with a selected 
"offset" value added thereto. For convenience of explanation, the sensor 
input signal 133 is considered as ranging from 0 to 13 sensor units, with 
a value of 13 representing the maximum possible sensor input signal (i.e., 
the highest possible physiological activity), and with a value of 0 
representing the minimum possible sensor input signal (i.e., no sensed 
physiological activity). 
With the sensor units defined as indicated above, the rate-responsive 
pacemaker of the present invention allows the Threshold control parameter 
131 to be selectively programmed to one of the values indicated in Table 
1. Note that the first four values listed in Table 1 are considered as 
"autothreshold" values. When one of the autothreshold values are selected, 
the Threshold 132 is determined by computing a long-term average of the 
sensor input signal 133, e.g., an 18 hour average, and by adding the 
indicated offset to such long-term average. When one of the fixed 
threshold values are selected, then the Threshold 132 assumes the value 
selected, and does not change. 
TABLE 1 
______________________________________ 
Threshold 
Value Offset 
______________________________________ 
AUTO +0.0 
+0.0 
AUTO +0.5 
+0.5 
AUTO +1.0 
+1.0 
AUTO +1.5 
+1.5 
1.0 (low) 
None 
1.5 None 
2.0 None 
2.5 None 
3.0 None 
3.5 None 
4.0 None 
4.5 None 
5.0 None 
5.5 None 
6.0 None 
6.5 None 
7.0 (high) 
None 
______________________________________ 
Referring next to FIG. 3, a representation of the sensor input signal 133 
is shown as it might appear as a function of time. Note that the signal 
133 dithers and varies a great amount, as would be expected for a typical 
sensor input signal. Also shown in FIG. 3 is a representation of a running 
long-term average (shown as a dotted line 150) of the sensor input signal 
133. By definition, over the long-term there are an equal number of 
excursions of the sensor input signal 133 above the long-term average 150 
as there are excursions below the long-term average 150. If the long-term 
average 150 were used as the Threshold 132, then any of the excursions 
above the average 150, represented as the shaded portion of the 
excursions, would cause the sensor index signal 140 (FIG. 2) to be 
erratically active, potentially causing too much sensitivity to low 
activity resulting in increases in the SIR signal 126 (although such 
changes could be tempered by proper programming of the Reaction and 
Recovery time sensor control parameters described above). 
To minimize oversensitivity of increases in the sensor index signal, the 
present invention advantageously utilizes a programmable offset 152 that 
is added to the long-term average 150 as shown in FIG. 4 whenever an 
autothreshold value is programmably selected as the Threshold parameter 
131. As indicated in Table 1, there are four possible offset values that 
may be selected when the autothreshold feature is selected. It is to be 
understood, however, that this is merely exemplary, as any number of 
offset values, having a wide range of values, could be made available for 
selection. Thus, by defining the Threshold level 132 (which the sensor 
input signal 133 must exceed in order to be considered as evidence of 
significant physiological activity) as the long-term average 150 plus the 
offset value 152, most of the minor and inconsequential fluctuations of 
the sensor input signal 133 are advantageously ignored insofar as 
influencing the sensor index signal 140 is concerned. However, all such 
fluctuations still contribute to the long-term average 150. 
In accordance with an alternative embodiment of the present invention, the 
offset value 152 that is added to the long-term average of the sensor 
input signal 133 may also be automatically determined. Such automatic 
determination may be based on any suitable processing routine. For 
example, the value of the offset 152 may be automatically determined as 
the long-term average of the peak values of the sensor input signal. 
Alternatively, the offset value 152 may be automatically determined as the 
peak-to-peak variation in the long-term average of the sensor input 
signal. Indeed, any processing method or technique that defines a 
meaningful offset value 152 that may to added to the long-term average of 
the sensor input signal 133 may be used. 
Referring next to FIG. 5, there is shown a flow chart that illustrates the 
manner in which the autothreshold feature of the present invention is 
utilized in order to arrive at a sensor level index signal, which sensor 
level index signal is used in conjunction with a Slope parameter to define 
a sensor indicated rate (SIR) signal for use by the rate-responsive 
pacemaker. In the flow chart of FIG. 5, each main step is illustrated as a 
block, or box, with each block having a corresponding number assigned 
thereto for reference purposes. 
As seen in FIG. 5, the present invention is invoked (when programmably 
selected) during the normal programmed operation of the pacemaker (or 
"pacer"). That is, the pacer operates in its programmed mode of operation 
in a normal manner (block 160). When, during the course of its programmed 
mode of operation, a ventricular event occurs (block 162), the sensor 
input signal is read in order to obtain a short-term average thereof 
(block 164). A short-term average is typically considered as a single 
reading of the sensor 26 (FIG. 1), which reading normally occurs once 
during each pacing cycle. A short-term average of the sensor signal may 
thus comprise the raw sensor signal as filtered, or otherwise processed, 
over the previous pacing cycle. That is, during the course of processing 
the raw sensor signal 27 to produce the sensor input signal 133, as will 
be explained below in conjunction with FIG. 6, the raw sensor signal is 
typically amplified, rectified and filtered, all of which tends to average 
or smooth the raw sensor signal. Such filtering and averaging is highly 
advantageous because the raw sensor signal itself can be highly erratic 
and unstable. Hence, in the normal process of obtaining the sensor input 
signal 133, an effective short-term average of the raw sensor signal is 
obtained (block 164). 
Alternatively or conjunctively, in some embodiments of the invention it is 
desirable to sample the sensor input signal at a prescribed sampling rate, 
e.g., once each ventricular or atrial event, and average the samples over 
a relatively short time period or number of samples in order to produce a 
short-term average. A short-term average may thus comprise m consecutive 
samples of the sensor input signal, where m is a small integer, e.g., 
3-10; or m consecutive samples of the raw sensor signal. 
After the short-term average of the raw sensor signal and/or the sensor 
input signal has been obtained (block 164), a determination is made as to 
whether a sufficient number of short-term averages have been obtained to 
provide a meaningful long-term average (block 166). That is, there must be 
some initialization process invoked in order to provide sufficient data 
for a long-term average to be computed. Thus, if such initialization has 
not been completed (block 166), then an average is computed based on 
whatever sensor readings have been made (block 180). Unless thirty (30) 
sensor readings have thus been taken (block 182), the pacemaker paces at 
the Base Rate (block 184), i.e., the pacemaker provides stimulation pulses 
on demand to either the atrium and/or the ventricle at the programmed Base 
Rate in accordance with the normal pacemaker operation in the programmed 
mode (block 160). 
If thirty (30) sensor readings have been included in the computation of the 
average of the sensor input signal (made at block 180), as determined at 
block 182, then such computed average is used as the start of a long-term 
average (block 186), and the initialization of the autothreshold program 
is deemed completed (block 188). Thereafter, upon continuing with the 
normal pacer operation (block 160), and updating the sensor short-term 
average (block 164), the determination is made that the initialization is 
complete (block 166). With the initialization complete, a long-term 
average of the sensor reading is begun by maintaining a running average of 
the short-term averages (block 168). That is, the sensor reading, or the 
sensor input signal, continues to be averaged over a specified number of 
pacing cycles or a specified time period. In the preferred embodiment, 
such running average is maintained for 18 hours. That is, the long-term 
average of the sensor input signal reflects the most recent 18 hours of 
operation, with the most current sensor reading always being included in 
the long-term average computation, and the oldest sensor reading (i.e., 
the one that occurred just over 18 hours ago) being deleted from the 
computation. Thus, it is seen that the long-term average of the sensor 
input signal is computed as an average of the last n short-term averages 
of such sensor input signal, where n is a large integer, e.g., greater 
than 30. 
At this point, it should be noted that other measures of the sensor input 
signal, in addition to, or in place of, a long-term average, could also be 
used. For example, a weighted average of the sensor input signal could be 
performed, giving greater weight to the sensor input signals from certain 
time periods of the day. Further, a least squares computation could be 
performed wherein the sensor input signals having a large variance from 
other sensor input signals are discounted. In other words, any processing 
method or technique that provides a meaningful measure of the variation 
and movement of the sensor input signal over the long-term time period of 
interest may be employed. 
Still referring to FIG. 5, after the long-term average has been updated 
with the most recent short-term average (block 168), a determination is 
made as to whether one of the autothreshold values has been selected 
(block 170). If not, then that means a fixed Threshold parameter has been 
programmed. In such case, the sensor level index signal, which (in the 
preferred embodiment) is a discrete number ranging from 0 to 31, is 
derived from the short-term average, less the fixed Threshold value (block 
172). Only if the short-term average is greater than the fixed Threshold 
value does a sensor level index signal result that is considered 
sufficiently large to evidence significant physiological activity. 
If one of the autothreshold values has been selected (block 170), then the 
sensor level index signal is derived from the short-term average, less the 
long-term average, less the selected value of offset. (block 178). Thus, 
only if the short-term average (which, as indicated above, is typically 
the value of the sensor input signal for the current pacing cycle) is 
greater than the long-term average and the selected (programmed) offset 
value does a sensor level index signal result that is considered 
sufficiently large to evidence significant physiological activity. 
Regardless of how the sensor level index signal is determined, whether 
using a fixed Threshold value (block 172) or an autothreshold value with 
offset (block 178), the sensor level index signal is then used in 
conjunction with a programmed Slope parameter, as described, e.g., in the 
'052 patent or one of the above-identified patent applications, to point 
to a sensor indicated rate (SIR) value (block 174). The SIR value is then 
used by the rate-responsive pacemaker, when operating in a rate-responsive 
mode, to determine the rate at which stimulation pulses are provided on 
demand (block 176). 
Turning next to FIG. 6, there is shown a block diagram that illustrates how 
the raw sensor signal 27 is processed to provide the sensor input signal 
133. As seen in FIG. 6, the sensor 26, which in the preferred embodiment 
comprises a piezoelectric crystal, generates the raw sensor signal 27. The 
raw sensor signal 27 is processed in pre-processing circuitry 190 
comprising an amplifier 192 and a rectification/filter circuit 194. The 
amplifier 192 has a bandwidth associated therewith that varies from about 
0.8 to 40 Hz. The raw signal is rectified and filtered using the 
rectification/filter circuit 194 substantially as described in the '053 
patent, or equivalent manner. Such rectification and filtering results in 
an analog signal 196 having an amplitude that varies as a function of the 
energy content of the raw sensor signal 27. 
The analog signal 196 is coupled to a voltage-controlled oscillator (VCO) 
198 which generates an output frequency signal 200 that is frequency 
modulated as a function of the analog signal 196. In the preferred 
embodiment, the VCO frequency increases as the absolute value of the 
voltage input to the VCO increases in magnitude, with the VCO frequency 
varying from about 0 Hz at 0 volts input (no activity sensed), to about 22 
KHz at approximately -2.66 volts (high level of activity sensed). Thus, 
the frequency signal 200 has a frequency varying from about 0 Hz to about 
22 KHz. It should be noted, however, that such VCO parameters are only 
exemplary, and that other parameters could also be used. 
The frequency signal 200 is counted in a counter circuit 202. The counter 
circuit 202 is reset each sampling period, e.g., each pacing cycle, by the 
pacer state machine and related logic 42 of the rate-responsive pacemaker. 
The contents of the counter circuit 202 at the conclusion of each sampling 
period thus comprise a digital word. Such digital word functions as the 
sensor input signal 133 for that sample period. Such sensor input signal 
133 may then be processed by the rate-responsive sensor processing 
circuits 124, as controlled by the sensor control parameters stored in the 
pacer memory 62, in order to perform the long-term averaging and offset 
addition in the manner programmed. 
In the preferred embodiment, the counter 202 is an eight bit counter. Such 
counter may thus contain a count that ranges from 0 to 255. During a 
typical autothreshold operation, the sensor input signal 133, i.e., the 
count obtained from the counter 202, after appropriate scaling, is 
averaged over a sufficiently long-term, e.g., at least 30 samples and no 
more than 18 hours, to produce a running, long-term average. The long-term 
average is then subtracted from the current sensor input signal, as is the 
selected offset value, and the resulting number, after appropriate scaling 
and processing, becomes the sensor level index signal, having a value 
ranging from 0 to 31. The sensor level index signal, in turn, points to 
the appropriate SIR value as a function of the programmed Slope parameter. 
As described above, it is thus seen that the present invention provides, in 
one embodiment, a rate-responsive pacemaker wherein a programmable offset 
value is added to an automatically generated baseline threshold value 
(long-term average) in order to define a Threshold reference level above 
which the sensor input signal of the rate-responsive pacemaker must go 
before such sensor input signal is considered as evidence of significant 
physiological activity. In another embodiment, the invention automatically 
determines an offset value that is added to an automatically generated 
baseline threshold value (long-term average) in order to define the 
Threshold reference level. 
As further described above, it is seen that the invention provides a 
programmable rate-responsive pacemaker that may be selectively programmed 
to provide either: (1) an automatically generated Threshold value, 
comprising a long-term average to which a selected one of a plurality of 
offset values may be added, or (2) a programmed Threshold value selected 
from a plurality of fixed baseline threshold values, to which no offset is 
added. Such programmably selected Threshold levels thus provide a means 
for effectively defining that threshold reference level which must be 
exceed by the sensor input signal before it is considered as an indication 
of significant physiological activity. Hence, such programmable Threshold 
levels provide an effective tool in programming the rate-responsive 
pacemaker for optimum operation with respect to a given patient. 
While the invention herein disclosed has been described by means of 
specific embodiments and applications thereof, numerous modifications and 
variations could be made thereto by those skilled in the art without 
departing from the scope of the invention set forth in the claims. For 
example, while the invention has been described substantially as a digital 
embodiment, particularly relative to the manner in which the sensor input 
signal is processed to produce a digital sensor input signal, and the 
manner in which the digital sensor input signal is processed to point to 
an appropriate SIR signal, such processing could also be carried out using 
equivalent analog circuitry, or hybrid circuitry (analog and digital).