Rate-responsive pacing method and system employing minimum blood oxygen saturation as a control parameter and as a physical activity indicator

A rate-responsive pacing method and system senses the minimum blood oxygen saturation in the right atrium of a patient's heart and uses such minimum blood oxygen saturation as a control parameter for indicating the muscular activity of a patient. Because the oxygen content of the venous blood in the right atrium varies significantly as venous blood from all parts of the body is introduced therein, evidencing differing levels of oxygen demand throughout the patient's body, the minimum oxygen content of the venous blood provides an accurate and reliable measure of those portions of the patient's body experiencing the greatest oxygen demand, i.e., experiencing muscular activity. A rate-responsive pacing system includes means for sensing the minimum oxygen content in the right atrium over a prescribed time interval, and using such minimum oxygen content as a control parameter for adjusting the rate of the pacemaker.

BACKGROUND OF THE INVENTION 
The present invention relates to rate-responsive pacing methods and 
systems, and more particularly to a rate-responsive pacing method or 
system wherein the minimum oxygen saturation level of the venous blood in 
the right atrium is used as a control parameter to adjust the rate at 
which electrical stimulation pulses are delivered to a patient's heart. 
A pacemaker is a medical device, usually an implantable medical device, 
that provides electrical stimulation pulses to a patient's heart at a 
controlled rate for the purpose of controlling the heart rate. Most modern 
implantable pacemakers can be programmed to operate in several modes, as 
required by the needs of a particular patient. Several common modes of 
operation provide stimulation pulses only when the patient's heart does 
not beat by itself at a minimum rate. In such mode(s), the stimulation 
pulses are provided only when needed, or "on demand", thereby preserving 
the limited power source of the implanted pacemaker for the longest 
possible time. Typically, the manner in which such demand pacemakers 
operate is to define a basic pacing interval (sometimes referred to as an 
"escape interval") and to wait and see if the heart beats during this 
interval. (A heart beat is determined by sensing a "P-Wave" indicating 
contraction of the atria, or an "R-wave", indicating contraction of the 
ventricles.) If so, the basic pacing interval starts over, and no 
stimulation pulse is provided. If not, a stimulation pulse is provided at 
the end of the pacing interval. In this manner, the pacemaker's pacing 
interval defines the rate at which stimulation pulses are provided to the 
heart in the absence of naturally occurring heart beats. It is noted that 
pacemakers may be employed that stimulate either, or both, chambers of the 
heart (i.e., either the right atrium and/or the right ventricle). 
A rate-responsive pacemaker is a pacemaker that automatically adjusts the 
pacing interval, or the rate at which stimulation pulses are provided to 
the patient's heart, as a function of the sensed physiological needs of 
the patient. That is, every person has times when his or her heart needs 
to beat fast, and times when his or her heart should beat slow. For 
example, physical activity causes a person's heart rate to increase in 
order to compensate for the increased oxygen demands of the muscle tissue 
undergoing the physical activity. Similarly, physical inactivity, such as 
prolonged periods of sleep or rest, allow a person's heart rate to 
decrease because the oxygen demands of the body tissue are less. A 
rate-responsive pacemaker thus attempts to sense the physiological needs 
of a patient at a particular time, e.g., by sensing physical activity or 
inactivity, and adjusts the pacing interval of the pacemaker accordingly. 
The operation and design of pacemakers, including rate-responsive 
pacemakers, are known in the art. See, e.g., Furman, et al., A Practice of 
Cardiac Pacing, (Futura Publishing Co., Mt. Kisco, N.Y. 1986); Moses, et 
al., A Practical Guide to Cardiac Pacing (Little, Brown & Co., 
Boston/Toronto 1983); U.S. Pat. No. 4,712,555 (Thornander et al); U.S. 
Pat. No. 4,856,523 (Sholder et al). U.S. Pat. No. 4,712,555 (Thornander et 
al.) is a particularly comprehensive reference explaining the general 
operation of a rate-responsive pacemaker, and the application of one 
particular type of physiological parameter (a timing interval) for 
controlling such pacemaker. U.S. Pat. No. 4,712,555 is incorporated herein 
by reference. Further, it is known in the art to sense several different 
physiological parameters as the control parameter of a rate-responsive 
pacemaker. One common type of sensor is an activity sensor that senses the 
physical activity level of the patient. See, e.g., U.S. Pat. No. 
4,140,132, issued to Dahl; and U.S. Pat. No. 4,485,813, issued to 
Anderson. 
Other types of sensors used in prior art rate-responsive pacers include 
sensors that sense respiration rate, blood and/or body temperature, blood 
pressure, the length of the Q-T interval, and the length of the P-R 
interval. 
Of particular significance to the present invention, it is also known in 
the art to use an implantable sensor to determine the oxygen content of 
blood and to use such sensor in a rate-responsive pacemaker. See, e.g., 
U.S. Pat. Nos. 4,202,339; 4,399,820; and 4,815,469. Further, recent 
studies have suggested that mixed venous oxygen saturation provides one of 
the best indications available of physiological need, especially for low 
and medium levels of exercise (physical activity). It has thus been 
suggested that mixed venous oxygen saturation, when combined with other 
parameters, provides a very useful control parameter for controlling a 
rate-responsive pacemaker. See, Stangl, et al., "A New Multisensor Pacing 
System Using Stroke Volume, Respiratory Rate, Mixed Venous Oxygen 
Saturation, and Temperature, Right Atrial Pressure, Right Ventricular 
Pressure and dP/dt," E, Vol. 11, pp 712-724 (June 1988). 
Unfortunately, while oxygen saturation may be one of the most sensitive 
parameters to indicate low and medium level exercise, the techniques 
heretofore used in the prior art to sense oxygen saturation have masked 
out the most beneficial information provided by this parameter. For 
example, oxygen saturation is typically sensed optically using a sensor 
that includes both a source of light, such as a light emitting diode 
(LED), and a means for detecting light, such as a phototransistor. The 
sensor, including both LED and phototransistor, is positioned in an 
appropriate location to sense venous oxygen saturation, e.g., in the right 
atrium. Light energy is directed to the blood in the right atrium from the 
light source. The amount of light energy reflected back to the 
phototransistor is a function of the properties of the blood, including 
the level of oxygen saturation of the blood. Thus, by monitoring the ratio 
of emitted light energy to reflected light energy, it is possible to 
measure the blood oxygen saturation level of the blood in the right 
atrium. However, because the return blood in the right atrium comes from 
all parts of the body, it contains significantly different levels of blood 
oxygen saturation, reflecting the different activity levels of various 
parts of the body. That is, if the patient is walking, the blood returned 
from the legs and arms (assuming the arms are swinging as the legs are 
walking) will have a significantly lower oxygen content than will blood 
from other parts of the body. This is because the leg and arm muscle 
tissue is working harder (and therefore consuming more oxygen) than is 
muscle tissue at other body locations. 
Hence, the blood oxygen saturation measured in the right atrium tends to 
fluctuate over a wide range, depending upon how thoroughly the blood is 
mixed at the time the measurement is made. To compensate for these 
fluctuations, the prior art teaches averaging or integrating the 
measurement over a sufficiently long period of time to smooth out such 
fluctuations. Disadvantageously, such averaging or integrating masks out 
the most beneficial portions of the measurement--the oxygen saturation 
level of the blood returned from the arms and legs, or other parts of the 
body that are experiencing physical activity. What is needed, therefore, 
is a technique or method for measuring the oxygen saturation of the blood 
returned from just those portions of the body undergoing the greatest 
physical activity, or otherwise isolating that portion of the fluctuating 
oxygen saturation measurement indicative of such physical activity. 
Further, when measuring blood oxygen saturation using an optical sensor 
that measures reflected light energy, and when such sensor is positioned 
in the heart, the amount of reflected light energy detected by such sensor 
is significantly influenced by optical reflections from the heart wall or 
valves. Such optical reflections disadvantageously give erroneously high 
readings. Hence, what is needed is a sensing method or system that senses 
only those optical reflections from returned blood, not from optical 
reflections occurring within the heart. More particularly, what is needed 
is a system and method for sensing optical reflections from blood returned 
to the heart from only those body portions undergoing the most strenuous 
physical activity. 
The present invention advantageously provides a system and method of blood 
oxygen saturation measurement that addresses the above and other needs. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a rate-responsive 
pacing method senses the minimum blood oxygen saturation level of the 
venous blood in the right atrium of a heart, and uses such minimum blood 
oxygen saturation level as a control parameter for indicating the muscular 
activity of a patient. As mentioned above, the oxygen content of the 
venous blood in the right atrium varies significantly as venous blood from 
all parts of the body returns thereto, but is not thoroughly mixed 
therein. Some of the venous blood exhibits a low oxygen content when 
returned from insulated parts of the body, such as the arms or legs, 
undergoing muscular activity. Other of the venous blood, from parts of the 
body not experiencing significant muscular activity, exhibits a higher 
blood oxygen content. Advantageously, the minimum oxygen content of the 
poorly mixed venous blood in the right atrium thus provides an accurate 
and reliable measure of muscular activity used by the pacing method of the 
present invention to adjust the rate at which pacing pulses are provided 
on demand to the patient. 
In accordance with another aspect of the present invention, a 
rate-responsive pacing system is provided that includes an implanted 
rate-responsive pacemaker coupled to an appropriate chamber(s) of the 
patient's heart. A blood oxygen sensor, e.g., a sensor that optically 
senses the oxygen content of the blood in contact therewith, is positioned 
so as to sense the oxygen content of the poorly mixed venous blood in the 
right atrium. Preferably, this oxygen sensor forms an integral part of the 
pacing lead that couples the rate-responsive pacemaker with the heart. The 
minimum oxygen content thus sensed is used as a control parameter to 
automatically adjust the pacing rate of the rate-responsive pacemaker, 
i.e., to automatically adjust the rate at which pacing or stimulation 
pulses are provided on demand by the pacemaker in order to meet the 
physiologic needs of the patient. 
An optical sensor is preferably used with the present invention to sense 
the oxygen content of the venous blood in the right atrium. 
Advantageously, when this is done, the minimum oxygen content thus sensed 
automatically rejects any erroneous high readings caused by optical 
reflections within the heart. 
One embodiment of the present invention may thus be characterized as a 
method of automatically controlling the rate at which a rate-responsive 
pacemaker delivers pacing pulses to a patient's heart. Such method 
comprises: (a) measuring the oxygen content of the blood in the right 
atrium of the patient's heart; (b) determining the minimum value of blood 
oxygen content measured in step (a) during a prescribed time period; and 
(c) using the minimum value of blood oxygen content determined in step (b) 
as a control parameter to adjust the pacing rate of the rate-responsive 
pacemaker. 
Another embodiment of the invention may be characterized as simply a method 
or system of determining the relative physical activity level of a 
patient. Such method or system includes the steps of or means for: (a) 
repeatedly measuring the oxygen content of venous blood in the patient; 
(b) monitoring the measurements made in step (a) over a prescribed 
interval; (c) ascertaining the minimum blood oxygen content measured 
during the monitoring interval of step (b); and (d) using the minimum 
blood oxygen content ascertained in step (c) as an indication of the 
physical activity level of the patient, where a greater physical activity 
is indicated by a lower minimum blood oxygen content. 
A preferred rate-responsive pacing system in accordance with the present 
invention includes: (1) a blood oxygen sensor, this sensor including light 
emitting means for emitting light energy therefrom, and light sensing 
means for sensing light energy directed thereto; (2) a sensor drive 
circuit for selectively causing the light emitting means to emit light 
energy; (3) a sensor process circuit for determining the amount of light 
energy sensed by the light sensing means corresponding to a given amount 
of light emitted by the light emitting means, and for converting the 
determined amount of light energy into a first measurement representative 
of the minimum amount of light energy sensed during a prescribed interval; 
(4) rate-responsive pacing means for generating stimulation pulses on 
demand at a rate controlled by the first measurement; and (5) lead means 
for delivering the stimulation pulses to a desired heart chamber. 
It is a feature of the present invention to provide an accurate and 
reliable system and/or method for determining the oxygen content of venous 
blood using optical measuring techniques, i.e., emitting light energy into 
the blood and sensing the amount of light energy reflected therefrom, 
despite reflections and other erroneous light energy sensings that may 
occur. 
It is another feature of the invention to provide a reliable and accurate 
system and/or method for measuring the oxygen content of blood returning 
from body tissue undergoing the greatest oxygen demand, e.g., experiencing 
the most physical exercise, even though such blood is in the process of 
being mixed with blood returning from body tissue not experiencing a high 
oxygen demand. In other words, it is a feature of the present invention to 
provide a system and/or method that sorts relevant blood oxygen 
measurements (i.e., from blood returning from active body tissue) from 
irrelevant blood oxygen measurements (i.e., from blood returning from 
non-active body tissue). 
It is a further feature of the invention to provide such a system and/or 
method for measuring relevant blood oxygen levels that can be used as a 
control parameter in a rate-responsive pacing system. 
It is yet another feature of the invention to provide a rate-responsive 
pacing system and/or method wherein stimulation pulses may be provided to 
a patient's heart on demand at a rate that is determined by the oxygen 
content of blood returning from those parts of the patient's body 
experiencing the greatest oxygen demand.

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. 
It is noted that in accordance with one aspect of the present invention, a 
rate-responsive pacing system is provided in which the oxygen content of 
blood is used as a physiological parameter to control the rate at which 
stimulation pulses are provided to a heart of a patient. As previously 
indicated, rate-responsive pacing systems are known and described in the 
art, and will not be described in any detail herein. While such systems 
take several forms, all employ some means for sensing one or more 
physiological parameters of the patient indicative of how fast or slow the 
patient's heart should beat. The present invention is directed primarily 
to the manner in which the oxygen content of blood can be accurately 
measured, and once measured, used as a physiological parameter for 
controlling a rate-responsive pacing system. 
In order to better appreciate the advantages associated with the use of the 
present invention, it will first be helpful to have a basic understanding 
of the manner in which oxygen content of blood is sensed. Accordingly, 
reference is made to FIG. 1, where there is shown a schematic diagram of 
an optical blood oxygen sensor of the prior art. The sensor includes two 
light-emitting diodes 20 and 22 connected in parallel, with the anode of 
diode 20 being connected to the cathode of diode 22, and the anode of 
diode 22 being connected to the cathode of diode 20. A phototransistor 24 
is connected in parallel with a resistor 26, and the collector of the 
phototransistor 24 is connected to the same node as is the anode of diode 
22 and the cathode of diode 20. The node comprising the anode of diode 20 
and the cathode 22 comprises one input terminal 28, and the emitter of 
phototransistor 24 and one side of the resistor 26 comprises another 
terminal of the sensor 30. 
In operation, a bi-phase voltage pulse is applied across terminals 28 and 
30. This bi-phase voltage pulse is also illustrated in FIG. 1 and includes 
a positive portion, having an amplitude of +V1; followed by a negative 
portion, having a negative amplitude of -V2. The positive portion of the 
bi-phase voltage pulse causes a current Il to flow through light-emitting 
diode 20, thereby causing light energy E1 to be emitted by the LED 20. The 
light E1 comes in contact with a desired body fluid 32, such as blood. 
Depending upon the properties of the fluid 32, a portion of the light 
energy El is reflected back to the phototransistor 24. In FIG. 1, as well 
as in the other figures, that portion of light energy reflected back to 
the phototransistor is identified as E2. Thus, in FIG. the amount of 
current I2 that flows through phototransistor 24 is a function of the 
light energy E2 that is incident upon the base of the phototransistor 24. 
The balance of the current Il that does not flow through phototransistor 
24, therefore, flows through the resistor 26. This current is identified 
as I3. Thus, it is seen that I1 is equal to I2 and I3. The current I2 
varies as a function of the light energy E2, thereby also affecting the 
amount of current I3 that flows through resistor 26. The voltage developed 
across terminals 28 and 30 (which voltage is a function of the forward 
drop across LED 20 and the voltage drop across resister 26 caused by the 
current flow I3) will thus vary as a function of the reflected light 
energy E2 that is incident upon the phototransistor 24. Hence, by 
monitoring the voltage across the terminals 28 and 30, it is possible to 
get an indication of the reflectance properties of the fluid 32. 
In order to determine the amount of voltage variation across terminals 28 
and 30 caused by the current I2, it is necessary to isolate other 
variations in this voltage from the measurement. This is typically done by 
causing current I4 to flow through resistor 26 and LED 22 during the 
negative portion of the bi-phase voltage waveform. During this portion of 
the waveform, both the phototransistor 24 and LED 20 are back biased, and 
therefore no current flows through either of these devices. The value of 
I4 is selected to be close to the value of I1 so that the forward voltage 
drop across LED 20 will be approximately the same as the forward voltage 
drop across LED 22. 
Because of some of the difficulties associated with using and operating the 
prior art sensor shown in FIG. 1, an improved blood oxygen sensor has been 
proposed as disclosed in U.S. Pat. No. 4,815,469, which patent is 
incorporated herein by reference. While the sensor described in the '469 
patent, or one equivalent thereto, is the preferred type of sensor for use 
with the present invention, it is to be emphasized that any type of sensor 
capable of sensing the oxygen content of blood may be used with the 
methods and systems of the present invention. 
Referring next to FIG. 2, there is shown a block diagram depicting a 
preferred manner of using a blood oxygen sensor 34 (such as the sensor 
described in the '469 patent), in a rate-responsive pacing system. The 
sensor 34 is positioned within an area of a patient's body where venous 
blood is able to come in contact with light energy E- emitted by the 
sensor. The preferred placement of the sensor is within a heart 36 of the 
patient, and more particularly within the right atrium 38 of the heart 36. 
An implantable rate-responsive pacemaker 40 is implanted in the patient in 
conventional manner. Included within the implantable pacemaker 40 is a 
sensor drive circuit 42, a sensor process circuit 44, and conventional 
rate-responsive pacemaker circuits 46. Also included in the pacemaker 40 
is a source of electrical energy, e.g., a battery 48. 
The drive circuit 42 provides the drive voltage necessary for operation of 
the sensor 34. The sensor process circuit 44, senses the returning signal 
from the sensor, e.g. the voltage potential at the output of the sensor 
34, in response to an applied drive voltage. As is known in the art, and 
as described in the '469 patent, this sensor output voltage varies as a 
function of the oxygen content of the blood from which the emitted light 
energy is reflected. Hence, by monitoring changes in this output voltage, 
a qualitative measurement of the oxygen content of the blood may be made. 
By using appropriate calibration techniques, a quantitative measurement of 
the oxygen content of the blood may be made. 
The drive circuit 42 and the sensor circuit 44 are coupled to each other 
and to the pacemaker circuits 46. Appropriate timing signals 50 are shared 
between the sensor drive circuit 42 and the sensor process circuit 44. 
Such timing signals assure that both circuits operate only at a desired 
time within the cardiac cycle or other control cycle. (The "cardiac cycle" 
is the time required by the heart 36 to complete one beat. This cycle is 
typically manifest by contraction or depolarization of the atria, 
evidenced by the generation of a P-wave, followed by contraction or 
depolarization of the ventricles, evidenced by the generation of an 
R-wave. P-waves and R-waves are evident by examining the patient's 
electrocardiogram, or ECG. The cardiac cycle is frequently measured from 
R-wave to R-wave, as the R-wave is the predominant wave, and thus the 
easiest to measure, in the ECG.) Further, in order to synchronize the 
sensing function of the sensor 34 with other events associated with the 
operation of the pacemaker circuits 46, the sensor drive circuit 42 and 
the sensor process circuit 44 receive a clock signal 52 and a timing 
reference signal 54 from the pacemaker circuits 46. Thus, for example, the 
timing reference signal 54 may be a signal indicating a cardiac event, 
such as a V-pulse or an R-wave signal, which signals indicate that the 
ventricle of the heart has either been paced (meaning that a stimulation 
pulse, e.g. a ventricular stimulation pulse, or V-pulse, has been provided 
by the pacemaker), or that a ventricular contraction, an R-wave, has been 
sensed. 
In operation, the clock signal 52, as well as a timing reference signal, 
such as a V/R signal, are provided from the pacemaker circuit 46 to the 
sensor drive circuit 42 and the sensor process circuit 44. A pacing lead 
60, connected to the pacemaker 40 by way of, e.g., a conventional bipolar 
pacer connector 62, allows the pacemaker to deliver stimulation pulses to 
the heart 36 at a distal electrode tip 66 through conductor 70. This same 
conductor 70 allows the pacemaker circuits 46 to sense cardiac events 
occurring near the lead tip 66. 
In a preferred embodiment, the sensor 34 is advantageously embedded within 
the pacemaker lead 60 at a location near the distal tip so as to place the 
sensor 34 in the right atrium 38 of the heart 36. Further, when positioned 
properly within the heart, the lead is formed in a manner that causes the 
sensor 34 to face blood (and therefore measure the oxygen content of 
blood) just after the blood enters the atrium 38, before such blood has an 
opportunity to become thoroughly mixed within the atrium. One terminal of 
the sensor 34 is connected to a separate conductor 68 of the lead 60. The 
other terminal of the sensor 34 is connected within the lead to the 
conductor 70. 
The sensor process circuit 44 develops a control signal 49 that is 
representative of the reflectance properties of the blood (and hence 
relatable to the amount of oxygen within the blood). This control signal 
49 is presented to the pacemaker circuits 46 and is used as a 
physiological parameter to control the rate at which the pacemaker 
circuits deliver a stimulation pulse to the heart. 
Referring next to FIG. 3A, a waveform diagram illustrating representative 
fluctuations in the output signal from the sensor 34 of FIG. 2 (when such 
sensor is placed in the right atrium 38 of a patient's heart 36) is 
illustrated. The horizontal axis in the diagram shown in FIG. 3A 
represents time, while the vertical axis represents the output signal, 
e.g., the output voltage, obtained from the sensor 34. As this output 
signal represents the optical reflectance properties of the blood, which 
properties are relatable to the oxygen content of the blood, the waveform 
shown in FIG. 3A thus depicts the variations in the oxygen content of the 
blood as a function of time. 
The blood oxygen content measured in the right atrium of the patient's 
heart fluctuates as a function of time for two reasons: (1) there are 
different oxygen demands placed on the patient's body tissue at different 
times of the day depending upon the activities of the patient; and (2) 
different body tissue within the patient undergoes different oxygen 
demands because of the location of the body tissue. The first variation is 
a relatively slow variation, and may be considered as the average oxygen 
demand. At certain times of the day, such as when the patient is sleeping, 
the average oxygen demand is lowest. At other times of the day, such as 
when the patient is exercising, the average oxygen demand increases 
significantly. The second variation is a relatively fast variation, and 
occurs due to the fact that the blood returning to the right atrium from 
various body tissue locations is rather poorly mixed. Thoroughly mixed 
blood, from all body tissue locations, would not exhibit the second 
variation. However, because the blood is never thoroughly mixed in the 
right atrium, some of the second variation is always present. 
In FIG. 3A, the first type of variation is predominantly illustrated. At 
time t-, for example, when the sensor output is high, the blood oxygen 
content is likewise high, indicating a time of relative inactivity of the 
patient. In contrast, at times t2 and t3, when the sensor output is low, 
the blood oxygen content is likewise low, indicating a time of relative 
activity of the patient. 
In FIG. 3B, the second type of variation is illustrated. That is, FIG. 3B 
depicts the type of variations in the blood oxygen measurement that may 
occur during a relatively short portion of the waveform of FIG. 3A, e.g., 
during the portion included within the circle B. As seen in FIG. 3B, such 
variations in the sensor output may be rather abrupt and sudden, 
evidencing the entry of blood into the right atrium from body tissue 
locations having markedly different oxygen content. A low sensor output, 
such as at the point P1, may be indicative of blood returning from a 
relatively active portion of the patient's body, such as an arm, where the 
oxygen demand of the body tissue is high. A high sensor output, such as at 
point P2, may be indicative of blood returning from a relatively inactive 
portion of the patient's body, such as the hip, where the oxygen demand of 
the body tissue is low. Alternatively, a high sensor output, such as at 
point P3, may be indicative of inappropriate reflection of light energy 
into the phototransistor of the sensor caused, e.g., by a moving heart 
valve. 
In operation, the sensor 34 does not typically operate continuously 
(although it could with appropriate circuitry). That is, the sensor is 
typically energized during a refractory period of the heart and/or 
pacemaker circuits, and a "sample" of the blood oxygen content at that 
measurement time is made. Such sample times, i.e., those times when a 
measurement is made, are represented in FIG. 3B as heavy dots equally 
spaced along the horizontal axis. Statistically, assuming the fast 
variations in the blood oxygen content are more or less random, some of 
these sample times occur when the blood oxygen content is low, and others 
occur when it is high. Hence, within a particular measurement window 70, 
which "window" 70 includes a plurality of sample times, there will be one 
sample measurement that has a lower value than the others. In FIG. 3B, 
this low or minimum measurement is the one made at point P1. It is a 
feature of the present invention, to identify the low or minimum 
measurement within a given measurement window 70, and to use such 
measurement as an indicator of the relevant blood oxygen content, i.e., to 
use such minimum value as an indicator of the oxygen content of the blood 
returning from the body tissue undergoing the highest oxygen demand. This 
minimum value can then be used as a reliable indicator of the 
physiological need to adjust the heart rate, e.g., as controlled by a 
rate-responsive pacemaker. 
It is to be noted that while FIG. 3B suggests that sample measurements made 
within the measurement window 70 be equally spaced in time, such equally 
spaced samples are not necessary. If sample measurements are taken, all 
that is necessary is that sufficient samples be obtained so that a 
statistically accurate minimum value will be obtained. (In contrast, if a 
continuous measurement is made, all that is required is that the minimum 
value of the blood oxygen content be determined for a prescribed 
measurement window.) For example, a plurality of discrete blood oxygen 
measurements could be made only during the refractory interval of a 
cardiac cycle. Such refractory interval may last, e.g., only 10-20 
milliseconds during an 800 millisecond cardiac cycle. However, during this 
10-20 milliseconds, several discrete measurements, e.g. 5-10, of the blood 
oxygen content can be measured. Alternatively, there is no requirement 
that the blood oxygen measurement be performed only during a refractory 
period. Thus, if desired, the blood oxygen measurement can be made at 
regular intervals throughout the cardiac cycle, either synchronous with 
the cardiac cycle, or asynchronous relative to the cardiac cycle. 
FIG. 4 shows a simplified flow chart depicting one method of determining 
the minimum blood oxygen measurement and using this minimum blood oxygen 
measurement to automatically adjust the pacing interval of a 
rate-responsive pacing system. As seen in FIG. 4, once rate-responsive 
(RR) pacing has been started, an initialization step is performed. Such 
initialization step may involve, e.g., calibrating the blood oxygen sensor 
against a standard. Further, such step may involve assigning nominal 
values of blood oxygen measurement values until actual measurements of 
blood oxygen can be made. Once initialized, the minimum value of the last 
n (where n is an integer) consecutive blood oxygen measurements (where the 
term "S" is used in FIG. 4 to signify a blood oxygen measurement) is 
selected. This minimum value of S is then used as the control parameter to 
adjust the pacing interval of the pacemaker. If RR pacing is to continue, 
then the next value of S is measured and the process repeats. 
To further illustrate the method depicted in FIG. 4, the following example 
is provided. Assume that twenty (n=20) consecutive measurements of S are 
to be made. Initialization of the method may thus involve assigning 
nominal values to represent the last 20 measurements. Once initialized, a 
first actual measurement is made. The minimum value of blood oxygen 
represented in this first actual measurement and 19 of the initialized 
nominal values is then selected as the control parameter for the RR 
pacing. A second blood oxygen measurement is then made. The minimum value 
of blood oxygen represented in the first and second actual blood oxygen 
measurements and 18 of the initialized nominal values is then selected as 
the control parameter for the RR pacing. This process continues until 
twenty actual measurements have been consecutively made and all twenty 
have been examined to determine the minimum blood oxygen measurement. Each 
time a new blood oxygen measurement is made, the oldest of the twenty most 
recent blood oxygen measurements is discarded. In this way, the minimum 
blood oxygen measurement is always selected from the twenty most recent 
measurements. The minimum blood oxygen measurement selected from the 
twenty most recent measurements will change only if the new blood oxygen 
measurement is less than the prior nineteen measurements. 
It is to be understood that the above example is only one possible 
implementation of the method shown in FIG. 4. Any value of n could be 
employed, from vary small values (e.g., n =3) to very large values (e.g., 
n=100). Further, the value of n may be automatically changed by the RR 
pacing circuits at certain threshold levels. For example, at normal heart 
rates, when the patient is at rest, one value of n may be used. At higher 
heart rates, when the patient is exercising, a different value of n may be 
used. 
Further, it is to be emphasized that the method shown in FIG. 4 is only one 
of several different types of methods or algorithms that may be used to 
select the minimum oxygen measurement. Any method of algorithm that 
systematically determines the minimum oxygen content measurement may be 
used. 
Referring next to FIG. 5, a simplified functional block diagram of a 
digital embodiment of the sensor process circuit 44 of FIG. 2 is shown. 
Such embodiment can be readily incorporated into a digitally controlled 
rate-rate-responsive pacemaker. (It is noted that most modern pacemakers 
are digitally controlled, many involving the use of a microprocessor, or 
equivalent, to control the operation of the pacemaker in accordance with 
prescribed operating programs, which programs may be altered as required 
to suit the needs of a particular patient.) 
As seen in FIG. 5, the output signal from the sensor 34, EOUT, is directed 
to an analog-to-digital (A/D) converter 80. The resulting digital output 
signal is stored in a first-in first-out (FIFO) register stack 82. The 
number of registers included in the FIFO register stack 82 may be selected 
to be any desired value, e.g., 32. The contents of the various registers 
within the FIFO stack 82 are compared in Minimum Value Logic circuitry 84. 
That is, circuitry 84 compares the contents of each register in the FIFO 
stack 82 and determines which one has the lowest or minimum value. This 
value is then selected and placed in a holding register 86. Further, 
during a calibration mode, as explained below, the circuitry 84 may be 
programmed to make an adjustment to the minimum value selected in order to 
compensate for variations that may occur over time to the sensor 34. 
During normal rate-responsive operation, the value held in the holding 
register 86 is selected by a multiplexer (MUX) circuit 88 and placed in a 
control register 90. The rate-responsive pacing circuits 46 (FIG. 2) look 
to the control register 90 for the control parameter that sets or controls 
the rate-responsive pacing interval. 
Still referring to FIG. 5, selection logic 92 is utilized to control the 
MUX 88. In addition to selecting the contents of the holding register 86, 
the MUX 88 may also select the contents of a calibration register 94. The 
calibration register 94 may have a fixed value loaded therein, or a 
programmed value loaded therein. If a programmed value is used, such may 
be loaded into the register 94 using conventional programming techniques 
known in the art. (See, e.g., U.S. Pat. No. 4,232,679 (Schulman) for a 
basic description of how an implantable medical device may be programmed 
using an external programmer.) Selection logic 92 is controlled by an 
appropriate select signal. This select signal may be a programmable 
command signal sent to the implantable pacemaker from a non-implantable 
programming device, in conventional manner. During calibration, i.e., when 
selection logic 92 selects the contents of the calibration register 94, 
processing circuitry 83 examines the measured blood oxygen to determine if 
the sensor 34 is functioning properly. As required, the processing 
circuitry makes programmed adjustments to the measurement value passing 
through the minimum value logic 84 in order to add or subtract an 
appropriate increment therefrom. 
In operation, the calibration circuitry shown in FIG. 5 functions as 
follows. With the patient at rest, or at some other known and controlled 
level of activity, an attending physician or cardiologist generates an 
appropriate command to cause selection logic 92 to select the contents of 
the calibration register 94 as the control parameter that is loaded in the 
control register 90. In such calibration mode, the pacemaker is 
essentially a non-rate-responsive pacemaker. Hence, the physician has some 
control as to the patient's heart rate. Further, in the calibration mode, 
the contents of the calibration register can be programmably altered to 
any desired value. In the calibration mode, the blood oxygen sensor 34 
measures the blood oxygen content, with the value of such measurements, 
after being digitized, being directed to processing circuitry 83. The 
processing circuitry 83 (which may be part of the processing circuitry of 
the pacemaker circuits 46, particularly when such circuits 46 include one 
or more microprocessors) then determines if, for the known level of 
activity and heart rate of the patient, the blood oxygen measurements are 
approximately where they should be. If not, the processing circuitry 83 
instructs the minimum value logic 84 to make whatever adjustments are 
needed to bring the blood oxygen measurements to an appropriate level 
before such measurements are placed in the holding register 86. In this 
manner, the measured value of blood oxygen may be adjusted, as required, 
in order to compensate for changes that may occur in the sensor over time. 
Initially, the contents of the registers in the FIFO register stack 82 are 
all loaded with a nominal value. However, as actual measurements of blood 
oxygen are made, the digital values corresponding to such measurements 
supplant the nominal values initially loaded. After a short time, all 
values held in the register stack 82 represent the most recent n 
measurements of blood oxygen, where n is number of registers in the FIFO 
register stack 82 that are used. In this manner, the minimum value logic 
84 always looks to the most recent n measurements in order to determine 
the minimum value that is to be used in accordance with the present 
invention. 
It is to be emphasized that that which is shown in FIG. 5 is functional, 
and numerous variations in the manner of calibration and operation could 
be readily performed by those skilled in the art. 
Referring next to FIG. 6A, a simplified electrical schematic diagram of an 
analog embodiment of the sensor process circuit 44 of FIG. 2 is shown, 
while FIG. 6B shows a timing waveform diagram illustrating the operation 
of the analog embodiment of FIG. 6A. In accordance with this embodiment, 
the output signal from the sensor 34 (FIG. 2) is directed to the inverting 
input of an input buffer amplifier 100. A peak detection circuit 102 then 
determines the peak of the input signal over a prescribed period of time, 
defined by a clock signal, and holds this value until the next period of 
time. 
The inverse of the input waveform directed to the amplifier 100 is shown as 
the signal waveform 104 in FIG. 6B. Thus, the waveform 104 represents the 
input signal, VIN, directed to the peak detection circuit 102. While this 
signal is shown as a continuous signal (suggesting continuous operation of 
the sensor 34), it is noted that it need not be continuous. Rather, the 
sensor may be continuous for only a small portion of a cardiac cycle, with 
the signal waveform 104 representing that small portion; or, the sensor 
may be sampled at an appropriate rate, with the signal waveform 104 
representing an extrapolated representation of the sampled signal. In 
either event, a clock signal, or equivalent, defines a time period T 
during which the waveform -04 is to be examined for a peak signal. (Note 
that a peak signal in the waveform -04 corresponds to a minimum signal in 
the output signal from the sensor 34, or a minimum blood oxygen 
measurement.) Once this peak signal has been found, it is held until the 
next period T. 
To illustrate, in FIG. 6B, the waveform 104 is generally increasing during 
the first period T. The peak value of the waveform 104 occurs at the end 
of this first period. Thus, at the conclusion of T, the output signal, 
shown as the waveform 106 in FIG. 6B, assumes a value corresponding to the 
value of the waveform 104 at the end of period T. Similarly, during the 
second period which ends at 2T, the peak value of the waveform 104 occurs 
at the end of the second period. Thus, at the conclusion of 2T, the output 
signal 106 assumes a value corresponding to the value of the waveform 104 
at the end of period 2T. During the third period, however, the peak of the 
signal 104 occurs somewhere near the start of the period. Thus, at the 
conclusion of the period 3T, the output waveform -06 assumes a value 
corresponding to this peak value. In a similar manner, the output waveform 
106 assumes a value at the conclusion of each period corresponding to the 
peak value of the waveform 104 during that period. The output waveform 106 
can then be used directly or indirectly (e.g., converted to a digital 
value) as a measure of the minimum oxygen content of the blood. As 
desired, such measure may also be used as the control parameter of a 
rate-responsive pacemaker. 
Those skilled in the art will recognize that the circuit shown in FIG. 6A 
and described in connection with FIG. 6B is simply a peak detection and 
hold circuit, sometimes referred to as a "boxcar" circuit (because the 
shape of the output waveform 106 resembles the profile of boxcars of a 
passing train, each loaded with cargo of differing heights). Such circuit, 
or equivalent, may advantageously be used as part of the present invention 
in order to readily determine the minimum value of blood oxygen during a 
particular time interval. 
FIG. 6C shows an exemplary embodiment of the peak detection circuit of FIG. 
6A; and FIG. 6D is a timing diagram illustrating the relationship between 
the clock signals used in the peak detection circuit of FIG. 6C. As seen 
in these figures, the peak detection circuit 102 includes an amplifier U1 
driving an emitter follower Q1. The output of the emitter follower Q1 is 
coupled to a holding capacitor C1. As long as the output voltage of the 
amplifier U1 is rising, the voltage applied to the capacitor C1 follows. 
However, as soon as the output voltage of the amplifier U1 falls below the 
voltage on the capacitor C1, the emitter-base junction of the follower Q1 
becomes back biased, thereby maintaining the prior voltage level on C1. At 
the appropriate time, e.g., at the end of the sample period, a switch Q3 
is turned on by clock signal, CLK A, allowing the voltage on C- to be 
passed, through output amplifier U2, to the output signal line 106. A 
short time thereafter, another clock signal, CLK B, turns on switch Q2, 
causing the voltage held on capacitor C1 to be discharged to ground. This 
action thus clears capacitor C1 of is previous voltage, thereby allowing 
the voltage on capacitor C1 to seek the peak value of the input voltage 
for the next period. 
FIG. 6D illustrates a preferred relationship between the clock signals CLK 
A and CLK B. CLK A defines the measuring period T. This clock may be 
obtained from the pacemaker circuits 46 (FIG. 2), or derived from an 
appropriate oscillator. For purposes of the present invention, the period 
T need not be precise. CLK B includes the same period as CLK A, but is 
delayed a slight amount, t, therefrom. The length of delay may be small, 
on the order of microseconds. Its purpose is simply to ensure that switch 
Q3 is turned OFF, after having been turned ON to present the output 
voltage to the amplifier U2, before the switch Q2 is turned ON to 
discharge capacitor C1. 
As described above, it is thus seen that the present invention provides an 
accurate and reliable system and/or method for determining the oxygen 
content of venous blood using optical measuring techniques, i.e., emitting 
light energy into the blood and sensing the amount of light energy 
reflected therefrom, regardless of reflections and other erroneous light 
energy that may be present at the time a particular measurement is made. 
Such an accurate and reliable measuring system or method is realized by 
making many measurements, e.g., spaced close together in time, and 
discarding all the measurements but the one(s) evidencing the lowest 
oxygen content. Thus, while the sensing of any erroneous light energy, 
caused by reflections or otherwise, may adversely affect one or more 
individual measurements (causing such measurements to indicate a higher 
than actual oxygen content), such affected measurements are discarded and 
not considered. 
As further described above, it is also seen that the present invention 
provides a reliable and accurate system and/or method for measuring the 
oxygen content of blood returning from body tissue undergoing the greatest 
oxygen demand, e.g., experiencing the most physical exercise, even though 
such blood is in the process of being mixed, e.g., in the right atrium, 
with blood returning from body tissue not experiencing a high oxygen 
demand. Advantageously, such system and/or method is again realized by 
making many measurements, e.g., spaced close together in time, and 
responding only to the minimum blood oxygen content measurement(s), and 
discarding or ignoring the non-minimum blood oxygen content measurements. 
The minimum blood oxygen measurement represents that portion of the poorly 
mixed blood having the lowest oxygen content, i.e., that blood from body 
tissue undergoing the greatest oxygen demand. In this way, the present 
invention advantageously provides a system and/or method that 
automatically determines or selects relevant blood oxygen measurements 
(i.e., from blood returning from active body tissue) from irrelevant blood 
oxygen measurements (i.e., from blood returning from non-active body 
tissue). 
Moreover, as also described above, it is seen that the present invention 
provides a system and/or method for easily and accurately measuring 
relevant blood oxygen levels and using such measurements as a control 
parameter in a rate-responsive pacing system. 
Finally, as further indicated above, the present invention provides a 
rate-responsive pacing system and/or method wherein stimulation pulses are 
provided to a patient's heart on demand at a rate that is determined by 
the oxygen content of blood returning from those parts of the patient's 
body experiencing the greatest oxygen demand. 
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.