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Claims  |
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We claim:
1. A metabolic-demand pacemaker comprising means for pacing a patient's
heart at a controlled rate, means for placement in a blood vessel in the
vicinity of the patient's pleural cavity and for monitoring the impedance
of the blood therein, the blood impedance varying in accordance with the
patient's pleural pressure, means responsive to said monitoring means for
determining the patient's minute volume, and means for changing said
controlled rate in accordance with the patient's minute volume.
2. A metabolic-demand pacemaker in accordance with claim 1 wherein said
determining means includes means for summing successive peak blood
impedance changes for a predetermined time interval.
3. A metabolic-demand pacemaker in accordance with claim 2 further
including means for representing a nominal standby rate, means responsive
to said determining means for calculating a long-term minute volume
average value, and means for calibrating said controlled rate to equal
said nominal standby rate for a minute volume which equals the long-term
average value, and wherein said changing means includes means for changing
said controlled rate in accordance with the deviation of the minute volume
from the long-term average value.
4. A metabolic-demand pacemaker in accordance with claim 3 further
including means for limiting the rate at which said controlled rate is
allowed to change.
5. A metabolic-demand pacemaker in accordance with claim 4 wherein said
pacemaker includes a case and said monitoring means includes first and
second electrodes for placement in said blood vessel, means for causing a
fixed current to flow through the blood in said vessel between said first
electrode and the pacemaker case, and means for measuring the voltage
across said second electrode and the pacemaker case to monitor the
impedance of said blood.
6. A metabolic-demand pacemaker in accordance with claim 5 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
first and second electrodes are fixed in position along said catheter.
7. A metabolic-demand pacemaker in accordance with claim 4 wherein said
pacemaker includes a case, said pacing means includes a first electrode,
and said monitoring means includes second and third electrodes for
placement in said blood vessel, means for causing a fixed current to flow
through the blood in said vessel between said second electrode and the
pacemaker case, and means for measuring the voltage across said third
electrode and the case to monitor the impedance of said blood.
8. A metabolic-demand pacemaker in accordance with claim 7 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
first, second and third electrodes are fixed in position along said
catheter.
9. A metabolic-demand pacemaker in accordance with claim 1 further
including means for representing a nominal standby rate, means responsive
to said determining means for calculating a long-term minute volume
average value, and means for calibrating said controlled rate to equal
said nominal standby rate for a minute volume which equals the long-term
average value, and wherein said changing means changes said controlled
rate in accordance with the deviation of the minute volume from the
long-term average value.
10. A metabolic-demand pacemaker in accordance with claim 9 wherein said
pacemaker includes a case and said monitoring means includes first and
second electrodes for placement in said blood vessel, means for causing a
fixed current to flow through the blood in said vessel between said first
electrode and the pacemaker case, and means for measuring the voltage
across said second electrode and the pacemaker case to monitor the
impedance of said blood.
11. A metabolic-demand pacemaker in accordance with claim 9 wherein said
pacemaker includes a case, said pacing means includes a first electrode,
and said monitoring means includes second and third electrodes for
placement in said blood vessel, means for causing a fixed current to flow
through the blood in said vessel between said second electrode and the
pacemaker case, and means for measuring the voltage across said third
electrode and the case to monitor the impedance of said blood.
12. A metabolic-demand pacemaker in accordance with claim 1 wherein said
pacemaker includes a case and said monitoring means includes first and
second electrodes for placement in said blood vessel, means for causing a
fixed current to flow through the blood in said vessel between said first
electrode and the pacemaker case, and means for measuring the voltage
across said second electrode and the pacemaker case to monitor the
impedance of said blood.
13. A metabolic-demand pacemaker in accordance with claim 12 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
first and second electrodes are fixed in position along said catheter.
14. A metabolic-demand pacemaker in accordance with claim 1 wherein said
pacemaker includes a case, said pacing means includes a first electrode,
and said monitoring means includes second and third electrodes for
placement in said blood vessel, means for causing a fixed current to flow
through the blood in said vessel between said second electrode and the
pacemaker case, and means for measuring the voltage across said third
electrode and the case to monitor the impedance of said blood.
15. A metabolic-demand pacemaker in accordance with claim 14 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
first, second and third electrodes are fixed in position along said
catheter.
16. A metabolic-demand pacemaker in accordance with claim 1 further
including means responsive to said determining means for calculating a
long-term minute volume average value, and wherein said changing means
includes means for changing said controlled rate in accordance with the
deviation of the minute volume from the long-term average value.
17. A metabolic-demand pacemaker in accordance with claim 16 wherein said
monitoring means includes a pair of electrodes for placement in said blood
vessel, and means coupled to said electrodes for causing a fixed current
to flow through the blood in said vessel and for measuring the voltage
across said blood to determine the impedance of said blood.
18. A metabolic-demand pacemaker in accordance with claim 17 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
electrodes are fixed in position along said catheter.
19. A metabolic-demand pacemaker in accordance with claim 1 further
including means for limiting the rate at which said controlled rate is
allowed to change.
20. A metabolic-demand pacemaker in accordance with claim 19 further
including means for representing a nominal standby rate, means responsive
to said determining means for calculating a long-term minute volume
average value, and means for calibrating said controlled rate to equal
said nominal standby rate for a minute volume which equals the long-term
average value, and wherein said changing means includes means for changing
said controlled rate in accordance with the deviation of the minute volume
from the long-term average value.
21. A metabolic-demand pacemaker in accordance with claim 19 wherein said
monitoring means includes a pair of electrodes for placement in said blood
vessel, and means coupled to said electrodes for causing a fixed current
to flow through the blood in said vessel and for measuring the voltage
across said blood to determine the impedance of said blood.
22. A metabolic-demand pacemaker in accordance with claim 21 wherein a
catheter is used to couple the pacemaker to the patient's heart, and said
electrodes are fixed in position along said catheter.
23. A method of determining a patient's minute volume comprising the steps
of placing electrodes in a blood vessel in the vicinity of the patient's
pleural cavity, monitoring the blood impedance between predetermined
points, the blood impedance varying in accordance with the patient's
pleural pressure, and deriving the patient's minute volume from the
variations in the monitored blood impedance.
24. A method in accordance with claim 23 wherein the patient's minute
volume is derived by summing successive peak blood impedance changes for a
predetermined time interval.
25. A method in accordance with claim 24 further including the step of
calculating a long-term minute volume average value from the monitored
blood impedance, and wherein the patient's minute volume is derived by
comparing variations in the monitored blood impedance with the average
value.
26. A method in accordance with claim 23 further including the step of
calculating a long-term minute volume average value from the monitored
blood impedance, and wherein the patient's minute volume is derived by
comparing variations in the monitored blood impedance with the average
value.
27. A method in accordance with claim 26 wherein said monitoring step
includes the sub-steps of causing a fixed current to flow through the
blood in said vessel, and measuring the voltage across two predetermined
points to determine the impedance of said blood.
28. A method in accordance with claim 23 wherein said monitoring step
includes the sub-steps of causing a fixed current to flow through the
blood in said vessel, and measuring the voltage across two predetermined
points to determine the impedance of said blood. |
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Claims |
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Description |
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DESCRIPTION
This invention relates to pacemakers, and more particularly to
metabolic-demand pacemakers and methods of using the same to determine a
patient's minute volume.
The term "demand" in the pacemaker art generally denotes a pacemaker which
stimulates the heart only when there is a need for stimulation. Typically,
in a VVI pacemaker, the timer which controls the generation of a
stimulating pulse is reset when an R wave is sensed, thus ensuring that a
stimulus is generated only when there is a demand for it. The term
"metabolic-demand" as used to characterize a pacemaker has a completely
different meaning. It refers to the rate at which the pacemaker paces the
heart, in the absence of spontaneous beating, as being dependent upon the
true metabolic demand of the human body.
The concept of providing a metabolic-demand pacemaker is very old. One of
the earliest approaches involved monitoring the respiration rate of the
patient; the faster the breathing, the greater the need for oxygen to be
transported in the blood throughout the body, and the faster the required
pacing rate. In the past few years, a number of other methods have been
proposed for making the pacing rate reflect the true metabolic demand. In
addition to respiration rate, the QT interval, body sounds, oxygen
saturation and blood temperature have all been used as measurable
parameters for controlling the pacing rate. The prior art methods were not
satisfactory, however, because either the relationships of the measured
parameters to true metabolic demand were not well defined, or there were
problems with sensor stability or complexity.
It has been suggested by others that a satisfactory parameter would be
minute volume. The minute volume is a measure of the average amount of air
being breathed by the patient; the greater the minute volume, the faster
should the heart beat. The relationship results from the fact that both
minute volume and heart rate are controlled by the nervous system, and the
same body parameters influence both. Minute volume is known from extensive
medical investigation to be an excellent indicator of metabolic demand.
All physical activity consumes oxygen, and oxygen consumption is directly
related to the replenishing process, i.e., minute volume. Measuring the
respiration rate alone is insufficient to indicate physical activity
because a patient may increase his air intake not only through rate, but
also through volume change. [The term "minute volume" would appear to
refer to the volume of air breathed in during the course of a minute. The
term as used herein is broader, however, in that it does not contemplate
measurements which necessarily extend for exactly one minute. It is simple
enough, for example, to determine the volume breathed in during the course
of one minute from a measurement taken over only thirty seconds. The term
minute volume as used herein refers to the average amount of air breathed
in by the patient, while the actual measurement interval may extend from
several seconds all the way to longer than a minute.]
Minute volume is a function of two factors--the volume of air breathed in
during each breath, and the rate at which the patient is breathing. Adding
up the individual breaths for a certain period of time provides a measure
of the minute volume. In general, doing this for too short an interval,
e.g., a few seconds, can give rise to inaccurate results. The reason for
this is that considerably different results will be obtained if the
measurement interval starts just before a breath or just after it. On the
other hand, performing the measurement over too long an interval is also
inadvisable because the response of the pacemaker will be too slow. In
general, a measurement interval of 20-30 seconds is preferred, although
intervals outside this range are also possible. The real problem, however,
is how to measure the amount of air breathed in by the patient. Prior art
techniques for measuring minute volume have not been satisfactory.
Generally, a transducer is placed across the rib cage of the patient, with
the movement of the ribs reflecting the expansion of the lungs. This
measurement technique suffers from the disadvantages of poor resolution
and susceptibility to motion artifacts, and is not sufficiently reliable
to find use in a pacemaker.
Our invention is based upon the realization that the volume of air in the
lungs is related to a corresponding pressure called pleural pressure. The
pleural pressure, in the pleural cavity, manifests itself in a change in
the diameter of blood vessels in the immediate vicinity of the cavity. The
blood in the vessels comprises a volume conductor, and its impedance is
measured by establishing a known current field and measuring the voltage
which develops in the field. The impedance change which is due to
respiration depends on the particular placement of the current source
electrodes as well as the voltage sense electrodes. Preferably, the
current source is established between the blood in the right ventricle and
the pacemaker case, and the voltage is measured between either the high
right atrium or the superior vena cava ("SVC") and the pacemaker case. If
there is no pacing catheter in the right ventricle, then the current may
be established between the blood in the atrium and the pacemaker case.
However, the preferred blood vessel for use is the SVC, one of the two
large veins which return blood to the heart. The SVC is also the vein
along which all transvenous electrodes for pacemakers pass to get to the
right ventricle, and that is why the SVC is the ideal blood vessel for use
in practicing our invention. The modulation in the impedance measurement
is a direct measure of the minute volume. By monitoring the degree of the
impedance change during the course of each breath, the volume of air
contained in the breath (called the "tidal volume") can be determined.
Simply by summing these values over a fixed period of time, a measure of
minute volume can be obtained.
While the basic operating principle of our invention is easy to understand,
there are other factors to consider. Consider a minute volume measurement
taken over a 20-second interval. If there is a doubling during a
measurement interval, it is not desirable to suddenly change the pacing
rate of the heart by a correspondingly significant amount. [Minute
ventilation can vary by a factor of about 4-14 depending upon the patient,
while the heart rate varies by about a factor of 2, depending upon the
patient. Although it is possible to construct a pacemaker which would
allow the function of minute volume versus rate to be programmed, in the
illustrative embodiment of the invention the pacing rate doubles for a
minute volume which increases by a factor of 8. Thus a doubling of the
minute volume does not result in a doubling of the rate but the change, if
abrupt, can still be undesirable.]
One of the most severe problems is that there is no permanent relationship
between the measured minute volume and the pacing rate. A typical
pacemaker is programmed to have a particular standby rate, the minimum
rate at which the heart should be paced. Theoretically, there is a minimum
minute volume, associated with minimal physical activity, which
corresponds with this programmed standby rate. As the patient exercises,
the minute volume increases and, in accordance with the principles of our
invention, so will the standby rate. It is found over long periods of
time, however, that the minute volume measurement does not return to the
original value. This is not to say that the real minute volume for minimal
physical activity has changed, but rather that the measurement value has
changed. This can be due to many factors, such as a change in electrode
position or even the conductivity of the blood. There must be some way to
accommodate for long-term changes in the minute volume measurement, that
is, changes which can properly be attributed to permanent or
semi-permanent factors, as opposed to instantaneous metabolic demand.
In accordance with the principles of our invention, the minute volume is
averaged over a long period of time in order to determine an average
value. This long-term average value is assumed to be the minimum minute
volume and to correspond with the programmed standby rate. An
instantaneous minute volume (measured over the 20-second measurement
interval) less than the long-term average does not affect the pacing rate;
the programmed (minimum) standby rate is used. It is only when the
instantaneous value exceeds the average value that it is assumed that
there is a short-term need for more oxygen, and the standby rate is
adjusted accordingly. The instantaneous rate can be increased or
decreased, to a degree dependent upon the amount by which the
instantaneous minute volume exceeds the long-term average. The long-term
average can increase or decrease, with the long-term average at any time
corresponding with the programmed standby rate.
It is in this way that the system adapts itself to permanent and even
long-term changes. Another way of looking at it is that the pacing rate is
calibrated to equal the nominal (or programmed standby) rate for a minute
volume which equals the long-term average value, with the instantaneous
rate being changed in accordance with the deviation of the minute volume
from the long-term average value.
The basic objective of our invention thus is to provide novel and improved
metabolic-demand pacemakers and methods of using the same to determine a
patient's minute volume.
Further objects, features and advantages of our invention will become
apparent upon consideration of the following detailed description thereof
when read in conjunction with the hereto annexed drawing, in which:
FIG. 1 depicts the input/output section of a pacemaker constructed in
accordance with the principles of our invention;
FIGS. 2, 2A and 2B depict a human heart and three possible approximate
placements of electrodes;
FIG. 3 depicts the signals which appear on the conductors labelled OP1 and
OP2 of FIG. 1;
FIG. 4 is the processing section of the illustrative embodiment of our
invention, having an input OP2 connected to the OP2 output of FIG. 1, and
two outputs MV and CLK; and
FIG. 5 is the third stage of the illustrative embodiment of our invention,
having two inputs, CLK and MV, connected to the outputs of the circuit of
FIG. 4, a VENT. EVENT input connected to a corresponding output in the
circuit of FIG. 1, and a STANDBY INTERVAL output connected to a
corresponding input in the circuit of FIG. 1.
As shown in FIG. 1, the pacemaker includes a case 10 and three electrodes
S1, S2 and S3. The electrodes are shown in FIG. 2 all disposed along a
single catheter 12 which is extended through the SVC to the patient's
heart. Electrode S1 is the conventional pacing/sensing electrode. The
indifferent electrode in the illustrative embodiment of the invention is
the case itself, as will be described. Electrode S2 is used for current
sourcing, i.e., to apply a current which flows in the blood between the
electrode and the pacemaker case, and electrode S3 is used to measure the
respiratory impedance between the electrode and the case.
Alternative electrode placements are also shown in FIGS. 2A and 2B. In the
arrangement of FIG. 2A, the impedance measurement electrode (which is
designated S3 in FIG. 1) may be placed, for example, in the SVC and is
designated S3'; in general, the electrode used for the impedance
measurement may range in position from the vicinity of the high right
atrium to 3-4 cm above the margin. In the arrangement of FIG. 2B, in the
absence of a ventricular pacing catheter, but where a conventional atrial
J electrode S4 is provided, an electrode S5 may be used for current
sourcing and an additional electrode such as S3' may be used for sensing.
All of electrodes S2, S3 and S5 are conventional ring electrodes. In
general, the sensitivity and the signal/noise ratio are compromised if the
current sourcing is done in the endocardium rather than in the blood. The
case is used for both current sourcing and impedance measurement not only
to save electrodes, but also because it has been found to provide an
improved signal/noise ratio. When the pacemaker also requires atrial
pacing or sensing, and utilizes a conventional atrial J electrode, as
shown in FIG. 2B, the positioning of electrode S3' is relatively simple
because the distance between the SVC-atrial margin and the atrial
appendage is relatively well defined. Thus when an atrial electrode is
used, the rings which comprise electrodes S5 and S3' may be fixed relative
to the atrial electrode, thereby simplifying their proper positioning in
the SVC. When only a ventricular electrode is used, as in FIG. 2A, the
distance between the SVC-atrial margin and the ventricular apex is
typically 10-18 cms. In this case, it will usually be necessary, after
placing the ventricular electrode, to fix the position of electrode S3 by
changing the curvature of the electrode portion within the heart.
All switches are under the control of microprocessor controller 14. One
output of the controller is shown extended to switch SW2, but it is to be
understood that the switches SW1, SW3, SW5, SW6 and SW7 are similarly
controlled. With switches SW1 and SW5 closed, and switch SW3 open, current
from constant-current source 16 flows from electrode S2 through the blood
to grounded case 10. Depending upon the blood impedance, the voltage at
electrode S3 relative to the case changes. This voltage is extended
through the buffer amplifier 20 and switch SW2 (which follows the state of
switch SW1), and sampled on capacitor C1 at the input of filter 23. The
waveform of the input to the filter, OP1, is shown in FIG. 3. Typically,
the switches SW1, SW2 and SW5 are closed for 20-60 microseconds, 100 times
per second. The resistors and capacitors associated with filter 23 pass
frequencies between about 0.05 Hz and 1.0 Hz, the standard range for
respiration rate measurements. The waveform of the output of the filter,
OP2, is also shown in FIG. 3. It is apparent that the OP2 signal is the
envelope, suitably filtered, free of the faster transients contained in
OP1. (The waveforms of FIG. 3 are not drawn to scale.)
During a measurement interval, switches SW4 and SW6 are open, and switch
SW7 is closed; pace and sense functions are briefly disabled. Although
sensing is disabled while the impedance measurement is being performed,
even 60 microseconds is so short an interval relative to heart signals
that disabling the sensing during this time is of no importance.
Unipolar pacing is used in the illustrative embodiment of the invention. In
order to pace the heart, switches SW4 and SW5 are closed. Switch SW5
connects the case to the circuit ground, so that the case serves as the
indifferent electrode during the pacing. Switch SW4 is closed by a signal
extended over control lead 22 by standby timer 24. The standby timer is a
conventional timer (whose function, like many of those depicted by circuit
blocks, can be controlled by a microprocessor) for controlling a pacing
pulse when a spontaneous beat has not been sensed for an interval equal to
the standby interval. The standby interval itself is derived from the
circuit of FIG. 5, as will be described, and is extended to the timer. The
pacer includes a standard storage capacitor 26 and charge circuit 28 for
furnishing the pacing pulse when needed. During pacing, the state of
switch SW3 is not important. Switches SW1 and SW2 should be open since
impedance measurements should not be performed during pacing. The states
of switches SW6 and SW7 are not critical, although preferably the former
is held open and the latter is held closed.
The pacemaker shown in the drawing is capable of unipolar sensing or
bipolar sensing; element 32 is an amplifier-filter commonly used for heart
signal detection. The status switch SW7 determines the sensing mode, and
switch SW7 can be a programmable parameter. (The programming circuit is
not shown. For example, referring to FIG. 5, it will be seen that there is
a register 30 which contains a standby rate. While it is fixed in the
illustrative embodiment of the invention, it is to be understood that it
can be a programmable parameter as is known in the art.) In the unipolar
mode, switch SW7 is closed. The plus input of difference amplifier 32 is
thus connected to circuit ground, and the minus input is connected to
electrode S1. Switch SW5 is closed to connect the case to circuit ground.
Switches SW1 and SW2 are, of course, held open since they are closed only
in order to conduct an impedance measurement. Switch SW5 should be closed
so that the case is grounded. Switch SW6 is held open, and unipolar
sensing proceeds with the plus input of amplifier 32 grounded through
switch SW7. The state of switch SW3 is not important.
When the system is operated in the bipolar sensing mode, switch SW7 is
open. Switch SW6 is closed during the sensing, so that electrode S2 is
connected to the plus input of difference amplifier 32. Electrode S1 is
still connected to the minus input, so that the voltage which is sensed is
that which appears across electrodes S1 and S2. Switch SW3 must be open in
this case so as not to short electrode S2 to the case. The case should be
grounded through switch SW5.
Although switch SW5 is always closed, as described, the switch is shown
should bipolar pacing be employed. In that case the switch should be
opened during pacing.
The remaining circuitry in FIG. 1 is straightforward. The output of sense
amplifier 32 triggers an IRP (interference reversion period) multivibrator
34. This multivibrator causes sensed signals which are too close to each
other to be ignored. The output of the multivibrator resets standby timer
24 to start a new ventricular-ventricular timing interval. The interval
itself is determined by the output of the standby in-use latch 36 in FIG.
5. This element will be described below. Whenever the timer times out,
switch SW4 is closed and a pacing pulse is generated. An OR gate 38 is
used to signal the occurrence of a ventricular event. The two inputs to
the OR gate are the output of the timer 24, representing the generation of
a pacing pulse, and the output of the IRP multivibrator 34, representing a
spontaneous beat. The output of the OR gate is extended to the clock input
of latch 36 in FIG. 5, the purpose of which will be described below.
It should be noted that four electrodes have been used in the past to
perform an impedance measurement to determine the volume of blood in the
right ventricle. (This has nothing to do with providing a measurement of
minute volume.) Only two electrodes are used to measure blood impedance in
the illustrative embodiment of the invention. The current source electrode
is preferably made with a larger surface area and the surface is prepared
with platinum black or sintering to increase the effective area. The
voltage sense electrode is ideally a small platinum electrode.
With reference to FIG. 3, it will be seen that the inspiration phase
corresponds to a decreasing blood impedance. It is during the inspiration
phase that the pleural pressure is decreased, which is why the lungs fill
up with air in the first place and increase in volume. A decrease in
pressure results in decompression of the veins and a decrease in the
impedance measurement.
It is the peak-to-peak transition in the OP2 signal of FIG. 3 which is
directly proportional to the tidal volume, that is, the amount of air
inhaled in one breath. If successive peak-to-peak measurements are summed,
the sum will represent the total amount of air breathed in during the
measurement interval. It is the circuit of FIG. 4 which both determines
the peak-to-peak values, and also adds them all up during a 20-second
measurement interval.
A preferred way to process a physiological signal is to use a delta
modulator. Delta modulator processing is in and of itself a standard
technique. Reference may be had, for example, to Pat. Nos. 4,466,440,
4,448,196 and 4,509,529. The OP2 signal is coupled through capacitor 50 to
delta modulator 52. The output of the delta modulator is a series of 1 and
0 bit values, as is known in the art. The bit stream is extended to the
input of slope detector 54. The output of the slope detector is energized
whenever the slope of the OP2 signal is negative. Slope detectors per se
are also well known in the art. (A typical strategy, for example, is to
determine the slope by monitoring whether 0's or 1's predominate over a
predetermined number of bits.) Cumulative counter 56 is enabled whenever
the slope of the OP2 signal is negative. For each bit at the output of the
delta modulator, the counter increments or decrements its count, depending
upon the polarity of the bit. Thus during the inspiration phase, the count
will increase (assuming that bits of value 1 correspond to decreases in
the OP2 signal) by an amount equal to the difference between the number of
1's and the number of 0's. This, in turn, is a function of the
peak-to-peak magnitude of the OP2 signal. Because the counter is enabled
during every negative sloping portion of the OP2 signal, the cumulative
count is a measure of the sum of the peak-to-peak values. The counter is
reset at 20-second intervals under control of timer 58, and thus the
output of counter 56 at the end of every measurement interval is directly
proportional to the average minute volume over the preceding 20 seconds.
(This is considered to be an "instantaneous" value.) At the end of every
measurement interval, the load input of latch 60 is pulsed and the final
count is loaded in the latch. The counter then resets so that another
measurement may be taken. Thus the multi-bit value on cable MV represents
the instantaneous minute volume, and a pulse on the CLK conductor is an
indication that a new value has been derived. These two signals are used
by the circuit of FIG. 5. This is the circuit which not only determines
the standby rate in accordance with the instantaneous minute volume, but
also calibrates the nominal standby rate to the long-term, or average,
value of minute volume.
The circuit of FIG. 5 includes a random: access memory 64 which contains 32
locations. Address counter 62 contains five stages so that its count
cycles between 0 and 31. Each CLK pulse advances the counter. Following a
count of 31, the next CLK pulse causes the counter to be reset to zero and
a carry pulse to be extended to address counter 78. This counter is
similar to counter 62, with the carry outputs of counter 62 serving as the
clock input. It is thus apparent that counter 62 cycles 32 times for each
cycling of counter 78. RAM 80 is similar to RAM 64. The address bits of
counter 62 identify a storage location in memory 64, while the address
bits of counter 78 select a storage location in memory 80.
Memory 64 is used to store the 32 most recent minute volume samples, that
is, the samples taken over the last 10 minutes and 40 seconds. These 32
values are used to form an average. The actual average is computed by
adding together the most recent 32 samples, and then dividing by 32. Each
time a new sample is derived, the oldest sample is discarded, and the
newest sample takes its place. It is in this way that the average reflects
the most recent samples. The running sum is contained in counter 72. This
counter can count up or down, depending upon whether its up or down count
input is energized at the same time that a sample is applied by gate 68 to
its data input. (Gate 68 and many of the other individual gates in FIG. 5
really represent multiple gates for transmitting bits in parallel. For the
sake of drawing simplicity, no distinction is made between single-bit and
multi-bit operations. In each case, when it is a multi-bit value which is
transmitted, it will be apparent that a group of gates must be used even
though only one is shown.)
With the arrival of each CLK pulse, address counter 62 advances, and thus
identifies the oldest sample in memory 64. The CLK pulse is applied to the
read input of the memory, and it also enables gate 68. Consequently, the
oldest sample stored in the memory is transmitted through the gate to the
input of counter 72. The CLK pulse is also applied to the down count input
of the counter, and thus the oldest sample is subtracted from the running
count maintained by the counter. Delay unit 66 delays the CLK pulse
slightly. The delayed pulse is applied to the write input of the memory.
The latest minute volume sample is applied to the data input of the
memory, and thus this sample gets stored in the memory. The same sample is
extended through gate 70, which gate is also energized by the delayed CLK
pulse, for application to the data input of counter 72. Because the
delayed CLK pulse now energizes the up count input of the counter, the
latest sample is added to the running count. It is in this manner that the
oldest sample is subtracted from the running count and the latest sample
is added to it, in order to update the average. The newest sample also
replaces the oldest sample in the memory so that 32 sample times from now,
the system will know the value of the oldest sample which should be
subtracted from the running count.
As described above, the most recent minute volume sample is always compared
with a long-term average in order to determine how the standby rate should
be controlled. In the preferred embodiment, the long-term average which is
used is taken over a much longer period of time than 10 minutes and 40
seconds, the time interval during which the 32 most recent samples were
taken. In order to form an average over a much longer period of time, the
second memory 80 is used. What is done is to take the 32 samples stored in
memory 64 and to average them. This is accomplished simply by taking the
count in counter 72 and dividing it by 32. The average value is then
stored in memory 80. The two memory circuits function in an almost
identical fashion, the only difference being that what is stored in memory
80 each time is an average value taken over 32 sample times.
At the end of each complete cycling of address counter 62, after 32 new
samples have been stored in memory 64 following the last complete cycling
of the address counter, a carry pulse is generated. This pulse serves to
trigger divider 76, which divides the count in counter 72 by 32. Thus the
data value at the output of the divider is comparable to the MV input to
the circuit of FIG. 5. The difference is that while the MV input
represents the latest minute volume sample, the output of the divider
represents the average value over the last 32 samples. The carry output
from address counter 62 also serves as a clock pulse, and is analogous to
the CLK input. The carry pulse cycles address counter 78 so that a new
location in memory 80 is identified. The carry pulse energizes the read
input of the memory as well as gate 84, so that the oldest average value
stored in the memory is transmitted through the gate to the data input of
counter 88. Because the carry output of address counter 62 energizes the
down count input of counter 88, the oldest average value is subtracted
from the running count. After a delay introduced by element 82, the newest
average value is extended through gate 86 to the data input of the
counter. At the same time, the up count input of the counter is energized
so that the newest average value is added to the count. Also, because the
write input of the memory is energized, the newest average value is stored
in memory 80.
Delay unit 74 serves to delay each incoming clock pulse slightly in order
to allow the count maintained by counter 88 to be up-dated should address
counter 62 have completed a cycle. The pulse at the output of delay unit
74 energizes divide-by-32 element 90 so that what appears on the AVG
output of the divider is an average value taken over the last 32.times.32
samples.
It should be appreciated that there is a delay between the long-term
average value at the output of divider 90 and the current sample. Counter
88 is up-dated only at intervals of 10 minutes and 40 seconds, and thus
the long-term average value is not even influenced by the most current
samples. This is of no moment, however. The only reason for maintaining a
long-term average value is to calibrate the system by establishing a
correspondence between the programmed standby rate maintained in register
30 and the long-term minute volume average value. The current minute
volume value is compared with the long-term value, and there is no reason
to be concerned about updating the long-term value rapidly.
The current value is applied to the B input of comparator 94, and the
long-term average value is applied to the A input of the comparator. The
comparator energizes one of its two outputs, as indicated. Should the
current value be less than or equal to the long-term average value, then
gate 91 is enabled. A data value is transmitted through this gate and OR
gate 98 to the input of multiplier 93; this data value is unity. The data
input to multiplier 93 is the programmed standby rate stored in register
30, and in this case the output of the multiplier is simply the
programmed, or nominal, standby rate.
On the other hand, if the current value is greater than the long-term
average value, gate 96 is energized. The MV and AVG inputs are applied to
divider 92, which divider forms the ratio shown. The ratio will be greater
than unity only if the current value is greater than the average value,
and it is only at this time that gate 96 is operated. Thus the upper input
of OR gate 98 causes a value to be transmitted to multiplier 93 only if
the value is greater than unity. The net result is that the input to
multiplier 93 is equal to the ratio of the current minute volume value to
the long-term average value, but the ratio never drops below unity.
The reason for this is that the output of multiplier 93 represents the
desired standby rate. The desired rate can never be less than the
programmed rate. Consequently, the input to the multiplier from OR gate 98
can never be less than unity. The output of the multiplier can only be
equal to or greater than the programmed rate, depending upon the degree by
which the current minute volume exceeds the long-term average.
The multiplier does not simply multiply its two inputs to derive an output.
As mentioned above, minute volume for a typical patient can vary between
limits which differ by as little as a factor of 4 or as much as a factor
of 14. It would hardly be desirable to increase the standby rate by even 4
times because this would certainly cause the patient to be paced at too
fast a rate. In the illustrative embodiment | | |
