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BACKGROUND OF THE INVENTION
This invention generally relates to the measurement of blood flow and more
particularly to the measurment of blood flow from the heart in a patient
being aided by an intra-aortic balloon pumping system.
An intra-aortic balloon pumping system is used in the treatment of ischemic
heart disease. The system includes a long slender balloon which is
inserted surgically into a patient's descending thoracic aorta. An
external system controller inflates and deflates the balloon in
synchronism with the patient's heartbeat. The alternate inflation and
deflation of the balloon boosts aortic diastolic pressure, lowers systolic
pressure, and increases coronary blood flow. As a result, the balloon
pumping system increases or maintains cardiac performance while reducing
the workload on the patient's heart.
The monitor functions in response to the signals normally present in an
intra-aortic pumping system: a pressure signal from a catheter in the
radial artery and associated transducer, a balloon pressure signal
representing the pressure within the balloon, and a balloon drive signal
which controls the inflation or deflation of the balloon.
It is, of course, extremely desirable to measure the effectiveness of the
balloon pumping system treatment. A key parameter in this regard is
cardiac output, which is the total blood flow rate from the heart.
Diverse, clinically acceptable methods for measuring cardiac output are
used and other methods have been proposed.
The oldest and most reliable method is known as the Fick method. This
method was first proposed in the late 1800's, but did not gain acceptance
as a clinical method until procedures for cardiac catheterization were
perfected. In accordance with this method, a catheter is implanted in the
pulmonary artery to measure the oxygen concentration in venous blood while
another catheter is implanted in the brachial, radial or femoral artery to
measure the oxygen concentration in arterial blood. Patient oxygen
comsumption is determined by breath analysis. The ratio of the
differential of the arterial and venous blood oxygen concentrations to the
patient oxygen consumption indicates cardiac output.
As can be appreciated, the Fick process is quite complicated and requires
expensive apparatus. Even though it is recognized as being reliable, a dye
method for determining cardiac output is now more popular. In accordance
with this method, known since the late 1940's, a dye solution is injected
through a catheter into the pulmonary artery. A constant flow blood sample
is then withdrawn from another artery to provide a time analysis of dye
concentration which leads to a determination of cardiac output. This
method, although widely used, has two disadvantages. It is less accurate
than the Fick method. Also, a patient may have an allergic reaction to the
dye solution, so the method can not be used universally.
Another clinical method which has been used since the introduction of
thermistors in the 1950's is a thermal dilution method. With this method,
a cold saline solution is injected through a catheter into the right
atrium. The dispersion of that solution is monitored by inserting a
catheter with a thermister into the pulmonary artery. Cardiac output can
then be determined by analyzing the temperature variations.
Other methods are experimental. In one such method, a magnetic flow meter
is implanted in the ascending aorta. Leads from the flow meter are then
connected to an external controller which indicates the blood flow at the
flow meter. With still another experimental technique, a catheter
implanted in the ascending aorta has means for obtaining spaced lateral
pressure taps. The differential pressures from these taps indicate cardiac
output.
All these foregoing clinical and experimental methods are invasive.
Impedance plethysmography is an experimental non-invasive method.
Energizing electrodes disposed on the skin are excited by an AC signal.
Sensing electrodes, also on the skin, receive the signals and couple them
to a controller which analyzes the exciting and receive signals for
displaying cardiac output. Although this technique is non-invasive, the
patient is subjected to electrical signals which, in some cases, may have
deleterious effects.
When a patient is undergoing treatment by means of an intra-aortic balloon
pumping system, all the foregoing methods for determining cardiac output
require additional external or internal connections to be made to the
patient. These connections can complicate treatment. None of the methods,
other than the Fick method, produces an extremely accurate indication of
cardiac output. As known, they are subject to rather substantial errors.
Moreover, none of the clinical methods permit or facilitate continuous
monitoring of cardiac output.
Therefore, it is an object of this invention to provide an improved method
and apparatus for measuring cardiac output in a patient undergoing
treatment by means of an intra-aortic balloon pumping system.
Another object of this invention is to measure cardiac output by using
signals which are presently available in an intra-aortic balloon pumping
system.
Still another object of this invention is to provide apparatus for
measuring cardiac output continuously.
Yet another object of this invention is to provide apparatus for measuring
cardiac output in an intra-aortic balloon pumping system without further
invasion into the patient.
Still yet another object of this invention is to provide apparatus for
measuring cardiac output which facilitates measurements and recording at
the time the measurements are made.
SUMMARY
In accordance with this invention, a cardiac output monitor receives
various system signals from an intra-aortic balloon pumping system
including a signal representing the pressure in a patient's artery,
normally the radial artery. The monitor determines the pressure which
characterizes a ventricle ejection pulse and the pressure which
characterizes the balloon deflation after a counterpulse. The monitor
processes these signals and other system signals to indicate, on an
interrupted or continuous basis, the patient's cardiac output.
This invention is pointed out with particularity in the appended claims.
The above and further objects and advantages of this invention may be
better understood by referring to the following description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a typical intra-aortic balloon pumping system
utilizing this invention;
FIG. 2 comprises Graphs 2A through 2D of typical signals for a patient
undergoing intra-aortic balloon pumping system treatment;
FIG. 3 comprises Graphs 3A and 3B of balloon drive and arterial pressure on
an expanded scale which are useful in understanding this invention;
FIG. 4 is a block diagram of the cardiac output monitor shown in FIG. 1;
FIG. 5, comprising FIGS. 5A through 5I, includes a flow chart of a sequence
for converting the incoming signals from an intra-aortic balloon pumping
system to obtain signals representing cardiac output;
FIG. 6 is a memory map for the monitor in FIG. 4; and
FIG. 7, comprising FIGS. 7A through 7D, is an alternate embodiment of the
cardiac output monitor shown in FIG. 4.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
A cardiac output monitor 10 constructed in accordance with this invention
is shown in FIG. 1. It connects to a balloon controller 11 which comprises
a pressure transducer 12 for converting pressures received from a catheter
13, normally implanted in the radial artery, to an electrical radial
artery pressure signal.
A control circuit 14 controls inflation and deflation of the intra-aortic
balloon 15 on the basis of signals from an electrocardiograph 16.
Typically, the control circuit 14 causes a pressure pump 17 of the
diaphragm type to shift between first and second positions, thereby
alternately to define two separate or distinct volumes within the pressure
pump 17. Thus, when the control circuit 14 issues an inflation command,
the pressure pump diaphragm shifts and reduces the volume in the pump to
compress a gas, normally helium. The diaphragm remains in that position
until a deflation command, whereupon the diaphragm retracts to its
original position. As the pump 17 alternates between these positions, the
gas in the intra-aortic balloon 15 and an interconnecting catheter 18
alternatively inflates and deflates the balloon 15.
Referring to FIG. 2, Graph 2A shows a typical BALLOON DRIVE signal wherein
the patient has a heartbeat of about 96 beats per minute. The control
circuit 14 in FIG. 1 transmits a positive signal to shift the pressure
pump diaphragm to its first position at time T.sub.A. As the catheter 18
in FIG. 1 constitutes a restricted flow orifice between the pump 17 and
the balloon 15, the pressure within the balloon pump 17 rises rapidly over
an initial portion A of the pressure curve shown in Graph 2B. During a
portion B of the curve, the gas inflates the balloon 15 as the gas passes
through the catheter 18. As the volume in the pump 17 is constant during
this interval, the pump pressure decreases over the portion B. Then the
rate of decrease slows at the point C of the curve as the pressure is
determined by the back pressure of the blood in the patient's aorta.
When the controller 14 issues a deflation command, the balloon drive signal
returns to zero as shown at T.sub.B in Graph 2A. The pressure pump 17
immediately shifts the diaphragm to its original position thereby to
expand the pressure pump volume. The pressure in the pump 17 thus
decreases rapidly over the curve portion D in Graph 2B. Then, as shown by
portion E, the pressure in the pump 17 increases as the gas flows from the
balloon 15 back through the catheter 18 into the pump 17. Finally, the
curve portion F depicts the pressure of the deflated balloon just prior to
the next inflation command.
As the pressure pump 17 receives successive BALLOON DRIVE signals and
inflates the deflates the balloon 15, the blood pressure in the aorta and
other arteries increases and decreases. Graph 2C depicts the corresponding
pressure variations in a patient's radial artery. The time delay between
the pressures in the balloon 15 and at the radial artery is due to the
physiological make-up of the arterial tree.
The BALLOON DRIVE and radial arterial pressure graphs shown in Graphs 2A
and 2C, are shown in expanded form in FIG. 3, where Graph 3A depicts the
BALLOON DRIVE signal while Graph 3B depicts variations in arterial
pressure represented by an AP signal from pressure transducer 12 in FIG.
1. From point G through points H and I to J, the AP signal represents a
pressure pulse produced when the balloon inflates. Once inflation has
completed, the radial arterial pressure remains relatively constant or
decreases slightly from point J to point K. Any loss in pressure results
from displacement of blood in the aorta into other portions of the
arterial tree. At point K, as the BALLOON DRIVE signal shifts, it produces
a deflation pulse, represented by the AP signal excursion from point K to
point L, as the pressure in the balloon 15 is reduced. This excursion is
characterized by the pressure P.sub.1. The AP signal from point L through
P.sub.2 to point G represents a ventricle ejection pulse. P.sub.2
represents the maximum ventricle ejection pressure excursion relative to
the minimum pressure at L.
In accordance with this invention, the pressure excursions at points
P.sub.1 and P.sub.2 (as measured from the minimum pressure at L), the
volume (BV) of the intra-aortic balloon 15, shown in FIG. 1, and the heart
rate (HR), which can be determined during an analysis of the radial and
arterial pressure or other system signals from the controller 11, are
indicative of cardiac output. More specifically:
Cardiac output = (P.sub.2 /P.sub.1) (BV) (HR)
the ratio of the two pressures P.sub.2 and P.sub.1 leads to reliable
estimates of the volume of blood ejected from the heart according to the
above-mentioned formula.
Further, several observations about the AP pressure signal can be made. For
example, the magnitude of the slope of the arterial pressure signal
between points J and K is often very low, apparently because the flow of
blood from the aorta during this interval is not reflected in a pressure
change at the radial artery. P.sub.1 occurs in time when various pressure
pulse reflections in the system tend to cancel each other, so the pressure
P.sub.1 is relatively independent of these effects. Also the ratio P.sub.2
/P.sub.1 generally has a value between 0.7 and 2 and often is close to
unity so that problems introduced by non-linearities in the aortic
compliance are minimized.
A preferred embodiment of the cardiac output monitor 10 is shown in FIG. 4.
It includes an input circuit 20 for receiving the arterial pressure (AP),
balloon pressure (BP), balloon drive (BD), and balloon volume (BV)
signals, and a processing unit 21 for controlling a cardiac output display
22 and a condition display 23.
Conditioning circuits 24 and 25 in the input circuit 20 condition the
analog AP and BP signals for use by the processing unit 21. It is assumed
that the BD and BV signals are in proper binary form when they reach the
processing unit 21, so they merely pass through the input circuit 20. If
any conditioning of these signals is necessary, other condition circuits
can be added to the input circuit 20. In the processing unit 21, gates 26
and 27 selectively couple the AP and BP signals respectively to an
analog-to-digital converter 30. The converter 30 produces at its output a
binary representation of its analog input.
The remaining elements in the processing unit 21 comprise a bus 35 that
interconnects a central processor unit 31, input ports 32, output ports 33
and a memory 34. Specific embodiments of these elements 31 through 35 are
described in "User's Manual, 8008 8-Bit Parallel Central Processor Unit"
published by Intel, Corp. in 1973. For purposes of the present
explanation, it is sufficient to know that the central processor unit 31
time-multiplexes address and data signals on the bus 35. In any reading or
writing cycle, the central processor unit 31 first transmits address
signals to select one of the input or output ports 32 or 33 or a location
in the memory 34. Then the data is placed on the same bus for transfer
either to or form the central processor unit 31. All data transfers are to
or from the central processor unit 31. Thus, to transfer the BD signal to
the memory 34, the central processor unit 31 first "reads" the data from
an appropriate one of the input ports 32 and then "writers" the data into
a specified location in the memory 34. In essence, the central processor
31 establishes various input and output paths and processes the input
signals from the input circuit 20 to produce cardiac output signals for
the display 22 and other signals for the condition display 23.
The operation of the processing unit 21 will be more fully appreciated by
referring to the flow diagram of FIG. 5. Compiler statements and
corresponding statement numbers for implementing this flow in the Intel
8008 central processor unit are also shown. These statements are converted
into machine instructions for the central processor unit 31 by a compiler
available from Intel Corp. As the correspondence between these complier
statements and steps in the flow diagram will be apparent, there is no
further discussion of these statements.
The processing unit 21 shown in FIG. 4 operates in two stages. In a first
stage, depicted in FIGS. 5A and 5B, the cardiac output monitor stores
periodic samples of the arterial pressure (AP) and the balloon drive (BD)
signals in binary form. During the second stage, the processing unit 21
processes the sampled data and transmits output signals to the cardiac
output display 22.
An operator obtains a reading of cardiac output by depressing a switch 36
in FIG. 4. A starting buffer circuit 37 then transmits a momentary or a
continuous START signal to the input ports 32. As shown in FIG. 5A, the
processing unit 21, while at rest, repeatedly couples the output from the
buffer circuit 37 through one of the input ports 32 for testing (Step 50).
When the switch buffer circuit 37 does transmit a START signal, Step 50
diverts to Step 51 which, in a monitor for taking a single sequence of
readings, "resets" the starting switch by clearing the START signal from
the buffer circuit 37. This is done by transmitting an output signal
through the one of the output ports 33 connected to the buffer circuit 37.
For continuous monitoring operations, Step 51 is eliminated.
Next, the processing unit 21 periodically samples the balloon drive (BD)
signal until controller 11 (FIG. 1) issues a balloon inflation command,
i.e., a BD signal transition from a ZERO level to a ONE level. Step 52
establishes the sampling interval while Step 53 transfers the BD signal
through one of the input ports 32 to the unit 31. If the transition does
not occur within a predetermined time (e.g., 1.5 seconds), Step 54
branches to point B, an entry to an OFF subroutine described later.
Otherwise, Step 55 actually tests the BD signal for a transition.
Step 55 branches back to Step 52 until the balloon deflation command
occurs, whereupon control shifts to Step 56. The balloon drive (BD) signal
shifts to a ZERO level in response to the balloon deflation command, and
this transition initiates the sampling operation. So long as the BD signal
is at a ONE level, Step 56 branches to Step 57, which turns off the
cardiac output monitor 10 (FIG. 1) if the balloon deflation command is
late by branching to point B, the OFF subroutine. Step 58 determines the
sampling interval and Step 59 transfer a new value of the BD signal
through the input ports 32 before returning the operation to Step 56.
When the controller 11 in FIG. 1 issues a balloon deflation command within
the time interval, Step 56 branches to point C in FIG. 5A and to Step 60
in FIG. 5B whereupon the sampling operation begins. Initially, the
processing unit 21, in Step 60, enables the gate 27 to transfer the BP
signal to the analog-to-digital converter 30 and the digital
representation of the balloon pressure to a BP location 300 (FIG. 6) in
the memory 34 through the input ports 32. Then, by processing Steps 61 and
62 iteratively, the processor unit 31 transfers the balloon drive signal
and the digital representation of the arterial pressure signal into a
series of BD (I) and AP(I) memory locations 301 and 302, respectively
(FIG. 6). When the samples are stored, Step 62 diverts to point D and Step
63 in FIG. 5C which is the first step in the second operating stage for
processing the sampled data to obtain and display cardiac output. When the
processing unit 21 completes the stage depicted in FIGS. 5A and 5B, the
memory 34 (FIG. 6) stores in the locations 300, 301, and 302 data in
binary form corresponding to the arterial pressure and balloon drive over
a measurement interval (e.g., 5 seconds) and balloon pressure at the
beginning of the measurement interval. Thus, the memory 34 constitutes an
historical model of patient conditions during the measurement interval.
In this specific embodiment, the balloon deflation pressure P.sub.1 and the
ventricle ejection pulse pressure P.sub.2 obtained for each heartbeat in
succession are stored in the memory 34 in Steps 63 (FIG. 5C) through 77
(FIG. 5E). Step 63 locates the T.sub.1 and T.sub.2 times thereby to define
a beat interval over which the radial artery pressure is analyzed and
increments the value in a KP location 303 (FIG. 6) thereby to count the
number of heartbeats. Normally Step 64 branches to Step 65 to ascertain
the location of the first maximum of arterial pressure which is point H in
Graph 3B. After the location of minimum arterial pressure is found (point
L in Graph 3B) in Step 66, the memory 34 (FIG. 6) contains the locations
of and values of the maximum and minimum pressures at locations 304
through 307.
If the magnitude of the slope of the AP signal between points H and I
(Graph 3B) is not below a predetermined level, the AP signal may be noisy,
system timing may be incorrect, the catheter 13 in FIG. 1 may be defective
or some other problem may exist. Step 70 diverts to a Step 94 in FIG. 5G
to indicate that a problem exists. Otherwise, the processing unit 21, in
Step 71, obtains the second order differential of the AP signal.
The maximum and minimum of the second order differential signal define
points J and K in Graph 3B. In Step 72, the processing unit establishes
these points and stores the locations in 12DMX and 12DMN locations 308 and
309 (FIG. 6). As previously discussed, the AP signal defines a plateau
between points J and K in Graph 3B. If a systm problem exists, then the AP
signal may be low or its plateau slope in this region may not be within a
normal range of values. In Step 73 the processing unit 21 tests the
magnitude of the AP signal and its slope and branches to Step 94 in FIG.
5G if a problem exists.
Assuming the AP signal slope is normal, the processing unit, in Step 74,
locates point P.sub.1 in FIG. 3B as a point of minimum slope between
points J and K. The value P.sub.1 which represents the balloon deflation
excursion pressure to the minimum pressure at point L, is stored in a
P.sub.1 location 310 (FIG. 6).
After this point is obtained, the processing unit 21 again checks the
system operation by examining the slope of the AP signal between points L
and M in Graph 3B. If the slope is too low, Step 75 in FIG. 5E branches to
Step 94 in FIG. 5G. Otherwise, the processing unit 21 uses Step 76 to
locate the next maximum of the AP signal which corresponds to the maximum
pressure of the ventrical ejection pulse. Then the processing unit 21
determines the difference between this pressure and the minimum pressure
at point L in Graph 3B and stores the difference in a P.sub.2 location 311
(FIG. 6).
In step 77, the processing unit 21 retrieves the numbers from location 310
and 311, determines the ratio, and stores the ratio for that heartbeat in
a KR(I) location 312 in the memory 34 of FIG. 6. Then the processing unit
21 returns to Step 63 to initiate an operation for a successive heartbeat.
When all the data stored in locations 301 and 302 of FIG. 6 corresponding
to the last sampled heartbeat has been converted, Step 64 in FIG. 5C
branches to Step 80 in FIG. 5E. At this time the KP location 303 has a
number representing the number of sampled heart beats; the BP location
300, the initial balloon pressure; and each KR location 312, the P.sub.2
/P.sub.1 ratio for a heartbeat.
In Step 80 (FIG. 5E) the processing unit 21 determines whether the number
in the KP location 303 (FIG. 6) represents a minimum number of heart beats
which should be sampled. Step 80 then branches to Step 94 in FIG. 5G to
indicate a problem or to Step 81 in FIG. 5E.
In Step 81, the processor unit 21 retrieves all the ratios from the KR
locations 312 (FIG. 6), obtains an average value for the ratios, and
stores the average ratio in an RA location 313 (FIG. 6). The processing
unit 21 might, as an alternative, obtain the median of the ratios. In the
following "average" includes either the average or "median" value. The
ratio is reduced if it is larger than a predetermined maximum in Steps 82
and 83 in FIG. 5F. It is assumed that the aorta and the rest of the
arterial tree constitute a linear fluidic system. In fact, they do not.
If, in a particular patient, the P.sub.2 /P.sub.1 ratio becomes overly
large, the effect of non-linearities must be corrected by reducing the
ratio by a fixed percentage.
In Step 84, processing unit 21 retrieves the value stored in the KP
location 303 of FIG. 6 to calculate the heart rate based upon the number
of recorded beats and the timing interval. An HR location 314 (FIG. 6)
stores this heart rate value.
Steps 85 and 86 perform another check on system conditions. In step 85 the
processing unit 21 determines the average plateau width between points J
and K in FIG. 3 and normalizes the average width with respect to the heart
rate. A PL location 305 (FIG. 6) receives the normalized plateau width
value. If this value is too large, the processing unit 21, in Step 87,
energizes a timing light in the condition display 23 (FIG. 4) to indicate
to the operator that the deflation commands are late.
Intral-aortic ballons have known volumes. In Step 88, the processing unit
21 couples the BV signals through the input ports 32. In one embodiment,
the BV signals are in the form of a code which the processing unit
converts into a balloon volume value for storage in a BV location 316
(FIG. 6).
When the processing unit 21 completes Step 88 in FIG. 5F, the BP, RA, HR
and BV locations 300, 313, 314 and 316 contain all the values for
calculating the cardiac output (CO) according to the equation
CO = (P.sub.2 /P.sub. 1) (BV) (HR)
this calculation is performed in Step 89 (FIG. 5G) and a CO location 317
(FIG. 6) stores this result. If the value of CO is overly large, the
processing unit 21, in Step 90, diverts to Step 94 to indicate a system
problem.
The actual inflation volume of a balloon depends upon the current pressure
in the aorta (i.e., aortic back pressure) against which the balloon must
fill. In accordance with another aspect of this invention, the magnitude
of the pumped balloon volume (BV) is increased or decreased about 1.5 c.c.
for each 10 millimeters of pressure below or above 75 millimeters of
pressure, respectively, to substantially correct the effect for the aortic
back pressure. After the processing unit 21 makes this correction in Step
91, it transfers the signals transmitted by the CO location 317 (FIG. 6)
through the output ports 33 in FIG. 4 to the cardiac output display 22 in
Step 92. Then the processing unit 21 returns to Step 50 in FIG. 5A. If the
switch 36 and buffer 37 transmitted a momentary START signal, the
processing unit 21 awaits a subsequent START signal. However, if the START
signal is continuous, Step 50 immediately and automatically diverts to
Step 52 (Step 51 is omitted). Such continuous operations are beneficial
when the display 22 is a chart recorder, for example. In such an
application, the switch 36 could be integral with a jack in the monitor
which receives a chart recorder plug.
If the tests performed at Steps 54 and 57 in FIG. 5A indicate no changes in
the BD signal for the predetermined periods, it is assumed that the
intra-aortic balloon pumping system is off. The processing unit 21, in
that case, diverts to Step 93 in FIG. 5G and transfers ZERO values through
the output ports 33 to the cardiac output display 22 in FIG. 4. A display
of ZERO therefore indicates that the system is off.
Similarly, if any test performed at Steps 70 and 73 in FIG. 5D, Steps 75
and 80 in FIG. 5E and Step 90 in FIG. 5G is failed, the processing unit
21, in Step 94, passes to the condition display 23 signals which energize
a trouble light thereby to alert the operator of some improper condition.
FIGS. 5H and 5I contain various compiler statements which constitute the
subroutines for finding a maximum, a minimum and the timing points.
Further, the statements required by the compiler language for declaring
variables are also shown. As they are straightforward statements, they are
not discussed further.
Thus, this embodiment of the invention provides a number of advantages.
First, the cardiac output is obtained without any further connection to a
patient as all the input signals are available from the intra-aortic
balloon pumping system. The resulting cardiac output is reasonably
reliable, and the results, in clinical conditions, compare favorably with
results obtained from using the standard clinical tests for cardiac
output. In addition, the system can continuously measure and display
cardiac output unlike present clinical methods. Moreover, the monitor
provides information about system conditions more rapidly than can be
obtained by operator observation.
FIG. 4 discloses a specific embodiment of the invention which uses advanced
microprocessor techniques. However, other implementations of this
invention are also possible. FIG. 7 depicts in basic block form one such
circuit comprising various conventional analog and digital circuitry.
The circuitry in FIGS. 7A and 7B samples AP and BD signals and stores the
sampled data in a memory for subsequent analysis by the circuitry in FIGS.
7C and 7D. Assuming a BUSY latch 100 in FIG. 7A is reset, a START signal
passes through AND gate 101 to clear BP register 102 and energize an OR
gate 103. The OR gate 103 enables an AND gate 104 to pass clocking pulses
of about 50% duty cycle from a pulse generator 105 to an AND gate 106. The
first such pulse from the AND gate 104 sets the BUSY latch 100. An
inverter 108 and flip-flop 109 block the first pulse from the pulse
generator 105. However, the trailing edge of that pulse sets the flip-flop
109 to enable the AND gate 106 and latch the circuit in FIG. 7A until a
DONE signal is transmitted. Then the second and subsequent pulses pass
through the AND gate 106 to trigger a monostable multivibrator 110. The
pulses from the generator 105 and multivibrator 110 overlap so an AND gate
111 triggers a monostable multivibrator 113 when the BD signal shifts to a
ONE level. An AND gate 114 receives the output from the multivibrator 113
as well as the pulses from the pulse generator 105, an inverted BD signal
from an inverter 115 and the Q signal from a flip-flop 116. Thus, the
flip-flop 116 sets whenever it is reset and the BD signal shifts to a ZERO
level. The output signal, a BD=0 signal, enables the remaining portions of
the circuit to begin reading data into memory. Thus, these circuit
elements in FIG. 7A function in accordance with FIG. 5A.
Once the BD signal shifts to ZERO, the BD=0 signal enables an AND gate 120
to pass subsequent clock pulses from the pulse generator 105. A flip-flop
121, set by the output from the AND gate 101, also enables the AND gate
120. The first pulse from the AND gate 120 clocks into the BP register 102
a digital representation of the balloon pressure (BP) signal supplied from
an analog-to-digital converter 122. An inverter 123 clocks the flip-flop
121 to a reset condition on the trailing edge of that pulse to disable the
AND gate 120.
Pulses from the pulse generator 105 also pass through another AND gate 124
in FIG. 7B. A flip-flop 125 in FIG. 7A, set on the leading edge of the
clocking pulse to the BP register 102, and the reset output of a DONE
flip-flop 126 in FIG. 7B, which the AND gate 120 resets, enable the AND
gate 124.
The clocking pulses thus passed by AND gate 124 advance a counter 127
through an OR gate 128. The counter 127 has a modulus which is two times
the number of samples to be taken and an inverter 130 couples the signal
from the most significant bit position to an AND gate 131 between the OR
gate 128 and the counter 127. Initially the counter 127 contains ZERO, so
the AND gate 131 is enabled. The remaining CTR signals from the counter
127 constitute address signals for a memory 132.
The memory receives, as data inputs, the BD signal and the binary
representations of AP signals from an analog-to-digital converter 133. A
writing flip-flop 134, set in response to a pulse from the AND gate 120,
conditions the memory 132 for writing operations. An AND gate 136 provides
the clocking pulses to write the data into the memory 132. The AND gate
136 passes delayed clocking pulses from an inverter 137 when the flip-flop
125 is set and the DONE flip-flop 126 is reset. Thus, during the first
half of a clock pulse interval from the pulse generator 105, the data to
be written and addressing signals are coupled to the memory 132, while the
data is written into the memory 132 during the second half of the clock
pulse interval.
When the counter 127 produces a ONE in the most significant bit position,
all the required samples have been taken and the functions of FIG. 5B are
completed. Then the inverter 130 disables the AND gate 131, so no further
operations can occur. This shift in the most significant bit also sets the
DONE flip-flop 126. The DONE signal resets the flip-flop 134 so no further
writing operations can occur and disables the AND gates 124 and 136. Thus,
when the flip-flop 126 transmits the DONE signal, the memory 132
constitutes an historical model of data sampled during the sampling
interval. The data is all in a binary form and corresponds to the arterial
pressure and balloon drive signals.
FIGS. 7C and 7D disclose the circuitry for processing the information in
the memory 132. Certain elements, including the counter 127 and the memory
132, are also shown in FIG. 7C. When an element is shown twice in FIG. 7,
it is identified by the same reference numeral.
Now referring to FIG. 7C, the DONE signal triggers a monostable
multivibrator 140 and clears the counter 127 so the inverter 130 enables
the AND gate 131. The DONE signal also enables an AND gate 150 to couple
clocking pulses from another clock 151 through the OR gate 128 to advance
the counter 127. The clock 151 has a much greater frequency than the pulse
generator 105 in FIG. 5A; for example, the pulse generator 105 could have
a frequency of 100 Hz and the clock 151, 500 kHz.
The circuitry in FIGS. 7C and 7D analyzes the data in two stages. In a
first stage, the reference points, such as T.sub.1 and T.sub.2 in FIG. 3,
are located. More specifically, as the counter 127 advances, a monostable
multivibrator 154 is connected to transmit a pulse at each transition of
the BD signal and clock a JK flip-flop 155. As the first location in the
memory 132 always contains a ZERO and the flip-flop 155 is reset by the
DONE signal, the flip-flop 155 always reflects the binary state of the BD
signal. A delay circuit 156 couples each pulse from the multivibrator 154
to routing AND gates 157 and 160. Thus, when the flip-flop 155 sets, the
AND gate 157 clocks a register 161 to store the CTR signals from the
counter 127 corresponding to the occurrance of a positive transition of
the BD signal. Likewise, a register 162 stores the CTR signals
corresponding to the occurrence of a negative transition.
The output from a delay circuit 163 energizes an AND gate 164 whenever the
flip-flop 155 is reset to produce a writing operation in a reference point
memory 165. Thus, immediately after the register 162 stores a transition
location, the AND gate 164 causes the reference point memory 165 to store
data in a location defined by a clock counter 166. The stored data is from
a parallel adder circuit 167 which sums the numbers in the registers 161
and 162. In a simple circuit, the sum is shifted to the ri | | |
