Note: Descriptions are shown in the official language in which they were submitted.
5~i~
ACKGRO~D OF THE INVENTION
The invention relates to dialysis machines,
and more particularly, to a negative pxessure valve system
and to alarm systems ~or use in such machines.
In dialysis, a patient's blood and dialysate
flow through a dialyzer which includes a semlpermeable
membrane for separating the blood and the dialysate.
Impurities and water from the blood cross the membrane
and enter the dialysate for disposal. The terms dialysate,
dialysis solution and dialyzing ~luid as may be used here-
inafter are intended to be synonomous.
In some dialyzers the dialysate is drawn through
the dialyzer under a negative pressure (i.e., below atmo-
spheric pressure). Such systems normally include a nega-
tive pressure pump positioned downstream of the dialyzer
for drawing the dialysate through the dialyzer and a
negative pressure valve positioned upstream of the dialyzer.
The negative pressure in the dialyzer is controlled by
adjustment of the negative pressure control valve. Although
these systems are commonly referred to as negative pressure
systems, there are certain conditions under which positive
dialysate pressures may be generated. U.S. Patent 3,878,095
discloses one such negative pressure system.
In some machines electromechanically operated
needle valves have been used as the negative pressure con-
trol valve. Such valves have an operating characteristic
such that as the valve moves from the open position toward
the closed position, the change in pressure is relatively
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small and linear. However as the valve is about to close,
the pressure becomes increasingly negative at a very rapid
rate until the valve closes. In other words, as the valve
closes, there are very great changes in pressure. This
steep change in pressure makes it difficult to accurately
control and maintain the negative pressure at highly nega-
tive levels (e.g., -400 to -500 mm Hg). This is particularly
true in an electromechanical system wherein gear tolerances
and changes in temperature also affect the control and
positioning of the needle valve and thus the negative
pressure.
Furthermore, the electromechanical system includes
a constant speed DC motor to operate the valve. Therefore,
since the valve characteristics are relatively linear, the
time necessary to induce large changes in negative pressure
may be relatively long. For example, the change rrom -200
mm H~ to 0 mm Hg may take on the order of two minutes.
It is the-efore desirable to provide a more
accurately controllable and responsive negative pressure
valve system.
In dialysis the pressure differential across the
semipermeable membrane (i.e., the difference in pressure
between the blood and the dialysate) is important. This
differenti21 may be referred to as the transmembrane pres-
sure. However, in the event that the dialysate pressure
exceeds the blood pressure, impurities in the dialysate
could undesirably pass through the membrane and into the
blood.
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It is desirable, therefore, that dialysis be
prevented in the event that the dialysate pressure exceeds
the blood pressure.
During dialysis, water is removed ~rom the blood
by a process known as ultrafiltration. The quantity of
water removed is directly related to the transmembrane
pressure. It is desirable to control the amount of water
removed since removal of too much water during dialysis
may result in undesirable side effects.
Therefore, it is desirable to maintain control
over the difference between the dialysate pressure and
blood pressure so as to control ultrafiltration.
Some prior art dialysis machines have included
txansmembrane pressure monitors, which merely measured
and displayed the transmembrane pressure. In another
machine, provisions were made for alarms in the event the
transmembrane pressure exceeded a predetermined value.
The alarms included a tolerance or alarm window of, for
example, 50 mm Hg above or below the predetermined value.
Therefore, in the event that the transmembrane pressure
was zero, it is possible tha with those tolerances dialysate
pressure could increase beyond the blood pressure level,
thereby permitting undesirable transfer from the dialysate
to the blood.
It is therefore desirable to provide an alarm
system for preventln~ dialysis if the dialysate pressure
exceeds the blood pressure.
In order to maintain a set or predetermined trans-
membrane pressure, both the dialysate pressure and the
blood pressuremust be monitored. In the event that the
blood pressure signal is not received by -the -trans-
membrane pressure control system, it is possible that
the actual transmembrane pressure could undesirably e~ceed
the set or prede~ermined pressure without providin~ any indi-
cation or alarm as to that acutal increase.
It is therefore desirable to provide a system whereby
the actual transmembrane pressure is maintained at a set or
predetermined level, and in the event of signal failure from
the venous pressure transducer, appropriate alarms and shut-
off mechanisms are activated.
SUMMARY OF THE INVENTION
According to the invention there is provided a dialysismachine for use with a negative pressure dialyzer which includes
a negative pressure control valve positioned upstream of a
dialyzer, a negative pressure sensing means operatively associ-
ated with said valve means positioned downstream thereof, and
a negative pressure pump for drawing dialysate through said
valve means and said dialyzer. The negative pressure valve
means comprises electromagnetically controllable flapper valve
means for accurately controlling negative pressure, said valve
means being effective to minimize the time in which said machine
responds to changes in negative pressure.
The negative pressure control system of this dialysis
machine provides for very accurate and very responsive control
of the negative pressure. The electromagnetically controlled
flapper valve has to permit very rapid response times in terms
of the stabilization of the negative pressure in the system.
This rapid response time (on the order of 30 seconds~ has
been found to be very desriable from physiologoical, safety
and/or convenience points of view.
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The -transmembrane pressure alarm systems disclosed
herein prevent the dialysate pressure from exceedi.ng the
blood pressure. Another feature of the transmembrane alarm
system ls that the transmembrane pressure will be maintained
at a set or predetermined level during operation of the machine,
and in the event that no venous pressure signal is received
or the venous pressure becomes negative below a predetermined
level, the system will alarm and prevent further dialysis.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a flow diagram depicting the fluid
flow path within the dialysis machine;
FIGURE 2 is a broken away and sectional view
showing the details of the negative pressure control flapper
valve;
FIGURE 3 is a front view of a transmembrane/
dialysate pressure module; and
FIGURE 4 is a block-type diagram showing the
transmembrane alarm systems.
DESCRIPTION OF THE P~EFERRED El~BODIMENT
.
I. In General
Referring now to Figure 1, the di.alysis machine
is shown in block diagram form.
Incoming water flows to the holding tank 10.
From the holding tank the water flows through a heat
exchanger 12, a first temperature sensor 14, through an
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electric heater 16, and to a second tempera-ture sensor 18.
The heated water then flows from the second temperature
sensor 18 ~o a degassing section 20, which includes a
restriction and degassing pump. Degassed water then flows
to a holding tank 22, which is connected at its upper end
to a pressure relief valve 24 and through a return line
26 to the input tank 10. Degassed water flows from the
holding tank 22 through a flow detector 28 and to a dialy-
sate mixing chamber 30. Dialysate concentrate flows to the
mixing chamber 30 from a concentrate pump 32 and mixes
with the degassed water. The amount of concentrate deli-
vered to the mixing chamber by the pump 32 is controlled
by a conductivity detector 34 that is positioned immediately
downstream of the mixing chamber 30.
The dialysate which has been prepared then flows
via line 36 to the negative pressure or dialysate pressure
control valve 38, through line 39 and to a ne~ative pressure
or dialysate pressure transducer 40. The transducer 40 and
negative pressure valve 38 are connected through a feedback
loop 42 for controlling the valve 38 and the negative pressure.
The dialysate then flows from the transducer
40 to a junction 44, at which point the flow lines divide
into two branches. One branch is a dialyzer bypass line
46 which includes a bypass control valve 48. The other
branch includes a dialyzer inlet line 50, having an inlet
control valve 52 positioned therein. A negative-pressure-
type dialyzer 54 is positioned downstream of the valve 52,
and the downstream end of the dialyzer connects to the
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outlet line 56, outlet control valve 58 and to the
junction 60.
A negative pressure pump 62 is positioned down-
stream of the junction 60 for drawing dialysate through
the system and part~ularly through the negative pressure
valve 38 and khe dialyzer 54. Used or spent dial~sate
flows from negative pressure pump 62 to the drain 64.
The dialyzer 54 includes a semipermeable mem-
brane, shown illustratively as 54a, which separates the
dialysate from the blood side of the dialyzer. A patient's
blood enters the dialyzer via the arterial line 66 and exits
the dialyzer via the venous line 68. A pressure transducer
70 is positioned in the venous line 68 to detect the blood
pressure at that point.
The mean transmembrane pressure within the dia-
lyzer is approximated by measuring the difference between
the pressure measured by the venous pressure transducer
70 and that measured by the negative pressure transducer
40. Both the venous pressure transducer 70 and the nega-
tive pressure transducer 40 are connected to the transmem-
brane/dialysate pressure module 200.
II. The Negative Pressure Control_System
Referring now to Figure 2, there is shown an
electromagnetically operated flapper type negative pres-
sure valve assembly 38, to which the input line 36 and
the output line 39 are connected. ~lectromagnetically-
operated flapper valves are sold by Hydraulic Servo System
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Corp~, 5800 Transit Road, Depew, New York 14042. ~Model
58 valve when modified, has been found to be generally
suitable for a negative pressure control valve in a
dialysis machine.)
The valve assembly 38 includes a magnet frame
100 within which is posltioned a magnet coil 102. An elon-
gated rod or armature 104 is provided and its upper section
is positioned in the coil. The armature is connected to
a flat leaf spring-like member 106 which permits the
armature to pivot and which is ad~usted to bias the arma-
ture to a first position.
A flow housing 110 is secured to the bottom of
the magnetic frame, the spring-like member 106 is secured
to the housing, and the O-ring 112 seals the armature 104
to the housing. The housing 110 includes: a central
bore 114; an inlet bore 116 and an outlet bore 118, which
are axially aligned; and a flow orifice or nozzle 120
which is positioned at the outlet end of the inlet bore
116. The inlet bore is connected to the line 36 and the
outlet bore 118 is connected to the line 39.
The armature 104 includes an elongated lower
section 104a which is positioned within the central bore
114. The lower end of the armature includes flat land
portion 104_ which faces the orifice 120 and is constructed
to seat thereagainst. The positioning of the land 104b
relative to the nozzle 120 establishes the pressure drop
or negative pressure across the negative pressure valve.
The level of negative pressure is related to the distance
between the land 104b and the orifice 120. In other words,
s~
the closer the land is to the orifice, the more negative
the pressure, and the further the land is from the orifice,
the less negative the pressure. In this application the
lower section 104a has a length greater than the length of
the upper section so as to permit control of the positioning
of the land and operation of the system under dialysate
positive pressure. The length of the lower section in this
valve is several times greater than khe length of the lower
section in the standard model 58 r and under the same system
constraints positive dialysate pressures could not be ob-
tained with the standard model 58. It has been found that
the valve disclosed herein has-an operating characterlstic
which is substantially linear.
The position of the armature relative to the
orifice is controlled by controlling the current flow
through the magnetic coil. The armature is biased by the
spring-like member 106 to a first position, such that when
there is no current flow, the valve is in an open position,
away from the orifice, and there is a very small pressure
drop across the valve.
The magnetic coil 102 is operatively associated
with the transducer 40 through a buffered operational
amplifier. It will be appreciated that the long length
of the lower section (i.e., the distance between the pivot
106 and the land 104) pe~mits of very carefully controlled
incremental changes in the distance between the orifice
and the land. Therefore, positioning of the land 104b
relative to the orifice 120 can be controlled by very small
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changes in the current flow to the magnetic coil 102.
This is -true throughout substantially the entire operating
range of the valve.
~ s the current flow through the coil increases,
the upper section of the armature is pulled against biasing
spring, and the land section 104b is moved toward the
orifice 120, which increases the negative pressure. It
has been Eound that negative pressure increases substan-
tially linearly wi-th increasing current flow through the
coil. Thus the positioning of the land and the negative pres-
sure can be controlled very accurately, even at highly
negative values, such as -400 or -500 mm Hg.
Furthermore, since the only moving part in the
system is the armature and the operating characteristics
are substantially linear, the response time to change the
negative pressure at the valve is very small. The time
required for the entire system to adjust to changes in
the negative pressure and stabilize are related to system
constraints. In this system, the response time to change
from -200 mm Hg to 0 mm Hg is on the order of 30 seconds.
Different systems may respond in longer or shorter times
depending upon system constraints.
Flapper valve assemblies, such as 38, are
believed to be readily interchangeable between dialysis
machines so as to avoid problems in calibration and
standardizing equipment when replacing the valve assem-
blies.
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III. Transmembrane Pressure Controls
The dialysis machine as used herein can be referred
to as a proportional dialysate delivery system as sold by
Baxter Travenol Laboratories, Inc., under the name Propor-
tionin~ Dialyzin~ Fluid Delivery System (5M 1352 - 5M 1355).
These machines are in modular form whereby various functions
can occur within the movable and serviceable modules.
Figure 3 shows the front panel of a transmembrane/dialysate
pressure module 200. The panel shows a negative pressure
gauge 201 which provides for negative pressure readings of
zero (0) to -500 mm Hg. A slide control 202 is provided
for setting transmembrane pressure, as will be described
hereinafter.
This module can be operated either in a dialysate
pressure mode or a transmembrane pressure mode. Indicator
light 204 will be lit when the machine is in the dialysate
pressure mode, and light 206 will be lit when the machine
is operated in the transmembrane pressure mode. Switch
208 permits selection of operation in either the trans-
membrane pressure or dialysate pressure mode. During
set-up and initial operation of the machine, the module
is operated in the dialysate pressure mode. Once stabilized,
the module may be switched to the transmembrane pressure
mode.
~he transmembrane pressure in the dialyzer is
approximated by measuring the difference between the pres-
sure indicated by the venous pressure transducer ~0 and
the negative pressure transducer 40. For example, if the
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venous pressure is +50 and -the negative pressure is -200,
the transme~brane pressure is 250.
The operator can select a desired transmembrane
pressure by use of the slide control 202. For example,
the transmembrane pressure can be set at 300 and this
pressure will be maintained automatically through the opera-
tion of the negative pressure control valve 38 as the venous
pressure varies. Use of the flapper valve 38 thus is very
advantageous in that the negative pressure changes can
quickly follow or "track" changes in the venous blood pres-
sure.
III. A. Zero Transmembrane Pressure Alarm ~ondition
There are circumstances in which the operator
desires that the transmembrane pressure be zero (i.e.,
no pressure differential across the membrane). However,
the dialysate pressure should never exceed the blood pres-
sure since undesirable impurities may then pass through the
semipermeable membrane and into the blood.
Under normal operating circumstances, an "alarm
window" of +50 mm Hg is provided. For example, at 200
mm Hg, alarms would be activated if the transmembrane
pressure is not within the pressure of 150-250 mm Hg.
However, with any such alarm window, at a transmembrane
pressure of 0 mm Hg, it is possible that the pressure on
the dialysate side of the membrane could undesirably exceed
the venous blood pressure.
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As described hereinaft~r, means are provided
for preventing dialysate pressure from exceeding the
blood pressure and appropriate alarms are activated.
In the event that the dialysate pressure exceeds the venous
pressure, further dialysis is prevented by opening bypass
valve 48 and closing the clialysis inlet and outlet valves
52 and 58. Thls effectively isolates the dialyzer and
prevents ~he undesirable situation in which the dialysate
pressure can increase above the blood pressure. Further-
more, audible and visible alarms are also activat~d.
Referring now to Figure 4, the dialysate flows
through negative pressure transducer 40. Negative pres-
sure transducer 40 includes an operational ampllfier that
develops a vol-tage which varies in accordance with the pres-
sure on the transducer. When the pressure on the transducer
40 is negative in nature, a positive voltage is developed
at the output of transducer 40; when the pressure on trans-
ducer 40 is positive, negative voltage is developed at the
output o transducer 40. In the preferred embodiment,
the signal developed at the output of negative pressure
transducer 40 will be +1.6 volts at -100 mm Hg. At 0 mm Hg,
the outputof pressure transducer 40 will be 0 volts, and
at ~100 mm Hg, the output will be -1.6 volts. This voltage
is coupled from transducer 40 to one input 300 of an adder
circuit 302 in pressure module 200.
Venous pressure transducer 70 includes an opera-
tional amplifier circuit that develops a voltage which
varies in accordance with the venous pressure at the trans-
ducer. In the preferred ernbodiment, the output of pressure
transducer 70 will vary over a range of -2 volts at 0 mm H
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to -8 volts at 300 mm Hg. This output signal is coupled
from pres-sure transducer 70 to an amplifier/inverter 304
in pressure module 200. Amplifier/inverter 304 provides
a scaling factor adjustment to its output signal in addi-
tion to an inversion. Consequently, at the output of
amplifier/invertex 304, the voltage will vary from 0 volts
at 0 mm Hg to +~.8 volts at 300 mm Hg. The signal developed
by amplifier/inverter 304 is coupled to a second input 306
of adder circuit 302.
Adder circuit 302 adds the signals coupled to
inputs 300 and 306 and develops an output signal correspond
ing to the sum of the signals at inputs 300 and 306. For
example, i the dialysate pressure is -100 mm Hg, the
voltage coupled to input 300 will be +1.6 volts. If the
venous pressure is also 100 mm Hg, +1.6 volts will be
coupled to input 306. The output of adder circuit 302
would then be 3.2 volts. The output of adder circuit 302
is coupled to an amplifier 308 where it is amplified and
coupled to a comparator 310.
Comparator 310 compares the signal coupled from
the amplifier 308 to a reference voltage. If the signal
from amplifier 308 exceeds the reference voltage, comparator
310 will develop an output signal which is coupled to the
alarm circuit 312 actuating audible and visible alarms.
Alarm circuit 312 is also coupled to valves ~8, 52 and 58
in order to open valve 48 and close valves 52 and 58.
In the preferred embodiment, should the dialysate
pressure become positive rather than negative, the voltage
-15
SS~
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developed at the output of transducer 40 will become posi-
tive. If, for example, the dialysate pressure becomes
+100 mm Hg, the voltage at input 300 will be -1.6 volts.
The blood pressure as sensed by transducer 70 remains at
100 mm Hg so that the dialysate pressure and blood pressure
are identical. In this circumstance, the output of ampli-
fier/inverter 304 will develop a voltage of ~1.6 volts which
is coupled to input 306 of adder circuit 302. Adder cir-
cuit 302 will add the voltages developed at the two inputs
and will develop an output voltage of 0 volts. The 0 volt
signalldeveloped by adder circuit 302 is coupled to ampli-
fier 308 and from amplifier 308 to comparator 310. A 0
volt signal, indicating identity of pressure between pres-
sure transducer 70 and pressure transducer 40, and any
more positive signal, indicating a greater pressure at
transducer 40 than at transducer 70,will actuate comparator
310 to develop an output signal and actuate the alarms in
alarm circuit 312. Alarm circuit 312 will open valve 48
and close valves 52 and 58, thus bypassing the dialyzer
54.
III. B. Alarm Conditions for Loss of
Venous Blood Pressure Signal
This alarm condition is intended to prevent the
transmembrane pressure from increasing beyond a set limit.
The venous pressure transducer 70 provides information to
the pressure module for comparison with the signal ~rom
the negative pressure transducer 40 SG as to maintain the
appropriate transmembrane pressure.
, -16-
It is possible ~hat signals ~rom the venous pres-
sure transducer may not be received in the transmembrane
pressure module, for example if there are faulty connections
between the various modules of the machine. In this parti-
cular machine, if no signal is received, the machines assumes
a venous pressure of -100 mm Hg. When operatlng properly
and if the venous pressure signal is ~200 and the desired
transmembrane pressure is set at 300, then the dialysate
pressure would be controlled to -100. However, if no
signal is received, the machine would assume a venous
pressure of -100 (even though the actual pressure was
~200). Based on the -100 indication, the transmembrane
pressure module operates the negative pressure controls
to permit the negative pressure to reach -400. Therefore,
the actual transmembrane pressure would be 600 (i.e., the
actual venous pressure of +200 less the actual dialysate
pessure of -400). However, the displayed transmembrane
pressure would be only 300 and no alarm would have been
activated.
As described hereinafter, there is provided
electronic circuitry for (1) preventing further dialysis
by bypassing and isolating the 2ialyzer through the valves
48, 52 and 58, and (2) activating audible and visual alarms
when no signal from the venous pressure transducer is applied
to the pressure module.
As previously noted, venous pressure transducer
70 includes an operational amplifier circuit that develops
a voltage which varies in accordance with the venous pres-
sure at the transducer. Further, at the output of amplifier/
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~ ~4 S~ ~
inverter 304, the voltage will vary from 0 volts for 0 mm
Hg to +4.8 volts for 300 mm Hg.
Resistor 320 is shown in Figure 4 as being coupled
between a reference potential and amplifier/inverter 304.
Resis~or 320 in the preferred embodiment is coupled to a
positive voltage potential and has a value of appro~imately
200K ohms. This resistor is generally termed in the art as
a "pull-down" resistor.
The output of amplifier/inverter 304, in addition
to going to adder circuit 302, also is coupled to the input
of a comparator circuit 322. A reference voltage also is
coupled to comparator 322. If the signal coupled to compara-
tor 322 exceeds the reference voltage, comparator 322 will
develop a comparison signal that is coupled to an alarm
circuit 324. The comparison signal will actuate the audi~
ble and visual alarms contained in alarm circuit 324.
Alarm circuit 324 also will open valve 48 and will close
valves 52 and 58, thus bypassing membrane 54.
Should venous pressure transducer 70 be inadver-
tently disconnected from amplifier/inverter 304, or should
the coupling therebetween be inadvertently broken, the
input voltage coupled to amplifier/inverter 304 will be
0 volts. With an open connection at the input of amplifier/
inverter 304, pull-down resistor 320 will cause the input
to become positive. In the preferred embodiment, this
will become positive with the voltage of approximately
+0.5 volts, corresponding to a blood pressure more negative
than -100 mm Hg. The output of amplifier/inverter 304 will
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55:9.
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now become negative ra-ther than its normal positive condi-
tion. The +0.5 volts coupled to the input of amplifier/
inverter 304 will cause the amplifier to develop -1.8 volts
approximately at its output. This -1.8 volt signal is
coupled to comparator 322 causing comparator 322 to develop
a comparison signal that is coupled to alarm circuit 324.
In the event that there is no open connection and the venous
signal is less -than O volts (i.e., -100 mm Hg), the alarms
will still be actuated. As previously noted, alarm circuit
324 will actuate its audible and visual alarms, open valve
48 and close valves 52 and 58, thus preventing excessive
transmembrane pressure.
It will be appreciated that numerous changes
and modifications can be made in the embodiment disclosed
herein without departing from the spirit and scope of this
invention.
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