Note: Descriptions are shown in the official language in which they were submitted.
24~9
FIELD 0~ THE INVF,NTION
Thic invention relates to artificial kidney
systems, and more particularly, to an improved apparatus
for establishing and controlling ultrafiltration rates.
BACKGROUND OF THE IN~ENTION
Artificial kidney systems include a dialyzer
and a dialysis machine whlch controls the operation of the
dialyzer. The dialyzer is used to treat a pa~ient's blood
so as to remove water and waste products therefrom. The
dialyzer includes a semipermeable memkrane which separates
blood and dialysis solution flowing through the dialyzer.
~aste product removal occurs ky mass transfer through the
membrane and water removal occurs by ultrafiltration through
the membrane.
In some dialysis machines the dialysis solution
is drawn through the dialyzer under a negative pressure
(i.e., below atmospheric pressure). One such machine is
disclosed in U.S. Patent 3,878,095 Frasier et al, and a
commercial machine embodying such a system is manufactured
and sold ~y Baxter Travenol Laboratories and is identified
as Proportioning Dialyzing Fluid Delivery System (5M 1352-5M
1355j. This machine can be referred to as a single-pass or
flow-through system in that the dialysis solution is con-
tinuously prepared, 10ws through the machine and dialyzer
and then flows to a discharge drain.
The removal of water from the blood by the pro-
cess of ultrafiltration relies on the pressure dif~erential
4~
across the semipermeabie membrane (i.e., -the difference in
the pressure of the blood flowing under positive pressure
through the dialyzer and he dialysis solution flowing
under a negative pressure through the dialy7er). This
pressure differential is commonly known as the transmem-
brane pressure, and the amount of water removed from the
blood is dlrectly related thereto.
It is known to be desirable to control the uitra-
filtra-tion rate so that the amount of and the rate at which
water is removed from the patient is controlled. U.S.
Patents 3,844,940; 3,979,284; and 3,990,973 appear to be
representative of patents disclosing ultrafiltration systems.
There also exists an apparatus used in negative-
pressure flow-through systems for establishing and controll-
ing ultrafiltration rates. In that system there is provi-
ded an ultrafiltration branch which is connected to the
flow system adjacent the dialysis solution inlet to the
dialyzer. The ultrafiltration branch includes a variable
speed pump for drawing liquid from the dialyzer at a rate
equivalent to the ultrafiltration xate, a flow meter which
displays the flow rate, and a drain to which the withdrawn
liquid flows.
In operation of that system, the operator adjusts
the pump and observes the flow meter until the desired flow
rate is obtained, which corresponds to the desired ultrafil-
tratlon rate~ Thereafter, the operator observes or deter-
mines the transmembrane pressure at which the desired ultra-
filtration rate is obtained, and the system is thereafter
controlled to that transmembrane pressure. In that system
,49L~
manual adjustments and observations may not provide the
accuracy sollgnt in some situations.
It is therefore an object of this invention to
provi.de a more accurate system for astablishing and con-
trolling the ultrafiltration rates.
~ his and other objects and advantages of this
invention will become apparent from the ~ollowing descrip-
tion and appende.d claims.
SUMMARY OF THE INVENTION
10 There is disclosed herein an improved system for
establishing and controlling the rate Gf ultrafiltration.
The system is useful in flow-through negati~7e-pressure types
of dialysis machines for establishing the ultrafiltration
ra-te and controlling the same. The system includes an ultra-
filtration branch having a very accurately manufactured
syringe-like single-acting piston-pump for wi~hdrawin~ liquid
from the dialyzer in very accurately controlled amounts at
very accurately controlled rates during the ultrafiltration
set-up mode. The pump is operated by any accurately-controlled
motor and stepper-motors have been found to be desirable.
This system does not require manual operations or observa-
tions which thus increases its accuracy. This system, in
addit-on to being useful with negative-pressure flow-through
dialysis machines, may be used with other types of machines,
such as batch, recirculating or positive pressure machines.
~lq~4~S~
BRIEF ~ESCRIPTION OF THE D.~AWINGS
FIGURE 1 is a diayrammatic representation of a
dialysis system which includes an ultrafiltration brancn
having the syringe-like piston-pump and stepper ~otor for
establishing the ultrariltration rate;
FIGURE 2 is a plan view showing the structural
details of the piston-pump;
FIGURE 3 is an enlaraed elevational view showing
a limit-switch control associated with the pump; and
FIGURE 4 is an end view o the pump showing a
gear reduction system for operating the pump.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawing, there is shown a
dialysis system 10 generally. The system has three main
sections, namely -- the standard flow system; the transmem-
brane control system; and the ultrafiltration branch.
The Flow System
The flow system includes a negative-pressure-type
dialyzer 12 that has a semipermeable membrane which sepa-
rates blood flowing through the dialyzer from dialysis
solution. Such dialyzers may be of either the parallel-
plate-type or the hollow-fiber type. Blood enters the dla-
lyzer via inlet 12a and exits via outlet 12b, and dialysis
solution enters via inlet 12c and exits via outlet 12_.
In normal operation, the diaIyzer may be tipped so that the
outlet 12d is above the inlet 12c so as to aid in removal
of any gas bubbles formed in the dialyzer during dialysis.
~Z~9
IJpstream of the dialyzer there is provided a
supply 14 for dialysls solution. The supply can either
be a reservoir for previously prepared batches of solution
or, in the alternative, can be freshly mi~ed by the known
proportioning-type devices. Fresh dialysis solution flows
from the supply L4, along a main flow conduit 16, through a
constant 1OW device 13 and to an inle' line 20 that leads
to the dialyzer 12. An inlet ~Talve 22 is positioned in the
inlet line upstream of the dialyzer for permitting or term-
inating flow to the dialyzer. Spent diaiysis solutionexits the dialyzer via outlet line 24 and an outlet valve
26 is provided for permitting or terminating flow o~ dia-
lysis solution from the dialyzer. A bypass line 28 is pro-
vided which also includes a bypass valve 30 that permits
flow to bypass the dialyzer. The bypass line 2~ bridges or
is connected across the inlet line 20 and outlet line 24.
An adjustable flow control valve 32 is positioned
downstream of the valves 26 and 30 for receiving and con-
trolling flow. A constant-speed negative-pressure pump 34
is positioned downstream of the control valve 32 for drawing
dialysis solution from the supply through the dialyzer.
Alternatively, a variable~speed pump can be substituted for
the control valve 32 and the constant-speed pump 34 to
control liquid flow. The spent or used dialysis solution
is discharged from the dialysis system to the drain 36.
Pressure Detectors
Pressure conditions within the dialyzer on both
the blood and the dialysis solution sides are continuously
~14~,~919
monitored. An arterial pressure transducer (APT) 38 monitors
the incoming blood pressure and the venous pressure trans-
ducer (VPT) 40 monitors returnlng blood pressure. Two nega-
tive pressure transducers are provided for measuring dia-
lysis solution pressure within the dialyzer. The upstream
negative pressure transducer (NPTl) 42 ~easures dialysis
solution pressure at the dialyzer inlet and downstream of
the inlet valve 22; while the downstream negative pressure
transducer (NPT2) 44 measures the pressure at the dialyzer
outlet and upstream of the outlet valve 26.
The transducers 42 and 44 have heen positioned
as close to the dialyzer as possible so as to maximize
accuracy of pressure determinations. Furthermore, the
negative pressure transducers 42 and 44 are positioned
adjacent the inlet and outlet of the diaiyzer so as to func-
tion during the ultrafiltration set-up mode as well as during
normal operation.
The Transmembrane Pressure Control
The transmembrane pressure control system in-
cludes a transmembrane pressure controller 46 which re-
ceives the output signal from each of the pressure trans-
ducers 38, 40, 42 and 44. A desired transmembrane pressure
can be entered into the controller and the controller can
adjust the actual transmembrane pressure to the desired
pressure.
The mean blood pressure within the dialyzer is
determined by averaging the outputs of the arteria7. pressure
4~
transducer 38 and venous pressure transducer 40. The mean
negative pressure in the dialvzer is determined by averag-
ing the outputs of the upstream and downstream negative
pressure transducers 42 and 44. These mean pressuras are
added algebraically so as to determine -the transmembrane
pressure.
The adjustable flow control valve 32 is also
connected ~o the transmembrane pressure controller 46.
Control of the actual transmembrane pressure is ~ade by
adjusting the valve 32 so as to maintain a predetermined
transmembrane pressure. The adjustment is made by the
controller comparing (1) the actual transmembrane pressure
as approximated by the mean blood pressure and mean negative
pressure with (2) the predetermined transmembrane pressure
value which is determined during ultraflltration set-up
and entered into the controller and (3) then opening or
closing the valve 32 to minimize any differences between
the actual and predetermined pressures. The specific con-
struction for such controls is known and can be either manu~l
or automatic.
The Ultrafiltration Branch
The ultrafiltration branch 48~includes li.nes 50a
and 50b, the piston-pump assembly 52, the stepper motor 54
and th~ indicator 56. The lines 50a and 50b are connected
to the piston-pump assembly 52 and to the dialysis solution
inlet line 20 at a position between the inlet valve 22 and
the dialyzer inlet 12c. The negative pressure transducer
42 is preferably positioned between the connection and the
dialyzer inlet.
The pi5 ton-pump assembly 52, the stepper motor 54
and the indicator 56 are mounted on a support plate 58 and
are shown in greater detail in Figures 2, 3 and 4.
The piston-pump assembly 52 is a single-acting
syringe-like device which includes a very accurately finished
cylinder 60 having at one end an end cap 6~ and a retractable
piston assembly 64. The piston assembly 64 includes the
piston 66 and a threaded piston rod 68. The inside of the
cylinder is very accurate and uniform in cross-sectional
area, and the piston carries seals for sealingly and slidably
engaging the interior surface. The cylinder, end cap and
piston define a chamber whose volume can be very accurately
determined and varied. The maximum volume of the piston
chamber is about 200 milliliters (ml), which provides suf-
ficient capacity to establish the desired ultrafiltration
rate.
The lines 50a and 50_ are connected to the end cap
so as to permit flow of liquid into and through the piston-
pump assembly 52.
Referring ~o Figure 3, an arm 70 is mounted to
the end of the piston rod 68 opposite t~e piston and is
arranged to engage a travel limit stop switch 72 mounted on
the support plate. The limit switch cooperates in stopping
retraction of the piston assembly when the piston reaches a
predetermined point (usually the end of the piston's travel
or stro~e).
The stepper motor 54 is connected to the piston
rod through a gear train 74. The gear train also connects
the motor 54 to a potentiometer or indicator 56 which indi-
cates whether or not the motor is functioning.
2g~49
The motor 54 is commonly referred to as a stepper
motor. In that type of motor, the armature rotates a fixed
incremental amount upon receiving a pulse signal. In other
words, for each pulse applied to the motor, the armature
rotates one ir.crement. Thus by accuratel~ controlling the
number of pulses delivered to the motor and their rate of
delivery, the operation of the motor is accurately controlled.
Since the operation of the motor is accurately controlled,
the rate of withdrawal of the piston is accurately controlled
and the change in volume of the piston-pump chamber is
accurately controlled. The motor used herein: is manufac-
tured by Rapidsyn, Dana Industrial, 11901 Burke St., Santa
Fe Springs, Ca. 90670; is described as a small angle DC
stepping motor, Series 23; i5 identified as model number
23 H-503; and reference is made to U.S. Patent 3,519,859.
In this particular motor the armature moves in
increments or steps of 1.8 degrees so that 200 steps con-
stitute one full revolution of the armature. Thus, one
pulse will cause the motor to move one step or 1.8 degrees.
The gear train 74 establlshes the relationship between the
rate of motor rotation and the rate of fluid withdrawal.
In this system the application of 10 pulses/second to the
motor corresponds with an increase in ch~.mber volume of
1 ml/minute, which, in turn, corresponds to a liquid flow
rate or ultrafiltration rate of 1 ml/minute (20 pulses/
second is 2 ml/minute, etc.).
The means by which the pulses are generated and
applied to the motor are known in the art and include a
keyboard entry system 76 and an encoder and pulse generator 78.
-10-
~4Z~
In such a system the desired flow rate is entered into the
keyboard, the corresponding number of pulses are generated
and then applied to the stepper motor 54. For example, if
2 ml/minute is entered into the keyboard 76, then 20 pulses/
second are applied to the motor 54.
It will be appreciated that, if necessary, the
gear train can be modified so as to obtain difrerent rela-
tionships between the motor steps and liquid withdrawal.
Since the motor armature moves in increments or
steps rather than continuously, it may be desirable to pro-
vide a flywheel or other damping device.
It should be appreciated that other drives or
motors can be substituted for the stepper motor so long as
those means can be accurately controlled so as to permit
accurate control of the rate of the piston's withdrawal.
The principal advantage of the foregoing system
is that the cross-sectional area of the cylinder is accu-
rately known and the rate of movement or withdrawal of the
piston can be accurately controlled. ~ence the rate of
liquid drawn into the piston pump can be very accurately
controlled. This system thus removes certain potential
errors from the setting and determination of the ultrafil-
tration rate.
Operation
.
In normal operation, sometimes referred to as
the dialyze mode, blood flows through the dialyzer and the
negative pressure pump 34 draws dialysis solution from the
~1~24~49
supply 1~, through the flow device 18, through the inlet
line 20 and ~hrough the dialyzer 12. From the dialyzer
the spent dialysis solution flows via line 24, through valve
32, pump 34 and to the drain. ~Tegative pressure within
the dialyzer is controlled by adjusting the control valve
18 to obtain the desired negative pressure. During normal
operation, the valves 22 and 26 are open so as to permit flow
to and from the dialyzer. However, under certain predeter-
mined conditions, the valves 22 and 26 are closed so as to
terminate flow to and from the dialyzer, thereby isolating
the dialyzer from the flow system and the bypass valve 30
is simultaneously opened so as to permit flow directly from
the supply 14 to the pump 34.
When it is desired to determine or set the ultra-
filtration rate, the machine is removed from the dialyze
mode and is operated in the ultrafiltration set-up mode.
In the set-up mode, valves 22 and 26 are closed so as to
isolate the dialyzer and bypass valve 30 is opened. Blood
still continues to flow through the blood side of the dia-
lyzer. The ultrafiltxaticn branch ~8 ls activated and themotor 54 is operated at a rate which corresponds to the
desired ultrafiltration rate.
In such operation, as the piston is retracted, it
withdraws liquid from the flow system which, in turn, causes
liquid to flow across the semipermeable membrane from the
blood side to the dialysis solution side. That rate of flow
is equivalent to the ultrafiltration rate. The only liquid
flowing into the branch is water flowing across the membrane
from the blood into the dialysis solution. When the desired
-12-
ultrafiltrarion rate is obtained, the transmembrane pres-
sure will stabilize. At that point the mean negative
pressure indicated by the negative pressure transducers
42 and 44 are noted and added algebraically to the mean
blood pressure as determined from the signals from the
transducers 38 and 40. This value is automatically deter-
mined and entered into the controller 46 and represents
the transme~brane pressure which will provide the desired
ultrafiltration rate.
When the machine is returned to the dialyze mode,
the valves 22 and 26 are opened, the bypass valve 30 is
closed, and the motor 54 pushes the piston 66 in the reverse
direction, thereby returning all withdrawn solution to the
system without any waste or requirement for drainage and the
pump 34 draws dialysis solution through the system.
In the dialyze mode the transmembrane pressure
control system compares the entered transmembrane pressure
value and actual pressure conditions as noted by the pres-
sure transducers 38, 40, 42 and 44, and then operates the
valve 32 to control the negative pressure, in such a manner
as to maintain the actual transmembrane pressure at a value
approximating the entered transmembrane pressure.
The operator of the dialysis machine may decide
at various times during the dialysis treatment that, be-
cause of changing conditions in the dialyzer, the patient's
condition, etc., that it is necessary to reset the ultrafil-
tration rate. In order to reset the ultrafiltration rate,
the system is returned to the ultrafiltration set-up mode
in which flow to the dialyzer is prevented by closing valves
2~9
22 and 26 and the ultrafiltration branch 48 and piston-pump assembly
52 are then activated. The previously described procedure is then
followed to establish an ultrafiltration rate and determine a
desired transmembrane pressure. Automatic operation is anticipa-
ted to achieve some functions which the operator may perform
manually.
It will be appreciated that numerous changes and modifica-
tions may be made.to the embodiment disclosed herein without
departing from the spirit and scope of this invention.
~.
