GB2053724A - Dialysis machine with means for controlling ultrafiltration rate - Google Patents

Abstract

The machine includes a flow system for drawing fresh dialysis solution from a source through a dialyzer 12. Inlet and outlet valves 22,26 can cut off flow to and from the dialyzer. An ultrafiltration branch 48 is provided between the valves and the dialyzer for drawing liquid from the dialyzer when the valves are closed. This branch includes a single- acting piston pump 52 operated by a stepper motor 54 to draw the liquid in accurately controlled amounts and at accurately controlled rates. <IMAGE>

Description

SPECIFICATION Dialysis machine with means for controlling ultrafiltration rate This invention relates to a dialysis machine for use with an artificial kidney dialyzer.

Artificial kidney devices include a dialyzer and a dialysis machine which controls the operation of the dialyzer. The dialyzer is used to treat a patient's blood so as to remove water and waste products therefrom. The dialyzer includes a semipermeable membrane which separates blood and dialysis solution flowing through the dialyzer. Waste product removal occurs by 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 petal, and a commercial machine embodying such a system is manufactured and sold by Baxter Travenol Laboratories and is identified as Proportioning Dialyzing Fluid Delivery System (5M 1352-5M 1355).

This machine can be referred to as a single-pass or flow through system in that the dialysis solution is continuously prepared, flows through the machine and dialyzer and then flows to a discharge drain.

The removal of water from the blood by the process of ultrafiltration relies on the pressure differential across the semipermeable membrane (i.e., the difference in the pressure of the blood flowing under positive pressure through the dialyzer and the dialysis solution flowing under a negative pressure through the dialyzer). This pressure differential is commonly known as the transmembrane pressure, and the amount of water removed from the blood is directly related thereto.

It is known to be desirable to control the ultrafiltration 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 negativepressure flow-through systems for establishing and controlling ultrafiltration rates. In that system there is provided 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 rate, 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 ultrafiltration rate. Thereafter, the operator observes or determines 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 manual adjustments and observations may not provide the accuracy sought in some situations.

It is an object of this invention to provide a more accurate system for establishing and controlling the ultrafiltration rates.

The present invention provides a dialysis machine adapted for use with an artificial kidney dialyzer and for establishing a desired ultrafiltration rate, said machine including: a flow arrangement for causing fresh dialysis solution to flow to a dialyzer and spent dialysis solution to flow from said dialyzer; valve means in said flow arrangement for termi nating flow to and from said dialyzer; and an ultrafiltration branch connected to said flow arrangement between said valve means and flow connections for communicating with said dialyzer; said branch including a single-acting piston-pump for drawing liquid from the flow arrangement and a motor connected to the pump for operating the pump to withdraw liquid from the flow arrangement in accurately controlled amounts and at an accurate ly controlled rate.

This machine does not require manual operations or observations which thus increases its accuracy.

This machine may be a negative-pressure flowthrough dialysis machine, on another type of machine, such as a batch, recirculating or positive pressure machine.

Reference is now made to the accompanying drawings, wherein: Figure 1 is a diagrammatic representation of a dialysis system which includes an ultrafiltration branch having a syringe-like piston-pump and stepper motor for establishing the ultrafiltration rate; Figure 2 is a plan view showing the structural details of a piston-pump of the system.

Figure 3 is an enlarged elevational view showing a limit-switch control associated with the pump; and Figure 4 is an end view of the pump showing a gear reduction system for operating the pump.

Referring now to the drawing, there is shown a dialysis system 10. The system has three main sections, namely -- a standard flow system; a transmembrane control system; and an ultrafiltration branch.

The flow system The flow system includes a negative-pressuretype dialyzer 12 that has a semipermeable membrane which separates blood flowing through the dialyzerfrom dialysis solution. Such dialyzers may be of either the parallel-plate-type or the hollow-fibre type. Blood enters the dialyzer via inlet 1 2a and exits åia outlet 12b, and dialysis solution enters via inlet 12c and exits via outlet 12d. In normal operation, the dialyzer may be tipped so that the outlet 1 2d is above the inlet 1 2c so as to aid in removal of any gas bubbles formed in the dialyzer during dialysis.

Upstream of the dialyzer there is provided a supply 14 for dialysis solution. The supply can either be a reservoir for previously prepared batches of solution or, in the alternative, can be freshly mixed by the known proportioning-type devices. Fresh dialysis solution flows from the supply 14, along a main flow conduit 16, through a constant flow device 18 and to an inlet line 20 that leads to the dialyzer 12.

An inlet valve 22 is positioned in the inlet line upstream of the dialyzer for permitting or terminating flow to the dialyzer. Spent dialysis solution exits the dialyzer via outlet line 24 and an outlet valve 26 is provided for permitting or terminating flow of dialysis solution from the dialyzer. A bypass line 28 is provided which also includes a bypass valve 30 that permits flow to bypass the dialyzer. The bypass line 28 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 controlling flow. A constant-speed negativepressure 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 monitored. An arterial pressure transducer (APT) 38 monitors the incoming blood pressure and the venous pressure transducer (VPT) 40 monitors returning blood pressure. Two negative pressure transducers are provided for measuring dialysis solution pressure within the dialyzer. The upstream negative pressure transducer (NPT1) 42 measures 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 been 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 dialyzer so as to function during the ultrafiltration set-up mode as well as during normal operation.

The transmembrane pressure control The transmembrane pressure control system includes a transmembrane pressure controller 46 which receives the output signal from each of the pressure transducers 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 arterial pressure transducer 38 and vanous pressure transducer 40. The mean negative pressure in the dialyzer is determined by averaging the outputs of the upstream and downstream negative pressure transducers 42 and 44. These mean pressures are added algebraically so as to determine the transmembrane pressure.

The adjustable flow control valve 32 is also connected to the transmembrane pressure controller 46. Control of the actual transmembrane pressure is made 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 means negative pressure with (2) the predetermined transmembrane pressure value which is determined during ultrafiltration 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 construction for such controls is known and can be either manual or automatic.

The ultrafiltration branch The ultrafiltration branch 48 includes lines 50a and SOb, the piston-pump assembly 52, the stepper motor 54 and the 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 piston-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, 3and4.

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 62 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 sufficient capacity to establish the desired ultrafiltration rate.

The lines 50a and 50b are connected to the end cap so as to permit flow of liquid into and through the piston-pump assembly 52.

Referring to Figure 3, an arm 70 is mounted to the end of the piston rod 68 opposite the 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 stroke).

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 indicates whether or not the motor is functioning.

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 increment. Thus by accurately controlling the number of pulses deli vered 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 manufactured by Rapidsyn, Dana Industrial, 11901 Burke St., Santa Fe Springs, Ca. 90670; is described as a small angle DC stepping motor, Series 23; is 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 constitute one full revolution of the armature. Thus, a one pulse will cause the motor to move one step or 1.8 degrees. The gear train 74 establishes 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 chamber volume of 1 ml/minute, which, in turn, corresponds to a liquid flow rate or ultrafiltration rate of 1 mI/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. 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 different relationships between the motor steps and liquid withdrawal.

Since the motor armature moves in increments or steps rather than continuously, it may be desirable to provide 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 accurately known and the rate of movement or withdrawal of the piston can be accurately control led. Hence 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 ultrafiltration 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 supply 14, through the flow device 18, through the inlet line 20 and through the dialyzer 12. From the dialyzer the spent dialysis solution flows via line 24, through valve 32, pump 34 and to the drain. Negative 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 predetermined 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 ultrafiltration 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 dialyzer. The ultrafiltration branch 48 is activated and the motor 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 ultrafiltration rate is obtained, the transmembrane pressure 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 deter mined from the signals from the transducers 38 and 40. This value is automatically determined and entered into the controller 46 and represents the transmembrane 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 with drawn 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 transmem brane pressure value and actual pressure conditions as noted by the pressure 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, because of changing conditions in the dialyzer, the patient's condition, etc., that it is necessary to reset the ultrafiltration rate. In order to reset the ultrafiltra tion rate, the system is returned to the ultrafiltration set-up mode in which flow to the dialyzer is pre vented by closing valves 22 and 26 and the ultrafil tration branch and pump 48 are then activated. The previously described procedure is then followed to establish an ultrafiltration rate and determine a desired transmembrane pressure. Automatic opera tion is anticipated to achieve some functions which the operator may perform manually.

Claims (10)

1. A dialysis machine adapted for use with an artificial kidney dialyzer and for establishing a desired ultrafiltration rate, said machine including: a flow arrangement for causing fresh dialysis solution to flow to a dialyzer and spent dialysis solution to flow from said dialyzer; valve means in said flow arrangementforterminating flow to and from said dialyzer; and an ultrafiltration branch. connected to said flow arrangement between said valve means and flow connections for communicating with said dialyzer; said branch including a single-acting piston-pulp for drawing liquid from the flow arrangement and a motor connected to the pump for operating the pump to withdraw liquid from the flow arrangement in accurately controlled amounts and at an accurate ly controlled rate.

2. A dialysis machine according to Claim 1, whe#rein said motor is a stepper motor.

3. A dialysis machine according to Claim 1 or 2, wherein said pump includes a chamber including a cylinder having an accurately determined inside surface; a piston slidably and sealingly engaging said inside surface; a piston rod connected to the piston for moving the piston within the cylinder; said motor being operatively associated with the piston rod for moving the rod.

4. A dialysis machine according to Claim 3 as appendantto Claim 2, wherein each incremental movement of said motor corresponds to a predeter mined change in chamber volume due to piston movement.

5. A dialysis machine according to Claim 4, wherein the rate of incremental movement is pre cisely controllable so as precisely to control the rate of fluid withdrawal.

6. A dialysis machine according to Claim 3,4 or 5, wherein a gear train is provided connecting the motor to the piston rod.

7. A dialysis machine according to any preceding Claim, wherein there is provided a potentiometer coupled to the motor for indicating operation of the motor.

8. A dialysis machine constructed substantially as herein described with reference to the accom panying drawings.

9. A method of determining the ultrafiltration rate in a negative-pressure type dialyzer which has a semipermeable membrane with dialysis solution flowing through said dialyzer on one side of the membrane under a negative pressure and blood flowing on the other side of said membrane and inlet and outlet ins for carrying blood and dialysis solution to and from said dialyzer, said method comprising the steps of:: (a) flowing blood and dialysis solution through said dialyzer; (b) terminating flow of dialysis solution to and from said dialyzer; (c) thereafter withdrawing liquid from said di alyzer in fixed incremental amounts at a predetermined rate; mined determining the transmembrane pressure rate; lated determiningto transmembrane said flow rate; (e) re-establishing dialysis solution flow through said re-establishing dialyzer; solutionand said thereafter controlling the actual transmem- brane pressure during normal operation so as to approximate the predetermined transmembrane pressure.

10. A method according to Claim 9, substantially as herein described with reference to the accompanying drawings.