Abstract:
A peritoneal dialysis (PD) system that includes a disposable PD cassette including a base having a substantially planar portion and a dome-shaped protrusion extending from the substantially planar portion and a flexible membrane attached to the base and covering a recessed region of the dome-shaped protrusion to form a pumping chamber between the flexible membrane and the recessed region of the dome-shaped protrusion. The system also includes a PD machine including a deck and a door hinged from one side to the deck. The door and the deck can cooperate to form a cassette compartment, and the door has a cylindrical recess positioned to receive the dome-shaped protrusion of the base of the disposable PD cassette when the disposable PD cassette is disposed in the cassette compartment and the door is closed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of and claims priority to U.S. application Ser. No. 11/515,359, filed on Aug. 31, 2006, entitled “Improved Cassette System for Peritoneal Dialysis Machine,” which is a continuation-in-part application of and claims priority to U.S. application Ser. No. 11/069,195, filed on Feb. 28, 2005, entitled “Portable Apparatus for Peritoneal Dialysis Therapy,” each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to apparatus for the treatment of end stage renal disease. More specifically, the present invention relates to portable apparatus for the performance of peritoneal dialysis. 
     Dialysis to support a patient whose renal function has decreased to the point where the kidneys no longer sufficiently function is well known. Two principal dialysis methods are utilized: hemodialysis; and peritoneal dialysis. 
     In hemodialysis, the patient&#39;s blood is passed through an artificial kidney dialysis machine. A membrane in the machine acts as an artificial kidney for cleansing the blood. Because the treatment is extracorporeal, it requires special machinery and a visit to a center, such as in a hospital, that performs the treatment. 
     To overcome this disadvantage associated with hemodialysis, peritoneal dialysis (hereafter “PD”) was developed. PD utilizes the patient&#39;s own peritoneum (a membranous lining of the abdominal body cavity) as a semi-permeable membrane. With its good perfusion, the peritoneum is capable of acting as a natural semi-permeable membrane. 
     PD periodically infuses sterile aqueous solution into the peritoneal cavity. This aqueous solution is called PD solution, or dialysate for short. Diffusion and osmosis exchanges take place between the solution and the blood stream across the peritoneum. These exchanges remove the waste products that the kidneys normally excrete. The waste products typically consist of solutes like urea and creatinine. The kidneys also function to maintain the proper levels of other substances, such as sodium and water, which also need to be regulated by dialysis. The diffusion of water and solutes across the peritoneal membrane during dialysis is called ultrafiltration. 
     In continuous ambulatory PD, a dialysis solution is introduced into the peritoneal cavity utilizing a catheter, normally placed into position by a visit to a doctor. An exchange of solutes between the dialysate and the blood is achieved by diffusion. 
     In many prior art PD machines, removal of fluids is achieved by providing a suitable osmotic gradient from the blood to the dialysate to permit water outflow from the blood. This allows a proper acid-base, electrolyte and fluid balance to be achieved in the body. The dialysis solution is simply drained from the body cavity through the catheter. The rate of fluid removal is dictated by height differential between patient and machine. 
     A preferred PD machine is one that is automated. These machines are called cyclers, designed to automatically infuse, dwell, and drain PD solution to and from the patient&#39;s peritoneal cavity. A cycler is particularly attractive to a PD patient because it can be used at night while the patient is asleep. This frees the patient from the day-to-day demands of continuous ambulatory PD during his/her waking and working hours. 
     The treatment typically lasts for several hours. It often begins with an initial drain cycle to empty the peritoneal cavity of spent dialysate. The sequence then proceeds through a succession of fill, dwell, and drain phases that follow one after the other. Each phase is called a cycle. 
     Unlike hemodialysis machines, which are operated by doctors or trained technicians, PD cyclers may be operated by the patient. Furthermore, many PD patients travel, which means taking their PD cyclers with them. Thus the insertion and operation of the cassette interface should be as ergonomic, safe and foolproof as possible, while exhibiting enhanced performance. The all-important design of the cassette itself should permit the maximum flexibility in functionality. 
     The intent of this invention is to provide improved PD equipment with a focus on the design of the cassette and cassette compartment of the PD cycler. 
     SUMMARY OF THE INVENTION 
     In one aspect the invention includes apparatus peritoneal dialysis apparatus including a disposable cassette compartment defined by a deck lying in a plane inclined from the vertical by about 10 to about 35 degrees, preferably about 20 to about 25 degrees, and more preferably about 22 degrees, having openings for valve actuators and piston heads and a door hinged from the side so as to close in parallel over the deck and enclose the cassette within the compartment. In one embodiment, the cassette has inlet/outlet connections along the bottom of the cassette, the compartment accommodating the connection of vertically hanging tubes to the inlet/outlet connections on the cassette so that preferably all of the inlet/outlet connections are in a line along the bottom edge of the cassette. In this configuration the lines are permitted to make a gentle bend substantially greater than 90 degrees when sitting on a flat surface. 
     In another aspect of the invention, a disposable PD solution routing cassette compartment is defined by a door and a cassette deck, and an inflatable pad carried by the door forces a cassette that fits into the compartment into sealing engagement with the cassette deck when the door is closed and the pad is inflated. In addition a door latch mechanism can be locked merely by the force of the inflatable pad tending to push the door away from the cassette deck. 
     In another aspect of the invention a disposable PD solution cassette defining channels, valves and pump chambers for routing PD solution to and from inlet/outlet connections on the cassette is arranged in a cassette compartment with a cassette deck for sealingly engaging the cassette, the cassette having a diaphragm covering at least one pump chamber facing the deck, the deck having a reciprocating piston head mounted for reciprocation in a cylindrical chamber, an annular space surrounding the piston head between the chamber walls, and a pneumatic system draws a vacuum in the cylindrical chamber, the vacuum drawing the diaphragm tight against the piston head so that the diaphragm retracts with the piston head. The pneumatic system can also be used to seal a pressure reading area of the cassette to a pressure sensor on the deck. 
     A further aspect of the invention is the design of a disposable cassette for routing PD solution with a molded plastic panel having a circumferential fluid channel defined along the perimeter of the panel. 
     Finally, another aspect of the invention involves a method of operating a PD machine, for example, using a cassette system with some of the features disclosed herein, to drain spent PD solution from the patient to an empty solution bag that had been filled with PD solution earlier that was used to infuse the same patient to take a sample of the used PD solution. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a PD cycler. 
         FIG. 2  is a perspective view of the PD cycler of  FIG. 1  on a special cart with a heater bag on the heater tray and additional PD solution bags for more exchanges hanging off the cart. 
         FIG. 3  is an end view of the PD cycler of  FIGS. 1 and 2  showing the angle of the front and the heater bag outlet. 
         FIG. 4  is a perspective view of the cassette holder of the PD cycler of  FIG. 1 . 
         FIGS. 5 and 6  are end views of the bracket and cassette deck of an embodiment of the cassette holder of  FIG. 4  showing the angle of the cassette deck. 
         FIGS. 7A and 7B  are exploded perspective views of the cassette holder of  FIG. 4 ,  FIG. 7A  showing the front of the cassette deck and door assembly, and FIG. B showing the back of the cassette deck and internal components behind the cassette deck, as well as the safety clamp 
         FIG. 8  is a perspective view of the front of the cassette deck of  FIGS. 5 and 6 . 
         FIG. 9  is a detail perspective view of the front of the cassette deck of  FIG. 8  with one of the mushroom piston heads removed. 
         FIGS. 10A and 10B  are perspective views of the cassette holder of the PD cycler of  FIG. 2  showing the displacement of the door cassette pad when pressurized from the retracted uninflated position in  FIG. 10A  to the fully inflated, extended position in  10 B, which of course only happens when the door is closed with the cassette in place. 
         FIG. 11  is a view of a cassette used in the apparatus of the invention, the view being of the side that faces the cassette deck, i.e., the machine, when inserted. 
         FIG. 11A  is a perspective view like those of  FIGS. 10A and 10B , but showing a cassette installed in the cassette compartment before closing the door. 
         FIG. 12  is a hydraulic schematic for the liquid lines of the cassette and tubing for the cycler of  FIG. 2 , indicating the valves by number on the cassette of  FIG. 11 . 
         FIGS. 13A ,  13 B and  13 C illustrate various PD solution flow paths through the cassette of  FIG. 11 . 
         FIG. 14  is a pneumatic schematic for the pressure and vacuum sides of the system for actuating the cassette valves and other pneumatic components of the cycler of  FIGS. 1 and 2 . 
         FIG. 15  is a schematic and block diagram of the electronic operation of the PD cycler of  FIGS. 1 and 2 . 
         FIGS. 16 and 17  illustrate aspects of the user interface. 
     
    
    
     Numbers referring to the same items in several drawings will bear the same reference numbers. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The Door Sealing Mechanism 
     Referring to  FIG. 1 , the portable PD apparatus of the invention is shown in an embodiment of a PD cycler  10 . The housing  20  holds a touch screen  22 , along with additional control buttons  22 A forming the control panel for the user interface operated by the patient. A cassette holder includes a hinged door  24  and a cassette support deck  26 . The cassette  28 , shown in  FIG. 4 , fits into the cassette support deck  26 . A cassette is inserted into the support deck  26  and the door  24  is closed on the cassette and securely latched, as will be described later. 
       FIG. 2  shows the PD cycler  10  with some of its accessories to illustrate how it used. The cycler  10  is seated on top of a cart  12  designed to accommodate the PD solution bags and associated tubing. The disposable cassette  28  ( FIG. 1 ) locked inside door  24  includes channels, flexible valve domes and diaphragm covered pumping chambers described below that are actuated by mating pneumatic valves and pistons interfacing with the cassette compartment to route the flow of PD solution from the bags through the cycler to the patient and from the patient to a drain. The cassette itself has tubing connectors  16  arrayed along its bottom edge. The connectors extend beneath the door  24  and are connected to tubing as shown in  FIG. 2 . 
     PD solution bags  18  are suspended from fingers on the sides of the cart  12  as shown. A heater bag  19  is shown lying in a shallow concave depression forming the heater tray  21 , which is sized and shaped to accommodate a typical 5 L bag of PD solution. The heater tray  21  has a plurality of heating coils (not shown) embedded below the surface. The surface of the tray  21 , as better shown in  FIGS. 1 and 3 , is slightly inclined downward to the right to assist in emptying the heater bag which is arranged so that the outlet  19 A of the heater bag is also at the right side, adjacent to a temperature sensor  23  positioned in the surface of the heater tray  21  to track the temperature of the solution in the heater bag for a thermostatic control circuit that turns the heating coils on and off as needed to maintain the PD solution at the desired temperature. A dual voltage heating system for the heater tray  21  is disclosed in accompanying application Ser. No. 11/513,618, filed the same day as this application, by Kulwinder Plahey, assigned to the same assignee, entitled “Peritoneal Dialysis Machine with Dual Voltage Heater Circuit and Method of Operation,” which is incorporated by reference herein in its entirety. The dual voltage heating system automatically reconfigures the heating circuit depending on detection of either 110 VAC or 220 VAC to deliver the same wattage for heating PD solution before delivery to the patient, thus facilitating use of the same machine in the United States and Europe. 
     The heater tray  21  is also mounted internally on a support equipped with a load cell (not shown) to provide an electrical signal indicating the weight of the contents of the PD solution bag to tell the cycler control system how full the heater bag is with PD solution. Referring to  FIGS. 4 ,  5 ,  6 ,  7 A,  7 B,  8 ,  9 ,  10 A and  10 B, the cassette compartment  60  will now be described in detail. Essentially, the cassette compartment  60  consists of a base  30  and door  24  hinged to the base  30  on the right side, as shown in  FIG. 4 . Base  30  incorporates two pumps  44  having exposed mushroom heads  32 . Mating with these heads are two cylindrical chambers  34  that accommodate the rigid domes for the pump chambers on the cassette  28  within door  24 . The base  30  also includes a pair of door latches  36  that mate with holes  38  in door  24 . The door also has a sliding latch  40  or catch slide. Microswitch  42  provides an electrical indication of whether the door is opened or fully closed. 
     It is necessary that a very tight, secure mechanical enclosure be provided with intimate contact with the cassette  28  ( FIG. 4 ) when the machine is in operation. Prior art PD machines provided this tight enclosure by using a tight door latch that had to be almost forced closed by the patient. This created a problem for elderly or very ill patients who lacked the strength to close the door. Alternatively, in other prior art PD machines, cassettes were inserted using a complicated mechanism, similar to a VCR, making servicing more difficult. Accordingly, the PD apparatus of this invention does not require the patient to close the door with sufficient force to make all the necessary seals. Furthermore, the cassette can be set directly into compartment  60  without use of the more complicated, VCR-like apparatus. 
     Door  24  is lightly latched using latch lever  40  and latch posts  36 , which loosely engage with holes  38 . Although the door easily “clicks” shut, the proper seals are not made by this closing. To insure that the cassette  28  is in intimate and sealed contact with both the base  30  and the door  24 , the PD apparatus of the invention uses an inflatable pad  47 , shown in  FIG. 7A . In front of the pad  47  is a displaceable spacer  49  mounted to the pad  47  by means of a plate  58 . One or more molded plastic pressure pads  51  are bonded to the front of the spacer  49  for engaging the cassette. The cassette is held in place between the cassette pad  51  and the cassette deck  26 , as shown in  FIGS. 4 and 7A . Once the door is lightly shut and latched by the patient, and the system receives a signal to that effect from microswitch  42 , pressurized air is pumped into pad  47 , squeezing the cassette between the door  24  and the cassette deck  26 , as shown in  FIG. 4 . The pressure applied should be adequate so that all necessary seals are made. Thus even though the cassette will wind up being under pressure, the patient does not need to exert any force on the door or latch to close the door. 
     To open door  24  to load a cassette, button  50  on the top left edge of the door ( FIG. 4 ) is depressed. This will disengage the door lock. The door then swings open from left to right. Cassette  28  ( FIG. 11 ) may then be loaded into cassette holder by putting the top of the cassette under the locating pins  52 . The bottom edge of the cassette will be snapped in place over a spring loaded center clip  53  ( FIG. 4 ). The door  24  closes from right to left pushing gently on it to automatically engage the door with latch posts  36 . The catch assembly is comprised of a catch slide  40  a mounting slide block  41  to which it is attached ( FIG. 7A ) and a catch tension spring (not shown). The block slides in a machined slot  54  on the left side of the door as viewed in a closed position ( FIG. 4 ). As the door swings shut, the catch slide comes in contact with the beveled end  56  of the latch posts  36 . The action of lightly pushing on the door to latch it also actuates the door safety switch  42 . The catch slide lowers and then springs upward in the notches formed by the latch posts  36 , coming to rest with the flat blade of the catch slide in contact with the forward planar wall of the notches in the latch posts. When the door is not pressurized the friction between the contact area of the latch post notch walls and the catch slide blade is easily overcome to re-open the door. However, when the compartment is pressurized by the inflatable pad  47 , the friction between these elements cannot be overcome by the user who will be unable to push the catch slide  40  downward with enough force to overcome the contact friction with the post notch walls. Thus the pressurization of the door acts as a safety interlock for the cassette compartment. 
     Once the door safety switch is closed, the system receives an electrical signal indicating that it is ready to clamp the cassette into the cassette holder by inflating the cassette clamping inflatable pad  47  (( FIG. 3A ) with approximately 37 psi pressure (which generates approximately 1000 pounds of force). This clamps the cassette  28  against the clamp pad  51  ( FIG. 3A ), thereby rigidly holding the cassette in place so that it can be operated by the valves and pistons in the cassette deck. The door locking mechanism is then immobilized, preventing the door from accidentally opening or even from being opened by the patient, for safety purposes. 
     There are several ergonomic features of the basic arrangement of the cassette compartment  60  and door  24 . As shown in the end views in  FIGS. 5 and 6 , the brackets  57  that hold the base  30  of the cassette deck and also support the hinges  59  for the door, are designed to hold the base and door at a 10 to 35 degree angle to the vertical, preferably 22 degrees. Thus the hinge line of the door itself is inclined rather than plumb and the deck  36  where the cassette is mounted is also in a reclining orientation. When the user opens the door as shown in  FIGS. 1 and 10A , for example, the door tends to hold itself open when opened past 90 degrees because of this inclination. In addition, the surface of the deck where the cassette is to be mounted is more easily viewed and accessed by the user because of the angle, particularly because the compartment would rarely be at eye level. The user must assure that the cassette is inserted correctly with the notches  28 A ( FIG. 11 ) under the pins  52  ( FIG. 4 ) and the lower center edge of the cassette  28 B snapped in place over the clip  53 . (Note that the side of the cassette  28  in view in  FIG. 11  is the one that fits against the cassette deck, so when in place, the cassette  28  will appear reversed.) This is more easily accomplished with the compartment at this approximate angle. 
     A further advantage of the cassette compartment design is achieved by virtue of the door being hinged from the side. With this arrangement, the cassette is free to have the tubing connections (inlets and outlets), of which there are typically seven in use, arrayed along the bottom edge of the cassette as shown in  FIGS. 1 and 11  with the tubing hanging straight down. This permits the tubes to hang free and untangled, straight down under the force of gravity if there is a slot on the table as shown in  FIG. 2  without any unnecessary bending likely to kink or constrict the lines. In combination with this bottom entry feature, the 22 degree angle of the door compartment better accommodates a bend in the lines if the cycler is sitting on a night table for example where the lines would extend downward and then across the table top for a few inches. If the compartment was vertical the lines would have to make a 90 degree turn. Instead they can take a gentler 112 degree turn on the table top or other flat surface and remain free of constriction. 
     The Pump 
     The pumps  44  (best seen in  FIG. 7B ) are controlled by stepper motors  45 . The details of the stepper motor control will be explained later. The PD apparatus of the invention uses two modes of pumping, simultaneous and alternating. With the alternating method, while one pump is protracted, the other is retracted. Simultaneous pumping is where both pump heads extend at the same time in the same direction, and both retract at the same time. Each pump has a piston with a mushroom shaped head  32  as shown in  FIGS. 4 and 5 . The mushroom head  32  has a threaded bore which screws onto a threaded post  65  on the piston shaft as shown in  FIG. 5 . The outer diameter of the head  32  is slightly less than the inner diameter of the cylinder  55  in which the head reciprocates as shown in  FIG. 9 . The inner wall of each cylinder has a slot (not shown) in the form of a circumferential arc in the wall to allow evacuation of air from the piston chamber, as described below. 
     To move fluid out of one of the pump chambers, the mushroom head  32  mated to that chamber is protracted all the way to the rigid back dome of the cassette  28 , but not touching it. To draw fluid into one of the pump chambers, the piston head  32  is pulled back by one of the stepper motors  45 . The vacuum in the piston chamber causes the diaphragm membrane covering the pump chamber on the cassette to be sucked flush against the spherical surface of the piston head. The diaphragm is exposed to the vacuum approximately −500 millimeters of mercury in the piston chamber by way of the annular space surrounding the circumference of the piston head where it comes closest to the cylindrical wall of the piston cylinder  55 . The periphery of the diaphragm remains sealed airtight against the cassette deck  26  because of the pressurized door due to its inflatable pad. Thus the vacuum in the piston chamber is bounded by the cylindrical wall, the cassette diaphragm and the piston itself. Thus when the piston head retracts, the vacuum continues to hold the diaphragm against the mushroom head and the diaphragm retracts with the piston to thus enlarge the chamber, drawing fluid into one of the chambers A or B of the cassette  34  through whichever valve is opened. 
     For draining fluids from the patient, an alternating pumping method is employed where one pump  44  extends while the other retracts. When the pump associated with chamber A is extending, the fluid in the chamber A is pushed out into a drain line of the cassette  28 . As the pump associated with chamber B retracts, fluid from the patient is drawn into chamber B. When this motion is completed, the pump associated with chamber A then retracts and draws fluid from patient while pump B protracts and transfers fluids out into the drain line. This process continues until the required volume of fluid from the patient is processed. 
     Initially, the pumps  44  are moved to a home position which is sensed by a conventional optical sensor, not shown. The pump controller encoder value is then set to zero. Next the pump is moved towards the cassette until it touches the cassette. This is the “OUT” position where the encoder is then set to a current encoder value less a maximum (calculated to be the maximum possible stroke, for example, an encoder count of 250). Then, the pump is moved backwards by 800 microsteps, or about an encoder count of 16000. The “HOME” position is then set to this encoder value. The stepper motor  45  next moves backward another 500 microsteps, or about an encoder count of 10,000. This is where the “IN” position is set. 
     Volume calculation is based on the fact that the cassette volume is a known value (based on its physical dimensions). The volume of the pump head is also a known value (again, the calculation of this volume is based on the physical dimensions of the pump head and chamber). If the whole mushroom head  32  is flushed against the cassette wall  46 , then no fluid volume can reside in the cassette chamber. As the mushroom head  32  is moved back, however, it draws fluid into the chamber of the cassette  28  ( FIG. 4 ). The volume of fluid drawn into the chamber is calculated by subtracting the volume of the mushroom head  32  that remains in the chamber from the volume of the chamber. To calculate how much volume of the pump head resides inside the chamber, the amount of linear travel of the pump is calculated, and this distance correlates to the distance of travel of the mushroom head. From that distance a formula is used to determine how much fluid volume still resides in the chamber. 
     The Electronic Controls for the Pump 
     The electronics board  101  of the PD apparatus of the invention is shown in  FIG. 6 . Stepper motor  100 , which drives each pump of the PD apparatus of the invention, is controlled conventionally using firmware with signals to stepper motor driver  108 . The firmware resides in two flash memories  102  and  104 . The firmware stored in flash memory  102  is used to program the bridge field-programmable gate array (FPGA)  106 . The firmware stored in the flash memory  104  is used to program the MPC823 PowerPC microprocessor  112 . 
     Referring to  FIG. 2 , a stepper motor  45  drives a conventional lead screw (not shown) which moves a nut (also not shown) in and out on the lead screw. The nut, in turn, is connected to a mushroom head  32  which actually makes contact with the membrane A or B on the cassette  28  ( FIG. 4 ). The stepper motor and lead screw are chosen to provide the required force to push fluid out of the cassette following the opening of fluid paths in cassette, as will be described later. The stepper motor  45  preferably requires 200 steps to make a full rotation, and this corresponds to 0.048″ of linear travel. Additionally, an encoder measures the angular movement of the lead screw. This measurement can be used to very accurately position the mushroom head assembly. 
     A stepper motor controller (not shown) provides the necessary current to be driven through the windings of the stepper motor. The polarity of the current determines whether the head is moving forward or backward. Rough positioning of the piston is aided by one or more opto-sensors (not shown). 
     Inside the FPGA  106 , there are two duplicate sets of control logic, one for each piston. The two-channel quadrature output of the linear encoder  110  ( FIG. 6 ) is converted into an increasing or decreasing count. The overall range of this count is from 0 to ˜65,000 (or, the count can be split in half about 0, from −32,499 to +32,500). This count is required to determine the current position and subsequent movement of the piston. There is a direct correlation between actual movement of the lead screw and an encoder value. 
     Referring again to  FIG. 6 , the FPGA  106  makes a comparison between the current encoder input and a target value. This is needed for automatic movement. A single command to the FPGA  106  initiates a complete cycle that ends with the piston being moved from its current position to newly designated position. Additionally, the FPGA  106  can automatically stop the motor movement. This is desirable, for example, where the pump head reaches its end of travel (sensed by end of travel switch  112 , or where the pumping action causes the pressure to be out-of-bounds. If the piston reaches an end-of-travel switch  112 , the automatic movement is halted. Likewise, if a pressure sensor  48  ( FIG. 2 ) determines that the pressure is outside of the prescribed, limited range, the motors  45  ( FIG. 2 ) can be halted to prevent a larger excursion, which might be harmful to the patient. 
     Another part of the FPGA firmware allows the speed of the stepper motors  45  to be controlled, as is well known in the art. By adjusting the motor pulse duration and time between pulses, the motor can run faster or slower to get a desired speed vs. torque balance. The speed the motor runs is inversely related to the torque it is able to apply to the pump head. This adjustment allows the machine to produce the desired amount of push on the fluid in the pump chambers A or B ( FIG. 4 ) so that it flows easily through the lines, but isn&#39;t forced so as to trigger pressure alarms or cause rupture of the lines. On the other hand, if you try to run the motor too fast, you may lose the necessary torque required on the pump head to move the fluid through the line. 
     In addition to the motor pulse, the FPGA  106  provides several control signals to the stepper motor controllers (not shown), for example, direction and step size. Depending on the values sent from the flash memories  102  and  104  to the FPGA  106 , the step size can be adjusted between full, half, quarter and eighth steps. Also, the motor controller can be sent a continuous sequence of pulses for rapid motor movement, or just a single pulse to make a single step. This is set conventionally by registers in the FPGA  106 . 
     The Cassette 
     The cassette itself is shown in more detail in  FIG. 11 . The cassette is a biocompatible plastic molded part which has a rigid plastic back facing away from the viewer in  FIG. 11 . The side that faces the cassette deck as shown in  FIG. 11  includes channels and small dome shaped flexible pod like diaphragms forming occludable valves numbered  1  through  16 . The intermediate size dome shaped diaphragms cover the pressure sensor chambers P on the cassette facing the deck  26 , and finally two large flexible diaphragms cover the clamshell (when expanded) shaped pumping chambers A and B. The diaphragms are facing the viewer in  FIG. 11  but would be flush against the piston heads and other mating components of the cassette deck when installed in the cycler. The inlet/outlet valves across the bottom of the cassette are from right to left as follows: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 6 
                 Patient line 
               
               
                 7 
                 N/A (pediatric option) 
               
               
                 11 
                 Solution bag No. 1 
               
               
                 12 
                 Solution bag No. 2 
               
               
                 13 
                 Solution bag No. 3 
               
               
                 14 
                 Last solution bag 
               
               
                 15 
                 Heater bag 
               
               
                 10 
                 Drain 
               
               
                 A 
                 Pump chamber 
               
               
                 B 
                 Pump chamber 
               
               
                 P 
                 Sensors 
               
               
                   
               
             
          
         
       
     
     The cassette  28  is shown installed in  FIG. 11A  with its rigid plastic back now facing the viewer and the lines reversed. The inlets and outlets as shown in  FIG. 11A  are formed with capsule like connectors  28 C that allow connection to the tubing set. The connectors  28 C project out of the plane of the cassette  28  and fit into mating recesses  51 C on the door plate  51 , as shown in  FIG. 11A . Also shown in  FIG. 11A  is the safety clamp  71  also shown in  FIGS. 4 and 7B . The clamp acts to close all of the inlet/outlet connections in an error situation as described in the description of the pneumatic system below. 
     The valves in the cassette control and route the flow of PD solution throughout the PD system under the control of a hydraulic network shown in  FIG. 12 . The valves are designated V 1 -V 16  and correspond to the numbered valves in  FIG. 11 . The flow lines in the schematic are implemented by the cassette&#39;s preformed channels. The valves are actuated pneumatically to case various sources and destinations to be placed in fluid communication. For example, for fluid to flow from Bag  1  (one of the bags  18  hanging on the cart  12  in  FIG. 2 ), valve V 11  is opened and pump valve V 1  is opened while the piston head for chamber A is retracting to fill the chamber, then V 1  is closed and V 2 , V 16 , V 9  and V 15  are opened while the piston head  32  protracts into the chamber A driving liquid out into the heater bag. Other examples are shown in  FIGS. 13A ,  13 B and  13 C. 
     One other design feature of the cassette  28  which is not found in other cassettes is the circumferential channel  28 D formed in the cassette. Channel  28 D actually circumnavigates the entire periphery of the cassette passing valves  16 ,  9 ,  5  and  8 . This channel also passes by all of the inlet/outlet ports on the bottom of the cassette. Thus the interconnected circumferential channel  28 D has multiple uses in delivering fluid to and from the pump chambers A and B. This arrangement also potentially affords an opportunity for flushing the all of the lines of the cassette by appropriate valve openings. For example, fluid could be introduced under pressure from the drain outlet  10  and forced all the way around the cassette and out the rest of the ports  6 ,  7  and  11 - 15 . 
     Description of Fluid Flow Through the Machine 
     The fluid flow through the disposable cassette  28  is illustrated in  FIGS. 13A-13C . The PD machines of the invention utilize six fluid-processing sequences: flush, prime, drain, fill, pause and dwell. The purpose of the flush sequence is to remove air from all the lines (except the patient line) and from the cassette. This is accomplished by pumping dialysate solution through the lines to be flushed. 
     The prime sequence removes air from the patient line by pumping dialysate solution through the patient line. The drain sequence is used to pump dialysate solution from the patient to the drain. The fill sequence is used to pump dialysate solution from the heater bag to the patient. The pause sequence allows the patient to disconnect from the PD machine once the patient has been filled with dialysate solution. While the patient is disconnected from the machine, the machine will be transferring dialysate solution from the solution bags to the heater bag. Finally, the dwell sequence is used to allow the dialysate solution to remain for a set time in the patient. Dwell sequences are identical to pause sequences with the exception that the patient does not disconnect from the machine. While a dwell sequence is occurring, the machine will be transferring dialysate solution from the solution bags to the heater bag. 
     Each figure contains a dashed or solid line, each line having arrows that indicate the direction of flow. All flow diagram lines that are the same pattern (i.e., either dashed or solid) occur at the same time during the process. The different line patterns thus represent alternate times. 
     For example in  FIG. 13A , in the “Heater to Patient” line diagram, when pump chamber A is filling, chamber B is emptying. The dashed lines indicate that pump A is retracting to pull dialysate solution from the heater bag. At the same time pump B is protracting to pump dialysate solution through the patient line. The solid lines indicate that pump A is protracting to push dialysate solution to the patient. At the same time, pump B is retracting and pulling dialysate solution from the heater bag. 
       FIGS. 13B and 13C  show more of the flush sequence as the dialysate solution comes from the supply and moves through the drain line. 
       FIG. 13A  illustrates the prime sequence as the solution from the heater bag pushes air out of the patient line, as well as the fill sequence where solution from the heater bag is pumped to the patient.  FIG. 13C  illustrates the drain sequence as the solution is pulled from the patient and pumped to the drain. 
     Solution may be pumped from a solution bag to the heater bag while the patient is disconnected (pause mode) or still connected (dwell mode), as shown in  FIGS. 13B and 13C . 
     Owing to the flexibility of the flow paths that can be created by manipulating the balloon valves in coordination with the pumps, any number of other flow paths can be utilized. One possibility would be to drain fluid from the patient during a portion of the drain operation to lines other than the drain line. For example, The patient line could be connected for a period of time during the drain mode to divert some of the spent PD solution from the patient line into one of the empty solution bags to collect a sample for testing. 
     The Pneumatic System 
     Referring to  FIGS. 4 and 14 , a pneumatic system provides pressure to operate the valves and fill the inflatable pad  47  to seal the door closed and vacuum to seal the flexible cassette diaphragms to the mating members on the cassette deck  26 , namely the mushroom heads and pressure sensors. The basic schematic for the components of the pneumatic system are shown in  FIG. 14 . A compressor pump is used to provide either air or a vacuum in corresponding reservoirs. On the right side of  FIG. 14  as shown, is the pressure tank which is drawn on as necessary to pressurize and maintain the pressure in the inflatable pad  47 . During the pumping sequence, this air and vacuum resource is used to inflate and deflate the balloon valves  48 . When inflated, a balloon valve will block the fluid from moving through the particular one of channels  1 - 16  ( FIG. 4 ) of the cassette that mates with the selected one of balloon valves  48 . When a balloon valve is deflated, the fluid can move freely through that particular channel controlled by that balloon valve. 
     Another function of the pneumatic system is to pressurize the safety clamp  71  shown in  FIGS. 4 ,  11 A and  7 B. As shown in  FIG. 7B , the bar shaped clamp below the cassette deck is spring loaded and acts like a “dead-man” brake switch. Pneumatic pistons operated by the pneumatic system retract the clamp against the spring force when pressurized thus withdrawing the clamp  71  away from the door  24 . As shown in  FIG. 11A , the clamp extends across all of the tubing connected to the cassette and in the absence of pressure will crimp closed all seven of the tubes shown in  FIG. 11A  against the bottom of plate  51  in the door  24 . This happens automatically when the machine&#39;s controller senses some out of bounds condition that makes it unsafe to continue the operation, such as over-temperature of the heater bag, or rupture of one of the lines or excessive patient pressure, or loss of power. 
     The Pressure Sensors 
     Referring to  FIGS. 4 and 11 , a very important requirement of the PD apparatus of this invention is the accurate measurement and control of pressure between the fluid reservoirs and the patient. If the pressure on a line to the patient increases above alarm limits, serious harm can be caused to the patient. The PD system itself needs to operate at pressures that far exceed the limit. These high pressures are needed for to operate the pressure sensors, balloon valves and other functions in the cassette. Therefore these pressures need to be kept independent from the pressures seen by the patient. Appropriate and reliable sealing and valving needs to be used to keep these high pressures away from the patient. 
     Referring to  FIG. 4 , to monitor the pressure in the system, two pressure sensors  33  are utilized to indirectly detect the pressure and vacuum within the patient&#39;s peritoneum. These sensors are preferably solid state silicon diaphragm infusion pump force/pressure transducers, for example Model 1865 made by Sensym Foxboro ICT. When cassette  28  ( FIG. 4 ) is inserted into the cassette compartment  60 , the pressure sensing areas “P” within the cassette  28  ( FIG. 11 ) line up and are in intimate contact with the two pressure sensors  33 . These sensing areas P are connected, respectively, directly to each chamber A and B through canals  62  and  64 , respectively, so that when fluid moves in and out of the chambers A and B, the pressure sensors  33  can detect its presence. The cassette membrane comprising two areas marked “P” adheres to the pressure sensors  33  using vacuum pressure in the same manner as the diaphragms of the pump chambers A and B are sealed against the mushroom head. Clearance around the pressure sensors communicates vacuum to the pressure dome diaphragms the circumferences of which are sealed airtight to the cassette deck by the pressurization of the door compartment. 
     The two pressure sensors  33  are connected to a high resolution 24 bit Sigma-Delta, serial output A-D converter (ADC)  103  on I/O board  101 . This ADC sends a signal from each of the two pressure sensors to the FPGA  106  on the board  101 . After the data ready signal is received by the FPGA  106 , the FPGA reads this ADC and transfers this data to be processed by the microprocessor  112 , which in the preferred embodiment of the invention is an MPC823 PowerPC device manufactured by Motorola, Inc. 
     On completion of the flush and prime processes, as is well known in the art, the cassette will be filled with solution. At this time, the line to the patient will be completely filled with solution. The pressure at this stage is detected and will be used as base line for static pressure. At that time, the patient&#39;s head height relative to the PD machine will be determined from the differential in the pressure reading. Preferably, this pressure differential is maintained below 100 mbar. 
     During the drain sequence, the maximum pump hydraulic vacuum is limited to −100 mbar to prevent injury to the patient. The vacuum in the peritoneum must be held at or above this value. The position of the patient below or above the PD machine level indicated by the static pressure measurement is compensated by adjusting the level of the vacuum. 
     By way of example, the target vacuum of the vacuum chamber can be based on the following equation:
 
Pstat=static hydraulic pressure(+1 meter=+100 mbar and −1 meter=−100 mbar)
 
Ppatmax=−100 mbar
 
Pvac=target vacuum of vacuum chamber
 
Pvac=Ppatmax+Pstat
 
     For example, where the patient is 1 meter above the PD machine, the differential pressure=+100 mbar; Pvac=−100 mbar+100 mbar=0 mbar. 
     Where the patient on same level than machine, the differential pressure=0 mbar;
 
Pvac=−100 mbar+0 mbar=−100 mbar.
 
     Where the patient is 1 meter below machine, the differential pressure=−100 mbar;
 
Pvac=−100 mbar+−100 mbar=−200 mbar.
 
     Since continuous flow through the various lines connected to the patient is essential to proper treatment of the patient, it is important to continuously monitor if a patient line is blocked, partially blocked or open. There are three different possible situations:
         1. the patient line is open;   2. the patient line is closed; or   3. the patient line is not completely open and therefore creates an undesired flow resistance (caused, for example by the patient is lying on the line).       

     The pressure sensors  33  ( FIG. 2 ) can be used to detect error conditions. Referring to  FIG. 5A , when the pump B is protracting and thereby pumping dialysate fluid into a line that is open to patient, it is very important that the patient pressure and the encoder values are carefully monitored, using the pressure sensors  33  described above. Three possible error situations may occur, for example, as a result of the following events:
         1. The patient line is open when pump B is protracting until a defined length value is reached, and the patient pressure is not increasing;   2. The patient line is closed, and the pump is not able to protract because the patient pressure increases to a defined alarm limit.   3. The pump protracts to produce an increasing patient pressure, but the pressure decreases slowly.       

     These error conditions may be sensed using the pressure sensors  33  of the invention, and corrective action can then be taken, either automatically or by sending an alarm to the patient, where the screen tells the patient what action to take. For example, the screen may tell the patient that he or she may be lying on a fluid line, and should move off of it. 
     Since the patient pressure sensors are critical components to patient safety, it is very important to monitor whether these sensors are functioning properly. Although prior machines have attempted to accomplish this monitoring by checking the pressure readings from the sensors, such tests are not foolproof, because the varied nature of the normal, expected readings may fool one to believe that the sensors are working properly when actually they are not. 
     Therefore this sensor monitoring should be independent of the pressure measurements. In a preferred embodiment of the invention, the pressure sensors are monitored through an A-to-D converter (“ADC”) having two dedicated current sources, one for each sensor. On command, each ADC will source current (instead of acquiring data, as is usual case) and monitor how this current flows (or fails to flow) through each sensor. This independent monitoring of the pressure sensors would guarantee patient safety. Since normal treatments typically run overnight, the ability to continually double-check the very pressure sensors that monitor patient safety is indeed desirable. 
     The User Interface 
     One important part of a patient-controlled PD machine is the user interface, shown in  FIG. 7 . A common problem with prior art machines is that the patient loses track of the mode in which the machine is operating. In the invention, the touch screen display has at least two portions: one is a mode-indicating portion  80 , and the other is an operation descriptive portion  82 . 
     The mode-indicating portion  80  has a plurality of touch sensitive indicia  84 ,  86 ,  88 ,  90 , and  92 , each indicating the mode in which the machine is operating to keep the patient continually informed of which one of at least three operating modes the machine is operating in. These modes as illustrated in the preferred embodiment shown in  FIG. 7 . By way of example and not of limitation, the modes may include: a treatment mode  84 , during which dialysis is taking place; a settings mode  86 , where the treatment type settings of the PD machine are displayed and can be modified by the patient; a diagnostic mode  88  where the operation of the machine is being diagnosed; a patient data mode  90 , where patient data is displayed; and treatment history mode  92 , where prior treatment of the patient is displayed. 
     During operation under any of these modes, the operation descriptive portion  82  of the display changes to display details of the specific operation being carried out within the selected mode. Generally, the descriptive portion shows helpful information to guide the user in operating the machine. For example, during treatment, when the treatment mode indicator is highlighted, as shown in  FIG. 7 , the descriptive portion  82  shows the patient that the next required step is to “Push open cassette door.” Alternatively, the descriptive portion may show the direction of fluid flow, or provide an indication of the extent of treatment completion or other description of the current stage of treatment. The same kind of descriptions is provided for various diagnostic operations which take place in the diagnostic mode. 
     All five illustrated mode indicia in the mode portion  80  of the screen, for each of the five operating modes of the preferred embodiment, always remain visible to the patient, with the mode that the machine is currently operating in being highlighted in some manner, as shown in  FIG. 7  for the treatment mode indicator  84 . 
     The operating mode is changed by the patient by touching one of the indicia on the screen different from the one (“treatment” in  FIG. 7 ) that is currently highlighted. Unless there is some reason, such as safety or otherwise, that the mode must not be changed at that time, the mode will change to the new mode when the patient touches the different icon, and the newly selected icon  88 , “diagnostics” as shown in  FIG. 8 , will be highlighted and the “treatment” icon  84  for the prior operating mode will no longer be highlighted, as shown in  FIG. 8 . 
     Then the descriptive portion  96  of the touch screen, shown in  FIG. 8 , will display information pertaining to the new “diagnostics” mode of operation, such as a “treatment recovery warning” shown in  FIG. 8 . Icons  84 ,  86 ,  90  and  92  for all the other four possible modes in the preferred embodiment will remain displayed, but not highlighted, so the patient always knows (1) what mode the machine is operating in; and (2) what other possible operating modes exist. 
     The invention has been described in terms of particular embodiments. Other embodiments are within the scope of the following claims. For example, steps of the invention can be performed in a different order and still achieve desirable results.