Patent Publication Number: US-2013237897-A1

Title: Apparatus and Method for Venting Gas from a Liquid

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/609,587, filed on Mar. 12, 2012. The entire teachings of U.S. Provisional Application No. 61/609,587, filed on Mar. 12, 2012 and U.S. Provisional Application No. 61/470,680, filed on Apr. 1, 2011, are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Hemodialysis is the diffusive transfer of small solutes out of blood plasma by diffusion across a selectively permeable membrane. Hemodialysis proceeds due to a concentration gradient across the selectively permeable membrane such that solutes diffuse from a liquid having a higher concentration to a liquid having a lower concentration. Hemodialysis removes toxic substances, metabolic waste, and excess fluid from the bloodstream using an extracorporeal circuit with components designed to perform ultrafiltration and diffusion on the blood. Before the blood is returned to the body, air bubbles are removed from the blood to inhibit embolisms. 
     Gas venting chambers for hemodialysis systems have been disclosed in the art. For example, a conventional system is disclosed in U.S. Pat. No. 7,871,391, which describes a chamber for use in an extracorporeal liquid system. The conventional system includes a semipermeable membrane at the top of the gas vent chamber that allows gas in the liquid to vent from the chamber. In such a system, it is important to minimize contact between the liquid (e.g., blood) and the semipermeable membrane. Should the blood contact the membrane, proteins present within the blood can be deposited on the membrane, thus clogging the membrane and decreasing the ability of gas (e.g., air) to exit through the membrane. 
     Despite the fact that there are existing systems for venting a gas from a liquid, there is a need for improved systems that are reliable, affordable, and simple to use in either a clinical setting or in the home. In particular, there is a need for an apparatus and method for venting gas from a liquid that prevents the liquid in the gas collection chamber from contacting a semipermeable membrane covering the outlet where the gas vents to the atmosphere. 
     In addition, normal operation of a hemodialysis machine results in gas build-up in a gas collection chamber that is vented through a semipermeable membrane. The continual release of gas through the semipermeable membrane can tend to degrade the membrane due to the pressure applied across the membrane by the gas. Thus there is also a need for an apparatus and method for prolonging the lifetime of the semipermeable membrane. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention relate to a gas venting apparatus and a method for venting gas, which are applicable to a wide variety of medical liquid delivery systems. The embodiments discussed below, however, are directed generally to dialysis, such as hemodialysis (“HD”). 
     In one embodiment of this invention, an apparatus for venting gas contained in a liquid flowing in a liquid flow circuit includes a gas collection chamber located within the liquid flow circuit so that liquid flows through the chamber allowing gas to separate from the liquid and establish a gas-liquid interface within the chamber. A gas vent chamber is provided at the top of the gas collection chamber through which gas within the chamber can be released. The gas collection chamber and the gas vent chamber can be a single unit. A lower detector located at either the gas collection chamber or the gas vent chamber, and an upper detector located at either the gas collection chamber or the gas vent chamber are provided. The lower detector is located below the upper detector. The lower and upper detectors are capable of detecting gas and liquid. A clamp is provided in the gas vent chamber either between the lower and the upper level detectors or above both level detectors. The apparatus also includes a control apparatus for opening and closing the clamp in response to whether the lower and upper detectors detect gas or liquid. 
     In another embodiment of this invention, an automated method for venting a gas contained in a liquid flowing in a liquid flow circuit includes flowing liquid into a gas collection chamber located within the liquid flow circuit so that liquid flows through the gas collection chamber allowing gas to separate from the liquid and establish a gas-liquid interface within the gas collection chamber, detecting whether liquid is present at a lower position in either the gas collection chamber or a gas vent chamber by a lower level detector for detecting gas and liquid, opening a clamp if a liquid is not present at the lower position, detecting whether liquid is present at an upper position in either the gas collection chamber or the gas vent chamber by an upper level detector for detecting gas and liquid, and closing the clamp if liquid is present at the upper position. The clamp can be located in the gas vent chamber either between the lower level detector and the upper level detector or above both level detectors. 
     In another embodiment of this invention, an apparatus for controlling pressure differential across a semipermeable membrane sealing an exit pathway from a gas venting chamber is disclosed. The apparatus includes a first pressure sensor that senses the pressure along an exit pathway from a gas venting chamber, a valve operably connected to a gas source, the exit pathway from the gas collection chamber, and the atmosphere, and a control apparatus connected to the first pressure sensor and the valve. The control apparatus can vary the gas source in order to vary the amount of backpressure applied across the semipermeable membrane. A gas pump or pressurized gas, such as pressurized air or nitrogen, can provide the gas source. The apparatus can further include a second pressure sensor that senses the inlet or outlet pressure. The control apparatus can compare the pressure measured by the first pressure sensor and the second pressure sensor and control the gas source so that the amount of pressure along the exit pathway is less than the pressure measured by the second pressure sensor. The pressure sensed by the second pressure sensor can be approximately 100 mmHg to approximately 500 mmHg, or approximately 100 mmHg to approximately 200 mmHg. The apparatus can further include a lower level detector located in either the gas collection chamber or the gas vent chamber, and an upper level detector located in either the gas collection chamber or the gas vent chamber. The lower level detector can be located below the upper level detector, and the lower and upper level detectors can be capable of detecting gas or liquid. The control apparatus can open the valve connection to the atmosphere when the lower level detector detects gas and close the valve connection to the atmosphere when the upper level detector detects liquid. 
     In another embodiment, an apparatus for controlling the pressure differential across a semipermeable membrane sealing an exit pathway from a gas venting chamber that receives liquid-releasing gas is disclosed. The apparatus includes a gas source for applying pressure to the exit pathway side of the semipermeable membrane, a valve operably connected to the gas source, the exit pathway, and the atmosphere, a first pressure sensor that senses the pressure along the exit pathway side, a second pressure sensor that detects the pressure in the gas venting chamber, and a control means connected to the first and second pressure sensors and the valve. The control means can vary the gas source in order to vary the amount of backpressure applied across the semipermeable membrane. 
     In another embodiment, an automated method for controlling pressure differential across a semipermeable membrane includes detecting an inlet or outlet pressure, detecting a pressure along an exit pathway, comparing the inlet or outlet pressure to the exit pathway pressure with a control apparatus, increasing the amount of pressure supplied by a gas source if the inlet pressure exceeds the exit pathway pressure by a first predetermined amount, and decreasing the amount of pressure supplied by the gas source if the inlet pressure is too close to the exit pathway pressure by a second predetermined amount. 
     In another embodiment, an automated method for controlling pressure differential across a semipermeable membrane includes detecting an inlet or outlet pressure, detecting a pressure along an exit pathway, comparing the inlet or outlet pressure to the exit pathway pressure with a control apparatus, opening a valve connection to the atmosphere if the inlet pressure exceeds the exit pathway pressure by a first predetermined amount, and closing the valve connection to the atmosphere if the inlet pressure is too close to the exit pathway pressure by a second predetermined amount. 
     In another embodiment, an automated method for controlling pressure differential across a semipermeable membrane includes detecting whether liquid is present at a lower level detector, opening a valve connection to the atmosphere if liquid is not present at the lower level detector, detecting whether liquid is present at an upper level detector, and closing the valve connection to the atmosphere if liquid is present at the upper level detector. 
     In another embodiment of this invention, an extracorporeal hemodialysis circuit includes arterial tubing for receiving unfiltered blood from a patient, venous tubing for providing filtered blood to a patient, a dialyzer, and an apparatus for venting gas contained in a liquid. The dialyzer and apparatus for venting gas are located within the extracorporeal hemodialysis circuit so that blood flows from the patient, through the arterial tubing, through the dialyzer, through the apparatus for venting gas, and towards the venous tubing. 
     In another embodiment of this invention, an extracorporeal hemodialysis circuit includes arterial tubing for receiving unfiltered blood from a patient, venous tubing for providing filtered blood to a patient, a dialyzer, and an apparatus for controlling pressure differential. The dialyzer and apparatus for venting gas are located within the extracorporeal hemodialysis circuit so that blood flows from the patient, through the arterial tubing, through the dialyzer, through the apparatus for controlling pressure differential, and towards the venous tubing. 
     Embodiments described herein can provide advantages in operating a hemodialysis machine. By opening and closing a clamp to control the height of a gas-liquid interface in the gas collection chamber, liquid (such as blood or saline) can be prevented from contacting the semipermeable membrane positioned at the upper portion of the gas vent chamber. By reducing the pressure differential across the semipermeable membrane, the lifetime of the semipermeable membrane can be prolonged. These advantages result in decreased operator supervision, maintenance, and operating costs, as well as improved sterility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a typical extracorporeal liquid circuit of a hemodialysis system. 
         FIG. 2  is a side view of a chamber for venting gas having two level detectors and a clamp. 
         FIG. 3  is a side view of a hemodialysis cassette for venting gas having two level detectors and a clamp. 
         FIG. 4  illustrates a control algorithm for venting gas in a system having two level detectors and a clamp. 
         FIG. 5  is a schematic illustrating the connectivity between a gas collection chamber, a pressure transducer, a gas pump, and a valve. 
         FIG. 6  illustrates a control algorithm for a first mode of operating an active gas venting system. 
         FIG. 7  illustrates a control algorithm for a second mode of operating an active gas venting system. 
         FIG. 8  illustrates a control algorithm for a third mode of operation of an active gas venting system. 
     
    
    
     The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Extracorporeal Circuit 
       FIG. 1  illustrates a typical extracorporeal hemodialysis circuit  100 , which includes tubing through which the blood flows and components for filtering and performing hemodialysis on the blood. Blood flows from a patient  105  through arterial tubing  110 . After exiting the patient, blood drips into a drip chamber  115  where a connecting tube  116  from the drip chamber  115  attaches to an arterial pressure sensor assembly  120  that determines the pressure of the blood on the arterial side of the circuit  100 . The arterial pressure assembly  120  can include a pressure transducer  130  so that the pressure of blood flowing through the circuit  100  on the arterial side can be monitored. 
     A pump  160 , such as a peristaltic pump, forces the blood to continue along the path through the circuit  100 . After exiting the drip chamber  115 , the blood then flows through tubing  117  to a dialyzer  170 , which separates waste products and excess fluid from the blood. After passing through the dialyzer  170 , the blood flows through venous tubing  180  towards a gas collection and venting chamber  230  in which gas (e.g., air) bubbles in the blood can separate from the blood and escape by passing across a semipermeable membrane  270  before the blood continues to the patient  105 . As used herein, the term “semipermeable membrane” refers a membrane that is permeable to gas, but impermeable to liquid. Thus, the semipermeable membrane operates to inhibit liquids within the chamber  230  from escaping while allowing gas to escape from the chamber  230 . After leaving the chamber  230 , the blood travels through a venous line  190  and back to the patient  105 . The gas collection apparatus and cassette subsequently described herein can be used with an extracorporeal hemodialysis circuit and device, as illustrated in  FIG. 1 , or with other blood dialysis systems. 
     Gas Collection Apparatus and Cassette 
       FIG. 2  illustrates an exemplary embodiment of a gas venting apparatus  200  having a chamber, two level detectors, and a clamp. The gas venting apparatus  200  has a liquid inlet  210  and a liquid outlet  220 . In  FIG. 2 , the liquid inlet  210  is positioned below the liquid outlet  220 , but the liquid inlet  210  can also be positioned above the liquid outlet  220  or at approximately the same height as the liquid outlet  220 . A liquid, such as blood, enters through the liquid inlet  210  and leaves through the liquid outlet  220 . The liquid enters the volume of the gas collection chamber  230  when the clamp  250  is in an open position. 
     The lower level detector  240  and the upper level detector  260  can detect the presence of a gas or a liquid. The clamp  250  can open or close based on signals from the lower level detector  240  and the upper level detector  260 . During operation, the gas collection chamber initially fills with a liquid, such as blood, if the clamp  250  is in an open position. Upon detecting the presence of liquid at the upper level detector  260 , the clamp  250  can close. The liquid can contain gas bubbles. Over time, the gas bubbles rise to the surface and begin to fill the gas collection chamber with the gas, thereby creating an interface between the gas and the liquid. As gas bubbles continue to rise to the surface, the interface between the gas and the liquid moves vertically down the gas collection chamber. 
     When the lower level detector  240  detects the presence of a liquid, the clamp  250  remains closed. When the gas-liquid interface crosses the location where the lower level detector is positioned, the level detector can send a signal indicative of the presence of a gas. The signal can be sent from the lower level detector  240  to a control apparatus, which is described with respect to  FIG. 4 , that receives the signal. Upon receiving the signal, the control apparatus can send a signal to the clamp  250  instructing the clamp to open. 
     Once the clamp  250  opens, the gas in the gas collection chamber  230  can travel through the gas venting chamber  270 . The gas venting chamber  270  has a semipermeable membrane  275  positioned at the upper portion and a gas outlet  280 . In some embodiments, the outlet can vent gas to the atmosphere. When the clamp  250  is open, the gas collection chamber  230  is in fluid communication with the gas vent chamber  270 , which in turn is in fluid communication with the atmosphere. 
     Ordinarily, the pressure in the gas collection chamber is greater than atmospheric pressure. Thus when the clamp  250  is open, the gas-liquid interface moves vertically up the gas collection chamber, which releases accumulated gas to the atmosphere. 
     Similar to the lower level detector, the upper level detector  260  detects the presence of a gas or a liquid. When the clamp  250  is open, the gas-liquid interface can move vertically up the chamber and can cross the location where the upper level detector  260  is positioned. When the upper level detector  260  detects the presence of a gas, the clamp  250  can remain open, thus permitting further venting of gas to the atmosphere. When the upper level detector  260  detects the presence of a liquid, the clamp  250  can close, thus preventing the liquid from reaching the gas outlet  280 . In some embodiments, the upper level detector can send a signal indicative of the presence of a gas or a liquid to the control apparatus, which is described with respect to  FIG. 4 . The control apparatus can then send a signal to the clamp that opens or closes the clamp. 
       FIG. 3  illustrates a gas venting cassette  300  having a chamber, two level detectors, and a clamp. The apparatus of  FIG. 3  is similar to that of  FIG. 2 , except the apparatus of  FIG. 3  is a cassette  300  including a liquid inlet  310 , a liquid outlet  320 , a gas collection chamber  330 , a lower level detector  340 , a clamp  350 , an upper level detector  360 , a gas vent chamber  370 , a semipermeable membrane  375 , and a gas outlet  380 . The embodiment of  FIG. 3  operates similarly to the embodiment of  FIG. 2 . The primary difference between the embodiments of  FIG. 2  and  FIG. 3  is in the shape of the gas collection chamber. 
     The term “clamp” is used in its broadest sense, meaning that it is an element that is capable of opening and closing the gas vent chamber. In one embodiment, the clamps ( 250  and  350 ) can be pinch clamps that, in a closed position, exert pressure on a tube to prevent the passage of gas or liquid. In another embodiment, the clamps ( 250  and  350 ) can be balloon clamps. A wide variety of suitable devices that can open and close the gas vent chamber in response to a signal can be used. The clamp can completely close and completely open the gas vent chamber, or the clamp can partially close and partially open the gas vent chamber. 
     In one embodiment, the level detectors ( 240 ,  260 ,  340 , and  360 ) can detect the density of a fluid. For example, the level detectors can be ultrasonic level detectors. Liquids have a higher density than gasses. Thus, the level detector can send a signal indicative of the density of a fluid, wherein the density is indicative of the presence of a gas or a liquid. In some embodiments, the liquid can be blood. In some embodiments, the gas can be air. 
     The gas bubbles can be dissolved in the liquid, or the gas bubbles can be too large to be considered dissolved in the liquid. In some cases, the gas bubbles can be observable with the naked eye. In other cases, the gas bubbles can on the order of magnitude of a millimeter or less. 
     The gas collection chamber ( 230  and  330 ) and the gas vent chamber ( 270  and  370 ) are not necessarily separate pieces. Rather, the two can be an integrated component. In other words, the gas collection chamber and the gas vent chamber can be a single, integral unit. While  FIGS. 2 and 3  illustrate the lower level detector ( 240  and  340 ) positioned at the gas collection chamber ( 230  and  330 ) and the upper level detector ( 260  and  360 ) positioned at the gas vent chamber ( 270  and  370 ), the lower and upper level detectors ( 240 ,  260 ,  340 , and  360 ) can be positioned at either the gas collection chamber ( 230  and  330 ) or at the gas vent chamber ( 270  and  370 ). 
     Typically, the gas outlet ( 280  and  380 ) is capped with a semipermeable membrane ( 275  and  375 ) made from polymers such as a polytetrafluoroethylene (PTFE) or polyethylene (PE), though other suitable semipermeable membranes can also be used. Typically, the semipermeable membrane is a hydrophobic membrane. Typically, the semipermeable membrane is a microporous membrane. Preferably, the semipermeable has a pore size between 0.1 micron to 0.22 microns. Suitable membranes are manufactured by W. L. Gore &amp; Associates, Inc., Millipore Corporation, and Pall Corporation. 
     The chamber embodiment of  FIG. 2  and the cassette embodiment of  FIG. 3  can be made of a wide variety of biocompatible materials suitable for medical applications, and can be formed into the appropriate shape by any processes suitable for medial applications. For example, the cassette can typically be formed from polymers such as polyvinyl chloride (PVC) and polycarbonate (PC). 
     Typically, the liquid inlet ( 210  and  310 ) and the liquid outlet ( 220  and  320 ) connect to tubing. As shown in  FIG. 1 , the liquid inlet ( 210  and  310 ) connects to tubing  180 , and the liquid outlet ( 220  and  320 ) connect to tubing  190 . The tubing can be of a wide variety of biocompatible materials suitable for medical application. 
     Operation of the Gas Collection Apparatus and Cassette 
       FIG. 4  is a flowchart illustrating steps in a method for venting gas in a system having a gas collection chamber, two level detectors, and a clamp. In one embodiment, the clamp is initially in a closed position. Optionally, the clamp can be closed if it is open (step  405 ). Liquid flows into the gas collection chamber (step  410 ). The liquid enters the gas collection chamber via a liquid inlet, such as liquid inlet  210  or  310 . The lower level detector detects the presence of a liquid or gas (step  420 ). If liquid is present at the lower level detector (i.e., if gas is not present), the clamp remains closed and liquid continues to flow into the gas collection chamber (step  410 ). If liquid is not present at the lower level detector (i.e., if gas is present at the lower level detector), the clamp opens (step  430 ). 
     While the clamp is open, gas in the gas collection chamber is in fluid communication with the atmosphere via a gas venting chamber, and gas can vent to the atmosphere. As gas vents to the atmosphere, the gas-liquid interface rises, and the upper level detector detects the presence of a liquid or a gas (step  440 ). If liquid is not present at the upper level detector (i.e., if gas is present), the clamp remains open (step  430 ), and the upper level detector continues to detect the presence of liquid or gas (step  440 ). If liquid is present at the upper level detector (i.e., if gas is not present), then the clamp closes (step  460 ). Liquid continues to flow into the gas collection chamber (step  410 ) and the cycle repeats. 
     While  FIG. 4  describes detecting whether liquid is present at the upper level detector at step  440 , the upper level detector can detect the presence of liquid or gas at the outset. However, the system and method do not need to consider the presence or absence of gas or liquid at the upper level detector until the clamp has opened. 
     While steps  420  and  440  describe detecting whether liquid is present at the level detectors, it is equivalent to detect whether gas is present at the level detectors. In such case, the relative placement of the “Yes” and “No” answers to the inquiry are reversed. In other words, if gas is present at the lower level detector (step  420 ), then the clamp opens (step  430 ), and if gas is not present, then the clamp remains closed (step  410 ). Similarly, if gas is present at the upper level detector (step  440 ), then the clamp remains open (step  430 ), and if gas is not present, then the clamp closes (step  460 ). 
     Apparatus for Controlling Pressure Differential 
     Another embodiment is an apparatus and method for controlling the pressure differential across a semipermeable membrane by providing backpressure to the semipermeable membrane (i.e., in the direction opposite to the direction applied by the flow of gas exiting the system). Thus, the present apparatus and method can prolong the lifetime of the semipermeable membrane as well as provide a means to actively control the level of liquid, such as blood and/or saline, in the gas venting chamber. 
       FIG. 5  is a schematic illustration of the connectivity of the active venting apparatus and method  500 . The gas collection chamber  530  collects gas that has been released from liquid entering the gas collection chamber  530  via inlet  510  and exiting via outlet  520 . Pressure sensors  511  and  521  can monitor the inlet and outlet pressures. In a typical hemodialysis setup, the liquid is blood  505  and saline  515  that meet at an interface  522 . The gas collection chamber  530  can also have a lower level detector  540  and an upper level detector  560 , as described above. As illustrated in  FIG. 5 , the upper level detector  560  is positioned to sense gas or liquid near the top of the gas collection chamber  530 . As previously described, the level detectors ( 540  and  560 ) can detect the density of a fluid. For example, the level detectors can be ultrasonic level detectors. 
     Near the top of the gas collection chamber is a semipermeable membrane  542  that serves as a barrier between the hemodialysis circuit, such as the extracorporeal hemodialysis circuit  100  of  FIG. 1 , and the environment. The semipermeable membrane  542  can be a hydrophobic membrane. When the valve  572  is open, gas passes across the semipermeable membrane  542  and along the exit pathway  545  toward branch point  550 . Branch  555  extends to a pressure sensor  561  that monitors the pressure along the exit pathway  545 . The pressure sensor  561  can be referred to as the machine-side pressure sensor to distinguish it from the bloodline pressure sensors (e.g.,  511  and  521 ). The machine-side pressure sensor  561  ordinarily does not directly contact biohazardous liquids. The exit pathway extends along pathway  565  into the T-connector  570 , which extends to the atmosphere  575  via valve  572  and to a gas pump  590  via pathway  580 . Alternatively, a source of pressurized gas, such as pressurized air or nitrogen, can be provided instead of a gas pump  590 . 
     During operation of a hemodialysis machine, the gas collection chamber  530  becomes increasingly filled with gas, and the gas-liquid interface  562  moves downward. Gas exits the gas collection chamber  530  by flowing across the semipermeable membrane  542  and along the exit pathway  545  and  565  into the T-connector  570 . The gas vents to the atmosphere  575  via valve  572 . 
     In a first and second mode of operation, the machine side pressure sensor  561  and the inlet pressure sensor  511  can be operably connected to a control apparatus, which is described with respect to  FIGS. 6 and 7 . The control apparatus can compare the inlet pressure as measured by the inlet pressure sensor  511  to the pressure as measured by the machine-side pressure sensor  561 . The pressure along the exit pathway  545  can be adjusted by controlling the gas source  590  to increase or decrease the pressure. The pressure along the exit pathway  545  can also be adjusted by opening or closing valve  572 . Thus, the amount of pressure applied to the backside of the semipermeable membrane can be adjusted so that it is only slightly less than the pressure applied on the gas vent side. Alternatively, the outlet pressure sensor  521  can be operably connected to a control apparatus, and the control apparatus can compare the outlet pressure as measured by the outlet pressure sensor  521  to the pressure as measured by the machine-side pressure sensor  561 . In addition, both the inlet and outlet pressures can be measured and compared to the machine-side pressure sensor  561 . Typical venous or arterial pressures are 100 mmHg to 500 mmHg, or more typically 100 mmHg to 200 mmHg. 
       FIG. 6  illustrates a control algorithm for a first mode of operating an active venting system, such as that illustrated in  FIG. 5 . The inlet pressure and the pressure along the exit pathway are detected (steps  610  and  620 ). Alternatively, the outlet pressure can be detected rather than the inlet pressure, or the outlet pressure can be detected in addition to the inlet pressure. The sequence of steps  610  and  620  can be reversed, such that the pressure along the exit pathway is detected before the inlet pressure is detected. Alternatively, steps  610  and  620  can be combined into a single step that is performed simultaneously. Next, the control apparatus compares the inlet or outlet pressure to the exit pathway pressure (step  630 ). If the inlet or outlet pressure exceeds the exit pathway pressure by a first predetermined amount, then the amount of pressure supplied to the exit pathway (e.g., by a gas pump or source of pressurized gas) is increased. If the inlet or outlet pressure is too close to the exit pathway pressure (i.e., within a second predetermined amount), then the amount of pressure supplied to the exit pathway is decreased (step  640 ). The first and second predetermined amounts can be selected by an operator of a hemodialysis machine. 
       FIG. 7  illustrates a control algorithm for a second mode of operating an active venting system, such as that illustrated in  FIG. 5 . The inlet pressure and the pressure along the exit pathway are detected (steps  710  and  720 ). Alternatively, the outlet pressure can be detected rather than the inlet pressure, or the outlet pressure can be detected in addition to the inlet pressure. The sequence of steps  710  and  720  can be reversed, such that the pressure along the exit pathway is detected before the inlet pressure is detected. Alternatively, steps  710  and  720  can be combined into a single step that is performed simultaneously. Next, the control apparatus compares the inlet or outlet pressure to the exit pathway pressure (step  730 ). If the inlet or outlet pressure exceeds the exit pathway pressure by a first predetermined amount, then the valve  572  can open the connection to the atmosphere  575 . If the inlet or outlet pressure is too close to the exit pathway pressure (i.e., within a second predetermined amount), then the valve  572  can close the connection to the atmosphere  575  (step  640 ). The first and second predetermined amounts can be selected by an operator of a hemodialysis machine. The valve  572  that opens and closes the connection to the atmosphere  575  need not be fully closed or fully opened, but can be partially opened or partially closed relative to the difference between the inlet pressure exceeds the exit pathway pressure. 
     In a third mode of operation, the lower level detector  540  and upper level detector  560  can detect the presence of a liquid or a gas, as described above. When the lower level detector detects the presence of a gas, a signal can be sent to a control apparatus, which is described with respect to  FIG. 8 . The control apparatus can send a signal to open and close the valve  572  controlling the connection to the atmosphere  575 . For example, the control apparatus can open the valve  572  when the lower level detector  540  detects gas and closes the valve  572  when the upper level detector  560  detects liquid. 
       FIG. 8  illustrates a control algorithm for a third mode of operating an active venting system, such as that illustrated in  FIG. 5 . At the outset, the valve connection to the atmosphere is closed (step  810 ). If liquid is present at the lower level detector, the valve remains closed (step  820 ). If liquid is not present at the lower level detector, the valve connection to the atmosphere opens (step  830 ). Then, if liquid is present at the upper level detector (step  840 ), the valve is closed (step  810 ). If liquid is not present at the upper level detector (step  840 ), then the valve connection to the atmosphere remains open (step  830 ). 
     The apparatus for controlling pressure differential can serve to reduce the pressure differential across the semipermeable membrane  542 , which can reduce the damage to the membrane caused by excessive pressure differential across the membrane. In the inventors&#39; experience, the semipermeable membrane can be fragile and can tend to leak liquid, particularly over the course of an operation of extended duration (e.g., 24 to 72 hours). By reducing or preventing the incidence of rips, tears, breaks, or other leaks in the membrane  542 , the operational lifetime of the membrane can be increased, thereby reducing operating costs and improving ease of operation in a clinical or home setting. The increased reliability provided by the active venting system and method can reduce the amount of operator intervention required during a typical hemodialysis procedure. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.