Patent Publication Number: US-2021187173-A1

Title: Pressure control unit for an ophthalmic surgical system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pressure control unit, in particular a pressure control unit for use in an aspiration/irrigation system, such as an ophthalmic aspiration/irrigation system. In a further aspect the present invention relates to a method for regulating the pressure in an ophthalmic aspiration/irrigation system. 
     BACKGROUND ART 
     During ophthalmic surgery, fluid is typically delivered into the eye and aspirated therefrom. Generally, a pressure source is used to move fluid to the eye and a pressure source or flow drain to move fluid from the eye. For aspiration procedures, a negative pressure may be employed to draw fluid from the eye into an aspiration chamber. For irrigation procedures, positive pressure is applied to deliver fluid from an infusion source to the eye. 
     U.S. patent publication 5,674,194 discloses a pressure control unit configured to generate a vacuum for a suction probe of which a desired vacuum level can be manipulated by an input device of the pressure control unit. The system comprises a regulated high pressure source connected via an input manifold to a proportional valve, wherein the degree to which the proportional valve is open determines the pressure and air flow rate through a venturi connected to an output of the proportional valve. The pressure control unit further comprises a cassette volume connected to the suction probe and to the venture, wherein the venturi allows a vacuum to be drawn at the suction probe. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved pressure control unit for an ophthalmic surgical system, such as an ophthalmic aspiration/irrigation system, wherein the pressure control unit minimizes air consumption and circumvents the use of an external air source. The pressure control unit of the present invention further aims to provide fast pressure control of aspiration or irrigation pressure with maximum precision. 
     According to an aspect of the present invention, there is provided a pressure control unit for an aspiration/irrigation system having a chamber with an upper part for storing air and a lower part for storing a surgical fluid to be irrigated or aspirated. The pressure control unit comprises a negative pressure source, a positive pressure source and an adjustable valve arrangement, wherein the adjustable valve arrangement comprises a vacuum port connected to the negative pressure source and a pressure port connected to the positive pressure source. The adjustable valve arrangement further comprises a main port in controllable fluid communication with the vacuum port and the pressure port, wherein the main port is connected to the chamber. 
     The adjustable valve arrangement is adapted to control the air pressure and air flow to and from the chamber through the main port corresponding to the intensity at which the negative pressure source and/or the positive pressure source is/are allowed to be active and supply the negative pressure and/or positive pressure to the vacuum port and pressure port, respectively. In other words the amount of opening ratio of each valve in the adjustable valve arrangement determines their individual contribution and as such the coupling between the pressure sources and the main port. 
     Since regulating the pressure within the chamber is achieved through both negative and/or positive pressure sources (negative and positive pressure being used with respect to an ambient pressure), the pressure control unit of the present invention is able to provide dynamic pressure control within the chamber with exceptionally short response times and precision, wherein the adjustable valve arrangement is configured to provide any required pressure to the chamber during an ophthalmic procedure. 
     Another advantage of the pressure control unit is that it is conveniently configured to connect to, for example, an existing chamber, which may be a disposable chamber, of an ophthalmic aspiration/irrigation system. In this way an ophthalmic surgical system can be upgraded using the pressure control unit for increasing irrigation and/or aspiration response times as well as increased accuracy at which required pressures can be provided to the chamber. 
     According to a further aspect of the present invention, there is provided a method for regulating the pressure in an ophthalmic surgical system comprising a chamber for exchanging surgical fluid and a pressure control unit having a negative pressure source, a positive pressure source and an adjustable valve arrangement, the valve arrangement comprising a vacuum port connected to the vacuum source and a pressure port connected to the pressure source, and wherein the valve arrangement further comprises a main port configured for fluid communication with the chamber; wherein the method comprises the steps of determining a desired chamber pressure; delivering the desired pressure to the chamber via the main port; measuring the pressure within the chamber; adjusting the adjustable valve arrangement to deliver positive and/or negative pressure to maintain the desired pressure within the chamber. 
    
    
     
       SHORT DESCRIPTION OF DRAWINGS 
       The present invention will be discussed in more detail below, with reference to the attached drawings, in which 
         FIG. 1  shows an embodiment of an aspiration system for aspirating ocular material during an ophthalmic surgical procedure; 
         FIG. 2  shows an embodiment of an irrigation system for irrigating the eye during an ophthalmic surgical procedure; 
         FIG. 3  shows a schematic view of a controller for use in an ophthalmic aspiration/irrigation system; and 
         FIGS. 4A and 4B  each show a schematic view of a pressure control unit for an ophthalmic aspiration/irrigation system according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described in detail. The skilled person will understand that devices and methods described herein are non-limiting exemplary embodiments and that the scope of protection is defined by the claims. For example, although the present invention is described with respect to ophthalmic aspiration and/or irrigation procedures, the skilled person readily understands that the present invention may be used in other applications, for example in other aspiration and/or irrigation systems, e.g. fine needle aspiration procedures. The skilled person will also understand that the features illustrated or described in connection with one exemplary embodiment may be combined with features described in other exemplary embodiments. Such modifications and variations are included within the scope of the present disclosure. 
     The pressure control unit according to the present invention may be employed in an exemplary embodiment of an aspiration system  100  shown in  FIG. 1  or in an exemplary embodiment of an irrigation system  200  as shown in  FIG. 2 . As shown, the aspiration system  100  and the irrigation system  200  according to embodiments of the present invention each comprise a cassette  8 , sometimes referred to as a surgical cassette, having a chamber  10 . The chamber  10  is configured to store a fluid F in a lower part  10   b  of the chamber  10  and air A in an upper part  10   a  of the chamber  10 , the upper part  10   a  being the remaining space at the top of the chamber  10  (see also the description of  FIGS. 4A and 4B  below). 
     A variable pressure source  20  is coupled to the chamber  10  to control the pressure within the chamber  10  and thus the flow of fluid between the chamber  10  and an eye  1  of a patient. The skilled person in the art will appreciate that in the aspiration system  100  shown in  FIG. 1  the variable pressure source  20  is at least capable of applying a negative pressure to the chamber  10  to draw fluid from the eye  1  into the chamber  10  via an aspiration line  116 , i.e. fluid moves in the direction of arrow F 1  in  FIG. 1 . 
     In an irrigation system  200 , the variable pressure source  20  is at least capable of applying a positive pressure to the chamber  10  to deliver fluid from the chamber  10  to the eye  1  via an irrigation line  214 , i.e. fluid moves in the direction of arrow F 2  in  FIG. 2 . Of course, a variable pressure source  20  capable of applying a positive pressure and a negative pressure to the chamber  10  is advantageously versatile and can be used in both irrigation and aspiration procedures. The cassette  8  as depicted in  FIGS. 1 and 2  may further comprise a pressure sensor  30  which is in fluid communication with the chamber  10  to monitor the air pressure within the chamber  10 . 
     The aspiration system  100  and the irrigation system  200  typically comprise a pump  212  that controls the egress and ingress of fluid F from the chamber  10 . The pump  212  may be any pump suitable for this purpose, such as a peristaltic pump of any type known in the art. The skilled person will appreciate that in an aspiration system  100  as shown in  FIG. 1 , the pump  212  is configured to deliver aspirated fluid from the chamber  10  to a drain  120 , which is coupled to the chamber  10  by way of a drainage line  118 . In the irrigation system shown in  FIG. 2 , the pump  212  is configured to deliver irrigation fluid from an infusion bottle  210  to the chamber  10  by way of an infusion line  218 . The cassette  8  also comprises a fluid level indicator  50  to indicate the level of fluid F within the chamber  10 . 
     As shown in  FIG. 2 , a controller  40  may be provided in communication with the variable pressure source  20 , pressure sensor  30 , the fluid level indicator  50 , and the pump  212 . The controller  40  is configured to control the pressure and the fluid level within the chamber  10  based on, for example, measurements taken by the pressure sensor  30  and/or the fluid level indicator  50 , and by controlling a velocity of the pump  212 . Note that the controller  40  can likewise be provided to the aspiration system  100  of  FIG. 1  and connected to the variable pressure source  20 , pressure sensor  30 , pump  212  and the fluid level indicator  50  to control the pressure and the fluid level within the chamber  10 . 
     Referring now to the schematic of  FIG. 3 , in an embodiment the controller  40  comprises a fluid level controller  43  for maintaining the fluid level within chamber  10  within a desired range and a pressure controller  42  for maintaining a desired pressure within the chamber  10 . The desired pressure within the chamber  10  is a negative or positive pressure depending on whether aspiration or irrigation is needed. 
     The fluid level controller  43  receives fluid level information from the fluid level indicator  50  and provides a set-point to a velocity controller  220 , which controls the pump  212  to maintain the fluid level within chamber  10  within a desired range. For example, in aspiration applications the velocity controller  220  controls the rate at which fluid F is drained from the chamber  10  into the drain  120 . If the controller  40  determines based on feedback from the fluid level indicator  50  that the fluid level within the chamber  10  is too high, the controller  40  adjusts the set point of the velocity controller  220  to increase the rate/speed at which pump  212  moves fluid F from the chamber  10  to the drain  120 . 
     In irrigation applications, the velocity controller  220  controls the rate at which fluid F enters the chamber  10  from the infusion bottle  210 . If, based on feedback from the fluid level indicator  50 , the controller  40  determines that the fluid level within the chamber  10  is too low, the controller  40  adjusts the set-point of the velocity controller  220  to increase the rate/speed at which pump  212  delivers irrigation fluid from the infusion bottle  210  to the chamber  10 . 
     The pressure controller  42  receives pressure information from the pressure sensor  30  and adjusts the pressure delivered by the variable pressure source  20  to maintain the pressure within the chamber  10  at the desired level. 
     Advantageously, the controller  40  shown in  FIG. 3  can also allow for calculation of the flow rate to and from the eye  1  without the need for a flow sensor within the aspiration line  116  or the irrigation line  214 . This is advantageous because ophthalmic surgical systems generally comprise narrow gauge irrigation/aspiration lines, across which accurate flow sensing can be challenging. However, in the aspiration and irrigation systems  100 ,  200  described above, the controller  40  may calculate the flow rate to or from the eye  1  based on all or some of the following known quantities: the fluid level within the chamber (measured by fluid level indicator  50 ); the pressure within chamber  10  (measured by pressure sensor  30 ); and the flow rate dictated by the pump  212  to and from the chamber  10 , system parameters relevant for pressure losses in the flow to/from the eye during use (e.g. tubing and needle length and diameters). 
     The above description of  FIG. 3  relates to a pressure mode of operation, wherein a user can input a set point for the desired pressure to pressure controller  42 . This may be applied both when the present invention embodiments are used for controlling irrigation to the eye, and for controlling aspiration from the eye. In a further embodiment, specifically suited for aspiration purposed, the present invention embodiments are operated in a flow control mode. In the flow control mode, a set point for the desired aspiration flow is input to the velocity controller  220 , for controlling the speed of the drainage pump  212 . The fluid level controller  43  uses the input from the fluid level indicator  50  to provide a pressure set point to the pressure controller  42  that subsequently control the variable pressure source  20  to ensure the fluid level is controlled to an internal defined set point. 
     It will be appreciated that to allow precise control of the pressure within the eye  1 , the variable pressure source  20  should be capable of providing fast pressure control with maximum precision within the chamber  10 . To that end reference is made to  FIGS. 4A and 4B , each of which show an embodiment of a pressure control system  21  for an ophthalmic irrigation and aspiration system  100 ,  200  having a chamber  10  with a lower part  10   b  for storing a surgical fluid F to be irrigated or aspirated. 
     According to the present invention, the pressure control unit  21  comprising a negative pressure source  22  (e.g. a vacuum source) and a positive pressure source  24  (e.g. a compressor). In an embodiment, the negative or positive pressure source  22 ,  24  is a membrane pump for example, although the skilled person will appreciate that other positive and/or negative pressure sources could be employed. The terms negative and positive pressure are being used herein with respect to an ambient pressure. As an alternative implementation one of the pressure sources  22 ,  24  could be actually at the ambient pressure. 
     The pressure control unit  21  is further provided with an adjustable valve arrangement  25 , wherein the valve arrangement  25  comprises a vacuum port  25   a  connected to the negative pressure source  22  and a pressure port  25   b  connected to the positive pressure source  24 . The adjustable valve arrangement  25  also comprises a main port  25   c  in controllable fluid communication with the vacuum port  25   a  and the pressure port  25   b,  wherein the main port  25   c  is connected to a chamber  10 , e.g. connected to the chamber  10  (more particularly to an upper part  10   a  of the chamber  10 ), for storing air A. As mentioned above, the chamber  10  is configured for storing and exchanging a surgical fluid F stored in a lower part  10   b  of the chamber  10 . 
     The adjustable valve arrangement  25  is adapted to control the flow of air to/from the chamber  10  through the main port  25   c  corresponding to the intensity at which the vacuum source  22  and/or the pressure source  24  is/are active. Since the valve arrangement  25  shown in  FIGS. 4A and 4B  couples the chamber  10  to a negative pressure source  22  as well as a positive pressure source  24 , the pressure control unit  21  is able to provide fast, dynamic pressure control within chamber  10  with remarkable precision. The adjustable valve arrangement  25  of the present invention is thus configured to provide virtually any pressure within the chamber  10  at any desired speed and accuracy. Factors influencing the speed and accuracy are the total air volume A and the presence (or absence) of a restriction (such as optional filter  29   a.  It is noted that the present application embodiments could also be used to control pressure in an infusion bottle instead of in the chamber  10  of the cassette  8 . 
     In an embodiment, the adjustable valve arrangement  25  is a proportionally adjustable valve arrangement allowing smooth and continuous changes in air pressure and air flow across the main port  25   c.  The proportionally adjustable valve arrange is capable of switching between and/or “blending” the negative and positive pressure sources  22 ,  24 , so that any desired pressure and air flow across the main port  25   c  can be achieved with great speed and precision. As an alternative implementation, a pulse width modulation (PWM) controlled on/off valve arrangement may be applied 
     In an exemplary embodiment as shown in  FIG. 4A , the adjustable valve arrangement  25  comprises a first adjustable valve R 1  connected between the vacuum port  25   a  and the main port  25   c  and a second adjustable valve R 2  is connected between the pressure port  25   b  and the main port  25   c.  This allows accurate selection and control of both negative pressure and positive pressure at the main port  25   c,  so that any desired pressure within the chamber  10  can be reached fast and accurately maintained (within the negative and positive pressure ranges of the vacuum source  22  and/or pressure source  24 ). 
     In an advantageous embodiment, the first adjustable valve R 1  is a first proportional valve and the second adjustable valve R 2  is a second proportional valve. Each of the proportional valves R 1 , R 2  allow for fast, continuous control to further increase speed and accuracy of the air flow across the main port  25   c.    
     Controlling the first and second adjustable/proportional valves R 1 , R 2  with e.g. a current source can be advantageous because current controlled valves may be less sensitive for temperature variations when compared to voltage controlled valves. Alternatively, the first and second adjustable/proportional valves R 1 , R 2  are position controlled valves. 
     In a further embodiment, the first and second adjustable/proportional valves R 1 , R 2  are biased with a first current to allow a bias flow in the flow path between negative pressure source  22  and positive vacuum source  24 , whilst maintaining a net zero flow through the main port  25   c  (keeping pressure in the chamber  10  at a constant level). In general, the current/flow characteristic of such a valve includes a threshold current below which the valve remains closed. Biasing the first and second adjustable/proportional valves R 1 , R 2  with a current at least equal to this threshold current allows to have a faster response time when further opening one of the valves R 1 , R 2  during control. 
     To allow control and proper setting of the adjustable valve arrangement  25 , the pressure control unit may further comprise a flow sensor  31  arranged in a bias flow path between the negative pressure source  22  and the positive pressure source  24 . The bias flow path comprises the direct connection parts between the negative pressure source  22  and the positive pressure source  24  in any of the exemplary embodiments described herein. E.g. the flow sensor  31  is arranged between the positive pressure source  24  and the pressure port  25   b  or between the vacuum source  22  and the vacuum port  25   a.  The flow sensor  31  can be any suitable flow sensor, e.g. an in-line flow sensor. 
     The flow sensor  31  may be connected to the controller  40  (or to a dedicated controller), and may be implemented as a mass flow sensor or as a volumetric flow sensor (e.g. a differential pressure based flow sensor which allows measuring a volumetric flow rate by measuring a differential pressure over a (fixed) restriction). The measurement data from the flow sensor  30  can then be used in a secondary control loop which actively controls the flow in the bias flow path (without affecting flow through the main port  25   c ). 
     Referring to  FIG. 4B , in an alternative embodiment, the adjustable valve arrangement  25  is a three-way valve arrangement T, which may be seen as a single, unitary valve manifold having three connecting ports and a valve insert adapted to control/divide air flow through and among these three connecting ports. For example, the three-way valve arrangement (T) may be seen as a single valve unit comprising the vacuum port  25   a,  the pressure port  25   b,  and the main port  25   c  to accurately control the degree in which the vacuum port  25   a  and/or the pressure port  25   b  communicate with the main port  25   c.    
     As with the first and second adjustable/proportional valves R 1 , R 2 , the three-way valve arrangement T may be current controlled, thereby reducing or avoiding temperature dependent valve sensitivities. Note that the skilled person in the art will appreciate that voltage control may still be effectively applied to control the adjustable valve arrangement  25 , in particular the first and second adjustable/proportional valves R 1 , R 2  as well as a three-way valve arrangement T as described above. 
     In order to monitor the pressure within the chamber  10 , an embodiment is provided wherein the pressure control unit  21  comprises a pressure sensor  30  in communication with the main port  25   c.  For example, in an embodiment the pressure sensor  30  is connected to a conduit arranged between the main port  25   c  and the chamber  10 . In an alternative embodiment the pressure sensor  30  may be directly connected to the upper part  10   a  of the chamber  10  as depicted in  FIGS. 4A and 4B  via a separate conduit. As mentioned earlier, a controller  40 , see e.g.  FIGS. 2 and 3 , can be provided and configured to adjust the adjustable valve arrangement  25  to maintain a desired pressure in the chamber  10  in response to the pressure measured by the pressure sensor  30 . 
     During a surgical procedure, the pressure sensor  30  provides feedback to the controller  40  for adjusting the valve arrangement  25  to maintain the pressure within chamber  10  at a desired level. For example, if the sensed pressure within the chamber  10  is lower than a required pressure, the pressure can be quickly increased by adjusting the valve arrangement  25  to deliver pressurised air from the positive pressure source  24  to the main port  25   c  and thus to the chamber  10 . Should the sensed pressure within the chamber  10  be higher than the required pressure, the pressure can be quickly decreased by adjusting the valve arrangement  25  such that a negative pressure is applied to the main port  25   c  and thus to the chamber  10 . This allows for rapid adjustment of the pressure within the chamber  10 . This may also be advantageous as it allows to deal with blockages and/or leaks within the ophthalmic aspiration/irrigation system  100 ,  200 . 
     In an embodiment, the controller  40  controls the first and second adjustable/proportional valves R 1 , R 2  such that if the desired pressure set point is achieved, the steady state air consumption is minimized to meet the flow capacity of the compressor and the vacuum source. It is only when the pressure in the chamber  10  needs to be changed that an air flow is applied. Once the pressure is reached the valves R 1 , R 2  can be closed such that the positive pressure source  24  (e.g. fed by a compressor) and negative pressure source  22  (e.g. fed by a pump) can maintain the respective pressure levels (even the compressor and/or pump can be turned off or operated at a lower power consumption level). 
     The pressure control unit  21  of the present invention may further comprise one or more air filters  29   a,    29   b  configured to filtrate air that during operation may come into contact with the surgical fluid F in the chamber  10 . In an exemplary embodiment, the pressure control unit  21  comprises a first air filter  29   a  that can be arranged in communication with the main port  25   c  of the adjustable valve arrangement  25  (as shown in  FIG. 4A and 4B  also in direct communication with the chamber  10 , e.g. the upper part  10   a  thereof). As shown in  FIGS. 4A, 4B , the first air filter  29   a  may be arranged in a conduit extending between the main port  25   c  and the upper part  10   a  of chamber  10 . The first air filter  29   a  is adapted to filtrate air entering the chamber  10  from the main port  25   c  and maintains appropriate sterility of the surgical fluid F within the chamber  10 . Appropriate sterility of the fluid F can be achieved through an embodiment wherein the first air filter  29   a  is a bacterial filter (e.g. 0.22 μm bacterial filters). This is useful in irrigation procedures in which the fluid from the chamber  10  is delivered to the eye  1 , but this also ensures appropriate sterility of backflush during aspiration procedures (which means irrigation for the aspiration line). 
     There are several ways in which the first air filter  29   a  can be arranged within the pressure control unit  21 . For example, in an embodiment the pressure sensor  30  may be connected to a conduit extending between the main port  25   c  and the chamber  10  (also see  FIGS. 1 and 2 ), wherein the first air filter  29   a  is arranged in the conduit between the pressure sensor  30  and main port  25   c.  In an alternative embodiment the first air filter  29   a  is arranged in the conduit between the pressure sensor  30  and the chamber  10 . 
     According to the present invention, the adjustable valve arrangement  25  allows for rapid and accurate control of negative and positive air pressure within the chamber  10 . To allow this fast pressure changes, short term mass flow rates are required. To be capable of providing these flow rates that might not match the flow capacity of the internal system pressure sources  22 ,  24 , a vacuum buffer  28   a  and a pressure buffer  28   b  are used. Furthermore, fast changes in air pressure can induce short bursts of relatively high mass flow rates and pressure ripples through the pressure control unit  21 . To provide dampening of such high mass flow rates and pressure ripples there is provided an embodiment wherein the pressure control unit  21  comprises a vacuum buffer  28   a  (e.g. a vacuum buffer tank) arranged between the negative pressure source  22  and the vacuum port  25   a.  In another embodiment a pressure buffer  28   b  (e.g. pressure buffer tank) may be provided and arranged between the positive pressure source  24  and the pressure port  25   b.  Of course, in an advantageous embodiment the pressure control unit  21  comprises both the vacuum buffer  28   a  and the pressure buffer  28   b,  so that mass flow bursts to and from the chamber  10  are dampened. The vacuum buffer  28   a  and the pressure buffer  28   b  each provide capacitance to momentarily absorb/store some of the mass flow rate from/to the chamber  10  and in doing so provide dampening of pressure ripples within the chamber  10 . It is noted that either the vacuum buffer  28   a  and/or the pressure buffer  28   b  may alternatively or additionally be formed by using the air volume within the pneumatic tubing between the negative/positive pressure sources  22 ,  24  and the main port  25   c.    
     Because the adjustable valve arrangement  25  allows for rapid changes in pressure within the chamber  10 , high pressure pulses may occur within the pressure control unit  21  during operation. Then to ensure operational safety (e.g. in case of failure of the control unit  21  or one or more of the pneumatic components), the adjustable valve arrangement  25  may further comprise one or more safety valves. In an exemplary embodiment, the adjustable valve arrangement  25  comprises a first safety valve configured for venting air to the ambient environment when a particular positive pressure is exceeded. In a further embodiment, the adjustable valve arrangement  25  comprises a second safety valve configured to block air flowing toward and/or from the chamber  10  when a particular negative pressure and/or positive pressure is exceeded within the chamber  10  to avoid damaging the ophthalmic surgical system. Advantageously, the adjustable valve arrangement  25  may comprise both the first and second safety valves for increased safety. 
     The skilled person will appreciate that in addition to maintaining the chamber pressure at a desired constant level, the pressure control unit  21  described herein may also allow for precise control of the chamber pressure according to a varying pressure profile (e.g. varying aspiration pressure according to a particular aspiration strategy). The arrangements shown in  FIGS. 4A and 4B  also minimize air consumption within the system, thereby eliminating the need for an external air source to maintain chamber pressure at the desired level. 
     An important advantage of the pressure control unit  21  of the present invention is that through the main port  25   c  it is easily connected to an existing chamber  10 , such as a disposable chamber, of an ophthalmic aspiration/irrigation system  100 ,  200 . In this way an existing ophthalmic surgical system can deploy the pressure control unit  21  for increasing irrigation and/or aspiration response times as well as increased accuracy at which required pressures can be provided within chamber  10 . 
     However, there may exist cases wherein an existing chamber  10  exhibits specific features or shortcomings that do not allow the pressure control unit  21  to be used for such a chamber  10 . In view of these difficulties the pressure control unit  21  comprises the chamber  10  as an integral part thereof. So according to the present invention, the pressure control unit  21  for an ophthalmic irrigation/aspiration system may comprise a negative pressure source  22 , a positive pressure source  24  and an adjustable valve arrangement  25 , wherein the adjustable valve arrangement  25  comprises a vacuum port  25   a  connected to the negative pressure source  22  and a pressure port  25   b  connected to the positive pressure source  24 . The adjustable valve arrangement  25  also comprises a main port  25   c  in controllable fluid communication with the vacuum port  25   a  and the pressure port  25   b.  Then the pressure control unit  21  further comprises a chamber  10  configured for exchanging a surgical fluid F. The chamber  10  comprises an upper part  10   a  for storing air A and a lower part  10   b  for storing the surgical fluid F to be irrigated or aspirated. The main port  25   c  of the adjustable valve arrangement  25  is then connected to the chamber  10 , in particular the upper part  10   a  of the chamber  10 . 
     The skilled person in the art will appreciate that all embodiments already described above for the pressure control unit  21  are readily applicable to the pressure control unit  21  in case the chamber  10  is provided as an integral part of the pressure control unit  21 . So the adjustable valve arrangement  25  may be a proportionally adjustable (or PWM controlled on/off) valve arrangement to allow for continuous and accurate control of air flow toward and from the chamber  10 . In a particular embodiment, the adjustable valve arrangement  25  may comprises a first adjustable valve R 1  connected between the vacuum port  25   a  and the main port  25   c  and a second adjustable valve R 2  connected between the pressure port  25   b  and the main port  25   c.  Note that it is also possible that adjustable valve arrangement  25  is a three-way valve arrangement T as depicted in  FIG. 4B . 
     As described earlier, the pressure control unit  21  may comprise a pressure sensor  30  in communication with the main port  25   c  to monitor the pressure in the chamber  10  and to control the negative and/or positive pressure based on the measured pressure. In a particular embodiment the pressure sensor  30  may be connected to a conduit extending between the main port  25   c  and the chamber  10 , i.e. the upper part  10   a  thereof. In an alternative embodiment the pressure sensor  30  may be directly connected to the upper part  10   a  of the chamber  10  for having a more direct measurement of the pressure within the chamber  10 . For maintaining a sterile and/or clean surgical fluid F in the chamber  10  during use, there is an embodiment wherein the pressure control unit  21  comprises a pressure sensor  30  directly connected to the upper part  10   a  of the chamber  10  and wherein a second air filter  29   b  is arranged between the pressure sensor  30  and the chamber  10 . This second filter  29   b  may be a bacterial filter to specifically maintain sterility of the surgical fluid F. Furthermore, the second filter  29   b  (e.g. a bacterial filter) in combination with a direct connection to the chamber  10  (instead of via the main port  25   c ) may be adapted to ensure high dynamic response of the pressure sensor  30  through minimizing air mass flow. 
     In some embodiments, the chamber  10  has a small air volume (i.e. less than 10 cc). However, the skilled person understands that the chamber  10  may have any air volume for which the control is needed. Note that the air volume is the volume of air within the chamber  10  during normal operation, i.e. when the chamber  10  is filled with fluid within the target range. The target volume of fluid may be 10-15 cc. which would still allow a fast priming (i.e. filling the cassette  8  and all connecting lines with fluid). The variation in amount of fluid in the chamber  10  during normal operation is e.g. about 1 cc, which would also still enable proper and accurate level sensing using the fluid level indicator  50 . In an exemplary embodiment, the total internal volume of the chamber  10  is about 25-30 cc. 
     With reference to  FIGS. 1-3, 4A and 4B , in a further aspect the present invention relates to a method for regulating the pressure in an ophthalmic aspiration/irrigation system. In light of the pressure control unit  21  as described above, the pressure in the chamber  10  is regulated by the adjustable valve arrangement  25  which allows for accurate control of the amount of negative pressure from the negative pressure source  22  and the amount of the positive pressure from the positive pressure source  24  at the main ports  25   c  to reach a required pressure within the chamber  10 . 
     According to the present in invention, the method for regulating the pressure in an ophthalmic aspiration/irrigation system comprises a chamber  10  configured for exchanging surgical fluid F and a pressure control unit  21  comprising a negative pressure source  22 , a positive pressure source  24  and an adjustable valve arrangement  25 , the valve arrangement  25  comprising a vacuum port  25   a  connected to the negative pressure source  22  and a pressure port  25   b  connected to the positive pressure source  24 , and wherein the valve arrangement  25  further comprises a main port  25   c  configured for fluid connection to the chamber  10 . The method comprises the steps of: determining a desired chamber pressure; 
     delivering the desired pressure to the chamber via the main port  25   c;    
     measuring the pressure within the chamber  10 ; and 
     adjusting the adjustable valve arrangement  25  to deliver positive or negative pressure to maintain the desired pressure within the chamber  10  based on the measured pressure within the chamber. 
     The method of the present invention allows the pressure in the chamber  10  to be controlled by both a negative pressure source  22  as well as a positive pressure source  24  through the adjustable valve arrangement  25 . This allows for extremely fast and accurate changes in chamber pressure and thus improving overall responsiveness of an ophthalmic aspiration/irrigation system. Furthermore, controlling both the negative and positive pressure sources  22 ,  24  to maintain desired pressures within the chamber  10  allows non-linearities and/or hysteresis phenomena of an ophthalmic surgical system to be mitigated.