AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Alcon, Inc. Actual Inventor(s): Frederick M. Reed, Roger D. Thomas, Nader Nazarifar, John C. Huculak Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: INTRAOCULAR PRESSURE CONTROL Our Ref: 956961 POF Code: 453863/459561 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1 6008I INTRAOCULAR PRESSURE CONTROL The present application is a divisional application from Australian patent application No. 2006295256, the entire disclosure of which is incorporated herein by reference. Field of the Invention The present invention generally pertains to microsurgical systems and more particularly to controlling intraocular pressure in ophthalmic surgery. Description of the Related Art During small incision surgery, and particularly during ophthalmic surgery, small probes are inserted into the operative site to cut, remove, or otherwise manipulate tissue. During these surgical procedures, fluid is typically infused into the eye, and the infusion fluid and tissue are aspirated from the surgical site. Maintaining an optimum intraocular pressure during ophthalmic surgery is currently problematic. When no aspiration is occurring, the pressure in the eye becomes the pressure of the fluid being infused into the eye. This pressure is typically referred to as the "dead head pressure". However, when aspiration is applied, the intraocular pressure drops dramatically from the dead head pressure due to all the pressure losses in the aspiration circuit associated with aspiration flow. Therefore, ophthalmic surgeons currently tolerate higher than desired dead head pressures to compensate for occasions when aspiration would otherwise lower the intraocular pressure to soft-eye conditions. Clinically, such over-pressurizing of the eye is not ideal. Accordingly, a need continues to exist for an improved method of controlling intraocular pressure during ophthalmic surgery. la Summary of the Invention In one aspect, the present invention provides an intraocular pressure microsurgical 5 system, comprising an infusion chamber, said infusion chamber having a first chamber for holding irrigating fluid and a second chamber for holding irrigating fluid, said first chamber not fluidly coupled to said second chamber. An infusion source is configured to provide an irrigating fluid to said first chamber and said second chamber. Fluid level sensors are provided for measuring a level of fluid in said first chamber and said second chamber. The microsurgical 10 system also includes a surgical device, a flow sensor, a pressurised gas source and infusion lines fluidly coupling the infusion source, infusion chamber, flow sensor and surgical device. A gas line fluidly couples the pressurised gas source and the infusion source; gas lines fluidly couple the pressurised gas source and the chambers; and gas lines fluidly couple the pressurised gas source and one or more actuators. A microprocessor is configured to receive a user input 15 via an input. The system is configured to initially prime the infusion lines, the chambers and the surgical device with a surgical irrigating fluid by pressurising the infusion source. After priming, in response to a desired intraocular pressure received via the input, the system provides said irrigating fluid to the surgical device from said first chamber during a microsurgical procedure. The system uses the flow sensor to measure a flow rate of the fluid 20 and provide a corresponding signal to the microprocessor, the microprocessor calculating a predicted intraocular pressure using the signal from the flow sensor and empirically determined impedance information from the microsurgical system. The microprocessor is used to send an appropriate feedback control signal to a proportional solenoid valve to maintain the predicted intraocular pressure at or near the desired intraocular pressure during all portions of the surgery. 25 The fluid level sensor determines when a level of irrigating fluid in said first chamber reaches said bottom limit and the system ends providing said irrigating fluid to the surgical device from said first chamber when said level of irrigating fluid in said first chamber reaches said bottom 2 limit. Upon said ending step, the system provides said irrigating fluid to said surgical device from said second chamber and refills said first chamber with said irrigating fluid from said infusion source. 5 Also described herein is a method of controlling intraocular pressure with a microsurgical system, comprising the steps of providing an infusion chamber, said infusion chamber having a first chamber for holding irrigating fluid and a second chamber for holding irrigating fluid, said first chamber not fluidly coupled to said second chamber; providing an irrigating fluid from an infusion source to said first chamber and said second chamber wherein 10 said infusion source is fluidly coupled to said first chamber and said second chamber during priming of said chambers; providing said irrigating fluid to a surgical device from said first chamber during a microsurgical procedure; ending said third providing step when said level of said irrigating fluid in said first chamber reaches a bottom limit; upon said ending step, providing said irrigating fluid to said surgical device from said second chamber; and refilling 15 said first chamber with said irrigating fluid from said infusion source during said fourth providing step. Also described herein is a method of controlling intraocular pressure with a microsurgical system. An infusion chamber containing an irrigating fluid is provided, and a desired intraocular pressure is selected. The infusion chamber is pressurized with a pressurized 20 gas to provide irrigating fluid to a surgical device. A flow rate of the fluid within a fluid line fluidly coupled to the surgical device is measured. A signal corresponding to the measured flow rate is provided to a computer. A predicted intraocular pressure is calculated with the computer in response to the signal. A level of the pressurized gas is adjusted in response to a second signal from the computer to maintain the predicted intraocular pressure proximate the desired 25 intraocular pressure. An irrigating fluid is provided from an infusion source to the first chamber and the second chamber. The irrigating fluid is provided to a surgical device from the first chamber 2a during a microsurgical procedure, and this step is ended when the level of the irrigating fluid in the first chamber reaches a bottom limit. Upon such ending, the irrigating fluid is provided to 5 the surgical device from the second chamber, and the first chamber is refilled with the irrigating fluid from the infusion source. Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of 10 the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereto. 2b Brief Description of the Drawina For a more complete understanding of the present invention, and for further objects and advantages thereof, reference is made to the following description taken in 5 conjunction with the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating infusion control in an ophthalmic microsurgical system; and Figure 2 is a schematic diagram illustrating infusion control and irrigation control in an ophthalmic microsurgical system. 10 Detailed Descriotion of the Preferred Embodiments The preferred embodiments of the present invention and their advantages are best understood by referring to Figures 1-2 of the drawings, like numerals being used for like and corresponding parts of the various drawings. As shown in Figure 1, ophthalmic 15 microsurgical system 10 includes a pressure cuff 12; an infusion source 14; a dual infusion chamber 16 having a chamber 16a and a chamber 16b; fluid level sensors 18 and 20; a flow sensor 22; filters 24 and 26; a surgical device 29; a computer or microprocessor 28; gas manifolds 30 and 32; a pressurized gas source 34; proportional solenoid valves 36, 38, and 40; "on/off" solenoid valves 42, 44, 46, 48, 50, 52, 54; 20 actuators 56, 58, 60, and 62; and pressure transducers 64, 66, and 68, Dual infusion chamber 16; fluid level sensors 18 and 20; portions of infusion fluid lines 70, 72, 74, 76, 78, and 80; and portions of gas lines 84 and 86 are preferably disposed in a surgical cassette 27. Infusion source 14; dual infusion chamber 16; flow sensor 22; filters 24 and 3 26; and surgical device 29 are fluidly coupled via infusion fluid lines 70-80. Infusion source 14, dual infusion chamber 16, gas manifolds 30 and 32; pressurized gas source 34; and actuators 56, 58, 60, and 62 are fluidly coupled via gas lines 82, 84, 86, 88, 90, 92, 94, and 96. Infusion source 14; fluid level sensors 18-20; flow sensor 22; microprocessor 5 28; proportional solenoid valves 36-40; on/off solenoid valves 42-54; actuators 56-62; and pressure transducers 64-68 are electrically coupled via interfaces 100, 102, 104, 106, 108, 110, 112, 114,116, 118, 120, 122, 124, 126, 128, 130, and 132. Infusion source 14 is preferably a flexible infusion source. Fluid level sensors 18 and 20 may be any suitable device for measuring the level of fluid in infusion chambers 10 16a and 16b, respectively. Fluid level sensors 18 and 20 are preferably capable of measuring the level of fluid in infusion chambers 16a and 16b in a continuous manner. Flow sensor 22 may be any suitable device for measuring the flow rate of fluid within fluid line 80. Flow sensor 22 is preferably a non-invasive flow sensor. Filters 24 and 26 are hydrophobic micro-bacterial filters. A preferred filter is the Versapor* membrane 15 filter (0.8 micron) available from Pall Corporation of East Hills, New York. Microprocessor 28 is capable of implementing feedback control, and preferably PID) control. Surgical device 29 may be any suitable device for providing surgical irrigating fluid to the eye but is preferably an infusion cannula, an irrigation handpiece, or and irrigation/aspiration handpiece. 20 In operation, fluid lines 70, 72, and 74; chambers 16a and 16b; fluid lines 76, 78, and 80; and surgical device 29 are all primed with a surgical irrigating fluid 140 by pressurizing infusion source 14. Surgical irrigating fluid 140 may be any surgical 4 irrigating fluid suitable for ophthalmic use, such as, by way of example, BSS PLUS@ intraocular irrigating solution available from Alcon Laboratories, Inc. The pressurizing of infusion source 14 is preferably performed by pressure cuff 12. More specifically, microprocessor 28 sends a control signal to open solenoid valve 5 42 via interface 106 and to close solenoid valves 44 and 46 via interfaces 108 and 110, respectively. Microprocessor 28 also sends a control signal to open proportional solenoid valve 40 via interface 104 so that manifold 30 supplies the appropriate amount of pressurized air to actuate pressure cuff 12. Pressure transducer 68 senses the pressure within gas line 82 and provides a corresponding signal to microprocessor 28 via interface 10 126. Solenoid valves 48-54 are initially open so that manifold 32 provides pressurized air to actuate actuators 56-62 to close fluid lines 72-78. Microprocessor 28 sends control signals to close solenoid valves 48-54 via interfaces 114-120. The closing of solenoid valves 48-54 actuates actuators 56-62 to open fluid lines 72-78, After all chambers and fluid lines are primed, microprocessor 28 closes actuators 56-62 and thus fluid lines 72 15 78. Alternatively, the pressuring of infusion source 14 may be performed solely via gravity. After priming, a user then provides a desired intraocular pressure to microprocessor 28 via an input 134. Input 134 may be any suitable input device but is preferably a touch screen display or physical knob. Chamber 16b is preferably the initial 20 active infusion chamber. Microprocessor 28 sends appropriate control signals to opcn solenoid valve 44 and to open proportional solenoid valve 36 (via interface 100) to provide an appropriate level of pressurized air to chamber 16b. Pressure transducer 64 senses the pressure within gas line 84 and provides a corresponding signal to 5 microprocessor 28 via interface 124, Microprocessor 28 also sends an appropriate control signal to open actuator 60 and thus fluid line 78. Chamber 16b supplies pressurized fluid 140 to the eye via fluid lines 78 and 80 and surgical device 29. Flow sensor 22 measures the flow rate of fluid 140 and provides a corresponding signal to microprocessor 28 via 5 interface 132. Microprocessor 28 calculates a predicted intraocular pressure using the signal from flow sensor 22 and empirically determined impedance information of microsurgical system 10. Microprocessor 28 then sends an appropriate feedback control signal to proportional solenoid valve 36 to maintain the predicted intraocular pressure at or near the desired intraocular pressure during all portions of the surgery. 10 Fluid level sensor 20 continuously monitors the decrease in the level of fluid 140 in chamber 16b during surgery and provides a corresponding signal to microprocessor 28 via interface 130. Microprocessor 28 performs adjustments to the air pressure provided to chamber 16b to accommodate for the difference in fluid head height as the level of fluid 140 decreases. When the level of fluid 140 in chamber 16b reaches a bottom limit 15 level, microprocessor 28 closes solenoid valve 44 and actuator 60 and opens solenoid valve 46 and actuators 58 and 62. Chamber 16a is now the active infusion chamber. Microprocessor 28 sends an appropriate control signal to proportional solenoid valvo 38 via interface 102 to provide an appropriate level of pressurized air to chamber 16a. Pressure transducer 66 senses the pressure within gas line 86 and provides a 20 corresponding signal to microprocessor 28 via interface 122. Chamber 16a supplies pressurized fluid 140 to the eye via fluid lines 76 and 80 and surgical device 29. Flow sensor 22 measures the flow rate of fluid 140 and provides a corresponding signal to microprocessor 28 via interface 132. Microprocessor 28 calculates the predicted 6 intraocular pressure as described above and the sends an appropriate feedback signal to proportional solenoid valve 38 to maintain the predicted intraocular pressure at or near the desired intraocular pressure during all portions of the surgery. Microprocessor 28 closes actuator 58 and fluid line 74 once chamber 16b is refilled with fluid 140. 5 Fluid level sensor 18 continuously monitors the decrease in the level of fluid 140 in chamber 16a during surgery and provides a corresponding signal to microprocessor 28 via interface 128. Microprocessor 28 performs adjustments to the air pressure provided to chamber 16a to accommodate for the difference in fluid head height as the level of fluid 140 decreases. When the level of fluid 140 in chamber 16a reaches a bottom limit 10 level, microprocessor 28 switches chamber 16b to active infusion, makes chamber 16a inactive, and refills chamber 16a with fluid 140 via fluid line 72. This cycling between chambers 16b and 16a continues throughout the surgery. Infusion source 14 is preferably monitored via a fluid level sensor (not shown) capable of providing a signal to microprocessor 28 via interface 112 when source 14 15 reaches a near empty limit. Chambers 16a and 16b also preferably each have a volume that enable infusion source 14 to be exchanged, when near empty, without interrupting the surgical procedure. More specifically, chambers 16a and 16b preferably each have a volume of about 30 cc. Such volume allows about two minutes for a near empty infusion source 14 to be exchanged during conditions of maximum flow (e.g. core vitrectomy). In 20 addition, once infusion source 14 is exchanged, all air bubbles within fluid lines 70, 72, and 74 will be automatically "scrubbed out" as the inactive chamber 16a or 16b refills, without the need for re-priming. 7 In the case of failure of either of chambers 16a or 16b, microprocessor 28 can preferably continue surgery with only one active chamber. In the case of failure of both chambers 16a and 16b, microprocessor 28 can preferably continue surgery using only infusion source 14, 5 Figure 2 shows a modified ophthalmic microsurgical system 10 a. Microsurgical system 10a is similar to microsurgical system 10 except that it has an irrigation system in addition to the infusion system described above for system 10. More specifically, system 10a is identical to system 10 except that system 10a also includes an irrigation source 200; fluid lines 202 and 206; gas lines 208 and 216; solenoid valves 210 and 218; 10 actuators 214 and 222; electrical interfaces 212 and 220; and a surgical device 224. As shown in Figure 2, irrigation source 200 is pressurized solely by gravity. As will be appreciated by one of ordinary skill in the art, microsurgical system 1 0a allows surgical irrigating fluid 140 to be delivered to surgical device 29 via fluid line 80 (infusion), and surgical irrigating fluid 140 to be delivered to surgical device 224 via fluid line 206 15 (irrigation), independently. Microprocessor 28 can calculate flow information for fluid 140 within fluid line 206 by continuously monitoring the volumetric change of fluid inside chamber 16b, as indicated by fluid sensor 20. From the above, it may be appreciated that the present invention provides an improved method of controlling intraocular pressure with a microsurgical system. The 20 present invention is illustrated herein by example, and various modifications may be made by a person of ordinary skill in the art. For example, while the present invention is described above relative to controlling intraocular pressure in an ophthalmic 8 microsurgical system, it is also applicable to controlling pressure within the operative tissue during other types of microsurgery. It is believed that the operation and construction of the present invention will be apparent from the foregoing description. While the apparatus and methods shown or 5 described above have been characterized as being preferred, various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the following claims 9