Patent Publication Number: US-2018028359-A1

Title: Pressure control in phacoemulsification system

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to phacoemulsification surgery and more particularly to the control of fluid flow during surgery. 
     The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (TOL). 
     In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. A typical surgical instrument suitable for phacoemulsification procedures on cataractous lenses includes an ultrasonically driven phacoemulsification hand piece, an attached hollow cutting needle surrounded by an irrigating sleeve, and an electronic control console. The hand piece is attached to the control console by an electric cable and flexible tubing. Through the electric cable, the console varies the power level transmitted by the hand piece to the attached cutting needle. The flexible tubing supplies irrigation fluid to the surgical site and draws aspiration fluid from the eye through the hand piece. 
     During a phacoemulsification procedure, the tip of the cutting needle and the end of the irrigation sleeve are inserted into the anterior segment of the eye through a small incision in the eye&#39;s outer tissue. The surgeon brings the tip of the cutting needle into contact with the lens of the eye, so that the vibrating tip fragments the lens. The resulting fragments are aspirated out of the eye through the interior bore of the cutting needle, along with irrigation fluid provided to the eye during the procedure, and into a waste reservoir. 
     Throughout the procedure, irrigating fluid is infused into the eye, passing between the irrigation sleeve and the cutting needle and exiting into the eye at the tip of the irrigation sleeve and/or from one or more ports or openings formed into the irrigation sleeve near its end. This irrigating fluid is critical, as it prevents the collapse of the eye during the removal of the emulsified lens. The irrigating fluid also protects the eye tissues from the heat generated by the vibrating of the ultrasonic cutting needle. Furthermore, the irrigating fluid suspends the fragments of the emulsified lens for aspiration from the eye. 
     Conventional systems employ fluid-filled bottles or bags hung from an intravenous (IV) pole as an irrigation fluid source. Irrigation flow rates, and corresponding fluid pressure at the eye, are regulated by controlling the height of the IV pole above the surgical site. For example, raising the IV pole results in a corresponding increase in head pressure and increase in fluid pressure at the eye, resulting in a corresponding increase in irrigation flow rate. Likewise, lowering the IV pole results in a corresponding decrease in pressure at the eye and corresponding irrigation flow rate to the eye. 
     Aspiration flow rates of fluid from the eye are typically regulated by an aspiration pump. The pump action produces aspiration flow through the interior bore of the cutting needle. The aspiration flow results in the creation of vacuum at the aspiration line. The aspiration flow and/or vacuum are set to achieve the desired working effect for the lens removal. The IV pole height and irrigation pump are regulated to achieve a proper intra-ocular chamber balance in an effort to maintain a relatively consistent fluid pressure at the surgical site within the eye. 
     While a consistent fluid pressure in the eye is desirable during the phacoemulsification procedure, a common phenomenon during a phacoemulsification procedure arises from the varying flow rates that occur throughout the surgical procedure. Varying flow rates result in varying pressure losses in the irrigation fluid path from the irrigation fluid supply to the eye, thus causing changes in pressure in the anterior chamber (also referred to as Intra-Ocular Pressure or IOP). Higher flow rates result in greater pressure losses and lower IOP. As IOP lowers, the operating space within the eye diminishes. 
     Another common complication during the phacoemulsification process arises from a blockage, or occlusion, of the aspirating needle. As the irrigation fluid and emulsified tissue is aspirated away from the interior of the eye through the hollow cutting needle, pieces of tissue that are larger than the diameter of the needle&#39;s bore may become clogged in the needle&#39;s tip. While the tip is clogged, vacuum pressure builds up within the tip. The resulting drop in pressure in the anterior chamber in the eye when the clog is removed is known as post-occlusion surge. This post-occlusion surge, in some cases, can cause a relatively large quantity of fluid and tissue to be aspirated out of the eye too quickly, potentially causing the eye to collapse and/or causing the lens capsule to be torn. 
     Various techniques have been attempted to reduce this surge, such as by venting the aspiration line or otherwise limiting the buildup of negative pressure in the aspiration system. However, there remains a need for improved phacoemulsification devices, including irrigation systems that reduce post-occlusion surge as well as maintain a stable IOP throughout varying flow conditions. 
     SUMMARY OF THE INVENTION 
     In one example of the present invention, a surgical system comprises: a pressurized irrigation fluid source, the pressurized irrigation fluid source comprising a flexible bag located between two opposing plates; an irrigation line fluidly coupled to the pressurized irrigation fluid source; a hand piece fluidly coupled to the irrigation line, the hand piece including an irrigation sleeve; and a controller for controlling the pressurized irrigation fluid source; wherein the controller controls the pressurized irrigation fluid source based on a rate of change of a source pressure associated with the pressurized irrigation fluid source and a rate of change of a linear displacement of one or both opposing plates. The controller may control the pressurized irrigation fluid source further based on compliance of the pressurized irrigation fluid source. The controller may control the pressurized irrigation fluid source further based on incision leakage. The controller may control the pressurized irrigation fluid source further based on sleeve compression. 
     In another example of the present invention, the surgical system further comprises a display and a controller input device. The controller input device may receive a desired intraocular pressure value and the controller may control the pressurized irrigation fluid source so as to maintain the desired intraocular pressure value. The controller input device may receive a desired intraocular pressure range and the controller may control the pressurized irrigation fluid source so as to maintain the desired intraocular pressure range. 
     In other examples of the present invention, the controller calculates intraocular pressure of an eye. The surgical system may comprise an irrigation pressure sensor located at or along the pressurized irrigation fluid source or irrigation line. The surgical system may further comprise: an aspiration line fluidly coupled to the hand piece; an aspiration pressure sensor located at or along the aspiration line; and an aspiration pump configured to draw fluid through the aspiration line. The controller may calculate an aspiration flow value based on a reading from the aspiration pressure sensor, a pump vacuum achievable by the aspiration pump, and a characteristic of the aspiration pump. The controller may use a reading from the aspiration pressure sensor to determine if an occlusion is present or if an occlusion break occurs. The controller may control the pressurized irrigation fluid source to accommodate for changes in fluid flow that result from the occlusion or the occlusion break. 
     In other examples of the present invention, the surgical system further comprises a source pressure sensor for measuring a pressure of the pressurized irrigation fluid source. The controller may calculate an irrigation flow value based on a reading from an irrigation pressure sensor, the source pressure sensor, and an impedance of the irrigation line. The controller may calculate intraocular pressure of an eye based on a reading from the source pressure sensor. 
     In other examples of the present invention, the controller may calculate an irrigation flow value based on the rate of change of the source pressure associated with the pressurized irrigation fluid source, the rate of change of the linear displacement of the one or both opposing plates and compliance of the pressurized irrigation fluid source. An incision leakage value may be calculated from the irrigation flow value and an aspiration flow value. The controller may control the pressurized irrigation fluid source further based on the incision leakage value. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a diagram of the components in the fluid path of a phacoemulsification system including a pressurized irrigation source according to the principles of the present invention. 
         FIG. 2  is a pressurized irrigation fluid source according to the principles of the present invention. 
         FIGS. 3 and 4  depict a hinged pressure sensor arrangement for a pressurized irrigation fluid source according to the principles of the present invention. 
         FIG. 5  is a diagram of system components in a pressurized irrigation fluid source control system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made in detail to the exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts. 
       FIG. 1  is a diagram of the components in the fluid path of a phacoemulsification system including a pressurized irrigation source according to the principles of the present invention.  FIG. 1  depicts the fluid path through the eye  1145  during cataract surgery. The components include a pressurized irrigation fluid source  1105 , a source pressure sensor  1110 , an irrigation pressure sensor  1130 , a three-way valve  1135 , an irrigation line  1140 , a hand piece  1150 , an aspiration line  1155 , an aspiration pressure sensor  1160 , a vent valve  1165 , a pump  1170 , a reservoir  1175  and a drain bag  1180 . The irrigation line  1140  provides irrigation fluid to the eye  1145  during cataract surgery. The aspiration line  1155  removes fluid and emulsified lens particles from the eye during cataract surgery. 
     When irrigation fluid exits pressurized irrigation fluid source  1105 , it travels through irrigation line  1140  and into the eye  1145 . An irrigation pressure sensor  1130  measures the pressure of the irrigation fluid in irrigation line  1140 . Irrigation pressure sensor  1130  may be located anywhere along the irrigation line  1140  or irrigation fluid path. If located close to the eye  1145 , irrigation pressure sensor may also be incorporated into the irrigation path of the hand piece  1150 . In some instances, the irrigation line  1140  may pass through and include a path in a fluidics cassette. In this case, the irrigation pressure sensor  1130  may be located in the fluidics cassette. For purposes of this description, irrigation line  1140  may comprise flexible tubing, a path through a fluidics cassette, rigid tubing, or other fluidic pathways that carry irrigation fluid from pressurized irrigation fluid source  1105  through hand piece  1150  and into eye  1145 . Source pressure sensor  1110  also measures the pressure of irrigation fluid at the pressurized irrigation fluid source  1105 . A three-way valve  1135  is provided for on/off control of irrigation and to provide a path to the drain bag  1180 . Irrigation pressure sensor  1130  and source pressure sensor  1110  are implemented by any of a number of commercially available fluid pressure sensors. Irrigation pressure sensor  1130  and/or source pressure sensor  1110  provides pressure information to a controller (shown in  FIG. 5 ) that operates pressurized irrigation fluid source  1105 . The pressurized irrigation fluid source  1105  controls the pressure and/or flow rate of the irrigation fluid exiting it. 
     In some embodiments of the present invention, the pressurized irrigation fluid source  1105  includes a flexible bag that contains irrigation fluid. In this case, the bag can be squeezed to pressurize the fluid it contains. For example, the bag may be located between two opposing plates that press together to pressurize the contents of the bag (as more fully described in  FIG. 2 ). In another example, a flexible band surrounds the bag and is tightened to squeeze the bag and pressurize its contents. In other embodiments of the present invention, the pressurized irrigation fluid source  1105  includes a bottle or other container that can be pressurized. In further embodiments of the present invention, the pressurized irrigation fluid source  1105  is pressurized using a pump or a compressed gas. 
     The source pressure sensor  1110  may be a single pressure sensor or an array of pressure sensors. The source pressure sensor  1110  may contact the pressurized irrigation fluid source  1105  to determine the pressure of its contents. For example, when the pressurized irrigation fluid source  1105  is a flexible bag located between two opposing plates, source pressure sensor  1110  may be located on one of the plates adjacent to the bag. As the plates travel, the bag is pressurized and source pressure sensor  1110  measures the pressure. In this case, the source pressure sensor  1110  may be an array of sensors located on the plate or a single sensor located on the plate. In another example, a hinged plated may be used as more fully described in  FIG. 4 . 
       FIG. 2  depicts pressurized irrigation fluid source  1105  as a flexible bag  1109  (e.g. an IV bag) located between two opposing plates  1106  and  1107 . One of the two plates  1106  or  1107  may be fixed while the other plate travels to compress or squeeze flexible bag  1109 . For example, plate  1106  may be fixed and plate  1107  may travel to compress flexible bag  1109 . In another example, both plates  1106  and  1107  are capable of traveling. In one embodiment, an actuator  1111  is coupled to plate  1107 . Actuator  1111 , which may be a stepper motor or similar actuator, moves plate  1107 . An optional sensor  1112  measures a rate of change of linear displacement of plate  1107 , a position of plate  1107 , or other position or movement information associated with plate  1107 . In another embodiment, the position or movement information associated with plate  1107  can be determined from actuator  1111 . For example, when actuator  1111  is a stepper motor, the position of the motor shaft or the current applied to the motor may be used to determine position or movement information associated with plate  1107 . In this manner, the position of plate  1106  with respect to plate  1107  can be determined. Further, the rate of change of the linear displacement of the plates may also be determined. For example, the movement of one plate with respect to the other plate can be calculated or determined from sensor  1112  or from the following formula: dX/dt where X is the plate position or the location of one plate with respect to the other plate. 
     In  FIG. 3 , plate  1106  has an array of source pressure sensors  1110  located on a surface that faces the flexible bag  1109 . In this manner, a reading from each of the four depicted source pressure sensors  1110  may lead to a more accurate pressure reading. In this example, a reading can be taken from each of the four source pressure sensors  1110 , and the readings averaged or an errant reading thrown out. In  FIG. 4 , a source pressure sensor  1110  (or an array of sensors) is located on plate  1106  under a hinged plate  1108 . The flat surface of the hinged plate  1108  contacts the source pressure sensor  1110 . In some cases, the surface of the flexible bag  1109  may become wrinkled or have creases when it is squeezed between plates  1106  and  1107 . These wrinkles or creases can lead to inaccurate pressure readings if a wrinkle or crease is located at a source pressure sensor  1110 . Using an array of sensors as shown in  FIG. 3  is one way to overcome this problem. Using a hinged plate  1108  is another way. When using a hinged plate  1108 , a flat uniform surface always contacts source pressure sensor  1110 . 
     Source pressure sensor  1110  is capable of providing a continuous pressure reading or a series of pressure readings over time from which a rate of change of source pressure can be determined. Source pressure sensor  1110  may be polled periodically over a period of time to provide a series of pressure readings. For example, source pressure sensor  1110  may be polled every ten or hundred milliseconds to provide a pressure profile for irrigation source  1109 . This series of pressure readings can then be used to calculate a rate of change of pressure and direction of change of pressure. For example, when source pressure sensor  1110  reads a series of increasing pressures, a rate of change of the increasing pressures can be calculated and a direction of change (in this case, increasing or positive) can also be determined. Likewise, when source pressure sensor  1110  reads a series of decreasing pressures, a rate of change of the decreasing pressures can be calculated and a direction of change (in this case, decreasing or negative) can also be determined. A controller, such as controller  1230 , may make these calculations. In other cases, an instantaneous source pressure may be provided by source pressure sensor  1110 . One or more instantaneous pressure readings can be used to determine the rate of change of source pressure: dP/dt where P is source pressure. 
     Irrigation pressure sensor  1130  is capable of providing a continuous pressure reading or a series of pressure readings over time from which a rate of change of source pressure can be determined. Irrigation pressure sensor  1130  may be polled periodically over a period of time to provide a series of pressure readings. For example, irrigation pressure sensor  1130  may be polled every ten or hundred milliseconds to provide a pressure profile for irrigation pressure or the pressure associated with the irrigation line  1140 . This series of pressure readings can then be used to calculate a rate of change of pressure and direction of change of pressure. For example, when irrigation pressure sensor  1130  reads a series of increasing pressures, a rate of change of the increasing pressures can be calculated and a direction of change (in this case, increasing or positive) can also be determined. Likewise, when irrigation pressure sensor  1130  reads a series of decreasing pressures, a rate of change of the decreasing pressures can be calculated and a direction of change (in this case, decreasing or negative) can also be determined. A controller, such as controller  1230 , may make these calculations. In other cases, an instantaneous source pressure may be provided by irrigation pressure sensor  1130 . One or more instantaneous pressure readings can be used to determine the rate of change of irrigation pressure: dP/dt where P is irrigation pressure. 
     In a similar manner, aspiration pressure sensor  1160  is capable of providing a continuous pressure reading or a series of pressure readings over time from which a rate of change of source pressure can be determined. Aspiration pressure sensor  1160  may be polled periodically over a period of time to provide a series of pressure readings. For example, aspiration pressure sensor  1160  may be polled every ten or hundred milliseconds to provide a pressure profile for irrigation pressure or the pressure associated with the aspiration line  1155 . This series of pressure readings can then be used to calculate a rate of change of pressure and direction of change of pressure. For example, when aspiration pressure sensor  1160  reads a series of increasing pressures, a rate of change of the increasing pressures can be calculated and a direction of change (in this case, increasing or positive) can also be determined. Likewise, when aspiration pressure sensor  1160  reads a series of decreasing pressures, a rate of change of the decreasing pressures can be calculated and a direction of change (in this case, decreasing or negative) can also be determined. A controller, such as controller  1230 , may make these calculations. In other cases, an instantaneous source pressure may be provided by aspiration pressure sensor  1160 . One or more instantaneous pressure readings can be used to determine the rate of change of aspiration pressure: dP/dt where P is aspiration pressure. 
       FIG. 5  is a block diagram representing some components of a phacoemulsification machine.  FIG. 5  shows an irrigation line  1140 , an irrigation pressure sensor  1130  in, along, or associated with the irrigation line  1140 , an aspiration line  1155 , an aspiration pressure sensor  1160  in, along, or associated with the aspiration line  1155 , a hand piece  1150 , a controller  1230 , a flow command input device  1210  (e.g. a foot pedal), a display  1220 , and an associated controller input device  1240  for entering data or commands for programming the system. 
     The irrigation line  1140  extends between a pressurized irrigation fluid source  1105  and the hand piece  1150  and carries fluid to the hand piece  1150  for irrigating an eye during a surgical procedure (as shown in  FIG. 1 ). In one example, the sterile fluid is a saline fluid, however, other fluids may be used. At least a portion of the irrigation line  1140  may be formed of a flexible tubing, and in some embodiments, the path  1140  is formed of multiple segments, with some segments being rigid and others being flexible. 
     The irrigation pressure sensor  1130  is associated with the irrigation line  1140  and performs the function of measuring the irrigation pressure in or associated with the irrigation line  1140 . In some embodiments, the sensor  1130  is a pressure sensor configured to detect current pressure conditions, instantaneous pressure conditions, a rate of change of pressure, a direction of change of pressure, or a series of pressure conditions over time. The sensor  1130  communicates signals indicative of the sensed pressure to the controller  1230 . Once received, the controller  1230  processes the received signals to determine whether the measured pressure is above or below a desired pressure or within a pre-established desired pressure range. The controller  1230  may also determine a rate of change of sensed pressure and/or a direction of change of sensed pressure. Although described as a pressure sensor, the irrigation pressure sensor  1130  may be another type of sensor, such as a flow sensor that detects actual fluid flow and may include additional sensors for monitoring additional parameters. In some embodiments, the sensor  1130  includes its own processing function and the processed data is then communicated to the controller  1230 . 
     The aspiration line  1155  extends from the hand piece to the drain reservoir  1180  (as shown in  FIG. 1 ). The aspiration line  1155  carries away fluid used to flush the eye as well as any emulsified particles. 
     The aspiration pressure sensor  1160  is associated with the aspiration line  1155  and performs the function of measuring the waste fluid pressure in or associated with the aspiration line  1155 . Like the sensor  1130  described above, the sensor  1160  may be a pressure sensor configured to detect current pressure conditions, instantaneous pressure conditions, a rate of change of pressure, a direction of change of pressure, or a series of pressure conditions over time. It communicates signals indicative of the sensed pressure to the controller  1230 . The sensor  1160 , like the sensor  1130 , may be any suitable type of sensor, such as a flow sensor that detects actual fluid flow and may include additional sensors for monitoring additional parameters. 
     The hand piece  1145  may be an ultrasonic hand piece that carries the irrigation fluid to the surgical site. The hand piece is configured as known in the art to receive and operate with different needles or equipment depending on the application and procedure being performed. It should be noted that although an ultrasonic hand piece is discussed, the principles of the invention are intended to cover the use of vitrectomy cutter hand pieces or other hand pieces known in the art. For ease of reference only, this application will refer only to the hand piece  1145 , recognizing that the system operates in a similar manner with other hand pieces. 
     In the example shown, the fluid command input device  1210  is typically a foot pedal. It can receive inputs indicative of a desired flow rate, desired pressure, or other fluid characteristic. It is configured to control the operational setting of the machine through a plurality of major control settings, including controlling the irrigation flow rate or pressure within each of the major control settings. In some embodiments, the flow command input device is not a foot pedal, but is another input device, located elsewhere on the machine. 
     The controller input device  1240  permits a user to enter data or commands that affect system programming. In this embodiment, the controller input device  1240  is associated with the display  1220 . However, it could be associated directly with the controller in a manner known in the art. For example, in some embodiments, the controller input device  1240  is a standard computer keyboard, a standard pointing device, such as a mouse or trackball, a touch screen or other input device. 
     As is apparent from  FIG. 5 , the controller  1230  communicates with the display  1220 , the flow command input device  1210 , the hand piece  1150 , the irrigation pressure sensor  1130 , the aspiration pressure sensor  1160 , the actuator  1111 , and the controller input device  1240 . It is configured or programmed to control the pressurized irrigation system based upon pre-established programs or sequences. 
     In use, the controller  1230  is configured to receive signals from the irrigation pressure sensor  1130  and process the signals to determine whether the detected irrigation pressure is outside of an acceptable range or above or below acceptable thresholds. The controller  1230  may also determine a rate of change of the irrigation pressure and a direction of change of the irrigation pressure. If the controller  1230  detects an unacceptable irrigation pressure, it controls the pressurized irrigation system to correct the pressure to a desired range. Likewise, in another example, the controller  1230  is configured to receive signals from the aspiration pressure sensor  1160  and process the signals to determine whether the detected pressure is outside of an acceptable range or above or below acceptable thresholds. The controller  1230  may also determine a rate of change of the irrigation pressure and a direction of change of the irrigation pressure. If the controller  1230  detects an unacceptable pressure, it controls the pressurized irrigation system to correct the pressure to a desired range. In this manner, the irrigation pressure sensor  1130  and/or the aspiration pressure sensor  1160  may be used to control the fluid pressure in the eye (IOP). 
     Returning to  FIG. 1 , aspiration pressure sensor  1160  measures the pressure in the aspiration line  1155  or aspiration pathway. Aspiration pressure sensor  1160  may be located anywhere along the aspiration line  1155  or aspiration pathway. If located close to the eye  1145 , aspiration pressure sensor may be located in the hand piece  1150 . Aspiration pressure sensor  1160  is implemented by any of a number of commercially available fluid pressure sensors. Aspiration pressure sensor  1160  provides pressure information to a controller (shown in  FIG. 5 ) that operates pressurized irrigation fluid source  1105 . 
     A hand piece  1150  is placed in the eye  1145  during a phacoemulsification procedure. The hand piece  1150  has a hollow needle that is ultrasonically vibrated in the eye to break up the diseased lens. A sleeve located around the needle provides irrigation fluid from irrigation line  1140 . The irrigation fluid passes through the space between the outside of the needle and the inside of the sleeve. Fluid and lens particles are aspirated through the hollow needle. In this manner, the interior passage of the hollow needle is fluidly coupled to aspiration line  1155 . Pump  1170  draws the aspirated fluid from the eye  1145 . An aspiration pressure sensor  1160  measures the pressure in the aspiration line. An optional vent valve can be used to vent the vacuum created by pump  1170 . The aspirated fluid passes through reservoir  1175  and into drain bag  1180 . 
     During a phacoemulsification procedure, the tip of the needle on hand piece  1150  may become occluded with a lens particle. This creates a condition that is called an occlusion. During an occlusion, less fluid is generally aspirated from the eye, and the vacuum pressure in aspiration line  1155  increases as a result of the occlusion. Accordingly, during an occlusion, aspiration pressure sensor  1160  detects the increased vacuum that is present in aspiration line  1155 . When the occlusion breaks (that is when the lens particle that causes the occlusion is broken up by the ultrasonic needle), a surge occurs. The increased vacuum in aspiration line  1155  creates a sudden demand for fluid from the eye resulting in a rapid lowering of IOP and shallowing of the operating space within the eye. This can lead to a dangerous situation in which various structures of the eye can be damaged. 
     Upon occlusion break, the aspiration pressure sensor  1160  detects a drop in pressure in aspiration line  1155 . A series of pressure measurements from aspiration pressure sensor  1160  can be used to determine a rate of change of the drop in aspiration pressure in aspiration line  1155 . Likewise, the irrigation pressure sensor  1130  also detects the pressure drop in irrigation line  1140  that occurs as a result of occlusion break. A series of pressure measurements from irrigation pressure sensor  1130  can be used to determine a rate of change of the drop in irrigation pressure in irrigation line  1140 . Signals from the irrigation pressure sensor  1130  and/or the aspiration pressure sensor  1160  may be used by the controller  1230  to control the irrigation source  1105  as more thoroughly described below. 
     The pressurized irrigation system of the present invention is capable of responding to the surge caused by occlusion break by increasing the irrigation pressure in irrigation line  1140 . When an occlusion breaks and a surge occurs, pressurized irrigation fluid source  1105  increases the pressure of the irrigation fluid in response. Increasing the irrigation pressure of pressurized irrigation fluid source  1105  meets the added fluid demand caused by occlusion break. In this manner, the pressure and resulting operating space in eye  1145  can be maintained at a relatively constant value which may be selected by the surgeon. 
     Likewise, when an occlusion occurs, irrigation pressure may increase as the fluid aspirated from the eye decreases. An increase in irrigation fluid pressure detected by irrigation pressure sensor  1130  can be used to control pressurized irrigation fluid source  1105  to regulate the pressure in eye  1145 —that is to keep the pressure in eye  1145  within an acceptable range. In such a case, the aspiration pressure sensor  1160  may also detect the presence of an occlusion and a reading from it may be used by controller  1230  to control pressurized irrigation source  1105 . In this case, the pressure in pressurized in pressurized irrigation fluid source  1105  is not increased but remains the same or is decreased. 
     Generally, control of the pressurized irrigation fluid source  1105  can be based on two parameters: (1) a pressure reading and (2) an estimate of irrigation flow based on flow through the system (or a measurement of actual flow through the system). The pressure reading may be from the irrigation pressure sensor  1130  (i.e. pressure in the irrigation line), the aspiration pressure sensor  1160  (i.e. pressure in the aspiration line) or the source pressure sensor  1110  (i.e. pressure at the pressurized irrigation source). 
     In one embodiment of the present invention, control of the pressurized irrigation fluid source  1105  can be based on irrigation pressure and flow through the system as modified by the compensation factor (as described in detail below). Irrigation pressure can be used to control for occlusion break and to maintain a constant IOP. Irrigation flow also determines IOP. Flow through the system as modified by the compensation factor (which equates to irrigation flow) can be used to control for incision leakage and sleeve compression. Collectively, these parameters can be used to maintain a constant IOP during the procedure. 
     Estimated flow through the system is generally the fluid flow from the pressurized irrigation source  1105  through the irrigation line  1140 , through the hand piece  1150 , into the eye  1145 , out of the eye  1145 , through the hand piece  1150 , through the aspiration line  1155  and into the drain bag  1180 . In operation, fluid may also be lost from the system by leakage from the eye  1145  or the wound through which the needle of the hand piece  1150  is inserted (also called “incision leakage”). In this manner the total fluid flow in the system is equal to the fluid that flows through the eye minus the fluid that is lost due to incision leakage. 
     Estimated fluid flow may be based on a number of different calculations. For example, flow can be estimated by any of the following:
         (1) A differential pressure measurement to calculate flow can be based on an aspiration pressure sensor reading plus pump impedance plus maximum vacuum attained by the aspiration pump. Flow can be calculated by the difference between the measured aspiration pressure at the aspiration pressure sensor  1160 , the maximum vacuum that can be created by the pump  1170 , and the pump impedance. The impedance of the pump  1170  is a known parameter and the maximum vacuum that the pump creates can be measured accurately as can the aspiration pressure (by the aspiration pressure sensor  1160 ). In this manner, flow is estimated by the difference in two pressures in the fluid path and the impedance of that path. In this case, the two pressures are the pressure measure by the aspiration pressure sensor  1160  and the maximum pressure achievable by the pump  1170 . The impedance in this example is the impedance of the pump  1170 .   (2) A differential pressure measurement to calculate flow can be based on the source pressure measured at the source pressure sensor  1110 , the irrigation pressure measured at the irrigation pressure sensor  1130 , and the impedance of the irrigation line (or irrigation path) from the irrigation source  1105  to the irrigation pressure sensor  1130 . Flow can be calculated by the pressure difference between the irrigation source  1105  and the irrigation pressure sensor  1130  and the impedance of the irrigation line  1140  between the irrigation source and the irrigation pressure sensor. In this manner, flow is estimated by the difference in two pressures in the fluid path and the impedance of that path.   (3) When the pressurized irrigation fluid source  1105  is a flexible bag  1109  located between two opposing plates  1106  and  1107  (as depicted in FIG.   2), the travel of plates  1106  and  1107  correspond to fluid flow through the system. Fluid flow and/or the volume of fluid used during the procedure can be estimated directly from the position of plates  1106  and  1107 .       

     Generally, during a procedure, plates  1106  and  1107  travel toward each other to squeeze fluid out of flexible bag  1109  at a desired pressure or flow rate. The total fluid that exits the flexible bag  1109  is directly related to the position of the opposing plates  1106  and  1107 . The closer plates  1106  and  1107  are together, the more fluid has left the flexible bag  1109 . In this manner, the position of plates  1106  and  1107  can also be used to indicate the amount of fluid left in the flexible bag  1109  and provide an indication to the surgeon of the fluid level in the flexible bag  1109  (for example, by displaying fluid level on the display  1220 ). Mathematically, irrigation flow can be expressed as: 
     
       
         
           
             
               Irrigation 
                
               
                   
               
                
               flow 
             
             = 
             
               
                 
                   C 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 
                   dP 
                   dt 
                 
               
               - 
               
                 
                   A 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 dX 
                  
                 
                   / 
                 
                  
                 dt 
               
             
           
         
       
         
         
           
             
               
                 where X is plate position (or the position of one plate  1106  with respect to the other plate  1107 ), 
                 dP/dt is the rate of change of the source pressure, 
                 dX/dt is the rate of change of the linear displacement of one or both plates, 
                 C(X) is the compliance of the pressurized irrigation fluid system  1105 , and 
                 A(X) is the cross-sectional area of the flexible bag  1109  in contact with plate  1106 , plate  1107 , or an average the cross-sectional area of the flexible bag  1109  in contact with the two plates  1106 ,  1107 . 
                 The compliance of the pressurized irrigation fluid system  1105  refers to compliance in the flexible components of the irrigation path which may include compliance of the bag  1109  and/or irrigation fluid path  1140  and/or other items located in or along the irrigation flow path. 
               
             
           
         
       
    
     Actual fluid flow through the system may also be affected by two different factors: incision leakage and sleeve compression. As noted above, the hand piece  1150  has a sleeve located around a needle. The sleeve provides irrigation fluid from irrigation line  1140  to the eye  1145 . The irrigation fluid passes through the space between the outside of the needle and the inside of the sleeve. Fluid and lens particles are aspirated through the hollow needle. During a procedure, the sleeve and needle are inserted into the eye through a small incision. In this manner, the sleeve contacts the eye tissue of the incision (or wound). Incision leakage describes the amount of fluid that exits the eye through the wound (or through the space between the sleeve and the eye tissue through which the wound is formed). During a procedure, fluid may exit the eye through the wound—such fluid loss exits the system (i.e. the fluid that exits the eye does not pass through the aspiration line  1155 ). Incision leakage typically results in the loss of a small amount of fluid thus decreasing the total flow through the system. Expressed mathematically, irrigation flow=aspiration flow+incision leakage. 
     Sleeve compression generally describes the condition in which the sleeve is pinched or compressed against the needle when inserted into the incision. Sleeve compression occurs more frequently with smaller incisions and may or may not result in less incision leakage. Sleeve compression can restrict fluid flow through the system. Since pinching the sleeve increases the flow resistance in the system, flow may be decreased when sleeve compression is present. 
     Generally, the losses due to incision leakage and sleeve compression are dependent on the type of needle and sleeve that is being used as well as surgeon technique. Flow profiles for various combinations of needles and sleeves can be determined experimentally and the resulting data incorporated into an algorithm or database for use in control of pressurized irrigation fluid source  1105 . Alternatively, such experimental data can be aggregated to provide a range of different compensation factors (as described in the next paragraph). Surgeon technique differs considerably among the population of ophthalmologists. During a procedure, some surgeons may move the needle in a manner that creates more sleeve compression. Surgeons also prefer different sizes of needles and sleeves as well as different incision sizes. These surgeon specific factors also impact incision leakage and sleeve compression. 
     A compensation factor may be implemented to compensate for these two different variables that result in a decrease in flow through the system: incision leakage and sleeve compression. Incision leakage may be compensated with an estimated incision leak rate factor (which can be implemented as an offset that is set as a default value). Alternatively, incision leakage may be calculated directly from irrigation flow and aspiration flow. Sleeve compression may be compensated with an estimated compression factor. The incision leak rate factor and the sleeve compression factor may collectively comprise the compensation factor. The compensation factor may be surgeon-adjustable. The compensation factor may be an offset that acts to either increase or decrease the pressure at the pressurized irrigation fluid source  1105 . For example, the compensation factor may be an integer from zero to seven (with zero being no compensation and seven being maximum compensation). 
     Irrigation flow can be estimated from the estimated flow through the system and the compensation factor. Since irrigation flow generally equals aspiration flow plus incision leakage. Therefore, irrigation pressure can be estimated from the compensation factor and estimated flow through the system. Alternatively, incision leakage can be calculated from the irrigation flow. 
     Generally, in order to compensate for the decreased flow (or losses) resulting from incision leakage and sleeve compression, the pressure in pressurized irrigation fluid source  1105  is increased slightly. Such increase in pressure may be implemented in an algorithm based on the compensation factor. In the above example, a surgeon may select a compensation factor of three to provide moderate compensation for incision leakage and sleeve compression. In this example, a compensation factor setting of three may correspond to a slight increase in pressure at the pressurized irrigation fluid source  1105 . In other words, the baseline pressure at the pressurized irrigation fluid source  1105  is increased slightly to compensate for these factors. 
     In another example, the compensation factor may be implemented by a default offset value that can be adjusted by the surgeon. A nominal constant may be the default offset value in the algorithm. The surgeon may adjust this default value by a factor (of between zero for no compensation and 2 for double the compensation). The default offset value can be determined by the experimental data relating to flow characteristics of various needle and sleeve combinations. Some needle and sleeve combinations are much more common than others, so that the most common combinations may be used to determine the default offset value. In other instances, an aggregation of this data may be used to determine the default offset value. 
     In another example, the incision leakage may be calculated and used to directly compensate for the fluid loss. The pressurized irrigation source may be controlled to provide extra fluid flow to compensate for incision leakage. For example, irrigation flow and aspiration flow can be determined as noted above. The difference between aspiration flow and irrigation flow is incision leakage. The calculated incision leakage can be used by controller  1230  to control pressurized irrigation fluid source  1105  to compensate by providing a slightly higher pressure or slightly more fluid flow (as incision leakage is typically a small percentage of the total fluid flow through the system during a procedure). 
     In another example, irrigation flow, aspiration flow, and incision leakage can be used to more precisely control IOP during a procedure. In a completely loss-free system with no flow restrictions, a given pressure provided by pressurized irrigation fluid source  1105  would result in a given IOP (pressure in the eye). Introducing incision leakage into such a system would result in a lower actual IOP. In order to precisely control IOP, it is desirable to know incision leakage. To maintain a desired IOP in the face of incision leakage, therefore, controller  1230  could control pressurized irrigation fluid source  1105  to provide a greater pressure to maintain a given IOP. 
     In another example, the surgeon may enter the type of sleeve and needle via controller input device  1240 . A bar code reader may be employed to scan the bar code from the surgical pack that includes the sleeve and needle as well. When the controller  1230  receives the needle and sleeve information, it can determine the flow characteristics associated with needle and sleeve (or look up the flow characteristics from a database) and select an appropriate compensation factor. In addition, doctor preferences and/or data from prior procedures can be used to select the proper compensation factor. For example, parametric data from prior procedures may be used to determine doctor technique and adjust, modify, or select the compensation factor. 
     Regardless of how the compensation factor is determined, the compensation factor may be used to compensate for flow losses. The compensation factor may be used to control the pressurized irrigation fluid source  1105  so as to provide an amount of fluid equal to that fluid lost due to incision leakage. The compensation factor may be used to control the pressurized irrigation fluid source  1105  so as to provide a slight increase in pressure to overcome the increased flow resistance caused by sleeve compression. In addition, since irrigation flow determines IOP, the compensation factor is used to adjust IOP as well as to compensate for flow losses. 
     Therefore, control of the pressurized irrigation fluid source  1105  can be based on irrigation pressure and flow through the system as modified by the compensation factor. Irrigation pressure can be used to control for occlusion break and to maintain a relatively constant IOP. Flow through the system as modified by the compensation factor can be used to compensate for incision leakage and sleeve compression and maintain a relatively constant IOP. Collectively, these parameters can be used to maintain a relatively constant IOP during the procedure. 
     Instead of using a compensation factor, the system may use calculated incision leakage to control IOP. Since incision leakage can be calculated as noted above, the incision leakage parameter may be used by controller  1230  to control pressurized irrigation fluid source  1105  in order to maintain a given IOP. 
     The estimation of IOP may be based on the irrigation pressure sensor  1130 . The pressure drop between the irrigation pressure sensor and the eye is known because the characteristics of the passage between the irrigation pressure sensor and the eye are known. For example, if the irrigation pressure sensor is located in a fluidics cassette that is connected to the hand piece  1150  through a length of irrigation line  1140 , then the flow impedance of the length of irrigation line  1140  and the irrigation pathway through the hand piece  1150  are both known (or can be measured). IOP can then be determined from the irrigation pressure sensor reading. The IOP reading may also be affected by sleeve compression (because the sleeve is in the irrigation path between the irrigation pressure sensor and the eye) and incision leakage. The compensation factor or calculate incision leakage may be used to adjust IOP for these losses (or changes in the impedance). 
     In one embodiment of the present invention, a surgeon selects a desired IOP. The pressurized irrigation fluid source  1105  is then controlled to maintain the desired IOP. Since IOP is based on a reading from the irrigation pressure sensor, the irrigation pressure sensor  1130  can be used to control the pressurized irrigation fluid source  1105 . In conjunction with irrigation pressure, flow through the system as modified by the compensation factor and/or incisions leakage and/or sleeve compression can also be used to control the pressurized irrigation fluid source  1105 . Irrigation flow also determines IOP. The flow through the system as modified by compensation factor and/or incisions leakage and/or sleeve compression equates to irrigation flow. When an occlusion is present (as detected by the irrigation pressure sensor  1130  or the aspiration pressure sensor  1160 ), IOP can be maintained by this control scheme. On occlusion break (as detected by the irrigation pressure sensor  1130  or the aspiration pressure sensor  1160 ), the pressurized irrigation fluid source  1105  can be controlled to maintain a relatively constant IOP. 
     Alternatively, source pressure sensor  1110  or aspiration pressure sensor  1160  may be used in place of irrigation pressure sensor  1130  in the control scheme above. Likewise, irrigation flow can be calculated as noted above and used in the control scheme. 
     The control of pressurized irrigation fluid source  1105  can also be described in three different states: steady state (when the needle is not occluded and flow through the system is relatively constant); occluded state (when the needle is occluded and there is little or no flow through the system); and occlusion break or surge (when there is a sudden and rapid flow through the system). An example of each state is described. 
     For example, in steady state, the pressurized irrigation fluid source  1105  is controlled to maintain a selected IOP. The irrigation pressure sensor  1130  may be used to provide an estimate of IOP. A pressure reading from irrigation pressure sensor  1130  is received by the controller  1230 . The desired IOP is also received by the controller  1230 . The controller directs the operation of pressurized irrigation fluid source  1105  so as to maintain the desired IOP. In steady state, the controller typically directs pressurized irrigation fluid source  1105  to provide fluid at a relatively constant pressure to maintain IOP. In addition, the controller calculates a value for estimated fluid flow as modified by the compensation factor. In this example, in steady state, flow may be estimated by a differential pressure measurement or by plate travel. In the case of a differential pressure measurement, the controller  1230  receives the pressure reading(s) needed for the differential pressure measurement and makes the calculation. In the case of plate travel, the controller  1230  receives readings from position sensors, pressure sensor, or the like and determines plate travel. The compensation factor is also received by the controller (as an input by the surgeon, for example). Since irrigation fluid flow (estimated flow through the system as modified by the compensation factor) is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that the compensation factor is used to adjust fluid pressure at the pressurized irrigation fluid source  1105  to compensate for flow losses. 
     In another example, the controller calculates irrigation flow and uses irrigation flow to control the pressurized irrigation fluid system  1105 . In this example, irrigation flow may be calculated as follows: 
     
       
         
           
             
               Irrigation 
                
               
                   
               
                
               flow 
             
             = 
             
               
                 
                   C 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 
                   dP 
                   dt 
                 
               
               - 
               
                 
                   A 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 dX 
                  
                 
                   / 
                 
                  
                 dt 
               
             
           
         
       
         
         
           
             where X is plate position (or the position of one plate  1106  with respect to the other plate  1107 ), 
             dP/dt is the rate of change of the source pressure, 
             dX/dt is the rate of change of the linear displacement of one or both plates, 
             C(X) is the compliance of the irrigation system, and 
             A(X) is the cross-sectional area of the flexible bag  1109  in contact with plate  1106 , plate  1107 , or an average the cross-sectional area of the flexible bag  1109  in contact with the two plates  1106 ,  1107 .
 
Aspiration flow may also be calculated as noted above. The difference between irrigation flow and aspiration flow is incision leakage. Further, sleeve compression can be estimated as a flow restriction in the flow path. Control of IOP can be based on any one or more of these parameters. Since irrigation fluid flow is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that calculated incision leakage is used to adjust fluid pressure (and associated flow) at the pressurized irrigation fluid source  1105  to compensate for flow losses. In this flow-based control, irrigation flow is increased to compensate for incision leakage. To maintain a given desired IOP, pressurized irrigation fluid source  1105  provides irrigation flow to meet the demand of the aspiration flow. Since incision leakage results in flow losses, irrigation flow is increased to meet these losses (plus the demand of the aspiration) to maintain a given IOP.
 
           
         
       
    
     When an occlusion occurs, the tip of the needle is wholly or partially clogged with a lens particle. In the occluded state, flow through the system is decreased. The irrigation pressure sensor  1130  provides an estimate of IOP. A pressure reading from irrigation pressure sensor  1130  is received by the controller  1230 . The desired IOP is also received by the controller  1230 . The controller directs the operation of pressurized irrigation fluid source  1105  so as to maintain the desired IOP. In an occluded state, the controller typically directs pressurized irrigation fluid source  1105  to provide fluid at a relatively constant pressure to maintain IOP. Maintaining pressure in an occluded state is likely to mean that the plates  1106  and  1107  maintain the flexible bag  1109  at a relatively constant pressure. In addition, the controller calculates a value for estimated fluid flow as modified by the compensation factor as detailed above. Since irrigation fluid flow (estimated flow through the system as modified by the compensation factor) is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that the compensation factor is used to adjust fluid pressure at the pressurized irrigation fluid source  1105  to compensate for flow losses (e.g. incision leakage). 
     In another example, the controller calculates irrigation flow and uses irrigation flow to control the pressurized irrigation fluid system  1105  during an occlusion. In this example, irrigation flow may be calculated as follows: 
     
       
         
           
             
               Irrigation 
                
               
                   
               
                
               flow 
             
             = 
             
               
                 
                   C 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 
                   dP 
                   dt 
                 
               
               - 
               
                 
                   A 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 dX 
                  
                 
                   / 
                 
                  
                 dt 
               
             
           
         
       
         
         
           
             where X is plate position (or the position of one plate  1106  with respect to the other plate  1107 ), 
             dP/dt is the rate of change of the source pressure, 
             dX/dt is the rate of change of the linear displacement of one or both plates, 
             C(X) is the compliance of the irrigation system, and 
             A(X) is the cross-sectional area of the flexible bag  1109  in contact with plate  1106 , plate  1107 , or an average the cross-sectional area of the flexible bag  1109  in contact with the two plates  1106 ,  1107 .
 
Aspiration flow may also be calculated as noted above. The difference between irrigation flow and aspiration flow is incision leakage. Further, sleeve compression can be estimated as a flow restriction in the flow path. Control of IOP can be based on any one or more of these parameters. Since irrigation fluid flow is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that calculated incision leakage is used to adjust fluid pressure (and associated flow) at the pressurized irrigation fluid source  1105  to compensate for flow losses. In this flow-based control, irrigation flow is increased to compensate for incision leakage. To maintain a given desired IOP, pressurized irrigation fluid source  1105  provides irrigation flow to meet the demand of the aspiration flow. Since incision leakage results in flow losses, irrigation flow is increased to meet these losses (plus the demand of the aspiration) to maintain a given IOP. In the case of an occlusion, aspiration flow typically decreases. During an occlusion, the controller  1230  controls pressurized irrigation fluid source to decrease irrigation flow to meet the demand of aspiration flow plus incision leakage.
 
           
         
       
    
     When an occlusion break occurs, the lens particle at the tip of the needle is dislodges and a surge of fluid exist the eye through the lumen of the needle. During occlusion break, flow through the system is increased. The irrigation pressure sensor  1130  provides an estimate of IOP. A pressure reading from irrigation pressure sensor  1130  is received by the controller  1230 . The desired IOP is also received by the controller  1230 . The controller directs the operation of pressurized irrigation fluid source  1105  so as to maintain the desired IOP. During occlusion break, the controller typically directs pressurized irrigation fluid source  1105  to provide fluid at an increased pressure to maintain IOP. Maintaining pressure during occlusion break is likely to mean that the plates  1106  and  1107  exert force on the flexible bag  1109  to increase the pressure in the irrigation line so as to provide the necessary fluid flow to meet the fluid demand of the surge. In addition, the controller calculates a value for estimated fluid flow as modified by the compensation factor as detailed above. Since irrigation fluid flow (estimated flow through the system as modified by the compensation factor) is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that the compensation factor is used to adjust fluid pressure at the pressurized irrigation fluid source  1105  to compensate for flow losses (e.g. incision leakage). 
     In another example, the controller calculates irrigation flow and uses irrigation flow to control the pressurized irrigation fluid system  1105  during occlusion break to accommodate the increased flow through the system. In this example, irrigation flow may be calculated as follows: 
     
       
         
           
             
               Irrigation 
                
               
                   
               
                
               flow 
             
             = 
             
               
                 
                   C 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 
                   dP 
                   dt 
                 
               
               - 
               
                 
                   A 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 dX 
                  
                 
                   / 
                 
                  
                 dt 
               
             
           
         
       
         
         
           
             where X is plate position (or the position of one plate  1106  with respect to the other plate  1107 ), 
             dP/dt is the rate of change of the source pressure, 
             dX/dt is the rate of change of the linear displacement of one or both plates, 
             C(X) is the compliance of the irrigation system, and 
             A(X) is the cross-sectional area of the flexible bag  1109  in contact with plate  1106 , plate  1107 , or an average the cross-sectional area of the flexible bag  1109  in contact with the two plates  1106 ,  1107 .
 
Aspiration flow may also be calculated as noted above. The difference between irrigation flow and aspiration flow is incision leakage. Further, sleeve compression can be estimated as a flow restriction in the flow path. Control of IOP can be based on any one or more of these parameters. Since irrigation fluid flow is related to IOP, the controller  1230  directs the operation of pressurized irrigation fluid source  1105  to maintain a flow rate consistent with the desired IOP. The net result is that calculated incision leakage is used to adjust fluid pressure (and associated flow) at the pressurized irrigation fluid source  1105  to compensate for flow losses. In this flow-based control, irrigation flow is increased to compensate for incision leakage. To maintain a given desired IOP, pressurized irrigation fluid source  1105  provides irrigation flow to meet the demand of the aspiration flow. Since incision leakage results in flow losses, irrigation flow is increased to meet these losses (plus the demand of the aspiration) to maintain a given IOP. In the case of an occlusion break, aspiration flow increases (typically in a rapid manner). During occlusion break, the controller  1230  controls pressurized irrigation fluid source to increase irrigation flow to meet the demand of aspiration flow plus incision leakage.
 
           
         
       
    
     In a further embodiment of the present invention, incision leakage may be determined as the difference between irrigation fluid flow and aspiration fluid flow. Irrigation fluid flow can be measured directly with a flow sensor, can be calculated using a differential pressure measurement, or can be calculated based on plate travel, all as noted above. In addition, a rate of change and direction of change of irrigation fluid flow and/or the parameters used to calculate irrigation fluid flow may be used. Readings from the source pressure sensor  1110  and the irrigation pressure sensor  1130  can be used to make a differential pressure measurement. In this case, the flow impedance between the source pressure sensor  1110  and the irrigation pressure sensor  1130  is known (or can be measured). The difference in the pressure readings measured by the source pressure sensor  1110  and the irrigation pressure sensor  1130  can be calculated and flow determined. In the case of plate travel, flow can be estimated from the position and/or movement of the plates  1106  and  1107 . In any of the prior cases, a rate of change and/or a direction of change of the parameters may be employed. For example, irrigation fluid flow can be calculated as follows: 
     
       
         
           
             
               Irrigation 
                
               
                   
               
                
               flow 
             
             = 
             
               
                 
                   C 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 
                   dP 
                   dt 
                 
               
               - 
               
                 
                   A 
                    
                   
                     ( 
                     X 
                     ) 
                   
                 
                 * 
                 dX 
                  
                 
                   / 
                 
                  
                 dt 
               
             
           
         
       
         
         
           
             where X is plate position (or the position of one plate  1106  with respect to the other plate  1107 ), 
             dP/dt is the rate of change of the source pressure, 
             dX/dt is the rate of change of the linear displacement of one or both plates, 
             C(X) is the compliance of the irrigation system, and 
             A(X) is the cross-sectional area of the flexible bag  1109  in contact with plate  1106 , plate  1107 , or an average the cross-sectional area of the flexible bag  1109  in contact with the two plates  1106 ,  1107 . 
           
         
       
    
     Aspiration fluid flow can also be calculated using a differential pressure measurement. Flow can be calculated by the difference between the measured aspiration pressure at the aspiration pressure sensor  1160 , the maximum vacuum that can be created by the pump  1170 , and the pump impedance. The impedance of the pump  1170  is a known parameter and the maximum vacuum that the pump creates can be measured accurately as can the aspiration pressure (by the aspiration pressure sensor  1160 ). In this manner, flow is estimated by the difference in two pressures in the fluid path and the impedance of that path. In this case, the two pressures are the pressure measure by the aspiration pressure sensor  1160  and the maximum pressure achievable by the pump  1170 . The impedance in this example is the impedance of the pump  1170 . In any of the prior cases, a rate of change and/or a direction of change of the parameters may be employed. 
     Using the calculated values for irrigation flow and aspiration flow, one can find incision leakage as the difference between irrigation flow and aspiration flow. This calculation of incision leakage may then be used to more accurately determine the compensation factor and/or control IOP. In one embodiment of the of the present invention, the compensation factor is determined dynamically based in part on the calculated incision leakage. 
     In another embodiment of the present invention, control of pressurized irrigation source  1105  may be based on the irrigation flow rate (i.e. the magnitude of the change and/or the direction of the change of irrigation flow). The irrigation flow rate can be used to compensate for incision leakage and/or sleeve compression. The irrigation flow rate can be used to calculate flow losses due to incision leakage and/or the change in flow resistance due to sleeve compression. In this manner, irrigation flow rate may be used by controller  1230  to control pressurized irrigation source  1105  to compensate for incision leakage and/or sleeve compression in order to control IOP. 
     From the above, it may be appreciated that the present invention provides an improved phacoemulsification system. The present invention provides active control of pressure in the eye during the surgical procedure. The present invention is illustrated herein by example, and various modifications may be made by a person of ordinary skill in the art. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.