Abstract:
A method for direct indication of the IOP level without requiring an additional pressure transducer being introduced into the irrigation path. The method of the present invention estimates resistance ratio for a particular setup, and thus does not assume a typical value; the estimation is performed in a pre-operational configuration that is the closest possible to the surgical configuration.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the field of cataract surgery and more particularly to an intraoperative pressure monitoring method for use with a phacoemulsification system. 
     The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of the lens onto the 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 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 (IOL). 
     In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. During this procedure, a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquefies or emulsifies the lens so that the lens may be aspirated out of the eye. The diseased lens, once removed, is replaced by an artificial lens. 
     A typical ultrasonic surgical device suitable for ophthalmic procedures consists of an ultrasonically driven handpiece, an attached cutting tip, and irrigating sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubings. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubings supply irrigation fluid to and draw aspiration fluid from the eye through the handpiece assembly. 
     The operative part of the handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece by flexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body&#39;s distal end. The nosecone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve. 
     In use, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined width in the cornea, sclera, or other location. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the cutting tip and horn bores and the aspiration line and into a collection device. The aspiration of emulsified tissue is aided by a saline flushing solution or irrigant that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the cutting tip. 
     During cataract surgery, it is necessary to control the intraocular pressure (“IOP”) within the eye. Lack of control over the IOP may impair the effectiveness or ease of the procedure, and in certain cases may result in damage to tissue, such are the result of a collapse of the eyeball with concomitant tissue damage. Conversely, over-pressuring the intraocular region may also result in damage to the sensitive retinal, optic nerve, or corneal tissue. However, it is occasionally desirable to apply controlled high pressure for a brief time period, for example, to staunch bleeding in the intraocular region. 
     One method of controlling pressure within the eye during surgery is disclosed in U.S. Pat. No. 4,041,947 to Weiss, et al. That patent discloses the use of limiting valves external to the eye on the infusion and aspiration lines. These limiting valves are designed to provide pressure relief if either the pressure in the infusion line exceeds a high limit, or if the pressure in the aspiration line exceeds a low limit. This device does provide some ability to maintain pressure within a predetermined range of values, but does not allow the surgeon to accurately know or set the IOP. 
     IOP can be directly measured by insertion of a pressure transducer into the eye. U.S. Pat. Nos. 4,548,205, 4,722,350, and 4,841,984 to Armeniades, et al., disclose direct measurement and control of the IOP. A pressure transducer is inserted into the eye as an independent tool or integrated into the cutting tool. Alternatively, a pressure transducer can be integrated into a separate tool that provides infusion or aspiration. However, there are several problems with tools which provide direct measurement of the IOP. If the pressure transducer is incorporated into the invasive portion of a tool, the tool must be made larger in diameter than is necessary to perform the actual surgery. This approach requires a correspondingly larger incision in the eyeball for tool insertion. Further, integration of a pressure transducer into another tool creates inaccuracies in the pressure readings caused by the proximity of the transducer to the operating infusion line, aspiration line, or surgical tool. 
     One solution to the size problem is to design a tool with a channel which is inserted into the eye and which provides fluid communication with a pressure transducer outside of the eye. However, this design suffers from the same accuracy problems detailed above, as well as problems caused by debris from the operation clogging the channel. This accuracy problem can be overcome by providing a separate tool that only contains a pressure transducer for insertion into the eye away from the operating tools. However, this approach is disfavored because it requires another incision into the eye. 
     Currently, no commercial surgical console provides any direct indication of the IOP level. Users control IOP by adjusting the irrigation source pressure (bottle height) to the level appropriate for combination of the settings used (aspiration rate, vacuum limit, tip, sleeve, etc.). Users evaluate and establish a certain IOP level based on their experience with a particular instrument. Although one commercially available surgical instrument, the INFINITI® Vision System, has an irrigation pressure sensor (“IPS”) that can be used as an indirect indicator of the IOP quality, the variables downstream of the sensor can distort the interpretation of the IPS reading. For example, given identical instrument setting, a more restrictive irrigation sleeve will result in a higher IPS reading, which can be misinterpreted as indicating a higher IOP level, whereas in reality the IOP will be lower than expected. 
     Therefore, a need continues to exist for a method of measuring IOP during ophthalmic surgery. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention improves upon the prior art by providing a method for direct indication of the IOP level without requiring an additional pressure transducer being introduced into the irrigation path. The method of the present invention estimates resistance ratio for a particular setup, and thus does not assume a typical value; the estimation is performed in a pre-operational configuration that is the closest possible to the surgical configuration. Although the method accuracy is affected by a variable that can not be evaluated in pre-op tests, i.e. incision/irrigation sleeve interface (namely how tight/loose the incision is), the estimated value can still be the closest direct IOP indication available, and can be interpreted as the best case IOP. 
     Accordingly, one objective of the present invention is to provide a surgical console control system. 
     Another objective of the present invention is to provide a surgical console control system having a method for determining IOP. 
     Another objective of the present invention is to provide a method for estimating the fluid resistance ratio for a particular setup. 
     These and other advantages and objectives of the present invention will become apparent from the detailed description and claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one embodiment of a control system that can be used with the method of the present invention during surgery. 
         FIG. 2  is a block diagram of one embodiment of a control system that can be used with the method of the present invention during system set-up and priming. 
         FIG. 3  is a graphical illustration of irrigation and aspiration pressure versus time during the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventor has discovered that the changes in irrigation and aspiration pressure during the priming cycle of a surgical system can be used to estimate the flow resistance in the system. Once the approximate flow resistance is known, that information can be used to estimate the pressure within the system and, as a consequence, the pressure at the operative site. 
     As best seen in  FIG. 1 , system  10  of the present invention generally includes pressurized source of irrigation fluid  12 , irrigation line  14  running from source  12  to handpiece  16 , irrigation pressure sensor (“IPS”)  18 , aspiration line  20  running from handpiece  16 , pump  22  for providing a vacuum to aspiration line  20  and aspiration pressure (vacuum) sensor  24 . During surgery, tip  26  on handpiece  16  is held within eye  28  so that there is continuous fluid communication from irrigation source  12  to drainage bag  30  through irrigation line  14 , handpiece  16 , aspiration line  20  and pump  22 . The fluid resistance in this continuous path is unknown, because of the various combinations of sleeves, tips, handpieces and tubings that can be used. Therefore, in order to estimate the IOP in eye  28 , the fluidic resistance must be determined. 
     As best seen in  FIG. 2 , system  10 ′ is identical to system  10  illustrated in  FIG. 1  except that  FIG. 2  represents a surgical system during pre-surgical set-up, or priming, so that test chamber  32  is substituted for eye  28 . During the priming operation, a sequence of steps is added to the existing priming steps. Test chamber  32  is evacuated by running pump  22 ′ with irrigation valve  34 ′ closed so that approximately 2-3 cc of fluid is evacuated without pulling a high vacuum in system  10 ′. At the end of the step test chamber  32  should be collapsed and vacuum in system  10 ′ is approximate 100-150 mm Hg. Pump  22 ′ is stopped and irrigation valve  34 ′ is opened to refill test chamber  32  with irrigation fluid from source  12 ′. Irrigation and/or aspiration pressure sensors  18 ′ and  24 ′ readings are monitored. When a flat (i.e. test chamber  32  refill) segment is detected in the pressure reading IPS  18 ′ pressure is recorded.  FIG. 3  illustrates the pressure curves for both aspiration pressure (vacuum) and irrigation pressure during this step. Flat segment  36  in  FIG. 3  illustrated test chamber  32  refill. Flat segment  36  indicates that test chamber  32  pressure (which is the pressure at the handpiece tip  26 ′ as well) is approximately equal to ambient, i.e. 0 mm Hg. Irrigation line  14 ′ resistance ratio (K R ) based on the pressure readings is estimated as follows: 
                     K   R     =       R   ADMIN       R   IRR                   =       Δ   ⁢           ⁢     P   ADMIN         Δ   ⁢           ⁢     P   IRR                     =         P   SOURCE   0     -     P   IPS   0           P   IPS   0     -     P   TIP   0                     =         P   SOURCE   0     -     P   IPS   0           P   IPS   0     -   0                   =         P   SOURCE   0       P   IPS   0       -   1                       Where   ⁢     :                     P   SOURCE   0     ⁢     -     ⁢   irrigation   ⁢           ⁢   source   ⁢           ⁢   pressure   ⁢           ⁢   during   ⁢           ⁢   the   ⁢           ⁢   test     ,     
     ⁢       P   IPS   0     ⁢     -     ⁢   IPS   ⁢           ⁢   reading   ⁢           ⁢   during   ⁢           ⁢   the   ⁢           ⁢   test   ⁢           ⁢   chamber   ⁢           ⁢   refill     ,     
     ⁢       P   TIP   0     ⁢     -     ⁢   pressure   ⁢           ⁢   at   ⁢           ⁢   the   ⁢           ⁢   handpiece   ⁢           ⁢   tip   ⁢           ⁢   during   ⁢           ⁢   test   ⁢           ⁢   chamber   ⁢           ⁢   refill     ,     
     ⁢     ≈     0   ⁢           ⁢   mm   ⁢           ⁢     Hg   .               
During surgery the IOP is estimated based on the current bottle height, current IPS  18  reading, and previously estimated resistance ratio (K R ) as follows:
 
     
       
         
           
             
               
                 
                   
                     P 
                     IOP 
                   
                   = 
                   
                     
                       P 
                       IPS 
                     
                     - 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         P 
                         IRR 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       P 
                       IPS 
                     
                     - 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           P 
                           ADMIN 
                         
                       
                       
                         K 
                         R 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       P 
                       IPS 
                     
                     - 
                     
                       
                         
                           P 
                           SOURCE 
                         
                         - 
                         
                           P 
                           IPS 
                         
                       
                       
                         K 
                         R 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               K 
                               R 
                             
                             + 
                             1 
                           
                           ) 
                         
                         ⁢ 
                         
                           P 
                           IPS 
                         
                       
                       - 
                       
                         P 
                         SOURCE 
                       
                     
                     
                       K 
                       R 
                     
                   
                 
               
             
           
         
       
       
         
           
             Where 
             ⁢ 
             
               : 
             
           
         
       
       
         
           
             
               
                 P 
                 IOP 
               
               ⁢ 
               
                 - 
               
               ⁢ 
               current 
               ⁢ 
               
                   
               
               ⁢ 
               IOP 
             
             , 
             
               
 
             
             ⁢ 
             
               
                 P 
                 SOURCE 
               
               ⁢ 
               
                 - 
               
               ⁢ 
               current 
               ⁢ 
               
                 
                     
                 
                 ⁢ 
                 
                     
                 
               
               ⁢ 
               irrigation 
               ⁢ 
               
                   
               
               ⁢ 
               source 
               ⁢ 
               
                   
               
               ⁢ 
               pressure 
             
             , 
             
               
 
             
             ⁢ 
             
               
                 P 
                 IPS 
               
               ⁢ 
               
                 - 
               
               ⁢ 
               current 
               ⁢ 
               
                   
               
               ⁢ 
               IPS 
               ⁢ 
               
                   
               
               ⁢ 
               
                 reading 
                 . 
               
             
           
         
       
     
     In general, the method utilizes compliance of the test chamber to instrument a transient condition in which the pressure at the handpiece is known, despite the absence of the direct measurement means. The compliance of the test chamber determines the refill pressure at the handpiece during the instrumented condition. For currently used test chambers (highly compliant) the pressure can be accurately assumed to be at ambient (i.e. 0 mmHg). For a lower compliance test chamber, the typical refill pressure value can be accurately established in lab testing. 
     This description is given for purposes of illustration and explanation. It will be apparent to those skilled in the relevant art that changes and modifications may be made to the invention described above without departing from its scope or spirit.