Abstract:
A method of testing a surgical system that takes advantage of the fact that in a balanced irrigation/aspiration system (inflow≧outflow) the duration of the aspiration pressure recovery to the irrigation fluid source pressure immediately following pump stop is independent of pump run time. This method provides a more reliable way of detecting restricted irrigation flow configurations not detectable by the current methods, such as marginal irrigation flow cases that could potentially lead to surgical complications (e.g. chamber collapse during post-occlusion break surge).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to the field of cataract surgery and more particularly to a surgical parameters control method for use with a phacoemulsification system.  
         [0002]     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.  
         [0003]     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).  
         [0004]     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.  
         [0005]     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.  
         [0006]     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. Ultrasonic handpieces and cutting tips are more fully described in U.S. Pat. Nos. 3,589,363; 4,223,676; 4,246,902; 4,493,694; 4,515,583; 4,589,415; 4,609,368; 4,869,715; 4,922,902; 4,989,583; 5,154,694 and 5,359,996, the entire contents of which are incorporated herein by reference.  
         [0007]     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 front 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.  
         [0008]     Prior to use in surgery, the various handpieces, tubings and fluid management cassettes all need to be purged of air or primed. During the priming stage, current phacoemulsification systems also run an aspiration system diagnostic step to test for leaks or blockages in the aspiration system. During this diagnostic step, the system pump is activated to generate a certain vacuum in the aspiration line. If the system is not able to reach the desired vacuum level, this indicates to the system that there is a leak somewhere in the aspiration system, and the system will sound a warning for the operator. On the other hand, inability to release previously build vacuum indicates that there is a blockage in the system, such as a kink in one of the tubings.  
         [0009]     Following the priming step, a flow check is performed specifically intended to verify an adequate fluid flow through the surgical handpiece. Current phacoemulsification systems use a small rubber test chamber that fits over the cutting tip and sleeve. The test chamber is filled with the irrigation fluid and when placed on the handpiece creates a closed compliant aspiration system. During this test an excessive vacuum level for a given pump speed would indicate a flow restriction in the fluidic path. Also, a manual check can be performed by the user to ensure that the test chamber is filled and pressurized upon test completion. A deflated test chamber would be an indication of the irrigation flow restriction. While this priming and diagnostic system procedure is effective, it can cause some compromises with current phacoemulsification system technology. For example, phacoemulsification tip technology has evolved over the years and many different tip styles and diameters are now available. As will be understood to one skilled in the art, an aspirating tip with a small diameter or bore will naturally have a higher resistance to flow than a large bore tip. Therefore, at any given pump speed, a small bore tip will create a higher vacuum in the aspiration line than a large bore tip. As a result, diagnostic settings that use a vacuum level compatible with a small bore tip may not be appropriate when a large bore tip is used, and visa versa. This can lead to inaccuracies and false warnings by the system. Similar inaccuracies can result from different sized tubings and handpieces. Also, a reliance on the user to verify a proper test chamber state following the diagnostics completion is subjective and susceptible to a human error.  
         [0010]     Therefore, a need continues to exist for a method of priming and testing phacoemulsification systems that is accurate for a wide variety of handpieces, tubings and tip.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention improves upon the prior art by providing a method of testing a surgical system that takes advantage of the fact that in a balanced irrigation/aspiration system (inflow≧_outflow) the duration of the aspiration pressure recovery to the irrigation fluid source pressure immediately following pump stop is independent of pump run time. This method provides a more reliable way of detecting restricted irrigation flow configurations not detectable by the current methods, such as marginal irrigation flow cases that could potentially lead to surgical complications (e.g. chamber collapse during post-occlusion break surge).  
         [0012]     Accordingly, one objective of the present invention is to provide a surgical console control system.  
         [0013]     Another objective of the present invention is to provide a surgical console control system having a method for priming a surgical system.  
         [0014]     Another objective of the present invention is to provide a more reliable method for priming a surgical system that can detect restricted irrigation flow configurations not detectable by the current methods.  
         [0015]     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  
       [0016]      FIG. 1  is a block diagram of one embodiment of a control system that can be used with the method of the present invention.  
         [0017]      FIG. 2  is a graphical illustration of a typical aspiration pressure system waveform having unrestricted irrigation.  
         [0018]      FIG. 3  is a graphical illustration of a typical aspiration pressure system waveform having restricted irrigation.  
         [0019]      FIG. 4  is a flow chart illustrating the steps of a first embodiment of the method of the present invention.  
         [0020]      FIG. 5  is a flow chart illustrating the steps of a second embodiment of the method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The inventors have discovered that in a balanced irrigation/aspiration (“I/A”) system (inflow≧outflow) the duration of the aspiration pressure recovery to the bottle pressure (immediately following pump stop) is independent of pump run time. In fact, the pressure recovery profile has a shape that can be approximated as an exponential settling:  
         P   ⁡     (   t   )       =       P   SRC     -       (       P   TEST     +     P   SRC       )     ·     ⅇ     -     t   τ                 
 
 Where: 
        P SRC —irrigation source pressure     P TEST —initial system pressure after pump stop     τ—time constant for a given setup 
 
 Given the approximation above, the recovery time does not depend on the initial vacuum, instead it&#39;s a function of the time constant only, and is equal to 5τ. The time constant depends on system configuration such as infusion sleeve, diameter of the tubing, infusion set drip chamber, etc. The method of the present invention does not require the knowledge of the exact recovery time or time constant, but rather utilizes the fact that recovery time is constant. Further, in the case of an imbalanced system, fluid starvation of the test chamber during pump run time extends the recovery time because not only is the test chamber recovery being slowed down by restricted infusion, but an additional amount of time is needed to refill the previously starved test chamber. As a result, test chamber refill duration is directly proportional to pump run time. Thus, by extending run time of the pump, the effect of test chamber starvation can be amplified for a definite detection. 
       
 
         [0025]     As best seen in  FIG. 1 , system  10  of the present invention generally includes surgical console  12  and cassette  14 . Console  12  may be any suitable commercially available surgical console, such as the SERIES TWENTY THOUSAND LEGACY®, INFINITI® or ACCURUS® surgical systems available from Alcon Laboratories, Inc., Fort Worth, Tex. Cassette  14  may be any suitable commercially available surgical cassettes, such as those described in U.S. Pat. Nos. 5,267,956, 5,364,342 and 5,499,969 (Beuchat, et al.), U.S. Pat. No. 5,899,674 (Jung, et al.), U.S. Pat. No. 6,293,926 B1 (Sorensen, et al.) and U.S. Patent Publication No. 2003/0190244 A1 (Davis, et al.), the entire contents of which being incorporated herein by reference. Cassette  14  is held in operative association with console  12  by means well-known in art.  
         [0026]     Console  12  generally contains aspiration pump mechanism  16 , which may be any suitable flow or vacuum based pump, such pumps being widely known in the art. For example, pump mechanism  16  may be a peristaltic pump roller head that interacts with a peristaltic pump tube formed by aspiration line  20  and aspiration exhaust line  34 . Aspiration line  20  is connected to surgical handpiece  22  on one end and to pump mechanism  16  on the other end so as to draw fluid through handpiece  22 . In fluid communication with aspiration line  20  is pressure sensor  26 , which may be one of a variety of invasive or non-invasive pressure sensors well-known in the art. Aspiration line  20  is intersected between handpiece  22  and pump mechanism  16  by aspiration vent line  24 .  
         [0027]     Cassette  14  generally contains aspiration exhaust line  34 , which fluidly connects to aspiration vent line  24  through vent valve  30  and to aspiration line  20  through pump mechanism  16 . Aspirant or exhaust from pump mechanism  16  is directed into drain bag  29  through aspiration exhaust line  34 .  
         [0028]     System  10  of the present invention also include irrigation fluid container  32  that is connected through line  36  to pressurized air source  38 . Alternatively, fluid container  32  may be gravity-fed as is well-known in the art. Fluid container  32  is fluidly connected to handpiece  22  through line  40  and valve  42  and to aspiration line  34  through irrigation vent line  44  and valve  46 .  
         [0029]     As discussed above, while it is preferred that pump mechanism  16  be a peristaltic roller head and aspiration line  20  and aspiration exhaust line  34  be formed in one continuous length so as to form a peristaltic pump tube that interacts with pump mechanism  16 , one skilled in the art will recognize that aspiration line  20  and aspiration exhaust line may be formed as a separate piece or pieces or may be formed integrally with cassette  14  and that pump mechanisms  16  other that peristaltic pump roller heads may be used, such as linear peristaltic pumps.  
         [0030]     In addition, pressure sensor  26  is depicted as being contained within console  12 . One skilled in the art will recognize that portions of pressure sensor  26 , such as a pressure diaphragm (not shown) may be contained in or on cassette  14  and interact with a force transducer or other means (not shown) contained within console  12 .  
         [0031]     As best seen in  FIG. 2 , for an I/A system, such as system  10 , having unrestricted infusion, the settling time (T S ) of the pressure in aspiration line  20  (as indicated by aspiration pressure sensor  26  or “APS”) at the end of the Flow Check cycle (pump  16  stops after running at some practical rate, for example 60 cc/min) is constant regardless of pump  16  run duration (by way of example, t 60  may be 3 seconds). For an I/A system, such as system  10 , having restricted infusion, the overall settling time (t SR ) is greater than that of unrestricted system, and it can be expressed as:
 
 t   SR   =t′   S   +t   F 
 
 where: 
        t′ S —slowed down settling time (t′ S &gt;T S ),     t F —test chamber refill time 
 
 While the slowed down settling time (t′ S ) is also remains constant for a given restriction, test chamber refill time (if applicable) depends on the degree of restriction and more importantly on pump  16  run time (t 60 ). The applicability of test chamber  28  refill time depends on whether test chamber  28  was starved during pump  16  run time. If, despite the restriction, the infusion fluid flow through line  40  and into test chamber  28  was able to keep up the aspiration fluid flow through line  20  caused by pump  16 , then t F =0. This case can be assumed as being clinically safe. The cases with a more significant restriction (i.e. when test chamber  28  is starved during pump  16  run time) present a greater risk. In these cases, test chamber  28  refill time can be estimated as follows:  
         t   F     =         V   Starved       F   Infusion       =         (       F   Aspiration     -     F   Infusion       )     ·     t   Test         F   Infusion             
 
 where: 
    V Starved —test chamber volume lost during pump run time     F Aspiration —aspiration flow rate during the test     F Infusion —infusion flow rate     t Test —pump run time, i.e. test duration        
 
         [0038]     As best seen in  FIG. 3 , for a restricted infusion flow, the overall settling time (t SR ) is greater than that of unrestricted infusion flow, and it can be expressed as:
 
 t   SR   =t′   S   +t   F 
 
 where: 
        t′ S —slowed down settling time (t′ S &gt;T S ),    t F —test chamber refill time        
 
         [0041]     Based of this information, the method of the present invention operates as shown in  FIG. 4 . At Step  1 , the pressure of irrigation fluid container  32 , as seen by aspiration pressure sensor  26  is recorded for future use (P SRC ). The value is measured with irrigation valve  42  open and pump  16  stopped; throughout the test, irrigation valve  42  stays open and vent valve  46  stays closed. The measured value is essentially the pressure of irrigation fluid container  32  as seen by aspiration pressure sensor  26 , and it is also the recovery pressure of the aspiration pressure any time pump  16  stops. At this time, a “traditional” vacuum limit test is still used for gross error detection (e.g. aspiration side clog) provided that prior to proceeding, Step  2  of the test method of the present invention requires that pump  16  is run at some high rate for several seconds. For example a “high rate” may be the maximum unoccluded rate configurable by the user, such as 60 cc/min. The duration of pump  16  run time should be sufficient for the restricted irrigation flow to have some measurable effect (i.e. partial collapse of test chamber  28 ). The longer the duration, the higher the resolution in reliably detecting the failure; on the other hand, in the case of a non-failing setup, prolonged pump run time wastes irrigation fluid and slows down the user. Therefore, a duration that is a reasonable compromise between the two may be between 2 seconds and to 6 second. At the end of Step  2 , pump  16  is stopped and irrigation valve  42  stays open to allow the aspiration pressure in system  10  to recover back to the pressure in irrigation fluid container  32 . In Step  3 , the time that it takes from the point when pump  16  is stopped to the point when the aspiration pressure crosses 0 mm Hg is measured (t 01 ). The value of 0 mm Hg value is chosen out of convenience, and other values may be used. The requirement for the value is that at this point, system  10  has gone through test chamber  28  recovery/refill (if any) and incurred pressure recovery delay. When test chamber  28  is being refilled the pressure at test chamber  20  is approximately 0 mm Hg and aspiration pressure reads a negative value at this point. Consequently, when aspiration pressure reads 0 mm Hg, test chamber  28  (upstream) pressure is above zero or positive, which means test chamber  28  is fully inflated. Therefore, a non-negative aspiration pressure reading is a suitable test point. Also, during the previous step (pump  16  run time) the aspiration pressure reading is typically well below 0 mm Hg (i.e. vacuum) and the pressure of irrigating fluid container  32  is some positive value, and system  10  pressure necessarily has crossed 0 mm Hg at some point during recovery. In Step  4 , after a predefined time period, the recovered aspiration pressure is compared against the expected P SRS  value. A reasonable value for the recovery period may typically be between 0.5 to 1.5 seconds, and may be determined experimentally for the worst case combination of consumable products (cassette, tip, sleeve, etc.). The allowable differential between the actual and expected recovered values depends on sensor  26  accuracy, bottle spike, fluid level in the irrigating fluid container  26 , etc; but the practical values are generally in the range of 10-15 mm Hg. If the recovered pressure is within tolerance, system  10  has passed the flow check test and no further fluidic setup is required.  
         [0042]     As seen in Step  5 , if the recovered aspiration pressure failed the test in Step  4 , then the difference in recovered value is compared against some experimentally determined threshold. A recovery above this threshold represents a marginally acceptable irrigation setup. The threshold value itself is not used to control the ability of system  10  to detect a faulty setup. Instead, this value allows system  10  to reject a bad setup sooner, without going though additional steps. Having a value that is too low makes system  10  go through extra steps before rejecting an obviously bad setup; having a value that is too high can cause false positives. A practical value may be approximately 50% of irrigating fluid container  32  pressure. The practicality of the value can be determined by simulating a worst case acceptable irrigation restriction. If the recovered aspiration pressure does pass the test in Step  5 , indicating that the difference in recovered value as compared against some experimentally determined, threshold value is potentially acceptable, then pump  16  is restarted to repeat the last part of Step  2  or the “standard flow check”. In this Step  6 , pump  16  run time is significantly different from the standard run time used in Step  2  in order to amplify the effect of the “inflow&lt;outflow” imbalance by aggravating test chamber  28  starvation, and consequently prolonging the recovered aspiration pressure as compared against the expected P SRS  value after stopping pump  16 . By way of example, doubling pump  16  run time to 6 seconds produces a discernable difference for a faulty setup. To be able to meaningfully compare the recovery times, pump  16  rate should be set to the same value as in Step  2 . Step  7  is then carried out, with Step  7  being similar to Step  3  in that a recovery period to 0 mm Hg is measured (t 02 ). In Step  8 , both recovery times (t 01  and t 02 ) are compared. A significant difference (in terms of the accuracy of the measurement) between two time indicates a longer test chamber  28  recovery/refill duration, i.e. indicates a significant “outflow&gt;inflow” imbalance. Assuming that the overall recovery duration has two components: 1) system (tubing, cassette, etc) recovery; and 2) test chamber  28  recovery or refill, the system component is close to being a constant for any given system configuration, while test chamber  28  recovery is a variable determined by the pre-existing test chamber  28  collapse. Test chamber  28  recovery duration can be roughly approximated as the time it takes irrigation free flow to fill the deficit previously created by the outflow/inflow imbalance. Doubling pump  16  run time simplifies the math in estimating the imbalance:  
                   t   01     =         t   System     +     t     TestChamber   ⁢           ⁢   1         =       t   System     +         (       F   Aspiration     -     F   Infusoin       )     ·     t   1         F   Infusoin                         t   02     =         t   System     +     t     TestChamber   ⁢           ⁢   2         =       t   System     +           (       F   Aspiration     -     F   Infusoin       )     ·   2     ⁢     t   1         F   Infusoin                   ⇒     
     ⇒     Δ   ⁢           ⁢   t       =         t   02     -     t   01       =             (       F   Aspiration     -     F   Infusoin       )     ·     t   1         F   Infusoin       ⇒       F   Infusion       F   Aspiration         =       t   1         t   1     +     Δ   ⁢           ⁢   t                 
 
         [0043]     Ideally, given the proper setup, Δt should be 0, and F Infusion /F Aspiration  ratio should be 1. Outside any pressure and time measurement inaccuracy, any Δt indicates some irrigation flow restriction. For example, if a flow check run at 60 cc/min for 3 and 6 seconds produces 0.5 seconds recovery differential, then F Infusion /F Aspiration  ratio can be approximated to 86%, or irrigation flow restriction of approximately 14%. Given pressure sensor  26  accuracy, time measurement accuracy, and other factors on commercially available system, approximately 15% (or a Δt of approximately 0.5 second) can be used as a practical pass/fail criteria. If the test in Step  8  passes, then the same test as in Step  5  is applied for assurance that system  10  pressure recovers above a minimum threshold.  
         [0044]     The flow chart in  FIG. 5  illustrates an alternative or second implementation of the method of the present invention. The main difference between the method illustrated in  FIG. 4 , and the method illustrated in  FIG. 5 , is that the second implementation is time based only. The method illustrated in  FIG. 5  uses an expected recovery periods T 0  and T 1  as pass fail criteria in the earlier steps of the test.  
         [0045]     One skilled in the art will recognize that the methods of the present invention are based on the principle that system  10  aspiration pressure recovery following pump  16  stop is constant for a given proper (unrestricted) setup that includes cassette, tip, irrigation sleeve, handpiece, and fluidics module. Pressure recovery in time can be approximated and an exponential approach to irrigating fluid container  32  pressure:  
         P   ⁡     (   t   )       =       P   SRC     -       (       P   TEST     +     P   SRC       )     ·     ⅇ     -     t   τ                 
 
 Where: 
        P SRC —irrigation source pressure     P TEST —initial system pressure after pump stop     τ—time constant for a given setup 
 
 Given this approximation, the restricted setup recovery can be modeled to have similar exponential component plus a “test chamber refill” component, which is approximately linearly proportional to pump  16  run time. The fact that the exponential component of the restricted system has a greater time constant τ (i.e. slower response) than the unrestricted does not matter to the method, because a comparison of the two setups is not being made. Whatever that exponential component is, it stays constant for a given setup, so when two recoveries after two different pump run times are compared, the constant part is eliminated, thus allowing to check for the presence of the linear component. 
       
 
         [0049]     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.