Abstract:
An outflow rate regulator system for use in a phacoemulsification system to prevent the anterior chamber collapses that occur after occlusion breaks caused by fluid surges in the aspiration line. The outflow rate regulator system consisting in a flow limiting device installed in the aspiration line capable of varying the section or the extension of a fluid passage as a function of the pressure difference across the outflow rate regulator access and exit sides. The device is designed to reduce the outflow fluid passage area as a function of an increasing pressure difference across the outflow rate regulator. Alternatively, the effective extension of a narrow fluid passage is designed to increase as the pressure difference across the outflow rate regulator increases. Resistance to flow is increased with increasing pressure differences across the device in reversible manner. Clogging of the narrow fluid passages is avoided by upstream removal of solid particles above a determined size by a retaining filter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a flow-rate control system and more particularly is related to an outflow rate control system for ophthalmic surgical equipment of the kind used for crystalline lens removal such as phacoemulsification equipment. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typically, cataracts, or crystalline manifestations, in an eye are removed by fragmentation thereof which may include a hollow needle inserted into the eye through a small incision. Removal of the fragmented lens is effected through a centre hole in the needle and involves continuous circulation of fluid through the eye provided by positive pressure fluid irrigation and vacuum fluid aspiration which is provided to the hollow needle inserted therein. Ultrasound, water-jet, laser and other forms of energy can be transferred to the lens tissue by the hollow needle inserted in the eye to help fragment, disrupt and emulsify the cataract material to facilitate the removal of the crystalline lens fragments through the needle conduct together with the circulating fluid. Flow rate entering the aspiration line must be controlled to prevent excessive outflow that produces instability and collapse of the anterior chamber of the eye. This condition is particularly prone to occur after the breaking of occlusions that occur at the hollow needle tip by crystalline lens material. When an occlusion occurs, vacuum rises inside the aspiration system by the action of the aspiration pump located in the unit console. During vacuum rise a contraction occurs in the elastic walls of the aspiration system that is a function of the magnitude of the vacuum. Also, bubbles in the aspiration line will expand by the action of vacuum. Expansion of the contracted walls and contraction of the expanded bubbles when pressure drops creates a volume deficit that has to be filled by volume from the eye chamber. The release of the needle tip blockage allows fluid to travel from the anterior chamber of the eye towards the aspiration line at high flow rates because of the high pressure gradient created during occlusion. Compliance of the aspiration line will determine how fast and how much volume is needed to restore balance. Compliance will depend on rigidity of the walls of the aspiration line and the eventual presence of bubbles in the line. After occlusion break the outflow rate can overshoot to a flow rate that is higher than the console preset outflow rate value (above 60 cc/min peak). This peak in aspiration flow rate rapidly drops to the steady state outflow rate that is equal or lower that the console preset outflow rate depending on the outflow system resistance. Normally, the irrigation system is too slow to fully compensate he fluid void inside the eye chamber created by the outflow system peak suction. The current trend to reduce the incision size for lens removal procedures has further reduced the capabilities of the irrigation system to compensate post occlusion surges because of the increasing resistance to inflow at the incision level. This increases the chances of a negative fluid balance and a transient collapse of the anterior chamber of the eye that can lead to serious complications. The appearance and the magnitude of a post-occlusion surge will be determined by a series of factors such as infusion line pressure (irrigation bottle height), infusion resistance, aspiration line outflow rate, vacuum in the aspiration line at the moment of occlusion break, tubing material and structure, phacoemulsification needle tip resistance, presence of an aspiration bypass systems and eventual bubbles in the aspiration line. One way to reduce post-occlusion surge has been to increase irrigation bottle height but this condition over-pressurizes the eye with unknown consequences. Several active and passive post-occlusion surge reducing devices have been proposed to increase the vacuum level safely in order to remove the crystalline lens fragments with reduced amounts of energy. For example one passive device to reduce surge consists in coiling the outflow tubing to exponentially increase resistance as flow rate increases. This system increases the length of the tubing making it uncomfortable for the user. Another passive surge control system consists in a stricture in the aspiration line (i.e. 0.35 mm diameter port) that has high resistance to high flow rates (Cruise Control System, Staar, USA.). This system increases resistance and reduces maximum flow rate under non occlusion conditions affecting performance. Also, active post-occlusion surge limiting devices have been proposed usually based on feed-back loops that adjust flow rate or vent the aspiration line when an occlusion related state is detected to reduce the post-occlusion surge phenomenon. As an example, an aspiration line pressure sensing method and active flow control has been proposed for phacoemulsification systems in U.S. Pat. No. 5,392,653 entitled “Pressure transducer magnetically-coupled interface complementing minimal diaphragm movement during operation”. The above-referenced patent is incorporated herein by specific reference thereto. It is desirable to provide a surge control system that is inexpensive, simple, and does not affect performance of the lensenctomy system under non occlusion conditions. 
       SUMMARY OF THE INVENTION 
       [0003]    According to the principles of the present invention, an outflow rate regulator is provided for use with an ophthalmic surgical instrument having a hand-piece with a lens removing hollow needle in fluid communication with an aspiration line adapted to carry the fluid and particles of emulsified lens debris away from the surgical site. In accordance with one aspect of the present invention, the outflow rate regulator includes a flow limiting device adapted to be placed in fluid communication with the aspiration line that connects the aspiration pump and the hollow needle. The flow limiting device defines a fluid passage offering a variable resistance to flow that limits post occlusion surge in the anterior chamber of the eye following an occlusion break occurring at the distal portion of the aspiration line The fluid passage section of the outflow rate regulator is designed to vary resistance to flow across the device as a function of the difference in pressure between an access side and an exit side of the flow rate controlling device. The fluid passage can be acted upon to vary resistance to flow either by modifying the section of the fluid passage, by modifying the length of a narrow fluid passage or a combination of both as a function of the difference in pressure between an access side and an exit side of the flow rate controlling device. In this way increasing flow rates encounter a progressive resistance to flow produced by reduction of the fluid passage section or increased length produced by a mechanism that reacts to an increment in a pressure difference between an access and an exit side as sensed by a differential pressure sensor element. By varying several design aspects of the outflow rate regulator, different free flow-rate versus real flow-rate curves can be achieved that can better adapt to different real word surgical settings and instrumentation to prevent anterior chamber collapse caused by post occlusion surge. The free flow-rate versus real flow-rate function can deviate from linearity in several forms and can include hysteresis on purpose by variations in design. A single outflow rate regulator can incorporate an adjustment feature to program a desired performance of the device to accommodate to different surgical environments. This adjustment can be factory made or user selectable. Proper operation of the outflow rate regulator of the present invention requires that the fluid entering the narrow fluid passages is free from solid particles of sizes that could block the narrow fluid passages of the system. A particle retainer preferably consisting in a low resistance particulate material filter must be installed between the surgical hand-piece and the outflow rate regulator device to ensure proper operation of the outflow rate regulator. Among the advantages of the present invention it can be mentioned that it is low cost, simple, effective to reduce post-occlusion surge, reliable and that it does not affect performance of the lensectomy apparatus while operating in non-occlusion conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a schematic drawing of an ophthalmic surgical system including the outflow rate regulator of the present invention. 
           [0005]      FIG. 2A  is a detailed longitudinal view of one embodiment of an outflow rate regulator of the present invention. 
           [0006]      FIG. 2B  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention. 
           [0007]      FIG. 2C  is a graph depicting the pressure difference versus flow rate obtained by using the outflow rate regulator depicted in  FIGS. 2A and 2B . 
           [0008]      FIG. 3A  is a detailed longitudinal view of one embodiment of an outflow rate regulator of the present invention. 
           [0009]      FIG. 3B  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention. 
           [0010]      FIG. 23  is a graph depicting the pressure difference versus flow rate obtained by using the outflow rate regulator depicted in  FIGS. 23 and 23 . 
           [0011]      FIG. 4A  is a detailed longitudinal view of one embodiment of an outflow rate regulator of the present invention. 
           [0012]      FIG. 4B  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention. 
           [0013]      FIG. 4C  is a detailed longitudinal view of one embodiment of the outflow rate regulator of the present invention including an adjustment knob for a user to select a desired performance pattern. 
           [0014]      FIG. 4D  is a graph depicting the pressure difference versus flow rate obtained by using the outflow rate regulator depicted in  FIGS. 4A ,  4 B and  4 C. 
           [0015]      FIG. 4E  is a graph depicting a trans device pressure difference versus flow rate obtained by using the outflow rate regulator of the present invention and showing an hysteresis curve that is on purpose obtained by design characteristics 
           [0016]      FIG. 5A  is a detailed longitudinal view of one embodiment of an outflow rate regulator of the present invention incorporating a compression low compliance bellows. 
           [0017]      FIG. 5B  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention incorporating a disc shaped low compliance bellows 
           [0018]      FIG. 5C  is an enlarged cross sectional view of variable section fluid passage portion of the embodiments shown in  FIGS. 5A and 5B   
           [0019]      FIG. 6  is a graph depicting a pressure difference versus flow rate obtained by using the outflow rate regulator of the present invention depicted in  FIG. 5 . 
           [0020]      FIG. 7A  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention including a solid residue retainer and using an extension bellows. 
           [0021]      FIG. 7B  is an enlarged cross sectional view of the variable section fluid passage portion of the embodiments shown in  FIG.7A . 
           [0022]      FIG. 8A  is a detailed cross sectional view of one embodiment of an outflow rate regulator of the present invention including a solid residue retainer and using a diaphragm to vary the extension of a narrow fluid passage. 
           [0023]      FIG. 8B  is an upper view at section level with label ‘b’ of the flow regulating device shown in  FIG. 8A . 
           [0024]      FIG. 9A  is a lateral sectional view of an alternative embodiment incorporating a movable ball and spring. 
           [0025]      FIG. 9B  is an axial view from the access side of the embodiment depicted in  FIG. 9A . 
       
    
    
     NUMERALS FROM FIGURES 
       [0026]    Particle retainer  10 , retainer in port  12 , particle retaining chamber  14 , low resistance filtering membrane  16 , clean fluid exit side  18 , retainer out port  20 , outflow rate regulator  30 , regulator in port  32 , regulator out port  34 , diaphragm  40 , calibrated permanent fluid passage  42 , blocking fluid passage  44 , diaphragm bed  46 , access side  48 , exit side  50 , blockable fluid passage  52 , blockable fluid passage  54 , blockable fluid passage  56 , slit  60 , slit non blocking portion  62 , diaphragm  70 , calibrated bellows  76 , variable area fluid passage  78 , variable section flow regulator needle  80 , reflux bypass  83 , reflux valve  84 , adjustment element  86 , phacoemulsification surgical system  100  hand-piece  102 , phacoemulsification needle  104 , infusion bottle  106 , infusion line  108 , infusion sleeve  109 , infusion solenoid valve  110 , phacoemulsification needle  11 , aspiration line  112 , aspiration line sensor  114 , aspiration pump  116 , waste fluid outlet  117 , collector bag  11   8 , adjustment knob  120 , console controls  122 , fluid passage  130 , fluid narrow channel  132 , spring  140 , body  142 , body guides  144 , septum  146 , spring holder  148 , walls  150 , clear space  152   
       DETAILED DESCRIPTION 
       [0027]    In  FIG. 1 , there is shown a phacoemulsification surgical system  100  in which an outflow rate regulator  30  of the present invention may be used to advantage. Surgical system  100  has an infusion bottle  106  connecting through an infusion line  108  to an infusion sleeve  109  to perfuse the anterior chamber of the eye. 
         [0028]    Alternatively line  108  can connect to a secondary port infusion instrument such as an anterior chamber maintainer or irrigating instrument for the same purpose. An infusion line solenoid valve  110  has a clamping action upon infusion line  108 . A hollow phacoemulsification needle  104  placed at the distal end of a phacoemulsification hand-piece  102  operates with the distal end placed at the anterior chamber of the eye. Needle  104  is proximally in fluid connection with a solid particle retainer  10  which is in fluid downstream connection with outflow rate regulator  30  of the present invention. The output of outflow rate regulator  30  is in fluid connection with an aspiration line  112  connecting downstream to an aspiration pump  116  having a waste fluid outlet  117 . Waste fluid outlet connects to a waste fluid collector bag  118 . A set of controls  122  allows an operator to program and activate surgical system  100 . An outflow rate regulator system in accordance with the present invention generally includes a flow rate regulator device  30 . It is desirable for proper operation of the flow regulator device  30  that fluid passing through the flow rate regulator device  30  is free of solid material above a critical particle size preferably  50  microns. As shown in  FIG. 2A ,  FIG. 5A  and  FIG. 5B  particle retainer  10  is always provided that is mainly composed of a retainer input port  12  opening to a particle retaining chamber  14 . A low insertion resistance filtering membrane  16  is placed fully across the fluid path of particle retainer  10 . A clean fluid exit side  18  directs the filtered fluid to retainer output port  20 . One embodiment of flow regulator device  30  shown in  FIGS. 2A and 2B  is composed of a regulator input port  32  communicating with an access side  48 . An exit side  50  conducts the exiting fluid to output port  34 . A movable diaphragm  40  is disposed to progressively displace towards a diaphragm bed  46  occluding a blocking fluid passage  44  when deforming or displacing in response to a pressure difference between access side  48  and exit side  50 . A non-blocking fluid passage  42  is placed between chambers  48  y  50  and is designed to maintain the device permanently patent to fluid flow avoiding latch-up. Another embodiment of flow regulator device  30  is shown in  FIGS. 3A and 3B  composed of a regulator input port  32  communicating with an access side  48 . An exit side  50  conducts the exiting fluid to output port  34 . A flexible diaphragm  40  is disposed to progressively displace towards a diaphragm bed  46  occluding in sequence a series of fluid passages  52 ,  54 ,  56  when bending by the action of a pressure difference between access side  48  and exit side  50 . A non-blocking fluid passage  42  is placed between chambers  48  y  50  and designed to maintain permanently patent to fluid flow avoiding latch-up. One preferred embodiment of flow regulator device  30  shown in  FIGS. 4A and 4B  and is composed of a regulator input port  32  communicating with an access side  48 . An exit side  50  conducts the exiting fluid to output port  34 . A flexible membrane is disposed o progressively displace towards a membrane bed progressively occluding slit shaped fluid passage  60  when bending by the action of a pressure difference between access side  48  and exit side  50 . A non-blocking portion  62  of the fluid passage is unreachable to membrane this portion maintaining permanently patent to fluid flow.  FIG. 4C  depicts a variation in design that further included s adjustment knob  120  providing the manufacturer or a user means to vary the angle between diaphragm  40  and diaphragm bed  46  being this one method to adjust the dynamic response curve for flow regulator device  30  to a desired pattern according o surgical conditions. Another possible embodiment of a flow regulator device is shown in  FIGS. 5A ,  5 B and  5 C and s and composed of a regulator input port  32  communicating with an access side  48 . An exit side  50  conducts the exiting fluid to output port  34  toward the aspiration pump. A diaphragm  70  is attached to a calibrated deformable bellows  76 , both elements separating access side  48  and exit side  50 . Diaphragm  70  has a calibrated opening that in combination with an axially disposed variable section needle  80  constitutes a variable section fluid passage  78  between chambers  48  and  50 . Needle  80  is usually cone shaped with the wider portion oriented towards the side of chamber  50 . As an option a secondary calibrated opening  42  can be included to prevent latch up. Also as an option an adjustment element  86  can be included to regulate the response curve. In one configuration shown in  FIG. 5A  diaphragm  70  is mounted over a calibrated compression bellows. Alternatively, as shown in  FIG. 5B  diaphragm  70  is mounted over a calibrated disk shaped bellows. Also as an option an adjustment element  86  can be included. Optionally a calibrated spring can be added to support the diaphragm from either side to alter the pressure versus flow response curve of the device in a favourable manner (not shown).  FIGS. 5A and 5B  incorporate a reflow duct  83  and reflow valve  84  operable during reflow conditions to avoid waste fluid to deliver lens particles back to the eye chambers. As shown in  FIG. 5C , variable section needle  80  is disposed to centrally cross in a perpendicular direction the calibrated opening of diaphragm  70  with the wider section towards chamber  50 . The variation of the section of needle  80  along its main axis is designed to provide a desired performance curve when operating in combination with the calibrated perforation of diaphragm  70  determining a fluid passage  78  of variable area. Variation in fluid passage area  78  occurs by relative displacement of diaphragm  70  and its calibrated opening along the variable section fixed needle  80 .  FIGS. 7A and 7B  illustrate one preferred embodiment that incorporates solid particle retainer system  10  to the body of an outflow rate regulator  30 . Added features are the optional adjustment feature provided by adjustment element  86  operable to modify the resting relative position of diaphragm  70  and its calibrated opening over fixed variable section needle  80 .v  FIGS. 8A and 8B  illustrate another embodiment that incorporates a solid particle retainer membrane  14  within an outflow rate regulator device  30 . A diaphragm  70  is operable to displace towards a flat bed with a calibrated fluid channel  132  as a function of the pressure difference between an access side  48  and an exit side  50 . A fluid passage  130  communicates access side  48  and exit side  50  in a way that when contact occurs between diaphragm  70  and the flat bed, fluid passage  130  delivers fluid to narrow fluid channel  132 . 
         [0029]    OPERATION: Infusion bottle  106  provides pressurized inflow fluid by gravitational or other forces to infusion line  108 . Solenoid valve  110  opens and closes inflow to the eye by clamping infusion line  108  on console command. Infusion line  108  is in fluid communication with the anterior chamber of the eye through infusion sleeve  109  or other infusing devices providing pressurized fluid to the anterior chamber of the eye. Aspiration pump  1   6  produces a vacuum in aspiration line  12  that is transmitted upstream to hollow phacoemulsification needle  104  tip. A fluid outflow and a vacuum at the tip of needle  104  removes lens fragments. Lens fragments are retained by particle retainer  10  to avoid clogging the narrow fluid passages downstream. The filtered fluid travels across outflow rate regulator  30  and is conducted through aspiration line  112  to aspiration pump  16 . Aspiration pump  16  delivers the waste fluid to a waste fluid outlet  117  and is collected by waste fluid collector bag  118 . During unobstructed operation of the phacoemulsification system, aspiration line  112  vacuum remains relatively low and the actual outflow rate can increase almost linearly with the console preset flow rate. In a standard system, upon occlusion of phacoemulsification needle  104  by lens material, aspiration line  112  vacuum increases by the sustained action of aspiration pump  116  partially collapsing aspiration tubing  112  and expanding bubbles eventually present in the aspiration ducts. After an occlusion breaks, fluid rapidly exits the anterior chamber into the aspiration line and a peak of outflow rate is observed through hollow needle  104  to fill the fluid void produced by the expansion of the partially collapsed tubing  112  and contracting bubbles. This peak of fluid outflow is known as post-occlusion surge and can collapse the anterior chamber of the eye and promote complications. The incorporation of the outflow rate regulator  30  of the present invention allows to significantly reduce the post-occlusion surge even when operating at the very high vacuum levels (i.e. above 600 mmHg) available in the most modern phacoemulsification systems available today. Operation of all embodiments depicted in  FIGS. 2 ,  3 , and  4  consider displacement of a flexible membrane or diaphragm  40  progressively occluding one or more fluid passages between an access side  48  and an exit side  50 . In this way, flow across the outflow rate regulator is incrementally restricted according to the pressure gradient across rate regulator  30  access and exit sides. As the pressure gradient is reduced, the occluded fluid passages reopen allowing higher flow rates. A calibrated permanent fluid passage  42  permits a controlled equilibration of the pressure difference minimizing the surge phenomenon and avoiding latch up of the displacing membrane or diaphragm. As the pressure gradient drops, diaphragm  40  returns to incrementally less occluding positions restoring operation at normal flow rate with low pressure differences across device  30 . Preferred embodiment depicted in  FIGS. 4A ,  4 B and  4 C is designed to provide a graded response. Incremental deformation of flexible or movable membrane or diaphragm  40  produces incremental occlusion of fluid passage  60  in a selected pattern determined by membrane  40  elastic properties, architecture, membrane bed  46  shapes, membrane  40  relative position, access side  48  three-dimensional architecture among others. Graphs depicted in  FIGS. 2C ,  3 C,  4 D and  4 E illustrate possible pressure gradient versus flow rate curves corresponding respectively to the outflow rate regulator embodiments shown in  FIGS. 2 ,  3  and  4 . Preferred embodiment depicted in  FIGS. 5  and  FIGS. 7  are designed to provide a graded response to post-occlusion surge. During occlusion conditions the pressure gradient between chambers  48  and  50  is near zero. During normal, non-occlusion operation conditions a pressure gradient appears between chambers  48  and  50  that may displace to some extent diaphragm  70  and its calibrated opening crossed by needle  80 . This displacement occurs along the axis of needle  80  in a zone where needle  80  section is designed not necessarily to contribute to increase resistance to flow. When occlusion breaks with high vacuum in the aspiration line, the pressure difference between access and exit sides  48  and  50  steeply increases producing a proportional displacement of diaphragm  70  with calibrated opening into a wider section of needle  80  narrowing the fluid passage  78  in a way that resistance to fluid flow between chambers  48  and  50  increases. In this manner net flow is limited reducing the rate of fluid extraction from the anterior chamber and avoiding anterior chamber collapse after the occlusion break. With device  30  in operation, the post-occlusion break peak outflow is clamped to moderate flow rates (i.e. &lt;60 cc/min) allowing fluid from infusion line  108  to timely refill the eye chambers preventing collapse. As fluid traverses through the transiently increased resistance between chambers  48  and  50 , the pressure difference reduces allowing the moving parts return to their pre-surge position, increasing in the area of variable fluid passage  78 , returning to non-occlusion normal flow rate operation conditions. The graph depicted in  FIG. 6  illustrates a typical pressure gradient versus flow rate curve corresponding to the performance of the outflow rate regulator  30  embodiment shown in  FIG. 5  and  FIG. 7 .  FIGS. 2C ,  3 C,  4 D, and  5 C include ruler markings X 1 , X 2 , Y 1 , Y 2  that allow a better description of the pressure gradient versus flow rate curve of outflow rate regulators of the present invention. 
         [0030]    As can be interpreted from the graphs shown, flow rate across outflow rate regulator devices  30  of the present invention will increase almost linearly with the pressure gradient when in non-occlusion operation up to a desirable level typically about 40 to 60 ml/min. When the pressure gradient across device  30  exceeds a preset value, the fluid passage will progressively narrow increasing resistance and reducing the flow rate. In this way post-occlusion surge is prevented. 
         [0031]    Alternative embodiment depicted in  FIGS. 8A and 8B  operates by varying the resistance to flow by the action of a diaphragm  70  that progressively contacts a flat bed with a narrow fluid channel  132  in a way that an increasing pressure difference between an access side  48  and an exit side  50  produces an increasing contact zone increasing the effective length of narrow channel  132  increasing resistance to flow. An important design aspect is to produce the modifications in the fluid passage section with minimal volume compliance, to obtain fast responses to variations in pressure differences. 
         [0032]    Similarly, embodiment shown in  FIGS. 8A and 8B  is designed to produce an increase in effective length of narrow channel  132  as a function of pressure differences across diaphragm  70  with minimal volume compliance, to obtain fast variations in flow resistance in response to variations in pressure differences. 
         [0033]    The embodiment shown in  FIGS. 9A and 9B  includes a spring  140  holding a movable body  142  suspended between guides  144  and leaving a clear space  152  for free fluid flow. Septum  146  incorporates permanent fluid passage  42  and blockable fluid passage  52 . A spring holder rim  148  houses the fixed end of spring  140 . The complete system is enclosed by wall  150  having a diameter between 3 and 6 millimetre comparable to standard aspiration line diameters. During operation, normal flow rates (i.e. below 50 cc/min) maintain body  142  at sufficient distance from blockable passage  52  opening. 
         [0034]    Depending on design characteristics of an outflow rate regulator  30 , the pressure gradient versus flow rate curve for a particular device  30  can vary in several ways determining different thresholds, inflections and slopes of the flow versus pressure gradient curve. 
         [0035]    Also depending on variations in design, a different curve can be traced when plotting while moving from a low to high vacuum difference and when plotting moving from a high to low vacuum difference, a phenomenon known as hysteresis and that can be used with advantage upon design. 
         [0036]    Dynamic behaviour can be adjusted by design in a way that different curves can be traced for a single device  30  depending on the rate of change of the pressure gradient across the device. 
         [0037]    It will be understood for those skilled in the art that this description contains many specific details relevant only to the described embodiments. Other embodiments can be construed following the same principles of operation without departing from the present invention. For example the moving part of the variable area fluid passage  78  can be the variable section needle  80  with the diaphragm remaining fixed. The movable portion can be ball shaped. A spring can be part of the deformable portion to adjust the response curve. The permanent fluid passage can be a non-blockable portion of a bigger, partially blockable fluid passage. 
         [0038]    Manufacturing of the present invention can be made using traditional construction techniques and/or micromachining technologies.