Abstract:
A method of and apparatus for removing a thrombus deposit from the cardiovascular system of a patient without the need to surgically access the location of the thrombus deposit via a cut-down or other surgical procedure. A catheter is inserted percutaneously into the patient at a convenient location either directly or over a previously positioned guide wire. The distal end of the catheter is advanced under fluoroscopy to the site of the thrombus deposit. A balloon is inflated to stabilize the position of the distal end of the catheter within the center of the vessel lumen. A flexible metal tube conveys an extremely high pressure stream of sterile saline solution to at least one jet at the distal end of the catheter. At least one jet positions the thrombus deposit for emulsification by at least one other jet. By directing the jets toward the orifice of the large evacuation lumen of the catheter, a stagnation pressure is induced which propels the emulsion proximally for disposal. The rate of proximal flow of effluent is metered to correspond with the distal flow of saline solution to ensure minimal local impact on the vasculature at the site of the thrombus deposit.

Description:
CROSS REFERENCE TO APPLICATIONS 
     This application is a division, of application Ser. No. 08/006,076, filed Jan. 15, 1993 now U.S. Pat. No. 5,370,609, which is a continuation of Ser. No. 07/563,313 filed Aug. 6, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to medical devices and procedures, and more particularly, relates to medical devices and procedures for removing thrombus deposits from the cardiovascular system. 
     2. Description of the Prior Art 
     Procedures and apparatus have been developed for ease in removing tissue and various deposits. U.S. Pat. No. 4,790,813 issued to Kensey and U.S. Pat. No. 4,842,579 issued to Shiber describe techniques for the removal of plaque deposited in arteries by mechanical ablation using rotating cutting surfaces. These relatively traumatic approaches are directed to the treatment and removal of very hard substances. 
     In current medical procedures, thrombus deposits are often removed using a catheter such as is described in U.S. Pat. No. 4,328,811 issued to Fogarty. In this system, a surgical cutdown is performed to access the vessel and allow catheter entry and advancement to a point beyond the deposit. The balloon is inflated and the catheter is withdrawn pulling the deposit along with it. 
     Pressurized fluids have also been used in the past to flush undesirable substances from body cavities. U.S. Pat. No. 1,902,418 describes such a system for domesticated animals. The more modern approaches tend to use vacuum rather than gravity as the primary means for removal of the deposits or tissue and relatively low fluid pressures to cut into and fragment the substances to be ablated. 
     U.S. Pat. No. 3,930,505 issued to Wallach describes a surgical apparatus for the removal of tissue from the eye of a patient. As with similar systems, Wallach uses a relatively low pressure jet of water (i.e. 15 to 3500 psi) to disintegrate the tissue, and a suction pump to perform the actual removal. 
     A similar approach applied to the cardiovascular system is discussed in U.S. Pat. No. 4,690,672 issued to Veltrup. Veltrup also provides a much lower pressure jet of water (i.e. less than 450 psi) to fragment deposits. As with Wallach, Veltrup uses a vacuum pump for evacuation of the fragments. The distal end of the Veltrup catheter is readily repositionable to permit manual entrapment of the deposits to be fragmented. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art systems by performing the entire procedure at positive pressures. This eliminates the need for a vacuum pump and provides the added safety feature of an intravascular environment which is always positively pressurized as during normal functioning of the cardiovascular system. This tends to prevent collapse of the vessel. The system also controls the exposure of the vessel to over pressurization and prevent distension. 
     According to the present invention, the only energy added to the system is via an extremely high pressure stream of saline solution. This stream serves to dislodge thrombus deposits, position them, and then emulsify them. Thrombus particles are attracted to the jet due to the localized high velocity and low pressure. Recirculation patterns and fluid entrainment bring the thrombus continually into close proximity of the jet. Once emulsified by the jet, the particles are removed by flow through the evacuation lumen generated as a result of stagnation pressure which is induced at the mouth of the evacuation lumen by the action of at least one fluid jet directed at and impinging on the lumen mouth. 
     The procedure is practiced by percutaneously or intraoperatively entering the vascular system of the patient at a convenient location with a cannula. The catheter is inserted either directly or over a previously positioned guide wire and advanced under fluoroscopy to the site of the vascular occlusion or obstruction which generally contains an aggregation of blood factors and cells or thrombus deposit, which is normally identified by angiography. One or more balloons may be inflated to stabilize the distal end of the catheter and provide a degree of isolation of the area to be treated. 
     Sterile saline is pressurized by a disposable pump and directed through a flexible metallic tube within the catheter. One or more jets at the distal end of the catheter direct the pressurized stream generally in the direction of the mouth of the evacuation lumen at the distal end of the catheter with a component directed toward the vessel wall. One function of the jet(s) alone or in combination with a distal balloon, is to dislodge thrombus deposits from attachment to the vessel wall. Other functions of the jet(s) are to attract and emulsify the thrombus deposits and create the stagnation pressure which evacuates the emulsion. 
     A metering device is utilized at the proximal end of the evacuation lumen to regulate the flow rate of the emulsified thrombus out of the catheter. Because the entire system operates at a positive pressure, the output must be metered to prevent excess evacuation. Safety monitors turn the system off if one of the lumens or jets becomes clogged. An optional monitor at the distal end of the catheter can monitor power delivery and degree of blockage. An alternative embodiment of the invention provides an extra lumen for monitoring of temperature and/or pressure at the site of the thrombectomy. The evacuation lumen permits the passage of an angioplasty dilatation catheter or angioscope for intravascular viewing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a schematic diagram of the overall system employing the present invention; 
     FIG. 2 a  is a mechanical view of disposable pump; 
     FIG. 2 b  is a cross-sectional view of the disposable pump; 
     FIG. 2 c  is a conceptual view of the safety monitor; 
     FIG. 2 d  is a cross-sectional view of an alternative source of pressurized fluid; 
     FIG. 3 is a cross-sectional view of the manifold; 
     FIG. 4 is a conceptual view of the operation of the manifold; 
     FIG. 5 a  is a close up view of the distal end of the catheter system of the present invention; 
     FIG. 5 b  is a longitudinal sectioned view of the distal end of the catheter system; 
     FIG. 5 c  is a view from the distal end of the catheter system; 
     FIG. 6 is a cross-sectional view from immediately proximal of the balloon; 
     FIG. 7 is a cross-sectional view across the balloon inflation port; 
     FIG. 8 is a cross-sectional view taken distal of the balloon; 
     FIG. 9 is a cross-sectional view taken near the distal tip of the catheter system; 
     FIG. 10 is a longitudinal sectioned view of the distal end of a catheter system employing an alternative embodiment of the present invention; 
     FIG. 11 is a cross-sectional view taken proximal to the proximal balloon of the alternative embodiment; 
     FIG. 12 is a cross-sectional view of the alternative embodiment from the inflation port of the proximal balloon; 
     FIG. 13 is a cross-sectional view of the alternative embodiment taken distal of the proximal balloon; 
     FIG. 14 is a cross-sectional view of the alternative embodiment taken distal of the mouth of the evacuation lumen; 
     FIG. 15 is a cross-sectional view of the alternative embodiment taken proximal of the distal balloon; 
     FIG. 16 a  is a sectioned view of the effluent safety switch; and, 
     FIG. 16 b  is a cross-sectional view of the effluent safety switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view of the preferred embodiment of catheter system  10  employing the present invention. The details supplied herein should be taken as representative and not limiting of the many embodiments which may be efficaciously employed within the scope of the present invention. 
     Catheter system  10  has a standard two lumen catheter  12 , which is extruded of a flexible material, such as polyolefin, PTFE, PVC, polyurethane, or other suitable material in the normal fashion. Near the distal end of catheter  12  is located inflatable balloon  14 , which is preferably an elastic balloon having no predefined outside diameter size limitation upon inflation. In this manner, balloon  14  can conform to the exact dimensions of the vessel to hold distal end  16  of catheter  12  in a fixed position. Alternatively, inflatable balloon  14  can be an inelastic balloon with a predefined shape and size to permit it to be also used for dilatation as in translumenal angioplasty. Distal end  16  of catheter  12  is described in more detail below. 
     Guide wire  18 , as manipulated by knob  38 , is optionally available for positioning catheter  12  as an over-the-wire system. Guide wire  18  passes through the larger of the two lumens of catheter  12  as described in more detail below. 
     Manifold  20  is molded of a rigid plastic. The main branch couples to the larger of the lumens of catheter  12  and has a standard seal assembly  72  applied to the proximal end to sealingly engage guide wire  18 . 
     Secondary branch  24  is also coupled to the larger lumen to provide for evacuation of the emulsified thrombus deposits. Secondary branch  24  sealingly engages distal end  42  of effluent tubing  54  via seal assembly  40 . The operation of safety monitor  44 , monitor switch  50 , and cable  52  are explained in further detail below. 
     Flexible effluent tubing  54 , including distal end  42 , is coupled to safety monitor  44  as described in more detail below. The flow of effluent through flexible effluent tubing  54  is metered by rollers  62 ,  64 , and  66  as rotated by rotor  60  in the direction of arrows  68 . It must be emphasized that the effluent in flexible effluent tubing  54  is under pressure and, therefore, need not be pumped by peristaltic pump assembly  58 , which merely restricts and meters the flow. This metering could equally well be accomplished with a timed mechanical valve (not shown) which controls the outflow rate. After metering, the effluent from flexible effluent tubing  54  is deposited in disposal bag  56 . 
     Secondary branch  26  of manifold  20  is sealingly coupled to inflation tubing  30  by seal assembly  28 . Inflation and deflation of inflatable balloon  14  is controlled by plunger  34  of syringe  32  in the customary manner. Syringe  32  is sealingly coupled to inflation tubing  30  by coupling assembly  36 . 
     The saline solution used to emulsify the thrombus deposit is derived from standard sterile saline bag  94 , which may be commercially available. The saline solution is transferred to disposable pump  80  via hypodermic needle  90  and tubing  88  and couplings  92  and  86 . This is a low pressure fluid path. 
     Disposable pump  80  is a positive displacement piston pump. It is made to be completely disposable for sanitary reasons. Disposable pump  80  is driven reciprocally as shown by arrows  84  by a motor driven cam (not shown) against cam bearing surface  82 . As a convenient means to correlate infused volume of saline solution with volume of evacuated effluent, a single electric motor can be used to drive both disposable pump  80  and rotor  60 . Control of these volumes is important to prevent rupture or collapse of the vessel wall. Closer tolerance control can be achieved at greater complexity using pressure and/or flow meters. 
     The high pressure output of disposable pump  80  is coupled to tubing  76  by high pressure coupling assembly  78 . Tubing  76  has a flexible metallic inner tube inside of a flexible plastic or rubber outer tube as shown in more detail below. Tubing  76  is sealingly coupled to secondary branch  25  of manifold  20  by seal assembly  74 . Safety monitor  96  operates as explained below to turn off the drive motor if the tubing or jets become clogged. 
     FIG. 2 a  is a partially sectioned view of disposable pump  80 . As explained above, disposable pump  80  is designed to be discarded after a single use for sanitary reasons. It is a positive displacement piston pump. All referenced components are as previously described. 
     High pressure coupling assembly  78  is shown in greater detail to highlight that metallic tube  75  is brazed at point  77  to produce the required high pressure joint. Outer tubing  71  is a low pressure connection which may be attached with adhesive. The entire high pressure coupling assembly  78  is attached to disposable pump  80  with threads  73  and compressing a high pressure seal. 
     Safety monitor  96  comprises two safety features. Pressure plug  100  is attached to disposable pump  80  by threads  98 . Pressure plug  100  is designed to release and vent the system to the atmosphere at pressures above 30,000-40,000 psi. The second safety feature serves to electrically disconnect the drive motor whenever the pressure is too high. Increased pump pressure forces contact  104  toward electrical contact with contact  106  as a result of pushing out of pressure plug  100  as attached at point  102  (shown in detail below), thereby closing the electrical circuit to a relay and turning off the drive motor. Insulators  95  and  97  maintain contacts  104  and  106  open under normal pressure conditions. 
     The saline input to the disposable pump includes a hypodermic needle  90  which penetrates a puncture port on a bag of saline. The saline is delivered through coupling  92  to tube  88  and through coupling  86  into the inlet of the disposable piston pump  80 . 
     FIG. 2 b  is a cross-sectional view of disposable pump  80 . As a matter of convenience the disposable pump  80  is oriented slightly different from FIG.  1 . All referenced components are as previously described. 
     Cam  310  is rotated by a drive motor (not shown) as discussed above. The action of cam  310  imparts a reciprocal motion to cam bearing surface  82  causing connecting rod  302  to move horizontally. This moves piston  300  in the direction of arrows  311 . Movement to the left enlarges the effective volume of chamber  305  creating a relatively low pressure. This permits entry of sterile saline fluid from fluid entry port  312  (see also FIG. 1) through ball valve  306  under tension of spring  307 . 
     Movement of piston  300  to the right decreases the effective volume of chamber  305  forcing sterile saline solution to exit via ball valve  309  under sufficient pressure to overcome the tension of spring  308 . Note that ball valve  306  will be forced closed as piston  300  is moved to the right. The saline solution is expelled through high pressure tube  75 . 
     Seals  301  and  303  and springs  307  and  308  are selected consistent with the fluid pressures to be developed. Bellows  304  provides an additional seal for the system. Cam  310  may be designed to provide a relatively smooth flow of sterile saline, or it may be implemented as a Geneva or similar cam to enhance the pulsatile delivery of the sterile saline to change the emulsification action at the distal tip of catheter  12 . 
     Pressure plug  100  can be adjusted so that if the pressure reaches an upper limit, such as 30,000-40,000 psi, the pressure will be released and the safety monitor  96  will turn the motor off. 
     FIG. 2 c  is a schematic view of safety monitor  44 . The emulsified thrombus is evacuated in line  314 . If the entrance to the evacuation port becomes blocked, the pressure in line  314  will drop and cause membrane  322  to retract around line  314  which has an opening port  324  which has passage to the membrane. As the membrane retracts due to a blockage in the evacuation tube, the contacts  340  and  336  are opened and thereby trigger a relay  329  which will turn off the drive motor  328 . 
     FIG. 2 d  is a cross-sectional view of an alternative source of pressurized fluid. This approach replaces the function of disposal piston pump  80 . Using this technique, the high pressure tubing  118 , plugs  53  and  48 , tapered ring  119 , and saline bag  95  are inserted into the conformal housing  46  and tightened down using threads  45 . Chamber  43  is pressurized by supplying pressurized non-sterile water or other fluid through inlet  51  forcing sterile saline to exit from port  49  of tubing  118 . A seal  81  is made between the bag  95  and the high pressure tubing  76  which delivers the high pressure saline. The high pressure tubing  118  is brazed into a tapered sealing ring  119 . A seal is made between the bag  95  and the ring  119  and also between the bag  95  and end plug  48  by tightening down plug  53 . The outer plastic tubing  76  is adhesively bonded to plug  53 . Bottom plugs  61  and  69  are held in place by threads  63 ,  65 , and  67  and sealed by seal  83  as plug  69  is tightened down. 
     Whenever employing this alternative embodiment, care must be exercised not to rupture sterile saline bag  95  under the extreme pressures required by the present invention. High pressure fluid is supplied to tubing  85  from a positive displacement pump (not shown). 
     FIG. 3 is a cross sectional view of manifold  20 . Because this component is molded as two halves, which are solvent-bonded together, the view also happens to show one of the two halves. As explained above, catheter  12  is a two lumen catheter. In the preferred mode, each of the two lumens has two distinct functions. Therefore, manifold  20  serves to provide passage for a high pressure tubing and balloon inflation through one lumen and passage of a guide wire and evacuation through the other lumen. 
     The larger lumen of catheter  12  is lumen  110 . It is used for passage of guide wire  18  (not shown in this view) and for evacuation of effluent and possible passage of an angioplasty dilatation catheter or angioscopic probe. Lumen  110  terminates inside the manifold  20  at the proximal end of the flexible tubular member and provides passage of a guide wire or other diagnostic or therapeutic device. Guide wire  18  is sealed by compressible circular seal  136  which is compressed by surface  140  as threaded knob  72  is tightened on threads  138 . It is important to seal guide wire  18  in this way as guide wire  18  must be movable with respect to catheter  12  to properly manipulate distal tip  16  of catheter  12  into position. 
     Lumen  110  is also terminated at secondary branch  24 . This is accomplished by removing a portion of the outer wall of lumen  110  at point  120 . This provides fluid coupling between lumen  110  and lumen  134  of secondary branch  24 . 
     The smaller lumen of catheter  12  is lumen  112 . One of its functions is as a fluid passageway for the inflation of balloon  14 . This function is accomplished by removing a portion of the outer wall of lumen  112  at point  114  to fluid couple lumen  112  to lumen  116  of secondary branch  26 . 
     The remaining purpose of lumen  112  is to provide for passage of metallic tubing  118 . Because of the extremely high pressures involved, the saline solution is conveyed in a metallic tubing  118 , which is preferably stainless steel hypo tubing. To handle the pressures involved, the hypo tubing is run as a continuous length along catheter  12 . The proximal end of metallic tubing  118  passes through the outer wall of lumen  112  and into secondary branch  25 . A larger diameter hypo tube is brazed onto hypo tube  118  at point  123 . This larger tubing is covered by protective plastic tubing  71 . Manifold  20  is solvent-bonded together prior to assembly of the catheter, and points  124 ,  126 ,  128 ,  130  and  133  are used to introduce an adhesive which serves as a seal to separate each path and each lumen. Point  132  shows the bonding of the outer plastic tube which surrounds the high pressure supply tube to the manifold. 
     FIG. 4 is a schematic view of manifold  20  wherein all referenced elements are as previously described. This figure is purposely not drawn to scale to better illustrate the operation of manifold  20 . 
     FIG. 5 a  is a close up view of balloon  14  and distal tip  16  of catheter  12 . Attachment between catheter  12  and balloon  14  occurs at overlap points  144  and  146 . These overlap points are sealingly attached with adhesive or heat sealing. 
     Catheter  12  is a dual lumen catheter extruded from rubber or a polymer as described above. Cap  150  is fixedly attached at the distal tip of catheter  12  as shown. Preferably cap  150  is made of a radiopaque metal such as platinum, tantalum or stainless steel to provide ease of location under fluoroscopy. 
     Extending beyond cap  150  is metallic tubing  118 . This is necessary to permit the jet or jets which dispense the saline solution to be directed at cap  150  (i.e. the distal tip of lumen  110 ). Because metallic tubing  118  is so flexible, it must be backed by metal plate  156  to provide the necessary rigidity. Metallic tubing  118  is bent as explained below. To conform, metal plate  156  is angled to form rounded distal surface  160 . This annular shaped tip would allow passage of a guide wire, angioscope, or angioplasty dilatation catheter. 
     FIG. 5 b  is a longitudinal sectioned view of the structure of FIG. 5 a , wherein referenced elements are as previously described. Also shown in this view is balloon inflation port  148  which provides fluid communication between lumen  112  and balloon  14 . Septum  142  separates lumen  110  from lumen  112 . Lumen  112  is sealed distal to the balloon using an adhesive seal  113  attaching the high pressure tube  118  and filling lumen  112 . 
     Metallic tubing  118  is bent into a circular shape perpendicular to the axis of the catheter beginning at bend  152 . Metal plate  156  bends at point  158  to provide rigidity at that point. Jet  164  is a small diameter orifice on the order of 0.0005 to 0.003 of an inch. It directs a stream of saline solution at cap  150  (i.e. mouth of lumen  110 ) at 5,000 to 30,000 psi. This pressure is sufficient to emulsify thrombus deposits located between jet  164  and cap  150 . This stream of saline solution also creates a stagnation pressure about cap  150  sufficient to propel the emulsion into and through lumen  110  (see also FIG.  1 ). This stream of saline solution is of high velocity which creates a localized area of low pressure around the stream which attracts thrombus deposits for emulsification and removal. 
     FIG. 5 c  is a view from the distal end of catheter system  10  wherein referenced elements are as described above. The central opening would allow passage of a guide wire, angioscope or angioplasty dilatation catheter. 
     FIG. 6 is a cross-sectional view from the proximal end of catheter  12  to balloon  14 , wherein referenced elements are as previously described. Metallic tubing  118  is shown within lumen  112 . The cross-sectional area of lumen  112  which is in excess of that needed for metallic tubing  118  provides the fluid passageway for inflation of balloon  14 . Lumen  162  of metallic tubing  118  has a diameter of about 0.003-0.010 inch. It conveys saline solution at 1,000 to 30,000 psi through the main body of the catheter. 
     FIG. 7 is a cross-sectional view taken through balloon  14  and balloon inflation port  148 . The remaining elements are as previously described. 
     FIG. 8 is a cross-sectional view taken distal to balloon  14 . The distal end of the balloon inflation lumen is plugged with adhesive  113  to provide an enclosed space for balloon inflation. 
     FIG. 9 is a cross-sectional view taken just proximal of the saline solution jets. Shown in addition to jet  164  are jets  154   a ,  154   b ,  154   c , and  154   d , which are similar in size and range to jet  164 . Jet  164  is directed generally back toward the evacuation channel to generate a stagnation pressure, create a localized area of low pressure to attract thrombus deposits, emulsify any thrombus which is brought into its path and keep the opening to the evacuation lumen clean and open. 
     Jets  154   a ,  154   b ,  154   c , and  154   d  can number from zero to eight with a preferred number of three to six jets, although not limiting and are directed with some radial component toward the vessel wall as drawn and may also have some axial direction towards the evacuation opening. These jets remove thrombus which is attached to the vessel wall and establish a recirculation pattern which entrains thrombotic material and brings it into contact with jet  164  for further emulsification and removal. 
     FIG. 10 is a close up view of the distal end of an alternative embodiment of the present invention including balloon  204  which is located distal to the active components. Balloon  204 , along with balloon  202 , can be used to isolate a portion of the vessel during the procedure. Fluid recirculation between the balloons brings the thrombus into contact with the jet for emulsification and removal. In this embodiment, lumens  206  and  208  function as lumens  110  and  112 , respectively. Cap  220  is similar to cap  150 . A thermistor (not shown) can be used with either the preferred or alternative embodiment. The thermistor concept should only be added as a possibility which will help in monitoring the degree of occlusion and/or power delivery. Metal tube  212  has the same function as metal plate  156  in the preferred embodiment. Metallic tubing  210 , bend  214  and jet  216  directly correspond to similar components in the preferred embodiment. An adhesive  221  and  223  is applied in the lumen of distal tubing  225  to provide an enclosed space and allow balloon  204  to be inflated through the distal balloon inflation port  219 . 
     FIG. 11 is a cross-sectional view of the alternative embodiment from proximal to balloon  202 . In this embodiment, a three lumen catheter is used. Lumen  206  is the largest lumen, which is used for passage of the guide wire and evacuation of the effluent. Annular space  222  is used for inflation of balloon  202  and for passage of metallic tubing  210 . Lumen  226  provides for inflation of balloon  204 . Annular space  224  could be used to permit an external device (not shown) to measure the pressure and/or temperature within the treatment area to determine when thrombus deposits are completely emulsified. 
     FIG. 12 is a cross-sectional view of the alternative embodiment as viewed through balloon  202 . Shown is balloon inflation port  218 . 
     FIG. 13 is a cross-sectional view of the alternative embodiment as viewed distal of balloon  202 . Guide wire  228  is shown located within lumen  206 . 
     FIG. 14 is a cross-sectional view of the alternative embodiment as viewed distal to cap  220 . 
     FIG. 15 is a cross-sectional view of the alternative embodiment as viewed proximal to balloon  204 . Shown is a jet  216  directed back towards the evacuation lumen and a plurality of jets numbered  217   a ,  217   b ,  217   c  and  217   d , which are directed with some radial component toward the vessel wall. These outwardly directed jets may not be necessary since the distal balloon can be used to dislodge the thrombus off of the wall. Metal tubing  212 , extends to the distal balloon for inflation. 
     FIG. 16 a  is a longitudinal sectioned view of safety monitor  44 . It is placed over flexible membrane  330  of distal end  42  and flexible effluent tubing  54 . Fluid communication is supplied by port  332 . 
     FIG. 16 b  is a cross sectioned view of safety monitor  44 . It functions much as a safety monitor with contacts  336  and  340  being closed whenever pressures are reduced due to blockage of the evacuation tube. Fluid communication to the membrane is supplied by port  332 . 
     Various modifications can be made to the present invention without departing from the scope thereof.