Patent Publication Number: US-2005119588-A1

Title: Microdialysis probe with inserting means and assembly

Description:
FIELD OF THE INVENTION  
      The present invention relates to a microdialysis probe and an assembly comprising a microdialysis probe and a cannula.  
     BACKGROUND OF THE INVENTION  
      Microdialysis probes are used, for instance, in collecting tissue fluid samples from humans and animals for diagnostic purposes, either intermittently or continuously. A microdialysis probe comprises a semipermeable membrane, one face of which is in contact with tissue whereas its other face is in contact with a solvent flowing inside of the probe. The solvent which is water or saline or similar is adduced to the probe via a first tube and passes the membrane. Depending on the pore size of the membrane small and medium size molecules in the tissue fluid can penetrate the membrane and are carried away by the probe solvent, thus forming a sample solution. The sample solution is carried away from the probe by a second flexible tube or similar. The analytes in the sample solution can be analyzed in a conventional manner, either continuously or discontinuously. For instance, the sample solution is made to flow through a quartz cell for detection and/or measurement of analytes that absorb UV light of a certain wavelength.  
      There are some important restrictions of design in regard of microdialysis probes. In general, the diameter of a probes has to be small to allow it to be inserted into tissue by means of a cannula provided with a sharp tip.  
      The use of a cannula is due to the fragile nature of the microdialysis probe, in particular of its membrane. Upon insertion the cannula has to be withdrawn to expose the membrane to the tissue. The design of thin microdialysis cannulae known in the art and used in clinical practice however requires the use of a particular kind of cannula with a longitudinal slit, such as the cannula disclosed in WO 95/20991. The reason for this is that the proximal (rear) end portion of state-of-the-art cannulae is rather bulky due to the mounting of the solvent-adducing tube and the sample-removing at the probe. For instance, in the microdialysis probe of this design disclosed in WO 95/20983 said tubes are mounted at a distal end piece the diameter of which (in a transverse direction to the extension of the probe) is substantially greater than the diameter of the probe. The distal end piece thus is too large to allow a cannula of a physiologically acceptable diameter (about 1 mm or less) to be drawn over the end piece rearwards, and thus withdrawn. The microdialysis probe of WO 95/20983 thus necessitates the use of a cannula having a longitudinal slit, such as the cannula disclosed in WO 20991.  
      U.S. Pat. No. 5,106,365 (Hernandez) discloses a microdialysis probe which is substantially rotationally symmetrical and comprises an outer sleeve at the distal end of which a dialysis membrane is fixed in such a manner so as to avoid extra thickness at the junction between the membrane and the sleeve. At the proximal end the sleeve is mounted in a second sleeve of substantially larger diameter. The tubes for delivery and removal of dialysis fluid are made of a stiff material. No technique for insertion of this known probe into tissue so as to avoid potential damage is disclosed.  
      The present invention seeks to avoid the aforementioned problems.  
     OBJECTS OF THE INVENTION  
      It is an object of the present invention to provide a microdialysis probe which does not require the use of a cannula that needs to be split longitudinally for removal.  
      It is an object of the present invention to provide a microdialysis probe which can be easily produced.  
      It is another object of the present invention to provide an assembly of such microdialysis probe and a corresponding cannula.  
      Further objects of the invention are evident from the following short description of the invention, the attached drawings illustrating a preferred embodiment, the detailed description thereof, and the appended claims.  
     SAY OF THE INVENTION  
      The dialysis probe of the invention, like dialysis probes known in the art, has substantial longitudinal extension, that is, extension in a proximal/distal direction, whereas its extension in a transverse direction, that is, its diameter, is comparatively small.  
      According to the present invention is provided a microdialysis probe comprising a tubiform probe body, a tubiform dialysis membrane disposed distally of the probe body, and flexible conduits for adducing and abducing dialysis fluid, distal end portions of which are disposed in the probe body, the probe being, apart from the conduit portions disposed externally of the probe body, substantially rotationally symmetrical, one of said conduits comprising an S-shaped portion inside of the probe adjacent to the distal end of the other conduit. It is preferred for the conduit comprising the S-shaped portion to be the abducing conduit and to have a distal end portion extending distally of the probe body. Preferably the sum of the outer diameters of the conduits corresponds to the inner diameter of the probe body. It is preferred for the probe to have a maximum outer diameter of 1.5 mm or less, in particular a diameter from 0.8 to 1.2 mm.  
      According to a first preferred aspect of the invention the microdialysis probe comprises a rear fitting connected to the probe body at the proximal end thereof so as to make a proximal end portion of the probe body to be enclosed by the rear fitting thereby forming a first chamber communicating via the probe body lumen with a second chamber formed by the membrane which is connected to the probe body at a distal end portion thereof. It is preferred to make a probe body distal end portion to be enclosed by a proximal end portion of the membrane. At its distal end the membrane is plugged or closed in any other convenient manner.  
      According to a second preferred aspect of the adducing and abducing flexible conduits are sealingly fastened at the rear fitting, in particular at a proximal end portion thereof, so as to make one of them extend through the rear fitting and the probe body and having its front end located in the front chamber adjacent to the distal end of the membrane, and to make the other of them having its front end disposed in the rear chamber. It is preferred for the first and second tubes to be sealingly fastened to the rear fitting at a proximal portion thereof.  
      According to a third preferred aspect of the invention it is preferred for the probe body to be radiologically opaque, in particular in regard of the other elements of the probe and the tissue to which it is intended to be inserted, such as fat or connective tissue. It is particularly preferred for the probe body to consist of a polymer or a mixture of polymers and radiologically opaque material finely dispersed therein, in particular barium sulphate.  
      According to a fourth preferred aspect of the invention is disclosed the combination of the microdialysis probe of the invention and a cannula the lumen of which is larger than the maximum outer diameter of the probe. It is also preferred for the microdialysis probe to be received in the cannula in its entirety except for proximal end portions of the adducing and abducing flexible tubes, and in combination with which it can be inserted into living tissue and from which it is slidingly removable in a proximal direction. It is preferred for the cannula to comprise a holding means disposed near its proximal end.  
      According to a fifth preferred aspect of the invention is disclosed the combination of the probe and the cannula in a configuration ready for insertion packaged in a sterile condition.  
    
    
     SHORT DESCRIPTION OF THE FIGURES  
      The invention will now be explained in more detail by reference to a preferred but not limiting embodiment illustrated in a drawing showing in  
       FIG. 1 a  microdialysis probe according to the invention, in a side view;  
       FIG. 2  the probe of  FIG. 1 , in a longitudinal section;  
       FIG. 3  the probe of  FIGS. 1 and 2 , provided with a cannula according to the invention for insertion into tissue, in the same view as in  FIG. 1 ;  
       FIG. 4  the probe of  FIG. 1 , in an enlarged transverse section A-A. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT  
      The microdialysis probe  1  of the invention shown in the Figures comprises a substantially rigid cylindrical probe body  2  to which a proximal sleeve  3  is attached by gluing  17  at the sealed rear opening of the proximal sleeve  3  protrude first and second flexible tubes  6 ,  7 , the proximal ends of which are not shown. The distal end portion of the probe body  2  is covered by a dialysis membrane  4 . The sealing gluing  17  at the rear opening of the proximal sleeve  3  is shown in  FIG. 4 . It may be provided, for instance, by a two-component polyurethane glue. Similarly, the membrane  4  is sealed at its front end by a polyurethane plug  5 . The probe body  2  can be made of any suitable stiff but resilient plastic material, such as polyamide, but also of metal such as, for instance, stainless steel. The probe body of plastic material can be made opaque to radiation, such as X-rays, by incorporation of finely dispersed barium sulphate. The proximal sleeve  3  is made of polyimide. The flexible tubes  6  and  7  are made of fluorinated polyethylene/propene. The maximum width of the probe  2  (at the proximal sleeve  3 ) was determined to be 1.0 mm.  
      As shown in  FIG. 2  the interior of the probe  1  comprises three communicating compartments: (a) a proximal chamber  9  defined by the proximal sleeve  3 , its sealed rear opening and the probe body  2 , (b) the interior of the probe body  2 , (c) a distal chamber  8  defined by the membrane  4  and the membrane plug  5 . The first flexible tube  6  for carrying away dialysis fluid extends through the proximal chamber  9  and the probe body  2  into the distal chamber  8 ; its front opening  10  is disposed adjacent to the membrane plug  5 . The portion of the first flexible tube  6  disposed in the proximal chamber  9  and in close proximity thereof has an oblong S-formed configuration with bends at  6 ′ and  6 ″. In other words, the first flexible tube  6  has two parallel (but not coaxial) sections, one of them located in the probe body  2 , the other in the proximal portion of the proximal sleeve  3  through which the distal end portion of the second flexible tube  7  extends; the portion of the flexible tube  6  extending between these parallel sections forms an oblique, angle with the probe axis. The lumen of the probe body  2  is slightly greater than the outer diameter of the first flexible tube  6  thus providing for communication between the proximal chamber  9  and the distal chamber  8  through the interstice  12  formed between the outer wall of the first flexible tube  6  and the inner wall of the cylindrical probe body  2 . The second flexible tube  7  extends into the proximal chamber  9  where its distal opening  11  is disposed. Dialysis fluid adduced through the second flexible tube  7  emerges at its distal end  11  into the proximal chamber  9 , flows from there through the interstice  12  and enters the distal chamber  8 . During its passage through the distal chamber  8  the dialysis fluid takes up material, such as small and medium size molecules, from body fluid surrounding the probe  1  that diffused through the membrane  4 ; the technique can however also be used for simultaneous administration of drugs and other compounds dissolved in the dialysis fluid to the surrounding tissue. Finally the dialysis fluid loaded with said material and enters the distal opening  10  of the first flexible tube  6  by which it is emptied from the probe  1 . Normally the dialysis fluid is a physiological aqueous sodium chloride solution. Cellulose acetate (manufactured by Akzo N. V., Netherlands), polyethersulphone and other semipermeable membranes of various kind and various cut-off pore sizes determining the size of molecules that can pass through the membrane  4  are known in the art and can be used in the probe  1  of the invention. The dialysis fluid loaded with those molecules is fed to an analysis unit (not shown) disposed downstream of the first flexible tube  6 . The dialysis fluid is fed into the probe  1  via the second flexible tube  7  by pumping means (not shown) disposed upstream of the probe  1 , such as, for instance, a fluid reservoir kept under a positive pressure, or a mini-roller pump at a selected feed rate, in particular at a selected constant feed rate.  
      The transverse dimensions of the probe  1  are generally kept as small permitted by considerations of mechanical stability, membrane area, and flow rate. The microdialysis probe  1  of the invention can be inserted into tissue by appropriate means, such as the cannula means disclosed in WO 95/20991. However, the design of the probe  1  of the invention allows the use of a cannula means that need not be split longitudinally for withdrawal. Such a cannula of the invention  13  is shown in  FIG. 3 . It is of simple design, comprising a stainless steel cannula body  14  having a pointed tip  15  at its distal end and a holder  16  attached near its proximal end. In  FIG. 3 a  combination of the microdialysis probe  1  of the invention and the cannula  13  of the invention is shown in state ready for insertion into living tissue, the distal portion of the probe  1  being disposed in the lumen of the cannula  13 . Since the lumen of the cannula is wider than the widest portion of the probe  1 , the distal sleeve  3 , the cannula  13  can be removed by withdrawal in a proximal direction upon insertion but before the proximal ends (not shown) of the flexible tubes  6 ,  7  are attached to the dialysis pump (not shown) and the analysis unit (not shown), respectively. The use of a cannula for insertion is due to the limited mechanical stability of the microdialysis probe; however, under certain conditions, such as with tissue that offers little mechanical resistance, in particular neural tissue, the use of a cannula can be dispensed with.