Abstract:
A device ( 1 ) for controlling the rate of flow of a fluid such as fluid which is diverted from a part of a human or animal body. The device comprises: an inlet ( 2 ); an outlet ( 4 ); and a resistance member ( 6 ), operatively connected to the inlet ( 2 ) and the outlet ( 4 ), the resistance member ( 6 ) comprising: a first plate ( 8 ), a surface of which comprises a groove which defines a resistance flow channel, an entry of the flow channel being in fluid communication with the inlet ( 2 ) and an exit of the flow channel being in fluid communication with the outlet ( 4 ); and a second plate ( 12 ), the second plate being held in abutment with the grooved surface of the first plate ( 8 ) so as to define a resistance (tube  14 ).

Description:
[0001]    The present invention relates to a device for controlling the rate of flow of a fluid and may for example, be used to divert fluid from the ventricles of the brain to another part of the body. 
       BACKGROUND 
       [0002]    Hydrocephalus is a defective condition of the brain caused by an imbalance between production of cerebral spinal fluid (CSF) within the brain and the capacity of the brain to re-absorb such fluid at normal pressure. Hydrocephalus may be congenital, accidental or age related and can result in loss of a wide range of physical and mental faculties. The accepted method of treatment of the condition is to divert excess fluid which the brain, through its impairment, is unable to absorb, to some other part of the body such as the right atrium or the peritoneal cavity, where the fluid can re-enter the blood stream. The primary technical challenge associated with this method of treatment is developing the capacity to control the conditions of pressure and flow within the brain in such a manner as to enable lost faculties to be restored, depending on the severity of the condition. This challenge is exacerbated by the greatly differing range of impairment between different patients. 
         [0003]    Various devices have been developed to control the pressure and flow conditions within the brain to desired levels. Such devices are commonly known as valves. If the flow imbalance referred to above is not addressed, pressure within the brain rises to an abnormal level. Such increased pressure causes the ventricles of the brain to expand and causes abnormal stress and damage within the brain tissue. Consequently, many valve devices focus on a technology that controls the pressure within the ventricles of the brain directly, restricting that pressure to a desired level and allowing flow rates to vary to accommodate the target pressure. Such devices are referred to as pressure control valves and commonly comprise an orifice that may be forced open by fluid pressure within the brain against some form of resistant mechanism such as a ball and spring or a slit in a tubular member. The majority of valves in use at the present time are pressure control valves. Difficulties with this type of valve arise in being able to set the spring pressure accurately. In addition, the valves are susceptible to malfunction when subject to pressure waves which occur in the brain and they are also very sensitive to changes in pressure due to body posture. 
         [0004]    An alternative type of valve directly controls the amount of fluid diverted. Control is effected in such a manner that the desired pressure conditions within the brain are achieved as a secondary effect. Such devices are referred to as flow control valves. Currently available valves control flow by some form of restrictive orifice, which in some cases can be varied to give different flow rates. However, at the pressures existing in the brain (around 12 cm H 2 O in a normal supine person), and at the very low diverted flow rates involved (some fraction of 0.35 ml/min) the size of orifice is minute, and it is difficult to predetermine accurately the resulting flow. 
       SUMMARY OF INVENTION 
       [0005]    According to the present invention, there is provided a device for diverting fluid from a part of a human or animal body such as the ventricles of the brain, the device comprising: an inlet; an outlet; and a resistance member, operatively connected to the inlet and the outlet, the resistance member comprising: a first plate, a surface of which comprises a groove which defines a resistance flow channel, an entry of the flow channel being in fluid communication with the inlet and an exit of the flow channel being in fluid communication with the outlet; and a second plate, the second plate being held in abutment with the grooved surface of the first plate so as to define a resistance tube. 
         [0006]    The device may divert the fluid to another part of the human or animal body. 
         [0007]    A resistance tube such as that provided by the present invention limits flow through the tube by frictional resistance on the walls of the tube. The device of the present invention provides highly predictable and stable control of pressure and flow within the brain of a patient. Due to the nature of the device, it is not substantially influenced by pressure waves within the fluid it diverts and it is far less sensitive to body posture than a pressure control valve. 
         [0008]    The device may be configured to be implanted under the skin of, for example, the scalp or chest of a person. It is an advantage of the device that it may be constructed in a highly compact fashion, accommodating the required length of resistance tube within a relatively small surface area. The device can therefore be made sufficiently small and compact to be implanted under the skin of a patient. 
         [0009]    The effective hydraulic mean diameter is defined as four times the cross sectional area of the flow in the channel divided by the wetted perimeter of said channel (for example, the hydraulic mean diameter of a circular cross section is the diameter). The effective hydraulic mean diameter of the resistance tube may be between 0.3 mm and 0.8 mm. Optionally, the effective hydraulic mean diameter of the resistance tube is between 0.4 mm and 0.5 mm. For example, the effective hydraulic mean diameter of the resistance tube may be 0.45 mm. The suggested ranges of effective hydraulic mean diameters ensure a desired flow rate is maintained within a device that is of a size suitable for implantation under the skin. 
         [0010]    The resistance tube may define a convoluted fluid flow path. For example, it may be folded back on itself at least once. Such a convoluted flow path enables the device to accommodate the high length to diameter ratio required for functionality whilst remaining compact and occupying a relatively small surface area. 
         [0011]    The exit of the resistance tube may communicate with the outlet via a selection means. The selection means imparts an element of selectivity and flexibility to the device. 
         [0012]    The resistance tube may comprise at least two exits along its length, each of which may be placed in fluid communication with the outlet by the selection means so as to vary the effective length of the resistance tube. By varying the effective length of the resistance tube, it is possible to tailor the device to treat patients having greatly differing levels of impairment. 
         [0013]    The exit may comprise an orifice in one of the first and second plates, which orifice may be covered or uncovered by the selection means. 
         [0014]    The selection means may comprise a rotor mounted adjacent the resistance member and having a through passage, the rotor being mounted in the device for rotation relative to the resistance member, such that an exit of the resistance tube may be brought into fluid communication with the outlet via the through passage. 
         [0015]    The selection means may be magnetically excitable, such that it may be operated via a magnet held proximate the device. The device may therefore be operated transcutaneously. Once implanted, the device may be adjusted to suit the changing needs of a patient in situ, without the need for further surgical intervention. 
         [0016]    The resistance tube may define a fluid flow path comprising a plurality of sectors, each sector having an associated exit that may be selected by the selection means. 
         [0017]    The selection means may be a sliding fit with the resistance member. Alternatively or in addition, the device may comprise a biasing means, which resiliently urges the selection means into engagement with the resistance member. 
         [0018]    The biasing means may comprise a spring, a leaf spring or a helical spring, a Belleville washer or a resilient element. 
         [0019]    The device may further comprise a biased ratchet or brake that releasably retains the selection means in a selected rotational orientation. The selection means is thus securely held in its desired position when not being adjusted. 
         [0020]    The selection means may further comprise a magnetic marker, which may be provided in the selection means, such that the orientation of the selection means may be determined through use of a compass or magnetic indicator held proximate the device. Such a marker assists in monitoring the operation of the device and in adjusting the device if necessary. 
         [0021]    The device may further comprise a filter at the inlet of the device. The filter reduces the possibility of the resistance tube becoming occluded. 
         [0022]    The device may further comprise an anti reflux mechanism at the outlet of the device. 
         [0023]    Such a mechanism ensures that reflux back towards the origin of the fluid cannot take place for any reason. 
         [0024]    The groove on the first plate may be formed in any appropriate manner. For example, the grove may be milled, laser cut, electro-discharge machined, electro-chemically machined, chemically etched, or moulded. Such techniques enable the resistance channel, and hence the resistance tube, to be machined to the extremely high tolerances required to achieve accuracy of flow control. 
         [0025]    The device may be formed from a biocompatible material selected from titanium, stainless steel alloy, or composite material. 
         [0026]    The device may further comprise a fixing element that maintains the first and second plates in abutment. For example, the fixing element may comprise one of a screw, a weld, a glue joint, a brazed joint or a cap. 
         [0027]    The inlet and the outlet may each define an axis and the axes of the inlet and outlet may not be parallel. Such positioning of the inlet and outlet facilitates routing of tubing to the area of the body where fluid is to be discharged. 
         [0028]    The device may further comprise at least one additional plate, held in abutment with one of the first and second plates. 
         [0029]    A surface of the first plate opposite the grooved surface may also comprise a groove which defines a resistance flow channel, and the additional plate may be held in abutment with this second grooved surface of the first plate so as to define a further resistance tube. 
         [0030]    The additional plate may be held in abutment with the second plate, a surface of the additional plate may comprise a groove which defines a resistance flow channel, such that the groove on the additional plate, in combination with the second plate that is held in abutment with the grooved surface of the additional plate, defines a further resistance tube. The further resistance tube provides additional resistance length and adds flexibility to the device without causing the device to increase in size significantly. 
         [0031]    The further resistance tube may be in fluid communication with the first resistance tube. The resistance tube and further resistance tube are thus connected in series to form a compound resistance tube of greater length and hence greater resistance to flow. 
         [0032]    The further resistance tube may be in fluid communication with the inlet, such that the resistance tube is in fluid communication with the inlet via the further resistance tube. 
         [0033]    In a preferred embodiment, the device diverts fluid from the ventricles of the human or animal brain to another part of the human or animal body, such as the peritoneal cavity or right atrium. 
         [0034]    The first, second or additional plates may be provided with a rim about an outer edge, such that the rim may be arranged to contain the one or more other plates. For example, the rim may be formed on one side of the first, second or additional plates, such that the remaining plates may fit inside the rim. Alternatively, the rim may extend on both sides of the first plate, such that the second and additional plates may fit inside the rim on either side of the first plate. The rim may also extend on both sides of the second plate, such that the first and additional plates may fit inside the rim on either side of the second plate. The first, second or additional plates may therefore form a module separate from the main casing, which may be flow pressure tested independently prior to assembly in the main casing. 
         [0035]    Surfaces of the resistance member may be provided with an antibiotic coating. In particular, surfaces of the first, second and/or additional plates may be provided with an antibiotic coating, such that the surfaces of the resistance tube and/or further resistance tube are coated with said antibiotic coating, thereby preventing the growth of any undesirable bacterial deposits. The inlet, outlet, anti-reflux mechanism and/or filter may also be coated with an antibiotic coating. 
         [0036]    According to another aspect of the present invention, there is provided a method of treating hydrocephalus by diverting fluid from the ventricles of the brain to another area of the body via a resistance tube, the resistance tube being formed by a grooved surface of a first plate that is held in abutment with a planar surface of a second plate. 
         [0037]    According to another aspect of the present invention, there is provided a flow control device comprising: an inlet; an outlet; and a resistance member, operatively connected to the inlet and the outlet, the resistance member comprising: a first plate, a surface of which comprises a groove which defines a resistance flow channel, an entry of the flow channel being in fluid communication with the inlet and an exit of the flow channel being in fluid communication with the outlet; and a second plate, the second plate being held in abutment with the grooved surface of the first plate so as to define a resistance tube, wherein the exit of the resistance tube communicates with the outlet via a selection means, and wherein the resistance tube comprises at least two exits along its length, each of which may be placed in fluid communication with the outlet by the selection means so as to vary the effective length of the resistance tube. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:— 
           [0039]      FIG. 1  is a sectional view of a device for diverting fluid. 
           [0040]      FIG. 2  is a partially cut away sectional view of the device of  FIG. 1 , taken at 90 degrees to the view of  FIG. 1 . 
           [0041]      FIG. 3  is a sectional view of an alternative embodiment of device for diverting fluid. 
           [0042]      FIG. 4   a  is a sectional view of the device of  FIG. 3 , taken along the line XX of  FIG. 3 . 
           [0043]      FIG. 4   b  is a sectional view of the device of  FIG. 3 , taken along the line YY of  FIG. 3 . 
           [0044]      FIG. 5  is a sectional view of another alternative embodiment of device for diverting fluid. 
           [0045]      FIG. 6  is a sectional view of the device of  FIG. 5 , taken along the line XX of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0046]    With reference to  FIGS. 1 and 2 , a device  1  for diverting fluid from the ventricles of the brain comprises an inlet  2 , an outlet  4  and a resistance member  6  operatively coupled between the inlet  2  and the outlet  4 . The resistance member  6  comprises an upper member  8 , a lower member  10  and a substantially circular plate  12  sandwiched between the upper and lower members  8 ,  10 . The upper and lower members  8 ,  10  each have a substantially circular, planar surface in contact with the plate  12 . The planar surfaces of the upper and lower members  8 ,  10  each comprise a groove or channel, which forms a continuous closed resistance tube  14 ,  16  when the grooved surface is in abutment with the plate  12 . The lower resistance tube  16  formed by the lower member  10  follows a convoluted flow path from a fluid entry point  18 , at a radially outer position on the planar surface of the lower member  10 , to a fluid exit point  20 , at a radially inner position on the planar surface of the lower member  10 . The fluid entry point  18  of the lower resistance tube  16  is in fluid communication with the inlet  2  of the device  1 . The upper resistance tube  14  formed by the upper member  8  follows a convoluted flow path from a fluid entry point  22 , at a radially inner position on the planar surface of the upper member  8 , to a fluid exit point  24 , at a radially outer position on the planar surface of the upper member  8 . The fluid exit point  24  of the upper resistance tube  14  is in fluid communication with the outlet  4  of the device  1 . The fluid exit point  20  on the lower resistance tube  16  corresponds to and is aligned with the fluid entry point  22  on the upper resistance tube  14 . The exit/entry point  20 / 22  comprises an orifice  26  in the plate  12 , permitting fluid communication between the two resistance tubes  14 ,  16  such that the two resistance tubes  14 ,  16  form a compound resistance tube having two superimposed layers. The illustrated convoluted flow paths of the resistance tubes  14 ,  16  comprise a series of arcs, each flow path folding back upon itself at the tip of each arc and thus meandering from a radially outer position to a radially inner position or vice versa. Convoluted flow paths between the entry and exit point of the resistance tubes  14 ,  16  having different shapes and/or orientations are also contemplated. 
         [0047]    The device  1  may comprise additional members (not shown) within the resistance member  6 , each having a grooved surface in abutment with a planar surface in order to form a further length of resistance tube. In this manner a compound resistance tube of many layers may be provided without increasing the surface area occupied by the device  1 . 
         [0048]    The plate  12  and upper and lower members  8 ,  10  of the resistance member  6  are maintained in abutment with one another by a single central screw  28 . Alternatively, the components of the resistance member may be held together by laser welding, brazing, gluing or any other appropriate means of fixation. The lower member  10  of the resistance member  6  is integrally formed with the inlet  2  and the outlet  4 . A rim  30  extends around the circumference of the resistance member  6  and, together with the integral structure of the lower member  10 , the inlet  2  and the outlet  4 , forms a casing of the device  1  within which the remaining components are mounted. 
         [0049]    A filter  32  is mounted in the inlet  2  of the device  1 , in the fluid flow path from the inlet  2  to the entry  18  to the lower resistance tube  16 . An anti-reflux mechanism  34  is mounted in the outlet  4  of the device  1 , in the fluid flow path from the exit  24  of the upper resistance tube  14  to the outlet  4 . The anti-reflux mechanism may be a non-return ball valve and may be spring-loaded. Alternatively, the mechanism  34  may be any other suitable anti-reflux mechanism. 
         [0050]    In the illustrated embodiment, the axes of the inlet  2  and outlet  4  are parallel. However, to facilitate routing of tubing leading from the outlet  4  to the area of the body selected for discharge of the fluid, it may be desirable for the axes of the inlet  2  and outlet  4  be angled with respect to one another. 
         [0051]    In an advantageous application, the device  1  is mounted under the skin of a patient, but for tests or other purposes the device may be mounted externally, such as on the skin or even remote from the body, for example on clothing. CSF fluid from the brain enters the device  1  via the inlet  2  and passes through the filter  32 . The filter  32  prevents occlusion of the resistance tubes  14 ,  16  by filtering from the CSF any particles of debris that might block the resistance tubes. Such debris may be particularly prevalent immediately after surgery. From the filter  32 , CSF flows through the entry point  18  and into the lower resistance tube  16 . CSF flows through the lower resistance tube  16  to the exit point  20 . At exit point  20 , CSF passes through the orifice  26  in the plate  12  and through the entry point  22  into the upper resistance tube  14 . CSF then flows through the upper resistance tube  14  to the exit point  24 . From the exit point  24 , CSF flows through the anti-reflux mechanism  34  and passes out of the device  1  via the outlet  4 . The anti-reflux mechanism prevents any reflux of CSF back towards the brain as a result of pressure changes caused by changes in body posture or for any other reason. The outlet  4  may be connected to appropriate tubing (not shown) to convey the CSF from the device  1  to the desired discharge area. 
         [0052]    Frictional resistance from the walls of the resistance tubes  14 ,  16  limits CSF flow through the device to the desired flow rate. In order to achieve consistency of flow for a given length of resistance tube and pressure drop over the tube, the grooves on the upper and lower members  8 ,  10  that form the resistance tubes  14 ,  16  must be made to very close tolerances. From Poiseuille&#39;s equation, flow rate varies as the fourth power of channel diameter for a given length, pressure drop and fluid viscosity. Consequently, a small channel of approximately 0.5 mm effective diameter must be made to extremely close tolerances if the desired level of flow control is to be achieved. A preferred method of producing the grooves that form the resistance tubes, enabling the production of various contours over the length of the grooves, is CNC milling using a small end mill. This method of production is flexible, enabling the production of varying shapes and cross sections as desired. Other possible methods for forming the grooves include laser cutting, electro-discharge or electro-chemical machining, chemical etching or moulding from a die if a suitable material is chosen. Suitable biocompatible materials for the device include titanium or stainless steel. Alternatively, a biocompatible composite would be required for moulding, for example High Density Polyethylene (HDPE), Ultra High Molecular Weight Polyethylene (UHMWPE) or Polyetheretherketone (PEEK). 
         [0053]    The device described above and illustrated in  FIGS. 1 and 2  has fixed characteristics of pressure and flow in accordance with the length and effective diameter of the upper and lower resistance tubes  14 ,  16 . In normal cases of hydrocephalus, such fixed characteristics are acceptable. However, in certain more complicated cases, it may not be possible to predict with accuracy the flow conditions required to compensate for the impairment of the brain. Consequently, it is desirable to be able to vary the pressure and flow characteristics of the device post operatively in a non-invasive manner. The characteristics of the device could then be adapted according to postoperative observations of a patient. This can be achieved using the device  101  of  FIGS. 3 ,  4   a  and  4   b.    
         [0054]    With reference to  FIGS. 3 ,  4   a  and  4   b , the alternative device  101  for diverting fluid from the ventricles of the brain comprises an inlet  102 , an outlet  104  and a resistance member  106  operatively coupled between the inlet  102  and the outlet  104 . The resistance member  106  comprises an upper member  108 , a lower member  110  and a substantially circular plate  112  sandwiched between the upper and lower members  108 ,  110 . The lower member  110  is integrally formed with the inlet  102  and the outlet  104 . A rim  130  extends around the circumference of the resistance member  106  and, together with the integral structure of the lower member  110 , the inlet  102  and the outlet  104 , forms a casing of the device  101  within which the remaining components are mounted. 
         [0055]    The upper and lower members  108 ,  110  each have a substantially circular, planar surface in contact with the plate  112 . The planar surfaces of the upper and lower members  108 ,  110  each comprise a plurality of grooves or channels, each of which forms a continuous, closed resistance tube when the grooved surfaces are in abutment with the plate  112 . Each groove on the planar surface of the upper member  108  forms an upper resistance tube that follows a convoluted flow path occupying a single sector of the plane circular area of the adjacent surface of the plate  112 . Similarly, each groove on the planar surface of the lower member  110  forms a lower resistance tube that follows a convoluted flow path occupying a corresponding sector of the plane circular area of the opposite surface of the plate  112 . 
         [0056]    With reference to  FIG. 4   b , each lower resistance tube defined by the lower member  110  follows a convoluted flow path from an entry point  118  at a radially outer position on the member  110 , to an exit point  120  at a radially inner position on the member  110 . An entry point  118   a  of one of the lower resistance tubes is in fluid communication with the inlet  102  of the device  101 . With reference to  FIG. 4   a , each upper resistance tube defined by the upper member  108  follows a convoluted flow path from an entry point  122  at a radially inner position on the member  108  to a connection point  123  at a radially outer position on the member  108 . From the connection point  123 , each upper resistance tube defined by the upper member  108  then follows a direct flow path to an exit point  124  at a radially inner position on the upper member  108 . The exit point  120  of each lower resistance tube corresponds to the entry point  122  of a corresponding upper resistance tube. Each pair of corresponding entry and exit points is in fluid communication via an orifice in the plate  112  that is sandwiched between the upper and lower members  108 ,  110 . Similarly, the entry point  118  of each lower resistance tube corresponds to the connection point  123  of a different corresponding upper resistance tube. Each pair of corresponding entry and connection points is in fluid communication via an orifice in the plate  112 . In this manner, a compound resistance tube of two layers is formed in which the fluid flow path occupies first the lower and then the upper resistance tube of each sector of the device  101  sequentially. 
         [0057]    The device  101  may comprise additional members (not shown) within the resistance member  106 , each having a grooved surface in abutment with a planar surface in order to form a resistance tube. In this manner a compound resistance tube of many layers may be provided without increasing the surface area occupied by the device  101 . 
         [0058]    The illustrated convoluted flow paths of the upper and lower resistance tubes comprise a series of sector arcs, each flow path folding back upon itself at the tip of each arc and thus meandering from a radially outer position of the sector to a radially inner position of the sector or vice versa. Convoluted flow paths between the entry and exit points of the lower resistance tubes and entry and connection points of the upper resistance tubes having a different shape or orientation are also contemplated. 
         [0059]    The exit point  124  of each upper resistance tube may be placed in fluid communication with the outlet  104  of the device  101  via a selection rotor  140 . The selection rotor  140  is mounted for rotation about the centre point of the resistance tube sectors and comprises a planar face that is held in abutment with, and may be spring loaded against, the surface of the upper member  108  that is opposite to the grooved surface. The rotor  140  comprises a single through passage  142 , which passage is in fluid communication with the outlet  104  via the chamber  105  within which the rotor  140  is mounted. The opening of the passage  142  on the planar face of the rotor  140  is at the same radial position as the exit points  124  of the upper resistance tubes. The opening of the through passage  142  may therefore be brought into alignment with the exit point  124  of any one of the upper resistance tubes merely by moving the rotor  140  to the appropriate rotational position. The exit points  124  of the remaining upper resistance tubes are sealed off by the planar face of the rotor  140 . The compound resistance tube formed by the upper and lower resistance tubes thus has a fixed entry point  118   a , that is in fluid communication with the inlet  102  of the device  101 , and a series of possible exit points  124 , each of which may be placed in fluid communication with the outlet  104  of the device  101 . The effective length of the compound resistance tube, and hence the pressure and flow characteristics of the device  101 , may therefore be varied by bringing different exit points  124  into fluid communication with the outlet  104 . 
         [0060]    The rotor  140  is magnetically excitable, so that it may be moved to a different rotational position through the correct orientation of a magnet brought into close proximity to the rotor  140 . In addition, the position of the rotor  140  may be determined even when the rotor is obscured from view (for example when the device is implanted under the skin) by holding a compass adjacent the rotor. The rotational position of the rotor may be stabilised by a spring-loaded ratchet (not shown) operating between the rotor and the surrounding casing such that a substantial torsional magnetic force must be exerted on the rotor  140  to move it from a particular rotational position. Alternatively, it may be located simply by frictional forces caused by the spring loading. 
         [0061]    A filter  132  is mounted in the inlet  102  of the device  101 , in the fluid path from the inlet  102  to the entry point  118   a  of the lower resistance tube that is connected to the inlet  102 . An anti-reflux mechanism  134  is mounted in the outlet  104  of the device  101 , in the fluid path from the through passage  142  of the rotor  140  to the outlet  104 . The mechanism may be a non return ball valve and may have a light spring loading. Alternatively, the mechanism  134  may be any other suitable anti-reflux mechanism. 
         [0062]    In the illustrated embodiment, the axes of the inlet  102  and outlet  104  are parallel. However, to facilitate shaping of tubing leading from the outlet  104  to the area of the body selected for discharge of the fluid, it may be desirable for the axes of the inlet  102  and outlet  104  be angled with respect to one another. 
         [0063]    In an advantageous application, the device  101  is mounted under the skin of a patient. CSF fluid from the brain enters the device  101  via the inlet  102  and passes through the filter  132 . The filter  132  prevents occlusion of the resistance tubes by filtering from the CSF any particles of debris that might block the resistance tubes. From the filter  132 , CSF flows through the first entry point  118   a  and into the first sector of the compound resistance tube. CSF initially enters the first lower resistance tube  150 . CSF flows through the first lower resistance tube  150  to the exit point  120  of the first lower resistance tube  150 . CSF then flows through the adjacent orifice in the plate  112  and into the first upper resistance tube  152  via the associated entry point  122 . CSF then flows through the first upper resistance tube  152  to the connection point  123  of the first upper resistance tube  152 . If the rotor  140  has placed the exit point  124  of the first upper resistance tube in fluid communication with the outlet  104 , then the CSF flows from the connection point  123  to the exit point  124  and via the through passage  142  to the anti-reflux mechanism  132  and out of the device  101  via the outlet  104 . If, however, the rotor  140  has sealed off the exit point  124  of the first upper resistance tube  152 , then the CSF flows from the connection point  123  through the adjacent orifice in the plate  112  and into the second sector of the compound resistance tube. CSF initially enters the second lower resistance tube  154  via the entry point  118  of the second lower resistance tube  154 . CSF then flows sequentially through the sectors of the compound resistance tube (entering each new sector via the lower resistance tube and exiting each sector via the upper resistance tube) until the sector of compound resistance tube that is in fluid communication with the outlet is reached. 
         [0064]    Before implantation, the effective length of compound resistance tube required for a particular patient is estimated and the rotor  140  is moved to place the appropriate exit point  124  in fluid communication with the outlet  104 . If, after implantation of the device  101 , it is determined that the length of compound resistance tube needs to be adjusted, in order to provide different pressure/flow characteristics, a compass is placed adjacent the skin covering the device  101  in order to determine the rotational orientation of the rotor  140 . A magnet is then brought into close proximity with the device and used to move the rotor  140  to the desired new rotational position. 
         [0065]    In a variation of the embodiment of  FIGS. 3 and 4 , the components of the device, specifically the upper and lower members of the resistance member  106  and the rotor  140 , may be arranged as a sliding fit, so that biasing means is not required. 
         [0066]    In the embodiments described above and illustrated in  FIGS. 1 to 4   b , the upper and lower resistance tubes, or sectors of resistance tube, are formed by grooved surfaces of the upper and lower members  8 ,  10 ,  108 ,  110  of the resistance member  6 ,  106  being held in abutment with planar surfaces of a central plate  12 ,  112 . In alternative embodiments, the groves that define the resistance tubes may be formed on opposite surfaces of the plate  12 ,  112 . The upper and lower resistance tubes, or sectors of resistance tube, may then be formed by holding the opposite grooved surfaces of the plate  12 ,  112  in abutment with planar surfaces of the upper and lower members  8 ,  108 ,  10 ,  110  of the resistance member  6 ,  106 . A device  1 ,  101  manufactured in this manner functions substantially as described above. However, in the event of damage to the grooves, or if a different groove diameter is required, the plate  12 ,  112  may simply be exchanged for a new plate  12 ,  112 , without requiring any other component of the device  1 ,  100  to be changed. An example of such an alternative embodiment of device is illustrated in  FIGS. 5 and 6 . 
         [0067]    With reference to  FIGS. 5 and 6 , the alternative device  201  is substantially similar in construction and operation to the device  101 . The device  201  comprises an inlet  202 , an outlet  204  and a resistance member  206  operatively coupled between the inlet  202  and the outlet  204 . The resistance member comprises an upper member  208 , a lower member  210  and a substantially circular plate  212  sandwiched between the upper member  208  and the lower member  210 . The lower member  210  forms an integral structure with the inlet  202  and the outlet  204  and with a rim  230  that extends around the circumference of the resistance member  206 . The integral structure comprises a casing within which the remaining components of the device  201  are mounted. 
         [0068]    The plate  212  has substantially circular upper and lower surfaces. The upper surface of the plate  212  is in contact with a planar surface of the upper member  208  and the lower surface of the plate  212  is in contact with a planar surface of the lower member  210 . The upper and lower surfaces of the plate  212  each comprise a plurality of grooves or channels, each of which forms a continuous closed resistance tube when the grooved surface is in contact with a planar surface of a respective upper or lower member  208 ,  210 . Each groove on the upper surface of the plate  212  forms an upper resistance tube that follows a convoluted flow path occupying a single sector of the plane circular area of the upper surface of the plate  212 . Similarly, each groove on the lower surface of the plate  212  forms a lower resistance tube that follows a convoluted flow path occupying a corresponding sector of the plane circular area of the lower surface of the plate  212 . 
         [0069]    As illustrated in  FIG. 6 , the upper and lower resistance tubes are arranged in a configuration equivalent to that of the upper and lower resistance tubes of the device  101  described above, with corresponding entry and exit points  218 ,  220 ,  222  and  224 . Fluid flowing through the device  212  therefore follows the same convoluted flow paths as previously described. Each exit point  224  of an upper resistance tube is aligned with a corresponding through passage  209  in the upper member  208 . 
         [0070]    The device  212  further comprises a selection rotor  240  that is mounted for rotation about the centre point of the resistance tube sectors and comprises a planar face that is held in abutment with, and may be spring loaded against, the surface of the upper member  208  that is opposite to the plate  212 . The rotor  240  comprises a single through passage  242 , which passage is in fluid communication with the outlet  204  via the chamber  205  within which the rotor  240  is mounted. The opening of the passage  242  on the planar face of the rotor  240  is at the same radial position as the exit points  224  of the upper resistance tubes and corresponding through passages  209  in the upper member  208 . The opening of the through passage  242  in the rotor  240  may therefore be brought into communication with the exit point  224  of any one of the upper resistance tubes merely by moving the rotor  240  to the appropriate rotational position. The exit points  224  of the remaining upper resistance tubes are sealed off by the planar face of the rotor  240  sealing off the corresponding through passages in the upper member  208 . The remaining functionality of the selection rotor  240  is substantially the same as that of the rotor  140  previously described. 
         [0071]    The components of the device  201 , including the rotor  240 , are held within the casing by a cap  260 . The cap  260  is a substantially circular planar element having a circumferential skirt  262 . The skirt carries an external thread that engages with a corresponding internal thread on the rim  230  to form a threaded connection  264 . 
         [0072]    If the resistance tubes defined by the grooves on the plate  212  become occluded or damaged, or if a different diameter of resistance tube is required, it is possible to replace the plate  212  independently of the remainder of the device  201 . Once the cap  260  is removed, the rotor  240  and upper member  208  can easily be lifted out of the device to allow the plate  212  to be removed and replaced. The rotor  240  and upper member  208  are then returned to their positions and the device is held together by screwing the cap  260  into place. 
         [0073]    A device  212  may be supplied with a series of plates  212 , each having grooves of a different diameter or configuration. This enables a medical practitioner to select the most appropriate plate for a particular patient. 
         [0074]    It will be understood that while the device  201  illustrated comprises resistance tube sectors and a selection rotor, a device having resistance tube configurations as illustrated in  FIG. 2  may also be implemented with a grooved plate  12  contacting planar surfaces of upper and lower members. 
         [0075]    To avoid unnecessary duplication of effort and repetition in the text, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention.