Abstract:
A portable medication delivery system enables a patient to elect slow continuous auto-administration or rapid bolus self-administration of a fluid medication. The medication delivery system has an infusion pump, which includes a fluid storage chamber and a spring-driven piston positioned in the fluid storage chamber. The fluid storage chamber is initially charged with the medication and the infusion pump automatically delivers a continuous dosage of the medication at a first flow rate over a long time from the fluid storage chamber to a treatment site in the patient. A bolus injector is positioned in series or in parallel with the infusion pump and is charged with a bolus dosage of the treatment fluid from the fluid storage chamber for alternate manual self-injection of the medication into the treatment site at a higher second flow rate for a short time. The infusion pump flowpath includes a drip chamber having an in-line flow restriction, which converts a continuous flow stream from the fluid storage chamber to a drip stream. A sight window in the drip chamber permits the user to visually confirm the flow of medication through the infusion pump.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a system for delivering medicine to a patient, and more particularly, to a system having in combination a spring-driven infusion pump and a bolus injector, which enable a user to selectively deliver a predetermined dosage of a fluid medication to a treatment site either automatically at a moderate flow rate over a long time or manually at a higher flow rate over a short time. 
     BACKGROUND OF THE INVENTION 
     Pain management is an important aspect of post-operative recovery from surgery. Pain management usually begins immediately following the surgical procedure with the administration of narcotics or other pain control medications to the patient while the patient is under the direct supervision of the health care provider. The pain control medications are most commonly administered either orally or by injection. 
     The proliferation of less-invasive arthroscopic techniques for the surgical repair of many joint or soft tissue injuries and ailments has significantly reduced post-operative recovery times and the attendant pain experienced by the patient. The current trend toward arthroscopic techniques frequently enables surgical procedures to be performed on an outpatient basis or with shortened post-operative hospital stays. 
     As a result, the bulk of the post-operative recovery time is spent in the home or even in the workplace. One goal of home recovery is to phase the patient back into routine physical activities relatively quickly as a means of shortening the post-operative recovery time. 
     Since the patient is generally not under the direct supervision of the health care provider when in the home or workplace, the responsibility for administering pain control medications falls on the patient in these environments. The vast majority of self-administered pain control medications are oral medications because most individuals lack the requisite knowledge, skill, and experience to self-administer pain control medications by injection. Unfortunately, however, pain control medications administered orally are transported throughout the body and correspondingly affect the entire body, often causing undesirable side effects such as drowsiness, disorientation, nausea, constipation or vomiting. In contrast, injected pain control medications can be administered more locally than orally administered medications, thereby frequently avoiding the undesirable side effects of oral medications. In addition, injected pain control medications reach the treatment site more rapidly and in greater concentrations than oral medications, rendering injected pain control medications a more effective pain control therapy. 
     To exploit the advantages of injected pain care medications, devices have been developed to inject the pain care medication into the treatment site in an automated manner, which requires minimal patient intervention. Such devices typically meter the pain care medication to the treatment site continuously over a long period of time. Automated continuous injection devices are, nevertheless, not entirely satisfactory. It has been found in many instances that pain care medication is most effective if periodically injected into the treatment site as a single relatively large bolus dosage whenever the patient senses the need rather than continuously injecting the same overall dosage of medication into the treatment site over a relatively long period of time. However, if the patient is allowed to self-administer injection of the pain care medication on an as needed basis in the absence of supervision of a health care provider, the risk of overmedication is significant. 
     The present invention recognizes a need for a fluid injection device, which selectively enables effective auto-administration of a fluid medication or, in the alternative, effective self-administration of the fluid medication by a patient while diminishing the risk of overmedication, even when the patient lacks the requisite knowledge, skill or experience to perform injections. Accordingly, it is an object of the present invention to provide a medication delivery system, wherein operation of the system is selective between an automated or extended mode and a manual or instantaneous mode. More particularly, it is an object of the present invention to provide such a medication delivery system, wherein the patient elects extended injection of a predetermined dosage of a fluid medication into a treatment site at a moderate flow rate over a long time or, in the alternative, elects to effect instantaneous injection of the predetermined dosage of the fluid medication into the treatment site at a higher flow rate over a short time. It is another object of the present invention to provide such a medication delivery system, wherein the system can effectively reduce the risk of overmedication even when the patient operates the system in the manual or instantaneous mode. It is still another object of the present invention to provide such a medication delivery system, wherein the system can be effectively monitored and operated by a patient lacking any specific medical knowledge, skill or experience in performing injections. It is yet another object of the present invention to provide such a medication delivery system, which is fully portable while operating so that the system can be used by the patient during normal daily activity. 
     These objects and others are accomplished in accordance with the invention described hereafter. 
     SUMMARY OF THE INVENTION 
     A first embodiment of the present invention is a medical infusion pump comprising a fluid storage chamber, a pump outlet, and a pump flowpath positioned between the fluid storage chamber and the pump outlet to provide fluid communication therebetween. The pump flowpath includes a flow restriction, a drip chamber, an outlet tube and a sight window. The flow restriction exits into the drip chamber and the sight window is oriented to enable visual contact with the drip chamber. The flow restriction is sized to convert a continuous stream of fluid entering the flow restriction from the fluid storage chamber to a drip stream exiting the flow restriction into the drip chamber. The outlet tube is positioned beneath the flow restriction in the drip chamber and separated from the flow restriction by a drip gap. The outlet tube is configured to revert the drip stream exiting the flow restriction to a reverted continuous stream. The infusion pump further comprises a displacement piston displacably positioned in the fluid storage chamber and an elastic member engaging the displacement piston and transitionable between a more stressed position and a less stressed position to displace the displacement piston. The elastic member is preferably a coil spring. 
     Another embodiment of the present invention is a medication delivery system comprising an infusion pump and a bolus injector. The infusion pump includes a fluid storage chamber, a pump outlet, a pump flowpath providing fluid communication between the fluid storage chamber and the pump outlet, a displacement piston displacably positioned in the fluid storage chamber, and an elastic member engaging the displacement piston and transitionable between a more stressed position and a less stressed position to displace the displacement piston. The bolus injector is positioned in series with the infusion pump and is a flexible bladder enclosing a bolus chamber. The bolus chamber has a fluid capacity substantially less than the fluid storage chamber. The bladder may have an elastic memory to restore the bladder to an initial configuration after the bladder is deformed by compression. The bolus injector has an injector inlet into the bolus chamber and an injector outlet out of the bolus chamber. The injector inlet is connected to the pump outlet. The pump flowpath may include a flow restriction, drip chamber, outlet tube and sight window substantially as recited above. 
     A further embodiment of the present invention is a medication delivery system comprising a first infusion pump, a second infusion pump, a bolus injector, a junction and a common flow tube. The first infusion pump includes a first fluid storage chamber, a first pump outlet, a first pump flowpath providing fluid communication between the first fluid storage chamber and the first pump outlet, a first displacement piston displacably positioned in the first fluid storage chamber, and a first elastic member engaging the first displacement piston and transitionable between a more stressed position and a less stressed position to displace the first displacement piston. The second infusion pump similarly includes a second fluid storage chamber, a second pump outlet, a second pump flowpath providing fluid communication between the second fluid storage chamber and the second pump outlet, a second displacement piston displacably positioned in the second fluid storage chamber, and a second elastic member engaging the second displacement piston and transitionable between a more stressed position and a less stressed position to displace the second displacement piston. The bolus injector is positioned in series with the second infusion pump and is substantially as recited above. The second pump outlet is connected to the injector inlet and the junction connects the first pump outlet with the injector outlet. The common flow tube exits the junction and is in fluid communication with the first pump outlet and the injector outlet. The first pump flowpath may include a flow restriction, drip chamber, outlet tube and sight window substantially as recited above. 
     Yet another embodiment of the present invention is a medication delivery system comprising an infusion pump, a bolus injector, a junction and a common flow tube. The infusion pump includes a fluid storage chamber, a first pump outlet and a second pump outlet, a pump flowpath providing fluid communication between the fluid storage chamber and the first pump outlet, a displacement piston displacably positioned in the fluid storage chamber, and a elastic member engaging the displacement piston and transitionable between a more stressed position and a less stressed position to displace the displacement piston. The bolus injector is substantially as recited above. The second pump outlet is connected to the injector inlet and the junction connects the first pump outlet with the injector outlet. The common flow tube exits the junction and is in fluid communication with the first pump outlet and the injector outlet. The first pump flowpath may include a flow restriction, drip chamber, outlet tube and sight window substantially as recited above. 
     Still another embodiment of the present invention is a method for delivering a fluid medication to a treatment site of a patient. A bolus injector is charged with a fluid medication. The bolus injector is a flexible bladder enclosing a bolus chamber and having an injector inlet into the bolus chamber and an injector outlet out of the bolus chamber. A fluid storage chamber serially positioned upstream of the bolus injector is also charged with the fluid medication. The fluid storage chamber is in fluid communication with a pump outlet via a pump flowpath and the pump outlet is in fluid communication with the injector inlet. A displacement force is applied to the fluid medication in the fluid storage chamber from an elastic member transitioning from a more stressed position to a less stressed position. The displacement force serially displaces the fluid medication from the fluid storage chamber and the pump flowpath into the bolus chamber. An outlet valve positioned at the injector outlet which is biased closed is opened in response to the ambient pressure of the fluid medication contacting the outlet valve to discharge the fluid medication from the injector outlet. The method may further comprise connecting the injector outlet with an inlet end of a catheter, positioning an outlet end of the catheter in a treatment site of a patient, and displacing the fluid medication through the catheter to deliver the fluid medication to the treatment site. 
     In accordance with specific aspects of the present embodiment, the bolus injector is charged with the fluid medication by displacing the fluid medication from the pump flowpath into the bolus chamber. Alternatively, the bolus injector is charged with the fluid medication by injecting the fluid medication into the bolus chamber from a source downstream of the infusion pump. In accordance with another specific aspect of the present invention, the fluid medication is preferably displaced from the fluid storage chamber as a continuous stream. The continuous stream of the fluid medication is then driven into a flow restriction in the pump flowpath and the fluid medication exits the flow restriction as a drip stream. 
     Another embodiment of the present invention is a method for delivering a fluid medication to a treatment site of a patient, which comprises charging a bolus injector and a fluid storage chamber serially positioned upstream of the bolus injector with a fluid medication substantially as described above. The practitioner then selects between an extended mode and an instantaneous mode of delivering the fluid medication to a treatment site. The extended mode is performed by applying a first displacement force to the fluid medication in the fluid storage chamber from an elastic member transitioning from a more stressed position to a less stressed position. The first displacement force serially displaces the fluid medication from the fluid storage chamber through the pump flowpath, the bolus chamber and the injector outlet into the treatment site at a first flow rate over a long time. The instantaneous mode is performed by applying a second displacement force to the bolus injector sufficient to deform the bolus injector. The second displacement force displaces the fluid medication from the bolus chamber and discharges the fluid medication from the injector outlet into the treatment site at a higher second flow rate over a short time. The method further comprises recharging the bolus injector with the fluid medication after the instantaneous mode of operation by applying the first displacement force to the fluid medication in the fluid storage chamber. The first displacement force serially displaces the fluid medication from the fluid storage chamber through the pump flowpath and injector inlet into the bolus chamber. 
     In accordance with a specific aspect of the present embodiment, the fluid medication is preferably displaced from the fluid storage chamber as a continuous stream. The continuous stream of the fluid medication is driven into a flow restriction in the pump flowpath and the fluid medication exits the flow restriction as a drip stream. 
     The present invention will be further understood from the drawings and the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a medication delivery system of the present invention in a partially disassembled inactive state. 
     FIG. 2 is an elevational view of the medication delivery system of FIG. 1 in an active state immediately after charging the system with a fluid medication, wherein the infusion pump and bolus injector are shown in sectional. 
     FIG. 3 is an elevational view of the medication delivery system of FIG. 1 during a an essentially steady-state automated mode of operation, wherein the infusion pump and bolus injector are shown in sectional. 
     FIG. 4 is an elevational view of the medication delivery system of FIG. 1 during a manual mode of operation, wherein the infusion pump and bolus injector are shown in sectional. 
     FIG. 5 is an elevational view of an alternate embodiment of the medication delivery system of the present invention in an active state immediately after charging the system with a fluid medication, wherein the infusion pump is shown in sectional. 
     FIG. 6 is an elevational view of another alternate embodiment of the medication delivery system of the present invention in an active state immediately after charging the system with a fluid medication, wherein the infusion pump is shown in sectional. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a medication delivery system of the present invention is shown and generally designated  10 . The medication delivery system  10  comprises an infusion pump  12  and a bolus injector  14 . The medication delivery system  10  further comprises a connective tube  16 , which provides fluid communication between the infusion pump  12  and the bolus injector  14 , and a catheter  18 , which provides fluid communication between the bolus injector  14  and a treatment site (not shown) in a user. The infusion pump  12  has a rigid opaque housing  20 , which comprises a series of interconnected components. In particular, the housing  20  includes a top end cap  22 , a bottom end cap  24  and a front face plate  26 . The housing  20  inter alia functions as a structural frame and a protective shield for the remaining components of the infusion pump  12 . The housing  20  is adapted to the external profile of the infusion pump  12 , which is compactly sized for portability, enabling ready transport of the medication delivery system  10  by the user during normal daily activities. The housing  20  may be fitted with one or more optional accessories (not shown), which facilitate transport of the medication delivery system  10  by the user. For example, the housing  20  may be fitted with an external strap for wearing the pump  12  on the body of the user or the housing  20  may be fitted with an external clip for clipping the housing  20  to an article of clothing worn by the user, such as a trouser waistband or a belt. 
     A pump inlet port  28  extends through the top end cap  22  of the housing  20  and a pump outlet port  30  correspondingly extends through the bottom end cap  24  of the housing  20 . The front face plate  26  is provided with an opening  32 , which exposes a discharge sight window  34 . The discharge sight window  34  is a light transmissive window, i.e., a transparent or translucent window, fitted into, or integral with, the surrounding wall of a drip chamber  36  enclosed within the housing  20  and described in greater detail below. The front face plate  26  is also configured to expose first and second storage sight windows  38   a ,  38   b . The first storage sight window  38   a  is a light transmissive window fitted into, or integral with, the surrounding wall of a first medication storage chamber  40   a  retained by the housing  20 . The second storage sight window  38   b  is similarly a light transmissive window fitted into, or integral with, the surrounding wall of a second medication storage chamber  40   b  retained by the housing  20 . The discharge sight window  34 , in cooperation with the opening  32 , renders the interior of the drip chamber  36  visible to the user. The drip chamber  36  contains a drip tube  42  and a chamber outlet tube  44 , which are described in greater detail below. The first and second storage sight windows  38   a ,  38   b  render the interiors of the first and second medication storage chambers  40   a ,  40   b , respectively, visible to the user. An overlay (not shown) in the form of a paper or plastic sticker or the like, which displays a gradient of fluid level markings, may optionally be positioned adjacent to each storage sight window  38   a ,  38   b  to facilitate measurement of the fluid levels in the first and second medication storage chambers  40   a ,  40   b  without preventing the user from viewing the interiors of the first and second medication storage chambers  40   a ,  40   b.    
     The connective tube  16  is a flexible transparent tube, which has an inside diameter, for example, of about 0.060 inches. The connective tube  16  has an inlet end  46  and an outlet end  48 , wherein the inlet end  46  is removably coupled with the pump outlet port  30  by means of a conventional tube/port coupling  50 , such as male and female Luer lock fittings. The bolus injector  14  is a fluid-tight flexible bladder, which has an injector inlet port  52  and an injector outlet port  54 . The bolus injector  14  is formed from a flexible, preferably transparent, material, which renders the bolus injector  14  manually compressible when a fluid is contained therein. The bolus injector  14  typically has a fluid capacity in a range of about 4 to 6 ml. In accordance with the embodiment shown in FIG. 1, the bolus injector is an elastomeric squeeze bulb having an elastic memory, which returns the squeeze bulb to its original shape after deformation. Although not shown, a plastic bag or the like is an alternate bolus injector within the scope of the present invention. The alternate bolus injector is likewise flexible, but not substantially elastic. 
     The injector inlet port  52  of the bolus injector  14  is integrally connected with the outlet end  48  of the connective tube  16  in a substantially fixed manner and the injector outlet port  54  is removably coupled with an inlet end  56  of the catheter  18  by means of a conventional tube/port coupling  57 , such as male and female Luer lock fittings. The catheter  18  is a flexible transparent tube preferably having an inside diameter substantially less than the inside diameter of the connective tube  16 . For example, the inside diameter of the catheter  18  is about 0.025 inches. The catheter  18  has an outlet end  58 , which is open to enable fluid flow therethrough. As noted above, the infusion pump  12  is designed to be worn by the user or otherwise connectively supported by the user. The bolus injector  14 , however, is preferably freely suspended from the outlet end  48  of the connective tube  16  with the inlet end  46  of the connective tube  16  connected to the infusion pump  12 . 
     The first and second medication storage chambers  40   a ,  40   b  are retained in parallel relation to one another by the top and bottom end caps  22 ,  24 , which are fastened to the first and second medication storage chambers  40   a , 40   b  by top retention screws  59  (shown in FIG. 2) and bottom retention screws (not shown), respectively. The first medication storage chamber  40   a  has a first piston, seal and spring set  60   a ,  62   a ,  64   a  positioned therein and the second medication storage chamber  40   b  has a corresponding second piston, seal and spring set  60   b ,  62   b ,  64   b  positioned therein. The first piston, seal and spring set  60   a ,  62   a ,  64   a  and second piston, seal and spring set  60   b ,  62   b ,  64   b  are substantially identical to one another. Accordingly, the following description of the first piston, seal and spring set  60   a ,  62   a ,  64   a  applies equally to the second piston, seal and spring set  60   b ,  62   b ,  64   b . The first piston  60   a  is cooperatively configured so that the first piston  60   a  is slidably displaceable up and down within the first medication storage chamber  40   a  in response to expansion or compression of the first spring  64   a  as described below with respect to operation of the system  10 . The first piston  60   a  has an outside diameter slightly less than the inside diameter of the first medication storage chamber  40   a  and the first seal  62   a  is an elastomeric O-ring positioned around the first piston  60   a  to maintain a fluid-tight seal between the wall of the first medication storage chamber  40   a  and the first piston  60   a . The first spring  64   a  is a coiled metal spring fitted in the first medication storage chamber  40   a  and has an outside diameter less than the inside diameter of the first medication storage chamber  40   a . The top end of the first spring  64   a  engages the first piston  60   a  and the bottom end of the first spring  64   a  engages the bottom of the first medication storage chamber  40   a , which is integral with the vented bottom end cap  24 . 
     Further details of the medication delivery system  10  are described below with reference to FIG.  2 . The first and second medication storage chambers  40   a ,  40   b  have first and second tubular walls  66   a ,  66   b , respectively. The first and second medication storage chambers  40   a ,  40   b  are substantially identically configured and each has a fluid capacity several times greater than the fluid capacity of the bolus injector  14 . For example, each of the medication storage chambers  40   a ,  40   b  may have a fluid capacity of about 50 ml for a combined fluid capacity of about 100 ml. A first storage chamber outlet  68   a  is positioned at the top of the first medication storage chamber  40   a  and a second storage chamber outlet  68   b  is likewise positioned at the top of the second medication storage chamber  40   b . The first and second storage chamber outlets  68   a ,  68   b  open into a manifold  70  which functions as a receiving chamber for the first and second storage chamber outlets  68   a ,  68   b . A filter  72  is positioned in series downstream of the manifold  70 . The filter  72  is a conventional in-line fluid filter which is designed to trap solid particles exceeding an appropriate maximum size, such as 2 microns, and prevent such particles from continuing downstream of the filter  72 . 
     The filter  72  discharges into the drip tube  42  positioned in the drip chamber  36 . 
     The drip chamber  36  has a tubular wall  74  of substantially uniform inside diameter along its entire length and an inside diameter substantially greater than each of the outside diameters of the drip tube  42  and chamber outlet tube  44 . The drip tube  42  functions as a flow restriction, having a substantially uniform inside diameter, which is significantly less than the inside diameter of the connective tube  16  or catheter  18 . For example, the inside diameter of the drip tube  42  is about 0.002 inches. The drip tube  42  has an outlet end  76 , which extends downwardly into the drip chamber  36  from the top of the drip chamber  36 . The chamber outlet tube  44  has an inlet end  78 , which correspondingly extends upwardly into the drip chamber  36  from the bottom of the drip chamber  36 . The chamber outlet tube  44  has a substantially uniform inside diameter, which significantly is greater than the inside diameter of the drip tube  42 . For example, the inside diameter of the chamber outlet tube is about 0.060 inches. A drip gap  80 , which is a void space on the order of about 0.5 inches or more in length, is provided between the outlet end  76  of the drip tube  42  and the inlet end  78  of the chamber outlet tube  44 . The drip gap  80  is aligned with the discharge sight window  34  and opening  32  (shown in FIG. 1) so that the drip gap  80  is visible to the user. Upward extension of the inlet end  78  of chamber outlet tube  44  into the bottom of the drip chamber  36  defines a fluid accumulation annulus  82  between the wall  74  of the drip chamber  36  and the chamber outlet tube  44 . 
     The bolus injector  14 , which in the present embodiment is a squeeze bulb, has a flexible wall  84  enclosing a bolus chamber  86 . The flexible wall  84  is formed from an elastomeric material, which is capable of deformation when a sufficient displacement force is applied to it, but has an elastic memory, which returns the wall  84  to its original configuration after the displacement force causing the deformation is removed. When the wall  84  is not compressed to the point of deformation, the internal volume of the bolus chamber  86  is equal to the above-recited fluid capacity of the bolus injector  14 , i.e., in a range of about 4 to 6 ml. When the wall  84  is compressed past the point of deformation, the internal volume of the bolus chamber  86  correspondingly decreases. 
     For purposes of illustrating its operation, the above-described system  10  is characterized in terms of three functionally distinct sub-assemblies, i.e., a system flowpath, a pair of automated fluid drive mechanisms and a manual fluid drive mechanism. The system flowpath is an essentially passive or static sub-assembly, whereas the automated and manual fluid drive mechanisms are essentially active or dynamic sub-assemblies. The system flowpath comprises in series the manifold  70 , filter  72 , drip tube  42 , drip chamber  36 , chamber outlet tube  44 , connective tube  16 , bolus chamber  86 , and catheter  18 . The two automated fluid drive mechanisms comprise in parallel the first piston, seal and spring set  60   a ,  62   a ,  64   a  and the second piston, seal and spring set  60   b ,  62   b ,  64   b , respectively. The manual fluid drive mechanism comprises the flexible wall  84  of the bolus injector  14 . 
     FIG. 1 shows the medication delivery system  10  in an inactive or passive state, wherein the inlet end  46  of the connective tube  16  is uncoupled from the pump outlet port  30  and the inlet end  56  of the catheter  18  is uncoupled from the injector outlet port  54 . When the automated fluid drive mechanisms are in the inactive state, the first and second medication storage chambers  40   a ,  40   b  are substantially devoid of any fluid medication. Operation of the system  10  is initiated with a start-up procedure, wherein the medication delivery system  10  is charged with a desired fluid medication, such as a pain care medication. 
     The start-up procedure comprises placing the outlet end  58  of the catheter  18  in the treatment site, which is typically a surgical wound site. Placement of the outlet end  58  in the treatment site is effected by any conventional technique. A preferred technique for placing a catheter in a surgical wound site is described in U.S. Pat. No. 6,270,481, which is incorporated herein by reference. In accordance with this technique, a concentrically fitted introducer needle and insertion catheter (not shown) are simultaneously pierced through the outer surface of the skin adjacent to the surgical wound site and pushed through the skin until they enter the wound. The introducer needle is then removed while the insertion catheter remains in place. The outlet end  58  of the catheter  18  is threaded from the outer surface of the skin through insertion catheter into the wound. Finally, the insertion catheter is removed leaving the catheter  18  in place with the outlet end  58  in the wound and the remainder of the catheter  18  extending out through the skin. 
     The start-up procedure continues by coupling the inlet end  56  of the catheter  18  with the injector outlet port  54  by means of the tube/port coupling  57 . The bolus injector  14  is primed with the fluid medication by injecting the fluid medication into the inlet end  46  of the connective tube  16  until the connective tube  16 , bolus chamber  86  and catheter  18  are charged, preferably at or near their fluid capacity, with the fluid medication. Although the sequential order of the above-recited steps is preferred, the present invention is not so limited and alternate sequences of these steps are within the scope of the present invention. 
     The start-up procedure further comprises placing a charge of the fluid medication in the first and second medication storage chambers  40   a ,  40   b . The volume of the charge typically approximates the total combined capacity of the chambers  40   a ,  40   b  and manifold  70 , although the volume of the charge may alternatively be less than the total capacity of the chambers  40   a ,  40   b  and manifold  70 , if desired. Placement of the fluid medication in the first and second medication storage chambers  40   a ,  40   b  is effected by injecting the fluid medication through the pump inlet port  28  using an injection means (not shown) such as a syringe or the like. The injection means discharges the fluid medication into the pump inlet port  28  at a pressure, which causes the fluid medication to urge open an inlet valve  87 , which is a one-way check valve positioned at the pump inlet port  28 . The inlet valve  87  is normally biased closed when fluid medication is not being injected into the pump inlet port  28 . 
     The open inlet valve  87  enables the fluid medication to pass through the pump inlet port  28  into the manifold  70 . The bulk of the charge is displaced under the pressure of the injection means from the manifold  70  through the first and second storage chamber outlets  68   a ,  68   b  into the first and second medication storage chambers  40   a ,  40   b , respectively. The remainder of the charge remains in the manifold  70  or is diverted from the manifold  70  into the filter  72  under the pressure of the injection means. However, this remainder is very small relative to the bulk of the charge because the flow resistance into the filter  72  is substantially greater than the flow resistance into the first and second medication storage chambers  40   a ,  40   b . Once the first and second medication storage chambers  40   a ,  40   b  are charged with the fluid medication, the inlet valve  87  closes and the injection means is withdrawn from the pump inlet port  28 . The inlet end  46  of the connective tube  16  is then coupled with the pump outlet port  30  by means of the tube/port coupling  50 . 
     Operation of the automated fluid drive mechanisms in cooperation with the first and second medication storage chambers  40   a ,  40   b  is described hereafter with respect to the first piston, seal and spring set  60   a ,  62   a ,  64   a  and first medication storage chamber  40   a , it being understood that the description applies equally to the second piston, seal and spring set  60   b ,  62   b ,  64   b  and second medication storage chamber  40   b , which are substantially identical to the first. The first medication storage chamber  40   a  has a volume which varies as a function of the position of the first piston  60   a  relative to the fixed wall  66   a  of the first medication storage chamber  40   a . When the automated fluid drive mechanism is in the inactive state, the first medication storage chamber  40   a  is at its minimum volume, typically at or approaching zero. At this point the first spring  64   a  is expanded to a substantially more relaxed or less stressed position and the first piston  60   a  is in an extended upward position. When the automated fluid drive mechanism transitions to the active state as shown in FIG. 2, the first medication storage chamber  40   a  is at its charge volume, which typically exceeds the minimum volume of the first medication storage chamber  40   a  by slightly less than one-half the total volume of the charge of fluid medication to the system  10 , the remainder of the total volume going to the second medication chamber  40   b  and the manifold  70 . At this point the first spring  64   a  is compressed to a substantially more stressed or less relaxed position and the first piston  60   a  is in a depressed downward position. 
     The compressed first spring  64   a  exerts an upward expansion or displacement force on the first piston  60   a  when the automated fluid drive mechanism is in the active state, which biases the first piston  60   a  toward its extended upward position. Consequently, the displacement force of the first spring  64   a  against the first piston  60   a  in cooperation with the first seal  62   a  displaces the fluid medication from the first medication storage chamber  40   a  back through the first storage chamber outlet  68   a  in an automated manner, which requires no user intervention or additional driving force. The second piston, seal and spring set  60   b ,  62   b ,  64   b  likewise displace the fluid medication from the second medication storage chamber  40   b  back through the second storage chamber outlet  68   b  in the same manner. The pressure created by the automated fluid drive mechanisms directs displacement of the fluid medication past the closed inlet valve  87  at the pump inlet port  28  through the manifold  70  into the filter  72 . 
     With reference to FIG. 3, displacement of the fluid medication from the first and second medication storage chambers  40   a ,  40   b  through the manifold  70  and filter  72  by means of the automated fluid drive mechanisms creates a substantially continuous uninterrupted stream of fluid medication in this portion of the system flowpath. However, the relatively small inside diameter of the drip tube  42  creates a flow restriction of sufficient degree to convert the continuous steam of fluid medication to a discontinuous drip stream at the outlet end  76  of the drip tube  42 . The drip tube  42  preferably has a smaller inside diameter than any other components of the system flowpath. Thus, the fluid medication is discharged from the outlet end  76  of the drip tube  42  downward into the drip gap  80  within the drip chamber  36  as a periodic series of droplets  88 . Although the inlet end  78  of the chamber outlet tube  44  is aligned with the outlet end  76  of the drip tube  42 , the bulk of the droplets  88  in the drip stream falling through the drip gap  80  are deflected into the fluid accumulation annulus  82  upon impact with the inlet end  78  rather than flowing into the chamber outlet tube  44 . 
     When a sufficient volume of droplets  88  accumulate in the annulus  82  to fill the annulus  82 , the fluid level  89  in the annulus  82  reaches the inlet end  78  of the chamber outlet tube  44 . Ultimately the fluid medication spills over the inlet end  78  and continues through the chamber outlet tube  44  along the system flowpath into the injector inlet port  52 . The fluid medication spillover into the chamber outlet tube  44  is substantially continuous, thereby converting the discontinuous drip stream back to a continuous stream in the chamber outlet tube  44 . The ambient pressure of the fluid medication in the system flowpath urges open an outlet valve  90  positioned at the injector outlet port  54 . The outlet valve is a one-way check valve, which is biased closed in the absence of the fluid medication. The ambient pressure of the fluid medication contacting the outlet valve  90  alone is sufficient to overcome the biasing force of the outlet valve  90  without reliance on any other external forces. 
     The open outlet valve  90  enables the fluid medication to exit the bolus chamber  86  via the injector outlet port  54  and flow as a substantially continuous stream of an extended dosage through the catheter  18  and out the outlet end  58  to the treatment site in an essentially steady-state manner. Delivery of the extended dosage of the fluid medication to the treatment site is characterized by a relatively moderate first flow rate over a relatively long time, for example, about 2 ml per hour over about 2 days. 
     The above-described operating mode of the system  10  shown in FIG. 3 is termed an automated or extended mode insofar as the system  10  operates in this mode by default without any need of user intervention once the fluid medication is charged to the system  10 . In the absence of user intervention, the system  10  maintains the automated mode of operation in the essentially steady-state manner until substantially all of the fluid medication is displaced from the first and second medication storage chambers  40   a ,  40   b  or until the springs  64   a ,  64   b  reach their expansion limit, whichever occurs first. The automated mode of operation is deemed essentially steady-state because the system  10  discharges an extended substantially continuous stream of fluid medication to the treatment site at a relatively constant first flow rate for the duration of the automated mode of operation. An exemplary first flow rate of fluid medication from the system  10  is in a range of about 2 to 5 ml per hour. 
     The term “essentially steady-state” as used herein encompasses operating conditions, wherein the automated mode is not precisely steady-state due to relatively small fluctuations or perturbations, which may occur in the first flow rate of the fluid medication or which may occur in the continuity of the stream of fluid medication from the system  10 . For example, if the frictional forces between the first and second walls  66   a ,  66   b  and the first and second pistons  60   a ,  60   b  and seals  62   a ,  62   b  remain constant while the displacement forces of the springs  64   a ,  64   b  decline with time throughout the automated mode, the first flow rate of the fluid medication from the system  10  may exhibit a relatively small correspondent decline with time. An exemplary decline rate of the flow rate under such conditions is relatively small, e.g., on the order of about 1% per hour. 
     An advantageous feature of the present system  10  is the passive conversion by the system flowpath of a continuous fluid medication stream to a more visible drip stream within the drip chamber  36 . As noted above, the discharge sight window  34 , in cooperation with the opening  32 , enables the user to observe the interior of the drip chamber  36 . However, it would be difficult to detect the presence of a continuous fluid stream within the drip chamber  36  due to the absence of light contrast between the continuous stream and the drip chamber wall  74 . The intermittent droplets  88  of the drip stream provide greater light contrast than a continuous stream, which enables the user to visually monitor whether fluid medication is flowing through the system  10  or not in a relatively simple manner without disrupting operation of the system  10 . The first and second storage sight windows  38   a ,  38   b  also advantageously enable the user to easily visually monitor the remaining level of fluid medication in the first and second medication storage chambers  40   a ,  40   b.    
     Another advantageous feature of the present embodiment is the raised position of the inlet end  78  of the drip chamber outlet tube  44 , which is approximately at the volumetric center of the drip chamber  36 . If the infusion pump  12  is inadvertently overturned during user activity, the configuration of the inverted drip chamber  36  nevertheless maintains the ratio of air to liquid in the drip chamber  36  constant at about 1 to 1 by trapping an air pocket in the fluid accumulation annulus  82 . Therefore, fluid medication cannot drain back into the drip chamber  36  via the chamber outlet tube  44  because it is unable to displace the air pocket out the inlet end  78 . If the drip chamber  36  were not so configured, the entire drip chamber  36  could fill with fluid medication upon inversion and remain in the drip chamber  36  even after the drip chamber  36  is restored to its upright position. If the drip chamber  36  is filled in its entirety with fluid medication, the user is unable to visually detect fluid flow through the drip chamber  36 . 
     The manual fluid drive mechanism is transitioned to an active state during an alternate mode of operation termed the manual or instantaneous mode, which is described below with reference to FIG.  4 . The manual mode of operation enables the user to manually override the automated mode of operation and provide instantaneous delivery of a bolus dosage of the fluid medication to the treatment site when desired. The manual mode can be performed at any time when the bolus chamber  86  is charged with the fluid medication, and preferably when the bolus chamber  86  is charged at or near its fluid capacity. Operation in the manual mode is effected by manually applying a sufficient displacement force to the flexible wall  84  of the bolus injector  14  to deform the wall  84  and collapse the bolus chamber  86 , which contains a volume of the fluid medication. The displacement force is typically applied by squeezing the wall  84  in the hand  92  of a user, e.g., between the thumb and fingers as shown. Collapse of the bolus chamber  86  applies a displacement force to the fluid medication therein, which maintains the outlet valve  90  at the injector outlet port  54  open and instantaneously drives substantially all of the fluid medication, or at least the bulk of the fluid medication, downstream from the bolus chamber  86  through the injector outlet port  54  and into the catheter  18 . Essentially none, or relatively little, of the fluid medication residing in the bolus chamber  86  is driven upstream from the bolus chamber  86  when the displacement force is applied to the wall  84  because of the severe flow restriction provided by the drip chamber  36  and in particular, the drip tube  42 . 
     The fluid medication manually driven from the bolus chamber  86  through the catheter  18  to the treatment site is termed the bolus dosage. In contrast to the extended dosage, delivery of the bolus dosage to the treatment site may generally be characterized by a relatively higher second flow rate over a relatively short time, for example, about 4 ml instantaneously. Thus, the bolus dosage is essentially delivered to the treatment site in a single large pulse. The volume of the fluid medication in the bolus dosage is preferably approximately equal to the fluid capacity of the bolus injector  14 , i.e., in a range of about 4 to 6 ml. 
     Once the bolus dosage is delivered to the treatment site, the displacement force is withdrawn from the wall  84  and the outlet valve  90  at the injector outlet port  54  closes. The bolus chamber  86  begins recharging with the fluid medication from the first and second medication storage chambers  40   a ,  40   b  in accordance with the automated mode of operation if fluid medication is present in the medication storage chambers  40   a ,  40   b . In addition to the displacement force applied to the system flowpath by the automated fluid drive mechanisms upstream of the bolus injector  14 , which drives the fluid medication from the first and second medication storage chambers  40   a ,  40   b  into the bolus chamber  86 , the elastic memory of the bolus injector  14  may apply a suction force to the system flowpath both upstream and downstream of the bolus injector  14 . However, the closed outlet valve  90  at the injector outlet port  54  negates the downstream effect of the suction force, blocking the backflow of fluid into the bolus chamber  86  from the catheter  18  or treatment site. The drip chamber  36  negates the upstream effect of the suction force, preventing the bolus injector  14  from drawing the fluid medication into the bolus chamber  86  at a faster rate than is dictated by the drip tube  42 . 
     When the bolus chamber  86  is recharged preferably at or near its fluid capacity, the ambient pressure of the fluid medication reopens the outlet valve  90  at the injector outlet port  54 , enabling the fluid medication to resume flow as a substantially continuous stream to the treatment site. As is apparent from above, the drip chamber  36  limits the rate at which the bolus chamber  86  can recharge to substantially prevent a user from overmedicating oneself by attempting to repeat the manual mode of operation in a relatively short time period. 
     The medication delivery system  10  has been described above as comprising a single infusion pump  12  having two medication storage chambers  40   a ,  40   b , respectively. However, the present invention is not so limited. It is readily apparent to the skilled artisan that the present invention additionally encompasses alternate embodiments of the medication delivery system  10 , wherein the infusion pump has a single medication storage chamber or includes three or more medication storage chambers. Such alternate embodiments require only minor modifications of the present teaching in a manner within the purview of the skilled artisan. The first and second springs  64   a ,  64   b  have also been described in the embodiment of the invention set forth above as being identical. The present invention additionally encompasses alternate embodiments of the medication delivery system  10  wherein the first spring  64   a  has a different displacement force than the second spring  64   b . For example, the first spring  64   a  could be selected with a displacement force substantially greater than the displacement force of the second spring  64   b  so that all or some of the fluid medication would be discharged from the first medication storage chamber  40   a  before any fluid medication would be discharged from the second medication storage chamber  40   b . If there is a decline in the fluid flow rate from the system  10  during the automated mode as described above, the practitioner can alter the decline by selecting the first and second springs  64   a ,  64   b  with various balanced or unbalanced displacement forces as desired. 
     Referring to FIG. 5, an alternate embodiment of a medication delivery system of the present invention is shown and generally designated  110 . The present medication delivery system  110  differs somewhat from the medication delivery system  10  described above. The medication delivery system  10  described above employs a single infusion pump  12  having two medication storage chambers  40   a ,  40   b  and two automated fluid drive mechanisms. The medication storage chambers  40   a ,  40   b  of the medication delivery system  10  are in fluid communication with one another via a single flowpath upstream of the bolus injector  14 . Thus, the infusion pump  12  in cooperation with the bolus injector  14  serves both the automated and manual modes of system operation. In contrast, the medication delivery system  110  of the present embodiment employs separate first and second infusion pumps  112   a ,  112   b , which are in fluid isolation from one another because each has a separate flowpath upstream of a bolus injector  114 . The first infusion pump  112   a  has a first medication storage chamber  140   a , which serves the manual mode of system operation exclusively, while the second infusion pump  112   b  has a second medication storage chamber  140   b , storage chamber outlet  168   b , and manifold  170 , which serve the automated mode of system operation exclusively. The remainder of the flowpath of the second infusion pump  112   b  is substantially similar to that described in the medication delivery system  10 . Accordingly, the elements of FIG. 5, which are common to FIGS. 1-4, are denoted by the same reference characters. 
     The primary functional difference between the system  110  and the system  10  is that the automated and manual modes of the system  110  operate independently in parallel, whereas the automated and manual modes of the system  10  operate cooperatively in series. To enable independent operation of the system  110 , the first medication storage chamber  140   a  is provided with a separate inlet port  128  having an one-way inlet valve  187 , an outlet port  130  and a tube/port coupling  150 , which are similar to the corresponding elements employed in the second medication storage chamber  140   b . However, the outlet port  130  has a flow restriction (not shown) positioned therein to regulate the fluid flow rate therethrough and to substantially prevent backflow. An injector inlet tube  116   a  connects the first medication storage chamber  140   a  with the bolus injector  114  via the outlet port  130  and the injector inlet port  152 . An injector outlet tube  116   b  connects the bolus injector  114  with a “Y” fitting junction  194  via the injector outlet port  154 . It is noted that the outlet valve (not shown) positioned at the injector outlet port  154  has a stronger pressure rating than that of the system  10  so that it only opens in response to a displacement force on the bolus injector  114 . A connective tube  116   c  connects the second infusion pump  112   b  with the “Y” fitting junction  194  via the pump outlet port  30 . The tubes  116   b  and  116   c  are joined at the “Y” fitting junction  194  and a common flow tube  118 , preferably a catheter, exits the “Y” fitting junction  194  to a treatment site. One apparent advantage of the present system  110  is that different fluid medications can be stored in each of the medication storage chambers  140   a ,  140   b  and independently delivered to the treatment site. 
     The medication delivery system  110  has been described above as having a single medication storage chamber  140   a  or  140   b  for each infusion pump  112   a  or  112   b , respectively. However, the present invention is not so limited. It is readily apparent to the skilled artisan that the present invention additionally encompasses alternate embodiments of the medication delivery system  110 , wherein the infusion pump includes two or more medication storage chambers. Such alternate embodiments require only minor modifications of the present teaching in a manner within the purview of the skilled artisan. 
     Referring to FIG. 6, another alternate embodiment of a medication delivery system of the present invention is shown and generally designated  210 . The medication delivery system  210  employs a single infusion pump  212  having a single medication storage chamber  240 , piston, seal and spring set  260 ,  262 ,  264 , storage chamber outlet  268 , and manifold  270 . The automated and manual modes of operation are served by the same infusion pump  212 , but via separate flowpaths. To enable separate flowpaths, an outlet port  230  is positioned in the top of the medication storage chamber  240 . In all other respects the medication delivery system  210  is substantially the same as the medication delivery system  110 . Accordingly, the elements of FIG. 6, which are common to FIG. 5, are denoted by the same reference characters. 
     While the forgoing preferred embodiments of the invention have been described and shown, it is understood that alternatives and modifications, such as those suggested and others, may be made thereto and fall within the scope of the invention.