Technical Field

The present invention pertains to medication infusion systems, and more particularly to a fluid handling system for a medication infusion system.

Background Art

This is a continuation- in- part o ^' f patent application, Serial No. 34,155, filed on April 27, 1979.

The desirability of being able to infuse medication into a human or animal body has been recognized by prior and contemporary technology. Seme such apparatuses are totally i plantable while in others the pump and control apparatus' is external to the living body and a catheter or the like is used to supply the medication to be infused into the liv¬ ing body. In either case delivering a medication flow hich is controlled is imperative. Additional- ly, for infusion of medication such as insulin it is important that the infusion flow profile mimics that of insulin production in a normal person.

- Another consideration, especially in implanted pumps, is that power consumption be kept to a minimum to insure adequate battery life.

To control the flow of medication from an infusion pump it has been suggested that a flow

restrictor can be employed. Such flow restrictors are taught, in United States patents 3,731,681, 3,894,538, 3,951,147 and 4,077,405. United States patent 4,299,220 teaches the use of a pressure regu- lation system in combination with a passive pump for infusing drugs .

The use of a fluid accumulator in combination with a fluid flow restrictor is taught in United States patents 4, 192, 397 and 4, 221, 219. Both of these patents employ a passive pump which operates under the principal of having pressure applied to a reservoir such that fluids stored within the reser¬ voir is forced thereout. While such a pump configur- at-ion does not in and of itself require electrical consumption it basically is uncontrollable unless valves or the like are used to control flow from the pu p.

The present invention solves the problems associated with the prior art by using a pulsatile pump in conjunction with an accumulator flow restric¬ tor network to give the advantage of low power con¬ sumption since a pulsatile pump as taught herein is economical in electrical consumption by virtue of the use of spring force to pump and which also provides, in combination with the accumulator flow restrictor a delivery of medication which has a profile that is smoothed and therefore desirable ^" for certain appli¬ cations such as infusion of insulin.

A pulsatile pump is shown in the United States patent 4, 152, 098 but its use in combination with an

accumulatσr flow restrictor is not shown or suggested .

Disclosure, of Invention

Therefore, a primary object of the present invention is to provide an infusion apparatus for providing medication to a living body of patient which can be configured to have a smooth flow of medication for infusion.

A further object of the present invention is to provide an infusion apparatus for providing medica¬ tion to a living body of a patient hich has minimal electrical consumption so that the longevity of its power source is maximized.

A still further object of the present invention is to provide an infusion apparatus for providing medication to the living body of a patient which mimics the natural flow profiles of body produced substances such as insulin or reproductive sex ho mones .

Still another object of the present invention is to provide greater flow smoothing at slow medication .delivery rates while still allowing faster delivery rat€s when that is require .

Still another object of the present invention is to provide an infusion apparatus for providing medi¬ cation to the living body of a patient which can be practiced with sufficient design flexibility to

permit the crucial components .thereof to be incor¬ porated either in the main housing thereof- or in the catheter thereof ^" used to deliver the medication to the living body.

-_ _^ Still another object of the present invention is to provide an infusion apparatus for providing medi¬ cation to a living body of" a patient which permits exquisitely precise control of medication dosing.

Still another object of the present invention is to provide medication flow rates which are not alter¬ ed by normal variations in barometric pressure or pressure variations occurring from a rapid change in ^■ the patient' s altitude.

Still another object of the present invention is to provide an infusion apparatus for providing medi¬ cation to a living body.

These objects, as well as further objects and advantages of the present invention will beccme readily apparent after reading the ensuing descript- ion of a non-limiting illustrative embodiment and viewing the acccmpanying drawings. An infusion apparatus for providing medication to a living body of a patient according to the principles of the present invention, comprises: a medication reservoir for storing selected medication; means for pumping said selected medication; said pump means operating

" in a pulsatile mode capable of producing a pulsing output flow, the input of said pump means being in

* ^• cαmnunicatioon with said reservoir; means for accixn— ulating said selected medication, the input of said

accumulator means being in communication with the output of said pump means ; means for restricting the flow of said selected medication, the input of said flow restrictor means being in communication with the ^' output of . said accumulator means, said flow restrict¬ or in combination with said accumulator means smooth¬ ing the pulsatile nature of the flow of said selected medication; and means for communicating said selected medication from the output of said flow restrictor means to said living body possibly through a catheter connected to the output of the said flow restrictor. The present invention also provides for additional accumulator flow restrictor combinations to be put in s.eries with a first accumulator flow restrictor assembly to further enhance the smoothing character- istics of-the apparatus if desired.

Brief Description of Drawings

In order that the present invention may be more fully understood, it will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a prospective view of an implantable infusion apparatus constructed in accordance with the principles of the present invention;

Figure 2 is a cross-sectional view of the apparatus of Fig.ure 1;

Figure 3 is a schematic drawing of the apparatus of the present invention;

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Figure 4 is a cross-sectional view in elevation of a pump incorporating the principles of the present in ention;

^' Figure 5 is ~ ^* a schematic view of an accumulator incorporating the principles of the present invention;

Figure 6 illustrates the behavior of an accumulator showing fluid time constant versus accumulator volume in an accumulator constructed in accordance with the principles of the present in ention;

Figure 7 is a graph of accumulator volume versus time for an accumulator constructed with the prin¬ ciples of the present invention;

Figure 8 is a g ^~ raph of the fluid flow rate out of an accumulator flow resistor combination constructed in accordance with the principles of the present invention;

Figure 9 is a schematic representation of an infusion apparatus incorporating dual acc mulator flow restrictor assemblies one of which assemblies is integral with a fluid catheter; and

Figure 10 is a graph of the flow rate characteristics of the apparatus of Figure 9.

Best Mode for Carrying Out the Invention

For purposes of illustration the present invention will be described as employed in an im- plantable infusion apparatus. However, it is to be understood that the teachings herein can also be applied to an apparatus where seme or all of the components thereof are situated such that they are external to a living body. Referring now to the figures, and more particularly to Figures 1 and 2 thereof, there is illustrated therein an implantable infusion apparatus 10. The infusion apparatus 10 is configured for implantation inside a living body and .includes a housing 12 formed of a biocompatible material such as titanium or the like. The housing 12 incorporates a medication reservoir section 14 which includes a flexible diaphragm 16 that divides the medication reservoir section 14 into a medication chamber 18 and a liquid-vapor chamber 20. The medi¬ cation chamber 18 is for storage of the medication to be infused into the living body and the liquid-vapor chamber 20 is filled with a saturated vapor and some liquid of a fluorocarbσn such as Freon 113 or seme other appropriate pressurant. Over normal body tem¬ peratures, Freon 113 can readily change from a liquid to vapor and visa versa and therefore, at the essen¬ tially constant temperature of the human body it will maintain the liquid- vapor chamber 20 and -therefore the medication chamber 18 at an essentially fixed pressure regardless of the amount of medication dis- posed within the medication chamber 18.

As the medication chamber 18 is filled with medication, as hereinafter described, the flexible

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diaphragm 16 distends downward (with reference to the Figure) toward the bottom of the housing 12 and, eventually ccmes in contact with a limit switch 22 which senses that the medication chamber 18 has reached a p ^' reselected degree of fullness . As medi¬ cation is drawn from the medication chamber 18 by a pulsatile pump 52 (not shown in Figure 2) which is illustrated in Figure 4, the flexible diaphragm 16 moves upward from the base of the housing 12. Fill- ing of the medication chamber 18 is accomplished through a fill port 24 closed by a self-sealing septum 26. A needle is inserted through the fill port 24 and through the septum 26 so that it can communicate fluid to an antechamber 28. The ante- chamber 28 is provided with a filter 30 for filtering the medication after which the medication is collect¬ ed in a manifold 31 and the medication then passes through a conduit 32 and a one-way check valve 34 after which it enters the medication chamber 18.

Medication is drawn from the medication chamber

18 by the pulsatile pump 52 illustrated schematically in Figure 3, and further in Figure 4 and still further described in conjunction therewith and then is delivered to an accumulator 36. The flow from the pulsatile pump is delivered to the accumulator 36 through an input 38. Fluid leaves the accumulator 36 from an output 41 and then travels to a flow restric¬ tor further illustrated in Figure 3. The accumulator 36 forms a variable chamber 40 therein. The variable ^* volume chamber 40 includes flexible walls 42 which move outwardly as the variable volume chamber is filled. The variable volume chamber 40 returns to a preselected minimum volume when at rest, and, after

expansion returns to such a minimum volume due to a force having a spring constant determined by the characteristics of the flexible walls 42. Because of the design of the present invention, the accuπulator 36 returns to its initial minimum volume due to a force having a spring constant dictated by the flexi¬ ble walls 42 and/or a second spring constant dictated by a plurality of leaf springs 44. The leaf springs 44 axe positioned adjacent to the upper flexible wall 42 such then when a preselected degree of expansion of the wall 42 has taken place the wall 42 ccraes in contact with the leaf springs 44. The spring con¬ stant of the leaf springs 44 is greater than the -spring constant of the flexible walls 42. As a result, when the variable volume chamber 40 is filled to a volume such that the flexible wall 42 does not touch the leaf springs 44 the accumulator 36 has one spring constant. When the variable volume chamber 40 is filled and expands such that the flexible wall 42 cones in contact with and is under the influence of the leaf springs 44 the accumulator 36 has another spring constant which is higher . The output 41 of the accumulator 36 is in* communication with a flow restrictor, as hereinafter described. The acσurtu- lator flow restrictor combination has two different time constants, the first one of which is longer for small volumetric displacements of the variable volume

-chamber 40 and another which is shorter for larger volumetric displacements of the variable volume chamber 40. The shorter time constant occurring after the upper flexible wall 42 has ccme in contact with the leaf springs 44.

Of course, it is to be understood that the different spring constants necessary for the func¬ tioning of the accumulator 36 can be provided thereto in manners other than the inherent flexibility of the walls 42 and the use of the leaf springs 44. For

• instance, Figure 5 illustrates an accumulator in which the variable volume chamber 40 is a bellows having a first spring constant and as the bellows expands it contacts leaf springs 44 which supply a second higher spring constant. Other spring arrange¬ ments can be employed so long as a differential between two spring rates is provided. It is also within the contemplation of this invention that a multiplicity of spring rate associated with different volume levels in the variable volume chamber can ^" be used to provide various degrees and methods of fluid flow smoothing ♦

Since the ambient pressure to which a living body is subject can vary in the course of a day because of normal barometric changes as well as travel in an elevator, a pressurized airplane cabin, or travel to different altitudes it is possible that the fluid flow out of a dual-rate accumulator as above de¬ scribed could be effected by such pressure changes . In order for the fluid delivery rate to be impervious to- sudden or gradual changes in ambient pressure, it is desirable to reference the flexible wall 42 of the accumulator to ambient body pressure. This is accom¬ plished by the provision of a flexible membrane 46 mounted on the housing 12, the membrane 46 being exposed to the ambient pressure 43 of the living body. Behind the, flexible member 46 is a fluid chamber 48 filled with a suitable fluid 50. The

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fluid 50 fills the fluid chamber 48 and also contacts the exterior of the flexible walls 42 so that the flexible walls are referenced to body pressure 43 through transmission of pressure from the living body through the flexible membrane 46 and the fluid 50 to the flexible walls 42.

With reference to Figure 3, the overall operation of the system can be understood. Beginning at the fill port 24 the sharp end of a non-coring hypodermic needle is shown inserted through the septe 26 and into the antechamber 28 so that a desired medication can be passed through the filter 30, into the ani- fold 31 and thence to the medication reservoir 14 by way of the check valve 34. Medication flows from the output o ^* f the medication chamber 18 when drawn there¬ from by a pulsatile pump 52 further illustrated in Figure 4. The pulsatile pump 52 includes a bellows chamber 54 which varies in volume as a result of the motion of the flexible wall 56 that is connected to a magnetizable armature 66 that is driven by the perme¬ able core coil 68. The bellows chamber 54 is connected on the input 58 thereof to the medication chamber 18 through a.. one-way valve 60. The output 62 of the pump 52 is connected to the bellows chamber 54 through a one-way valve 64. One-way valves 60 and 64 -are of the ball and spring type and operate such than an increase in volume of the bellows chamber 54 causes opening of the valve 60 so that medication can be drawn therein. A decrease in the volume of the bellows chamber 54 causes the closing of valve 60 and the opening of valve- 64 so that medication can be delivered to the input 38 of the accumulator 36.

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The volume of the bellows chamber 54 is varied by the flexing, of the flexible wall 56, this flexing being accomplished by the drawing of a magnetizable armature 66 toward a permeable core coil 68 and the returning of the-- armature 66 to a rest position as a result of the inherent spring constant of the flexi¬ ble wall 56 which could be supplemented by additional leaf springs or other types of springs. Ther-efore, when at rest, the bellows chamber 54 is at a minimum volume and when the permeable cove coil 68 is ener¬ gized the magnetizable armature 66 is drawn .adjacent thereto to put the bellows chamber 54 at a . maximum volume. When the coil 68 is de- energized, the magne¬ tizable armature 66, by virtue of the inherent spring - constant of the flexible wall 56 returns to its rest ¹ position and the bellows chamber 54 returns to mini¬ mum volume. The maximum volume which the bellows chamber 54 can assume is limited by the maximum travel of the armature 66 and, as illustrated this is - accomplished through the fixed relative position of _- the armature ^" 66 to the flexible wall 56 and the abutment of the armature 66 against the surface of the permeable core of the coil 68.

A preferred embodiment for the flexible wall 56 is a convoluted diaphragm which is rigidly mounted at its edges and is therefore a movable section that is in contact with the medication disposed in the bellows chamber 54. The flexible wall 56 is inhibit¬ ed from moving when the pressure P in the bellows - chamber means exceeds the spring force F of the flexible wall 56 divided by the effective area of the . surface of ^" the flexible wall in contact with the

^: selected medication ^" , i.e., when ?_ ma_„x ^* _ ~ F/A.

Effective area is defined as the projected area of a ^• portion of the flexible wall in contact with the selected medication and which moves such that it is perpendicular to the spring force of the flexible wall 56. In more general terms, effective area will be the surface area in contact with the medication which is perpendicular to the spring force of the variable volume.

Should a malfunction occur in the electronics and 0 a continuous sequence of rapid pulses be introduced to the permeable core coil 68, causing the armature

66 to reciprocate, the return of the flexible wall 56 to. its original position would be inhibited once the i pressure in the bellows chamber 54 exceeds P . The i ^c max S pressure builds up rapidly because of the flow re¬ strictor 72. Therefore, the possibility of intro¬ ducing drugs or other medication at a high rate, which would be unsafe for the patient, is essentially eliminated.

0 The pump 52 pumps a fixed volume of fluid each time the coil 68 is energized because of constraint on the movement of the armature 66. Once the elect¬ rical pulse current with the coil exceeds a certain valve, the stroke volume of the pump 52 is constant 5 and independent of any increase in the electrical pulse current or pulse width or pulse energy into the coil ^' 68. Therefore a known dosage of medication is released with each pulse as long as a certain minimum value of pulse current is exceeded.

0 In designing the pump, one could select a nominal pulse current that is twice the value needed to

maintain a constant stroke volume. This then would assure the constant volume per stroke even if the pulse current fell to 1/2 its nominal value.

The medication reservoir 14 is preferably maintained at a pressure level below the internal pressure of the living ^" body and this characteristic will not have an effect on the pressure within the pump since the pump pressure is independently gener¬ ated by the spring force of the flexible wall 56. As 0 the medication leaves the pump output 62 it enters the input 38 of the accumulator 36 and flows into the variable volume chamber 40 thereof as illustrated both in Figure 3. Upon expansion of the variable volume chamber 40 the flexible wall 42 is subject to

15* a spring force 43, such spring force may have a single spring constant, or it may have two spring constants for different volumetric expansions of the variable volume chamber 40 (illustrated in Fig. 5), or it may have two spring . constants and also be

20 _. responsive to the ambient atmospheric pressure 43 (illustrated in Figure 2) . Flow from the accumulator 36 leaves the output 41 thereof and enters a filter 70. The filter 70 is provided to preclude debris in the medication from clogging a flow restrictor 72

25 when the medication is supplied thereto . The flow restrictor 72 is essentially a length of tube in which a portion of the internal diameter thereof sized to partially restrict the flow of medication therethrough. Alternately, the filter 70 can act by

30 -itself as a flow restrictor and can be used alone or in combination with a constricted tube. Addition¬ ally, other means of restricting liquid flow are well

known to any person of ordinary skill in the fluid systems art.

By use of an accumulator having two different time constants in conjunction with a suitably sized flow restrictor, the pulsatile flow of medication leaving the pump 52 is smoothed. The accumulator acts in a manner analogous to an electrical capacitor and the flow restrictor acts in a manner analogous to an electrical resistor, the two working together to create an RC time constant which results in this smoothing. To satisfactory mimic the natural deliv¬ ery of certain substances such as insulin to a living -body, it is desirable to deliver the medication rather smoothly as compared to pulsatile delivery. When the pulsatile pump 52 pumps at very slow rates, a long time constant is needed to smooth the flow. When pumping . takes place at a rapid rate a long time constant is no longer desired and a short time con- stant is desired to obtain rapid flow of medication. The design of the present accumulator provides long time constants for small displacements and shorter time constants for greater displacement of the vari¬ able volume chamber 40. With reference to Figure 6, this performance characteristic is shown. As an example, for the accumulator characteristic illustra¬ ted at stored volumes of six microliters or less the time constant in the -accumulator when combined with the flow restrictor is 60 minutes . For stored volumes between 6 and 48 microliters, the time constant of the accumulator is reduced to 15 minutes.

Figure 7 shows the stored volume in the accumulator plotted against time, the accumulator

-16- prσviding two different time constants depending on accumulator volume as shown in Figure 6. In Figure 7, at low vol mes of the accumulator (below 6 ul_) , the accumulator is subject to a lower spring constant which results in. a slow drug flow. However, when one exceeds a stored volume of 6 microliters, for this particular accumulator, the spring constant increa¬ ses, the time constant immediately becomes shorter, and the medication is delivered at a faster rate. By this construction, it is possible to provide a smooth basal flow rate of medication while at the same time providing for a comparatively high rate of flow when desired to infuse supplemental medication ( for example, insulin immediately after eating) .

Figure 8 ^' shows flow rate out of the flow restrictor as a function of time for an accumulator flow restrictor combination wherein a single accumu¬ lator and single flow restrictor is employed. In Figure 8, the accumulator remains at low volumetric displacement and the time constant therefore does not shift. It can be seen that the medication flow rate profile has been smoothed by the accumulator flow restrictor combination.

It has been found that employment of a second accumulator and flow restrictor in series with the first one achieves an increased smoothing of the flow of medication. With reference to Figure 9 there is illustrated therein an alternate embodiment of the present invention which employs a pulsatile pump 74 having the output 76 thereof coupled to a valve 78 and then to a first accumulator 80. The accumulator in turn is in communication with a flow restrictor 82

and the output of the flow restrictor 82 is in communication with a second accumulator 84. The output of the second accumulator 84 is in communica¬ tion with .a second flow restrictor 86. The output of the second flow restrictor 86 flows through a check valve 88 and into the end of a catheter 90 as illus¬ trated. The first accumulator 80 and first flow restrictor 82 are disposed within a housing 92 and the second accumulator 84 and second flow restrictor 86 are disposed within a catheter 90. While this has been shown for matter of convenience it is to be understood that the multiple flow restrictors and accumulators may be entirely disposed within the housing 92 or can be disposed entirely in the cathe- ^" ter 90. This is also the case with a configuration employing a single accumulator and ^" a single flow restrictor such that either, (1) both the accumulator and the flow restrictor can be disposed within the housing, or (2 ) the accumulator can be disposed within the housing and the flow restrictor within the catheter or, (3 ) both the accumulator and flow re¬ strictor can be in the catheter. The catheter 90 is also illustrated with a pyrolytic carbon tip 94 a material known to be the least thrombogenic of any known material. Check valve 88 is provided to ensure that body fluids cannot flow into the infusion appa-

. ratus . The use of such a check valve and the pyro-

^* lytic carbon tip is also applicable to a single accumulator flow restrictor configuration. The accumulators 80 and 84 function in the same manner as the accumulator 36 hereinbefore described, and both can include one or two spring rates to provide one or two time constants in conjunction with their associ¬ ated restrictors 82 and 86.

With reference to Figure 10, the performance characteristics of the system of Figure 9 are illus¬ trated where the flow rate has been graphed against time. This flow is even smoother as compared to that which is achievable with a single accumulator, flow restrictor as previously discussed in conjunction with Figure 8.

It should be apparent that additional accumulator flow restrictors can be used in series with the two accumulator flow restrictors previously discussed to provide even more dramatic smoothing when desirable, i.e., when a medication will only be effective when administered at a very smooth and controlled rate. Although specific accumulator volumes and flow rates have been illustrated it is to be understood that these are for purposes of sho-wing the principles of the present invention and are not to be limiting to the manner in which these principles can be applied to a broad range of differently configured and sized accumulators and flow restrictors . Furthermore it will be understood that various changes in the de¬ tails, materials, arrangements of parts and opera¬ tional conditions which have been herein described and illustrated in order too explain the nature of the invention may be made by those skilled in the art within the principles and scope of the present inv ention .

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