GB2071222A - Medical infusion system and method of operation - Google Patents

Abstract

A medical infusion system has a pump and cassette combination for conducting fluid or semi-solids from an upstream flexible conduit portion (11) to a downstream flexible conduit portion (13) of the system. The cassette (18) includes a rigid enclosure defining a pumping cavity (25) and has a window with a diaphragm (26) spanning the window. The diaphragm is engaged by an actuator (28) on the pump for varying the volume of the pumping cavity. Restrictors (21, 22) at the upstream and downstream portions close off and open, respectively, the flow into and out of the pumping cavity in a sequence related to the actuator movement such that the pumping cavity alternatively is filled and substantially emptied. To minimize the detrimental effects of air in the pumping cavity: 1) the actuator is operated to substantially increase the volume of the pumping cavity prior to opening of the upstream portion; and 2) the actuator is operated to reduce the volume of the pumping cavity to a residual volume of less than about 0.07 ml subsequent to closing of the upstream portion and opening of the downstream portion. <IMAGE>

Description

SPECIFICATION Medical infusion system and method of operation This invention relates to medical infusion systems and, more particularly, to an improved system and method for operating the system.

US Patent Application Serial No. 79,806 describes a medical infusion system which operates to pump fluid or semi-solids from an upstream portion to a downstream portion. The pump includes upstream and downstream conduit means and a removable cassette communicates between the two. The cassette has a window therein and a diaphragm spans the window.

Means on the pump engage the diaphragm for varying the volume of the pumping cavity formed by the cassette. The cassette has inlet and outlet openings valvelessly communicating with the cavity. Fluid in the conduit is pumped by restricting and opening the conduit means at upstream and downstream locations and displacing the contents of the cassette pumping chamber or cavity appropriately.

The medical infusion system described in the foregoing application provides significant advantages over many prior art constructions.

There are no valves employed in the cassette, thus significantly improving the reliability and reducing the complexity of manufacture and therefore the cost. Increase accuracy over prior cassette designs is also achieved. By properly controlling the pinchoff of the tubes at the inlet and outlet of the cassette, good constant flow rates may be achieved, even to very low rates.

It has been observed that, in the infusion system described above, the presence of air in the cassette pumping cavity can detrimentally affect the operation of the device. Such affects result from three phenomena. Firstly, at high back pressures, the fluid level in the drip chamber feeding the system rises, due to expansion of the compressed air as the inflow valve opens prior to filling. Such back flow of fluid up into the drip chamber, can unnecessarily trigger warning devices such as an empty bottle alarm. Secondly, flow output decreases as the back pressure and volume of air increases, affecting accuracy.

Thirdly, at low or negative back pressures (e.g.

with low needle levels of minus 12" to minus 36" below the cassette) the delivered flow rate is higher, due to the expansion of the air upon opening of the outflow valve, also affecting accuracy.

The effects of air in the cassette can be substantially reduced or eliminated by placing the outlet from the pump chamber in such a position that air bubbles migrate to the outlet and are pumped out with the expelled fluid or semi-solid.

Such a principle is employed in the cassette described in U.S. Patent No. 4,140,118 issued February 20, 1979. Although successful, such an arrangement may lead, from time to time, to confusion on the part of hospital personnel as to which is the inlet. This is because most conventional and prior art drug infusion systems are arranged so that the flow is continuously downward. Additionally the cassette of U.S.

Patent No. 4,140,1 18 contains discrete inlet and outlet valves and is consequently expensive for an application where the cassette is disposed of after each use or every 24 hours for reasons of sterility.

The invention provides, in one aspect, a medical infusion system for pumping fluids or semi-solids from an upstream flexible conduit portion of the infusion system to a downstream flexible conduit portion thereof, comprising, a pump having means for supporting said conduit portions, first restricting means for selectively restricting and opening said upstream conduit portion, second restricting means for selectively restricting and opening said downstream conduit portion, a cassette having means for connecting to said upstream and downstream conduit portions, said cassette defining a pumping cavity and having a flexible diaphragm defining a portion of said pumping cavity, means on said pump for releasably supporting said cassette in a position between said upstream and downstream conduit portions actuator means for engaging said membrane and being movable to displace, from a fully retracted position to a fully extended position, a predetermined volume in said pumping cavity, said predetermined volume being such as to leave a residual volume in said pumping cavity of less than about 0.07 ml., and means for operating said first and second restricting means and said actuator means in a sequence such as to pump fluids or semi-solids from said upstream conduit portion to said downstream conduit portion, said sequence being such that said actuator means move toward the fully retracted position to substantially increase the volume of the pumping cavity prior to opening of said upstream conduit portion by said first restricting means.

In another aspect, the invention provides a method for operating a medical infusion system for pumping fluids or semi-solids from an upstream flexible conduit portion of the infusion system to a downstream flexible conduit portion thereof, including a pump having means for supporting said conduit portions, first constricting means for selectively restricting and opening said upstream conduit portion, second restricting means for selectively restricting and opening said downstream conduit portion, a cassette having means for connecting to said upstream and downstream conduit portions, said cassette defining a pumping cavity and having a flexible diaphragm defining a portion of said pumping cavity, means on said pump for releasably supporting said cassette in a position between said upstream and downstream conduit portions, actuator means for engaging said membrane and being movable to displace, from a fully retracted position to a fully extended position, a predetermined volume in said pumping cavity, and means for operating said first and second restricting means and said actuator means, said method comprising operating said first and second restricting means and said actuator means in a sequence such as to pump fluids or semi-solids from said upstream conduit portion to said downstream conduit portion, said predetermined volume being such as to leave a residual volume in said pumping cavity of less than about 0.07 ml, said sequence being such that said actuator means move toward the fully retracted position to substantially increase the volume of the pumping cavity prior to opening of said upstream conduit portion by movement of said first restricting means.

An embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a schematic view of a medical infusion system constructed in accordance with the invention: Figure 2 is an exploded cross-sectional view of a cassette of the system of Figure 1; Figures 3 and 4 are schematic cross-sectional views illustrating the operation of the pump of Figure 1; and Figure 5 is a graph illustrating the operation of the cam driving means illustrated in Figure 3.

Very generally, the medical infusion system of the invention (Figure 1) operates to pump fluid or semi-solids from an upstream flexible conduit portion 11 to a downstream flexible conduit portion 13. The upstream conduit portion 11 may be in fluid communication with a source 12 of fluid, and the downstream flexible conduit portion 13 may be secured suitably for delivering the fluid or semi-solid to a patient 14. The system comprises a pump 15 which includes (see Figure 3) means 16 and 17 for supporting the conduit portions 11 and 13, respectively. First constructing means 21 are provided for selectively restricting and opening the upstream conduit portion 11. Second restricting means 22 are provided for selectively restricting and opening the downstream conduit portion 13.A replaceable cassette has means 23 and 24 connected to the upstream and downstream conduit portions 11 and 13, respectively. The cassette 18 defines a pumping cavity 25 and has a flexible diaphragm 26 defining a portion of the pumping cavity.

Means 27 releasably support the cassette in a position between the upstream and downstream conduit portions for connection thereto. Actuator means 28 engage the membrane and are movable from a fully retracted position (Figure 3) to a fully extended position (Figure 4) to displace a predetermined volume in the pumping cavity.

Means 29 operate the actuator means and the first and second restricting means in a sequence such as to pump fluids or semi-solids from the upstream conduit portion to the downstream conduit portion. To minimize the detrimental effects of air in the pumping cavity: 1) the actuator means are operated to substantially increase the volume of the pumping cavity prior to opening of the upstream portion for filling the cavity; and 2) the actuator means are operated to reduce the volume of the pumping cavity to a residual volume of less than about 0.07 ml subsequent to closing of the upstream portion and opening of the downstream portion for expelling the contents of the cavity. Consequently the maximum amount of air that can remain in the cassette is about 0.17 ml since the inflow and outflow can also trap about 0.10 ml of air.This small amount of air (0.1 7 ml) has a negligible effect on flow accuracy.

Referring now more particularly to Figure 1 , the invention is shown in the form of an intravenous delivery system for delivering fluid from a fluid reservoir or storage means 12 to a patient 14. The fluid is introduced intravenously through a suitable catheter 30 attached to the downstream portion 13 of the delivery system. The catheter is held in place by adhesive tape 31 on the arm of the patient 14 as is well known in the art. The fluid reservoir 12 may be a conventional intravenous delivery system bottle suspended on a stand 32. A drip chamber 33 is attached to the lower portion of the bottle 12 and may be of conventional construction. An empty bottle alarm, not shown, of suitable design may be employed beneath or attached to the drip chamber to signal when the contents of the bottle 12 have been drained.The contents of the bottle 1 2 pass through the upstream conduit portion 11 of the delivery system, the upstream conduit portion constituting, in the illustrated emhodiment, a flexible hose.

Although the bottle 12 is shown positioned on the support 32 in an elevated condition with respect to the patient 14, as is typical of many intravenous delivery systems, it is not critical in the system of the invention that the bottle be so elevated inasmuch as the fluid is conveyed to the patient by the positive pumping action of the pump 15. The pump 15, shown in Figure 1, may be suitably contained in a housing 34 having control knobs, for example a knob 35. The pump 15 may be supported on a bedside table 39 or other suitable structure and is located between the upstream portion 11 and the downstream portion 13 of the delivery system. Grommets 41 form the entrance and the exit to the housing 33 for the upstream and downstream portions 11 and 13, respectively.

The system of the invention employs a replaceable cassette 18 in the pump 15. The cassette utilized in the system of the present invention does not require any valves, vastly simplifying the system and drastically lowering the cost of the replaceable portion thereof. In addition, the system of the present invention employs a cassette in which a substantial displacement occurs during each pumping stroke. This stroke displacement in the system of the present invention is preferably about at least 0.25 ml.

The cassette is specifically illustrated in Figure 2. Basically, the cassette is in only three pieces. The main piece is a molded housing 43 of a suitable medical grade plastic. The outline of the housing is of generally circular shape with slightly squared but nevertheless still rounded corners. A nipple 23 is provided for connection to the upstream conduit portion of the infusion system, and a nipple 24 (on the opposite side of the housing 43) is provided for connection to the downstream conduit portion of the infusion system. Unlike many prior art cassettes, the nipples contain no valves. The housing 43 defines the pumping chamber 25 which is generally in the shape of a cylinder having one open side. An annular recess 45 surrounds the open side of the pumping chamber and receives the annular lip 47 of a flexible diaphragm 26.The diaphragm 26 is comprised of a suitable medical grade flexible material which is impervious to the fluids or semisolids being pumped and which is capable of flexing as described below sufficiently so as to enable the desired displacement, also described in detail below. For the purpose of holding the flexible diaphragm in place, a mounting ring 49 seats in and mates with a projecting annular lip 51 on the housing 43 and is suitably sealed to the housing, such as by ultrasonic welding, to form a seal and to hold the diaphragm 26 in place spanning the open side of the pumping chamber 25.

As previously mentioned, restricting means 21 and 22 are employed to sequentially restrict and open the upstream and downstream conduit portions 11 and 13, respectively, in a sequence such as to provide a valve type action to enable fluid to be pumped from the upstream conduit portion 11 to the downstream conduit portion 13 of the delivery system. Although any suitable means for constricting the conduit portions in the manner described below may be employed, in the apparatus illustrated in Figures 3 and 4, the restricting means 21 and 22 are the tapered ends of movable rods or bars 63 and 65, respectively.

The bars are movably supported, by suitable means not shown, inside the housing 34 of the pump 15. The rounded tips of the tapered ends 21 and 22 of the bars 63 and 65 engage the respective upstream conduit portion 11 and downstream conduit portion 13. The conduit portions are supported in mating grooves or channels in a chassis 16 inside the housing 34 of the pump 15. A boot 53 extends across the chassis 16 as shown to provide a seal between the region of the cassette 1 8 and the drive means described below.

The actuator means 28 comprise the rounded end of an elongated bar 67 movably mounted in a suitable support to the chassis 16, not shown. The rounded tip of the actuator means 28 of the bar 67 engages the diaphragm 26 over the open side of the pumping chamber 25 of the cassette 18.

When the cassette is properly mounted, the actuator means 28 distend the diaphragm 26 slightly inward even at maximum retraction; thus constantly being in engagement with the diaphragm throughout the pumping stroke. The volume of the pumping cavity in this maximum retracted position of the bar 67 is at its maximum operating volume. As the bar 67 moves inwardly the membrane is distended inwardly as shown in Figure 4, thereby reducing the volume of the pumping chamber 25.

The pump 15 includes a hinged door 27 which opens to allow insertion and removal of the cassette 18 and the regions of the upstream and downstream portions 11 and 13 which are inside the pump housing 34. As may be seen in Figures 3 and 4 when the door 27 is closed, a coil spring 55 held in a recess 57 of the door, the latter being covered by a plate 59, presses a biasing cup 61 against the cassette housing 43, thus holding the housing in position against the chassis 16.

Pressure blocks 62 and 64 are provided attached to the door 27 by adjusting screws 66 and 68, respectively. The block 62 is positioned on the opposite side of the upstream conduit portion 11 from the rod 21. Similarly, the block 64 is positioned on the opposite side of the downstream conduit portion 13 from the rod 22 By suitably adjusting the position of the blocks 62 and 64 using the screws 66 and 68, the pressure exerted by the restricting means 21 and 22 during the operation described below may be adjusted during manufacture to insure complete seal off of the tube and thus total closure, as described.

For moving the restricting means 21 and 22, and the actuator means 28 in the desired sequence, the ends of the movable bars 63, 65 and 67 opposite the cassette and conduit portions carry suitable cam followers 71 biased (by springs, not shown) against the surface of cams 91, 93 and 95. The cams 91, 93 and 95 are mounted on a cam shaft 97 which is rotated by a driving motor 103. The driving motor is a stepping motor to provide incremental rotation of the shaft and therefore incremental movement and control over the movable bars.

In Figure 3, the lower or second restricting means 22 is in a position such as to restrict or pinch-off the downstream flexible conduit portion 13. Due to the suction (negative pressure) created by the retraction of the member 28, as explained below, the pumping chamber 25 of the cassette 18 fills. The pumping stroke is illustrated in Figure 4 wherein the second restricting means 22 are retracted to fully open the conduit portion 13 and the first restricting means 21 have closed to restrict or occlude the upstream conduit portion 11. Movement of the actuator means 28 inwardly reduces the volume of the pumping chamber 25 causing a fixed or preset amount of the contents to be expelled toward the downstream portion of the delivery system. The distance which the actuator means 25 move inwardly determines the displacement volume during the pumping stroke.

The filling and pumping strokes are conducted in sequence and repeated according to the rotation of the cam shaft 97 and the configuration of the cams 91,93 and 95.

Referring to Figure 5, a plot is provided illustrating the motion versus degrees of shaft rotation for the three sliding bars 63, 65 and 67, respectively. At the left-hand edge position or 0 position shown in the plot, the upstream bar 63 is closed whereas the bars 65 and 67 are open.

Movement of the bar 67 from the fully open position to the occluded position displaces a precise predetermined amount of the contents of the pumping chamber 25. Following this movement, the bar 65 moves to the occluded position to close the downstream conduit portion 13. Once this occurs, the bar 67 moves toward the open position. The resilience of the diaphragm causes expansion of the pump cavity volume. The bar 63 starts moving to the open position after the bar 67 has retracted about 20% of its total stroke, allowing filling of the pumping chamber once again. At about 2700, the upstream movable bar 63 moves from the open position to the closed position, and following this, the downstream movable bar 65 moves to the open position. This places the system in readiness for the next displacement stroke beginning with 00.Because the bar 67 retracts the plunger 28 to substantially increase the volume of the pumping cavity prior to opening of the upstream portion, back-filling of the drip-chamber is avoided. By "substantially", it is meant an amount sufficient to accommodate the maximum expected expansion of air in the cavity. It has been found that about a 20% increase in pump cavity volume before opening the upstream portion is typically satisfactory.

Typical applications of the invention would involve maximum pressures of about 25 psi.

Accordingly, the "rigidity" of the cassette 1 8 is selected appropriately. It is preferred that the actuator means engage the diaphragm throughout its stroke, and that there always be a slight inward loading on the diaphragm. For satisfactory accuracy, it is preferred that the unsupported or unengaged area of the diaphragm not exceed 75 per cent of the total diaphragm (window) area.

Rather than the cam drive illustrated, other means for operating the restricting means may be utilized, such as a lead screw drive. However, incremental control over the diaphragm movement is significant in achieving proper accuracy and control. By suitable design, a desired displacement volume may be selected for each step. Digital control is then readily possible with commercially available and relatively inexpensive micro-computer chips. Functions such as flow rate, total volume delivered and flow error monitoring may easily be handled by known digital techniques.

It is usually preferable to design the system to operate such that the return strokes of the bars 63, 65 and 67 be of the same duration regardless of the delivery rate and volume. Thus, the flow may be more even where the actuator means are retracted at a relatively quick fixed rate as opposed to a varying inward (delivery) rate. Digital capability makes such operation easily achievable.

The size of the outlet and inlet openings relative to the diaphragm size is of significance. With inlet and outlet openings which are too large, regurgitation becomes a large enough factor to deleteriously affect accuracy. Thus, it is preferred that each of the inlet and outlet openings have a cross-sectional area having a ratio to the diaphragm or window area of not greater than about 0.3.

As is the case with any cassette type pump, the presence of air in the cassette can affect the accuracy of the pumping rate. To avoid such problems, the system of the present invention combines two features which significantly enhance accuracy when used together. First of all, the member 67 is moved to a partially (about 20%) retracted position before opening the upstream portion by retracting the member 63.

This prevents expanding air in the pumping cavity from raising the fluid level in the drip chamber.

Secondly, the residual volume of the pumping cavity with the member 67 in the full in position is selected (by appropriate choice of stroke distance and pumping cavity geometry) to be less than about 0.07 ml, and preferably, 0.05 ml. The tubing between the cassette and the pinch-off regions adds, typically, a volume of 0.1 ml, giving a maximum volume of trapped air of 0.17. At this level, the air will affect flow accuracy at tolerable levels. Any air in the cassette which exceeds the residual volume plus tubing volume will be forced out of the cassette and may be suitably detected if excessive.

In addition to the foregoing, the medical infusion system of the invention provides other significant advantages over many prior art constructions. By properly designing the cams and the size of the movable restricting elements, good constant flow rates may be achieved, even to very low rates. For example, the system of the invention is capable of delivering at one ml per hour flow rate with an incremental delivery every 26 seconds and a refill time of only one second.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

Claims (8)

1. A medical infusion system for pumping fluids or semi-solids from an upstream flexible conduit portion of the infusion system to a downstream flexible conduit portion thereof, comprising, a pump having means for supporting said conduit portions, first restricting means for selectively restricting and opening said upstream conduit portion, second restricting means for selectively restricting and opening said downstream conduit portion, a cassette having means for connecting to said upstream and downstream conduit portions, said cassette defining a pumping cavity and having a flexible diaphragm defining a portion of said pumping cavity, means on said pump for releasably supporting said cassette in a position between said upstream and downstream conduit portions actuator means for engaging said membrane and being movable to displace, from a fully retracted position to a fully extended position, a predetermined volume in said pumping cavity, said predetermined volume being such as to leave a residual volume in said pumping cavity of less than about 0.07 ml., and means for operating said first and second restricting means and said actuator means in a sequence such as to pump fluids or semi-solids from said upstream conduit portion to said downstream conduit portion, said sequence being such that said actuator means move toward the fully retracted position to substantially increase the volume of the pumping cavity prior to opening of said upstream conduit portion by said first restricting means.

2. A medical infusion system according to Claim 1 wherein said predetermined volume is at least about 0.25 ml.

3. A medical infusion system according to Claim 1 wherein said connecting means and said conduit means between said pumping cavity and said restricting means define a volume, contiguous with said pumping cavity, and wherein said contiguous volume and the residual volume of said pumping cavity are less than about 0.17 ml.

4. A method for operating a medical infusion system for pumping fluids or semi-solids from an upstream flexible conduit portion of the infusion system to a downstream flexible conduit portion thereof, including a pump having means for supporting said conduit portions, first constricting means for selectively restricting and opening said upstream conduit portion, second restricting means for selectively restricting and opening said downstream conduit portion, a cassette having means for connecting to said upstream and downstream conduit portions, said cassette defining a pumping cavity and having a flexible diaphragm defining a portion of said pumping cavity, means on said pump for releasably supporting said cassette in a position between said upstream and downstream conduit portions, actuator means for engaging said membrane and being movable to displace, from a fully retracted position to a fully extended position, a predetermined volume in said pumping cavity, and means for operating said first and second restricting means and said actuator means, said method comprising operating said first and second restricting means and said actuator means in a sequence such as to pump fluids or semi-solids from said upstream conduit portion to said downstream conduit portion, said predetermined volume being such as to leave a residual volume in said pumping cavity of less than about 0.07 ml, said sequence being such that said actuator means move toward the fully retracted position to substantially increase the volume of the pumping cavity prior to opening of said upstream conduit portion by movement of said first restricting means.

5. A medical infusion system according to Claim 4 wherein said predetermined volume is at least about 0.25 ml.

6. A medical infusion system according to Claim 4 wherein said connecting means and said conduit means between said pumping cavity and said restricting means define a volume, contiguous with said pumping cavity, and wherein said contiguous volume and the residual volume of said pumping cavity are less than about 0.17 ml.

7. A medical infusion system substantially as herein described with reference to and as shown in the accompanying drawings.

8. A method of generating a medical infusion system substantially as herein described with reference to and as shown in the accompanying drawings.