WO2009041826A1

WO2009041826A1 - Infusion pump mechanism

Info

Publication number: WO2009041826A1
Application number: PCT/NZ2007/000284
Authority: WO
Inventors: Robert Stephen Murphy
Original assignee: Robert Stephen Murphy
Priority date: 2007-09-28
Filing date: 2007-09-28
Publication date: 2009-04-02

Abstract

An infusion pump comprising two linear actuators, two syringes, motorised rotary valve, fluid channels and connectors allows for continuous fluid flow by repeated fill and emptying of each syringe in a sequence that allows for the filling of one syringe while the other is being emptied. The mechanism of an infusion pump having two syringes (5) and (6) with outlets connected by rigid tubing (9) and (10) to a rotary valve flow director (7). The rotary valve is connected via connection point (13) to a reservoir fluid container and via connection point (14) to an extension line that is then connected to the patient infusion site. A linear actuator is attached to the pusher piece (1) of the first syringe and a separate identical linear actuator is attached to the pusher piece (2) of the second syringe.

Description

INFUSION PUMP MECHANISM

STATEMENT OF CORRESPONDING APPLICATIONS

This application is not based on any priority applications.

TECHNICAL FIELD

This invention relates to an infusion pump mechanism which may be incorporated into the design of a medical infusion pump suitable for precision dosing of fluid to a connected patient according to suitable electronic control.

BACKGROUND ART

Syringe pumps suffer from the disadvantage that the syringe must be manually refilled or replaced on expiry of the travel of the plunger.

The repeated use of disposable syringes to deliver accurate fluid dosing at low rates has been discouraged by the literature¹. The syringe plunger consists of a rubber construction that will change its frictional properties when re-used. In addition, the plunger is elastic in nature and will deform slightly under applied pressure. The result of these characteristics is a reported irregularity of flow known as 'stiction' where flow momentarily ceases then resumes. This invention uses syringes of small bore and volume typically less than 2 ml. The small syringe plunger offers less surface area to the syringe barrel, requires less motive force to maintain flow and deformation of the plunger under applied pressure is insignificant resulting in no perceived 'stiction' and consequent disruption to flow.

The complete extra-corporeal fluid circuit of a syringe infusion pump system comprising mechanical actuator, syringe, connectors and extension line to the patient will change slightly in internal fluid volume under applied external pressure. This change in volume, known as system 'compliance', will cause a consequent change in flow to the patient that is significant at low set flow rates i.e. less than 5 ml/h say. The change in external pressure can result from a change in height of the

¹D. F. Capes, D. Herring et al, 'The effect on syringe performance of fluid storage and repeated use: implications for syringe pumps", PDA J Pharm Sci Technol. 1996 Jan-Feb; 50(l):40-50. pump relative to the height ² of the patient site or by any change in the patient venous pressure during infusion³. All types of infusion pump systems, including syringe pumps, exhibit 'compliance' and hence will exhibit degradation low flow accuracy when acted on by change of height or patient venous pressure. Compliance also contributes to other unwanted flow phenomena such as bolus fluid being released after line blockage or occlusion, slow start of delivery at low set flow rates, undesirable delay in recognising blockage or occlusion to flow and continued flow for a small time after the pump has been stopped.

All references, including any patents or patent applications, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the reference states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertiency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms parts of the common general knowledge in the art, in New Zealand or in any other country.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

²K.H. Krauskopf, J. Rauscher, L.Brandt, "Disturbance of continuous, pump administration of cardiovascular drugs by hydrostatic pressure", Der Anaesthesist. 1996 May; 45(5):449-52.

³R. Murphy, "50ml syringe pumps-- Are they suitable for 'high risk' infusions?", Society for Technology in Anesthesia Annual Scientific Meeting Miami Jan 2005 - Conference Proceedings. DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided an infusion pump mechanism including:

• a first and second syringe, and

• rotary flow-directing valve that operates under electronic control

wherein each syringe can be separately filled and then emptied at a preset rate via the rotary valve.

Preferably, the rotary flow-directing valve is bidirectional such that a first position connects the first syringe to a reservoir fluid container to enable a linear actuator connected to the first syringe to withdraw the plunger and fill the syringe whilst the second syringe is connected to the patient infusion site via the valve and a linear actuator connected to the second syringe acts on the syringe plunger to deliver fluid flow to the patient site at a predefined rate and the second position reverses the above arrangement to connect the second syringe to the reservoir fluid container whilst the first syringe delivers fluid flow to the patient infusion site.

Preferably, small bore syringes with capacity typically less than 2ml and rigid components so as to minimise error due mechanical 'compliance' i.e. the expansion or contraction under applied operating pressures of the fluid compartment, caused by pump height change or changes in patient venous pressure, and flow errors due to syringe 'stiction'.

Preferably, the minimising of mechanical 'compliance' results in reduction in the time taken for the pump to deliver fluid at the set rate and in the time taken to detect an occlusion of the patient site and a reduction in errors caused by pump height change or change in venous pressure.

Preferably, the linear actuator, used to drive each syringe, is operated to displace syringe fluid back into the fluid reservoir after syringe filling in order to take up any residual system 'backlash' before connecting the syringe via the rotary flow- directing valve to the patient circuit to deliver set flow. Thus, preferred embodiments of the present invention may have a number of advantages over the prior art which can include:

• a reduction in errors caused by pump height change or change in venous pressure;

• reduction in the time taken for the pump to deliver fluid at the set rate and in the time taken to detect an occlusion of the patient site, and

• deliver fluid to the patient in a continuous manner requiring no manual intervention to refill or replace any syringe.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 : show's the infusion pump mechanism of the present invention;

Figure 2: shows a first position of the rotary valve of the mechanism shown in Figure 1 , and

Figure 3: shows a second position of the rotary valve of the mechanism shown in Figure 1.

BEST MODES FOR CARRYING OUT THE INVENTION

This invention uses two syringes operated in sequence to deliver fluid to the patient in a continuous manner requiring no manual intervention to refill or replace any syringe.

The invention may use individual components of syringes, fluid routing lines, rotary valve and connectors. However, the preferred embodiment of this invention encapsulates most of these elements (with the exception of a fluid line connecting the pump to the patient site) within a disposable plastic block and is described with reference to the accompanying drawings in which: A disposable syringe system comprising two syringes is connected to a motorised rotary flow director by suitable rigid tubing and from the flow director fluid from each syringe is routed via suitable rigid tubing either to a connection point attached to a fluid filling reservoir or to a connection point that is connected via suitable low volume rigid extension line to the patient infusion site. The rotary flow director can be operated to rotate clockwise by 90° then to return to the original reference position. In this manner, fluid routing can connect a syringe to the patient site whilst the other syringe is connected to a fluid reservoir container or vice versa. Each syringe is acted on by a separate mechanical linear actuator attached to its pusher piece such that movement of the syringe pusher at an electronically controlled rate translates to precision fluid flow either into or out of the syringe. By electronic control, fluid can fill the first syringe whilst the second is emptying to the patient after which, by changing the position of the rotary valve, the second syringe can be acted on to empty fluid to the patient whilst the first syringe is permitted to fill. This reciprocating sequence allows for continuous accurate fluid delivery.

This invention uses small volume syringes offering negligible compliance along with other components also selected to minimise compliance. A rotary valve is used, manufactured from hard plastic and offering a constant volume when operated i.e. the amount of fluid constrained by the valve is not altered when the rotary valve is caused to move between its two stable positions. The connecting lines used for fluid routing to and from the valve are rigid and the extension line connecting the system fluid output to a patient is of low volume non-compliant material.

The mechanical 'backlash' inherent in any form of linear actuator will add to overall system compliance. This invention uses two precision, minimal backlash actuators to operate on each syringe to fill and expel fluid. Whilst the backlash can be minimised, it cannot be completely removed. For this reason, this invention requires that, once a syringe has been filled from the connected reservoir, the linear actuator will be reversed in direction causing fluid to be returned into the reservoir. This allows any backlash to be taken up before the rotary valve is operated connecting the filled syringe, operating at the set rate, to the patient.

This invention differs from so-called piston or cassette pumps that use miniature syringes and flow directing valves in that such systems do not operate the syringe as a linear dosing component. The captive syringe in these devices is filled and expelled as a bolus and flow rate is calculated as the number of bolus ejections from the system in a given interval.

Figures 1 to 3 show an infusion pump mechanism having two syringes 5 and 6, mounted in an encapsulating block 15, with outlets connected by rigid tubing 9 and 10 to a rotary valve flow director 7. The rotary valve director 7, pivots on point 8 and is connected via connection point 13 and line 11 to a reservoir fluid container and via connection point 14 to an extension line 12 that is then connected to the patient infusion site. A linear actuator is attached to the pusher piece 1 of the first syringe and a separate identical linear actuator is attached to the pusher piece 2 of the second syringe. The actuators are capable of movement in the length axis of the syringes. The system allows for fluid from syringe 6 to be displaced via the rotary valve to a connection point 14 connected to the patient via a suitable extension line. This fluid movement is achieved by the action of the attached linear actuator causing the syringe plunger 4 to be moved in the direction of the syringe outlet. Whilst the fluid is being displaced at a predefined set rate towards the patient, the second syringe is filled by action of the linear actuator attached to pusher piece 1 causing the plunger 3 to move away from the syringe outlet. When the syringe is empty, the rotary valve is acted on by a motor to rotate through 90 such that the fluid routing channels of the valve now connect syringe 6 to the filling reservoir attached to connection 13 and connect syringe 5 via the connection point 14 to the patient. The system, comprising two linear actuators, two syringes, motorised rotary valve and described fluid channels and connectors allows for continuous fluid flow by repeated fill and emptying of each syringe in a sequence that allows for the filling of one syringe whilst the other is being emptied.

Rotary valve is operated to the position shown by Figure 2 and syringe 6 is acted on by the attached linear actuator such that pusher piece 2 is moved at a rate proportional to the commanded flow rate towards the syringe outlet. Fluid is thus forced at the commanded rate via the rotary valve 7 and patient connector 14 towards the patient. At the same time, the linear actuator connected to the pusher piece 1 of syringe 5 is operated to move the pusher piece in a direction away from the syringe outlet at a rate in excess of the commanded flow rate. Fluid from the reservoir container connected to connector 13 enters the syringe 5 via the rotary valve 7 and fluid routing channels. Once the syringe 5 is filled, the actuator reverses direction to force fluid back through the rotary valve towards the reservoir container at the commanded flow rate in order to take up any mechanical backlash or 'dead-space'.

When the syringe 6 has reached the empty state, the rotary valve is operated on to assume the orientation shown by Figure 3. This now connects syringe 5 to the patient and syringe 6 to the reservoir container. The actuator attached to syringe 6 is now operated in the reverse direction at a rate in excess of the commanded rate to fill syringe 6 from the reservoir. Once completed, this sequence is repeated for the duration of the patient infusion.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.

Claims

THE CLAIMS DEFINING THE INVENTION ARE:

1. An infusion pump mechanism including:

• a first and second syringe, and

• rotary flow-directing valve that operates under electronic control

2. An infusion pump mechanism, as claimed in claim 1 , wherein the rotary flow-directing valve is bidirectional such that a first position connects the first syringe to a reservoir fluid container to enable a linear actuator connected to the first syringe to withdraw the plunger and fill the syringe whilst the second syringe is connected to the patient infusion site via the valve and a linear actuator connected to the second syringe acts on the syringe plunger to deliver fluid flow to the patient site at a predefined rate and the second position reverses the above arrangement to connect the second syringe to the reservoir fluid container whilst the first syringe delivers fluid flow to the patient infusion site.

3. An infusion pump mechanism, as claimed in claim 1 or claim 2, wherein the capacity of the first and second syringe are less than 2ml.

4. An infusion pump mechanism, as claimed in claim 3, wherein the first and second syringe components are rigid.

5. An infusion pump mechanism, as claimed in claim 2, wherein the linear actuator, is operated to displace syringe fluid back into the fluid reservoir after syringe filling in order to take up any residual system 'backlash' before connecting the syringe via the rotary flow-directing valve to the patient circuit to deliver set flow.