In medical practice, syringe pumps are used for drugs which need high accuracy and have a short half-life in the body. In operating theatres and intensive care units, syringe pumps are mounted in stacks of in particular six to eight pumps that, however, require a lot of space and allow only low visibility and recognition of the indication of a drug provided on the syringe. Apart from occupation of large space syringe pumps have more problems, since a nominal trumpet curve and, thus, a constancy index cannot be achieved in practice due to the fact that the plunger usually made of rubber sticks to the walls of the syringe and therefore advances in pulses rather than continuously. Further, syringe pumps also have low sensitivity in occlusion pressure reading, that becomes a problem in neonatal infusions and recently with the use of wearable bolus large volume injectors. Syringe pumps are extensively used mainly in Europe, where about 40% of the beds in each hospital are equipped with a syringe pump, and for insulin infusions and most of immunoglobulin and Parkinson's disease infusions. It has been proposed to replace insulin syringe pumps by diaphragm pumps that, however, cannot be realized in practice since insulin crystallizes and renders active and passive valves of the mechanism leak.
Pre-filled syringes are part of a growing pharmaceutical delivery sector and work well for injections, but are problematic for longer term infusions, since they become bulky especially for newer biological drugs and have a volume limit of about 60 to 100 ml. So, the pumps become bulky as the needed infusion volume increases.
Syringe pumps are used because prior art peristaltic pumps had a low accuracy and causes a pulsatile flow and, hence, not a linear flow per infusion cycle, wherein during a part of the cycle there is no infusion but sometimes even a backflow, so that their constancy index is high for short half-life drugs. Short-term accuracy can be expressed by the concept of constancy index. This is the shortest period during a steady-state operation of a pump over which a measurement of output consistently falls within +/−10% of the mean rate. These data are derived from flow tests performed over 24 hours at 1 ml/h, wherein the flow is recorded at 30 seconds intervals during the final 18 hours period and the average rate is compared with the flow over each short period.
Peristaltic pumps comprise a housing and a compressible tubing arranged within the housing. Basically, there are two different embodiments of the peristaltic pump, wherein in the one embodiment the tubing is arranged along a straight track, whereas in the other embodiment the tubing is formed as a loop resulting in a more economical design with a smaller physical size and less producing costs. The former embodiment which is called a linear peristaltic pump is mainly used nowadays, while the present invention deals with the latter embodiment. The tubing is to be filled with a fluid to be delivered from its inlet to its outlet. The fluid is caused to move through the tubing by engagement elements, typically in the form of rollers driven by rotary means such an electric motor or a mechanically driven shaft. The engagement elements cause an occlusion of the tubing by squeezing it against a wall or track within the housing so that the fluid is forced through the tubing due to the movement of the engagement elements. The use of a peristaltic pump is advantageous in so far as the fluid does not come into contact with the operating environment, which renders the peristaltic pump suitable for medical applications like infusion of drugs where it is important to avoid contact of the fluid with the environment. Further, the mechanical components of a peristaltic pump do not come into contact with the fluid. So, the components of a peristaltic pump remain free from contamination by the fluid. As a result, a peristaltic pump is easy to clean and to sterilize because the tubing can be simply discarded after use, and a new tubing can be provided for the next use.
However, a disadvantage is that it is difficult with a peristaltic pump to achieve a constant or pulseless flow of the fluid through the tubing. Pulses are created when the engagement elements disengage from the tubing and, therefore, the occlusion is removed with the result of that a void is generated in the disengagement region of the tubing. Namely, in this region the tubing returns to its normal round shape resulting in an increase of the volume which are filled by fluid from the outlet of the tubing. This leads to a reduction of the flow rate of the fluid at the outlet of the tubing for the duration of the pulse.
In other words, the pulsatile behavior of a rotary peristaltic pump results from the alternation of a forerunner or leading engagement element by the next follower or trailing engagement element. V is the volume which is encapsulated between two neighboring engagement elements. In case of a rotary peristaltic pump, with φ defining an angular position so that the angular position of the trailing engagement element is φ1 and the angular position of the leading engagement element is φ2, the volume V encapsulated between both these engagement elements extends along an angular distance which is defined by the difference between both the aforementioned angular positions φ2 and φ1, i.e. Δφ=φ2−φ1. +ΔV/Δφ represents an increase of the volume defining a so-called frontwave which is displaced in front of each engagement element and is advanced by it. −ΔV/Δφ represents a decrease of the volume defining a so-called depression which arises behind each engagement element. The enclosed volume V between two neighboring engagement elements squeezing the tubing with unchanged distance between them is constant so that the fluid is just transported, if +ΔV/Δφ and −ΔV/Δφ are the same so thatV+ΔV/Δφ−ΔV/Δφ=V, resulting in that the pressure remains constant as well.
If otherwise an upstream portion of the tubing is larger than a downstream portion so that it isΔV2/Δφ>ΔV1/Δφ andV+ΔV2/Δφ−ΔV1/Δφ=V+Vdifference,wherein ΔV1/Δφ represents an increase of volume defining a front wave in front of the forerunner or leading engagement element and ΔV2/Δφ represents an increase of volume defining a front wave in front of the next follower or trailing engagement element, the pressure is increased by elastic tubing forces of the inflated portion due to increase of volume.
At the moment the leading engagement element stops squeezing the tubing and a front/back communication through a thin film of fluid is established under it, the frontwave +ΔV/Δφ suddenly disappears (so that it cannot push fluid anymore) and is replaced by the frontwave in front of the trailing engagement element which now takes over relay of infusion. Also −ΔV/Δφ disappears behind it. But due to the disengagement of the leading engagement element a new additional difference volume ΔVd/Δφ is built up and continues to be present until the disengagement of the leading engagement element is fully completed. A void creating the aforementioned new additional volume difference ΔVd arises, as the resilient tubing becomes asymptotic or starts with a larger diameter disengagement curvature at this point, resulting in a creation of a negative pulse ΔV/Δφ−ΔVd/Δφ in the flow graph (V,φ) (where ΔVd depends on the geometry of the engagement element and the disengagement curvature Δr/Δφ of the housing and r is the radius of the disengagement curvature increasing so as to let the engagement element disengage).
The flow rate is defined as ΔV/Δt. By multiplying the nominator and the denominator each with Δφ, the above equation can be written asflow rate=(ΔV/Δφ)·(Δφ/Δt).
This makes evident that for a constant rotational movement Δφ/Δt, if ΔV is constant at the vicinity of an engaging element at any location along its movement path, the flow rate will be constant without any variation or pulse.
The lack of a constant fluid caused by pulses in the tubing renders a peristaltic pump unsuitable for certain precision applications. E.g., in applications where a small volume of fluid is required, such as where less than a complete revolution of the rotor is used, the effect of the pulses are particularly disadvantageous.
U.S. Pat. No. 5,533,878 A discloses a squeeze type pump wherein the resilient tubing has a larger diameter at the start of an infusion cycle than at its end.
U.S. Pat. No. 7,654,127 B2 discloses a peristaltic pump with increased dimensions in an upstream portion of the tubing so as to increase pressure inside the tubing before disengagement of a roller squeezing the tubing. However, the pressure is suddenly released when the leading roller is going to be disengaged from the tubing. Therefore, a perturbation of flow happens first with a sudden increase of flow and second with a decrease of it at same quantity due to disengagement of the roller from the tubing after some rotational degrees.
U.S. Pat. No. 3,826,593 A proposes the provision of a cam compressing the tubing at another point at the same time the engagement roller is disengaged from the tubing in a peristaltic pump. However, this solution requires higher costs and a larger number of assembly parts and is therefore not suitable for a disposable pumping mechanism.
U.S. Pat. No. 5,470,211 A teaches a roller pump with the provision of a controlled curvature at the inlet and the outlet of the tubing.
It is an object of the present invention to provide a continuously operating pulseless and accurate peristaltic pump having a small size so that it can replace many bulky syringe pumps in limited space.