Diffusion barrier in a delivery apparatus for pressurised medical liquids

Field of the invention

The present invention relates in general to delivery apparatus for pressurised medical liquids and in particular to an apparatus for pressurising a medical liquid to a delivery pressure.

Background

In delivery apparatuses for pressurised medical liquids it is known to maintain a medical liquid in a reservoir at a delivery pressure by means of pressurised gas. The gas is inlet to the reservoir from a pressurised gas source In the reservoir the gas exerts its pressure on the surface of the medical liquid and the pressurised liquid is delivered through a controllable outlet. Such an apparatus is used for example in a vaporiser for liquid anaesthetic wherein a delivery apparatus delivers the liquid via an injector coupled to the outlet and into a flow of breathing gas In this kind of use it is important that the delivery apparatus is capable of delivering a certain volume of liquid with a high degree of repeatability.

The basic idea for a delivery apparatus of this kind is that the delivered volume of liquid shall be dependent only on the differential pressure over the controllable outlet and on the time during which the outlet is open. In practice, however, a part of the pressurising gas dissolves in the liquid, which in its turn affects the delivered volume due to the fact that bubbles of gas appear in the injector and in its inlet ducts as well as in or in the vicinity of possible filters.

Simplified experiments with this type of vaporiser have shown that the delivered volume of liquid deviates with an increase by about 10 % when pressurising gas has been dissolved in the liquid until saturation More specifically, the medical liquid used in the experiments was anaesthetic agent Isofiuoran and the pressurising gas was air. The reservoir was maintained at an overpressure of 1 bar and the pulse time of the injector was set to 2.5 ms (milliseconds). In the experiment the time to saturation of the medical liquid was about 72 hours It should be noted that these figures are approximate and serve to give a general understanding of the diffusion process in this context,

In the clinical practice of vaporisers these are often used in lengthy surgical operations during which pressurising gas thus dissolve in the medical liquid and thereby affects the repeatability and accuracy of the delivered dose of the medical liquid into the breathing gas- In practice an

increase of 10% of the anaesthetic liquid delivered to the patient would not have a severe effect on the majority of patients. However, it is always an aim for the anaesthesiologist to maintain anaesthesia with as low dose as possible and to have an accurate control of the process, which is more difficult if the delivered dose increases over time without changing the settings of the apparatus.

Another problem is that bubbles that are created through a pressure drop in the injector can get jammed for some types of injectors.

There is therefore a need for improvement of the repeatability and accuracy of the delivered dose in such a delivery apparatus.

Prior Art

EP 1082973 discloses an anaesthetic vaporiser for dosing a liquid anaesthetic by means of a liquid pump. This piece of prior art seeks to solve the above mentioned problem of dosing accuracy by pressurising the anaesthetic agent with a liquid pump and a system of regulating means. In addition the apparatus is provided with a return tube in order to make it easier to control pressure downstream from the liquid pump. However, this solution is complicated and expensive due to high demands on the materials used and the limited life span of the used components. Moreover, this prior art does not overcome the problem of diffusion of gas into the anaesthetic liquid since the liquid reservoir is connected to an air tube for admitting inlet of air at atmospheric pressure in order to compensate for a negative pressure drop arising when liquid is pumped out of the reservoir

EP 1 300 172 discloses a delivery apparatus for pressurised liquid anaesthetics- In this piece of prior art a primary reservoir is devised with primary pressurising means in order to pressurise the liquid to a delivery pressure basically as described in the background section above. The primary reservoir in its turn is connected to a secondary reservoir so as to deliver pressurised liquid to the secondary reservoir. The secondary reservoir is provided with a secondary pressurising means devised to supply compensating pressure to the liquid in the secondary reservoir in order to maintain it at the delivery pressure when pressure is reduced in the primary reservoir. The purpose is to be able to temporarily allow interruption of the pressurising activity in the primary reservoir without interruption of delivery of the

pressurised liquid from the secondary reservoir Thereby the primary reservoir can be refilled at atmospheric pressure during operation of the delivery apparatus.

In a first embodiment the secondary pressurising means involves the arrangement of a movable section in the shape of a membrane that delimits an inner variable volume liquid containing space and that is used to put pressure on the liquid surface by means of a mechanical or pressurised gas biasing force. In fact the secondary reservoir is designed with a function very similar to that of a piston pump, where the movable section corresponds to the piston and there being provided an inlet valve from the primary reservoir and an outlet valve to the injector. Although not explicitly explained in the disclosure of EP 1 300 172, the movable section must be tightly sealed against the inner walls of the secondary reservoir in order to operate properly and maintain the delivery pressure as intended. For the further purpose of achieving a safety seal that prevents evacuation of content in the secondary reservoir, the movable section is provided with a sealing head that is pressed against an outlet when the movable section is moved to reduce the volume of the space to a minimum,

In a second embodiment the secondary pressurising means is based on a pressurised gas biasing force similar to that of the first pressurising means for pressurising the primary reservoir, namely by introducing pressurised gas through a gas port into the space above liquid in the reservoirs. As stated in the description, this embodiment may optionally also be provided with a movable section, in this instance in the shape of a float provided with a sealing surface, for the purpose of sealing the gas port against escape of liquid in case the space of the reservoir is filled with liquid to a maximum level This may for example occur if the differential pressure between the primary and the secondary reservoirs is accidentally unbalanced with a higher pressure in the primary reservoir than in the secondary reservoir.

Although the float in the secondary reservoir would prevent its liquid content from becoming saturated with pressurising gas, there is nothing to prevent saturation in the main, primary reservoir. This piece of prior art does not recognise the problem nor provide a solution with regard to repeatability and accuracy of the delivered dose due to diffusion of pressurising gas in the medical liquid and there is no teaching in EP 1 300 172 that would give guidance to solve that problem.

The article "The Oxford vaporiser No 2 " by SL Cowan et al, The Lancet, vol 238, issue 6151, 19 July 1941, pages 64-66 shows a vaporiser for a medical liquid with a small tank

provided with a float for the purpose of closing an orifice that leads to a pump in case the level of liquid in the tank falls to a certain level The purpose is to prevent entry of air into the pump. There is no mentioning of the problem with diffusion of gas into the medical liquid.

The article "A new respirator" by Clous Bang, The Lancet, vol. 261, issue 6763, 11 April 1953, pages 723-726 shows a respirator apparatus with an automatic valve for controlling the inlet of pressurised breathing gas into the lungs of a patient. The valve is controlled dependent on the pressure of the lungs by means of an arrangement of two connected and liquid filled tubes each being provided with a float. When the pressure in the lungs reaches lower and higher end values, respectively, one of the floats rises to a certain level and connects two electrodes to close an electric circuit that actuates the automatic valve to open or close. The use of floats in this piece of prior art is totally different from that of the present invention,

WO 89/03483 discloses a hydro-pneumatic accumulator of the float type, preferably for use to store energy in connection with a hydraulic system Hydraulic or hydro-pneumatic accumulators are used in hydraulic systems to receive and deliver large quantities of working fluid during a short time. The accumulator consists of a vertical container, and a free swimming float which serves to separate the accumulator's content and gas and fluid, and to work together with a seal ring in the end cover as a valve which prevents the gas from escaping from the accumulator if it is completely emptied for fluid. This prior art is mainly directed to the problem of providing a float that has a low specific weight and at the same time is strong enough to withstand the high pressures that appear in hydraulic systems. Although this piece of prior art deals with problems that are partly similar to those of the present invention, it is not relevant foi the skilled person working with anaesthesia delivery apparatuses The skilled person would not seek solutions from the technical area of high pressure hydraulic systems and therefore this piece of prior art is not relevant as prior art for the present invention.

WO 99/13228 discloses another hydro-pneumatic accumulator for storing pressure in a large volume container for hydraulic oil. The accumulator is provided with a device that separates the fluid area from the gas area. The separating device contains submerging floating bodies arranged on the upper surface of the fluid volume and are situated in the fluid level itself In addition, the separating device contains at least one protecting body which interacts with the floating bodies and is configured as a shield for further reduction of the contact surface

between gas and fluid. This piece of prior art is directed to the problems met in connection with large volume containers and presents a floating device with a separation element mounted on floating bodies. An important issue foi this prior art is that the separating device should be possible to introduce through an inspection opening, usually a manhole. As the previously mentioned prior art, this piece of prior art is directed to high pressure and large scale hydraulic system, and for the same reasons it is not relevant as prior art for the present invention.

Thus, none of the prior art addresses the problem of repeatability and accuracy of delivered dose in a medical delivery apparatus with regard to diffusion of pressurising gas into a medical liquid

Object of the Invention

The problem to be solved by the present invention is therefore to improve the repeatability and accuracy of the delivered dose in a delivery apparatus for delivering a medical liquid. Aspects of the problem concern the repeatability and accuracy of the delivered dose in a delivery apparatus with medical liquid subjected to gas at atmospheric pressure and gas pressurised medical liquids, respectively.

Summary of the Invention

According to the present invention the problem is solved by prolonging the diffusion time for the gas, i.e. the time it takes the gas to dissolve in the medical liquid until saturation is reached. The problem is perhaps most accentuated in delivery apparatuses operating with pressurising gas and the invention will therefore mainly be described and discussed in connection with such apparatuses using pressurising gas, although it is also applicable for gas at atmospheric pressure _^

The invention is further based on the fact that time to saturation of a liquid with dissolved pressurising gas is largely dependent on the size of the area of the liquid surface that is in direct contact with the pressurising gas. In accordance with the invention a prolonged diffusion time is achieved by minimising the medical liquid surface area that is in exposed to the pressurising gas by means of a floating diffusion barrier that is configured to cover substantially the whole surface of the medical liquid in the reservoir. The barrier prevents diffusion of pressurising gas into the medical liquid.

According to an aspect of the invention, the floating diffusion barrier is configured to enable refilling the reservoir from the top, i.e. from an inlet above the diffusion barrier. For this purpose the diffusion bairier is configured such that there is a suitably sized gap between the diffusion barrier and inner wall of the reservoir to allow medical liquid to pass the diffusion barrier.

According to another aspect of the invention, the floating diffusion barrier is configured to freely follow the surface of the medical liquid as it sinks when doses are delivered and rises when the reservoir is refilled. For this purpose and for the purpose of minimising the above mentioned gap between the diffusion bairier and the inner wall of the reservoir, the diffusion barrier is preferably shaped as a convex lens with a thin peripheral section in the part of the lens shaped diffusion barrier that is close to the inner wall of the reservoir. This has the effect to prevent the diffusion barrier from sticking to the inner wall of the reservoir in case that the diffusion barrier accidentally gets into an angled position with regard to the horizontal plane of the medical liquid surface.

The latter aspect may be further enhanced by configuring the floating diffusion barrier such that it has a centre of gravity located below the plane defined by the peripheral section and such that the barrier has a balanced floating characteristic.

Yet another aspect of the invention addresses the problem aspect of enabling transport of gas bubbles that occur in the medical liquid past the diffusion barrier.

Furthermore, the diffusion barrier enables refilling of medical liquid in a more controlled way. The barrier will prevent the medical liquid from being stirred up and also prevent bubbles from entering the medical liquid already present in the reservoir.

The invention is advantageously applied in an apparatus for pressurising a medical liquid to a delivery pressure or directly in a delivery apparatus for such pressurised medical liquids. With a diffusion barrier applied in accordance with the invention the contact surface between the pressurising gas and the medical liquid is considerably diminished, and thereby the time to saturation is significantly prolonged. Experiments similar to those described above have shown that the time to saturation and in this experiment thereby the time to 10 % deviation

fiom the intended dose of the delivered medical liquid with diffusion barrier of the invention is about 480 hours. This a significant difference compared with the prior art type of pressurising apparatus where the corresponding time to saturation and the time to 10 % deviation from the intended dose of the delivered medical liquid is about 72 hours.

Brief Description of Drawings

The invention is further explained with reference to the accompanying drawings, in which: Fig 1 schematically shows a medical liquid reservoir provided with a surface covering diffusion barrier in accordance with an embodiment of the invention; Fig 2A and 2B show a diffusion barrier in accordance with an embodiment of the invention, and Fig 3 shows another variety of a diffusion barrier in accordance with the invention.

Detailed Description of Embodiments Fig 1 shows the invention applied in an apparatus for pressurising a medical liquid with a reservoir 1 for a medical liquid 2. The reservoir has preferably and exemplified herein the shape of a cylinder. The reservoir 1 is in its upper part provided with an inlet 3 for pressurised gas that is controllably inlet through a gas inlet valve 10 from a source 12 of pressurised gas The inlet 3 is also used for controllable evacuation of gas from the reservoir through a gas outlet valve 14 to a gas evacuation system 16. The invention is typically applied in an anaesthesia system, in which case the gas pressure provided to the reservoir 1 is preferably the driving pressure of the anaesthesia system. This driving gas pressure usually has a suitable somewhat higher level than the delivery pressure chosen to fit the individual anaesthesia system.

The reservoir 1 is in its lower part provided with an outlet 6 for egress of pressurised medical liquid via a controllable valve 8 that can have a number of functions The controllable valve 8 may be a safety valve connected to a secondary reservoir for an anaesthetic agent, a safety valve connected to an injector for delivering doses of pressurised medical liquid into a flow of breathing gas or be the injector valve itself. The reservoir 1 is furthei in its upper part provided with a refill inlet 18 provided for refilling the reservoir with medical liquid. The refill inlet is only schematically drawn but would typically be provided with a re-sealable port that can be opened to refill medical liquid preferably under about atmospheric pressure and sealed to withstand the pressurising pressure from the gas during operation.

The diffusion bairier provided reservoir is preferably intended to be used as a main reservoir in an anaesthesia system and could be the primary reservoir, such as in the prior art document EP 1 300 172, having a secondary reservoir close to the dispensing valve, but can of course be used in all systems having pressurised medical liquids without diverting from the inventive concept. The main or primary reservoir is generally understood as being the reservoir that stores the main portion of the medical liquid in an anaesthesia system, and may or may not be connected for example by tubing to one or a plurality of secondary reservoirs.

In accordance with the invention a diffusion barrier 20 is provided in the leservoir 1. The diffusion barrier 20 has a horizontal cross-section similar to the horizontal cτoss-section of the reservoir and is configured to cover substantially the whole surface of the medical liquid in the reservoir. The diffusion barrier 20 floats on the medical liquid surface and separates the medical liquid 2 from the pressurising gas that gathers in the space 4 above the diffusion bairier 20. During operation the gas exerts pressure on the diffusion barrier 20 that in its turn conveys the pressure to the medical liquid and simultaneously operates as a barrier against diffusion of gas into the medical liquid.

In order to enable refilling of the reservoir with medical liquid entered through the refill inlet at the top of the reservoir there is a suitably sized gap provided between the diffusion barrier 20 and the inner wall of the reservoir. This gap should be minimal but allow medical liquid to pass the surface barrier and gather in the lower part of the leservoir below the diffusion barrier. Furthermore, bubbles may occur when the reservoir is refilled due to the fact that the liquid is mixed with air during the filling process, and also from the pressure drop if the reservoir contains liquid that has been pressurised for a long period of time

A suitable gap is therefore achieved when there is a circumferential gap that is wide enough to allow medical liquid as well as gas bubbles to pass With a circular diffusion barrier for a cylindrical reservoir a difference between the inner diameter of the reservoir and the outer diameter of the diffusion bairier in the range of 0.1 - 1 millimeters may for example be suitable in order to enable a minimal gap while also enabling the diffusion barrier to freely follow the surface of the medical liquid as it sinks when doses are delivered and rises when the reservoir is refilled.

Fig 2A and 2B shows an embodiment of a diffusion barrier that for this purpose is shaped as a convex lens. Fig 2A thus shows a top view of a circular diffusion barrier 20 with a circumferential peripheral section 22. Fig 2B shows a side view of the diffusion barrier 20 with an upper part 24, a lower part 26 and a circumferential peripheral section 22 altogether giving the diffusion barrier the shape of a convex lens.

It is preferable to make the peripheral section of the lens shaped diffusion barrier that is close to the inner wall of the reservoir thin with a hard edge. This shape prevents the diffusion barrier from sticking to the inner wall of the reservoir in case that the diffusion barrier accidentally gets into an angled position with regard to the horizontal plane of the medical liquid surface. However, the peripheral section may also be thick with a maximum radius that is smaller than the innei radius of the cylindrical reservoir. With a thicker peripheral section it is important to adapt the dimension of the peripheral section in relation to a selected dimension of the gap in order to avoid capillary effects that may trap gas bubbles in the passage between the diffusion barrier and inner wall of the reservoir.

This effect is further enhanced in an embodiment as shown in Fig 2B and wherein the diffusion barrier 20 is configured such that it has a centre of gravity 28 located below the plane defined by the peripheral section and such that the diffusion barrier has a balanced floating characteristic In the embodiment shown in Fig 2B the lower part 26 of the convex diffusion barrier is larger than the upper part 24. By this shape a larger proportion of the diffusion barrier material is concentrated to the lower part and the centre of gravity 28 is positioned below the peripheral section plane. It is also conceivable to place a weight in the surface barrier in order to position the centre of gravity, for example in the lowest point of the convex lower part 26 as exemplified in Fig 2B. This measure counteracts any tendencies for getting stuck in an angling position since the weight in the centre of gravity forms a lever that pulls the diffusion barrier back into a straight position.

Fig 3 shows another variety of the diffusion barrier 20 with a thicker edge placed in a reservoir with schematically shown walls 30. The other reference numbers and features coπespond to those of Fig 2A-2B described above The vertical cross section of the edge preferably has the shape of a circle segment with a radius that is smaller than the radius of the circular horizontal cross section of the reservoir. For example, the shape of the diffusion baπier may be that of a sphere with a cut off upper section, slightly smaller than the upper

half of the sphere. As in the previously described embodiment, the centre of gravity is placed in the lower part of the diffusion barrier.

The gap and the convex surface of the diffusion barrier serve the purpose of enabling transport of gas bubbles occurring in the medical liquid and preventing bubbles from getting trapped on the lower side of the barrier In one embodiment the diffusion barrier is treated to have a reduced surface tension for the purpose of enhancing the transport of bubbles from underneath the barrier. It is usually sufficient if the lower side of the barrier has a smooth surface and not too wide an angle towards the reservoir surface, but the selection of material and chemical treatment could be considered.

The figures show a circular cross-section of the reservoir and surface barrier, but an arbitrary cross-section could of course be used without diverting from the inventive concept of reducing the gas saturation. However, the risk of having the diffusion barrier sticking to the wall will have to be dealt with in a manner dependent on the selected cross-section.

The invention has been described above in terms of a general embodiment. However, the invention may be applied in for example the following more specific embodiments

In one embodiment the invention is applied in an anaesthetic vaporiser identical with or similar to that described in EP 1 082 973. For the purpose of description EP 1 082 973 is in its entirety incorporated by reference In this embodiment the delivery apparatus for pressurised medical liquids comprises a reservoir for medical liquid, where the reservoir is provided with an inlet foi admitting inlet of air at atmospheric pressure and an outlet for the medical liquid. The outlet is connected to a pressurising means in the shape of a pump configured for pressurising medical liquid from the reservoir to a driving pressure. From the pressurising pump the medical liquid is fed to a dosing point from which doses of medical liquid is delivered at a delivery pressure into a flow of breathing gas, In accordance with the invention, the reservoir is provided with a reservoir diffusion barrier as described above and arranged to float on medical liquid in the reservoir. The diffusion barrier is disposed to cover substantially the whole surface of the medical liquid in the reseivoir for the purpose of preventing diffusion of gas present in the reservoir into the medical liquid In this instance the gas from which the medical liquid is protected is air

In another embodiment the invention is applied in a delivery apparatus for pressurised medical liquids identical with or similar to that described in EP 1 300 172. For the purpose of description EP 1 300 172 is in its entirety incorporated by reference. In this embodiment the delivery apparatus for pressurised medical liquids comprises a primary and a secondary reservoir for medical liquid. The primary reservoir is provided with a primary pressurising means configured for pressurising medical liquid in the primary reservoir to a driving pressure or a delivery pressure. The secondary reservoir has an inlet connected to receive pressurised medical liquid from the primary reservoir and an outlet for egress of the received medical liquid. The secondary reservoir is further provided with secondary pressurising means arranged for supplying a compensating pressure to the received pressurised medical liquid in order to maintain the pressure at substantially the delivered pressure. In accordance with the invention, the primary reservoir is provided with a reservoir diffusion barrier arranged to float on medical liquid in the primary reservoir. The diffusion barrier is disposed to cover substantially the whole surface of the medical liquid in the primary reservoir for the purpose of preventing diffusion of gas present in the reservoir into the medical liquid. In a variety of this embodiment also the secondary reservoir could be provided with a diffusion barrier In this instance the pressure is achieved by means of pressurised gas from a pressurised gas source and the gas from which the medical liquid is protected is thus the pressurising gas,

In general, the invention is applicable in an apparatus for pressurising a medical liquid wherein the apparatus is a system with a single reservoir, with a first and a second reservoir or a plurality of reservoirs. The reservoirs may have different configurations and purposes, and may be pressurised by different kinds of pressurising means. As mentioned above, the diffusion barrier in accordance with the invention is primarily intended to be applied in the main reservoir of the system.

An apparatus for pressurising a medical liquid in accordance with the present invention can be continuously used for several hours without any significant impact on the accuracy of delivered dose due to diffusion.