Device for monitoring pressure in a fluid flow system

Abstract: An arrangement for monitoring the pressure of fluid flowing through pliant tubing forming part of a system for the intravenous supply of fluids to a patient under medical treatment in which the tubing is caused to assume an elongate cross-section and a linear voltage displacement transducer 8 having a spring loaded plunger 9 is arranged to apply pressure in a flattened region of the tubing wall to provide indications of contact area movement and thus variations in fluid pressure. Compensation for movement of the contact area after pressure is applied due other than to variations in pressure is provided by a sampler 10 which regularly samples the output of transducer 8 and provides inputs to a comparator 13 direct and via a shift register 12 so that one sample is compared with an immediately preceding sample. If the difference exceeds the threshold of a threshold circuit 14, (indicative of fluid pressure change) alarm 15 is sounded.

Description: The invention relates to a device for monitoring pressure of fluid flowing through pliant tubing, especially in apparatus for intravenous supply of fluids to a patient under medical treatment. Particularly in such applications as last-mentioned, the fluid flow system will often be of the kind comprising pliant tubing, such as p.v.c. tubing, through which in operation fluid is passed. One object of the present invention is to provide improved devices for monitoring pressure in a fluid flow system of the above kind. DE-C-3838689 discloses pressure monitoring device in which a pliable tube is squeezed between a sensing member and a fixed bed, so as to cause flattening of a length of the tube. The sensing member is flat and is responsive to movement of the tube wall caused by variations in fluid pressure in the tube. The sensing member produces a signal indicative of such variations in fluid pressure. The precharacterising part of Claim 1 is based on this disclosure and provides a device for monitoring the pressure of a fluid flowing through pliant tubing, the device comprising first means for causing tubing to assume an elongate cross-section over a length thereof to define a relatively flattened region of a wall of the tubing within said length. The distinguishing features of the present invention are set out in the characterising part of Claim 1 and require second means comprising a pressure applying member for applying pressure to a part of the relatively flattened region, the pressure applying member (8) being responsive to movement of said part, and compensating means for compensating for movement of said part occurring after said pressure is applied and being due to causes other than variations in the pressure of the fluid, the compensating means comprising means for comparing an indication of the position of said part at a given time with an indication at a previous time and means responsive to the existence therebetween of a difference above a threshold level, for signalling that a change in the pressure of said fluid has occurred. The invention also relates to a detector for use in the device of claim 1 and a method of operating such a device. In liquid flow systems used in the intravenous supply of liquids to a patient under medical treatment, it is, as previously mentioned, common practice to use p.v.c. tubing in the system. Such tubing is usually treated as "disposable" being changed frequently. For this purpose, p.v.c. tubing is normally regarded as approaching the ideal, being both hygienic and cheap. However, if pressure is applied to an area of its wall and steadily maintained, the wall in the region of said area will firstly yield markedly as the pressure is first applied and then continue to yield decreasingly until it reaches a substantially stable state. With a monitoring device as described above such continued yielding causes continued movement of said area to which said pressure is applied and thus false indications of a variation in the pressure of said fluid. Such continued movement will be slight but it will be appreciated that even slight movements of said wall area due other than to pressure changes in the fluid flowing in the system will produce errors in monitoring which though they may be correspondingly slight may be unacceptable in applications, such as the aforementioned medical application, in which a high level of sensitivity to changes in pressure is required with fluids which in practice will be flowing relatively slowly. The objective of monitoring movement of an area within a relatively flattened region of the wall of the tubing created by causing said tubing to assume an elongate cross-section over a length thereof is to increase the tendency for movement of said area to occur with changes of pressure of said fluid by destroying the hoop strength of the tubing in that region. Preferably the indications of the position of said part of the flattened region are produced by sampling at regular intervals from the time that pressure is first applied to said area and continuing beyond the point in time at which the aforementioned substantially stable state is reached. Normally an indication produced by sampling is compared with an immediately preceding indication produced by sampling. In a practical example using standard p.v.c. tubing of outside diameter 4mm and inside diameter of 3mm, the rate of sampling is approximately once per second. Preferably said first means for causing said tubing to assume an elongate cross-section over a length thereof comprises a bed member having a non-resilient substantially flat surface across which said tubing may be positioned and a body member for flattening said tubing non-occlusively over a length thereof against said surface of said bed member, and the second means comprises a pressure-applying member for applying pressure to a point in said relatively flattened region. Preferably said pressure applying member comprises a linear voltage displacement transducer having a spring-loaded plunger and producing, in operation, an output voltage signal which is indicative of the extent to which said plunger extends beyond the body of said transducer, said plunger being arranged to be pressed into the relatively flattened region of the wall of said tubing at said point. Preferably said transducer is housed in the body member which is arranged to be moved towards and away from said bed member whereby said tubing may be engaged or released by said plunger. Preferably said body member includes a tubular projection extending beyond a cylindrical passage within said body member, said passage opening in a surface of said body member facing said bed member, and said tubular projection serving to flatten said tubing as aforesaid as said body member is moved towards said bed member. Preferably said body member and said bed member are hinged together and means are provided for securing the one to the other when the body member is closed up to said bed member in operation. In one arrangement in accordance with the present invention the output of said linear voltage displacement transducer arranged to be sampled by a sampling circuit the output of which is connected via an analogue-to-digital converter to the input of a unidirectional shift register and in parallel to one input of a comparison circuit a second input for which is derived from the output of said shift register whereby the digitalised output of said sampling circuit is compared with the immediately preceding output of said sampling circuit outputted from said shift register, the output of said comparison circuit being connected via a threshold circuit to operate a suitable alarm. In other devices in accordance with the present invention, constituents of the device as described above connected to take output from said linear voltage displacement transducer and operate said alarm are replaced by a microprocessor programmed to perform the component functions. The invention is illustrated in and further described with reference to the accompanying drawings in which:- Figure 1 shows a view in elevation of a liquid flow detector part of a monitoring device in accordance with the present invention; Figure 2 shows a section along the line x-x in Figure 1; Figure 3 illustrates in schematic form a liquid flow monitoring device in accordance with the present invention and Figure 4 is a graph illustrating the operation of the device of Fig 3. In all of the Figures like references are used for like parts. Referring to Figures 1 and 2, the liquid flow detector part of the device comprises a hard bed member 1 across the surface of which may be positioned a length of pliant tubing 2, in this case standard pvc tubing of outside diameter 4mm and inside diameter 3mm as used in the intravenous supply of liquid to a patient. Hinged to the bed member 1 by hinge 3 is a main body member 4 which has a circular cylindrical passage 5 extending orthogonally away from the surface of the bed member 1 across which said tubing 2 is positioned. The passage 5 exits from main body member 4 towards tubing 2 through a tubular projection 6 which, when the main body member 4 is closed up towards the bed member 1, flattens the tubing 2 against the surface of the bed member 1 over a short length 7 thereof. The extent to which tubular projection 6 projects towards bed member 1 is such that when the main body member 4 is fully closed up to the bed member 1 (and held by means of a suitable catch, not represented) the tubing 2 is flattened against the bed member 1 without occlusion occurring and without unduly impeding the flow of liquid through the tubing 2. Inserted within the passage 5 is a linear voltage displacement transducer 8 of known form. As known per se transducer 8 has a spring-loaded plunger 9 and produces an output digital voltage signal which is indicative of the extent to which plunger 9 extends beyond the body of the transducer 8. The output voltage signal of transducer 8 will thus vary with the displacement of a surface against which plunger 9 is pressed in the direction of action of said plunger. Transducer 8 is entered into and fixed within the passage 5 to an extent such that plunger 9 is spring-biased against the flattened surface of the tubing 2 when the main body member 4 is closed up and secured to the bed member 1, as previously described. In operation, any variation in the pressure of fluid passing through the tubing 2 will tend to cause expansions and contractions. It is believed that flattening the tubing as above described magnifies the resultant displacement of the relatively flat portion of the wall of the tubing with which the plunger 9 is in contact, thus enabling the transducer 8 to respond to such variations with a higher degree of sensitivity than would otherwise be the case. It has been found that following closing up of the main body member 4 to the bed member 1 a variation in wall displacement will occur, even though the flow of liquid through the tubing 2 remains constant, over a settling down period, as illustrated in the graph of Figure 4 which represents displacement against time in a typical case. The curve is, it will be noted, of increasingly decreasing slope as time progresses until a point in time at which a relatively stable condition is reached. Typically this is after a period of thirty minutes with an arrangement as described above. For medical applications such as the intravenous supply of fluid to a patient as previously mentioned it is required to monitor the flow of fluid through the tubing with a relatively high degree of sensitivity from the moment that the supply is set up. In order to meet this requirement the detector part of Figures 1 and 2 is connected in a device as illustrated in Figure 3. Referring to Figure 3 only transducer 8 and plunger 9 of the detector part of Figures 1 and 2 are represented. The analogue output voltage of transducer 8 is arranged to be sampled at the rate of once per second by a sampling circuit 10. The output of sampling circuit 10 is connected to the input of an analogue-to-digital converter 11. The output of analogue-to-digital converter 11 is connected to a unidirectional "count down" only shift register 12, where it is stored, and in parallel to one input of a comparator circuit 13. A second input for comparator circuit 13 is derived from the output of shift register 12 so that comparator circuit 13 compares the digital signal output of analogue-to-digital converter 1 derived from one sample taken by sampling circuit 10 with that derived from the sample taken one second before. The output of comparator circuit 13 is connected via a positive threshold circuit to a suitable visual and/or aural alarm 15. An understanding of the operation of the arrangement illustrated in Figure 3 is assisted by consideration of Figure 4 which shows a graph of the displacement of the point in the flattened portion 7 of the wall of the tubing 2 into which plunger 9 of transducer 8 is pressed, against time. As plunger 9 is pressed into the flattened portion 7 of the wall of the tubing 2 (with the flattening itself by tubular projection 6) the wall will first yield markedly over a very short period of time and then, with main body 4 of the monitoring device shown in Figures 1 and 2 fully closed up to bed member 1, will continue to yield further, but decreasingly so until a substantially stable state is reached at "S". From zero time to time "S" will typically be thirty minutes. Thus, save as the main body 4 is first closed up to bed member 1, the movement of plunger 9, second by second, will not be great unless there is a change of pressure in the fluid itself. Thus a comparison of the digital signal output of analogue-to-digital converter 11 at one sampling with that at the immediately preceding sample one second earlier will show a difference which is above a chosen threshold level (to which threshold circuit 14 is set) only if the pressure in the fluid flowing in the tubing 2 has changed. Any output from threshold circuit 14 above the set threshold will operate the alarm 15 and may be arranged to trigger any other event as may be desired. It will be appreciated that the sensitivity of the monitoring arrangement is dependent to a great extent on the number of samples taken by sampling circuit 10. Sensitivity may be increased by increasing the rate of sampling but in practice, with standard p.v.c. tubing as described, once per second has been found to give satisfactory results. The circuitry of Figure 3 is a discrete component embodiment. In other embodiments of the invention (not illustrated) the functions of components 10 to 14 are provided by a suitably programed microprocessor.

Claims: A device for monitoring the pressure of a fluid flowing through pliant tubing (2), the device comprising first means (1,4,6) for causing tubing to assume an elongate cross-section over a length thereof to define a relatively flattened region of a wall of the tubing within said length characterised by second means (8,9) comprising a pressure applying member (8) for applying pressure to a part of said relatively flattened region, the pressure applying member (8) being responsive to movement of said part and providing an indication of the position of said part, and compensating means (10-15) for compensating for movement of said part occurring after said pressure is applied by the pressure applying member (8) and being due to causes other than variations in pressure of the fluid, the compensating means comprising means (13) for comparing an indication of the position of said part at a given time with an indication at a previous time and means (14) responsive to the existence therebetween of a difference above a threshold level, for signalling that a change in the pressure of said fluid has occurred. A device as claimed in Claim 1 and wherein said first means (1,6) for causing said tubing to assume an elongate cross-section over a length thereof comprises a bed member (1) having a non-resilient substantially flat surface across which said tubing (2) may be positioned and a body member (4,6) for flattening said tubing non-occlusively over a length thereof against said surface of said bed member and the second means (8,9) comprises a pressure applying member (8) carried by the body member (4,6) for applying pressure to a point in said relatively flattened region. A device as claimed in Claim 2 and wherein said pressure-applying member (8) comprises a linear voltage displacement transducer having a spring-loaded plunger (9) and producing, in operation, an output voltage signal which is indicative of the extent to which said plunger extends beyond the body of said transducer, said plunger (9) being arranged to be pressed into the relatively flattened region of the wall of said tubing at said point. A device as claimed in Claim 3 and wherein said transducer (8) is housed in the body member (4,6) which is arranged to be movable towards and away from said bed member (1) whereby said tubing (2) may be engaged or released by said plunger (9). A device as claimed in Claim 4 and wherein said body member (4,6) includes a tubular projection extending beyond a cylindrical passage (5) within said body member, said passage opening in a surface of said body member facing said bed member (1) and said tubular projection (6) serving to flatten said tubing as aforesaid as said body member (4,6) is moved towards said bed member (1). A device as claimed in Claim 4 or 5 and wherein said body member (4,6) and said bed member (1) are hinged together and means are provided for securing the one to the other when the body member is closed up to said bed member in operation. A device as claimed in any of Claims 3 to 6 and wherein the output of said linear voltage displacement transducer (8) is arranged to be sampled by a sampling circuit (10), the output of which is connected via an analogue-to-digital converter (11) to the input of a unidirectional shift register (12) and in parallel to one input of a comparison circuit (13), a second input of which is derived from the output of said shift register (12), whereby the digitalised output of said sampling circuit is compared with the immediately preceding output of said sampling circuit outputted from said shift register, the output of said comparison circuit (13) being connected via a threshold circuit (14) to operate a suitable alarm (15). A device as claimed in Claim 7 and wherein the constituents connected to take output from said linear voltage displacement transducer (8) and operate said alarm (15) are replaced by a microprocessor programmed to perform the component functions. Intravenous fluid supply apparatus incorporating a device according to any one of the preceding claims and a pliant, fluid supply tube (2) engageable in the device. Apparatus as claimed in Claim 9 wherein said pliant tubing is p.v.c. tubing. A method of operating a device as claimed in Claim 1 wherein the indications are produced by sampling at regular intervals from the time that pressure is first applied to said part and continuing beyond the point in time at which the stable state of the tubing is reached. A method as claimed in Claim 11 wherein an indication produced by sampling is compared with an immediately preceding indication produced by sampling. A method as claimed in Claim 11 or 12 wherein the rate of sampling is approximately once per second. A detector for use in a device according to Claim 1, the detector comprising first means (1,6) for causing tubing (2) to assume an elongate cross-section over a length thereof to define a relatively flattened region of a wall of the tubing within said length, characterised by second means (8,9) for applying pressure to a part of the relatively flattened region, the second means being responsive to movement of said part to provide a signal, which is indicative of variations in the pressure of fluid in the tubing. A detector according to Claim 14, wherein said first means for causing said tubing to assume an elongate cross-section over a length thereof comprises a bed member having a non-resilient substantially flat surface across which said tubing may be positioned and a body member for flattening said tubing non-occlusively over a length thereof against said surface of said bed member and the second means comprises a pressure applying member carried by the body member for applying pressure to a point in said relatively flattened region. A detector according to Claim 15, wherein said pressure-applying member comprises a linear voltage displacement transducer having a spring-loaded plunger and producing, in operation, an output voltage signal which is indicative of the extent to which said plunger extends beyond the body of said transducer, said plunger being arranged to be pressed into the relatively flattened region of the wall of said tubing at said point. A detector according to Claim 16, wherein said transducer is housed in the body member which is arranged to be movable towards and away from said bed member whereby said tubing may be engaged or released by said plunger. A detector according to Claim 17, wherein said body member includes a tubular projection extending beyond a cylindrical passage within said body member, said passage opening in a surface of said body member facing said bed member and said tubular projection serving to flatten said tubing as aforesaid as said body member is moved towards said bed member. A detector according to Claim 18, wherein said body member and said bed member are hinged together and means are provided for securing the one to the other when the body member is closed up to said bed member in operation.

Inventors: DANBY HAL, DANBY, HAL
Applicants: BAXTER INT, BAXTER INTERNATIONAL INC.
Application year: 1991
Main classification: G01L9
IPC codes: G01L9, A61M5