Fluid management system

Abstract: There is disclosed an apparatus for supplying liquid to a body cavity (10) during an endoscopic procedure including a feedback loop-controlled liquid supply device inherently capable of supplying liquid at a substantially constant pressure substantially independent of the flow rate of liquid delivered by the liquid supply device within a relatively wide range of flow rate. Also disclosed are a disposable plastic pump cassette having an inflow pump in a housing, and an operative cannula. The surgical procedure is performed with continuous control over the body cavity pressure, regardless of the outflow flow rate.

Description: The present invention relates to apparatus for supplying liquid under pressure to a body cavity during an endoscopic procedure, and more particularly to an improved arthroscopic pump apparatus. Pumping systems have been previously proposed for supplying liquid under pressure to a body cavity by means of an inflow pump during an endoscopic procedure, such as arthroscopic surgery. Two prior art arthroscopic pumping systems are illustrative, namely that of DeSatnick et al. United States Patent 4,650,462, issued March 17, 1987 and Mathies et al. United States Patent 4,902,2777, issued February 20, 1990. DeSatnick et al propose the use of an open loop displacement pump as the outflow pump. Mathies et al propose the use of a feedback loop controlled inflow pump and an open loop suction pump that is automatically switched between an outflow cannula and a tool. European Patent No.0329599 discloses a system for supplying liquid to a body cavity using a pump constructed specifically as a peristaltic pump which is a displacement pump. International Patent Application No.WO86/01390 describes a system for supplying liquid to a body cavity using a magnet drive gear pump, which is also a displacement pump. Displacement pumps operate by providing volumes of fluid as a function of pump speed regardless of the outlet pressure of the pump. Thus, even at low pump speeds, displacement pumps are capable of generating pressures that can be too high, eg. where the outflow from the pressurised body cavity is curtailed by blockages or the like. The present invention provides a safer system by utilising a liquid supply means whose outflow rate is inherently inversely responsive to changes in the body cavity pressure. Preferably, the liquid supply means is inherently capable of supplying liquid at a substantially constant pressure substantially independent of the flow rate of liquid delivered by the liquid supply means within a relatively wide range of flow rate. The liquid supply means of the invention, when regulated by a feedback loop responsive to the pressure in the body cavity, can provide precise control of body cavity pressure regardless of changes in the volume of the body cavity or changes in the outflow flow rate from the body cavity, such as flow through a surgical tool, leakage or blockage or otherwise. According to the invention we provide apparatus for supplying liquid to a body cavity during an endoscopic procedure, which comprises: a) liquid supply means; b) means for generating a body cavity pressure signal representing the pressure in said body cavity; c) conduit means for communicating said liquid supply means (4) with said body cavity to deliver liquid under pressure to said body cavity; and d) control means responsive to said body cavity pressure signal for increasing or decreasing the pressure of said liquid delivered by said liquid supply means to thereby maintain said body cavity pressure at a predetermined value, characterised in that the liquid supply means is a dynamic pump means. According to a further feature of the invention we provide apparatus for supplying liquid to a body cavity during an endoscopic procedure as hereinbefore described, which comprises sensing means for generating a body cavity pressure signal representing the pressure in said body cavity. Preferably, the liquid supply means is a centrifugal pump. As used throughout the present application, the terms "dynamic pump" and "displacement pump" are as defined in the classification of pumps set forth in Pump Handbook, Edited by I J Karassik et al, Second Edition, McGraw-Hill Book Company, pages 1.2 - 1.5. A dynamic pump used in the present invention preferably provides a constant pressure head in a manner substantially insensitive to variations in flow rate. The apparatus of the present invention may include a suction pump for aspirating liquid from the body cavity. Alternatively, liquid may be aspirated from the body cavity from a suction source, such as wall suction. According to the present invention, outflow flow rate is independent of body cavity pressure and instead is chosen to optimize the performance of the surgical tool being used. The combination of a dynamic inflow (or irrigation) pump, such as a centrifugal inflow pump, controlled in response to body cavity pressure and an open loop aspiration of liquid from the body cavity provides a safe, stable, pressure-limited control system. It is a particular advantage of the present invention that it is preferably a two-portal system. That is, only two cannulas are inserted into the surgical site, an inflow cannula for the endoscope and an outflow cannula for the surgical tool. Liquid under pressure is normally supplied to the body cavity by the liquid supply means through the annulus between the inflow cannula and an endoscope in the cannula, while liquid is aspirated from the surgical site by suction via the outflow cannula or a surgical tool inserted into the outflow cannula. Alternatively, inflow liquid can be supplied via an endoscope having an internal conduit for supplying liquid to the body cavity. While not presently preferred, outflow liquid can simply drain from the outflow cannula. Moreover, body cavity pressure is calculated from the output of a pressure sensor in communication with the inflowing liquid at a location upstream of the body cavity, thus eliminating the need for a third portal for a pressure sensor inserted into the body cavity. In one embodiment of the invention, the pressure sensor is closely adjacent the body cavity, for example at the connection of an endoscope to the tubing supplying inflow liquid to the body cavity via the endoscope. In a presently preferred embodiment of the invention, the pressure sensor senses pressure at the outlet of the inflow pump located within a disposable cassette. The present invention will now be described by way of example in terms of a preferred embodiment with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the apparatus of the invention; Figure 2 is a block diagram of a control system of the present invention; Figure 3 is a curve showing the relationship between the pressure drop from the pressure sensing means to the inflow cannula and the inflow flow rate; Figure 4 is a family of curves showing the relationship between the inflow pump pressure and the inflow flow rate at various pump speeds; Figure 5 is a curve showing the relationship between the pressure drop from the inflow pump to the pressure sensing means and the inflow flow rate; Figure 6 is a view in section of an operative cannula; Figure 6a is an enlarged detail view of the seal in the operative cannula; Figure 7 is a view in section of an irrigation extender; Figure 8 is a view in section showing the operative cannula and irrigation extender coupled together to form an inflow cannula assembly, with an endoscope inserted in the inflow cannula assembly; Figure 9 is a detail view in section of a pressure sensing means; Figure 10 is a view in section of a blade of a surgical tool inserted in the operative cannula. Referring to Figure 1, the apparatus of the present invention includes a fluid source 1, usually two bags of sterile saline, communication via conduit 2 with the inlet of a centrifugal inflow pump 4 (Figure 2) housed within a cassette 200 in fluid management unit 5. Centrifugal inflow pump 4 supplies liquid under pressure through its outlet to conduit 7 and thence to the body cavity 10. As will be discussed in detail hereinafter, inflow cannula assembly 9 is inserted into body cavity 10 and endoscope 8 is placed within inflow cannula assembly 9 such that liquid under pressure is delivered by centrifugal inflow pump 4 to body cavity 10 via the annulus between the endoscope 8 and the inflow cannula assembly 9. Pressure transducer 11 is located upstream of the body cavity 10 and senses the liquid pressure in conduit 7 as will be described in detail hereinafter. In the embodiment of the invention shown in Figures 1 and 2, pressure transducer 11 is at the connection between conduit 7 and endoscope 8. The output from pressure transducer 11 is sent via line 12 to a pressure signal processor 13 (Fig. 2) within fluid management unit 5 for computing the pressure within body cavity 10. The desired pressure for the body cavity will be set by operating a selector 5a on fluid management unit 5 and display 5b will display the set pressure. Pressure controller 40 (Fig. 2) within fluid management unit 5 is responsive to the body cavity pressure signal generated by pressure signal processor 13 to adjust the speed of centrifugal inflow pump 4 to increase or decrease the outlet pressure of the liquid delivered by centrifugal inflow pump 4 and thereby maintain the body cavity pressure at the desired set value, as will be explained in detail hereinafter. An outflow cannula 14 (described in detail hereinafter) is inserted into the body cavity 10. Tool 15 is inserted via outflow conduit 14 into the body cavity 10, and the surgeon will manipulate tool 15 while viewing the procedure via endoscope 8, as is conventional. Tool control unit 21 powers tool 15 via power cord 22. Outflow conduit 16 connects the inlet of a suction pump 18 (Fig. 2) within unit 5 with the outlet of tool 15, whereby liquid within body cavity 10 may be aspirated and sent to waste container 19 via conduit 20. Fluid exiting body cavity 10 is filtered by tissue trap 16a located upstream of suction pump 18. Endoscope 8 and tool 15 are reused after sterilization. It is presently preferred that the pumps 4 and 18, the pressure transducer 11, and the conduits are disposable, as well as the blades for the tool 15. During the preoperative stage, the surgeon will select a desired pressure for the body cavity for the given surgical procedure by operating a set-pressure selector 5a, thereby causing set pressure generator 30 (Fig. 2) to generate a set pressure signal 31, which is applied to an input of pressure controller 40. To the other input of pressure controller 40 is applied signal 32a corresponding to the pressure in body cavity 10. Body cavity pressure signal 32a may be calculated by pressure signal processor 13 using a lumped parameter model as follows. By empirically determining the pressure vs. flow characteristics of each of the components in the path of fluid flow, (from fluid source 1 to distal end 92 (Fig. 1) of inflow assembly 9), one can determine body cavity pressure by calculating pressure drops across each component. One method takes into account the individual pressure vs. flow characteristics of centrifugal inflow pump 4, conduit 7 and inflow cannula assembly 9. While this will require more demanding software calculations, it will allow easier theoretical modeling of the effects of changing the pressure vs. flow characteristics of any component. Another method lumps the pressure vs. flow characteristics of the inflow pump 4 and conduit 7 up to the point of pressure measurement by transducer 11. This method simplifies the calculation of the body cavity pressure, but it is specific to the design of the inflow pump and conduit 7. Either method may be used in the present invention. For either method, the body cavity pressure, P c , is calculated by subtracting the pressure drop across the inflow cannula assembly 9 from the pressure, P s , sensed by transducer 11. The pressure drop across inflow cannula assembly 9, (P s -P c ), was empirically determined for various flows, Q in , by measuring the pressure at the outlet 92 of inflow cannula assembly 9 at a given flow, Q in , and subtracting it from the sensed pressure, P s , at that flow. A typical curve plotting (P s - P c ) versus flow is schematically shown in Fig. 3, and was accurately fit using the function: P s - P c = A 1 Q 2 in + A 2 Q in where A 1 and A 2 are constants, which can be rewritten as: P c = P s - A 1 Q 2 in - A 2 Q in The values for constants A 1 and A 2 are determined and stored in memory for use by pressure signal processor 13. A more generic method used to determine cavity pressure calculates the inflow, Q in , by solving two simultaneous non-linear equations as follows. The relationship between inflow, Q in , and pump pressure, P p , can be determined empirically by plotting P p vs. Q in for the inlet pump 4 for a range of pump speeds, (RPM). A family of curves was obtained for P p vs. Q in over a range of pump speeds (RPM), and is schematically shown in Fig. 4. For a given RPM, and using a centrifugal inflow pump connected to fluid source 1 via conduit 2 of 6.35mm (0.25 inch) ID and 2.29mm (90 inches) long and made of PVC, the equation was determined to be: P p = B o + B 1 Q 2 in + B 2 Q in which can be rewritten as: Q in = - B 2 + B 2 2 +4B 1 (B O -P p ) 2B 1 using the assumption that Q in must always be positive. In this equation, B o is the constant obtained at zero flow, which includes a component derived from the height of the fluid source (eg. the bag of saline 1), above the inflow pump 4. As the speed of motor 4a is varied, the values for B n change, but the general function does not. By repeating this pressure vs. flow experiment for many different motor speeds, functions are determined for B n as functions of motor speed. The speed of motor 4a (Fig. 2) can be determined by employing a feedback tachometer, but the actual motor speed is assumed to be a direct function of the pulse width modulation signal 61 (Fig. 2) used to drive motor 5a to be described in detail hereinafter. The functions for each B n can be expressed as a function of RPM by: B n = f(RPM) and RPM can be expressed as a function of the pulse width modulation signal, (PWM), by: RPM = C.PWM Pressure transducer 11 senses the pressure of liquid in conduit 7 at a point between centrifugal pump 4 and the inflow cannula 9. The relationship between the inflow, Q in , and the pressure drop from the centrifugal inflow pump 4 to the transducer 11 can also be determined empirically by measuring P p (pump outlet pressure) and P s (sensed pressure) over a range of flow rates and fitting the resulting (P p - P s ) vs. Q in data, such as schematically shown in Fig. 5, to an equation. Using a conduit 7 of 6.35mm (0.25 inch) ID and ten feet long made of PVC, the function was determined to be: P p - P s = D 1 Q 2 in + D 2 Q in which can be rewritten as: Q in = -D 2 + D 2 2 -4D 1 (P s -P p ) 2D 1 on the assumption that Q in must always be positive. Given the sensed pressure, P s , and the pulse width modulation signal, (PWM), cavity pressure, P c , is calculated in the following manner: a) Given PWM, calculate RPM from equation (6); b) Given RPM, calculate B n from equation (5); c) Given B n and P s , solve equations (4) and (8) for the two unknowns, Q in and P p ; d) Given P s , A n and Q in , calculate P c from equation (2). Pressure signal processor 13 may be provided with any suitable algorithm to solve equations (4) and (8) for unknowns P p and Q in . Suitable algorithms are commercially available. Equations (2), (5), and (6) are solved using appropriate software routines of a conventional nature. Pressure signal processor 13 then generates a signal corresponding to the body cavity pressure, P c , which is sent via lines 32 and 32a to the input of pressure controller 40 and via lines 32 and 32b to a pressure display 5b on the face of fluid management unit 5 (Fig. 1). Processor 13 can generate a signal representing the flow rate, Q in , of liquid into the body cavity, which can be sent via line 33 to a display 5b on unit 5 showing the inflow flow rate. If desired, inflow, Q in , can be integrated over time to estimate the total volume of liquid infused and the result displayed on unit 5 (not shown), so that the physician can monitor liquid usage and avoid unexpected depletion of the liquid supply. Since the pressure is maintained within narrow limits, Q in can be accurately measured. Pressure controller 40 compares the set pressure signal applied via line 31 and the calculated body cavity pressure signal applied via line 32a and generates a signal representing a pump speed necessary to raise or lower the calculated body cavity pressure to equal the set pressure. Pressure controller 40 thus sets the speed of motor 4a of centrifugal pump 4 and therefore the output from pressure controller 40 at any given time represents the actual speed of the pump 4 at that time. This pump speed output signal is supplied via lines 41 and 42 to an input of pressure signal processor 13 so that the processor 13 can select the constants B n of equation (4) using equation (5). That is, processor 13 can be provided with equation (5) for constants B n as a function of RPM. Cavity pressure P c can then be calculated using the method described above. A different and presently preferred method used to calculate cavity pressure, P c , eliminates the need to calculate Q in from P p and the pressure drop caused by conduit 7. Instead, the pressure vs. flow characteristics of the inflow pump 4 and conduit 7 are lumped together and empirically determined as a single component; therefore, Q in can be determined directly from P s . The pressure vs. flow characteristics of the inflow cannula assembly 9 are as described previously in equation (2): P c = P s - A 1 Q 2 in - A 2 Q in since P s is measured, and A n are predetermined empirically, P c can be calculated, knowing Q in . To calculate Q in , the pressure vs. flow characteristics of inflow pump 4 in communication with conduits 2 and 7 from fluid source 1 to the point of pressure measurement by transducer 11 were determined empirically over a range of speeds of motor 4a. This empirical pressure vs. flow relationship was accurately fit with the following function: P s = E 0 - E 1 Q 2 in - E 2 Q in which can be rewritten as: Q in = -E 2 + E 2 2 -4E 1 (P s -E 0 ) 2E 1 As described above the functions for each B n can be expressed as functions of RPM by: E n = f(RPM) and RPM can be expressed as a function of the pulse width modulation signal, (PWM), by: RPM = C · PWM Although the value for C in equation (12) is the same as the value for C in elation (6), the values for E n derived from equation (11) are not the same as the B n values derived from equation (5). Given the sensed pressure, P s , and the pulse width modulation signal, (PWM), body cavity pressure, P c , is calculated in the following manner: a) Given PWM, calculate RPM from equation 12; b) Given RPM, calculate E n from equation 11; c) Given E n and P s , calculate Q in from equation 10; and d) Given P s , A n and Q in , calculate P c from equation 2. Equations (2), (10), (11), and (12) can be solved using appropriate software routines of a conventional nature. AND gates 60a and 60b (Fig. 2) are software conditional statements, but could be implemented in hardware, if desired. To the input of AND gate 60a is applied the pump speed signal via lines 41 and 43. Unless an override signal is provided via line 52 at the other input of AND gate 60a, the pump speed PWM signal is sent via line 61 to a variable speed pump motor 4a to raise or lower the pump speed so that it equals the pump speed represented by the pump speed signal. A fault condition controller 50 is provided to generate an override signal if any of a number of system faults is detected. To the inputs 50a of controller 50 are applied signals representing faults such as excessive body cavity pressure, excessive inflow, sensor failure, etc. Controller 50 generates a fault signal in a conventional manner, which is sent via lines 51 and 52 to the AND gate 60a and via lines 51 and 53 to the AND gate 60b associated with the suction pump 18. The presence of a fault signal at the input of gates 60a and 60b provides a pump OFF signal as the output of gates 60a and 60b that will shut down motors 4a and 18a of centrifugal inflow pump 4 and suction pump 18, respectively. Gates 60a, 60b, processor 13 and controllers 40, 50 are preferably provided by a microprocessor. Tool 15 and tool control unit 21 are preferably provided by the PS 3500 motor drive and PS 3500 EP control unit, respectively, available from Smith & Nephew Dyonics Inc., Andover, Massachusetts. Tool 15 will thus contain a motor, a coupler for accepting a desired surgical blade and an internal passageway for the flow of liquid from the surgical site through the blade and tool to the conduit 16 and thence to suction pump 18. Tool control unit 21 contains a power source and a controller for the tool motor. As is known, the DYONICS PS-3500 control unit stores the ranges of tool motor speeds suitable for each of the DYONICS surgical blades that can be used with the DYONICS PS-3500 motor drive, and automatically displays this range to the surgeon on displays 6b (Fig. 1) after the blade is inserted into the PS-3500 motor drive. The surgeon selects a blade speed within this range by operating selector 6a (Fig. 1). See United States Patent 4,705,038, issued November 10, 1987. Control unit 21 generates output signals representing the blade selected and the speed of the tool motor, the signals being sent via line 23 to lines 23a and 23b (Fig. 2), respectively, and thence to suction pump controller 70. This information is processed by the suction pump controller 70, which sends an output signal representing the desired suction pump motor speed via line 70a to AND gate 60b. If the tool motor is OFF, the suction pump motor may be OFF or run at a low speed, such as up to about 200 RPM. If the tool motor is ON, depending on the blade selected, the suction pump motor speed will be in the range of about 500 to about 4,000 RPM. The suction pump motor speed signal is sent by AND gate 60b to pump motor 18a via line 70b, unless a fault signal has been applied to the input of gate 60b by line 53. No feedback loop is provided for motor 18a. The outflow rate of liquid is solely a function of the blade used in tool 15 and whether the motor (not shown) of tool 15 is running or idle. A manual override selector 71 is provided to enable the surgeon to override controller 70 to select a low, medium or high speed for motor 18a for different outflow scenarios. Fault condition controller 50 will also override controller 70 by sending a fault signal via line 53 to an input of AND gate 60b, which will shut down pump motor 18a, as described above. Figs. 6 and 7 show an operative cannula 9a and irrigation extender 9b that snap together to form the inflow cannula assembly 9 illustrated in Fig 8. Operative cannula 9a has a needle portion 91 terminating in distal end 92. Bores 93a and 93b are sealed by seal 94, which when opened by seal piercer 95 (Figs. 7 and 8) of extender 9b, allows fluid to flow through the cannula assembly 9 into body cavity 10. Extender 9b is provided with bore 96 that receives the proximal end 97 of cannula 9a. Latch 104 snaps into groove 98 of cannula 9a to hold the inflow cannula members 9a, 9b together. Seal 94 (Fig. 6A) has opposed faces 94a, 94b formed of an elastomer with transverse slits 94c, 94d formed therein and an outer mounting ring 94e. Seal 94 may be formed using a removable insert between the faces 94a, 94b. Seal piercer 95 passes through slits 94c, 94d to open the seal; the seal 94 being resealed when extender 9b is uncoupled from cannula 91. Extender 9b is provided with a rotatably mounted inlet 99 that communicates with the interior bore 99a via inlet bore 99b and apertures 99c. When endoscope 8 (Fig. 8) is inserted into the inflow cannula assembly 9, inflow liquid flows through bore 99b of inlet 99 and exits distal end 92 via apertures 99c and the annulus 99d between the tube 91 of cannula 9a and the endoscope 8. Endoscope 8 may be secured to extender 9b by a bayonet lock (not shown) using post 99e to facilitate locking of the extender 9b to endoscope 8. Endoscope 8 is a conventional endoscope having eyepiece 8b, light transmitting optics 8c and light inlet 8d. Referring to Fig. 9, a piezoresistive bridge pressure transducer 11 is carried by connector 100. Commercially available transducers can be used if modified to use biocompatible materials. Connector 100 is formed of a front portion 101 and rear portion 102. Front portion 101 has an internal bore 103 for receiving the inflow inlet 99 of extender 9b, which is held in place by the spring-loaded latch 104 being inserted into groove 98 in inflow inlet 99. Insertion of inflow inlet 99 into bore 103 will also open spring-loaded valve 101a in front unit 101. Rear unit 102 is provided with a stepped bore 104a, 104b connected by shoulder 104c. Transducer 11 is glued to the underside of unit 102 with bore 105 in unit 102 in liquid communication with bore 11a in transducer 11 such that liquid flowing through conduit 7 will fill bores 105 and 11a to come into direct contact with sensing diaphragm 11b of transducer 11. Bores 105 and 11a are of small diameter, such as about 1.0mm (0.04 inches) and diaphragm 11b is hence in contact with a small column of liquid that is at the same pressure as the inflow liquid. Transducer 11 transmits a voltage signal corresponding to the pressure, P s , sensed by diaphragm 11b to processor 13 via line 12. Of course, pressure transducer 11 can be located anywhere between the pump 4 and body cavity 10, with the body cavity pressure being calculated as described above. If the transducer 11 is located at the pump 4, then only the pressure drop between the pump 4 and cannula tip 92 need be taken into account. Alternatively, a pressure sensing tube (not shown) may be used, communicating at one end with the body cavity 10 or with conduits 7 or 16 immediately outward of body cavity 10 and utilizing air as the pressure transmission medium, as is known. See, eg. DeSatnick et al. US Patent 4,650,462. Other pressure sensors may also be used. In Fig. 10, outflow cannula 14 is shown assembled to blade 15a of a surgical tool 15. Outflow cannula 14 is preferably identical to the operative cannula 9a and has a luer taper 14a in bore 14b and a double lead screw 14c at its proximal end 14d. Blade 15a has a complementary luer taper 15b and double lead thread 15c so that blade 15a can be sealingly fastened to the cannula 14. Tubular portion 15d opens and passes through seal 94. Blade 15a is operatively connected by shank 15e to a tool 15 (Fig. 1). Suction applied by suction pump 18 will aspirate fluid from body cavity 10 through the tubular portion 15d into tool 15. Fluid exits tool 15 via outlet 15f (Figure 1) into conduit 16. The apparatus of the present invention may be operated as follows. Inflow cannula assembly 9 and outflow cannula 14 are inserted into the body cavity 10, endoscope 8 is inserted into body cavity 10 via inflow cannula assembly 9 and conduit 7 is connected between fluid management unit 5 and the inlet 99 of extender 9b. If the surgeon intends to examine the site before inserting tool 15 into cavity 10, then outflow conduit 16 is preferably connected directly between fluid management unit 5 and outflow cannula 14. In such a case, the appropriate speed for pump 18 is determined by manual override 71. Otherwise, outflow conduit 16 is connected between fluid management unit 5 and the outlet 15f of tool 15 as shown in Fig. 1. In either case, the desired pressure in body cavity 10 is maintained by the feedback loop described above, while the outflow flow rate is determined independently of the pressure in body cavity 10 by the nature of the blade in tool 15 and the speed of the motor in tool 15. Set pressure generator 30 is then operated to select a pressure suitable for the surgical site. For example, the selected pressure may be within the ranges set forth below: Surgical Site Pressure Range (mmHg) Ankle 80-150 Knee 35-90 Shoulder 80-150 Wrist 30-80 User defined <150 eliminated and/or replaced. Another simplification was made because it was empirically determined that, for a given centrifugal pump design, the coefficient B 0 in equation 4 and E 0 in equation 10 were strictly dependent on TAC and the height of the fluid supply 1 above the pump 4, but were not dependent on flow. The remaining variables B 1 , B 2 , E 1 , and E 2 were actually constants that were dependent only on the design of the tubing, cannula and pump are fixed, so will be the associated variables, B 1 , B 2 , E 1 , and E 2 . The variables B 0 and E 0 described previously are dependent on the height of fluid source 1 above pump 4, the pump speed, RPM, and the design of the centrifugal pump 4. Therefore, since the pump design will be fixed and the pump speed is measurable, the height of bags 1 above pump 4 must be either fixed or a measurable variable. Since the presently preferred embodiment fixes the height of fluid supply bag 1 at 610mm (two feet) above pump 4 (45 mmHg), the processor 13 software contains the variable P o (bag offset) preset to a constant of 45 mmHg, but the bag height offset may be used as a variable input with appropriate change in the processor software. Given a fixed centrifugal pump design, the peak pressure that the centrifugal pump can generate with no flow through it is a function of tachometer speed, TAC, by: P z = f(TAC) = G 1 (TAC) 2 + G 2 (TAC) The peak zero flow pressure that the system can generate is also dependent on the fluid supply bag offset, P o ; therefore the total zero flow pressure, P t , that the system can generate is defined by: P t = P z + P o The pressure drop upstream of the sensor, P 1 , across the tubing 2 and centrifugal pump 4, is related to the flow through them by, P 1 = P t - P s = H 1 Q in 2 + H 2 Q in where P s is the sensed pressure, and P t is the total zero flow pressure described earlier. Equation C is the same relationship as equation 9 where P t =E o , H 1 =E 1 , and H 2 =E 2 . The values for H 1 and H 2 are dependent on the placement of the pressure sensor 257 in the outflow path of the centrifugal pump, the geometry of the centrifugal pump, the geometry of tubing 2 and the size of the spikes (not shown). Because the entire inflow tube set 2 and pump 4 are manufactured under tight tolerancing, it has been found that the values for H 1 and H 2 do not change significantly from assembly to assembly. Therefore the following pressure conversion algorithm can apply to any such pump and inflow tubeset assembly as long as the values for A 1 and A 2 in equation 1 and H 1 and H 2 in equation C are not significantly changed from setup to setup. The pressure drop, P 2 , downstream of the sensor across the tubing 7 and cannula 9, is related to the flow through them by equation D, which is the same as equation 1: P 2 = P s -P c = A 1 Q in 2 + A 2 Q in where P s is the sensed pressure, and P c is the pressure at the distal end 92 of the cannula 9. The constants A 1 and A 2 are dependent on the size of the tubing 7 and the size and design of the arthroscope 8 and inflow cannula 9, and therefore care must be taken so that variations in design or manufacturing of the different components do not significantly affect the overall pressure vs. flow relationship, equation D, over the desired ranges. If changes do occur, then processor 13 will be provided with software that will update the processor with the different constants A 1 and A 2 . Although this is not the presently preferred embodiment, if one were able to directly measure both the flow rate, Q in , and pressure, P s , then P c could be estimated from equation 2 directly, P c = P s - A 1 Q in 2 - A 2 Q in . Alternatively, the total pressure drop through the system, (P t -P c ) could be used, which equals P 1 plus P 2 and can be represented as, P t -P c =(A 1 +H 1 )Q in 2 + (A 2 +H 2 )Q in in which case only the flow rate, Q in , would be needed to estimate body cavity pressure, P c , because P t is known as a function of TAC. This was not implemented due to the high cost of using both a pressure sensor and a flow sensor in the system, and because a means was found to estimate the flow, Q in , from the known characteristics of the centrifugal pump and tubing 2,7. The presently preferred embodiment effectively estimates the body cavity pressure, P c , by calculating the pressure drop downstream of the pressure sensor, P 2 , from the measured pressure drop upstream of the pressure sensor, P 1 . This is possible because since the flow through the bag 1-tubing 2-pump 4 and the flow through the tubing 7-cannula 9 are the same, there must be a direct relationship between the pressure drop across the bag 1- tubing 2- pump 4, P 1 , and the pressure drop across the tubing 7- cannula 9, P 2 . Since P 2 is a function of flow, Q in , and P 1 is also a function of flow, Q in , then P 2 must be a function of P 1 by the relationship P 2 =k(P 1 ) which can be predetermined and stored in the program memory for processor 13. Pressure signal processor 13 receives a pressure signal, P s , from pressure transducer 257 via line 12. Pressure signal processor 13 scales pressure signal P s to obtain a pressure value in units of mmHg pressure P s (mmHg)=P s /16. Pressure signal processor 13 also receives a tachometer signal, TAC, from tachometer 406 from line 42d. Pressure signal processor 13 calculates the zero flow pump pressure, P Z , using equation A, P Z = f(TAC) = G 1 (TAC) 2 + G 2 (TAC) and than calculates the total zero flow pump pressure, P t , using equation B, where P 0 is the known height of bag 1 above pump 4: P t = P Z +P 0 = G 1 (TAC) 2 + G 2 (TAC)+P 0 . By definition P 1 is the pressure drop upstream of the pressure sensor, as described by equation C, P 1 = P t -P s (mmHg). From equation D, P 2 =P s- P c , and from equation F, joint pressure P c can be defined in terms of the calculatable pressure drop P 1 by, P c =P s -k(P 1 ) which can then be sent via 32a to be represented in pressure display 5b. Pressure signal processor 13 also communicates with pressure controller 40 via lines 32 and 32b. Pressure controller 40 receives the values for P 1 from pressure signal processor 13 and calculates the total pressure drop, P d , through the pump, tubing and cannula by, P d = P 1 +P 2 = P 1 +k(P 1 ). A running average of 16 calculations per second of P d is maintained to filter out turbulent noise from the motor 4a. From equation E and knowing the desired set pressure, P set , pressure controller 40 calculates the required peak pump pressure, P 1 Z , to overcome the total pressure drop, P d , through the system by, P 1 Z = P set + P d - P 0 Because a centrifugal pump cannot produce negative pressures, P 1 Z is bounded to positive values. The TARGET tachometer speed is calculated by solving equation B for TAC, which can be written as TAC 1 = TARGET = -G 1 G 1 2 + 4G 1 P 1 Z 2G 2 Since P 1 Z is known from equation I, equation J can be solved for TARGET. The TARGET pump speed is then sent to the motor speed controller 407 via lines 41 and 61. If TARGET is more than 100, then pressure controller 40 produces a TARGET signal equal to 100, to maintain a maximum pump speed of 4000 RPM, since for the motor 4a used RPM=40 TAC. It is presently preferred to solve equations A and I by means of a lookup table stored in memory listing values of P z and their corresponding TAC values To summarise, processor 13 and pressure controller 40 calculate the TARGET speed of motor 406a and hence the speed of pump 4 using as inputs the motor speed, TAC, the sensed pressure, P s , and the set pressure, P set , based upon the pressure drops upstream of the sensor to the fluid supply and downstream of the sensor to the inflow cannula tip. In the preferred embodiment described, motor 4a is a brushless, three-phase DC motor, obtained from BEI KIMCO Magnetics Division, San Marcos, California, Part No.DIH 23-20-BBNB, controlled by a microprocessor. While conventional microprocessor controls can be used, it is presently preferred to use the brushless motor control system described in the US Patent No.US-A-5270622.

Claims: Apparatus for supplying liquid to a body cavity during an endoscopic procedure, which comprises: a) liquid supply means (4); b) means for generating a body cavity pressure signal (11) representing the pressure in said body cavity (10); c) conduit means (7) for communicating said liquid supply means (4) with said body cavity (10) to deliver liquid under pressure to said body cavity (10); and (d) control means (40) responsive to said body cavity pressure signal for increasing or decreasing the pressure of said liquid delivered by said liquid supply means to thereby maintain said body cavity pressure at a predetermined value, characterised in that the liquid supply means is a dynamic pump means. Apparatus for supplying liquid to a body cavity during an endoscopic procedure according to claim 1, which in addition comprises sensing means (11) for generating a body cavity pressure signal representing the pressure in said body cavity (10). Apparatus for supplying liquid to a body cavity during an endoscopic procedure, according to claim 1 wherein a) said liquid supply means (4) are for supplying liquid under pressure; b) said means for generating a body cavity pressure signal (11) representing the pressure in said body cavity comprises sensor means in communication with liquid under pressure delivered by said liquid supply means at a location upstream of said body cavity (10) and pressure signal processing means (13) for calculating the value of the pressure in said body cavity (10) from said sensed pressure and for generating a calculated body cavity pressure signal as said body cavity pressure signal. Apparatus as claimed in claim 1, 2 or 3, wherein said means for generating said body cavity pressure signal is a pressure sensing means. Apparatus as claimed in claim 3, wherein said sensor means comprises a pressure transducer means (11) and an operative pressure-sensing diaphragm, and said pressure sensing means includes channel means for directing a column of liquid from liquid flowing through said conduit means to said diaphragm. Apparatus as claimed in claim 5, wherein said sensor means comprises a chamber having an open end, a thin, flexible diaphragm closing said open end, channel means for directing liquid delivered from said liquid supply means to said chamber, and pressure transducer means in operative contact with said flexible diaphragm. Apparatus as claimed in claim 1, wherein said dynamic pump means is a centrifugal inflow pump. Apparatus as claimed in claim 3 wherein the location of a sensor means is adjacent the outlet of said pump means. Apparatus as claimed in any one of the preceding claims, wherein a liquid outflow cannula (14) is provided for insertion into said body cavity (10) and a source of suction (18) is provided communicating with said outflow cannula (14) for aspirating liquid from said body cavity (10) via said liquid outflow cannula (14). Apparatus as claimed in claim 9, wherein said source of suction is a suction pump. Apparatus as claimed in claim 9 or 10, wherein tubing means (16) connects said source of suction (18) to said liquid outflow cannula (14) and means is provided selectively to crimp said tubing and thus selectively restrict flow of liquid aspirated from said body cavity. Apparatus as claimed in claim 7 wherein said liquid supply means includes a motor for driving said centrifugal inflow pump, a tachometer for generating a tachometer signal indicating the actual speed of said motor, and wherein pressure signal processing means comprises input means for receiving said tachometer signal, a signal indicating said sensed pressure, and a signal indicating a desired set pressure and a signal indicating a desired set pressure, means for calculating from said input signals a target tachometer signal as said calculated body cavity signal as a function of the pressure drops between said liquid supply means to said sensor means and between said sensor means to said body cavity, said target tachometer signal representing the motor speed necessary to increase or decrease the pressure of liquid delivered by said centrifugal inflow pump means to thereby maintain said body cavity pressure at said predetermined value.

Inventors: BEISER DAVID G, KRAUSE KENNETH W, WOOLFSON STEVEN B, BEISER, DAVID G., KRAUSE, KENNETH W., WOOLFSON, STEVEN B.
Applicants: SMITH & NEPHEW INC, SMITH & NEPHEW, INC.
Application year: 1992
Main classification: A61M1
IPC codes: A61B17, A61M31, A61M1, A61B1, A61M3