Ultrasonic air-in-line detector for a medication infusion system

Abstract: An ultrasonic air-in-line detection system for use in detecting air bubbles in the fluid line (306) of a disposable cassette mounted on a main pump unit which can accurately detect air bubbles in any type of fluid, whether clear or opaque. The system uses a windowing technique to determine when a threshold amount of air bubbles in a predetermined voulime of material pumped occurs, with the alarm being sounded only when this threshold amount of air bubbles in present in the window volume. The system of the present invention thus avoids so-called nuisance alarms due to the presence of small isolated air bubbles in the fluid due to the fluid pumped not having degassed.

Description: The present invention relates generally to an ultrasonic system for detecting the presence of air in a fluid line, and more particularly to a system which determines when a threshold quantity of air is present with the alarm being sounded only when this threshold quantity is present, thus avoiding nuisance alarms. In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results. The second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid. As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are used to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic a natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis. One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which the medication is pumped must be sterile, in most applications of modern medication infusion equipment, some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device should be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit. It is clearly desirable to have a simple disposable cassette design to minimise the cost of its construction, using the minimum number of parts necessary in its design. The design of the cassette should be mass produceable, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include more than just a fluid pump; other features which have formerly been included in peripheral devices should also be included in the cassette. Such a system has been disclosed in the present Applicants' co-pending applications published under numbers EP-A-321120, EP-A-319269, EP-A-319279, EP-A-319278, EP-A-319276, EP-A-320168, EP-A-319273, EP-A-319274, EP-A-319268, EP-A-319277, EP-A-319275, EP-A-341346, EP-A-346548, EP-A-319272 and EP-A-319267. An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles. Although steps may be taken to minimize the possibility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remaining in the fluid to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement. One type of air-in-line detector which has been used in the past is an ultrasonic detector, which employs an ultrasonic transmitter located on one side of a fluid line and an ultrasonic receiver located on the other side of the fluid line. Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Examples of ultrasonic air-in-line detectors include US Patent No. 4,764,166, to Spani, and US Patent No. 4,821,558, to Pastrone et al. Such ultrasonic air-in-line detectors work very well in detecting air bubbles in the fluid line. In fact, fairly small air bubbles can be detected with an ultrasonic detector. The problem with such ultrasonic detectors is that they are too good at detecting air bubbles. In fact, many medications are not degassed and do contain some air, which may form small bubbles, particularly during the pumping process as performed in a medication infusion pump. In most patients it is not harmful to pump such small amounts of air intravenously. In fact, only very few patients are unable to tolerate any air being introduced into their venous systems, examples might be neonates, pediatrics, and those patients having septal defects. Other than when infusing fluid into these patients, or performing an intra-arterial infusion, the introduction of very small quantities of air is not believed to be particularly harmful. In such cases where even small bubbles may be harmful, air eliminating filters may be used. Thus, the situation presented is that the ultrasonic air-in-line detection system will alarm for any amount of air in the fluid line, irrespective of whether that amount is insignificant or large. Typically, hospital personnel tend to see such alarms for tiny amounts of air as nuisance alarms, since they tend to occur frequently due to the virtual inevitability of small bubbles being present. In fact, if a high number of nuisance alarms occurs, the device is likely to be seen as less desirable than a competing device with a poorer air-in-line detection system which does not alarm as often. DE-A-3344804 discloses a system for detecting the presence of air in liquid passing through a fluid passageway in an implantable insulin pummp. In this system, the time taken by a bubble to pass between two spaced-apart electrodes located in the fluid path is measured. It is suggested that the bubble volume is proportional to the time taken by the bubble to pass between the two electrodes and the bubble volume is thus estimated using the measured time and the volume of fluid pumped. This system is used to indicate whether the presence of a gas bubble is a random occurrence or a systematic effect. It is therefore the primary objective of the present invention to provide an air-in-line detection system which will not alarm merely due to the presence of a tiny amount of air detected in the fluid path. The system of the present invention should be of a design retaining all of the advantages of known ultrasonic air-in-line detection devices, and should also provide a number of additional advantages and improvements. Specifically, the air-in-line detection system of the present invention should be capable of detecting even very small air bubbles in the fluid line of disposable cassette near the output end of the cassette, after the pumping operation has been performed. Then, the system of the present invention should be able to discriminate between a situation in which isolated tiny air bubbles are present and the situation in which a significant amount of air is present in the fluid line, not alarming in the latter case. Several other additional features are desirable in the design of a cassette and a main pump unit making up an air-in-line detection system. Examples of such features are the ability to detect air bubbles whether the flow rate of the fluid in the cassette is fast or slow, and the ability to detect air in the fluid line even when the interior of the fluid line remains coated with liquid. The system should be capable of accurately and effectively detecting air bubbles in any type of fluid which may be infused, whether the fluid is clear or opaque, as in the case of lipid solutions. The system should accomplish all these objects in a manner which will retain and enhance all of the advantages of reliability, durability, and safety of operation. The system of the present invention should provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage. According to the invention, there is provided a system for detecting the presence of a significant amount of air in liquid passing through a fluid passageway in a medical infusion system, comprising a sensor disposed adjacent the fluid passageway for providing an air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway; means for monitoring the volume pumped through the fluid passageway, the monitoring means providing a volume output signal indicative of the volume pumped; means responsive to the air-in-line output signal and the volume output signal, for estimating a volume of air in the fluid passageway, and for providing an output representing the said volume of air; and means for providing an alarm signal if the volume of air output is equal to or greater than a volume representing a second predetermined volume; characterised in that the sensor is a single ultrasonic sensor which comprises a transmitter for transmitting an ultrasonic signal applied to the fluid passageway at a single location and a receiver for detecting whether the ultrasonic signal passes through the fluid passageway at a single location and for providing the air-in-line signal indicating the current presence of liquid or air in the fluid passageway at the single location monitored by the ultrasonic sensor; the means responsive to the air-in-line output signal and the volume pumped signal being calculating means arranged to calculate the volume of air contained in the last predetermined volume pumped through the fluid passageway, whereby the volume of air output represents the volume of air contained in the last predetermined volume pumped. Preferably, the transmitter comprises a first ultrasonic transducer located on one side of the fluid passageway and means for driving the first ultrasonic transducer to generate ultrasonic vibrations which are transmitted to the said one side of the fluid passageway and the receiver comprises a second ultrasonic transducer located on the other side of the said fluid passageway, the ultrasonic vibrations passing through the fluid passageway and being received by the second ultrasonic transducer when there is liquid in the fluid passageway, the ultrasonic vibrations substantially not passing through the fluid passageway and not being received by the second ultrasonic transducer when there is an air bubble in the fluid passageway and, the receiver also including means for detecting the ultrasonic vibrations received by the second transducer and providing the air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway between the first and second transducers. Preferably the first and second ultrasonic transducers have a resonant frequency and the driving means generating an oscillating signal which sweeps from a first frequency to a second frequency, the resonant frequency being between the first and second frequencies. The detecting means may comprise an amplifier for amplifying an electrical signal generated by the second ultrasonic transducer, a rectifier for rectifying the amplified electrical signal generated by the second ultrasonic transducer, and a first comparator for producing a first comparator output signal when the rectified, amplified electrical signal generated by the second ultrasonic transducer exceeds a first threshold. The driving means may operate periodically to drive the first ultrasonic transducer to generate ultrasonic vibrations, preferably on a one-eighth duty cycle. Preferably, the driving means operates at a frequency sufficiently high to ensure that an air bubble will be detected at even the highest possible rate of fluid flow through the fluid passageway, for example a frequency of approximately 750Hz. Conveniently, the system includes a piston pump for pumping fluid through the fluid passageway and means for driving the pump and the monitoring means comprises an encoder for providing position information describing the position of the piston pump and a decoder for decoding the position information to determine how much volume has been pumped from a first item of position information to a second item of position information. The system may also include means for measuring the pressure in the fluid passageway; and means for modifying the volume as determined by the decoder according to the measured pressure in the fluid passageway to produce the volume output signal, and also means for selecting the size of the last predetermined volume pumped from a plurality of selections. Preferably, the size of the last predetermined volume is less than the volume of the fluid passageway from the location monitored by ultrasonic sensor to a patient, and is for example between approximately 100 millilitres and 300 millilitres. Conveniently, the second predetermined volume is between one-twentieth and one-sixth of the size of the last predertermined volume, and is preferably one-sixth of the size of the last predetermined volume. According to another aspect of the invention, there is provided a method of detecting the presence of a significant amount of air in fluid passing through a fluid passageway in a medical infusion system, which comprises providing an air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway; monitoring the volume pumped through the fluid passageway and providing a volume output signal indicative of the volume pumped; estimating from the air-in-line output signal and the volume output signal, a volume of air in the fluid passageway and providing an output representing the said volume of air; and providing an alarm signal if the volume of air output is equal to or greater than a second predetermined volume; characterised in that the sensor is a single ultrasonic sensor disposed adjacent the fluid passageway at the particular location; the ultrasonic sensor comprising a transmitter which transmits an ultrasonic signal applied to the fluid passageway at the particular location, and a receiver which detects whether the ultrasonic signal passes through the fluid passageway at the particular location and which provides the air-in-line signal to indicate the current presence of liquid or air in the fluid passageway at the particular location; the method being further characterised in that the estimation step comprises calculating the volume of air contained in the last predetermined volume pumped through the passageway, whereby the volume of air output represents the volume of air contained in the last predetermined volume pumped. The disadvantages and limitations of the background art discussed above are overcome by teh present invention. With this invention, an ultrasonic air-in-line detector is used to detect all air bubbles contained in the fluid line leaving a disposable cassette containing a fluid pump. A signal from the air-in-line detector circuitry indicates either the presence or absence of air bubbles contained in the portion of the fluid pathway covered by the ultrasonic sensor. The system functions to evaluate the present indication of the presence or absence of air bubbles together with a past history of the presence of air bubbles during a window of a particular volume pumped, to determine whether sufficient air bubbles are present to initiate an alarm and to shut down the pump. The window is a volume less than the total volume of the outlet tubing in the preferred embodiment, to enable an alarm and pump shutdown to occur before air can reach the patient. If, during the pumping of the last window volume quantity passing the ultrasonic sensor, less than a threshold amount of air exists, the system is allowed to continue operating. If, on the other hand, the threshold amount of air is detected during the pumping of the last window volume quantity, the system is shut down and an alarm is sounded. The system automatically remembers the volume of air while that air is contained within the window. When the air has been pumped beyond the window, it is automatically "forgotten" since it is no longer within the window. In this manner, the system should be able to continue pumping fluid even though a tiny amount of air is contained in the fluid being pumped, thus avoiding nuisance alarms. However, teh threshold should be set low enough so that any significant quantity of air will be quickly detected and acted upon. In this manner, false alarms due to tiny quantities of air which are not dangerous to most patients will be avoided, thus avoiding nuisance alarms caused by insignificant amounts of air reaching the patient. The system remains capable of detecting even very small air bubbles, and of accurately determining when those small bubbles together represent a significant amount of air requiring an alarm to be activate. Thus an excellent compromise is reached between alarming at all air bubbles and not alarming unless there is a large quantity of air in the fluid line. It may therefore be appreciated that the present invention provides an air-in-line detection system which will not alarm merely due to the presence of a tiny amount of air detected in the fluid path. The system of the present invention is of a design retaining all of the advantages of known ultrasonic air-in-line detection devices, and also provides a number of additional advantages and improvements. The air-in-line detection system of the present invention is capable of detecting even very small air bubbles in the fluid line of a disposable cassette near the output end of the cassette, after the pumping operation has been performed. The system of the present invention can discriminate between a situation in which isolated tiny air bubbles are present and a situation when a significant amount of air is present in the fluid line, not alarming in the former case and uniformly alarming in the latter case. Several other additional features are included in the air-in-line detection system of the present invention. Examples of such features are the ability to detect air bubbles whether the flow rate of the fluid in the cassette is fast or slow, and the ability to detect air in the fluid line even when the interior of the fluid line remains coated with fluid. The system is capable of accurately and effectively detecting air bubbles in any type of fluid which may be infused, whether the fluid is clear or opaque. The system accomplishes all these objects in a manner which retains and enhances the advantages of reliability, durability, and safety of operation inherent in an ultrasonic detector. The system of the present invention provides all of these advantages and overcomes the limitations of the background art without incurring any relative disadvantage. All the advantages of the present invention result in a superior medication infusion system having a number of advantages which make the system a highly desirable alternative to systems presently available. The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings. In this description of the preferred embodiments, a uniform directional system is sued in which front, back, top, bottom, left and right are indicated with respect to the operating position of the cassette and main pump unit when viewed from the front of the main pump unit. In the accompanying drawings: Figure 1 is a top plan view of a disposable cassette body showing most of the fluid path through the cassette; Figure 2 is a top plan view of a disposable cassette body shown in Figure 1; Figure 3 is a rear-elevation (inverted) of the cassette body shown in Figures 1 and 2; Figure 4 is a bottom view of the cassette body shown in Figures 1 to 3; Figure 5 is a right side view (inverted) of the cassette body shown in Figures 1 to 4; Figure 6 is a left side view of the cassette body showing in Figures 1 to 5; Figure 7 is a partially cutaway view from the front of the cassette body shown in Figures 1 to 6, showing the bubble trap used to remove air bubbles from the fluid supplied to the cassette; Figure 8 is a partially cutaway view from the right side (inverted) of the cassette body shown in Figures 1 to 6, showing the cylinder of the fluid pump contained in the cassette; Figure 9 is a top plan view of a valve diaphragm used to seal the passageways on the top surface of the cassette body showing in Figure 1; Figure 10 is a bottom view of the valve diaphragm shown in Figure 9; Figure 11 is a vertical section through the valve diaphragm shown in Figures 9 and 10, viewed from the rear; Figure 12 is a partially cutaway view from the right side of the valve diaphragm shown in Figures 9 and 10; Figure 13 is a top plan view of a valve diaphragm retainer used to retain the valve diaphragm shown in Figures 9 to 12; Figure 14 is a bottom view of the valve diaphragm retainer shown in Figure 13; Figure 15 is a rear elevation of the valve diaphragm retainer shown in Figures 13 and 14; Figure 16 is a front elevation of the valve diaphragm retainer shown in Figures 13 to 15; Figure 17 is a right side view of the valve diaphragm retainer shown in Figures 13 to 16; Figure 18 is a left side view of the valve diaphragm retainer shown in Figures 13 to 17; Figure 19 is a vertical section through the valve diaphragm retainer shown in Figures 13 to 18, viewed from the front; Figure 20 is a partially cutaway view from the left side of the valve diaphragm retainer shown in Figures 13 to 19; Figure 21 is a partially cutaway view from the right side of the valve diaphragm retainer shown in Figures 13 to 20; Figure 22 is a top view of a bubble chamber cap; Figure 23 is a bottom view of the bubble chamber cap shown in Figure 22; Figure 24 is a left side view of the bubble chamber cap shown in Figures 22 and 23; Figure 25 is a cutaway view from the rear of the bubble chamber cap shown in Figures 22 to 24; Figure 26 is a cutaway view (inverted) from the right side of the bubble chamber cap shown in Figures 22 to 24; Figure 27 is a top plan view of a slide latch used both the lock the cassette in place on a main pump unit, and to pinch off the IV outlet line prior to installation on the main pump unit; Figure 28 is a right side view of the slide latch shown in Figure 27; Figure 29 is a bottom view of the slide latch shown in figures 27 and 28; Figure 30 is a rear view of the slide latch shown in Figures 27 to 29; Figure 31 is a front elevation of the slide latch shown in Figures 27 to 30; Figure 32 is a cutaway view (inverted) from the left side of the slide latch shown in Figures 27 to 31; Figure 33 is a side view of the piston cap and boot seal, which function both as a piston and as a bacterial seal; Figure 34 is a view from above of the piston cap and boot seal shown in Figure 33; Figure 35 is a view from below of the piston cap and boot seal shown in Figures 33 and 34; Figure 36 is a vertical section through the piston cap and boot seal shown in Figures 33 to 35; Figure 37 is a rear view of a piston for insertion into the piston cap and boot seal shown in Figures 33 to 36; Figure 38 is a front view of the piston shown in Figure 37; Figure 39 is a top view of the piston shown in Figures 37 and 38; Figure 40 is a left side view of the piston shown in Figures 37 to 39; Figure 41 is a bottom view of the piston shown in Figures 37 to 40; Figure 42 is a cutaway view (inverted) from the right side of the piston shown in Figures 37 to 41; Figure 43 is a perspective view from above of a tubing adaptor for installation in the outlet tube below the slide latch; Figure 44 is a cutaway view of the tubing adaptor shown in Figure 43; Figure 45 is a perspective view from above of an assembled cassette using the components shown in Figures 1 to 44, with the slide latch in the opened position; Figure 46 is a bottom view of the assembled cassette shown in Figure 45, with the tubing adaptor removed for clarity and the slide latch in the opened position; Figure 47 is a perspective view from above of the assembled cassette shown in Figures 45 and 46, with the slide latch in the closed position; Figure 48 is a bottom view of the assembled cassette shown in Figures 45 to 47, with the tubing adaptor removed for clarity and the slide latch in the closed position; Figure 49 is a left side view of the latch head used to capture and actuate the piston; Figure 50 is a right side view of the latch head shown in Figure 49; Figure 51 is a bottom view of the latch head shown in Figures 49 and 50; Figure 52 is top view of the latch head shown in Figures 49 to 51; Figure 53 is a cutaway view from the right side of the latch head shown in Figures 49 to 52; Figure 54 is a right side view of the spring retainer to be mounted in the latch head shown in Figures 49 to 52; Figure 55 is a front view of the spring retainer shown in Figure 54; Figure 56 is a left side view of the latch jaw to be mounted on the latch head shown in Figures 49 to 52; Figure 57 is a bottom view of the latch jaw shown in Figure 56; Figure 58 is a rear view of the latch jaw shown in Figures 56 and 57; Figure 59 is a left side view of the jaws assembly in the open position, the jaws assembly being made up of the latch head shown in Figures 49 to 52, the spring retainer shown in Figures 54 and 55, the latch jaw shown in Figures 56 to 58, a latch spring, and pins to assemble the various components together; Figure 60 is a bottom view of the jaws assembly shown in Figure 59, with the jaws assembly being shown in the open position; Figure 61 is a left side view of the jaws assembly shown in Figures 59 and 60, with the jaws assembly being shown in the closed position (and in the open position in phantom lines); Figure 62 is a bottom plan view of the main pump unit chassis; Figure 63 is a front view of the main pump unit chassis shown in Figure 62; Figure 64 is a top plan view of the main pump unit chassis shown in Figures 62 and 63; Figure 65 is a rear view (inverted) of the main pump unit chassis shown in Figures 62 to 64; Figure 66 is a perspective top view of the cassette guide used to position the cassette of Figures 45 to 48 on the main pump unit; Figure 67 is a sectional view of the cassette guide shown in Figure 66; Figure 68 is a top view of the cassette guide shown in Figures 66 and 67; Figure 69 is a bottom view of the cassette guide shown in Figures 66 to 68; Figure 70 is a side view of the pump shaft on which the jaws assembly shown in Figures 59 to 61 is mounted; Figure 71 is a right side view of the slide lock used to retain the cassette shown in Figures 43 to 48 in position on the main pump unit; Figure 72 is a bottom view of the slide lock shown in Figure 71; Figure 73 is a left side view (inverted) of the slide lock shown in Figures 71 and 72, showing the bevel used to reflect the light beam from the optical light source away from the optical light sensor when the slide lock is in the open position; Figure 74 is a top view of the slide lock shown in Figures 71 to 73, showing the reflective surface used to reflect the light beam from the optical light source to the optical light sensor when the slide lock is in the closed position; Figure 75 is a front view of the slide lock shown in Figures 71 to 74; Figure 76 is a rear view of the slide lock shown in Figures 71 to 75, showing the slanted surfaced used to reflect the light beam away from the corresponding sensor when the slide lock is in the open position; Figure 77 is a perspective view from above of the upper sensor housing; Figure 78 is a vertical section through the upper sensor housing shown in Figure 77; Figure 79 is a top plan view of the upper sensor housing shown in Figures 77 and 78; Figure 80 is a bottom plan view of the upper sensor housing shown in Figures 77 to 79; Figure 81 is a perspective view from above of the lower sensor housing; Figure 82 is a vertical section through the lower sensor housing shown in Figure 81; Figure 83 is a perspective view from beneath of the lower sensor housing shown in Figures 81 and 82; Figure 83A is a bottom plan view of the lower sensor housing shown in Figures 81 to 83; Figure 84 is a top plan view of a portion of a flex circuit used to interface electrically with a pair of ultrasonic transducers; Figure 85 is a partially exploded perspective view showing how the ultrasonic transducers are attached to the flex circuit using conductive transfer tape; Figure 85A is a partially exploded perspective view showing an alternative embodiment in which portions of the flex circuit and the conductive transfer tape on the rear sides of the ultrasonic transducers have apertures therethrough; Figure 86 is a perspective view from beneath showing the assembly of Figure 85 installed in the upper sensor housing; Figure 87 is a perspective view from beneath showing a miniature circuit board installed on the flex circuit of the assembly of Figure 86; Figure 88 is a front elevation of an optical sensor module; Figure 89 is a side elevation of the optical sensor module shown in Figure 88; Figure 90 is a top plan view of the optical sensor module shown in Figures 88 and 89; Figure 91 is a front view of a valve actuator; Figure 92 is a side view of the valve actuator shown in Figure 91; Figure 93 is a bottom plan view of the valve actuator shown in Figures 91 and 92; Figure 94 is a top plan view of one of the actuator guides used to guide and retain in position the valve actuators for one cassette; Figure 95 is a side view of the actuator guide shown in Figure 94; Figure 96 is a top plan view of a pressure transducer; Figure 97 is a side view of the pressure transducer shown in Figure 96; Figure 98 is a bottom plan view of the pressure transducer shown in Figure 96; Figure 99 is a bottom plan view of the elastomeric valve actuator seal used to bias the valve actuators into an upward position; Figure 100 is a cutaway view of the valve actuator seal shown in Figure 99; Figure 101 is a perspective view of the main pump unit chassis having the various components for one pump mounted thereon; Figure 102 is a bottom view of the main pump unit chassis having the various components for one pump mounted thereon, with the slide lock in the open position ready to receive a cassette; Figure 103 is a bottom view of the main pump unit chassis shown in Figure 102, with the slide lock in the closed position as it would be if a cassette were installed and latched onto the main pump unit; Figure 104 is a side view showing a cassette in position to be installed on the main pump unit; Figure 105 is a side view showing the cassette as it is engaging the main pump unit, with the tubing adaptor engaging the flared recess in the bottom of the sensor housing to draw the outlet tube into engagement between the ultrasonic transducers; Figure 106 is a side view showing the cassette fully installed on the main pump unit with the slide latch closed and the outlet tube in full engagement between the ultrasonic transducers in the sensor housing; Figure 107 is a block diagram of the entire operating system of the infusion pump of the present invention, showing the ultrasonic air-in-line detector system and self test therefor; Figure 108 is a schematic diagram of the transmitting circuitry for the ultrasonic air-in-line detector system for all three channels; Figure 109 is a block diagram of the receiver circuitry for one channel, the circuitry having an output signal; Figure 110 is a schematic diagram of the processing circuitry used to process the output signal from the receiver circuitry to produce an AILD Output signal for each channel and an interrupt signal indicating a change in state of the AILD Output signal of one of the three channels; Figure 111 shows various wave forms generated by the circuitry of Figures 108, 109 and 110; Figure 112 is a simplified flow diagram illustrating the operation of the air-in-line detector monitoring system; and Figure 113 is a simplified flow diagram illustrating the operation of the air-in-line detector self test system. The Cassette The preferred embodiment of the cassette of the present invention includes all of the features described above in a single compact disposable cassette constructed of seven parts. The basic construction and operation of the cassette is the subject of EP-A-319278 in the name of the present Applicants and entitled "Disposable Cassette for a Medication Infusion System". Referring to Figures 43 and 44, a tubing adaptor 301 is shown located between an outlet tubing 306 extending from an assembled cassette 302 and a delivery tubing 303 which leads to the patient. The tubing adaptor 301 is essentially cylindrical, and is hollow throughout allowing the inlet tubing 306 and the delivery tubing 303 to be inserted into it. The inlet tubing 306 and the delivery tubing 303 in the preferred embodiment are adhesively secured in the tubing adaptor 301. Located at the top end of the tubing adaptor 301 there is an outer tapered portion 305 having a smaller outer diameter as it approaches the top end. Located below the tapered portion 305 is a radially outwardly extending flange 307. The assembly and configuration of the cassette will now be described with reference to the assembled cassette 302 shown in Figures 45 to 48. A retainer cap 190 is located over the cassette body 100, and is secured in place by ultrasonic welding. While adhesive sealing may be used, it is more difficult to ensure the consistent hermetic seal required in the construction of cassette 302. An inlet tube 304 is adhesively secured to the inner diameter of the inlet aperture 238 in the bubble chamber cap 230, in fluid communication with the bubble chamber 106. An outlet tube 306, extending through the wider portion of the aperture 258 and is adhesively secured to the inner diameter of the outlet tube mounting cylinder 144 in the cassette body 100, in fluid communication with the fourth passageway 140 through the aperture 142. The tubing adaptor 301 is connected to the other end of the outlet tube 306 and the delivery tube 303 is also attached to the tubing adaptor 301. The inlet tube 304 and the outlet tube 306 are shown in the figures only in part; on their respective ends not connected to the assembled cassette 302 they may have connector fittings such as standard luer connectors (not shown), which are well known in the art. The adhesives used to attach the inlet tube 304 and the outlet tube 306 to the assembled cassette 302 also represent technology well known in the art. For example, adhesives such as cyclohexanone, methylene dichloride, or tetrahydrofuron (THF) may be used. The Main Pump Unit The preferred embodiment of the main pump unit of the present invention includes a number of components used to hold, latch, and drive the cassette 302 described above. These components will not be described in detail since they are not closely related to the subject matter of the present invention. For a complete description of the structure and function of these components, EP-A-319279, mentioned above, may be referred to. The power module to drive the main pump unit will not be described in detail since it is not closely related to the subject matter of the present invention. For a complete description of the construction of the power module, EP-A-319277, mentioned above may be referred to. Figures 77 to 80 show an upper ultrasonic housing 800. The upper ulstrasonic housing 800 is hollow, and is open at the bottom. The upper surface of the upper ultrasonic housing 800 has a U-shaped ridge 802 and a straight ridge 804, with a rectangular aperture 806 located between them in the upper surface. The U-shaped ridge 802 and the straight ridge 804 are sized to fit within the lower segment 511 of the cassette guide 510 (Figure 69). Located in the front of the upper ultrasonic housing 800, there is a slot 808 for receiving the outlet tube 306 of the assembled cassette 302. The slot 808 is deeper than it is wide, and has a funnel-shaped entrance to allow the outlet tube 306 to be directed easily into the slot 808. In the preferred embodiment, the width of the slot 808 is narrower than the outside diameter of the outlet tube 306, so that when the outlet tube 306 fits in the slot 80, it is deformed somewhat. The interior of the upper ultrasonic housing 800 may be thought of as three areas, one on each side of the slot 808, and a third area representing that portion of the upper ultrasonic housing 800 into which the slot 808 does not extend. The first two areas are locations in which ultrasonic transducers (not shown) will be located, and the third area will be the location of a miniature printed circuit board (not shown). Referring particularly to Figure 80, the first area, in the front and on the right-hand side of the upper ultrasonic housing 800, is bounded by a wall 810 on the right of the slot 808. The second area, in the front and on the left-hand side of the upper ultrasonic housing 800, is bounded by a wall 812 on the left of the slot 808. Figures 81 to 83 show a lower ultrasonic housing 814 which will be mounted onto the bottom of the upper ultrasonic housing 800. Like the upper ultrasonic housing 800, the lower ultrasonic housing 814 is hollow, but the lower ultrasonic housing 814 is open at the top. The front portion of the lower ultrasonic housing 814 (the portion which will be under the first two areas inside the upper ultrasonic housing 800) is shallow, while the rear portion of the lower ultrasonic housing 814 is deeper. The lower ultrasonic housing 814 also has a slot 816 which will be located under the slot 808 in the upper ultrasonic housing 800 when the lower ultrasonic housing 814 is mounted on the upper ultrasonic housing 800. The slot 816 also has a funnel-shaped entrance, like the slot 808. Beneath the portion of the lower ultrasonic housing 814 having the slot 816, there is a recessed area 818. The recessed area 818 is located on both the left and the right-hand sides of the slot 816. In the preferred embodiment, the recessed area 818 is frustroconically shaped, as best shown in Figures 83 and 83A. The frustroconically shaped recessed area 818 is spaced slightly away from the front of the lower ultrasonic housing 814. Located on the bottom and at the front of the lower ultrasonic housing 814 on each side of the slot 816 there are two ramps 820 and 822 which are inclined towards the frustroconically shaped recessed area 818. The recessed area 818 and the two ramps 820 and 822 are designed to capture and retain the tapered portion 305 of the tubing adaptor 301 (Figure 43). Accordingly, the size of the recessed area 818 is substantially the same as the size of the tapered portion 305 of the tubing adaptor 301. The two ramps 820 and 822 are located as shown in Figure 83A to draw the tapered portion 305 of the tubing adaptor 301 from a position on the two ramps 820 and 822 to a position in contact with the recessed area 818. This operation of engagement of the tapered portion 305 of the tubing adaptor 301 with the recessed area 818 will be discussed further in detail below. Figure 84 shows a portion of a two-piece flex circuit 824 and 825. The flex circuit 824 may be thought of as a straight base portion having four orthogonally extending side arms. An exposed circular conductive pad 826, 828, 830, 832 is located at the end of each of the four arms. A series of four terminals 834, 836, 838, 840 are located on the flex circuit 824 on the base portion near its centre. The conductive pads 826, 828, 830, 832 are electrically connected to the terminals 834, 836, 838, 840 by conductors 850, 852, 854, 856 respectively. The flex circuit 825 is a long tail segment having four terminals 842, 844, 846 and 848 on the end adjacent the flex circuit 824. The base portion of the flex circuit 824 and the flex circuit 825 are to be located close together, and thus effectively form a "T". Four more conductors 858, 860, 862 and 864 are located in the flex circuit 825. The conductors 858, 860, 862, 864 are electrically connected to the terminals 842, 844, 846, 848, respectively. It will be appreciated by those skilled in the art that the conductors 850, 852, 854, and 856 and the conductors 858, 860, 862 and 864 are electrically insulated on both sides. Figure 85 shows the assembly of two ultrasonic transducers 866 and 868 to the flex circuit 824. The transducers 866 and 868 are typically ceramic ultrasonic transducers. In a typical known assembly of ultrasonic transducers, soldering is used, with the danger of possible damage to the ceramic ultrasonic transducer. The present invention instead uses conductive adhesive transfer tape, which has adhesive on both sides and is electrically conductive. Such conductive transfer tape is commercially available from 3M under the product identification number 9703. A disc-shaped segment of conductive transfer tape 870 both secures the conductive pad 826 to the one side of the ultrasonic transducer 866 and makes electrical contact between the conductive pad 826 and the one side of the ultrasonic transducer 866. Similarly, a disc-shaped segment of conductive transfer tape 872 is placed between the conductive pad 828 and the other side (the front side) of the ultrasonic transducer 866. A disc-shaped segment of conductive transfer tape 874 is placed between the conductive pad 830 and one side (the front side) of the ultrasonic transducer 868. A disc-shaped segment of conductive transfer tape 876 is placed between the conductive pad 832 and the other side (the back side) of the ultrasonic transducer 868. Thus, the ultrasonic transducers 866 and 868 are assembled and electrically connected to the flex circuit 824. The disc-shaped segments of conductive transfer tape 870, 872, 874 and 876 are used in the preferred embodiment. Instead of using conductive transfer tape, conductive epoxy could be used, although the conductive transfer tape is preferred. Referring next to Figure 86, the ultrasonic transducers 866 and 868 are assembled into the upper ultrasonic housing 800. The portion of the flex circuit 824 on the side of the conductive pad 828 opposite the ultrasonic transducer 866 is adhesively bonded to the wall 812, thus securing the ultrasonic transducer 866 to the wall 812. Similarly, the portion of the flex circuit 824 on the side of the conductive pad 830 opposite the ultrasonic transducer 868 is adhesively bonded to the wall 810, thus securing the ultrasonic transducer 868 to the wall 810. The adhesive used is preferably an elastomeric adhesive which can be applied in a thin coat with no air pockets. One such adhesive is Black Max adhesive. A small block of foam 878 is used to bear against the ultrasonic transducer 866 and the associated attached portions of the flex circuit 824. Similarly, a small block of foam 880 is used to bear against the ultrasonic transducer 868 and the associated attached portions of the flex circuit 824. The flex circuit 825 is directed through the rectangular aperture 806 in the upper ultrasonic housing 800. The connectors 858, 860, 862 and 864 are electrically connected to a connector 882. A small printed circuit board 884 having various components thereon (Figure 87) is electrically connected to the terminals 834, 836, 838 and 840 (Figure 84) on the flex circuit 824 and the terminals 842, 844, 846 and 848 on the flex circuit 825. The printed circuit board 884 then rests in the third area in the upper ultrasonic housing 800, as shown. In an alternative embodiment, illustrated in Figure 85A, there are apertures in the conductive pads and the disc-shaped segments of conductive transfer tape located on the back sides of each of the ultrasonic transducers 866 and 868. Thus, the conductive pad 826 and segment 870 each have an aperture extending through to the back side of the ultrasonic transducer 866. Similarly, the conductive pad 832 and the segment 876 each have an aperture extending through to the back side of the ultrasonic transducer 868. The apertures allow the ultrasonic transducers 866 and 868 to flex more freely, and the strength of the output signal is approximately doubled by using these apertures. The apertures in the conductive pads 826 and 832 and in the disc-shaped segments of conductive transfer tape 870 and 876 are located centrally. The diameters of the ultrasonic transducers 866 and 868, and the diameters of the conductive pads 826, 828, 830 and 832, are approximately 0.21 inches (5.3mm). In the preferred embodiment, the diameters of the apertures in the conductive pads 826 and 832 and in the segments 870 and 876 are approximately 0.125 inches (3.2mm). The size of the apertures is dictated on the one hand by the desire to maintain a low resistance connection and on the other hand by the desire to maximise the amount of flexion in the ultrasonic transducers 866 and 868. The upper ultrasonic housing 800 and its associated components as shown in Figure 87 are then covered by attaching the lower ultrasonic housing 814. In the preferred embodiment, one of three manufacturing techniques may be used to attach the upper ultrasonic housing 800 and the lower ultrasonic housing 814 together. They may be adhesively secured together, they may be ultrasonically welded together, or a potting material may be used to fill the interiors of both components to produce a potted assembly. The upper ultrasonic housing 800 is then adhesively attached to the cassette guide 510, with the flex circuit 825 extending through the chimney 515 of the cassette guide 510. The U-shaped ridge 802 and the straight ridge 804 fit into the interior of the lower segment 511 of the cassette guide 510, and the adhesive securely attaches the upper ultrasonic housing 800 to the cassette guide 510. The cassette guides 510 together with the slide locks 560 may then be mounted into the three pump positions on the pump chassis 370. In the first pump position, the flex circuit 825 which extends through the chimney 515 of the cassette guide 510 is fed through a rectangular aperture 427 in the pump chassis 370. A screw is introduced through the aperture 514 in the cassette guide 510 into the threaded aperture 402 in the pump chassis 370, and a second screw is introduced through the aperture 512 in the cassette guide 510 into the threaded aperture 404 in the pump chassis 370. In the second pump position, the flex circuit 825 which extends through the chimney 515 of the cassette guide 510 is fed through a rectangular aperture 429 in the pump chassis 370. A screw is introduced through the aperture 514 in the cassette guide 510 into the threaded aperture 406 in the pump chassis 370, and a second screw is introduced through the aperture 512 in the cassette guide 510 into a threaded aperture 408 in the pump chassis 370. In the third pump position, the flex circuit 825 which extends through the chimney 515 of the cassette guide 510 is fed through a rectangular aperture 431 in the pump chassis 370. A screw is introduced through the aperture 514 in the cassette guide 510 into the threaded aperture 410 in the pump chassis 370, and a second screw is introduced through the aperture 512 in the cassette guide 510 into the threaded aperture 412 in the pump chassis 370. By way of example, the cassette guide 510 and the slide lock 560 are shown mounted in the first pump position in Figure 101. The installation of the assembled cassette 302 at the first pump position, will now be described. The installation of the assembled cassette into the other two pump positions is identical to the installation into of the first pump position. With the slide latch 240 pulled back fully away from the front of the assembled cassette 302 (Figures 45 and 46), the wider portion of the elongate, tear-shaped aperture 258 in the slide latch 240 will close the tube 306, preventing fluid from flowing though the assembled cassette 302. The inlet tube 304 is connected to a fluid source such as an IV bag (not shown), and the delivery tube 303 is connected to a fluid delivery device such as an injection set (not shown), the use of which is well known in the art. The slide latch 240 is opened, together with any other closures in the IV bag line, and fluid fills the lines, the assembled cassette 302, and the injection set. By tapping or shaking the assembled cassette 302 any residual air bubbles will flow out through the line. The slide latch 240 is then pulled back and the outlet tube 306 is closed, and the system is in a primed condition with the assembled cassette 302 ready to be installed onto the main pump unit. Referring now to Figure 104, the rearmost edge of the assembled cassette 302 is tilted upwards in front of the first pump position. The angled position of the tubing adaptor 301 should also be noted. The rearmost edge of the top of the assembled cassette 302 is then placed against the bottom of the pump chassis 370 between the pressure transducer 660 (mounted flush with the bottom of the pump chassis 370) and the top side of the cassette guide 510, as shown in Figure 105. As the assembled cassette 302 is so positioned, the outlet tube 306 will begin to move into the funnel-shaped entrances to the slots 808 and 816 in the upper ultrasonic housing 800 and the lower ultrasonic housing 814, respectively. Simultaneously, the top of the tapered portion 305 of the tubing adaptor 301 will contact the ramps 820 and 822 on the lower ultrasonic housing 814, as shown in Figure 105. This engagement is key, since the ramps 820 and 822 will urge the tapered portion 305 of the tubing adaptor 301 rearwards toward the recessed area 818. The rear-most portion of the top of the assembled cassette 302 is slid towards the back of the pump chassis 370 into position between a left lateral support wall 384 on the left-hand side and a right lateral support wall 390 on the right-hand side. As this movement of the assembled cassette 302 rearwards into engagement with the main pump unit is occurring, the outlet tube 306 will continue to be pulled into the slots 808 and 816 in the upper and lower ultrasonic housings 800 and 814, respectively. The tapered portion 305 of the tubing adaptor 301 will slide back into the recessed area 818, as shown in Figure 106. Thus, the installation of the assembled cassette 302 into the main pump unit will automatically engage the outlet tube 306 in position between the ultrasonic transducers 866 and 868. The outlet tube 305 is deformed slightly in the slots 808 and 816 since the width of the slots 808 and 816 is less than the outer diameter of the outlet tube 306. This ensures good contact of the outlet tube 306 with the walls 810 and 812 in the upper ultrasonic housing 800, and thus good contact with the ultrasonic transducers 866 and 868. When the assembled cassette 302 is pushed fully back into place, the front of the assembled cassette 302 is tilted upwards against the bottom of the pump chassis 370, stretching slightly the outlet tube 306. The pumping operation of the system described above will not be described fully. Rather, for a complete description of the pumping operation, EP-A-319277 may be referred to. The air-in-line detector of the present invention uses the pair of ultrasonic transducers 866 and 868 (Figure 86) to detect the presence of air in the outlet tube 306 of the assembled cassette 302 (Figure 106). The basic principle of operation is simple: liquids readily propagate ultrasonic energy while air or foam is a poor conductor of ultrasonic energy, in practice, several orders of magnitude poorer than liquids. In the discussion of the operation of the system, it will be assumed that the ultrasonic transducer 866 is the transmitter and the ultrasonic transducer 868 is the receiver. When the ultrasonic transducer 866 is driven by an oscillating signal at a resonant frequency, it will vibrate at that frequency. As the driving frequency moves away from the resonant frequency, the vibration will diminish to a very small value at some distance away from the resonant frequency. Thus, the strength of the vibrations is at a maximum at the resonant frequency, and will diminish as the driving frequency moves either higher or lower than the resonant frequency. In order for the system to function at its optimum, the ultrasonic transducers 866 and 868 should have approximately the same resonant frequency. The vibrations from the ultrasonic transducer 866 are directed through a segment of tubing to the ultrasonic transducer 868, where they will cause an output from the ultrasonic transducer 868 which is proportional to the strength of the vibrations received by the ultrasonic transducer 868. If there is a good conduit of vibrations between the transducers 866 and 868, the output from the transducer 868 will closely resemble the resonant input signal used to drive the transducer 866. When ultrasonic vibrations are generated by the transducer 866, they must pass through the outlet tube 306 to reach the transducer 868. If the outlet tube 306 contains liquid at the location between the ultrasonic transducers 866 and 868, the ultrasonic vibrations will easily pass through. On the other hand, if there is air in the outlet tube 306 at the location between the transducers 866 and 868, the ultrasonic vibrations will become greatly attenuated and a much lower signal (two orders of magnitude lower) will be detected. A simplified overview of the operation of the entire pump system is illustrated in Figure 107. A pump control system 886 is used to drive a power module 888, which in turn operates a pump 890. An encoder 892 is used to supply position information from the power module 888, which position information will indicate both the position of the pump 890 (which in the present system is a piston-type pump located in the assembled cassette 302) and the amount of fluid pumped by the pump 890. The pump 890 pumps fluid from a fluid input through a pressure transducer 894, and then through an ultrasonic air-in-line detector (AILD) 896 to a fluid output. The encoder 892 provides an encoder output which is supplied to the pump control system 886 as a feedback signal. The pressure transducer 894 provides a pressure output signal which is supplied to the pump control system 886 for use in monitoring the pressure to detect an occluded line situation. The AILD scheme used by the system of the present invention has two additional components, namely an AILD monitoring system 898 and a self-test system 900. The ultrasonic AILD 896 supplies two signals to the AILD monitoring system 898 and the self-test system 900, specifically, an interrupt signal and an AILD output signal. The nature of these two signals will become evident in the detailed discussion below. The AILD monitoring system 898 is used to monitor the signals from the ultrasonic AILD 896 to determine when air is present in the liquid line. More particularly, in the preferred embodiment the AILD monitoring system 898 will be used to determine when a predetermined amount of air has passed through the line during the passage past the sensor of a particular quantity of pumped volume, which is called a volume window. When the predetermined amount of air has been present in the liquid line during a volume window, an alarm will be sounded and the pumping of fluid will be halted. The concept of a volume window will be explained in detail below. The self-test system 900 is used periodically to ensure that the ultrasonic AILD 896 is functioning properly, and not giving false assurances that there is liquid in the line when in fact there is air in the line. The self-test system 900 functions by providing a test signal to the ultrasonic AILD 896 causing it to operate during the self-test at a frequency which is not resonant. Thus, during the self-test procedure a signal should be generated which would otherwise indicate the presence of air in the line. The generation of an air-in-line signal during the self-test procedure is an indication that the system is functioning properly. Referring next to Figure 108, a clock having an operating frequency of 3.072 MHz is used to drive the transmitter circuitry. The clock signal is supplied to a duty cycle generator 902, which generates a 166 uS low pulse once every 1.33 mS (750 Hz). The 750 Hz rate is chosen to be sufficiently often to detect a bubble at even the highest flow rates through the outlet tube 306. The pulse is thus on a one-eighth duty cycle, which is used to conserve power in the system. The output pulse train of the duty cycle generator 902 is supplied as the inhibit input to a voltage controlled oscillator (VCO) 904. The output pulse train from the duty cycle generator 902 is also supplied as an input to an inverter 906. The output of the inverter 906 is supplied to one side of a resistor 908, the other side of which is connected to the VCO in pin of the VCO 904. A capacitor 910 is connected on one side to the VCO in pin of the VCO 904, and on the other side to earth. The resistor 908 and the capacitor 910 act as an RC integrator to integrate the inverted inhibit waveform. The inhibit waveform supplied to the VCO 904 and the VCO input wave form supplied to the VCO 904 are illustrated in Figure 111. The output of the VCO 904 will be a variable frequency sweeping from a lower frequency to a higher frequency. The resonant frequency of the ultrasonic transducers 866 and 868 is nominally 1.8 MHz. Unless the ultrasonic transducers 866 and 868 are high precision devices, the exact resonant frequencies may vary somewhat, and may also vary slightly over a period of time. Thus, the VCO 904 is used to generate a variable frequency sweeping from, for example, 1.3 MHz to 2.3 MHz, a sweep which is certain to include the resonant frequency of the ultrasonic transducers 866 and 868. This sweep will be generated on the one-eighth duty cycle as shown in Figure 111, thereby conserving the energy required by the VCO 904 while repeating the sweep on a 750 Hz frequency to detect bubbles even at the fastest flow rate. Referring again to Figure 108, the output of the VCO 904 is supplied to one input side of three single-pole, double-throw switches 912A, 912B, 912C. The other input side of these switches 912A, 912B, 912C is connected directly to the 3.072 MHz clock. The outputs of the switches 912A, 912B, 912C may thus be switched between the output of the VCO 940 and the 3.072 MHz clock. Normally, the outputs of the switches 912A, 912B, 912C are connected to the output of the VCO 904. Only when the self-test is to be performed are the outputs of the switches 912A, 912B, 912C connected to the 3.072 MHz clock signal. The outputs of the switches 912A, 912B, 912C are connected to the input side of three inverters 914A, 914B, 914C, respectively. The outputs of the three inverters 914A, 914B, 914C are connected to the inputs of three buffers 916A, 916B, 916C, respectively. The three buffers 916A, 916B, 916C are each contained on one of the printed circuit boards 884 (Figure 878) used for the three channels. The outputs of the three buffers are connected to one side of three (one for each channel) ultrasonic transducers 866A, 866B, 866C, respectively. The other sides of the three ultrasonic transducers 866A, 866B, 866C are earthed. Referring again to Figure 111 in addition to Figure 108, it is apparent that the three ultrasonic transducers 866A, 866B, 866C will be excited by the sweeping frequency from 1.3 MHz to 2.3 MHz on a one-eighth duty cycle once every 1.33 ms (750 Hz). This is frequent enough so that even at the maximum pumping rate only a small amount of liquid can pass the position of the ultrasonic transducer pairs between sequential ultrasonic transmissions. The one-eighth duty cycle conserves energy used by both the VCO 904 and the three ultrasonic transducers 866A, 866B, 866C. Figure 109 illustrates the receiver circuitry used for one of the three channels, with the other two channels using identical circuitry. The receiving transducer for the first channel is the ultrasonic transducer 868A, the output of which is supplied to a cascode preamplifier 918A. The output of the cascode preamplifier 918A will be a signal increasing in strength at the resonant frequency when fluid is present, and thus having a triangular envelope as illustrated in Figure 111. The output of the cascode preamplifier 918A is supplied to a detector/rectifier 920A, the output of which is the rectifier output shown in Figure 111. The output of the detector/rectifier 920A is supplied to a first comparator 922A, which produces the waveform shown in Figure 111 when the envelope from the detector/rectifier 920A is below a threshold. The output from the first comparator 922A is supplied to an RC timer/second detector 924A, which integrates the output from the first comparator 922A, as shown in Figure 111. The integrated output is reset each time there is a signal from the ultrasonic transducer 868A which is over the threshold of the first comparator 922A. When there is air in the line, the integrated signal will not be reset, causing it to reach the threshold of the second comparator. At this point, the output of the sensor A circuitry will go low. In summary, when there is liquid in the outlet tube 306, the ultrasonic transducer 868A will receive a strong signal, and a high sensor A output will be given indicating the presence of liquid in the outlet tube 306. When there is air in the outlet tube 306, the ultrasonic transducer 868A will receive a weak signal, and a low sensor A output will be given indicating the presence of air in the outlet tube 306. Circuitry identical to that shown in Figure 109 is used for the other two channels. Figure 110 shows additional processing circuitry used to obtain the two signals used by the AILD monitoring system 898 and the self-test system 900 of Figure 107. The sensor A output is supplied to the D input of a latch 924A, the output of which is AILD output A. AILD output A will be low when there is liquid in the outlet tube 306, and high when there is air in the outlet tube 306. AILD output A is supplied to an edge detector 926A (one possible circuit for which is illustrated), the output of which will be a Channel A edge signal indicating either a rising or a falling edge in AILD output A. Thus, whenever an air/liquid interface is detected, the edge detector 928 A will produce an output signal. The other two channels use similar circuitry to produce corresponding signals. Thus, an AILD output and a Channel B edge signal will be produced by circuitry for Channel B. Similarly, an AILD output C and a Channel C edge signal will be produced by circuitry for Channel C. The Channel A edge signal, the Channel B edge signal, and the Channel C edge signal are supplied to an OR gate 930. The output of the OR gate 930 will be high if any of the three inputs are high. Thus, whenever an edge is present in any of AILD output A, AILD output B, or AILD output C, the output of the OR gate 930 will be high. The output of the OR gate 930 is used to latch a latch 932 high, to generate an interrupt signal AILD IRQ. This interrupt signal indicates that a change in state of one of AILD output A, AILD output B, or AILD output C has occurred. Thus, the circuitry of Figure 110 will generate two signals. The first signal indicates the presence of air or liquid in the outlet tube 306 of a channel, and the second signal indicates a change in state in one of the three channels. The first signal thus comprises the signals AILD output A, AILD output B, or AILD output C, while the second signal is the interrupt signal AILD IRQ. For the rest of the explanation of the operation of the system, only the first channel (channel A) will be discussed. The operation of the other two channels (channels B and C0 is identical to the operation of the first channel. Prior to a discussion of the operation of the AILD monitoring system 898, the concept of controlling the amount of air which may be pumped into a patient must first be discussed. First, it must be realised that it is not harmful to pump a small amount of air intravenously into many patients; in fact, many medications are not degassed and will contain air which may form small bubbles. Some patients (who are few in number) can tolerate the introduction of no air whatsoever into their venous systems, such as neonates, pediatrics, and those patients having septal defects. Other than when infusing fluid into such patients, or performing an intra-arterial infusion, the introduction of very small quantities of air is not believed to be particularly harmful. The attending physician also has the option of using air eliminating filters in such patients. The other problem faced in monitoring air in the fluid line to a patient is that it is undesirable to have too many alarms due to extremely small amounts of air being infused into most patients. The professional staff in most hospitals tend to view such frequent alarms as nuisance alarms which are undesirable and serve no useful purpose. Thus, the real purpose of an AILD system is to prevent unduly large, potentially dangerous quantities of air from being pumped into a patient. It is therefore necessary for the AILD system to allow some air past it without giving an alarm, since a failure to do so could result in a large number of nuisance alarms. The AILD system must always alarm at some threshold, which has been selected at a high enough value to prevent nuisance alarms but yet is low enough to sense uniformly an amount of air presenting even a remote threat to the health of the patient. This objective may be implemented by using the concept of windowing. The concept of windowing is when the passage of air bubbles in the immediately preceding preset volume of fluid is remembered. Such a window is used to monitor the amount of air which may be included in the most recent amount of a particular volume pumped to the patient. For example, in the last 2 millilitres of volume pumped, less than 100 microlitres of air may be present without an alarm. As soon as 100 microlitres of air is present in the last 2 millilitres of volume pumped, an alarm is to be given. This may be seen as a "forgetting" factor in which all air bubbles pumped prior to the last 2 millilitres of volume pumped are forgotten by the system. Such a volume window allows a particular amount of air less than a predetermined volume to be pumped within the last predetermined window volume. In the preferred embodiment the predetermined volume is one-twentieth (0.05) of the window volume. The window volume may be up to three millilitres, which is less than the volume of the delivery tubing 303. Thus, for a 50 microlitre predetermined volume, the window volume would be 1 millilitre, and for a 100 microlitre predetermined volume, the window volume would be 2 millilitres. In some circumstances a larger predetermined volume may be appropriate. In any event, it will be realised by those skilled in the art that the proportion could be varied from perhaps one-one hundredth (with a substantial increase in the number of nuisance alarms) to perhaps as low as one-sixth (with special precautions such as the use of an air filter being taken). The preferred proportion is approximately one-twentieth. The windowing scheme used by the present invention uses two pieces of information to determine whether the system has just pumped air or liquid in the immediately preceding time period since the net previous update. First, the sensor will detect whether there is currently air in the line at the sensor location. The second piece of information is whether at the immediately preceding time period at which information was being gathered there was air or liquid at the sensor location. This second information will thus indicate whether the bubble currently sensed is a continuation of a bubble started earlier, or the leading edge of a new bubble. Thus, whether the system has just been pumping liquid or air in the immediately preceding time interval since the last update may be determined. For example, if the current sensor reading indicates air in the line and the immediately previous reading was also air, then there is at the present time a continuing air bubble present in the line. If the current sensor reading indicates air in the line and the immediately previous reading was liquid, then the leading edge of an air bubble has been sensed. If the current sensor reading indicates liquid in the line and the immediately previous reading was air, then the trailing edge of an air bubble has been sensed. If current sensor reading indicates liquid in the line and the immediately previous reading was also liquid, then there is at the present time a continuing segment of liquid present in the line. The operation of the AILD monitoring system 898 will now be discussed with reference to the flow chart of Figure 112. The operation is a circuitous one, repeating at a high frequency, and beginning at block 934. Since the system discussed is a three channel system, only the operation of the first channel (Channel A) will be discussed; the operation of the other two channels (Channels B and C) is identical. In block 934 it is determined whether an interrupt signal AILD IRQ has been generated. If no interrupt signal has been generated, the operation goes to block 936. If an interrupt signal has been generated, the latch 932 (Figure 110) is reset by an AILD IRQ CLR signal on pin C. The operation would then proceed to block 938. In block 936 it is determined whether the end of a delivery stroke in the pump 890 (Figure 107) has been reached. If the end of a delivery stroke has not been reached, the operation would then proceed to block 938. Thus, it is apparent that the chain of events beginning at block 938 will be initiated either if an interrupt signal is generated or if the end of a delivery stroke has been reached. In block 938 the AILD output is read; for channel A, AILD output A would be read. Then, in block 940, the encoder ouput (for encoder A) is read. This will indicate how much volume has been pumped since the last time the operation occurred. Then, in block 942, the pressure output (for channel A) is read. This may be used to normalize the volume pumped using Boyle's law ( P¹*V¹=P²*V² ). Then, in block 944, a determination is made whether AILD output A indicates that there is currently air in the line at the sensor location. This is the first piece of information mentioned above, and it enables the system to divide into one of two branches depending on the outcome of the determination. If there is currently air in the portion of the liquid line where the sensor is located, the system moves to block 946; if there is currently no air in the portion of the fluid line where the sensor is located, the system moves to block 948. The operations which follow block 946 thus follow a determination that there is currently air in the tubing at the sensor location. Similarly, the operations which follow block 948 follow a determination that there is currently no air in the tubing at the sensor location. In each case, the second piece of information, whether in the immediately preceding time period at which information was gathered there was air or liquid at the sensor location, must next be evaluated for each of the two possibilities in blocks 946 and 948. First in block 946, a determination is made as to whether in the immediately preceding cycle during which information was gathered there was air or liquid at the sensor location. If the determination is made that there was air in the tubing at the sensor location at the time of the next previous update, the system will move to block 950. If, on the other hand the determination is made that there was no air in the tubing at the sensor location at the time of this next previous update, the system will move to block 952. Thus, block 950 will be reached if the current sensor reading indicates air in the line and the immediately previous reading also indicated the presence of air in the line. In this case, there is an air bubble in the line which existed at the next previous sensor reading and which still exists. Thus, in the block 950 the additional volume of the air bubble between the time of the next previous sensor reading and the present time is computed. Then, in block 954, the window is updated to calculate how much of the volume window is currently air bubbles. In block 954 the additional volume of the air bubble between the time of the next previous sensor reading and the present time is added to the volume of air contained in the volume window, and air bubbles now beyond the back edge of the window are subtracted from the volume of air contained in the volume window. In this manner, the volume window is updated to determine the volume of gas bubbles in the last volume window volume to pass through the ultrasonic sensor. The sequence would then move to block 960, in which a determination is made as to whether the portion of the volume window which is air bubbles exceeds the predetermined maximum. If the portion of the volume window which is air bubbles does exceed the predetermined maximum, the system moves to block 962, and an alarm is sounded and the pumping of liquid by the system will be halted. If the portion of the volume window which is air bubbles does not exceed the predetermined maximum, the system moves back to block 934. Block 952 will be reached if the current sensor reading indicates air in the line and the immediately previous reading indicated the presence of liquid in the line. In this case, there is an air bubble in the line which did not exist at the next previous sensor reading, but rather has just started (the starting edge of the bubble has been detected). Thus, in block 952 the additional volume of the liquid between the time of the next previous sensor reading up to the beginning of the bubble is computed. Then, in block 956, the window is updated to calculate how much of the volume window is air. In the preferred embodiment, an allowance is made for the fact that an air bubble must be at least a minimum size before it can be detected. Thus, when an air bubble is first detected, it is assumed that it is at least this minimum bubble size up to this point. The minimum bubble size used in the preferred embodiment is 6 microlitres. In block 956, since there is liquid between the time of the next previous sensor reading and the present time, only the minimum bubble size of 6 microlitres is added to the volume of air contained in the volume window, and air bubbles now beyond the back edge of the window are subtracted from the volume of air contained in the volume window. In this manner, the volume window is updated to determine the volume of air bubbles in the last volume window volume to pass through the ultrasonic sensor. In block 958, the window information is switched to indicate that the present information, soon to become the next previous update, indicates the presence of air. Thus, the next time the system moves through the loop, the second piece of information will indicate that at the previous update, there was air present in the tubing. The sequence would then move to block 960, in which a determination is made as to whether the portion of the volume window which is air bubbles exceeds the predetermined maximum. If the portion of the volume window which is air bubbles exceeds the predetermined maximum, the system moves to block 962, and an alarm is sounded and the pumping of fluid by the system will be halted. If the portion of the volume window which is air bubbles does not exceed the predetermined maximum, the system moves back to block 934. Alternatively, if there is presently no air in the line in block 944, the system would have moved to block 948. In block 948, a determination is made as to whether at the immediately preceding time period at which information was gathered there was air or liquid at the sensor location. If the determination is made that there was air in the tubing at the sensor location at the time of this next previous update, the system will move to block 964. If, on the other hand the determination is made that there was no air in the tubing at the sensor location at the time of this next previous update, the system will move to block 966. Thus, the block 964 will be reached if the current sensor reading indicates a lack of air presently in the line, but the immediately previous reading indicated the presence of air in the line. In this case, there was an air bubble in the line which existed at the next previous sensor reading, but which bubble ended (the trailing edge of an air bubble has been detected). Thus, in the block 964 the additional volume of the gas bubble between the time of the next previous sensor reading and its ending point at the present time is computed. Then, in block 968, the window is updated to calculate how much of the volume window is air bubbles. In block 968 the additional volume of the air bubble from the time of the next previous sensor reading which ended at the present time is added to the volume of air contained in the volume window, and air bubbles now beyond the back edge of the window are subtracted from the volume of air contained in the volume window. In this manner, the volume window is updated to determine the volume of air bubbles in the last volume window volume to pass through the ultrasonic sensor. In block 972, the window information is switched to indicate that the present information, soon to become the next previous update, indicates the absence of air. Thus, the next time the system moves through the loop, the second piece of information will indicate that at the previous update, there was no air present in the tubing. The sequence would then move to block 960, in which a determination is made as to whether the portion of the volume window which is air bubbles exceeds the predetermined maximum. If the portion of the volume window which is air bubbles exceeds the predetermined maximum, the system moves to block 962, and an alarm is sounded and the pumping of fluid by the system will be halted. If the portion of the volume window which is air bubbles does not exceed the predetermined maximum, the system moves back to block 934. It must be realised that the flow chart of Figure 112 represents a highly simplified example of how the system may be implemented to perform the windowing function. Those skilled in the art will immediately understand the principles behind this operation, and will be able to implement it in a variety of ways. The advantages of the technique are self-evident; the pumping of an excessive amount of air into a patient is avoided, while the occurrence of nuisance alarms is also avoided. Turning now to Figure 113, the operation of the self-test system is illustrated in a simplified manner. The self-test is performed in the preferred embodiment once per cycle after it has been determined that the end of a delivery cycle has been reached, assuming that the portion of the volume window which is air bubbles did not exceed the predetermined maximum. The initial determination is made in block 980 whether the end of a delivery cycle has been reached. If the end of a delivery cycle has been reached, the system moves to block 982. If the end of a delivery cycle has not been reached, the system moves back to the beginning of block 980. A determination is made in block 982 whether AILD output A indicates that there is currently air in the line at the sensor location. If there is air in the line, the self-test may not be run, and the system moves back to the beginning of block 980. If there is not currently air in the sensor, the system moves to block 984. In block 984, the frequency supplied to the ultrasonic transducer 866A is changed to a non-resonant frequency. (Referring briefly to Figure 108, the switch 912A would be switched to connect the 3.072 MHz clock to the inverter 914A.) This frequency is far enough from the resonant frequency that the ultrasonic transducer 868A will not resonate. At this point, the AILD output A should indicate air and an interrupt signal should quickly be generated. If a signal is generated by the ultrasonic transducer 868B, this would indicate that there is a failure in the ultrasonic transducer 868B or in the associated electronics. Accordingly, in block 986, if the interrupt signal does not appear within a preset time it will be apparent that there is an error, and the AILD fault signal 987 will be sounded and the pumping operation halted. If the interrupt signal appears within the preset time, it is an indication that the system is functioning properly, the system will move on to block 988. In block 988, the frequency supplied to the ultrasonic transducer 866A is changed back to the periodic resonant frequency encompassing sweep. (Referring briefly to Figure 108, the switch 912A would be switched to connect the output of the VCO 904 to the inverter 914A.) The system will move back to the beginning of block 980, and the sequence will be repeated. Through the above discussion of the entire system, it will be appreciated that the present invention provides a unique air-in-line detection system which will not alarm merely due to the presence of a tiny amount of air detected in the fluid path. The system of the present invention is designed to retain all of the advantages of known ultrasonic air-in-line detection devices, and it also provides a number of additional advantages and improvements. The ultrasonic air-in-line detection system of the present invention is capable of detecting even very small air bubbles in the fluid line of a disposable cassette near its output end, after the pumping operation has been performed. The system of the present invention discriminates between a situation when a significant amount of air is present in the fluid line, not alarming in the former case and invariably alarming in the latter case. Examples of other additional features included in the air-in-line detection system of the present invention are the ability to detect air bubbles whether the flow rate of the fluid in the cassette is fast or slow, and the ability to detect air in the fluid line even when the interior of the fluid line remains coated with liquid. The system is capable of accurately and effectively detecting air bubbles in any type of fluid which may be infused, whether the fluid is clear or opaque. The system accomplishes all these objects in a manner which retains and enhances the advantages of reliability, durability, and safety of operation inherent in an ultrasonic detector. The system of the present invention provides all of these advantages and overcomes the limitations of the background art without incurring any relative disadvantage. The present invention therefore results in a superior medication infusion system having a number of advantages which make the system a highly desirable alternative to systems presently available.

Claims: A system for detecting the presence of a significant amount of air in liquid passing through a fluid passageway (306) in a medical infusion system, comprising a sensor (896) disposed adjacent the fluid passageway (306) for providing an air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway (306); means for monitoring (898) the volume pumped through the fluid passageway, the monitoring means providing a volume output signal indicative of the volume pumped; means responsive to the air-in-line output signal and the volume output signal, for estimating a volume of air in the fluid passageway, and for providing an output representing the said volume of air; and means for providing an alarm signal (962) if the volume of air output is equal to or greater than a volume representing a second predetermined volume; characterised in that the sensor (896) is a single ultrasonic sensor which comprises a transmitter (866) for transmitting an ultrasonic signal applied to the fluid passageway (306) at a single location and a receiver (868) for detecting whether the ultrasonic signal passes through the fluid passageway (306) at a single location and for providing the air-in-line signal indicating the current presence of liquid or air in the fluid passageway at the single location monitored by the ultrasonic sensor; the means responsive to the air-in-line output signal and the volume pumped signal being calculating means arranged to calculate the volume of air contained in the last predetermined volume pumped through the fluid passageway, whereby the volume of air output represents the volume of air contained in the last predetermined volume pumped. A system as claimed in Claim 1, characterised in that the transmitter comprises a first ultrasonic transducer (866) located on one side of the fluid passageway and means for driving (904) the first ultrasonic transducer (866) to generate ultrasonic vibrations which are transmitted to the said one side of the fluid passageway (306), and the receiver comprises a second ultrasonic transducer (868) located on the other side of the said fluid passageway (306), the ultrasonic vibrations passing through the fluid passageway and being received by the second ultrasonic transducer (868) when there is liquid in the fluid passageway, the ultrasonic vibrations substantially not passing through the fluid passageway and not being received by the second ultrasonic transducer when there is an air bubble in the fluid passageway and, the receiver also including means for detecting the ultrasonic vibrations received by the second transducer and providing the air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway between the first and second transducers. A system as claimed in Claim 2, characterised in that the first and second ultrasonic transducers (866, 868) have a resonant frequency and the driving means (904) generates an oscillating signal which sweeps from a first frequency to a second frequency, the resonant frequency being between the first and second frequencies. A system as claimed in Claim 2 or Claim 3, characterised in that the detecting means comprises an amplifier (918A) for amplifying an electrical signal generated by the second ultrasonic transducer (868), a rectifer (920A) for rectifying the amplified electrical signal generated by the second ultrasonic transducer, and a first comparator (922A) for producing a first comparator output signal when the recitified, amplified electrical signal generated by the second ultrasonic transducer exceeds a first threshold. A system as claimed in any of Claims 2 to 4, characterised in that the driving means (904) operates periodically to drive the first ultrasonic transducer (866) to generate ultrasonic vibrations, preferably on a one-eighth duty cycle. A system as claimed in any of the Claims 2 to 5, characterised in that the driving means (904) operates at a frequency sufficiently high to ensure that an air bubble will be detected at even the highest possible rate of fluid flow through the fluid passageway (306), for example a frequency of approximately 750Hz. A system as claimed in any preceding Claim, charaterised in that it additionally comprises a piston pump (890) for pumping fluid through the fluid passageway (306) and means (888) for driving the pump, and the monitoring means comprises and encoder (892) for providing position information describing the position of the piston pump (890) and a decoder (898) for decoding the position information to determine how much volume has been pumped from a first item of position information to a second item of position information. A system as claimed in Claim 7, characterised in that it additionally comprises means for measuring the pressure (894) in the fluid passageway, and means (898) for modifying the volume as determined by the decoder according to the measured pressure in the fluid passageway to produce the volume output signal. A system as claimed in any preceding Claim, characterised by means for selecting the size of the last predetermined volume pumped from a plurality of selections. A system as claimed in any preceding Claim, characterised in that the size of the last predetermined volume is less than the volume of the fluid passageway from the location monitored by the ultrasonic sensor to a patient, and is for example between approxiamtely 100 millilitres and 300 millilitres. A system as claimed in any preceding Claim, characterised in that second predetermined volume is between one-twentieth and one-sixth of the last predertermined volume, and is preferably one-sixth of the size of the last predetermined volume. A system as claimed in any preceding Claim, characterised in that the means for providing an alarm signal (962) provides an audible or visible alarm signal and shuts down the pumping operation through the fluid passageway (306). A method of detecting the presence of a significant amount of air in fluid passing through a fluid passageway (306) in a medical infusion system, which comprises providing an air-in-line output signal indicative of whether there is presently air or liquid in the fluid passageway; monitoring the volume pumped through the fluid passageway and providing a volume output signal indicative of the volume pumped; estimating from the air-in-line output signal and the volume output signal, a volume of air in the fluid passageway and providing an output representing the said volume of air; and providing an alarm signal if the volume of air output is equal to or greater than a second predetermined volume; characterised in that the sensor (896) is a single ultrasonic sensor (896) disposed adjacent the fluid passageway (306) at the particular location; the ultrasonic sensor comprising a transmitter (866) which transmits an ultrasonic signal applied to the fluid passageway at the particular location, and a receiver (868) which detects whether the ultrasonic signal passes through the fluid passageway at the particular location and which provides the air-in-line signal to indicate the current presence of liquid or air in the fluid passageway at the particular location; the method being further characterised in that the estimation step comprises calculating the volume of air contained in the last predetermined volume pumped through the passageway, whereby the volume of air output represents the volume of air contained in the last predetermined volume pumped.

Inventors: HENKE JAMES L, KOENIG PAUL A, HENKE, JAMES L., KOENIG, PAUL A.
Applicants: IVAC CORP, IVAC CORPORATION
Application year: 1990
Main classification: A61M5
IPC codes: A61M5, G01N29