Determination of pressure conditions in an infusion system

Abstract: An apparatus for determining if fluid is being properly supplied through a fluid delivery system to a patient allows fluid to be infused into a patient so as to create a pressure wave response (67) in the delivery system. The apparatus comprises means (59) responsive to the pressure in the delivery system when the pressure wave response (67) is not present to ascertain if pressure conditions in the delivery system are suitable for detecting if fluid is being improperly supplied by the delivery system to the patient; and means (59) responsive to the pressure wave response for detecting if fluid is being improperly supplied by the delivery system to the patient.

Description: The invention concerns an apparatus for determining if fluid is being properly supplied to a patient. An apparatus comprising the features specified in the preamble of Claim 1 is disclosed in US-A-4 648 869. It is often necessary or desirable to infuse a flowable material or fluid, which may be liquid, a gas or a combination thereof, into a patient. One example is the administration of parenteral fluids to a patient. A typical infusion system includes an infusion device for delivering the fluid and conduit means for conducting the flowable material from the infusion device to the patient. The conduit means typically comprises flexible tubing leading from the infusion device and a cannula, such as a needle or catheter, for insertion into the vascular system of the patient. In normal operation, the infusion device delivers the fluid through the tubing and the needle to the vascular system of the patient. One problem with infusion systems of this type is a condition known as infiltration. Infiltration is a condition in which infused fluid finds its way into extravascular tissues rather than simply being released into the blood stream. Such a situation occurs when the needle is not in communication with the interior of the vessel into which the fluid is to be infused. When this occurs, fluid is infused into the interstitial spaces between layers of tissues. Thus, the patient is deprived of proper intravenous drug administration and is further subjected to possible toxic or caustic effects associated with infused fluids being in direct contact with body tissues. Infiltration is not the only possible type of anomaly associated with intravenous therapy which can cause the fluid to be improperly supplied to the patient. Other conditions which can cause abnormal infusion, i.e., the fluid to be improperly supplied to the patient, include venous inflamation and swelling at the infusion site (phlebitis), clotting and a wide variety of obstructions of the conduit means, such as kinking of the tubing which supplies the fluid to the patient. Many of these affect fluid flow characteristics in a manner similar to infiltration and can, therefore, be detected by infiltration detection devices. The goal of an infiltration detection system is to identify an abnormal infusion as early as possible without generating an excessive number of false alarms. Early detection allows the attending medical staff to rectify the problem before significant damage has been done by the infiltration and before the patient has been deprived of a significant amount of the intravenous therapy. On the other hand, if the detection system is too sensitive, false alarms will result. This is very undesirable since, from a clinical perspective, establishing a new intravenous site can be difficult and time consuming. During the time necessary to start the new IV, which can be hours in some cases, the patient is not receiving the desired treatment. Bobo U.S. Patent No. 4,648,869 discloses a significant advance in the field of infiltration detection systems and methods. According to the Bobo patent, an infusion system infuses a test pulse of fluid to a patient. The test pulse creates a pressure wave response which can be monitored and used to detect if abnormal infusion has occurred. Such a system is also known from EP-A-0 248 633. Butterfield US Patent No 4 710 163 discloses an infiltration detection system which uses the test pulse-pressure wave response concept of the Bobo patent. However, the Butterfield system compares the pressure wave response with a reference pressure wave response which represents the normal response when there is no infiltration. Specifically, the area between two curves representing these responses is used to attempt to detect infiltration. Thus, the Butterfield approach has the disadvantage of requiring that a normal pressure wave response be first determined and then stored for later comparison. The present invention provides an apparatus for determining if fluid is being properly supplied through a fluid delivery system to a patient, said apparatus comprising: means for delivering fluid through the delivery system so as to create a pressure wave response in the delivery system and means responsive to the pressure wave response for detecting if fluid is being improperly supplied by the delivery system to the patient; characterized in that the apparatus further comprises: further means responsive to the pressure in the delivery system, during the normal delivery pattern when the pressure wave response is not present, and for comparing a function of said pressure to a threshold to ascertain if pressure conditions in the delivery system are suitable for detecting if fluid is being improperly supplied by the delivery system to the patient. It has been found that the occurrence of a baseline pressure during the normal delivery pattern which is unusually large in magnitude or which fluctuates excessively, suggests that the pressure conditions in the conduit are not then suitable for detecting abnormal infusion. Such baseline conditions may be the result of a transient condition, such as movement by the patient. When these pressure artifacts occur, either the application of the test pulse is deferred or the data derived from such test pulse is ignored. The presence of these unacceptable pressure disturbances in the baseline indicate that the pressure conditions are not suitable for detecting abnormal infusion, and one reason for this is that these pressure artifacts would be superimposed upon the pressure wave response. This would tend to unacceptably alter the pressure wave response such that the pressure wave response would not be indicative of the health of the IV site, but rather indicative of some other condition, such as unusual patient movement. This feature of the invention is applicable to virtually any abnormal infusion detection system, which is responsive to the pressure wave response, and is not limited to use with a system which detects abnormal infusion by integration techniques. More specifically, the suitability of the pressure conditions in the conduit can be determined by comparing a function of the pressure of the fluid in the conduit during the normal delivery pattern to a threshold. This function of the pressure may he one or more pressure values or may be a function which is derived from one or more pressure values. In a preferred technique, the function of the pressure equals B1+K(B2) where B1 is the baseline pressure at an instant prior to the test pulse, K is a constant, and B2 is the rms value of a plurality of segments of the baseline pressure prior to the test pulse, and with at least one of the segments being prior to such instant. The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings. Fig. 1 is a block diagram illustrating one form of infusion system constructed in accordance with the teachings of this invention. Figs. 2-2E are plots of volume delivered by the infusion device versus time illustrating different example of test pulses. Fig. 3 is a plot showing one pressure wave response indicative of the fluid being properly supplied to the vessel of a patient and a second pressure wave response indicative of infiltration. Fig. 4 is a plot showing the relationship of the pressure response to the infusion rate for infiltration conditions and normal conditions. Fig. 5 is a flow chart showing how the system functions to detect infiltration. Fig. 1 shows an infusion system 11 which comprises a source 13 of a parenteral fluid, an infusion device 15 for delivering the parenteral fluid through conduit means 17 to a patient. The conduit means 17 may comprise flexible tubing or other flow channels for suppying the parenteral fluid to the patient. The conduit means terminates in a needle 18, such as an I.V. needle, which is adapted to be inserted into a vessel of the patient's vascular system so that the open distal end of the needle communicates with the interior of the vessel. In this embodiment, the needle 18 is inserted into a vein. If the open distal end of the needle communicates with tissue, as when the needle is forced completely through the vessel wall, infiltration has occurred. The infusion device 15 may be any infusion device which is controllable to produce a test pulse 19 (Fig. 2) and, as such, may include an infusion pump, a controller, syringe or the like. In this embodiment, the infusion device 15 includes a motor, such as a stepping motor 21, for driving the pump and pump controls 22 for controlling the motor. The pump 20 is a positive displacement pump, and accordingly, its output can be controlled by controlling the speed of the motor 21. The pump controls 22 control the motor speed as described more fully hereinbelow to provide the infusion device with the desired output. In a preferred construction, the infusion device 15 is a peristaltic pump of the type disclosed in Application Serial No. 661,032 entitled Continuous Delivery Peristaltic Pump and filed on October 15, 1984. Such an infusion pump has a normal delivery pattern 25 which, in this example, is essentially constant as shown by the flat portions of the pump delivery curve of Fig. 2. This is the result of accelerating through the deadband of the peristaltic pump. The curve of Fig. 2 is somewhat idealized in that the preferred infusion pump provides periodic short spikes and valleys of exceedingly short duration; however, these are sufficiently insignificant so that the normal delivery pattern of the pump can be considered as essentially constant, although a constant flow rate during the normal delivery pattern is not required. The pump control 22 periodically, and/or on demand, increase the speed of the stepping motor 21 to cause the infusion pump 20 to provide the test pulse 19 which, in the illustrated embodiment of Fig. 2, is in the form of an essentially square wave having a duration of approximately four seconds. As described more fully in Bobo Patent No. 4,648,869 the infusion rate, and hence the volume delivered, during the test pulse preferably varies with the selected infusion rate for the infusion device 15. However, the duration of the test pulse 19 may be constant for all selected infusion rates. Selection of the infusion rate also results in selection of the associated flow rate for the test pulse 19. In this regard, the pump controls 22, as is common for infusion devices of this type, are programmable to enable the attendant to select or punch in a desired or selected infusion rate. The pump controls 22 reduce the speed of the stepping motor 21 just before and just after each test pulse 19 to cause the infusion pump 20 to provide separating regions, which in this embodiment, are leading and trailing infusion valleys 27 and 29, respectively, contiguous to, and on opposite sides of, the test pulse 19. The valleys 27 and 29 are square waves of short duration during which the infusion rate is reduced sufficiently to wholly or partially compensate for the increased infusion rate which takes place during the test pulse 19. Preferably, the valleys 27 and 29 reduce the total flow by the same amount that the test pulse increases it so that the average or net effect across the valleys 27 and 29 and the test pulse 19 is an infusion rate equal to the rate represented by the normal delivery pattern 25. For example, each of the valleys 27 and 29 may have a duration which is twice as long as the duration of the test pulse 19, with such duration being 8 seconds in this embodiment and constant for all selected infusion rates. The test pulse 19 and the valleys 27 and 29 constitute an altered pattern of flow. Test pulses can be provided in various different ways, and additional examples of test pulses, which can be distinguished from the normal delivery pattern, are shown in Figs. 2A-2E. Portions of the curves shown in Figs. 2A-2E corresponding to portions of the curve shown in Fig. 2 are designated by corresponding reference numerals followed by the letter "a", "b", "c", "d", and "e", respectively. In Fig. 2A, the test pulse 19a is separated from the normal delivery pattern 25a by leading and trailing valleys 27a and 29a in much the same manner as disclosed in Fig. 2. However, the infusion rate during the test pulse 19a is the same as the infusion rate during the normal delivery pattern 25a. Although the infusion rates during the valleys 27a and 29a can be "0" or negative, preferably, the infusion rates during these times are positive. Also, although the infusion rates during the valleys 27a and 29a can be different, preferably, they are essentially the same. In Fig. 2B, the test pulse 19b is negative, i.e., the infusion pump 20 is reversed to create the infusion pulse. Although the infusion rate during the valleys 27b and 29b may be either positive or negative, in this embodiment, they are essentially "0." In Fig. 2C, a plurality of test pulses 19c is provided in relatively rapid succession before the infusion rate returns to the normal delivery pattern 25c. In this event, the valleys 27c and 29c between adjacent test pulses 19c constitute both trailing and leading valleys as shown. In Fig. 2D, the separating regions 27d ant 29d are not distinguishable from the normal delivery pattern 25d, and so the altered pattern of delivery consists only of the test pulse 19d. This can be contrasted with the embodiments described above in which the altered pattern comprises both the leading and trailing separating regions and the test pulses. In Fig. 2E, the separating regions 27e and 29e represent periods during which the flow rate is greater than during the normal delivery pattern 25e, and the test pulse 19e represents a decreased flow rate which may be equal to, greater than or less than the flow rate during the normal delivery pattern. As illustrated, the flow rate during the test pulse 19e is greater than the flow rate during the normal delivery pattern 25e. Generally, test pulses of the type shown in Figs. 2B and 2E, which have flow rates less than the adjacent separating regions, are not preferred. The presence of the test pulse 19 in the conduit means 17 creates a pressure wave response which has different characteristics depending upon whether or not the flowable material is being improperly supplied by the conduit means 17 to the patient. Fig. 3 shows examples of pressure wave responses 31 and 33 which indicate in-the-vessel and infiltration conditions, respectively. Although the pressure wave responses 31 and 33 are both shown in Fig. 3, they are not in scale in relation to each other. In reality, Fig. 3 shows curves which represent the pressure wave responses, but the curves, is desired, can be considered as the pressure wave responses. The pressure provided by the the pressure wave response 31 rises rapidly and almost instantaneously along a rising edge 35 and decays at about the same rate as represented by a falling edge 37. In between the edges 35 and 37, the pressure remains approximately constant, except for a short duration valley 39 which is representative of the deadband of the peristaltic pump being employed. In this regard, the pressure wave response 31 was generated by a peristaltic pump having a deadband and operating at about 350 cc's per hour, which is a high-delivery rate. If a peristaltic pump which accelerates through the deadband were employed, the duration and magnitude of the valley 39 would be greatly reduced. For this reason, it is preferred to utilize a peristaltic pump which does accelerate through the deadband so that any valley 39 would be of substantially less magnitude and duration than illustrated in Fig. 3. However, the pressure wave response 31 is essentially a square wave if the valley 39 is ignored. With the open distal end of the needle 18 of the conduit means 17 properly communicating with the interior of the vessel of the patient's cardiovascular system, the pressure wave response 31 is simply the result of forcing the additional fluid into the fluid carried by the vessel. For example, for an infusion rate of 5 cc/hour, the pressure wave response 31 may rise about 5 mm Hg above a base line. When infiltration occurs, the open distal end of the needle 18 is out of the interior of the vessel and communicates with tissue. As a result, the pressure wave response 33 is created in the conduit means 17. Specifically, the pressure wave response 33 rises along a rising edge or leading portion 43 and falls along a falling edge or trailing portion 45 with both the rise time and fall time being much greater than for the pressure wave response 31. In addition, the pressure wave response 33 has a maximum pressure or peak value 47 which is much higher than the maximum pressure or peak value of the pressure wave response 31. For example, for an infusion rate of 5 cc/hour, the pressure wave response 33 may rise about 20 mm Hg above a baseline 49. The rising edge has a discontinuity 48 which is the result of using a peristaltic infusion pump to generate the pressure wave response 33 which did not accelerate through the deadband and, therefore, did not have an essentially constant delivery rate. The pressure, or pressure response, of the parenteral fluid in the conduit means 17 can be monitored in various different ways, such as by a pressure transducer 51 which provides an electronic analog pressure signal which is related to the presure in the conduit means 17. In this embodiment, the pressure signal from the transducer 51 is amplified by an amplifier 53, conditioned in signal conditioning circuitry 55 and sampled in an analog-to-digital converter (A/D converter) 57 which provides the samples to a microprocessor 59. The signal conditioning circuitry 55 is conventional and is provided for the purpose of adjusting or compensating for various variables, such as temperature. Of course, if these variables are not considered significant, the signal conditioning circuitry 55 can be eliminated. The samples of the pressure signal from the transducer 51 may be taken continuously or taken only during the sample time. In the former case, the microprocessor 59 is used to separate the samples taken during the sample time from those which are not. However, in this embodiment, the A/D converter 57 samples the pressure signal continuously and provides the samples in a digital format to the microprocessor 59. The sampling frequency of the A/D converter 57 is preferably higher than the sampling frequency required for detecting infiltration or other abnormal infusion. For example, if infiltration detection requires about one sample per second, samples may be taken at, for example, 5 samples per second, 40 samples per second, etc. The samples can then be combined in any of a variety of ways to produce an overall sample value for each second by the microprocessor 59. In any event, the microprocessor 59 performs an integration function as described below to determine if infiltration has occurred. Fig. 4 shows a pressure response 65 in the conduit 17 as measures by the transducer 51 in response to infusion of fluid through the conduit to the patient. As shown, the pressure generally diminishes during the valley 27 until the leading edge of test pulse 19, at which time a pressure wave response 67 is initiated. The response 67 is indicative of abnormal infusion, e.g., infiltration. The pressure wave response 67 rises, with modest interruption to a peak value 69 of pressure which is reached at approximately the end of the test pulse 19. The pressure then slowly decays during the valley 29, again with modest interruption, to the end of the valley 29. Thus, the pressure wave response 67 has a leading portion 71 that extends from an initial point 73 at the beginning of the test pulse 19 to the peak value 69 and a trailing portion 75 that extends from the peak value 69 to a baseline 77. The baseline 77 can be established by the microprocessor 59 in various different ways and is preferably a function of the pressure response 65 intermediate test pulses 19. More specifically, the baseline 77 can be established by suitably smoothing the pressure response 65 between test pulses 19, and this function can be carried out by the microprocessor. Accordingly, the baseline 77 lags the pressure response 65. However, at the initiation of the test pulse 19, the baseline 77 then existing is frozen by the microprocessor such that it remains essentially constant throughout at least the test pulse 19 and the trailing valley 29. Various different techniques can be used to smooth the pressure response 65 between test pulses 19. According to one preferred smoothing technique, the pressure samples from the A/D converter 57 are slew-rate limited and averaged to provide pressure values for updating the baseline 77. By way of illustration and not by way of limitation, the A/D converter 57 may provide 40 pressure samples per second, and the slow-rate limiting processing limits each sample to a value which is no greater than 1.25 times, and no less than 0.75 times, the value of the previous sample. Following this, a 1/4 second average of ten of the samples is taken, and this average value provides a new point for updating the baseline 77. With this technique, four new smoothed pressure readings are provided each second. The baseline 77 is then typically derived by passing these samples through an additional smoothing process, such as a single-pole low-pass filter with a time constant of about 2 seconds. A primary advantage of slew-rate limiting is to eliminate spurious spikes that may occur in the pressure response 65 that might be incorrectly interpreted as the peak value 69. In addition, slew-rate limiting and averaging provides a desired smoothing effect for the baseline 77 to reduce transient irregularities that might otherwise occur in the baseline 77. To determine if infiltration or other abnormal infusion has occurred, the area between the pressure wave response 67 and the baseline 77 is calculated by the microprocessor between the initial point 73 and a truncation point 79. In this embodiment, the truncation point 79 is the time at which the pressure on the trailing portion 75 drops to 50 percent of the peak value 69. More generally, the truncation point 79 is the first to occur of a pre-established percent of the peak value 69 and the end of the trailing valley 29. Because in the example of Fig. 4, the predetermined percent of the peak value 69 occurred first, the integration is truncated at this point along a line 81. Alternatively, or in addition thereto, the integration of the pressure wave response 67 may proceed from the initial point 73 to the peak value 69 to provide a front end area. This front end area may be used in lieu of, or in addition to, the truncated area, i.e., the area obtained from integrating from the initial point 73 to the truncation point 79, to determine whether or not infiltration has occurred. Fig. 4 also shows a somewhat idealized pressure wave response 83 in dashed lines which is indicative of the proper supply of fluid to the patient. The pressure wave responses 67 and 83 would, of course, not exist simultaneously, but they are shown together for comparative purposes. Applying the same criteria for integration of the pressure wave response 83, virtually the entire area between the pressure wave response 83 and the baseline 77 would be determined. However, this area is much less than the area obtained by the truncated integration of the pressure wave response 67. Accordingly, the microprocessor 59 can readily determine whether or not infiltration has occurred. The area information can be used in various different ways to arrive at a decision regarding the presence or absence of infiltration or other abnormal infusion. For example, both the truncated area and the front end area may be normalized and compared with known thresholds, and an alarm condition may be declared in response to one or more of these areas exceeding the established threshold. In a preferred decision-making technique, an alarm condition is not declared until three consecutive normalized truncated areas or three consecutive normalized front end areas resulting from three consecutive test pulses have exceeded their respective thresholds. More specifically, in one technique, the truncated area is normalized by dividing it by a divisor which is a function of the difference between the infusion rate during the test pulse and the infusion rate during either of the valleys 27 and 29. Preferably, the function is simply the difference betweeen these infusion rates. The quotient or normalized area is then compared with a known threshold. The front end area is normalized in the same way and compared with its threshold. It has been found that the threshold varies with a number of factors, the most important of which is the gauge or size of the needle 18. The areas can be effectively normalized for needle (or other cannula) size by employing different thresholds for different ranges of needle size. Examples of suitable thresholds for both truncated area and front end area as a function of needle gauge are shown in the table below: Thus, if a 16-gauge needle is used, an alarm condition indicative of abnormal infusion would be declared if any three-consecutive test pulses yielded a normalized truncated area of greater than .2 or a normalized front end area of greater than .1. In the above table, it is assumed that the units on the area of pressure wave response are mmHg-seconds and that the infusion rates are in milliliters per hour. Of course, different units can be employed, and the thresholds adjusted accordingly. According to the invention it is determined if pressure conditions in the conduit means 17 are suitable for detecting if fluid is being improperly supplied by the conduit means to the patient. Although this invention is particularly adapted for use with the integration technique of this system, its use is not so limited, and it can be used with other techniques for determining whether or not abnormal infusion exists. The suitability of the pressure conditions in the conduit means 17 is evaluated by comparing a function of the pressure of the fluid in the conduit means 17 during the normal delivery pattern 25 to a threshold. Although the function of the pressure of the fluid in the conduit means 17 can be the raw pressure samples from the A/D converter 57 per se, preferably this function includes some form of smoothing. According to a preferred technique, the following equation is used: BQI = B1 + K(B2) where, BQI is a baseline quality index, B1 is the magnitude of the baseline 77 prior to the valley 27, K is a constant which may be, for example, 3 and B2 is the rms value of a plurality, such as 120, of baseline 77 magnitude occuring just prior to the baseline magnitude representing B1. Thusn BQI is a function of the pressure in the conduit means 17. Assuming that the pressure response 65 is slew-rate limited, averaged and low-pass filtered to determine the baseline 77 as described above, then B1 is the last baseline 77 value prior to the valley 27, and B2 is the RMS value of 120 baseline values immediately preceding the value used for determining B1. It has been found that, if BQI as calculated using the equation set forth above exceeds a threshold, the pressure conditions in the conduit means 17 are not suitable for determining whether or not abnormal infusion exists. Although this threshold can be adjusted as desired, by way of example, a threshold of 85 mmHg is suitable for many applications. Fig. 5 is a flow chart showing the basic steps in the truncated integration process of this invention. As shown in Fig. 5, when the end of the valley 29 occurs, a new BQI calculation commences using the BQI equation set forth above. When the time for the next site check, i.e., the time to initiate the altered infusion pattern represented by the valley 27, the test pulse 19, and the valley 29 occurs, it is allowed to proceed only is the BQI is below the threshold magnitude as described above. In this regard, the next site check referred to in Fig. 5 may be manually initiated by entering information into the infusion device 15 or in response to programming of the infusion device 15 to provide site checks at specified times or at specified intervals. In any event, the infusion device 15, and in particular, the infusion pump 20 will provide the reduced infusion represented by the valley 27 only if the BQI is below the specified threshold. When the end of the valley 27 is detected, i.e., the start of the test pulse 19, the baseline 77 is frozen and intregration of the difference between the baseline and the pressure wave response commences as shown in Fig. 5. The integration continues from the initial point 73 beyond the end of the test pulse 19 until the first to occur of a pressure less than 50 percent of the peak value 69 or the end of the trailing valley 29. At the end of the integration, the truncated area and the front end areas are normalized for infusion rate and needle-size effects as described above. Following this, the microprocessor 59 makes a decision concerning abnormal infusion as described above, and if infusion is abnormal, the alarm 63 is energized and/or infusion is terminated. Of course, the microprocessor may require information from multiple site checks before declaring abnormal infusion. The abnormal infusion may be the result of any condition, such as infiltration, occlusion of the conduit means 17, clotting or phlebitis, which leads to the creation of the pressure wave response 67 (Fig. 4). In any event, once the abnormal infusion is declared, an attendant can determine the particular cause and seek to remedy it. Although an exemplary embodiment of the invention has been shown and described, many changes, modifications and substitutions may be made by one having ordinary skill in the art without necessarily departing from the scope of this invention.

Claims: An apparatus for determining if fluid is being properly supplied through a fluid delivery system to a patient, said apparatus comprising: means (15) for delivering fluid through the delivery system so as to create a pressure wave response (67) in the delivery system and means (59) responsive to the pressure wave response (67) for detecting if fluid is being improperly supplied by the delivery system to the patient; characterized in that the apparatus further comprises: further means (59) responsive to the pressure in the delivery system, during the normal delivery pattern when the pressure wave response (67) is not present, and for comparing a function of said pressure to a threshold to ascertain if pressure conditions in the delivery system are suitable for detecting if fluid is being improperly supplied by the delivery system to the patient. An apparatus as defined in Claim 1 wherein said means for delivering the fluid is an infusion device (15), said infusion device (15) including means for delivering the fluid in a normal delivery pattern (25) and for delivering a test pulse (19) of the fluid with the test pulse (19) being distinguishable from the normal delivery pattern (25); and the apparatus comprises: conduit means (17) for conducting the fluid from the infusion device (15) to the patient, said test pulse (19) creating a pressure wave response (67) in the conduit means (17). An apparatus as defined in Claim 1 or 2, wherein said function of the pressure equals B1 + K(B2) where B1 is the baseline (77) pressure at an instant prior to the test pulse (19), K is a constant and B2 is the rms value of a plurality of segments of the baseline (77) pressure prior to the test pulse (19) and with at least one of said segments being prior to said instant.

Inventors: BOBO DONALD E JR, LAPP THEODORE R, SEGUINE DENNIS R, BOBO, DONALD E., JR., LAPP, THEODORE R., SEGUINE, DENNIS R.
Applicants: BAXTER INT, BAXTER INTERNATIONAL INC.
Application year: 1989
Main classification: A61M5
IPC codes: A61M5