Abstract:
In an infusion pump operable to deliver fluid to a patient at a programmed therapy flow rate, air-in-line sensing is improved by commanding the pumping mechanism to deliver a bolus volume of fluid at a flow rate higher than the therapy flow rate when an uninterrupted volume of air is detected that exceeds a first threshold. In many cases, the bolus will be effective to clear microbubbles from an observation zone of the air-in-line sensor to avoid an air-in-line alarm condition. If the uninterrupted volume of air continues to grow beyond a second threshold in spite of the bolus, then an alarm may be triggered. The invention reduces false alarms.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of medical infusion pumps, and more particularly to air-in-line sensing and management methods for medical infusion pumps. 
       BACKGROUND OF THE INVENTION 
       [0002]    Programmable infusion pumps for delivering nutritional liquids and medicine to patients in accordance with predetermined liquid delivery parameters are in wide usage. One type of infusion pump is a peristaltic pump arranged along flexible connective tubing carrying liquid from a liquid source to the patient. The peristaltic pump has a pumping mechanism for progressively squeezing successive portions of the tubing to cause fluid to flow through the tubing in a flow direction toward the patient. In a common arrangement, the pumping mechanism includes a motor-driven wheel having radial fingers or rollers that engage a segment of the tubing arranged about a circumferential portion of the wheel. As the wheel rotates, fluid is pumped through the tubing to the patient. The tubing segment arranged about the pump wheel may be held in a U-shaped configuration by a cassette designed for receipt in a channel or receptacle area of the pump. The cassette may provide terminals for connecting an incoming line of tubing coming from the liquid source and an outgoing line of tubing going to the patient to opposite ends of the U-shaped tubing segment received by the pump. 
         [0003]    A recognized safety concern when pumping nutritional liquids for enteral feeding or medicinal fluids for intravenous therapy is the formation of air bubbles in the liquid being pumped into the patient. As a safety measure, it is known to provide an air-in-line sensor on the infusion pump for detecting an air-in-line condition and triggering an alarm. For example, the air-in-line sensor may include an ultrasonic transmitter arranged to direct ultrasound through the tubing and a receiver on an opposite side of the tubing from the transmitter for receiving the ultrasound waves after passage through the tubing and the fluid carried thereby. The receiver generates an output signal indicating whether the ultrasound signal passed through liquid or air as it travelled from the transmitter to the receiver. 
         [0004]    The air-in-line sensor output is sampled regularly as fluid is pumped through the tubing to observe each incremental volume of fluid passing through the sensor&#39;s zone of observation. In known air bubble detection algorithms, an air-in-line alarm condition is detected when a series of consecutive sensor readings indicate that a predetermined volume of air has passed the sensor (e.g. 1.5 milliliters) without the presence of a predetermined volume of liquid (e.g. 0.375 milliliters). 
         [0005]    A problem has been identified that occurs when food bottles containing a nutritional liquid are vigorously shaken to mix the contents. In such cases, micro-bubbles may collect at the downstream side of the air-in-line sensor and may eventually cause an air-in-line alarm. The delivery of fluid by the pump may be implemented in discrete time segments during which the pump&#39;s motor is actually on and pumping only a small portion of the time segment, and is off for the remainder of the time segment. Due to gravity, air micro-bubbles caused by shaking may float upstream and gather at the air-in-line sensor, potentially causing detection of an air-in-line condition which will trigger a “false” alarm. 
         [0006]    A need exists to prevent this type of false alarm, preferably without changing the pump hardware or sensor hardware. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention addresses the problem mentioned above, and does so without changes to the pump hardware or sensor hardware, which are optimized for other key considerations. 
         [0008]    In one aspect, the invention provides a method of detecting an air-in-line condition with respect to flow of liquid through tubing connected to an infusion pump. The method generally comprises the steps of (i) providing an air-in-line sensor at a sensing location along the tubing, the air-in-line sensor generating a signal indicating whether a volume of fluid observed by the sensor at a given time is air or liquid; (ii) operating the pump to deliver fluid at a therapy flow rate; (iii) sampling the sensor signal as fluid flows past the sensor; (iv) calculating a total volume of air observed by the sensor since the sensor last observed liquid; (v) operating the pump to deliver a bolus volume of fluid at a bolus flow rate greater than the therapy flow rate when the total volume of air exceeds a first threshold; and (vi) detecting the air-in-line condition when the total volume of air exceeds a second threshold greater than the first threshold. In the method above, the bolus delivery in step (v) is often effective to clear accumulated air bubbles to avoid an air-in-line condition requiring an alarm. 
         [0009]    The method summarized above may further comprise the step of operating the pump to deliver fluid at a reduced flow rate less than the therapy flow rate after delivery of the bolus volume in order to compensate for excess volume delivered via the bolus volume. The pump may be operated at the reduced flow rate until the excess volume delivered relative to the therapy flow rate as a result of the delivery of the bolus volume is compensated for, and then at the therapy flow rate to resume the programmed therapy. 
         [0010]    In another aspect, the present invention provides a method of clearing air microbubbles from an observation zone of an air-in-line sensor arranged to observe fluid flowing through tubing connected to an infusion pump. The method generally comprises the steps of (i) calculating a total volume of air observed by the sensor since the sensor last observed liquid; and (ii) operating the pump to deliver a bolus volume of fluid at a bolus flow rate greater than a programmed therapy flow rate when the total volume of air exceeds a predetermined threshold. The bolus flow rate may be substantially equal to a priming flow rate used for priming the pump. 
         [0011]    In another aspect, the invention encompasses an infusion pump generally comprising (i) a pumping mechanism operable to cause fluid flow through tubing connected to the pumping mechanism, the pumping mechanism including a motor and a motor controller for energizing the motor; (ii) an air-in-line sensor arranged at a sensing location along the tubing to observe fluid flowing through the tubing, the air-in-line sensor generating a signal indicating whether a volume of fluid observed by the sensor at a given time is air or liquid; (iii) a memory module; and (iv) a microprocessor connected to the memory module, the pumping mechanism and the air-in-line sensor, wherein the microprocessor is programmable to command the pumping mechanism deliver fluid at a therapy flow rate, wherein the memory module stores programming instructions causing the microprocessor to command the pumping mechanism to deliver a bolus volume of fluid at a bolus flow rate greater than the therapy flow rate in response to signals from the air-in-line sensor indicating an uninterrupted volume of air flowing through the tubing is greater than a predetermined first volume threshold. 
         [0012]    The memory module may also store programming instructions causing the microprocessor to register an air-in-line alarm condition in response to signals from the air-in-line sensor indicating an uninterrupted volume of air flowing through the tubing is greater than a predetermined second volume threshold greater than the first volume threshold. 
         [0013]    To compensate for excess volume delivered by the bolus, the memory module may store programming instructions causing the microprocessor to command the pumping mechanism to deliver fluid at a reduced flow rate less than the therapy flow rate after delivery of the bolus volume. The reduced flow rate may be a predetermined percentage of the therapy flow rate, for example 50%. The memory module may also store further programming instructions causing the microprocessor to command the pumping mechanism to deliver fluid at the therapy flow rate after excess volume compensation is complete. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0014]    The invention is described in detail below with reference to the following figures: 
           [0015]      FIG. 1  is schematic representation of an infusion pump formed in accordance with an embodiment of the present invention, wherein a cassette and tubing are shown installed in the pump to illustrate basic operation; 
           [0016]      FIG. 2  is an electronic block diagram of the infusion pump shown in  FIG. 1 ; 
           [0017]      FIGS. 3A-3C  are a flow diagram illustrating methodology for monitoring an air-in-line sensor of the infusion pump and detecting and air-in-line condition during therapy in accordance with an embodiment of the present invention; 
           [0018]      FIG. 4  is a flow diagram of a microbubble detection routine that determines if a volume of fluid observed by the air-in-line sensor is air or liquid in accordance with an embodiment of the present invention; 
           [0019]      FIG. 5  is a flow diagram illustrating bolus compensation logic implemented by the pump in accordance with an embodiment of the present invention; and 
           [0020]      FIGS. 6A-6D  are graphs showing motor rate versus time under various conditions wherein a bolus is delivered to remove microbubbles from the air-in-line sensor zone and subsequently compensated for in accordance with exemplary embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIGS. 1 and 2  schematically depict a programmable infusion pump  10  embodying the present invention. Infusion pump  10  includes a housing  12 , a pump wheel or rotor  14  and a cassette receptacle  16  on an external face of the housing, and a door  18  connected to the housing to open and close over the cassette receptacle and pump wheel. As shown in  FIG. 1 , an administration set may be installed in association with the pump for carrying fluid from a fluid source to a patient. The administration set may include upstream tubing  4  running from the fluid source to the pump, downstream tubing  8  running from the pump to a patient, a cassette  5  received in cassette receptacle  16 , and a U-shaped tubing segment  6  arranged around pump wheel  14 . Cassette  5  is configured with connection terminals  5 U and  5 D for connecting upstream tubing  4  to an upstream end of tubing segment  6  and downstream tubing  8  to a downstream end of tubing segment  6  to complete a flow path from the upstream tubing to the downstream tubing via the pump. 
         [0022]    Pump wheel  14  is part of a pumping mechanism operable to cause fluid flow through the tubing in an intended flow direction. The pumping mechanism further includes an electric motor  20  connected to pump wheel  14  and operable to rotate the pump wheel about its axis. Pump wheel  14  has radial fingers or rollers (not shown) that engage tubing segment  6  arranged about a circumferential portion of the wheel. When pump wheel  14  rotates, successive portions of tubing segment  6  are progressively squeezed to cause fluid to flow through the tubing in a flow direction toward the patient. The flow rate of infused fluid may be controlled by controlling the rate at which motor  20  is driven and/or the length of time motor  20  is driven at a given rate. Those skilled in the art will understand that variations of the peristaltic pumping mechanism described above are possible. For example, motor  20  may drive a cam member connected to a series of parallel fingers or rollers arranged side-by-side, whereby peristaltic pumping action is applied to a straight segment of tubing instead of a curved segment of tubing as shown in  FIG. 1 . The present invention is not limited to a specific pumping mechanism configuration. 
         [0023]    Infusion pump  10  may be provided with an upstream occlusion sensor  22  at a location along tubing segment  6  upstream from pumping wheel  14  and a downstream occlusion sensor  24  at a location along tubing segment  6  downstream from pumping wheel  14 . Upstream sensor  22  and downstream sensor  24  each provide a respective sensor signal indicative of a respective local fluid pressure in the tubing. For example, upstream and downstream sensors  22 ,  24  may be transducers or strain gauges arranged to engage an outer wall of tubing segment  6  to detect deflection of the flexible tubing wall caused by fluid pressure within the tubing and provide an electronic signal proportional to the deflection. 
         [0024]    Infusion pump  10  further includes an air-in-line sensor  26  for detecting whether a volume of fluid observed by the sensor at a given time is air or liquid. In the present embodiment, air-in-line sensor  26  may comprise an ultrasonic transducer which includes a pair of piezoelectric ceramic elements  26 A and  26 B opposing each other across a portion of tubing segment  6 . One ceramic element  26 A is driven by microprocessor  30  at a frequency that sweeps through the resonance which lies within the frequency range. The ultrasonic energy is transmitted by element  26 A into one side of the tubing and a portion of the energy is received by element  26 B on the other side. If liquid is present in the tubing, the ultrasonic energy received by element  26 B will be greater than a preset comparator threshold and is then converted into a logic level of “High”. If air is present in the tubing, the medium for propagating the ultrasonic energy is less dense and the signal generated by element  26 B is attenuated below the threshold and is converted into a logic level of “Low”. Thus, in the embodiment just described, the amplitude of the ultrasonic energy which is received by element  26 B is the main principle for determining the difference between liquid and air within the tubing. The tubing may be dry-coupled to the air-in-line sensor elements  26 A and  26 B; i.e. the sensor arrangement does not require the use of ultrasonic gel. 
         [0025]    As seen in  FIG. 2 , infusion pump  10  is configured to permit a user to select and/or create, and then run, an infusion therapy protocol determining the amount of liquid to be delivered to the patient and the rate at which the liquid is to be delivered. 
         [0026]    Infusion pump  10  includes a microprocessor  30  connected to a user interface  32  having input devices such as a keypad, switches and dial controls. Infusion pump  10  also includes a display  34  connected to microprocessor  30 . Display  34  may be a touch screen display acting at times as part of user interface  32 . Microprocessor  30  is connected to a motor controller  36  for driving electric motor  20  to administer a chosen therapy protocol. One or more memory modules  38  are connected to or integrated with microprocessor  30  for storing instructions executable by the microprocessor for controlling pump operation. The stored instructions may be organized in software routines. Among the stored software routines are routines that detect possible microbubbles, attempt their removal through release of a bolus, and compensate for excess fluid delivered by the bolus to achieve the programmed therapy delivery rate. These routines are described in detail below. For purposes of the present invention, microprocessor  30  receives the signal from air-in-line sensor  26 . Microprocessor  30  is also connected to upstream occlusion sensor  22  and downstream occlusion sensor  24 . Analog-to-digital conversion circuitry  23  is shown for converting the analog voltage signals from the occlusions sensors to digital form for use by microprocessor  30 , however other forms of occlusion sensor and microprocessor interfaces may be used. Infusion pump  10  may also include an audible signal generator  35  connected to microprocessor  30 . 
         [0027]    In an embodiment of the present invention, fluid delivery is implemented in regular time segments, for example one-minute segments. A therapy flow rate may be selected within a range of 0.1 milliliters per hour (ml/hr) to 400 ml/hr. Motor  20  may be operated at a given rotational speed, for example 40 rpm. By way of example, each motor rotation may include  12  incremental rotational motor steps or “ticks”, wherein the resolution of fluid delivery is 18 microliters per tick. Thus, approximately 56 ticks are required to pump 1 milliliter of fluid. If the selected therapy rate is 60 ml/hr, then an average of 1 milliliter must be pumped during each one-minute segment. Assuming the motor is operating at 40 rpm for an entire one-minute segment, it would provide 480 ticks and deliver too much fluid for the selected flow rate. Consequently, the motor may be controlled such that it is active for only a portion of each time segment necessary to deliver 1 milliliter, and is inactive for the remainder of the time segment. In the present example, 1 milliliter is delivered in approximately 56 ticks, equivalent to about 7 seconds at a motor speed of 40 rpm. 
         [0028]    During the remaining 53 seconds of the time segment, the motor is inactive. As may be understood, the therapy delivery rate may be adjusted without changing the motor speed (rpm) by changing the length of time the motor is active during each time segment. 
         [0029]    As will be described in detail below, the present invention is embodied by a method wherein a fluid bolus is commanded and delivered at a higher flow rate if air-in-line exceeds a first threshold, and excess fluid delivered by the bolus is compensated for by temporarily reducing the flow rate relative to the selected therapy flow rate. In an embodiment of the present invention, the bolus may be 1.0 milliliters of fluid delivered at the priming flow rate of the pump, for example 700 ml/hr, which is higher than the maximum selectable flow rate for therapy. Of course, other bolus volumes and bolus flow rates may be used without straying from the invention. 
         [0030]    Attention is now directed to  FIGS. 3A-3C , which generally illustrate air-in-line detection logic implemented by software routines stored and executed by the pump in accordance with an embodiment of the present invention. In the embodiment shown, the air-in-line sensor  26  is sampled in block  120 . As described above, air-in-line sensor  26  provides a digital signal indicating that the sensor observed either air or liquid. In block  122 , a microbubble routine is called which includes logic for disregarding very small foam bubbles. A form of the microbubble routine is described in detail below with reference to  FIG. 4 . If the incremental volume of fluid observed by sensor  26  is air, decision block  124  directs flow to blocks  126 ,  128 , and  130 . In block  126 , a variable VOL_LIQ, which tracks the total volume of liquid observed since the sensor  26  last observed air, is set to zero. In block  128 , a variable VOL_AIR, which tracks the total volume of air observed since the sensor  26  last observed a continuous threshold volume of liquid (e.g. 0.375 ml), is incremented by the addition of incremental volume VOL_INC, which corresponds to the volume moved past sensor  26  by one incremental step or “tick” of pump motor  20 . By way of the example, in a current pump embodiment, the incremental volume is approximately 18 microliters. Thus, if the sampled sensor signal indicates air, in the present example, VOL_AIR is increased by 18 microliters. Decision block  130  checks whether VOL_AIR exceeds a first predetermined threshold, for example 1.0 milliliters. If not, flow loops back to handle the next sampled value from air-in-line sensor  26 . 
         [0031]    Returning to decision block  124 , if the incremental volume of fluid observed by sensor  26  is liquid instead of air, then VOL_LIQ is incremented by VOL_INC in accordance with block  132 . Decision block  134  determines if VOL_LIQ exceeds a predetermined threshold, which in the present embodiment is 0.375 ml. If so, VOL_AIR is set to zero in block  136  before flow loops back to handle the next sampled value from air-in-line sensor  26 . If not, then decision block  134  bypasses block  136 . 
         [0032]    If decision block  130  determines that VOL_AIR exceeds the first threshold of 1.0 milliliters, then an inventive approach of the invention is used in an effort to avoid an air-in-line alarm condition if the accumulated air is due to microbubbles congregating at sensor  26 . More particularly, when the total continuous volume of air exceeds the first threshold, the pump is commanded to deliver a bolus volume of fluid at a bolus flow rate greater than the therapy flow rate in an effort to clear the microbubbles away from the sensor. Decision block  138  in  FIG. 3B  checks the value of a Boolean variable bBOL_ACTIVE indicating whether a bolus is currently being delivered. If not, then flow moves to block  140  to set the value of bBOL_ACTIVE to True and then to block  142  to start the bolus delivery. Once the bolus is started, flow loops back to block  120 . 
         [0033]    If decision block  138  finds bBOL_ACTIVE to be True, it means bolus delivery was already commanded. In such a case, decision block  144  checks whether VOL_AIR exceeds a second predetermined threshold, for example 1.5 milliliters. If the second threshold is exceeded, then a delivered bolus failed to remove the air-in-line. Accordingly, an alarm condition is registered in block  150  and pumping is stopped in block  152 . If VOL_AIR does not exceed the second threshold, then decision block  144  directs flow to block  146  to increment a variable VOL_BOL which tracks the bolus volume. In the present example embodiment, a bolus volume of 1.0 milliliters is used. Thus, decision block  148  loops flow back to block  120  until the fluid delivered in the bolus reaches 1.0 milliliters, at which point decision block  148  advances flow to block  154  in  FIG. 3C . In block  154 , the value of Boolean variable bBOL_ACTIVE is set to False now that bolus delivery is complete. 
         [0034]    Next, the value of Boolean variable bBOL_COMP is checked in decision block  156 . The value of bBOL_COMP indicates whether bolus compensation is underway. If the value of bBOL_COMP is False, then flow is directed to block  158  to set the value of bBOL_COMP to True and then to block  160  to start bolus compensation. Bolus compensation schemes embodying the present invention are described later with reference to FIGS.  5  and  6 A- 6 D. If the value of bBOL_COMP is True at decision block  156 , then flow branches to decision block  162  to check if bolus compensation is completed. If so, the pump is returned in block  164  to its originally selected pumping rate for the therapy. 
         [0035]    Finally, a decision block  166  evaluates whether the programmed therapy is finished. If not, flow loops back to block  120  in  FIG. 3A . 
         [0036]    As mentioned above, a microbubble routine may be executed at block  122  to account for foam bubbles. Foam bubbles may form if the liquid source, such as a container of nutritional liquid, is vigorously shaken to mix the contents. A microbubble routine suitable for practicing the present invention is illustrated in  FIG. 4 . The routine may accept inputs AIRIN and LIQIN, which represent the volume of air and the volume of liquid, respectively, in the sampled incremental volume of fluid moved by the latest pump tick and observed by sensor  26 . In the present example where each motor tick corresponds to about 18 microliters, AIRIN will have either a value of 18 microliters if air-in-line sensor  26  sees air or a value of zero if air-in-line sensor  26  sees liquid. Conversely, LIQIN will have a value that is either zero if air-in-line sensor  26  sees air or 18 microliters if air-in-line sensor  26  sees liquid. 
         [0037]    The microbubble routine returns outputs AIROUT and LIQOUT. The routine is designed to look for consecutive occurrences of air until a predetermined threshold volume is reached before returning a non-zero value of AIROUT. In a current embodiment, the value of AIROUT is held at zero until AIRIN indicates air for four consecutive calls of the routine, at which point the sensor readings are deemed to indicate a real air bubble that may possibly trigger an air-in-line alarm, rather than merely indicating foam bubbles. At this point, the four readings are accumulated into a single AIROUT value (e.g. 72 microliters). Thus, the value of AIROUT will initially jump from zero to four times the volume resolution (e.g. 72 microliters) when a significant volume of air is detected. Once this threshold has been reached, AIROUT is set to AIRIN in subsequent calls of the routine until the chain of consecutive air readings is broken by a liquid reading. If successive values of AIRIN fluctuate between zero and a nonzero value (e.g. 18 microliters) without reaching four consecutive nonzero values, it is an indication that foam bubbles are present, and the AIRIN values will be disregarded. If the value of LIQIN is greater than zero, then the value of LIQOUT will be set equal to the value of LIQIN. As may be appreciated, the microbubble routine helps reduce false air-in-line alarms by disregarding small air bubbles indicative of foam. 
         [0038]    An embodiment of the microbubble routine is shown in  FIG. 4 . An initial block  200  of the depicted microbubble routine sets the values of AIROUT and LIQOUT to zero. A decision block  202  checks the value of AIRIN. If the value of AIRIN is greater than zero (e.g. 18 microliters), sensor  26  sees air rather than liquid in the sampled volume increment, and flow proceeds to decision block  204 . In decision block  204 , the value of a variable LASTAIROUT is compared to zero. LASTAIROUT stores the value of AIROUT resulting from the previous call of the microbubble routine. So, decision block  204  determines whether the previous call of the routine found air. If air was seen in the previous call, then flow branches to block  206 , wherein the value of AIROUT is set equal to the value of AIRIN. In other words, the routine keeps counting air if air was found previously. 
         [0039]    If LASTAIROUT equals zero at decision block  204 , then flow is directed to block  208  to set the value of a variable BUBBLE, which accumulates an air bubble volume over successive calls of the routine. Block  208  increments the value of BUBBLE by the value of AIRIN. Decision block  210  compares the value of BUBBLE to a predetermined threshold volume. In the present example, the threshold volume is 55 microliters, however another threshold volume may be chosen. As may be understood, four consecutive air readings of 18 microliters are required for the value of BUBBLE to surpass the threshold volume of 55 microliters. If the threshold is not reached, flow bypasses blocks  212  and  214 , and the value of AIROUT remains at zero. If, however, decision block  210  finds the threshold has been reached, then block  212  sets the value of AIROUT equal to the value of BUBBLE, and block  214  resets the value of BUBBLE to zero. 
         [0040]    Attention is returned now to decision block  202 . If sensor  26  sees liquid instead of air, then AIRIN will equal zero and decision block  202  will direct flow to blocks  216  and  218 . Block  216  resets the value of BUBBLE to zero, and block  218  sets the value of LIQOUT equal to the value of LIQIN. 
         [0041]    Regardless of the logic flow path, flow will reach block  220  wherein the value of LASTAIROUT is set equal to AIROUT before the routine returns the values of AIROUT and LIQOUT to the calling program. 
         [0042]    Description of bolus compensation according to an embodiment of the present invention will now be provided with reference to FIGS.  5  and  6 A- 6 D. When a bolus is delivered by the pump in accordance with block  142  of  FIG. 3B , subsequent pump control and operation must be modified to compensate for excess fluid delivered in the bolus “ahead of schedule.” An exemplary embodiment of bolus compensation logic implemented by the pump is illustrated in  FIG. 5 . In block  300 , a calculation of excess fluid volume delivered in the time segment in which the bolus was delivered is made. In some cases, the bolus can be delivered within the normal segment volume for the programmed therapy, such that the excess volume is zero. In these cases, there is no need for compensation. Thus, and initial decision block  302  checks whether the calculated excess volume is greater than zero, and if not, then bolus compensation is completely bypassed. If the excess volume calculation results in a volume greater than zero, then flow proceeds to block  304 , wherein a calculation of the bolus volume as a percentage of the normal segment volume is performed. Decision block  306  then branches flow based on whether the bolus proportion calculated in block  304  is greater than a predetermined threshold percentage, for example 25%. If not, then flow branches to block  308  and the next segment volume is reduced by the excess volume calculated in block  300 . In other words, the entire bolus compensation is achieved in the segment immediately following the bolus delivery segment. If decision block  306  determines that the bolus proportion is greater than the predetermined threshold percentage (e.g. 25%), then compensation for bolus overage will be spread over a plurality of subsequent segments by implementing a reduction rule. For example, the volume delivered in the next segment is reduced by 50% or some other factor as indicated in block  310 . The reduction rule is implemented in successive segments until the excess volume is compensated for as confirmed by decision block  312 . 
         [0043]      FIGS. 6A-6D  provide four examples of how the bolus compensation logic operates under actual pumping conditions. In  FIG. 6A , it is assumed that the therapy flow rate of the pump is selected to be less than 60 milliliters per hour. Fluid delivery is scheduled in one-minute segments, wherein block  402  represents the period of motor activity during a normal segment of the therapy. In the next segment, delivery of a bolus occurs about half-way through the scheduled pumping period  404  as represented by block  400 . As may be seen, the motor rate is increased during bolus delivery relative to the motor rate used during normal therapy delivery in order to achieve a high flow rate in excess of the maximum selectable therapy flow rate so that the bolus volume is delivered in a short period of time. The volume delivered by the bolus (e.g. 1.0 milliliter) is greater than 25% of the volume that would be delivered during the segment at the normal therapy flow rate (less than 1.0 milliliters assuming therapy flow rate is less than 60 milliliters per hour). Consequently, under the logic of  FIG. 5 , the volume pumped during subsequent segments is reduced by 50% relative to the normal segment volume until the excess volume delivered from the bolus is compensated for; this may be seen in blocks  406  and  408 , which are not as wide as block  402 , indicating that pump activity time is reduced for these segments. If an alarm is avoided, the segments will return to the selected therapy pumping rate (e.g. block  402 ) once compensation is complete. 
         [0044]      FIG. 6B  illustrates a situation wherein the selected therapy flow rate is greater than 60 milliliters per hour and the bolus  400  is delivered relatively early within a segment. In this case, the entire bolus volume is delivered within the segment volume such that the total volume delivered during the segment is equal to the volume which was already scheduled (block  414 ) under the selected therapy flow rate. In this situation, no compensation is needed and the logic of  FIG. 5  bypasses compensation. Consequently, blocks  412 ,  416 , and  418  are identical and correspond to the therapy flow rate. 
         [0045]      FIG. 6C  represents a situation similar to that of  FIG. 6A , however the therapy flow rate is greater than 150 milliliters per hour. The higher therapy rate can be understood by comparing block  422  to block  402  to observe that the motor is kept active for a longer period of time during the segment associated with block  422 . The bolus  400  is delivered during scheduled block  424 . Despite the higher flow rate, the bolus volume is still greater than 25% of the scheduled segment volume, and therefore subsequent segments are subject to the 50% volume reduction until compensation for excess volume is complete. The reduction can be seen in the shorter duration of blocks  426  and  428  relative to block  422 . The time required to complete compensation decreases as therapy flow rate increases. 
         [0046]      FIG. 6D  illustrates a situation wherein the therapy flow rate exceeds 240 milliliters per hour. The motor is kept active for a longer period of time within each segment, as indicated by the width of block  432 , to achieve the therapy flow rate. A bolus  400  is triggered near the end of the active pumping period  434 . In this case, the bolus volume (e.g. 1.0 milliliter) is less than 25% of the segment volume delivered under the selected therapy rate (greater than 4.0 milliliters assuming therapy flow rate is greater than 240 milliliters per hour). Here, the logic of  FIG. 5  will cause the next segment to be reduced by the entire bolus volume, as illustrated by block  436 , such that compensation is achieved entirely within one segment. Block  438  corresponds to the scheduled segment volume in accordance with the therapy flow rate, and thus block  438  is identical to block  432 . 
         [0047]    The present invention is embodied as methods and a pump apparatus programmed to perform the methods. Example embodiments of the methods and pump apparatus of the present invention are described in detail herein, however those skilled in the art will realize that modifications may be made without straying from the spirit and scope of the invention as defined by the appended claims.