Abstract:
A method of monitoring pressure inside a fluid line and a system for implementing the method. The method is applicable to syringe pump systems. The method includes the steps of measuring a force value caused by a pressure inside the fluid line; collecting the measured force values during at least two consecutive moving time windows; calculating a slope of a best-fit line within each time window; calculating a slope difference of the slopes of the best-fit lines; comparing the slope difference with a pre-determined threshold gradient value; defining a baseline force as the detected force value when the slope difference is equal to the threshold gradient value; determining a relative force value by subtracting the baseline force from the detected force value; and, triggering an alarm if the relative force is greater than a pre-defined threshold force

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority based upon U.S. Provisional Patent Application Ser. No. 61/154,033 filed on Feb. 20, 2009. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       TECHNICAL FIELD 
       [0003]    The present invention generally relates to a method for detecting occlusions. More particularly, the present invention relates to a software algorithm that detects occlusions in the fluid lines of a syringe pump system. 
       BACKGROUND 
       [0004]    Modern medical devices, including medical pumps, are increasingly being controlled by microprocessor based systems to deliver fluids, solutions, medications, and drugs to patients. Different types of medical pump systems are used depending on factors such as the dosage of fluid to be delivered, the rate of fluid delivery, the duration, and the volume of a fluid to be infused into a patient. 
         [0005]    One example of a medical pump system used to gradually deliver small amounts of fluid to patients is a syringe pump. A typical syringe pump system includes a syringe with a plunger mounted to a housing, a motor, a pump mechanism, a pump mechanism controller, a user interface, and an alarm. The pump mechanism exerts force on the syringe plunger, and forces fluid out of the syringe into fluid lines leading to the patient. The pump mechanism includes anti-free flow claws, and a force-detecting sensor, such as a loadcell sensor. 
         [0006]    One concern associated with using syringe pump systems is that an occlusion may occur in any of the fluid lines attached to the pump. An occlusion will cause under-delivery of the fluid to the patient, and, at the same time, pressure will build up inside the syringe and fluid lines. The built-up pressure will cause a significant bolus of fluid to be expelled through the line after the occlusion is relieved. Therefore, it is essential that the syringe pump include an occlusion detecting mechanism. One example of an occlusion detecting mechanism may be a syringe pump mechanism controller including a sensor that detects force inside the fluid lines, means for monitoring the sensor readings, and an alarm that signals to the user when a certain threshold force or pressure level has been exceeded. 
         [0007]    One method of occlusion detection is to calculate the force on the sensor due to fluid pressure: F pressure . In a typical syringe pump system, as shown in  FIGS. 1 and 2 , the following relationships are established: 
         [0000]    
       
      
       F 
       loadcell 
       =F 
       claws 
       +F 
       stiction 
       +F 
       pressure 
               
       F 
       pressure 
       =F 
       loadcell 
       −F 
       claws 
       −F 
       stiction  
      
     
         [0008]    Where F loadcell  is the total force sensed by the loadcell. F claws  is the portion of the total force caused by the anti-free flow claws, and F stiction  is the portion of the total force caused by stiction. The pressure of the fluid flow in the line, P liquid , is calculated according to the formula 
         [0000]    
       
         
           
             
               ⇒ 
               
                 P 
                 liquid 
               
             
             = 
             
               
                 F 
                 pressure 
               
               
                 A 
                 syringe 
               
             
           
         
       
       
         
           
             
               where 
                
               
                   
               
                
               
                 A 
                 
                   syringe 
                    
                   
                       
                   
                 
               
             
             = 
             
               π 
               × 
               
                 
                   ( 
                   
                     
                       ID 
                       syringe 
                     
                     2 
                   
                   ) 
                 
                 2 
               
             
           
         
       
     
         [0009]    Where A syringe  is the area of the syringe and ID syringe  is the internal diameter of the syringe. 
         [0010]    However, there are variations in stiction caused by the rubber tip of the plunger, and varying tolerances in the force caused by the anti-free flow claws. Therefore, F pressure  typically cannot be used as the single parameter to trigger the pressure alarm because there would be too many false alarms. Accordingly, there is a need for a method of monitoring F pressure  that also allows for variations in stiction and spring force in the anti-free flow claws to avoid triggering false alarms. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention generally provides an improved method of detecting occlusions in the fluid lines of a medical infusion system, and a computer software product that performs the method. The improved method offers increased sensitivity and accuracy, without a corresponding increase in false alarms. 
         [0012]    According to one embodiment, a detected force value caused by pressure inside a fluid line is collected. The detected force values are collected during at least two consecutive moving time windows. The slope of a best-fit line for the detected force values is calculated within each time window. In one embodiment, the best-fit line is determined by a least squares method. A slope difference of the slopes of the best-fit lines is then calculated. The slope difference is then compared with a pre-determined threshold gradient value. A baseline force is defined as the measured force value when the slope difference is equal to the threshold gradient value. A relative force value is determined by subtracting the baseline force from the detected for value. An alarm is triggered if the relative force is greater than a pre-defined threshold force. 
         [0013]    According to another embodiment, the system comprises a syringe pump, a syringe, a processor, and a loadcell sensor operatively connected to the syringe. The system performs calculations using at least two consecutive moving time windows to process measured force values detected by the loadcell sensor. The system further: calculates the slope of a best-fit line within each time window, calculates a slope difference of the slopes of the best-fit lines, selects a threshold gradient value based on the size of the syringe; compares the slope difference to the threshold gradient value, defines a baseline force value as the measured force value when the slope difference is equal to the threshold gradient value, compares the measured force value with the baseline force value to calculate a relative force value, and triggers an alarm if the relative force value exceeds a pre-determined threshold force value. 
         [0014]    According to another embodiment, the measured force value is converted to a pressure value. The pressure is compared to a pre-determined occlusion pressure level, and an occlusion alarm is triggered if the pressure value is greater than the threshold value. 
         [0015]    According to another embodiment, the pump mechanism controller automatically stops the pump motor when pressure inside the fluid lines reaches the pre-determined occlusion pressure level. 
         [0016]    Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which: 
           [0018]      FIG. 1  is a schematic block diagram of an exemplary syringe pump system. 
           [0019]      FIG. 2  is a diagram showing the forces detected by the loadcell sensor of the syringe pump system shown in  FIG. 1 . 
           [0020]      FIG. 3  is a graphical illustration of the moving windows and slope differences on a measured force versus time graph. 
           [0021]      FIG. 4  illustrates how the baseline is established. 
           [0022]      FIG. 5  illustrates how an occlusion is detected according to one method of the present invention. 
           [0023]      FIG. 6  is a plot of maximum slope difference versus flow rate. 
           [0024]      FIG. 7  shows how the threshold gradient changes with respect to syringe size. 
           [0025]      FIG. 8  shows the relationship between window size and flow rates. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. 
         [0027]    Referring now to the Figures, and specifically to  FIG. 1 , there is shown one embodiment of a syringe pump system  10  that may be used to deliver medications to a patient  8 . This embodiment of the syringe pump system  10  includes a syringe  12  holding a fluid medication  13 , a housing  14 , a plunger  16 , a pump mechanism  18 , a pump mechanism controller  20 , a user interface display  22 , and a fluid line  24  leading from the pump system  10  to the patient  8 . The syringe pump system  10  may further include a processor  64 , a communications port  28 , a sensor  30  and anti-free flow claws  32 . In one embodiment, the sensor  30  may be a loadcell sensor. Loadcell sensor  30  may be located at the end of pump mechanism  18  where it contacts the plunger  16 . 
         [0028]    As shown in the system of  FIG. 2 , there are at least three forces that act on a syringe loadcell sensor  30  when it is located on the end of pump mechanism  18 : a fluid pressure  42 , a stiction force  44 , and a force  46  from the anti-free flow claws  32 . The combination of these forces results in a total force  40  detected by the loadcell sensor  30 . Stiction, or static friction, force  44  is the force required to overcome static cohesion between the plunger  16 , fluid  13  and the walls of syringe  12 . The force  46  from the anti-free flow claws  32  is a spring force caused by resistance of the anti-free-flow claws  32 . 
         [0029]    Referring to  FIG. 3 , there is shown an example of a force-time-curve  50 . The force-time curve  50  is a plot of the force reading  40  measured by loadcell sensor  30  over time. A best-fit line for the force-time curve  50  may be calculated by a linear regression equation for different segments of force-time curve  50 . In one embodiment, the best-fit line is determined by a least squares method. As shown in  FIG. 3 , the force-time curve  50  has a turning point  52 . According to one embodiment, a first moving time window  54  and a second moving time window  55  are selected. In one embodiment, the moving time windows  54  and  55  are consecutive. A slope  62  of the best-fit line  64  for the force-time curve  50  in moving window  54  may be calculated. Similarly, a slope  66  of the best-fit line  65  for the force-time curve  50  within moving window  55  may also be calculated. Based on the slope  62  of the best-fit line  64  for the force-time curve  50  in the first window  54  and the slope  66  of the best-fit line  65  for the force-time curve  50  in the second window  55 , a slope difference  60  may then be calculated. The slope difference  60  is calculated by subtracting the slope  62  from slope  66 . Additionally, a threshold gradient  70  may also be calculated. The threshold gradient  70  is the value of slope difference  60  at the turning point  52  of force-time curve  50 . 
         [0030]    Referring again to  FIG. 3 , a baseline force  72  may also be established. The baseline force  72  is set as the total force  40  measured by the loadcell sensor  30  when the slope difference  60  is greater than the threshold gradient  70 . A threshold force  74  may be determined based on the occlusion settings and the size of the syringe  12 . Additionally, a trigger force  76  may also be calculated. The trigger force  76  is defined as the sum of the baseline force  72  and the threshold force  74 . According to one embodiment, when the total force  40  detected by the loadcell sensor  30  is equal to or greater than the trigger force  76 , an occlusion alarm is triggered. The pump mechanism controller  20  may also stop the pump motor  22  automatically when the occlusion alarm is triggered. 
         [0031]    Alternately, a pressure value  80  may be calculated from total force  40 . The pressure value  80  may then be compared to a pre-determined occlusion threshold pressure value  82 . If the current pressure  80  is greater than the occlusion threshold pressure  82 , then an occlusion alarm is triggered. Similarly, the pump mechanism controller  20  may also stop the pump motor  22  automatically when the occlusion alarm is triggered. 
         [0032]    Referring now to  FIG. 4 , there is shown a flowchart illustrating the steps taken to establish the baseline force  72  according to one embodiment. A total force reading  40 , as measured by the loadcell sensor  30  is sampled every 100 milliseconds. Multiple loadcell force readings  40  are stored in a Window Queue  112 . The loadcell force readings  40  are stored in reverse chronological order, i.e., the most recent force reading is stored in a first position of the queue, and the oldest force reading is stored in a last position of the queue. Slope values  62 ,  66  are calculated and stored in a Slope History Queue  114 . Slope values are also stored in reverse chronological order in the Slope History Queue  114 . 
         [0033]    In step  110 , the most recent loadcell force  40  reading is recalled from Window Queue  112 . A current loadcell force  40  reading is retrieved in step  120 , and the last (oldest) loadcell force reading  40  is removed from the last position of Window Queue  112  in step  130 . In step  140 , the current loadcell force  40  reading is put into the first position of Window Queue  112 . A current slope value (cur_slope) of the best-fit line for the force readings stored in Window Queue  112  is calculated in step  150 . In step  160 , a last slope value (last slope) is removed from a last position in Slope History Queue  114 , and in step  170 , the current slope value (cur_slope) is added to a first position of Slope History Queue  114 . Then, in step  180 , the slope difference  60  between the current slope value (cur_slope) and the last slope value (last_slope) is calculated, and compared to the threshold gradient  70 . If the slope difference  60  is greater than the threshold gradient  70 , the calculated pressure corresponding to the current measured force is set as the baseline pressure  82  (0 psi) in step  190 . In step  200 , the process starting with step  110 , is repeated for each pair of moving time windows. If the slope difference  60  is not greater than the threshold gradient  70 , step  190  is not performed, and the process is repeated beginning with step  110  for the next total force reading  40 . 
         [0034]      FIG. 5  is a flowchart illustrating the steps for detecting occlusions according to one embodiment. Specifically, after detecting the turning point  52  and establishing the baseline force  72 , the following steps shown in  FIG. 5  monitor the current pressure  80 . The total force readings  40  measured by the loadcell sensor  30  are stored in a Window Queue  112 . Starting with step  210 , the most recent loadcell force reading  40  is recalled from Window Queue  112 . Then, a current loadcell force reading  40  is retrieved in step  220 . The current loadcell force reading  40  is converted to a current pressure value  80  in step  230 . In step  240 , the current pressure value  80  is compared to an occlusion threshold pressure  82 . If the current pressure value  80  is greater than the occlusion threshold pressure  82 , an occlusion alarm is triggered. If the current pressure value  80  is less than the occlusion threshold pressure  82 , the process repeats, starting with step  210 . 
         [0035]      FIG. 6  is a plot of the maximum observed slope differences  61  for a 30 cc syringe running at different flow rates  86 . As is shown in  FIG. 6 , as the flow rate increases, a maximum slope difference  61  also increases, according to approximately a 1/x ratio. It has been observed that doubling the flow rate  86  doubles the maximum slope difference  61 . In one embodiment, the threshold gradient  70  is set as 50% of the maximum observed slope difference  61  at a particular flow rate  86 . This threshold gradient value accounts for variation in the maximum slope difference due to differences across pumps and syringes. 
         [0036]      FIG. 7  is a bar graph showing how the threshold gradient  70  changes in relation to the size of syringe  12 . As shown in  FIG. 7 , the change in threshold gradient  70  in relation to syringe size is not linear. Thus, the value at which the threshold gradient  70  should be set is different for different syringe sizes. To account for such different threshold gradients  70 , the processor  64  of the system  10  may include a lookup table  90 , and different threshold gradient values corresponding to different syringe sizes may be stored in the lookup table  90 . In one embodiment the processor  64  sets the threshold gradient  70  based on the size of syringe  12 . Syringe size may be input by the user or automatically detected by the processor  64 . One example of lookup table  90  is Table 1, below: 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Formulation for setting the threshold gradient for different syringe sizes 
               
             
          
           
               
                   
                 Syringe Size 
               
             
          
           
               
                   
                 60 cc 
                 30 cc 
                 20 cc 
                 10 cc 
                 5 cc 
                 3 cc 
                 1 cc 
               
               
                   
                   
               
             
          
           
               
                 % of gradient 
                 80% 
                 100% 
                 107% 
                 89% 
                 80% 
                 49% 
                 18% 
               
               
                 threshold 
               
               
                   
               
             
          
         
       
     
         [0037]    Similarly, the size of the moving windows  54  and  55  must be selected. The accuracy of turning point  52  corresponds to the size of windows  54 ,  55 . Accordingly, if the windows  54  and  55  are small, the possibility of false alarms may increase because turning point  54  will be subject to more noise and may be less accurate. Conversely, if the windows  54  and  55  are large, the turning point  52  will be more accurate, but the time required to establish the baseline  72  will increase. This can lead to an increased risk that the preset pressure triggering level  82  or force triggering level  76  will be reached before baseline  72  can be calculated. Since an alarm may not be triggered before the baseline  72  is established, the occlusion could go undetected. Thus, an optimal window size  87  produces the most accurate turning point  52 , but the time taken to establish turning point  52  will be relatively long. A minimum window size  88  is the smallest window required in order to find the turning point  52 . 
         [0038]    Referring now to  FIG. 8 , there is shown a plot of optimal window size  87  versus flow rate  86 . As is shown in  FIG. 8 , the optimal window size  87  decreases as the flow rate  86  increases. According to one embodiment, the selected window size  89  is calculated by finding the best-fit line for an average between the optimal window size  87  and the minimum window size  88 . In one embodiment, the best-fit line is determined by a least squares method. The selected window size  89  may be a compromise between processing time for calculating the baseline  72  and the accuracy of determining the turning point  52 . 
         [0039]    According to one embodiment, the processor  64  determines the size of the moving windows  54  and  55  for a particular flow rate  86 . The processor  64  calculates the turning point  52  based on the threshold gradient value  70  selected for the particular size syringe  12  according to the look-up table  90 . The processor  64  stores the measured force values  70  detected by the loadcell sensor  30 . The processor  64  then calculates a slope difference  60  of the slope of the best-fit lines  64 ,  65  within each moving window  54 ,  55 . The processor  64  compares the slope difference  60  to the threshold gradient value  70 . The processor  64  sets the baseline force  72  as the measured force value  40  at the point when the slope difference  60  is greater than the threshold gradient value  70 . After the baseline force  72  is set, the processor  64  compares the measured force value  70  to the baseline force  72 . The processor  64  triggers an alarm if the measured force value  70  is greater than the trigger force  76  (trigger force  76  is calculated as the threshold force  74  plus the baseline force  72 ). Threshold force  74  is calculated based on the pressure occlusion settings for a particular syringe size  12 . 
         [0040]    Several alternative embodiments and examples have been described and illustrated herein. A person of ordinary skill in the art would appreciate the features of the individual embodiments, and the possible combinations and variations of the components. A person of ordinary skill in the art would further appreciate that any of the embodiments could be provided in any combination with the other embodiments disclosed herein. Additionally, the terms “first,” “second,” “third,” and “fourth” as used herein are intended for illustrative purposes only and do not limit the embodiments in any way. Further, the term “plurality” as used herein indicates any number greater than one, either disjunctively or conjunctively, as necessary, up to an infinite number. Additionally, the term “having” as used herein in both the disclosure and claims, is utilized in an open-ended manner. 
         [0041]    It will be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein. Accordingly, while the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the invention.