Patent Application
20050214129
The present invention relates to a means of determining and controlling the operating condition of a medical pump. More particularly, this invention relates to a means of adjusting a stroke frequency of a pump to compensate for individual variation in the stroke volume delivered by the medical pump.
Modern medical care often involves the use of medical pump devices to deliver fluids and/or fluid medicine to patients. Medical pumps permit the controlled delivery of fluids to a patient, and such pumps have largely replaced gravity flow systems, primarily due to the pump's much greater accuracy in delivery rates and dosages, and due to the possibility for flexible yet controlled delivery schedules. Of the modern medical pumps, those incorporating a diaphragm are often preferred because they provide a more accurate controlled rate and volume than do other types of pumps.
A typical positive displacement pump system includes a pump device driver and a disposable fluid or pumping chamber, defined in various forms including but not limited to a cassette, syringe barrel or section of tubing. A disposable cassette, which is adapted to be used only for a single patient and for one fluid delivery cycle, is typically a small plastic unit having an inlet and an outlet respectively connected through flexible tubing to the fluid supply container and to the patient receiving the fluid. The cassette includes a pumping chamber, with the flow of fluid through the chamber being controlled by a plunger or pumping element activated in a controlled manner by the device driver.
For example, the cassette chamber may have one wall formed by a flexible diaphragm that is reciprocated by the plunger and the driver to cause fluid to flow. The pump driver device includes the plunger or pumping element for controlling the flow of fluid into and out of the pumping chamber in the cassette, and it also includes control mechanisms to assure that the fluid is delivered to the patient at a pre-set rate, in a pre-determined manner, and only for a particular pre-selected time or total dosage.
The fluid enters the cassette through an inlet and is forced through an outlet under pressure. The fluid is delivered to the outlet when the pump plunger forces the membrane into the pumping chamber to displace the fluid. During the intake stroke the pump plunger draws back, the membrane covering the pumping chamber pulls back from its prior fully displaced configuration, and the fluid is then drawn through the open inlet and into the pumping chamber. In a pumping stroke, the pump plunger forces the membrane back into the pumping chamber to force the fluid contained therein through the outlet. Thus, the fluid flows from the cassette in a series of spaced-apart pulses rather than in a continuous flow.
One of the requirements for a medical pump is that it is able to deliver precise volumes at precise delivery rates. Conventional pumps, in general, rely on nominal or empirical data to estimate the delivery volumes and delivery rates, and do not provide mechanisms for adjusting an actual delivery due to variations from this nominal or empirical data. This lack of adjustment during an actual delivery limits the accuracy of these pumps.
It is therefore a principal object of this invention to provide means for adjusting a stroke frequency of the pump to compensate for variation in the stroke volume based on pressure data from the medical pump.
These and other objects will be apparent to those skilled in the art.
A medical pump with a closed loop stroke feedback system and method, for use with a fluid chamber or pumping chamber, is disclosed. The fluid or pumping chamber can be included in or defined by a cassette, syringe barrel, or section of tubing. The pump includes a pumping element that intermittently pressurizes the pumping chamber during a pumping cycle. A pressure sensor detects the pressure exerted by the pumping element on the pumping chamber. A position sensor detects the position of the pumping element. A processing unit processes pressure data from the pressure sensor and position data from the position sensor to determine a calculated stroke volume of the pump for a pump cycle, and to adjust a stroke frequency of the pump to compensate for variation in the stroke volume.
In operation, the processing unit sets a stroke frequency for a desired dosage rate based on a nominal stroke volume, determines when an outlet valve of the pumping chamber opens, determines a calculated pressurization volume from a beginning of the pump cycle to the point when the outlet valve opens, determines a change in pressurization volume by subtracting the calculated pressurization volume from a nominal pressurization volume, determines a change in stroke volume by multiplying the change in pressurization volume by a ratio of pumping chamber expansion under pressure at the middle of the pumping cycle to pumping chamber expansion under pressure at the start of the pumping cycle, determines a calculated stroke volume based on the change in stroke volume, and adjusts the stroke frequency to compensate for variation between the calculated stroke volume and the nominal stroke volume.
With reference to
The medical pump 10 and cassette 12 are shown with several components for implementing the present invention. Those of ordinary skill in the art will appreciate that the pump 10 and cassette 12 may include many more components than those shown in
With reference to
The cassette 12 may include an inlet 14 and an outlet 16 formed in main body 18. An inlet fluid line 20 couples the inlet port 14 on the main body 18 to a fluid source (not shown) such as an IV bag or other fluid container. Similarly, an outlet fluid line 22 couples the outlet port 16 on main body 18 to the body of a patient (not shown).
With reference to
The inlet valve 26 resides between inlet port 14 and the pumping chamber 24. Inlet valve 26 operates to physically open and close the fluid communication between inlet port 14 and pumping chamber 24.
Similarly, the outlet valve 28 resides between the pumping chamber 24 and outlet port 16. Outlet valve 28 operates to physically open and close the fluid communication between pumping chamber 24 and outlet port 16. The pumping chamber 24, inlet valve 26, and outlet valve 28 are all operatively associated with the pump 10 to control the flow of fluid through the cassette 12. As will be understood from the description below, the cassette is a passive valve system requiring pressurization of the pumping chamber 24 prior to fluid delivery. Inlet valve 26 and outlet valve 28 react to the pressure of the pumping element 44 on the pumping chamber 24.
In operation, fluid enters through the inlet 14 and is forced through outlet 16 under pressure. The fluid is delivered to the outlet 16 when the pump 10 displaces the membrane 23 and thereby compresses the pumping chamber 24 to expel the fluid. As shown in
Referring to
A memory 34 communicates with the processing unit 30 and stores code and data necessary for the processing unit 30 to calculate and output the operating conditions of pump 10. More specifically, the memory 34 stores a programming code 36 formed in accordance with the present invention for processing data to determine and control the operating condition of the pump 10.
A clock 37 is used to keep time in the pump 10. The clock 37 is connected to the processing unit 30, and provides the processing unit 30 with time information for correlating data over time or conducting time sensitive activities.
An electric motor 38 is controlled by processing unit 30 and is energized by a power supply 40 to serve as a prime mover for rotatably driving a shaft 42 connected to the motor 38. The processing unit 30 orders the motor 38 to run at different speeds depending on the flow rate desired through the pump 10. The down-stroke or delivery portion of the stroke has the motor 38 running directly from power supply 40. The up-stroke, retract or fill portion of the stroke is run at a voltage set by the processing unit 30, so that the retract times are varied by the processing unit 30, where higher desired flow rates require faster retract speeds.
A pumping element 44 is operatively associated with the shaft 42. When energized, the pumping element 44 reciprocates back and forth to periodically down-stroke, causing pumping element 44 to press on pumping chamber 24, and expel fluid therefrom. On an up-stroke, pumping element 44 releases pressure from pumping chamber 24 and thereby draws fluid from inlet port 16 into pumping chamber 24. Thus, the pumping element 44 intermittently pressurizes the pumping chamber 24 during a pumping cycle.
A pressure sensor 46 is operatively associated with the pumping element 44 to detect the force or pressure exerted by the pumping element 44 on the pumping chamber 24. As shown, the pressure sensor 46 is directly connected to the pumping element and is positioned in-line with the pumping element 44, between the pumping chamber 24 and the shaft 42. The pressure sensor 46 is the only pressure sensor included in the medical pump 10, and operates to sense the force on pumping element 44 as well as to generate a pressure signal based on this force. The pressure sensor 46 is in electronic communication with the processing unit 30 to send the pressure signal to the processing unit 30 for use in determining operating conditions of pump 10. One of ordinary skill in the art will appreciate that the pressure sensor 46 may be a force transducer or any other device that can operatively sense the pressure brought to bear on the pumping chamber 24 by pumping element 44.
A position sensor 48 is operatively associated with the pumping element 44 to directly or indirectly detect the position of the pumping element 44. The position sensor 48 tracks the pumping cycle of pump 10 by detecting the position of the pumping element 44. As shown, the position sensor 48 is associated with the shaft 42. The position sensor 48 generates a position signal by detecting the rotational position of the shaft 42. The position sensor 48 is in electronic communication with the processing unit 30 to send the position signal to the processing unit 30. The processing unit 30 utilizes this information to associate the incoming pressure data with a particular portion of the pump cycle. One of ordinary skill in the art will appreciate that the position sensor 48 could alternatively track a cam attached to the shaft 42 or the pumping element 44. Additionally, one of ordinary skill in the art will appreciate that the position sensor 48 as used herein includes but is not limited to mechanical indicators such as pivoting dial indicators, electronic switches, Hall Effect sensors, and optical based position detectors.
Referring to
Referring to
Referring to
Referring to
The curve starts at 0 degrees or Bottom Dead Center (BDC) with the pumping element 44 deflecting the diaphragm 23 of the pumping chamber 24 a minimal amount at this point. The position of the pumping element 44 at 0 degrees, and the resultant displacement of pumping chamber 24 can be seen in
Cycle portion A shows the pressurization of the pumping chamber 24 and is shown in this example as occurring from 0 to 30 degrees. During the pressurization cycle portion A, the pumping element 44 moves into the cassette 12 (which is called the pressurization stroke because fluid is compressed in pumping chamber 24 of the cassette 12) building force within the pumping chamber 24, while the outlet valve 28 remains closed. The position of the pumping element 44 between 0 and 30 degrees, and the resultant displacement of pumping chamber 24 can be seen in
A delivery cycle portion B begins when the force within the pumping chamber 24 is sufficient to open the outlet valve 28. During the delivery cycle portion B, the pumping element 44 moves into the cassette 12 so as to build a force within the pumping chamber 24 sufficient to open the outlet valve 28 and expel fluids from the pumping chamber 24. The delivery cycle portion B is shown in this example as occurring from 30 to 180 degrees. The position of the pumping element 44 between 30 and 180 degrees, and the resultant opening of the outlet valve 28 can be seen in
The delivery cycle portion B ends at Top Dead Center (TDC), or 180 degrees of rotation, and a depressurization cycle portion C begins. The depressurization cycle portion C shows the depressurization the pumping chamber 24 and is shown in this example as occurring from 180 to 210 degrees. During the depressurization cycle portion C, the pumping element 44 moves out of the cassette 12 (which is called the up-stroke, depressurization or inlet stroke) and the force drops off. As the diaphragm 23 returns to its initial position, while the inlet valve 26 remains closed, negative pressure builds within the pumping chamber 24.
A refill cycle portion D begins when the negative pressure within the pumping chamber 24 is sufficient to the open the inlet valve 26. During the refill cycle portion D, the pumping element 44 moves out the cassette 12 building negative pressure within the pumping chamber 24 sufficient to open the inlet valve 26 and draw fluids into the pumping chamber 24. The refill cycle portion D is shown in this example as occurring from 210 to 360 degrees, or Bottom Dead Center (BDC). The position of the pumping element 44 between 210 to 360 degrees, and the resultant opening of the inlet valve 26 can be seen in
Referring to
Specifically, the processing unit 30 begins execution of the programming code 36 at a block 50 and proceeds to block 52 where the processing unit 30 sets a stroke frequency for a desired dosage rate. The stroke frequency is determined by the processing unit 30 based on a nominal stroke volume. This nominal stroke volume can be supplied from empirical evidence of an average normal stroke volume for all pumps of a particular type or for each individual pump. Once the stroke frequency is set, the processing unit 30 proceeds to block 54 where it determines a calculated stroke volume of the pump for a pump cycle based on the pressure data from the pressure sensor 46 and position data from the position sensor 48. Once the calculated stroke volume has been determined, the processing unit 30 proceeds to decision block 56 where it determines if the calculated stroke volume is greater than a given threshold value. One of ordinary skill in the art will understand that the threshold value disclosed herein is predetermined from experimental data, and will vary from pump model to pump model.
If the result from decision block 56 is negative, then the execution of the programming code 36 by the processing unit 30 is complete and ends in block 60. If the result from decision block 56 is positive, then the processing unit 30 proceeds to block 58 where it adjusts the stroke frequency to compensate for the variation between the calculated stroke volume and the nominal stroke volume. Once the stroke frequency has been adjusted, the execution of the programming code 36 by the processing unit 30 is complete and ends in block 60.
Referring to
Specifically, the processing unit 30 begins execution of the programming code 36 at a block 62 and proceeds to block 64 where the processing unit 30 sets a stroke frequency for a desired dosage rate. Like step 52 from
Once the stroke frequency is set, the processing unit 30 proceeds to block 66 where the processing unit 30 determines when an outlet valve 28 of the pumping chamber opens. The processing unit 30 determines the opening of outlet valve 28 based on data from the pressurization cycle portion A (as shown in
Specifically, by monitoring the slope of the pressure data over time, the opening of the outlet valve 28 can be determined. For instance, by taking the derivative of the pressure data over time, where the pressure derivate is greater than zero in the following equation:
the pressure within the pumping chamber 24 is increasing and the outlet valve 28 remains closed. Where the pressure derivate is less than zero in the following equation:
the pressure within the pumping chamber 24 is decreasing and the outlet valve 28 has opened, and the processing unit 30 determines at what angular position the pumping element 44 was in when the outlet valve 28 opened (i.e., where dp/dt changes from >0 to <0).
Once the outlet valve 28 opening is determined, the processing unit 30 proceeds to block 68 where the processing unit 30 determines a calculated pressurization volume from the beginning of the pump cycle to the point when the outlet valve opens. The processing unit 30 determines the calculated pressurization volume based on data from the pressurization cycle portion A (as shown in
Specifically, the processing unit 30 converts the angle at which the outlet valve 28 opens to a displacement distance of the pumping element 44 as follows:
xi=Lcam*(1−cos(θi))
where xi is the displacement distance, Lcam is half the distance of a stroke for the pumping element 44, and θi is the angular position of the pumping element 44 at which the outlet valve 28 opened. The processing unit 30 converts the displacement distance xi into a pressurization stroke volume based on a ratio of volume to displacement distance for the pump 10. This ratio is based on a nominal pressurization volume and a nominal displacement distance for a typical pump 10. This pressurization volume and a nominal displacement distance can be supplied from empirical evidence of an average normal stroke volume for all pumps of a particular type or for each individual pump.
Once the calculated pressurization volume is determined, the processing unit 30 proceeds to block 70 where the processing unit 30 determines a change in pressurization volume by subtracting the calculated pressurization volume from a nominal pressurization volume. Again, the nominal pressurization volume can be supplied from empirical evidence of an average normal stroke volume for all pumps of a particular type or for each individual pump.
The change in pressurization volume determined here is proportional to variations in the actual stroke volume, as a portion of compressive forces can be lost and not translated into fluid delivery. For instance, a portion of compressive forces can be lost where there is excessive compliance within mechanical components (such as the cassette 12) or air bubble entrained within the pumping chamber 24.
Once the change in pressurization volume is determined, the processing unit 30 proceeds to block 72 where the processing unit 30 determines a change in stroke volume by multiplying the change in pressurization volume by a ratio of pumping chamber expansion under pressure at the middle of the pumping cycle to pumping chamber expansion under pressure at the start of the pumping cycle. This “pumping chamber expansion under pressure” is also referred to as the compliance of the pumping chamber. For instance, the diaphragm 23 of the cassette 12 is constructed of various materials with corresponding spring rates. As pressure is placed on the pumping chamber 24, the volume of the chamber 24 changes according to the overall spring rate of the cassette 12. As the volume of the pumping chamber 24 will grow larger as pressure increases. However, the pumping chamber compliance is not the same throughout the pumping cycle. At the beginning of the pumping cycle (0 degrees) the compliance (ratio of volume change to pressure) is higher than the compliance at the middle of the pumping cycle (180 degrees). Again, the compliance can be supplied from empirical evidence of an average normal stroke volume for all pumps of a particular type or for each individual pump. Thus, to better estimate the change in stroke volume, the change in pressurization volume is multiplied by a ratio of compliance at the middle of the pumping cycle to the compliance at the beginning of the pumping cycle.
Once the change in stroke volume is determined, the processing unit 30 proceeds to block 74 where the processing unit 30 determines a calculated stroke volume based on the change in stroke volume. Specifically, the change in stroke volume is added to the nominal stroke volume to arrive at the calculated stroke volume. This calculated stroke volume provides a very close estimate of the actual individual stroke volume delivered.
Once the calculated stroke volume has been determined, the processing unit 30 proceeds to decision block 76 where it determines if the calculated stroke volume is greater than a given threshold value. One of ordinary skill in the art will understand that the threshold value disclosed herein is predetermined from experimental data, and will vary from pump model to pump model. If the result from decision block 76 is negative, then the execution of the programming code 36 by the processing unit 30 is complete and ends in block 80.
If the result from decision block 76 is positive, then the processing unit 30 proceeds to block 78 where it adjusts the stroke frequency to compensate for the variation between the calculated stroke volume and the nominal stroke volume. Once the stroke frequency has been adjusted, the execution of the programming code 36 by the processing unit 30 is complete and ends in block 80.
It will be understood that the threshold determination above from block 76, could be made on criterional information other than the calculated stroke volume above. For instance, the threshold determination 76 could similarly be made based on the angle at which the outlet valve 28 opens, the calculated pressure volume, the change in pressurization volume, or the change in stroke volume determined above. In any of these cases, the given threshold value would necessarily be scaled according to the designated alternative criterional information. Additionally, the timing of the threshold determination 76 could also be adjusted based on the designated alternative criterional information. For instance, were the calculated pressurization volume designated as the alternative criterional information, the threshold determination 76 could occur at any point after step 68.
Likewise, it will be understood that the stroke frequency adjustment above from block 78, could be made on criterional information other than the calculated stroke volume above. For instance, the stroke frequency adjustment 78 could similarly be made based on the angle at which the outlet valve 28 opens, the calculated pressure volume, the change in pressurization volume, or the change in stroke volume determined above. In any of these cases, the stroke frequency adjustment 78 would necessarily be scaled according to the designated alternative criterional information. Additionally, the timing of the stroke frequency adjustment 78 could also be adjusted based on the designated alternative criterional information. For instance, were the calculated pressurization volume designated as the alternative criterional information, the stroke frequency adjustment 78 could occur at any point after step 68.
Additionally, further restrictions can be placed on the execution of the stroke frequency adjustment 78 step. For example, the calculated stroke volume could comprise multiple calculated stroke volumes averaged together. This averaging of multiple calculated stroke volumes potentially reduces the effects that minor variations in stroke volume and/or statistical noise have to the overall pump 10 operation.
In operation, the above closed loop stoke feedback system embodiments provide several advantages. The first advantage is that the actual volume delivered per stroke can be used by the processing unit 30 to continuously adjust the stroke rate. The second advantage is that the detection of the pressure data profile and the determination of the opening of outlet valve 28 permits the processing unit 30 to determine lost stroke volume (i.e. calculated stroke volume as compared with the nominal stroke volume) and to use this as an indicator of presence of air in the pumping chamber 24, as well as determining the size of air bubbles in the set. These advantages of the present invention limit the effects of all causes of delivery error, including: compliance of physical components, air in the delivery fluid, variations in line pressure, and manufacturing variability of physical components (for example, in valve opening pressures).
In cassette type pumps the present invention is particularly advantageous. As the cassettes are disposable, the cassettes are produced in very high volumes there are limitations to reducing the manufacturing variability of the physical components and assemblies. The overall accuracy provided by present invention improves the ability to perform accurate deliveries within a broader range of these manufacturing variabilities of the physical components and assemblies.
The third advantage is that the detection of the pressure data profile and the determination of the opening of outlet valve 28 permits the processing unit 30 to deliver in smaller increments for very low flow rates in a more continuous manner (known as Low Flow Continuity). In general, Low Flow Continuity is defined as the ability of a pump to deliver at rates of 1 ml/hr to 0.1 ml/hr or less with periods of “no-flow” not exceeding 20 seconds and bolus volumes not exceeding 2 micro-liters. To meet the highest Emergency Care Research Institute (ECRI) industry standards for Low Flow Continuity and achieve an “Excellent” ECRI rating, the pump must deliver fluid in increments no greater than two micro-liters at a flow rate of 0.1 milliliter per hour with a maximum “no-flow” period of 20 seconds.
As shown in
Whereas the invention has been shown and described in connection with the embodiments thereof, it will be understood that many modifications, substitutions, and additions may be made which are within the intended broad scope of the following claims. From the foregoing, it can be seen that the present invention accomplishes at least all of the stated objectives.
