The present disclosure relates to fluid flow control systems, such as intravenous infusion pumps, and more particularly to feedback control infusion pumps with flow and volume sensing.
In some situations, such as with intravenous infusions, the flow rate range requirement is very broad, covering more than four orders of magnitude of rate, e.g., from about 0.1 ml per hour to about 6,000 ml per hour. If any sort of low flow rate sensitivity is to be expected, then the response of a flow sensor should be logarithmic or at least non-linear. The absolute calibration of such a flow sensor would be very challenging, especially at low flow rates. Even without the requirement for calibration, it is difficult to find a commercially available flow sensor that services this flow rate range.
Certain infusions have historically been managed by air pressure delivery systems, most commonly found in the operating room and in emergency situations. Prior art attempts have been made to determine the flow rate via pressure monitoring and control. For example, U.S. Pat. No. 5,207,645 to Ross et al. discloses pressurizing an IV bag and monitoring pressure to infer flow rates. However, the prior art systems lack independent flow sensing and, therefore, do not offer enough information to provide accurate and safe infusions. Under the best of circumstances, there is not enough information in the pressure signal alone to provide the accuracy needed for intravenous infusion therapy.
Furthermore, there are a number of likely failure modes that would go undetected using this pressure signal alone. An infusion pump must be able to respond to events in a relevant time frame. International standards suggest that a maximum period of 20 seconds can lapse before fluid delivery is considered “non-continuous.” As an example, for an infusion of 10 ml per hour, the system would want to resolve 20 seconds of flow, which corresponds to 0.056 mL. This volume represents one part in 180,000 of the total air volume. Temperature-induced change in pressure brought about by a normal air conditioning cycle is far greater than this signal. The measurement of pressure alone is not adequate for an intravenous infusion device. No general-purpose full-range, infusion devices using pressure-controlled delivery are known to be on the market.
It would therefore be useful to develop a fluid delivery device and a control system and method therefor that could control the rate fluid flow based on a high resolution, highly responsive fluid flow sensor that could be calibrated in situ by correlating its signal with changes in fluid volume over time.
In one aspect, a fluid control system that combines flow rate and absolute volume calculations is provided. In further aspects, a fluid delivery device and methods employing the same are provided.
An advantage of the present disclosure resides in its accuracy. The present disclosure offers two novel views of flow control, one operating over many minutes, and the other operating many times per second, which is analogous to having both a speedometer and an odometer, so that a steady speed may be maintained, while still making adjustments to arrive at one's destination on time. The high degree of accuracy is advantageous for infusion of vasoactive drugs, since the flow should be as continuous as possible and the flow rate resolution should be high enough to resolve a 1% change in requested flow rate. Likewise, for long term doses (e.g., 12-72 hours), it is important to complete the dose on time. Also, for longer-term infusions, the accuracy of the system should not degrade over time. There are clinical and operational problems with infusions that are completed too soon (e.g., increased chance of clotting off the IV line) or too late (e.g., unused medication, biohazard, etc.). For intermittent infusions, the time the dose starts is important, so that the peak serum levels of the intermittent medication occur as ordered. For market acceptance, the device should perform well according to worldwide testing standards, including the integrated flow volume “trumpet curve.”
An additional advantage of the present disclosure resides in its simplicity. Instead of a complex electromechanical assembly of gears, levers, motors, and switches, a sensor-based flow regulation system can be relatively simple and low cost in its construction.
In operation, a bag of fluid to be infused is set inside a rigid container. The device automatically measures the volume of liquid in the bag. A user input device is provided to allow the user to input (1) how fast the fluid should flow; (2) when the infusion should be complete; and/or (3) the amount of time the infusions should take to finish.
In certain embodiments, the present disclosure is capable of operating over an extended range of flow rates, e.g., from below 1 milliliter per hour to 6,000 milliliters per hour. At all flow rates, the fluid flows continuously without the characteristic flow disruptions of a typical motor driven large-volume pump.
Other benefits and advantages of the present disclosure will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.
The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.
Referring to the drawings, wherein like numerals reference numerals are used to indicate like or analogous components throughout the several views,
Referring now to
A calibration tank 60 of known volume is connected to the air pump 50 via a tank connection line 604, a tank valve 102, and a tank valve line 602. The tank 60 may be vented to atmosphere via a tank vent valve 104.
The liquid 40 is fluidically coupled to an output 500 via a liquid drain line 610, going through a fluid flow resistor 400 and through an output line 612. The liquid 40 may be, for example, a medication fluid, intravenous solution, or the like, and the output 500 may be, for example, a patient or subject in need thereof. An inline flow sensor 900 is provided in the line 612, as described in greater detail below.
The tank 60 is connected to a tank pressure sensor 204 and a tank temperature sensor 304. The bladder 20 is connected to a bladder pressure sensor 202 and a bladder temperature sensor 302.
Referring now to
With continued reference to
With reference now to
Ultimately, the objective of volume measurement is to know the quantity of liquid 40 remaining in an infusion and how that quantity changes over time.
The pressure frame 10 defines a rigid container of known volume, Vframe. This volume is known by design and is easily verified by displacement methods. Within the pressure frame 10, there is the air bladder 20, which has a nominal capacity greater than the volume Vframe. When expanded, the bladder must conform to the geometry of the rigid container and its contents. The volume of liquid 40 to be infused, Vtbi, is equal to Vframe, less the fixed and known volume of the bladder 20 itself, Vblad, less any incompressible materials of the bag 30, Vblag, and less the volume of gas in bladder 20, Vgas. Once the value Vgas is computed, then Vtbi may be computed as follows:
V
tbi
=V
frame
−V
blad
−V
bag
−V
gas
With the following method, at any given point in time, the volume of air contained in the bladder, Vgas, can be measured and Vtbi can be subsequently computed.
For purposes of economy and flexibility, the pump 50 may be an imprecise air pump, such as that of a rolling diaphragm variety, although other types of pumps are also contemplated. The output of such a pump may vary significantly with changes in back pressure, temperature, age of the device, power supply variation, etc. One advantage of the device and method disclosed herein is that they allow an imprecise pump to be used in a precision application, by calibrating the pump in situ.
The initial pressure in the bladder 20, Pbladder1, is measured using the bladder pressure sensor 202. The tank valve 102 is set to a closed state via the tank control valve line 702 from the processor 700. The bladder valve 106 is set to an open state via the tank control valve line 706 from the processor 700. The pump 50 is activated by the processor 700 via the pump control line 750 for a period of time, Stest, nominally, for example, about 250 milliseconds. A new measurement of the pressure in the bladder 20 is made, Pbladder2. Based on the percent of pressure change from this pumping action, a new pump activation time, Spump, will be computed. This calculation needs no precision; it is only intended to find an amount of pumping that provides a significant change in pressure, Pdeltatarget, in the bladder 20, for example, on the order of about 10%.
In step 804, the pump 50 or the tank vent valve 104 are activated to increase or decrease, respectively, the pressure, Ptank, in the tank 60, so that it approximately equals the pressure, Pbladder, in the bladder 20. The combination of valve and pump settings required for such adjustments are shown in the table below:
Adjustments made in step 804 can be made iteratively until Ptank is roughly equal to Pbladder, for example, within about 5% of the relative pressure measured in Pbladder. This does not need to be a precise process. Following the adjustment, the pressure in tank 60, Ptank2, is recorded.
In step 806, the system is configured to increase the pressure in tank 60, as shown in the above table. The pump 50 is activated for a time period equal to Spump. After a delay of approximately five seconds, the pressure in the tank 60 is measured, Ptank3. This delay is to reduce the effect of an adiabatic response from the increase in pressure in the tank 60.
In step 808, the system is configured to increase the pressure in the bladder 20, as shown in the above table. The pump 50 is activated for a period equal to Spump. After a delay of approximately five seconds, the pressure in the bladder 20 is measured, Pbladder3. This delay is to reduce the effect of an adiabatic response from the increase in pressure in the bladder 20.
Because the initial pressures in the bladder 20 and the tank 60 were approximately equal, the quantity of air mass injected into the tank 60 in step 806 and into the bladder 20 in step 808 will be roughly equal, even though the pump 50 need not be a precise metering device.
We take advantage of several simplifications. First, the ambient temperature for sequential steps 806 and 808 is unchanged. Second, the atmospheric pressure during sequential steps 806 and 808 is unchanged. These conditions simplify the ideal gas law formula and allow the use of gauge pressure measurements, rather than absolute pressure.
In step 810, the volume of gas in the bladder 20, Vgas, can be calculated with a reduced form of PV=nRT:
As examples of this calculation, if the pressure change were the same in the bladder 20 and the tank 60, then Vgas would be equal to Vtank. If the pressure change in the bladder 20 were 20% as large as that in the tank 60, then Vgas would be 5 times greater than Vtank.
Step 812 derives the value for Vtbi from Vgas, using known values for Vframe, Vblad, and Vbag and using the calculated value of Vgas, from step 810.
V
tbi
=V
frame
−V
blad
−V
bag
−V
gas
The valves 102, 106, 104, and 108 can be configured in many ways, including multiple function valves and or manifolds that toggle between distinct states. The depiction herein is made for functional simplicity, not necessarily economy or energy efficiency.
The exploded view of
The interior of the proximal housing 932 is shown in
Referring to the section view of
The graph depicted in
In operation, the light source 921, which may be, for example, an LED array, transmits light through the first optical rib 933a and into the cavity 955. The light incident upon the ball 942 is transmitted through the ball 942 and through the second optical rib 933b to form a light intensity pattern on the photosensor array 922. The photosensor array 922 may be, for example, a charge-coupled device (CCD) array, photodiode array, complimentary metal oxide semiconductor (CMOS) digital detector array, or the like.
The optical transmitter may include one or more light source elements having a wavelength, for example, in the infrared (IR), visible, or ultraviolet (UV) region and the housing and ball member may be formed of a material that optically transmits light of the light source wavelength. The light source may be an array of light elements, such as LEDs, or laser, etc. The light source may be segmented along the axis or may be a continuous, e.g., scanned or otherwise optically formed beam. The light source may illuminate the detector array along its length simultaneously or by sequentially scanning along its length. The refractive effect of a transparent ball member may have a focusing effect on the light passing therethrough that may be detected by the photosensor array. Alternatively, a nontransmissive ball may be employed and the ball position may be determined by detecting the position of a shadow cast by the ball on the photosensor array. In still further embodiment, the ball member may have reflective surface and the optical sensor array may be positioned to detect light reflected from the surface of said ball.
The output of the photosensor array 922 may be passed via the data line 710 to the processing system 700, which may include a position-detection module or circuitry wherein the axial position of the ball 942 within the channel 955 is determined. The axial position of the ball 922 may in turn be used to determine a flow rate and/or calibrate or correlate ball 922 positions with known flow rates calculated by other means such as plural volume measurements made using the method outlined in
In certain embodiments, the known flow rates corresponding to axial ball positions may be stored in a memory of the processing system 700, for example, in a table, database, or the like. In such embodiments, when an axial position of the ball 922 is measured, the measured position of the ball may be compared with the table of known flow rates and the flow rate corresponding to the measured axial position is then determined. In other embodiments, calibration measurements of axial ball position and known flow rates may be used to derive an algorithmic formula for mapping a measured axial ball position to a corresponding flow rate. In such other embodiments, when an axial position of the ball 922 is measured, the derived algorithmic formula may then be used to determine the flow rate.
While the present disclosure provides a currently preferred implementation of the system herein, it will be recognized that alternate inline flow sensors are also contemplated.
Once the fluid volume has been computed, then multiple measurements made over time will yield knowledge of fluid flow rate, which is, by definition, fluid volume changing over time. Repeated measurements of volume over time provided more and more resolution of average flow rate. The average flow rate and the volume of liquid 40 remaining to be infused can be used to estimate the time at which the fluid volume will be delivered. If the infusion is to be completed within some specified period of time, any error between the specified time and the estimated time can be calculated and the flow rate can be adjusted accordingly.
There are situations where the short-term flow rate is of interest. Rather than make repeated volume measurements over a short period of time, there is an alternative approach. Once the gas volume in bladder 20 is known, then the observation of pressure decay in the bladder can be converted directly to a flow rate. It is important to know that the measurement of pressure decay, by itself, is not adequate to compute flow rate. For example, if the pressure were decaying at a rate of 10% per hour, this information cannot be converted into flow rate, unless the starting gas volume is known. As an example, if Vgas has been measured to be 500 ml and the absolute pressure is decaying at a rate of 5% per hour, then the flow rate is 5% of 500 ml per hour or 25 ml per hour. The knowledge of the initial volume is critical to compute fluid flow rate.
The measurement of pressure decay is a simple procedure of observing the time the absolute pressure of Pbladder to drop by a small, but significant, amount, preferably for example about 2%. Because the processor 700 is capable of measuring times from microseconds to years, this measurement carries a very wide dynamic range. By observing a 2% drop, the change in pressure is well above the noise floor of the pressure measurement system.
A flow chart outlining an exemplary process 1000 for calculating flow rate by monitoring the rate of pressure decay in the bladder 20 is shown in
The relationship and purpose of having two independent measurement methods for determining flow rate is best described by referring to
One purpose of the two measurement systems is to calibrate flow measurement 865 with repeated values over time from primary volume measurement 861. Flow measurement 865, e.g., as determined as described above by way of reference to the flow sensor 900, is a measurement of flow rate or the first derivative of fluid quantity with respect to time. If one were to integrate the value of flow measurement 865 over time, the result would be a quantity of fluid. Any errors in this signal would accumulate, providing decreasing volume accuracy over time.
In contrast, an integral signal, such as that from primary volume measurement 861, e.g., calculated using the volume measurement method as described herein, has a fixed error that does not accumulate over time. In fact, as a percentage, the error obtained with an integral signal will decrease over time. As an analogy, if one were to attempt to reach a certain distance in a determined period of time, the use of a speedometer alone would lead to an obvious and significant error. Using this analogy, if one were to use integral measurements, such as those provided by an odometer and a clock, the resultant accuracy would be high.
Flow measurement 865, as described above, operates over a very wide flow rate range and cannot, in any practical way, be calibrated in advance to accommodate manufacturing variances and other environmental factors such as fluid viscosity. For any given fluid flow rate, the signal from flow measurement can be measured and correlated with repeated measurements over time from primary volume measurement 861. For example, if the measurement from flow measurement 865 was observed to a value “x” over a period of ten minutes and a measurement made by primary volume measurement 861 at the beginning of this period was 100 mL and a subsequent measurement made by primary volume measurement 861 at the end of this period was 90 mL, a correlation could be made between flow signal “x” and a flow rate of 10 mL per 10 minutes, or, 60 mL per hour. Flow rate calibration data may be maintained in memory, preferably a nonvolatile memory, of the processing system 700.
Another purpose of the dual measurement system is to distinguish between two sources of fluid directed to the same output. For purposes of distinguishing the source of fluid, assume that flow measurement 865 has been calibrated at various flow rates as described above. If a secondary fluid source 862 is connected to the system, as shown in
Yet another purpose of the dual measurement system is to detect a condition where gas is expressed from the primary infusion liquid. If a quantity of air leaves the system by way of an in-line air elimination filter 864, then an increased pressure drop will be observed. By itself, this increased pressure drop would indicate that the fluid flow rate increased proportionally. If air were to escape the system from air elimination filter 864, the signal from flow measurement 865 would remain unchanged, providing an indication that the pressure drop should be interpreted as an escape of air, not an increased in fluid flow. In this circumstance, without flow measurement 865, the pressure signal would be interpreted incorrectly.
Yet another purpose of the dual measurement system is to detect a condition where a leak in the pneumatic system exists. If an air leak occurs in the system, a pressure drop will be observed. By itself, this pressure drop would indicate that fluid is flowing from the system. If air were leaking, the signal from flow measurement 865 would be zero, providing an indication that the pressure drop should be interpreted as a leak of air, not as fluid flow. In this circumstance, without flow measurement 865, the pressure signal would be interpreted incorrectly.
The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
Number | Date | Country | Kind |
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PCT/US2007/002039 | Jan 2007 | US | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US07/04945 | 2/27/2007 | WO | 00 | 8/27/2008 |
Number | Date | Country | |
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60777193 | Feb 2006 | US |