Method And Apparatus For Fluid Flow Control

Abstract

A method and apparatus for parenteral administration of medical fluids, wherein a normally shut-off intravenous feeding tube is selectively opened at a frequency and open period duration automatically regulated by a digital control system to establish a fluid flow rate at any selected rate over a wide dynamic range. Measured and desired flow rates are converted to digital electrical signals and compared, the electrical difference being used to vary a control voltage which establishes the width of energizing pulses controlling a member for opening the feeding tube. The frequency of the energizing pulses is a high, preferably non-integral, multiple of the desired drop flow rate. Appropriate alarms respond to out-of-limit conditions indicated by the magnitude of the control voltage.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to improvements in fluid flow control systems and, more particularly, to a new and improved automatic, self-regulating, highly accurate drop flow control system for parenteral administration of medical liquids over a wide range of fluid flow rates.

The usual medical procedure for the gradual parenteral administration of fluid into the human body, such as liquid nutrients, blood or plasma makes use of apparatus which is commonly referred to in the medical arts as an intravenous set. The intravenous set usually comprises a bottle of liquid, normally supported in an inverted position, an intravenous feeding tube, typically of plastic, and a suitable valve mechanism, such as a roll clamp, which allows the liquid to drip out of the bottle at a controlled rate into a drip chamber below the bottle. The drip chamber serves the dual function of allowing a nurse or other attendant to observe the rate at which the liquid drips out of the bottle and also creates a reservoir for the liquid at the lower end of the chamber to ensure that no air enters the main feeding tube leading to the patient.

While observation of the rate of drop flow via the drip chamber is a simple way of controlling the amount of liquid fed to a patient over a period of time, its utlimate effectiveness requires that a relatively constant vigil be maintained on the drop flow, lest it cease entirely due to exhaustion of the liquid supply or become a continuous stream and perhaps increase the rate of liquid introduction to the patient to a dangerous level.

By way of example, it has been the general practice in hospitals to have nurses periodically monitor drop flow rate at each intravenous feeding or parenteral infusion station. Such moinitoring of drop flow rate is a tedious and time consuming process, prone to error and associated, possibly serious consequences, and resulting in a substantial reduction of the available time of qualified medical personnel for other important duties. Typically, the nurse monitoring drop flow rate will use a watch to time the number of drips flowing in an interval of one or more minutes, and she will then mentally perform the mathematics necessary to convert the timed drop count to an appropriate rate, e.g., in cubic centimeters per hour. If the calculated flow rate is substantially different than the prescribed rate, the nurse must manually adjust the roll clamp for a new rate, count drops again, and recalculate to measure the new rate.

Obviously, each of the aforedescribed measurements and calculations and flow rate adjustments usually takes several minutes time which, when multiplied by the number of stations being monitored and the number of times each station is monitored per day, can result in a substantial percentage of total personnel time available. In addition, under the pressure of a heavy schedule, the mental calculations performed by a harried nurse in calculating flow rate may not always prove to be reliable and, hence, errors do occur resulting in undesired, possibly dangerous infusion flow rates.

In addition to the aforedescribed difficulties, the parenteral administration of medical liquids by gravity induced hydrostatic pressure infusion of the liquid from a bottle or other container suspended above a patient is very susceptible to fluid flow rate variation due to changes in the liquid level in the bottle, changes in temperature, changes in the venous or arterial pressure of the patient, patient movement, and drift in the effective setting of the roll clamp or other valve mechanism pinching the feeding tube. Moreover, there are a number of situations, such as in intensive care, cardiac and pediatric patients, or where rather potent drugs are being administered, where the desired flow rate must be capable of precise selection and must not drift beyond certain prescribed limits. In addition, it is extremely important in such situations for medical personnel to be informed of undesired fluctuations in flow rate, failure of the fluid delivery system, or exhaustion of liquid supply when the bottle is emptied.

It will be apparent, therefore, that some of the most critical problems confronting hospital personnel faced with an overwhelming duty schedule and limited manhour availability are the problems of quickly, easily, reliably and accurately monitoring and regulating drop flow rate in the parenteral administration of medical liquids. In recent years, a number of relatively complex and costly electrical monitoring systems, drop flow controllers and infusion pumps have been developed to accomplish the various tasks of sensing and regulating drop flow rates. Some of these devices have also been capable of activating alarms when a potentially dangerous condition exists, thus freeing medical personnel to some extent for other duties. However, while such monitoring and drop rate control devices have generally served their purpose, they have not always proven entirely satisfactory from the standpoint of reliability and accuracy over a wide range of selected flow rates. For example, some drop flow controllers of the prior art have been unable to prevent passage of two drops instead of one when the feeding tube is opened momentarily, and such controllers have also been troubled by inconsistent drop size.

Hence, those concerned with the development and use of parenteral fluid administration systems have long recognized the need for relatively simple, economical, reliable, and accurate method and apparatus for fluid flow control which would obviate the aforedescribed difficulties. The present invention clearly fulfills this need.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a new and improved method and apparatus for controlling drop flow in the parenteral administration of medical liquids, wherein the frequency and width of control pulses which open a normally shut-off feeding tube are controlled by a digital system capable of sensing and regulating drop flow rate accurately over a wide range of flow rates.

The system for establishing control pulse frequency is an open loop subsystem wherein the control pulse frequency representing desired flow rate is a relatively high, preferably non-integral, multiple of the actual drop flow rate frequency (typically expressed as DPM, or drops per minute) which results in less drop distortion and more consistently repeatable drop size from one drop to another.

Additional control over drop flow rate is accomplished by varying control pulse width, i.e., the open period duration for the feeding tube for each control pulse. Variation of control pulse width to regulate actual drop flow rate so that it is maintained within close tolerances at the desired flow rate is established by a closed loop subsystem.

In a presently preferred embodiment, by way of example, a feeding tube clamping member (normally in the tube shut-off position) is repeatedly moved to the tube-open position by a driver which is, in turn, energized by pulses from a variable pulse generator which produces control pulses at a frequency which is a high multiple of the desired drop flow rate. The width of each control pulse is determined by the amplitude of a control voltage produced by a rate memory which compares a pair of electrical signals proportional to the measured and desired drop flow rates, respectively, and integrates the electrical difference between these signals with the proper polarity to either increase or diminish the amplitude of the control voltage. In this way, precise regulation of the control pulses to the proper pulse width for establishing the desired drop flow rate is accomplished.

The control voltage amplitude is monitored so that out-of-limit conditions calling for a flow rate in excess of system delivery capability, or indicating a leakage flow rate which cannot be terminated by the clamping member, trigger appropriate alarm subsystems.

The new and improved fluid flow control system of the present invention is extremely accurate, reliable and easy to use. The system provides digital precision in selecting and maintaining drop flow rates throughout a wide range, and the system is quick to inform medical personnel of any conditions which might pose a hazard to the patient. Hence, the system of the present invention minimizes the time consuming and error prone aspects of human monitoring and flow rate adjustment and frees medical personnel for other duties.

These and other objects and advantages of the invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings of illustrative embodiments.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fluid flow control system in accordance with the present invention;

FIG. 2 is an electrical schematic diagram for one embodiment of a variable pulse width circuit suitable for use in the flow control system of the present invention; and

FIG. 3 is a graphical representation illustrating typical drop size as a function of a driver pulse frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now particularly to FIG. 1 of the drawings, there is shown a drop flow control system embodying the novel features of the present invention. In the ensuing description, while reference will be made to the term "IV" normally connoting intravenous administration, it is to be understood that this is by way of example only, and the flow control system of the present invention is suitable for other forms of parenteral administration as well as intravenous administration.

In order to control drop flow rate, it is necessary to continuously monitor the actual drop flow as it occurs in an IV administration set. This is accomplished in the system of FIG. 1 by a drop flow monitor which includes a drop sensor 11 and a pulse generator 12 adapted to detect each drop as it falls and generate an electrical pulse train at a frequency directly proportional to the drop flow rate.

The drop sensor 11 monitors drop flow in a drip chamber (not shown) of the IV administration set and typically may include a sensor housing (not shown) containing a reference light source located a fixed distance from a photocell to define an optical sensing gap therebetween, with a reference light beam normally impinging upon the photocell. The housing is appropriately clamped upon the drip chamber of the IV set with the drip chamber positioned within the sensing gap to intercept the reference beam. A falling drop of fluid within the drip chamber interrupts the reference beam, and the variation in the electrical response of the photocell is communicated to appropriate circuitry indicating the presence of a drop. One example of a suitable drop sensor is set forth in copending U. S. Patent application Ser. No. 685,928, inventor: Richard A. Cramer, filed Nov. 27, 1967. While a photocell monitoring device is ideally suited for the drop sensor 11, it will be appreciated that any drop sensing device capable of providing an electrical indication of the presnece of a drop may be used without departing from the spirit and scope of the invention.

The pulse generator 12 is typically a conventional Eccles-Jordan monostable flip-flop (one-shot) which provides an output pulse with a prescribed pulse width and amplitude each time a drop is detected by the drop sensor 11. The pulse generator 12 provides a positive going pulse train proportional to measured drop flow rate, as an electrical input over line 13 to a rate memory 14.

A second electrical input to the rate memory 14 is provided over line 15 in the form of a negative going pulse train from a variable pulse generator 16. The pulse generator 16 is typically a variable frequency square wave generator which generates a negative pulse train at a frequency determined by a conventional rate selector (not shown) which alters the control voltage that establishes the output frequency of the pulse generator 16.

The positive pulse train from the pulse generator 12, indicative of measured drop flow rate, and the negative pulse train from the pulse generator 16, indicative of desired drop flow rate, are combined and compared in the rate memory 14, the electrical difference between the signals indicating measured and desired rates being integrated in the rate memory with the proper polarity to either increase or diminish the amplitude of a d.c. output control voltage which is fed from the memory over line 18 as an electrical input to a pulse generator 19 having a selectively variable output pulse width.

One embodiment of electrical circuitry suitable for carrying out necessary functions of the rate memory 14 is illustrated within the dashed outline in FIG. 1 of the drawings. The negative pulse train from the pulse generator 16 is directed through a current determining resistor R1 and diode D1 as input to the negative channel of a conventional operational amplifier 20 which, together with a capacitor C1, is electrically wired in a conventional integrating configuration to provide the d.c. control voltage output over line 18. In a similar manner, the positive pulse train from the pulse generator 12 is directed through a current determining resistor R2 and diode D2 as an additional input to the same negative channel of the amplifier 20 as the negative pulse train passed by the diode D1.

If the measured and desired flow rates are the same, then the net electrical input to the amplifier 20 is zero, since the positive and negative pulses essentially cancel each other out, and the d.c. control voltage output over line 18 stays constant. If the desired rate is higher than the measured rate, the control voltage output drifts more positive while, on the other hand, the control voltage drifts more negative if the flow rate measured is higher than the desired flow rate. It will also be apparent that, in the event the electrical inputs to the amplifier 20 are disconnected, the d.c. control voltage output of the amplifier will hold constant at its last level prior to disconnection.

One example of electrical circuitry suitable for carrying out the necessary functions of the pulse generator 19 having variable output pulse width, is illustrated in FIG. 2 of the drawings.

The negative pulse train from the variable pulse generator 16 over line 21 is first differentiated in a conventional manner by a capacitor C2 and resistor R3 and passed by a diode D3 as trigger pulse input to a standard Eccles-Jordan monostable flip-flop or one-shot provided by resistors R4, R5, R6, R7, a capacitor C3, and a pair of transistors T1, T2. The resistor R4 and capacitor C3 determine the time constant of the one-shot and, hence, the width of the output pulses from the pulse generator 19. The latter pulse width is dependent upon the amplitude of the negative voltage charging the capacitor C3 through the resistor R4. Therefore, in order to render the conventional one-shot capable of variable pulse width output, the resistor R4 is connected to a variable control voltage as opposed to being returned to ground in the conventional manner. The variable control voltage is the output of the rate memory 14 over line 18.

As the d.c. control voltage over line 18 goes more positive, the pulse width of the one-shot increases, whereas lowering of the control voltage (control voltage going more negative and indicative of too high a measured drop rate) the shorter the pulse width for electrical output from the one-shot.

It will be apparent that other variable width pulse generating circuits susceptible to control by the control voltage from the rate memory 14 may be utilized for the variable pulse width generator 19 without in any way departing from the spirit and scope of the present invention.

The output pulses from the pulse generator 19 are directed over a line 22 as energizing pulse input to a driver 23 which, in turn, energizes an electromagnet 24 to move a clamping member 25 away from a flexible intravenous feeding tube 26, to thereby open the feeding tube for fluid flow. The clamping member 25 is normally spring-biased to a position which pinches the tube 26 in a shut-off state.

Each output pulse over line 22 causes the clamping member 25 to be retracted and thereby open the feeding tube 26 for the duration of the energizing pulse width. By way of example, the electromagnet 24 and clamping member 25 may be a solenoid controlled finger normally pressing the feeding tube 26 against an appropriate clamping surface provided by a rigid block 27 or the like. Other selective tube clamping expedients may be utilized, however, as long as they are susceptible to control by the pulse output over line 22 from the pulse generator 19.

In accordance with one aspect of the new and improved method of controlling drop flow, the pulse output from the pulse generator 16 directed over lines 15 and 21 is a relatively high multiple of the actual drop flow rate frequency desired. The reason for this will be apparent from FIG. 3.

As the ratio of driver pulse frequency to actual drop flow rate frequency increases, drop size distortion, and consequent lack of consistent drop size repeatability, diminishes. It will also be apparent, that best results are achieved with a high, non-integral ratio of driver pulse frequency to desired drop flow rate frequency, although good results can be obtained with a relatively high integral ratio. In the preferred embodiment of the present invention, a ratio of ten and one-half to one has proven very satisfactory.

Hence, in accordance with the present invention, each drop which flows through the intravenous feeding tube 26 is made up of a multiplicity of smaller drop portions which are attached to each other to form a contiguous fluid body making up the final drop which is thus grown in steps under the control of the energizing pulses from the pulse generator 19. The width of the latter pulses is varied by the closed loop system including the drop sensor 11, pulse generator 12 and rate memory 14 to ensure regulation of the actual drop flow rate measured to the desired drop flow rate indicated by the pulse output from the pulse generator 16.

The sizes of the current determining resistors R1 and R2 are selected in accordance with conventional design practices to compensate for the high ratio of pulse frequency over line 15 when compared with the pulse frequency over line 13, so that the average current flow into the summing junction 28 between the diode D1 and D2 is not affected by the ratio of frequencies.

The control voltage output from the rate memory 14 is also directed over a line 29 to any appropriate monitor and alarm system (not shown) for detecting out-of-limit conditions such as an over-speed or runaway condition indicated by an excessively high control voltage from the rate memory 14, or an unusually low level control voltage indicating leakage in the feeding tube 26 with the clamping member 25 in the tube shut-off position. Such monitor and alarm systems may take any form well known in the art, such as high and low level discriminators for selectively triggering aural or visual alarms.

The new and improved method and apparatus for drop flow control, in accordance with the present invention, satisfied a long existing need in the medical arts for an extremely accurate, relatively low cost, reliable, easy to use system providing digitial precision in selecting and maintaining drop flow rates over a wide range. The system of the present invention functions to maintain selected flow rates substantially independent of changes in temperature, crimps in the feeding tube, variations in venous or arterial pressure of the patient, muscular activity of the patient, or variations in the height of the IV bottle or solution level within the bottle.

It will be apparent from the foregoing that, while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except as by the appended claims.

Claims

I claim:

1. In the parenteral administration of medical fluids by an intravenous set including drop forming means and fluid conduit means coupled to said drop forming means,

a method of controlling the rate of drop flow through said fluid conduit means, comprising the steps of:

clamping said fluid conduit to a substantially shut-off state;

producing control pulses having a frequency higher than a desired drop flow rate through said fluid conduit: and

repetitively opening and closing said fluid conduit to flow in response to said control pulses, the frequency of opening said fluid conduit being at a higher frequency than the desired drop flow rate through said fluid conduit, whereby a plurality of control pulses and cycles of opening and closing said fluid conduit are required to produce each individual drop of flow.

2. A method as set forth in claim 1 wherein said control pulses are produced at a frequency that is a relatively high multiple of the desired drop flow rate.

3. A method as set forth in claim 1, further including varying the duration of said control pulses to regulate said drop flow rate.

4. A method as set forth in claim 1, wherein said control pulses are produced at a frequency that is an integral multiple of the desired drop flow rate.

5. A method as set forth in claim 1, wherein said control pulses are produced at a frequency that is a non-integral multiple of the desired drop flow rate.

6. A method as set forth in claim 1, further including the steps of:

generating a first signal proportional to the actual drop flow rate;

generating a second signal proportional to the desired flow rate;

comparing said first signal with said second signal to produce a control voltage proportional to the difference between the actual and desired flow rates; and

varying the duration of said control pulses in accordance with said control voltage.

7. A method as set forth in claim 6, wherein said control voltage is the integral of the difference between said first and second signals.

8. A method as set forth in claim 6, wherein the frequency of opening said tube is an integral multiple of the desired drop flow rate.

9. A method as set forth in claim 6, wherein the frequency of opening said tube is a non-integral multiple of the desired drop flow rate.

10. In the parenteral administration of medical fluids by an intravenous set including drop forming means and a flexible tube coupled to said drop forming means for carrying drop flow, a method of controlling the rate of drop flow through a flexible tube, comprising the steps of:

clamping said tube to a substantially shut-off state;

producing control pulses at a selected frequency greater than the desired rate of drop flow; monitoring the actual drop flow occurring through said tube;

selectively varying the duration of each of said control pulses in accordance with the actual drop flow; and

repetitively opening and closing said tube to fluid flow in response to said control pulses to regulate the actual drop flow rate so that it conforms to said desired drop flow rate.

11. A method as set forth in claim 10, wherein the frequency of opening said tube is a relatively high multiple of the desired drop flow rate.

12. A method as set forth in claim 11, wherein said multiple is approximately 10 1/2 times the desired drop flow rate.

13. A method as set forth in claim 10, further including the steps of:

generating a first signal proportional to said monitored actual drop flow;

generating a second signal proportional to said desired flow rate;

producing a control voltage proportional to the difference between said first signal and said second signal; and

varying the duration of said control pulses in accordance with said control voltage.

14. In a system for parenteral administration of liquids by drop flow, apparatus for controlling the rate of drop flow comprising:

drop forming means;

a feeding tube coupled to said drop forming means;

tube clamping means for closing said feeding tube to prevent liquid flow therethrough;

flow rate setting means for designating a desired flow rate; and

means responsive to said rate setting means for unclamping said tube at a selected frequency higher than the desired drop flow rate to allow repetative liquid flow through said feeding tube, whereby a plurality of successive cycles of unclamping said tube are required to produce each individual drop of flow.

15. Apparatus as set forth in claim 14, wherein said means for unclamping said tube operates at a frequency that is a relatively high multiple of the desired drop flow rate.

16. Apparatus as set forth in claim 15, wherein said frequency is an integral multiple of the desired drop flow rate.

17. Apparatus as set forth in claim 15, wherein said frequency is a non-integral multiple of the desired drop flow rate.

18. Apparatus as set forth in claim 14, wherein said means for unclamping said tube at a selected frequency comprises:

drive means for said tube clamping means;

means for producing a control pulse having said selected frequency; and

means for applying said control pulse to said drive means for unclamping said tube at said selected frequency.

19. Apparatus as set forth in claim 18, wherein said means for producing a control pulse includes:

variable frequency pulse generator means for generating a first electrical signal proportional to the desired drop flow rate.

20. In the parenteral administration of medical fluids by an intravenous set including drop forming means and fluid conduit means coupled to said drop forming means, a method of controlling the rate of drop flow through said conduit means, comprising the steps of:

clamping said fluid conduit to a substantially shut-off state;

selecting a desired drop flow rate; and

repetitively opening and closing said fluid conduit to flow, the frequency at which said conduit is opened being higher than the desired drop flow rate through said fluid conduit, whereby a plurality of successive cycles of opening and closing said fluid conduit is required to produce each individual drop of flow.

21. A method as set forth in claim 20, wherein said fluid conduit is automatically and continuously opened and closed to provide a continuing drop flow at the desired rate.

22. A method as set forth in claim 20, wherein said frequency is a relatively high multiple of the desired drop flow rate.

23. A method as set forth in claim 20, wherein said frequency is an integral multiple of the desired drop flow rate.

24. A method as set forth in claim 20, wherein said frequency is a non-integral multiple of the desired drop flow rate.

25. In a system for parenteral administration of liquids by drop flow through a feeding tube, apparatus comprising:

clamping means for clamping said feeding tube in a normally shut-off state;

electrical pulsing means for generating pulses at a rate greater than the desired drop flow rate to periodically energize said clamping means and thereby open said tube to liquid flow;

rate setting means for generating an electrical signal proportional to desired drop flow rate;

flow monitoring means for monitoring actual drop flow through said tube and generating an electrical signal proportional to measured drop flow rate; and

means responsive to both said rate setting means and said flow monitoring means for regulating the pulse length of pulses from said pulsing means.

26. Apparatus as set forth in claim 25, wherein said means responsive to both said rate setting means and said flow monitoring means includes rate memory means for comparing said signals proportional to desired drop flow rate and measured drop flow rate, respectively, and for generating a control signal for establishing the pulse length of said pulses.

27. Apparatus as set forth in claim 26, wherein said rate memory means integrates the difference between said signals proportional to desired drop flow rate and measured drop flow rate, respectively, and varies said control signal in accordance with the integrated difference between said signals.

28. Apparatus as set forth in claim 25, wherein said flow monitoring means includes a drop sensing means for generating a pulse train at a frequency proportional to measured drop flow rate.

29. Apparatus as set forth in claim 25, wherein said rate setting means includes a pulse generator for generating said electrical signal as a pulse train at a higher frequency than the desired drop flow rate.

30. Apparatus as set forth in claim 29, wherein said frequency is a relatively high multiple of the desired drop flow rate.

31. Apparatus as set forth in claim 29, wherein said frequency is an integral multiple of the desired drop flow rate.

32. Apparatus as set forth in claim 29, wherein said frequency is a non-integral multiple of the desired drop flow rate.

33. Apparatus as set forth in claim 32, wherein said non-integral multiple is approximately 10 1/2.

34. In a system for a parenteral administration of liquids by drop flow through a feeding tube, apparatus for controlling the rate of drop flow comprising:

tube clamping means normally maintaining said feeding tube in a substantially shut-off state;

driver means for selectively energizing said clamping means to unclamp said tube;

rate setting means for generating an electrical signal having a frequency greater than the desired drop flow rate;

flow monitoring means for monitoring actual drop flow through said feeding tube; and

control means responsive to both said rate setting means and said flow monitoring means producing a train of control pulses at the frequency of said electrical signal for regulating the duration of each period of energization of said clamping means by said driver means.