the flow monitoring device of the present invention is used in an implantable drug delivery system where it is important to measure fluid flow for proper operation of the drug delivery system . in this respect , failure to deliver a proper amount of medication to a patient can result in a lack of proper treatment producing serious complications . there are several conditions that can cause a drug delivery system to malfunction and not provide the required amount of drug at the prescribed rate . two of these conditions are obstructed tubing and leaky valves . another condition influencing drug delivery occurs in the presence of air in the system which can cause an air lock , flow variations , or blockage . consequently , in an implanted drug delivery system the operation of same may seem acceptable by the actual medication delivery may result in serious life threatening situations since the proper amount of drug may not be timely delivered . these aforesaid conditions and their related problems exist in drug delivery systems available on the market today . accordingly , it is desirable to provide a flow monitoring device which is able to measure the drug flow through a delivery tubing and determine if a failure has occurred . in addition , the flow monitoring device must be sufficiently sensitive in order to be capable of determining which condition is causing the failure , i . e ., a leaky valve , an obstruction , air in the system or other problem . in one preferred embodiment of the flow monitoring device of the present invention shown in fig1 a thermistor 11 is mounted on a hollow polymeric outflow tubing 12 and the flow of fluid therein , as shown by arrows , is measured through a reduced - in - thickness titanium wall area 13 mounted in an opening 14 in the tubing 12 . preferably , the thermistor 11 is a glass bead thermistor 11 which is mounted on the outside of the reduced - in - thickness wall area 13 mounted in the delivery tubing 12 by a conductive epoxy such as that sold under grade and name designation epo - tek h70e by epoxy technology , inc . of watertown , mass . where the outflow tubing 12 is internal with respect to the patient , it may be desirable to have two tubings to provide two lumens , one lumen being provided for drug infusion and the other lumen being provided for carrying wire conductors to the thermistor 11 . in addition , feedthroughs may be provided to hermetically seal the place where the wire conductors extend through a pump casing to the outflow tubing 12 . another preferred embodiment of the flow monitoring device of the present invention is shown in fig2 and 3 . here the sensor 11 is mounted on a pump casing 14 of a pump . although not shown , and not forming part of the present invention , it will be understood that a solenoid is mounted in the pump for causing a pumping action and each pump stroke causes the thermistor 11 to change in temperature ( approximately 0 . 02 ° c . resulting in a change in resistance of approximately 0 . 05 %). the flow monitoring device is preferably used in a medication delivery system which includes a solenoid actuated pump that provides a one microliter volume of medication with each pump stroke . the flow of medication starts a few milliseconds after the solenoid pump is activated . the thermistor 11 forms a flow sensor of the flow monitoring device and can be mounted on an outer surface 15 of a reduced - in - thickness wall area 16 of the pump casing 14 which can be mounted in a hermetically sealed pacer case or casing thereby eliminate the need for feedthroughs and the need for double lumen tubing . mounting the thermistor 11 on the pump casing 14 within a pacer can simplify manufacturing and sealing . the flow monitoring device requires circuit design techniques to enhance the signal as a result of the mechanical thermal coupling ( convective and conductive ) between the fluid being measured and the sensor 11 which generally yields very small signals . two parameters can be used to control the power and thus heat available at the thermistor 11 , namely constant temperature and constant current . the thermistor 11 may be heated , by applying current adequate to maintain the sensor a few degrees above the temperature of a pulsatile fluid flow being measured . as a result of pulses of fluid flowing through a channel 18 formed in a body member 20 within the pump casing 14 , heat is transferred from the flow sensor forming thermistor 11 through the reduced - in - thickness wall area 16 to the flowing fluid and absorbed by the fluid mainly by convection , although some conduction takes place in the wall area 16 of the pump casing 14 . this is possibly due to the great sensitivity of the system which results from the circuitry utilized with the thermistor 11 . it is desirable to have as large a signal output as possible from the thermistor 11 in order to have a high signal - to - noise ratio . mechanical signal amplification is achieved by reducing the wall thickness in the reduced - in - thickness area where the thermistor 11 is mounted . in particular a thickness a of 0 . 003 plus or minus 0 . 001 inch has been found to provide ease of manufacture and yield good sensitivity . the thermistor 11 is approximately 0 . 014 inch in diameter excluding a mound 42 of conductive epoxy mounting the thermistor 11 to the surface 15 . the thermistor 11 is mounted on the surface 15 of the reduced - in - thickness wall area 16 of the pumping casing 14 by the mound 42 of high thermally conductive epoxy adhesive in such a way as to form air pockets 44 on the sides of the mound 42 such that half of the thermistor 11 is exposed to a high thermally non - conductive medium which provides an insulative - insulating barrier to direct heat transfer . also thermally conductive epoxy forms a large heat transfer surface area adjacent surface 15 which results in augmentation of the output signal from the thermistor sensor 11 . this particular adhesive arrangement of a flow sensor forming thermistor 11 is useful to measure very minute , pulsatile flow which is typical of drug delivery systems . the advantage of the flow monitoring device construction and circuits 22 and 37 therefore described above is the great sensitivity which allows the device to detect very minute pulsatile flow volumes with a thermistor 11 mounted outside of the fluid medium whereby damage , as a consequence of fluid contact with the sensor forming thermistor 11 and / or wire conductor therefor , are avoided . additionally the sensor does not interfere with the fluid flow . however , utilization of the flow monitoring device is limited to conditions where the flow is pulsatile ( rather than continuous ) and the time at which the flow starts must be known with anticipation . the start of flow is detected with the activation of the pump solenoid . in fig4 is illustrated a simplified schematic diagram of a sensor circuit 50 . as shown , the thermistor 11 is powered by a constant current source 52 and the voltage developed in the thermistor 11 is amplified by a high gain operational amplifier 54 , which has a gain determined by the values of resistors 56 and 58 . in one preferred embodiment the gain was set to 2 , 200 . a switch 60 is a cmos bilateral switch 60 . the switch 60 with capacitor 62 and differential amplifier 64 form a sample and hold circuit 66 which is used for resetting amplifier 54 before each pump stroke of the solenoid pump . resetting the high gain amplifier 54 is necessary to provide a stable reference for the flow measurements due to the high gain of the system . if the output level were not reset , a minor change in temperature of the thermistor 11 ( due perhaps to a minute change in ambient temperature ) would cause a significant drift in the output of amplifier 54 . fig5 illustrates the signals necessary for operation of the thermistor 11 . the signal labelled 1 is for thermistor power application . the second signal labelled 2 is for the amplifier reset and the third signal labelled 3 is for the solenoid or pump activation . from the time relationships of these signals 1 , 2 and 3 , it is apparent that the power to the thermistor 11 is applied before the pump stroke occurs , approximately a half second before . this is necessary for the thermistor 11 and adjacent parts to rise in temperature above the steady state level . the amplifier reset signal can be applied any time between the start of the thermistor power on and a few milliseconds after the solenoid activation has ended . the precise duration of this signal is not important . the solenoid operation signal will active the pump and cause the flow the start subsequent to when this signal ends . note the time intervals b and c in fig5 . interval b is the reset time . during this time interval b , the amplifier gain is set at unity for a voltage follower function which causes output voltage to follow the thermistor voltage . the output voltage during this time will be almost constant since the thermistor voltage , if not amplified , will change little . time interval c is the measurement of the interval which starts when the reset signal ends . during this interval c , any change in the thermistor voltage , in the preferred embodiment , will be amplified by a factor of 2 , 200 with the output starting at the reset level . when the measuring interval c ends the thermistor 11 is powered off to conserve power . in fig6 are illustrated two output waveforms 70 and 72 , one waveform 70 for normal flow superimposed on the waveform 72 for a no - flow situation . in the case of no - flow , the sloping ( downward ) line indicates that the thermistor 11 continues to increase in temperature , and its resistance drops , i . e ., a negative volt - ampere condition . if there is flow , the thermistor 11 will cool off momentarily causing the peaking waveform shown . the actual volume of flow can be quantitatively measured by either the peak voltage pv shown in fig6 or by the length of time required for the signal to fall below a preset threshold level . this time interval is designated as d for the no - flow condition and e for the normal flow condition . the flow monitoring device of the present invention is particularly designed to detect changes in the flow rate of a liquid begin output or delivered by a drug delivery system . a particular application of such a drug delivery system is in the administration of fluids to patients by means of intravenous bottles of by an implanted device which can be mounted in a pacemaker case or casing and where it is necessary to detect continuity of pulsatile flow conditions . for example , it is desirable to detect a flow rate increase or decrease with respect to time . it is also desirable to detect leakage in any valves or delivery lines of the drug delivery system . further it is desirable to detect air bubbles or the blockage in the drug delivery line . the flow monitoring device of the present invention enables one to easily and automatically monitor such conditions . moreover , the flow monitoring device of the present invention has a number of advantages , some of which have been described above and others of which are inherent in the invention . in particular , the flow monitoring device of the present invention is highly sensitive to changes in pulsatile flow rate ; it functions independently of ambient conditions such as the temperature of the drug being delivered ; it has a wide dynamic range being capable of monitoring varying ranges of pulsatile flow rates ; and lastly , it is very compatible with an implanted drug delivery system . also it will be apparent to those skilled in the art that modifications can be made to the flow monitoring device of the present invention without departing from the teachings of the present invention . accordingly , the scope of the invention is only to be limited as necessitated by the accompanying claims .
the flow monitoring device is used with a pump casing on an outflow tube having a passageway therethrough with a wall thereabout . a solenoid pump communicates with the passageway for providing pulsatile liquid flow therethrough . a reduced - in - thickness area along the wall of the casing or outflow tubing has an outside surface establishing a site to measure pulsatile flow along an adjacent inside surface . a thermistor is mounted on the outside surface of the wall area for sensing the temperature thereof in response to pulsatile flow therebeneath . the thermistor is electrically energized at a predetermined level . a control circuit regulates the input of electrical energy to the thermistor for predetermined time periods . the control circuit includes a sample and hold circuit and a reset switching circuit responsive to the pump activation for supplying thermistor power after pumping begins . an amplifier in the control circuit compares differences in the input power to the thermistor during a selected time interval to measure the pulsatile flow rate through the passageway .