Apparatus and method for measuring drop size in an intravenous drip chamber

#The present invention relates generally to an apparatus and method for measuring fluid volume, and more particularly, to an apparatus and method that measures the volume of a drop of fluid falling through free space.

#Conventionally, in an intravenous (hereafter “IV”) infusion apparatus, an aqueous solution of medication in an inverted bottle, or other type of fluid reservoir, is supplied to a patient through a fluid administration set comprising tubing, a cannula for insertion into the patient's blood vessel, various fluid control devices such as clamps, injection sites, and at the upstream end, a drip chamber device. The drip chamber device includes a drop former at its upstream end, a transparent chamber through which drops are to fall, and an outlet port at its downstream end. The drop former portion is typically constructed to form drops having a predetermined volume. It has become typical for a drip chamber manufacturer to specify that a particular number of drops equals a particular volume of fluid. For example, a drop former may be constructed such that 20 drops equals a milliliter. As the fluid is supplied to the drip chamber from the fluid reservoir, the drop former generates drops of the fluid that fall through the transparent chamber to the outlet port. The very existence of drops indicates that fluid is flowing in the fluid administration system. The falling drops can be visibly observed in the transparent chamber and counted over a unit of time to calculate the flow rate. The flow rate can be adjusted by a clamp or other device upstream of the drip chamber device, or by downstream means such as an infusion pump. If an infusion pump is used, it will engage the administration set downstream of the drip chamber device and can be used to set a flow rate. The frequency of drops through the drip chamber will then depend on the flow rate set at the infusion pump.

#A need exists for a reliable system that can more accurately measure the rate of flow of infusion medication through a fluid administration set. Typically, a treatment fluid is prescribed at a particular flow rate for a patient. Staying at that flow rate is desired so that the prescribed treatment is delivered. In the case where a transparent drip chamber is used, an upstream valve or clamp can be adjusted to control the rate of flow. Drops are observed in the drip chamber and are counted in an effort to monitor that the prescribed flow rate has been set. However, there are many factors that tend to cause the rate of flow to change after it has been initially set. For example, the rate of drop formation is dependent on the head height/pressure of the fluid reservoir. Depletion of the fluid supply will decrease the head pressure on the drop former and will cause a diminution in the rate of drop formation and flow. Vibration or shock may cause the rate controlling clamp to change its adjustment. An obstruction may find its way into the drop former causing the formation of smaller drops thus changing the rate of fluid flow. Uneven pumping by a downstream infusion pump can also cause variances in the frequency and shape of the drops. It would be helpful to make it readily known when a change in the rate of flow has occurred so that restoration of the desired flow rate can be effected.

#FIG. 1 exemplifies apparatus for controlling the flow of fluid to a patient in accordance with aspects of the invention, and more particularly, shows a conventional drip chamber with apparatus and electronic circuitry used to measure the volume of a drop falling within the drip chamber in accordance with more particular aspects of the invention; and

#FIG. 2 exemplifies the electronic circuitry of FIG. 1 in more detail.

#FIG. 3 exemplifies a preferred embodiment of a resonant circuit having a resonant frequency and incorporating capacitive plates disposed about the drip chamber as shown in FIG. 1, wherein the resonant circuit is shown connected to a phase locked loop circuit with which the resonance frequency of the resonant circuit may be compared.

#Referring now to the drawings in detail, in which like reference numerals indicate like or similar elements among the several views, FIG. 1 shows a system 10 for measuring drop size in an intravenous drip chamber 12. Also shown is the measurement system 10 forming a part of a flow control system 14 where the flow is adjusted in accordance with the measurement system 10. The measurement system 10 in FIG. 1 comprises a sensor 16 mounted to the drip chamber 12 and a measuring circuit 18 that determines the volume of a drop in one embodiment and a flow rate in another embodiment.

#In the flow control system 14, the measurement system 10 is included along with a flow control device 20, such as a peristaltic infusion pump. The output of the measurement system 10 is used by the flow control system 14 to control the downstream flow control device 20 to achieve a programmed flow rate. At the upstream end, a medical fluid reservoir, which in this case is a bag 24, is positioned so as to feed medical fluid via a conduit 26 into the drip chamber 12. Between the bag 24 and the drip chamber 12 in this embodiment is positioned an upstream flow control device 22. In one embodiment, an upstream flow control device 22 may comprise an electrically-controlled clamp. An upstream flow control device 22 may be used in place of or in addition to the downstream flow control device 20. The output of the measuring system 10 may also be used by the flow control system 14 to achieve a programmed flow rate. In the typical case however, only a downstream flow control device 20 would be used.

#After the medication has passed through the upstream flow control device 22, it flows into the drip chamber 12 where an integral drop former at the upstream end 28 forms drops 30 of the medication whose path of fall is within a transparent enclosure 32 of the drip chamber. The drops of fluid collect at the downstream end 34 of the drip chamber 12 where there is located an output port 36. From the output port, the fluid flows downstream through a conduit 38, through a downstream flow control device 20, such as a large volume infusion pump, and through a cannula 40 into the vasculature of a patient.