This invention relates generally to improvements in mechanisms for screw drive engagement and disengagement, and more particularly, to a new and improved mechanical system for use in a syringe pump of the type requiring engagement and disengagement of a lead screw to facilitate installation of a syringe for parenteral administration (referred to herein as "intravenous administration" or "IV administration") of medical fluids.
The usual medical procedure for the gradual IV administration of fluids into the human body, such as fluid replacement, liquid nutrients, blood or plasma, makes use of apparatus which is commonly referred to in the medical arts as an intravenous solution administration set. Such a set typically is a disposable plastic product, and comprises a drop chamber adapted to be connected to a fluid source, a length of tubing extending from the chamber to the patient and a valve mechanism, such as a roller clamp on the tubing.
The drip chamber of the IV administration set serves a dual function of allowing a nurse or other attendant to observe the rate at which the fluid drips out of the fluid source and also creates a reservoir for the fluid at the lower end of the drip chamber to insure that no air enters the main feeding tube leading to the patient.
While observation of the rate of drop flow via the drop chamber is a simple way of controlling the amount of fluid fed to a patient over a period of time, its ultimate effectiveness requires that a relatively constant vigil be maintained on the drop flow, lest it cease entirely due to exhaustion of the fluid supply or vary unacceptably from the set rate.
In addition to the aforedescribed difficulties, the IV administration of medical fluids by gravity induced hydrostatic pressure infusion of the liquid from a fluid source suspended above a patient, may be susceptible to fluid flow rate variations due to changes in the fluid 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 roller 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 critical drugs are being administered, where the desired drop flow rate must be capable of rather precise selection and must not drift beyond certain prescribed limits in spite of varying load conditions.
In view of the foregoing, a number of electrical monitoring systems, drop flow controllers and infusion pumps have been developed in recent years to infuse medical fluids into patients at precisely regulated fluid flow rates. In particular, syringe pumps have been developed and have become popular in the IV administration of fluids into the human body, such syringe pumps typically embodying a motor driving a plunger within a syringe body to expel fluid from the syringe at a controlled rate through a length of tubing and into the patient.
One specific variety of syringe pump has a syringe plunger mover that is coupled for engagement with a rotary-driven lead screw for linear travel along the lead screw. These pumps are especially adapted to receive a self-contained syringe, installed to have the syringe plunger mover drive the syringe plunger. Because such syringes are typically pre-loaded with varying amounts of fluid and the syringe plunger mover may be in any position along its linear travel from a previous use, some mechanism is necessarily provided for selectively disengaging the syringe plunger mover from the lead screw to enable manual repositioning of it into alignment with the free end of the syringe plunger upon installation.
In the past, such syringe pumps have commonly used a spring-biased mechanism that required continuous actuation of a button or lever in order to maintain the syringe plunger mover in a state of disengagement from the lead screw while the mover was repositioned. Premature release of such a button or lever while attempting to reposition the syringe plunger mover, as was prone to happen, however, caused damage to the lead screw and to the nut or other component coupling the mover to the lead screw due to thread scraping, and otherwise rendered difficult the process of alignment. Also, the spring-biasing of these mechanisms created substantial thread collision forces upon re-engagement of the lead screw, particularly in instances of thread misalignment, tending to cause further excess wear and damage to the instrument.
Another common feature of existing syringe pumps has been "unbalanced" engagement with the lead screw, e.g., as by utilization of a half-nut coupled to the syringe plunger mover and threadedly engaged on just one side of the lead screw. To maintain engagement, substantial force must be exerted by the half-nut or its equivalent on the lead screw, which results in relatively great friction and bowing of the lead screw. At least one solution to this latter problem has been known, but it involves use of a complex and relatively expensive collet-type collapsible nut.
In many of these pumps, moreover, syringe installation has been made even more difficult by the requirement that the mover be properly aligned before the syringe is installed. This necessitates a time consuming trial-and-error procedure in which the syringe is held out of the way, at least partly above or to one side of the pump, with one hand, while the spring-biased disengagement button or lever is actuated and the mover is repositioned, all with the other hand. An attempt is then made to install the syringe, but if there is misalignment, the procedure must be repeated.
Hence, those concerned with the development and use of mechanical drive systems of the type suitable for use in IV fluid administration systems, and particularly those concerned with the design of IV syringe pumps, have long recognized the need for an improved, relatively simple, economical, durable and reliable mechanism for engagement and disengagement with a lead screw that also provides for convenient installation of a syringe with respect to a syringe plunger mover, in order to obviate the aforedescribed difficulties. The present invention clearly fulfills this need.