Disposable infusion cassette with low air bubble retention...

Pumps – Expansible chamber type – Distributor formed from integral portion of flexible wall...

Reexamination Certificate

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Details

C604S153000, C137S512400

Reexamination Certificate

active

06494694

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a positive displacement volumetric infusion cassette pump, and more specifically, to a disposable cassette adapted for use with such a pump, the cassette having an elastomeric membrane that is displaced by a plunger into a cavity to pump fluid and including integral inlet and outlet valve components formed on a surface of the elastomeric membrane.
BACKGROUND OF THE INVENTION
Various types of pumps are used by medical personnel to infuse drugs into a patient's body. Of these, cassette infusion pumps are often preferred because they provide a more accurately controlled rate and volume of drug infusion than other types of infusion pumps. A cassette pump typically employs a disposable plastic cassette coupled between a proximal liquid line extending from a drug reservoir or source, and a distal fluid line that is coupled to the patient's body. The cassette is adapted to engage and be driven by a pump mechanism that includes a control and an interface for setting the desired flow rate, volume of fluid, and other parameters that control the infusion process.
In one prior art design of a disposable cassette, the cassette comprises a plastic shell or housing having a facing member joined to a base. The cassette is inserted into an appropriate receptacle in a pump chassis that typically includes a microprocessor controller and a motor or solenoid-actuated driver. A thin elastomeric sheet or membrane is encapsulated between the two sections. Inlet and outlet flapper valves are formed on one side of this elastomeric membrane and seal against adjacent surfaces formed on the base. These flapper valves are actuated in response to fluid pressure and the force applied by a plunger against the elastomeric membrane. As the plunger withdraws following a pumping stroke, the inlet flapper valve opens, enabling liquid to flow from the drug reservoir through an inlet port of the cassette and into a pumping chamber defined in the base and covered by the elastomeric membrane. The plunger actuated by the motor or solenoid in the pump driver displaces the elastomeric membrane into the pumping chamber, opening the outlet flapper valve and forcing liquid under pressure from the pumping chamber, through an outlet port. The pump chassis thus provides the driving force that pumps liquid through the cassette. In addition, the pump chassis normally includes one or more pressure sensors and air bubble sensors for monitoring and controlling the drug infusion process to protect against potential problems that may arise during delivery of a medicinal liquid to a patient.
Cassette infusion pumps have been widely adopted by the medical profession, which uses millions of such disposable cassettes per year. As is common with other high volume production items, manufacturers continually strive to improve their products. For instance, in prototype unit developed by applicant as a replacement for an existing product, it was observed that during operation, the prototype cassette produced a surprising level of audible noise. Much of this undesirable noise can be attributed to the operation of the flapper valves. Also, in prior art cassettes, when exposed to higher than optimal pressure conditions, the flapper valves can be “blown,” that is, the flapper valves can be forced beyond their sealing surfaces by excess pressure. It would therefore be desirable to provide a cassette that includes valve elements that produce substantially less audible sound when operating, and are much less susceptible to being “blown.”
Improving the reliability of disposable cassettes is also a goal of both end users and manufacturers. As noted above, disposable cassettes frequently are adapted to engage with air or pressure sensors included on the pumping mechanisms used, which can trigger an alarm to alert an operator of an undesirable or unsafe operating condition. The disposable cassettes generally include sensor ports that enable the sensors disposed on the pumping mechanism to monitor a parameter such as pressure or the presence of air in a fluid line. Including the sensor ports within a disposable cassette can increase the size and cost of the cassette. It would be desirable to eliminate any air sensor ports from the disposable cassette, so that the size and cost of disposable cassettes can be substantially reduced. Instead, the air sensors should be positioned to sense parameters relating to the presence of air contained within a tube set that is connected to the outlet port of the cassette. For proper operation, it is important that the positional relationship between the external air sensors and tubing be stable and consistent, because if the tubing moves relative to the sensors, false alarms can be generated, and/or errors in the monitored parameters can result. It would thus be desirable to provide a cassette that incorporates elements, which ensure the fluid tubing remains in a predetermined position relative to external air sensors, to reduce the possibility of erroneous sensor readings and false alarms due to movement of the fluid tubing relative to the sensors.
Another goal in the further development of disposable cassettes is improving the accuracy with which a medicinal liquid is delivered. It is well understood that air bubbles within cassettes are undesirable for several reasons. While gross amounts of air bubbles, such as levels that pose a risk to a patient's health by causing an embolism, are not much of a risk in such systems, even smaller volumes of air retained within a cassette pumping chamber can adversely impact the accuracy with which medicinal liquid is delivered to a patient. Prior art cassettes typically attempt to prevent air bubbles in the system by using a combination of an integral air trap and appropriate cassette priming procedures. While air traps and proper priming techniques generally avoid the delivery of large volumes of air that can pose a health risk, smaller volumes of air bubbles that become trapped within the cassette are more difficult to remove. When air bubbles are present within the pumping chamber, accuracy is affected in several ways.
The volume of the pumping chamber is a critical parameter in the algorithm controlling the pump to achieve accuracy in delivering a desired volume of medicinal liquid. The presence of air bubbles within the pumping chamber effectively reduces the volume of the pumping chamber, so that less than a desired volume of fluid will be delivered each pump cycle. Increasing the complexity of this problem is that pressure conditions within the pumping chamber vary during the pumping cycle, so that the actual volume of a fixed mass of air within the pumping chamber is not constant. Thus, the actual volume of fluid delivered cannot be accurately determined and compensated, because the volumetric error caused by the mass of air trapped within the pumping chamber is not constant. Accordingly, it would be desirable to provide a disposable cassette including elements that reduce the generation and/or retention of air bubbles within the pumping chamber, and elements that promote the removal of any air bubbles that are present in the pumping chamber.
Another aspect of the accuracy of prior art disposable cassettes relates to the elastomeric membrane. In prior art cassettes, the elastomeric membrane typically requires a break-in period the first time the cassette is used. This break-in period is required to enable the elastomeric membrane to reach an equilibrium, so that the repetitive manipulation of the membrane during successive pumping cycles produces repeatable results. Generally, the break-in period is required to enable the membrane to become seated with respect to the facing member and base of the housing that retain the elastomeric membrane, so that repeated manipulation of the membrane does not result in any further stretching or movement of the membrane relative to the housing.
Clearly, it would be desirable to provide a disposable cassette that reduces operating noise, that provides enhanc

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