1.  Field of the Invention
The present invention relates to implantable apparatus for delivering beneficial agents, including drugs to a living body.  More particularly, the present invention relates to implantable flow control devices for controlling delivery of beneficial agents to a living body. 
2.  Description of the Related Art
It is known to provide implanted drug infusion pumps to deliver a controlled, sustained dosage of beneficial agent or drug to the living human body.  Such infusion pumps are generally classified as fixed rate infusion pumps or variable rate infusion pumps.  Fixed rate infusion pumps deliver drug-carrying fluid at a preset flow rate, which cannot be changed after manufacture.  Variable flow rate implantable pumps permit adjustment of the flow rate, but only prior to implantation.  Readjustment of the flow rate of variable rate pumps requires removal of the pump from the patient""s body and related surgery.  Because of the surgical intrusiveness typically required for flow rate changes for both fixed rate and adjustable rate pumps, there has developed a trend towards selectable-rate pumps, which permit flow adjustment while the pump remains implanted in the living body. 
Flow control in selectable rate pumps, however, is complicated by the need to limit or minimize the power consumption.  To this end, there have been efforts to provide passive flow control elements on selectable rate pumps for minimizing power consumption while providing flow control. 
For example, U.S. Pat.  No.  5,820,589, to Torgerson and McMullen, the subject matter of which is incorporated herein, in its entirety, discloses the concept of an implantable pump, which is provided with a passive regulator in the form of a manifold communicating with a restrictor network, with a number (n) of bi-stable valves with two flow states, or a number (n) of multi-stable valves with a number (m) of flow states.  With the bi-stable valve configuration, the combination overall allows for 2n flow rate options.  With the multi-stable valves, the system has mn flow rate options.  Ideally, such bi-stable or multi-stable valves would have no requirement for power except during flow state changes.  Power is typically provided via RF signal with suitable electronic implements provided on the pump for providing an induced voltage from the RF signal.  While such known passive flow control systems provide variability in flow rates, the number of attainable flow configurations is somewhat limited.  It would therefore be desirable to provide an implantable pump with a passive flow control system which provides increased adjustability in flow rates over known systems. 
In drug infusion applications, it is frequently desirable to provide for the introduction of a drug bolus to the patient.  Bolus dosage may be required, for example, when a patient""s activity results in increased pain that is not adequately controlled with normal dosage.  Known passive control systems do not provide for the administering of a drug bolus.  Thus, there is a need to provide an implantable pump with a passive flow control system which permits the metering and delivery of a bolus of drug. 
The present invention solves the aforementioned problems and others by providing an implantable drug infusion pump with a passive flow control device with increased variability in flow settings over heretofore known devices.  In a preferred embodiment, the invention provides a bi-stable valve which is duty-cycled by a control module to achieve a desired average flow rate over time.  The control module provides appropriate signals at appropriate times to open and close the valve to achieve a desired average flow rate over time.  A flow restrictor may be provided downstream of the valve as a safety feature to limit flow or to achieve a desired flow rate range. 
The invention also provides a drug infusion pump with a passive flow control device which provides for metering and delivery of a bolus of drug.  In a preferred embodiment, a flow conduit communicates with the pressurized reservoir and with a first normally closed, bi-stable valve.  The first bi-stable valve is in fluid communication with an accumulator for accumulating a bolus of drug.  A second bi-stable valve isolates the drug supply stored in the accumulator from a drug delivery catheter.  To meter a drug bolus into the accumulator, a control module provides an appropriate signal to open and close the first valve and permit ingress of a desired amount of drug to the accumulator.  The inlet valve is closed after the bolus has accumulated.  When a bolus delivery is desired, the control module delivers appropriate signals to the second valve to release the bolus from the accumulator.