Mechanical implantable pumps

The driving force for drag release from a pump is a pressure difference that causes the bulk flow of a drag, or drug solution, from the device at a controlled rate. This is in contrast to the polymeric controlled release systems described above, where the driving force is due to the concentration difference of the drag between the formulation and the surrounding environment. Pressure differences in an implantable pump can be created by osmotic or mechanical action, as described below. 

Figure 4.19 The cross-sectional view of the Arrow model 3000 implantable pump, showing the pumping mechanism...

Valve Problems. The primary solution to valve problems has been implantable replacement valves. The introduction of these devices necessitates open-heart surgery. There are two types of valves available tissue (porcine and bovine) and mechanical. The disadvantage of tissue valves is that these have a limited life of about seven years before they calcify, stiffen, and have to be replaced. The mechanical valves can last a lifetime, but require anticoagulant therapy. In some patients, anticoagulants may not be feasible or may be contraindicated. Of the valves which require replacement, 99% are mitral and aortic valves. The valves on the left side of the heart are under much greater pressure because the left ventricle is pumping blood out to the entire body, instead of only to the lungs. Occasionally, two valves are replaced in the same procedure. 

The mechanisms that control dmg deUvery from pumps may be classified as vapor-pressure, electromechanical, or elastomeric. The vapor-pressure controlled implantable system depends on the principle that at a given temperature, a Hquid ia equiUbrium with its vapor phase produces a constant pressure that is iadependent of the enclosing volume. The two-chamber system contains iafusate ia a flexible beUows-type reservoir and the Hquid power source ia a separate chamber (142). The vapor pressure compresses the dmg reservoir causiag dmg release at a constant rate. Dmg maybe added to the reservoir percutaneously via a septum, compressing the fluid vapor iato the Hquid state. 

The mechanical pump approach employs miniature mechanical devices, such as implantable and portable infusion pumps and percutaneous infusion catheters, to deliver drugs into appro priate blood vessels or to a discrete site in the body. When compared with the... 

Mechanical support therapy has been a significant addition to the armamentarium against refractory heart failure. However, surgically placed pumps are associated with significant morbidity and mortality, most related to the surgical implantation procedure... 

Drug delivery from this pump is powered by the Freon propellant. When the Arrow pump is implanted subcutaneously, it is warmed by the patient s body temperature so that the propellant-containing chamber expands and exerts pressure on the movable bellows. Infusate is thus forced out of the reservoir chamber to an attached catheter through a filter and flow restrictor. This mechanism allows the delivery of infusate at a fairly constant rate to surrounding tissues or blood vessels. It should be noted, however, that the vapour pressure exerted by the outer chamber can be affected by changes in altitude/elevation or body temperature. 

Mechanical pumps are usually miniature devices such as implantable infusion pumps and percutaneous infusion catheters which deliver drugs into appropriate vessels or other sites in the body. Several pumps, implantable catheters, and infusion devices are available commercially. Examples of applications of these devices are as follows ... 

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