INFUSION DEVICES
Manual Delivery
When an infusion of an intravenous anesthetic is administered,
the infusion regimen can be controlled by a variety of mechanisms. These mechanisms
vary from the simple CAIR clamp or Dial-a-Flo (Abbott Laboratories) to complex computer-controlled
infusion pumps. Simplicity of mechanical design, however, is not necessarily correlated
with ease of use, which has prompted ongoing advances in infusion device technology.
Infusion devices can be classified as either controllers or positive
displacement pumps. Explicit in their title, controllers contain mechanisms that
control the rate of flow produced by gravity, whereas positive displacement pumps
contain active pumping mechanisms.
The most commonly used pumps for the administration of intravenous
anesthetics are positive displacement syringe pumps that use a screw mechanism.
These pumps are extremely accurate and have the convenience of not requiring specialized
tubing. Many of these pumps have features that make them particularly suitable for
anesthetic delivery. An important advance has been the introduction of a calculator
feature within the pump so that the clinician can set the weight of the patient,
the drug concentration, and the infusion rate in dose/unit weight/unit time and the
pump then calculates the infusion in volume/unit time. In addition, these pumps
allow simple application of a staged infusion scheme (e.g., the two-stage approach
of Wagner[80]
) by allowing a loading dose and a
maintenance infusion rate to be programmed into the pump. Numerous syringe pumps
also now include automated recognition of syringe size. These modest advances in
pump technology and design enable intravenous anesthetics to be conveniently delivered.
However, the only commercially available devices for the delivery of intravenous
anesthetics that approach the convenience of the present-day vaporizer are target-controlled
infusion devices such as the "Diprifusor," which are available everywhere except
North America. These devices go beyond simple calculator pumps to create truly "smart"
pumps that use automated drug delivery.
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