Intravenous Pumps and Syringe Drivers

Abstract

Many infusions are given by gravity assisted, drip sets that give a flowrate dependent on the height of the reservoir above the patient, the length of the tubing, the bore of the IV cannula, the density and viscosity of the fluid being delivered, and the patient’s venous pressure. However there is an increasing tendency to use programmable volumetric intravenous pumps and syringe drivers to deliver intravenous anaesthesia, fluids, patient controlled analgesia, epidural infusions and other drugs. Not only are they programmable, but they can also be adjusted to give desired flowrates or volumes. Some infusion devices are powered only by gravity, but the flowrate is controlled by a photoelectric drip rate detector in conjunction with a microprocessor controlled drip occlusion device. Other infusion devices use a stepper motor to control the rate of infusion. A stepper motor is designed so that the rotation is by a fixed amount per supplied electrical pulse, independent of the mechanical load it is carrying. The pulses are controlled by a microprocessor in the pump and the rate of infusion is dependent on the stepper motor’s output. Syringe drivers are designed to use a range of syringe sizes and some require special delivery tubing. The flow is a continuous, pulsatile flow and accuracy is 2–5%. Some syringe drivers are driven by clockwork motors, others by a battery powered motor that is intermittently on and off, depending on required flowrate. The driving mechanism is usually by a screw threaded rod connected to the syringe plunger. Other syringe drivers use a stepper motor connected to the screw threaded rod. Care should be taken not to position the syringe driver above the patient’s venous cannula or the syringe may siphon a drug additional to that programmed on the driver, by virtue of the weight of the column of fluid in the tubing above the patient. Care should also be taken to avoid any bubbles in the syringe reaching the patient. Modern syringe drivers are usually sufficiently accurate over the desired range of infusion [Stokes et al. 1990]. However, there may be a delay before the drug is delivered to the patient as the parts attached to the syringe take up slack [O’Kelly et al. 1992].