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Types of pump
There are two basic classes of pumps. Large volume pumps can pump
nutrient solutions large enough to feed a patient. Small-volume
pumps infuse hormones, such as insulin, or other medicines, such
as opiates.
Within these classes, some pumps are designed to be
portable, others are designed to be used in a hospital, and there
are special systems for charity and battlefield use.
Large-volume pumps usually use some form of peristaltic
pump. Classically, they use computer-controlled rollers compressing
a silicone-rubber tube through which the medicine flows. Another
common form is a set of fingers that press on the tube in sequence.
Small-volume pumps usually use a computer-controlled
motor turning a screw that pushes the plunger on a syringe.
The classic medical improvisation for an infusion
pump is to place a blood pressure cuff around a bag of fluid. The
battlefield equivalent is to place the bag under the patient. The
pressure on the bag sets the infusion pressure. The pressure can
actually be read-out at the cuff's indicator. The problem is that
the flow varies dramatically with the patient's blood pressure (or
weight), and the needed pressure varies with the administration
route, making this quite risky for use by an untrained person. Pressures
into a vein are ususally less than 8 lbf/in² (55 kPa. Epidural
and subcutaneous pressures are usually less than 18 lbf/in²
(125 kPa).
Places that must provide the least-expensive care
often use pressurized infusion systems. One common system has a
purpose-designed plastic "pressure bottle" pressurized
with a large disposable plastic syringe. A combined flow restrictor,
air filter and drip chamber helps a nurse set the flow. The parts
are reusable, mass-produced sterile plastic, and can be produced
by the same machines that make plastic soft-drink bottles and caps.
A pressure bottle, restrictor and chamber requires more nursing
attention than electronically-controlled pumps. In the areas where
these are used, nurses are often volunteers, or very inexpensive.
The restrictor and high pressure helps control the
flow better than the improvised schemes because the high pressure
through the small restrictor orifice reduces the variation of flow
caused by patients' blood pressures.
An air filter is an essential safety device in a pressure
infusor, to keep air out of the patients' veins: doctors estimate
that 0.55 cm³ of air per kilogram of body weight is enough
to kill (200-300 cm³ for adults) by filling the patient's heart.
Small bubbles could cause harm in arteries, but in the veins they
pass through the heart and leave in the patients' lungs. The air
filter is just a membrane that passes gas but not fluid or pathogens.
When an large air bubble reaches it, it bleeds off.
Some of the smallest infusion pumps use osmotic power.
Basically, a bag of salt solution absorbs water through a membrane,
swelling its volume. The bag presses medicine out. The rate is precisely
controlled by the salt concentrations and pump volume. Osmotic pumps
are usually recharged with a syringe.
Spring-powered clockwork infusion pumps have been
developed, and are sometimes still used in veterinary work and for
ambulatory small-volume pumps. They generally have one spring to
power the infusion, and another for the alarm bell when the infusion
completes.
Battlefields often have a need to perfuse large amounts
of fluid quickly, with dramatically changing blood pressures and
patient condition. Specialized infusion pumps have been designed
for this purpose, although they have not been deployed.
Many infusion pumps are controlled by a small embedded
system. They are carefully designed so that no single cause of failure
can harm the patient. For example, most have batteries in case the
wall-socket power fails. Additional hazards are uncontrolled flow
causing an overdose, uncontrolled lack of flow, causing an underdose,
reverse flow, which can siphon blood from a patient, and air in
the line, which can starve a patient's tissues of oxygen if it floats
to some part of a patient's body.
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