Infusion pump
A type of infusion pump, manufactured by Fresenius.
An
infusion pump
infuses
fluids,
medication
or
nutrient
s into a
patient's
circulatory system
. It is
generally used
intravenous
ly, although
subcutaneous
,
arterial
and
epidural
infusions are occasionally used.
Infusion pumps can administer fluids in ways that
would be impractically expensive or unreliable if
performed manually by nursing staff. For example,
they can administer as little as 0.1 mL per hour
injections (too small for a drip), injections every
minute, injections with repeated
boluses
requested by
the patient, up to maximum number per hour (e.g. in
patient-controlled analgesia
), or fluids whose volumes
vary by the time of day.
Because they can also produce quite high but controlled
pressure
s, they can inject controlled amounts of fluids
subcutaneously (beneath the skin), or epidurally (just
within the surface of the
central nervous system
- a very
popular local spinal
anesthesia
for
childbirth
).
Types of infusion
The
user interface
of pumps usually requests details on the type of infusion from the technician or nurse that sets
them up:
•
Continuous infusion
usually consists of small pulses of infusion, usually between 500 nanoliters and 10000
microliters, depending on the pump's design, with the rate of these pulses depending on the programmed infusion
speed.
•
Intermittent infusion
has a "high" infusion rate, alternating with a low programmable infusion rate to keep the
cannula open. The timings are programmable. This mode is often used to administer
antibiotic
s, or other drugs
that can irritate a blood vessel.
•
Patient-controlled
is infusion on-demand, usually with a preprogrammed ceiling to avoid intoxication. The rate is
controlled by a pressure pad or button that can be activated by the patient. It is the method of choice for
patient-controlled analgesia
(PCA), in which repeated small doses of
opioid
analgesic
s are delivered, with the
device coded to stop administration before a dose that may cause hazardous respiratory depression is reached.
•
Total parenteral nutrition
usually requires an infusion curve similar to normal mealtimes.
Some pumps offer modes in which the amounts can be scaled or controlled based on the time of day. This allows for
circadian cycle
s which may be required for certain types of medication.
Infusion pump
2
Types of pump
A
Baxter International
Colleague CX infusion
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
hormone
s, such as
insulin
, or other medicines, such as
opiate
s.
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 usually 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
pathogen
s. When a 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.
Infusion pump
3
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