An infusion pump pumps fluids into a patient. It is usually used to administer medicine or nutrient, often through a vein.

Infusion pumps can adminster fluids in ways that would be impractically expensive or unreliable if performed by nursing staff. For example, they can administer 1ml/hour injections (too small for a drip), injections every minute, injections with boli (boluses) requested by the patient, up to maximum number per hour, or fluids whose volumes vary by the time of day.

Because they can also produce quite high but controlled pressures, 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).

Table of contents
1 Types of Therapy
2 Types of Pump
3 Safety features available on some pumps
4 Warning

Types of Therapy

Pumps usually ask questions (in text) as they are being set-up by nursing or pharmacy staff. The most critical is "what type of therapy?"

There are several common therapies:

Continuous infusion is just what it sounds like. In practice, most pumps generate small pulses of infusion, usually between 20 nanoliters and 100 microliters, depending on the pump's design, and then repeat these small infusions at varying timings.

Intermittent infusion has a "high" infusion rate, alternating with a low programmable infusion rate to keep the vein open. The timings are programmable.

Patient-controlled analgesia is a specialized application for patients in pain. In this mode, the pump will have a continuous programmable infusion mode, establishing a base of pain medication. Usually the minimum continuous rate is the rate to keep a vein open. A button is made available to the patient. Pressing the button causes the pump to release a programmed bolus of analgesic, usually an opiate or fentanyl. There will also be a limit to the number of times that the pump will infuse a bolus each hour. Medically, patients rarely push the button unless they have pain, so doctors should permit a generous number of boli. There's also usually a way for nursing staff to send a larger bolus. When dispensing opiates, theft by staff or visitors is a concern. The medication is usually provided in tamper-evident syringes, or locked assemblies of medication-bag and pump.

Total parenteral nutrition programs a pump to simulate the blood-borne effects of a normal meal. Usually the sequence is a delay with a minimal amount of infusion to keep the vein open. Next the infusion ramps up to a maximum. The maximum sustains for the time normally taken to digest a meal. Then the amount ramps down to the keep-vein-open amount.

Some pumps offer modes in which the amounts can be scaled or controlled based on the time of day. This has become more in demand as doctors and pharmacists have learned more about circadian cycles and their interactions with medicines.

Typical values to program are: The clock time to start the infusion; a keep-vein-open rate will be infused until then. The size of the bag to be infused, the infusion rate, the length of time for the infusion (if set, the rate may be changed), the keep-vein-open rate, and the time between infusions.

Many pumps have time-of-day electronic clocks to control the infusion.

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 8PSI. Epidural and subcutaneous pressures are usually less than 18PSI.

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 0.55cc of air per Kg of body weight is enough to kill (200-300cc 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 overdone, 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 of it floats to some part of a patient's body.

Safety features available on some pumps

The range of safety features varies widely with the age and make of the pump. A state of the art pump in 2003 may have the following safety features:

Warning

Infusion pumps are used to perform sterile medical procedures that should only be performed by trained personnel under medical supervision, with proper equipment.

See also intravenous drip, life support, total parenteral nutrition and medicine