Infusion Pumps

Since the late 1960s, infusion pumps have been in widespread use in hospitals. All the infusion pumps being presently marketed fall into one of the following three operating systems:

  1. positive displacement—includes syringe and cassette pumps;
  2. linear peristaltic—a number of “fingers” milk fluid down a straight channel; and
  3. roller or rotary peristaltic—where rollers push the fluid about 270° around a pump head.

 Some newer infusion pumps feature more than one channel, a drug library, and/or a panel-lockout option to prevent unauthorized adjustments of pump parameters.

At many hospitals, infusions pumps are the No 1 devices with which biomeds come into contact. Many of the problems associated with infusion pumps are not technical in nature but rather concern the user’s application of the technology.

Positive-Displacement Pumps
Positive-displacement pumps are probably the most common type of infusion pump in use today. They employ a special cassette set with a built-in small chamber that is placed on the pump. A small valve in the set, often called a “duckbill,” allows fluid to fill the chamber during a pause in pumping. When pumping restarts, the valve closes to force fluid toward the patient instead of back toward the bag.

The rate of flow is set by the speed of the pumping action and can range from less than 1 mL per hour to up to 1 L per hour. The flow rate contributes to the pressure used by a pump to overcome resistance to flow. Today, most pumps will not generate pressure above 500 mmHg (millimeters of mercury), or 10 PSI (pounds per square inch). Some of the early models could generate more than 50 PSI in pressure. This caused some major problems if the intravenous (IV) needle being infiltrated went through the vein into soft tissue. Most pumps now have built-in pressure sensors and allow for the setting of lower occlusion pressures. The occlusion pressures should be checked during your normal preventive maintenance (PM) cycle.

Other problems with cassette pumps, such as cost, slowed their acceptance for many years. Once the cost problems were solved, their higher degree of accuracy made this type of pump very popular.

The true syringe pump is the most accurate of all pumps, with some having accuracy on delivered volumes to four decimal points. Cassette units are generally ±2% to ±5% accurate, depending on the manufacturer.

Biomeds will encounter two types of syringe pumps in most hospitals. One type uses a standard syringe, ranging from 5 cc to 60 cc in size. Not all pumps can be used with all syringe sizes, which leads to the common problem of a clinical person trying to use a syringe size or brand that is not compatible with the pump.

The travel of the plunger on the syringe is calibrated to the delivery rate in a syringe pump (being a displacement pump). If a syringe has a larger diameter than what is programmed for the machine, the same travel distance on the plunger will deliver more drug to the patient. This problem is common when more than one brand of syringes is used in a hospital.

The second type of syringe pump is the patient-controlled analgesia (PCA) pump, which often uses a syringe specifically designed for that pump instead of a standard syringe. Again, if the wrong size or brand of syringe is used, the delivered drugs could be either overinfused or underinfused. There are generally two parameters that clinicians program into the pump prior to starting the infusion. The first is the rate in mL/hr. The second is volume to be infused (VTBI) in mL. Some hospitals may have a practice to set the VTBI at approximately twice the rate. This means that the pump will sound an alarm in about 2 hours, indicating that the infusion is complete. The pump will then automatically switch to the “keep vein open” (KVO) mode, which will infuse 1 mL/hr. This alarm alerts the clinician to silence the pump, reset the pump, and check the patient to make sure the infusion is OK.

Linear Peristaltic Pumps
Originally, linear peristaltic pumps used standard IV sets, had drop sensors, and were not pressure limited. With some designs, it was possible to obtain more than 100 PSI of delivered pressure. The majority of linear peristaltic pumps now in use are pressure limited, use special sets, and have delivered accuracies close to that of the positive-displacement pumps.

As the drive mechanism wears, accuracy can vary, especially at high occlusion pressures. What happens is that the “fingers” do not completely close the infusion set; and if the occlusion pressure of the patient is greater than the head pressure from the container, some fluid is forced back to the container. One indication of this problem is blood coming back into the IV line from the patient; another is underinfusion of the solution. In either case, you have a problem.

One way to test for excess wear, from the bench, is to put 15 PSI of air pressure on the patient end of the IV set and see if air bubbles work their way back to the IV container. If there are air bubbles, you need to adjust the “finger” on the pump to get total occlusion.

The delivered volumes for linear peristaltic pumps are the same as the cassette positive-displacement pumps, and most have adjustments for the occlusion pressure.

Roller or Rotary Peristaltic Pumps
The roller or rotary peristaltic pump is not commonly used for IV drug administrations but is more often used for pumping in feeding solutions for nutritional support, such as total parenteral nutrition. The roller pump also is widely used for blood in the operating room, during cardiac bypass, and in blood banks for processing or separating cells from the plasma.

The roller peristaltic pump is probably the least accurate of all the pumps, and calibrating the pump heads—especially for bypass pumps—can take time.

Some points to remember:

• A controller basically counts drops; it does not pump fluid into a patient but controls only the rate of flow. The maximum pressure generated is the height of the bag or bottle above the injection site. Controllers cannot be used for arterial infusions.

• The batteries on all battery-operated pumps must be replaced on a scheduled basis. Generally, batteries should be replaced between 18 and 24 months. Allowing the battery to fail before replacement may put the patient in danger.

• Some of the newer pumps on the market feature more than one channel, allowing a single pump to infuse up to four lines on the same patient. Certain pumps have a drug library from which a clinician can tell the pump what drug it is infusing. Most pumps also feature a panel-lockout option, which, when activated, disables the front panel. This prevents the patient or anyone else from adjusting the pump parameters while it is connected and infusing.

• Pumps that do not have free-flow protection built in must be clearly marked, and a training program must be in place for users. Most providers have addressed the free-flow problem since the Joint Commission on Accreditation of Healthcare Organizations made it one of the patient safety goals in 2002.

• Pumps often suffer from sudden deceleration, or hitting the floor, and should be mechanically inspected as part of the PM process.

• A high percentage of the problems reported on IV pumps cannot be duplicated in the shop—often indicating a problem with staff education. Work with staff-education personnel to cut down on these calls. 24×7

Review Questions

1) Can a syringe pump use any syringe?
     a. Yes
     b. No

2) Most IV pumps, in general, have pressure limits of about _____________________.
     a. 50 mmHg
     b. 50 PSI
     c. 500 mmHg
     d. 500 nanometers

Answers: 1-b, 2-c

David Harrington, PhD, director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, is a member of 24×7’s editorial advisory board.

Freeman “Skip” Sands is the TiM account manager at North Adams Regional Medical Center, North Adams, Mass.