Here at Duke University Medical Center, we routinely perform such painful procedures as total shoulder replacements, total ankle replacements and unicondylar arthroplasties on an outpatient basis, thanks in large part to an aggressive multimodal approach to pain control. The cornerstone of our program is peripheral nerve blockade, which we continue for 24 to 72 hours postoperatively via pain-pump infusion in nearly one-third of all blocked patients. Many studies and our own experience with more than 1,500 such patients show that this technique improves analgesia over traditional approaches and reduces opioid use, thereby reducing narcotic-related side effects.

If you are considering offering continuous local anesthetic infusion to your outpatients - whether for peripheral nerve block infusion or infiltration of surgical wounds or intra-articular spaces - here's what you need to know about how pain pumps work. Understanding them and their role in the outpatient setting is the first step toward making the best purchasing decision for your facility.

Kinds of pain pumps
Manufacturers first created infusion pumps for at-home chemotherapy and antibiotic infusions. In the late 1990s, they began marketing these pumps for post-op pain indications. Because there were already many products on the market - at least 20 pain pumps with various features are now available - selecting a pump is difficult.

To make sense of it all, it helps to understand the two basic types of pain pumps: mechanical and electronic.

• Mechanical pumps. They generate force with an elastic device (such as a rubber ball, balloon, spring, vacuum or a combination thereof) that produces a high pressure over a fixed resistance. Mechanical pumps tend to be fully disposable.
• Electronic pumps. They employ a programmable, motorized pump to propel the analgesic out and control flow rate. Electronic pumps are either fully or partially reusable; some have disposable drug cartridges.

Pros and cons
There are advantages and disadvantages to each type of pain pump:

• Mechanical pumps. They are very simple in design and, as a result, tend to be very reliable. They also tend to be safe because they are not vulnerable to programming errors and are less prone to patient tampering than electronic pumps. The downside is that most mechanical pumps are non-adjustable; although they come in many flow rates, the rate is fixed. The physician must pre-select one infusion rate and cannot change it should the patient require more or less analgesic.

Basic mechanical pumps do not let patients self-administer a bolus of medication should they anticipate or experience pain intensification (patient-controlled analgesia, or PCA). In addition, some mechanical pumps have small drug reservoirs that limit the infusion rate, necessitating a refill for long infusions. In an effort to address these limitations, manufacturers of mechanical pumps recently developed "hybrid" pumps that offer enlarged reservoirs for prolonged infusions, bolus capabilities and even variable bolus dosing. Hybrid pumps still retain many of the simplistic features of their purely mechanical precursors, like fixed flow rates, in large part because they are not programmable.

Most mechanical and hybrid pumps are also disposable. This helps simplify cost-accounting since the associated costs per case are straightforward, and it obviates the need to track down and reprocess reusable pumps. Disposable pumps are not necessarily less expensive in the long run, however, since your cost picture will depend on your price and number of infusions. Prices vary widely among manufacturers, and quantity discounts are also a factor. In addition, reusable electronic pumps frequently employ a disposable cartridge that may help recoup costs over time.

Finally, some of these pumps are less accurate than electronic pumps. Studies show that many mechanical pumps' flow rates can vary over time due to alterations in temperature and pump elevation. In my experience, though, this variability is not significant enough to restrict their clinical use.

• Electronic pumps. Electronic pumps tend to be very flexible, in that the physician can set the infusion rate and duration. Most also let the physician vary the infusion rate over time; for example, the pump may deliver more medication on post-op day one and less medication on post-op day two. Accordingly, electronic pumps usually hook up to large reservoirs (250 to 500 mL).

With these pumps, the physician can also determine the bolus amount, and some pumps even offer the ability to specify intervals between boli, vary bolus dosages over time, and/or set maximum limits on the cumulative bolus amount. Some physicians even want their patients to manipulate their own dosages depending on their level of pain, while others prefer pumps that can "lock out" patients. The downside of this flexibility is that clinicians can make programming errors.

Many electronic pumps also sound warnings in the case of malfunction. This helps identify problems like kinked catheters, although troubleshooting over the telephone is difficult and time-consuming. The beeping of the alarm can also lead to patient stress and annoyance.

Define the clinical need
To determine which type and model of pain pump you need, consider the pros and cons in light of the clinical indication(s) and physician preferences. In general, the simpler mechanical pumps work well for surgical site infusions. These sites are predictable in that they require a relatively low infusion rate, because they can absorb just a limited amount of local anesthetic before leakage occurs.

The more flexible hybrid and electronic pumps, on the other hand, work well for continuous infusion peripheral nerve blocks. Long, continuous peripheral nerve blocks typically require higher infusion rates to be clinically effective (5 to 10 mL/hour), so pumps with larger reservoirs are essential. Patients who receive these nerve blocks also benefit from PCA capability. Research shows that the ability to self-administer boli allows patients to experience good pain control with lower continuous infusion rates - resulting in a lower overall dose of local anesthetic. Some anesthesiologists may prefer electronic over hybrid pumps for continuous infusion peripheral nerve blocks, however, because of their programmability. For example, physicians often modify infusion rates because the amount of anesthetic varies depending on block type, each patient's pain tolerance and catheter placement. Many physicians also prefer to set their own bolus dosages and frequencies - parameters that, due to limited data, are largely a matter of physician preference.

The right direction
Although pain pump technology in and of itself is not a panacea for post-op pain, it is extremely effective when integrated into a comprehensive, multimodal approach to pain control. We integrate peripheral blocks into a regimen typically consisting of thorough patient screening and education, NSAIDs, cryotherapy and narcotics for breakthrough pain. We use these blocks whenever there is an accessible nerve plexus that serves the surgical area, and when the patient undergoes a procedure associated with prolonged, intense pain and is capable of managing the pump at home. Our data for more than 1,500 such outpatients shows that 95.4 percent reported a VAS score of zero at 24 hours (see "Patient Proof That Pain Pumps Work"). Until better options become available, we will continue to rely on continuous local analgesic infusions as an integral part of our post-op pain control regimen. [For more on Duke's approach to post-op pain control, see Outpatient Surgery's "Blocking Out Post-op Pain," January 2003.]