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Portable pumps used for local anesthetic infusion during continuous regional analgesia are gaining acceptance. These pumps are often used for ambulatory patients who are medically unsupervised throughout most of the infusion. However, the performance of these pumps, which infuse potentially toxic medication, has not been independently investigated. We investigated the flow rate accuracy, consistency, and profiles of various portable pumps often used for local anesthetic infusion during continuous regional analgesia. By using a computer/scale combination within a laboratory to record infusion rates, 6 pumps were tested with their flow regulators at expected (30°32°C) and increased (34°36°C) temperatures. Infusion rate accuracy differed significantly among the pumps, exhibiting flow rates within ±15% of their expected rate for 18%100% of their infusion duration. An increase in temperature also affected pumps to differing degrees, with infusion rates increasing from 0% to 25% for each model tested. These results suggest that factors such as flow rate accuracy and consistency, infusion profile, and temperature sensitivity should be considered when choosing and using a portable infusion pump for local anesthetic administration. IMPLICATIONS: Portable pumps often used for local anesthetic infusion during continuous regional analgesia exhibit varying degrees of delivery rate accuracy and consistency. Furthermore, increases in temperature result in an increased infusion rate for various pumps investigated. These factors should be taken into consideration when choosing and using a portable infusion pump.
Ansbro first provided continuous regional anesthesia more than 50 years ago (1). Since then, multiple techniques for local anesthetic delivery have been described, most of which used a polyamide catheter through which local anesthetic was infused. Initially, large, heavy, technically sophisticated infusion pumps were used. Subsequently, many of the procedures amenable to these techniques were moved to an outpatient setting. In the past decade, lightweight, portable pumps designed to infuse narcotics (2) or antibiotics (3) in ambulatory patients were introduced. Several investigators later adapted these portable pumps for regional analgesia. In 1998, Rawal et al. (4) first described using a portable pump to infuse local anesthetic for postoperative analgesia after day surgery. After this report, portable pumps of various designs were described to provide perineural (58), wound (9), and intraarticular infusions (10). Although these techniques seem to be gaining acceptance and usage for an increasing number of ambulatory patients, the infusion rate accuracy and reliability of the infusion pumps have not been independently investigated. We have had ambulatory patients using various portable pumps exhaust their local anesthetic reservoir after 50% to 150% of the expected infusion duration. This is consistent with the experience of other investigators (8). No apparent increase in anesthetic morbidity resulted from these highly variable infusion rates, yet the quality of analgesia lacked consistency, and infusion duration was unpredictable. We were concerned about possible local anesthetic toxicity in patients if infusion rates remained irregular. Many of these pumps regulate the infusion flow rate by using a temperature-dependent device calibrated to skin temperature. Therefore, we performed this laboratory study to define the flow rate accuracy, reliability, and profiles of various portable infusion pumps at expected and higher-than-expected temperatures.
Pump Selection Six small, lightweight, portable pumps marketed for local anesthetic infusion were selected for testing (Table 1, Fig. 1). The energy source to dispense anesthetic varies between pumps. These sources include elastic- or spring-based, pressure gradient (e.g., vacuum pump), and electrical (e.g., electronic pump) energy. At least one representative from each "category" of infusion drives was selected. All of these pumps are available in various reservoir volume and infusion rate combinations. The largest volume available and the rate most closely approximating 5.0 mL/h were selected for each infusion pump (Table 2). All pumps tested were previously unused and were filled to their recommended capacity with normal saline (NS) immediately before testing. New batteries were inserted in the Microject PCA, the only electronic pump tested, before each infusion.
Infusate Selection Infusate viscosity may influence the infusion flow rate of various pumps. Only McKinley Medical included information with their pumps about the effect that various infusates would have on its pump, stating that the Accufuser was calibrated by using 5% dextrose in water, and that the use of NS would increase the flow rate by 10%. Ropivacaine is one of the most commonly used local anesthetics described in continuous regional anesthetic techniques (5,7,8,10). Because viscosity data for ropivacaine are not available (personal communication with AstraZeneca LP, Wilmington, DE, October 2001), a trial at 32°C using the Accufuser was performed first with ropivacaine, then followed by NS. It demonstrated identical flow profiles for ropivacaine and NS, validating our extrapolation of this data obtained with NS infusions to those involving ropivacaine.
Study Apparatus
The collection bottle was placed on an electronic scale (Navigator Balance; Ohaus Corp., Florham Park, NJ) that was placed on the same surface as the infusion pumps. Once a test infusion began, data were logged onto an IBM-compatible personal computer (Dimension XPS 400; Dell Computer Corp., Round Rock, TX) by using an RS-232 cable. A software program (Software Wedge; TAL Technologies, Philadelphia, PA) provided entry from the serial port directly into the spreadsheet program (Excel 2000; Microsoft Corp., Seattle, WA). Mass of the infusate was measured every minute over the duration of the infusion by using the tared value of the bottle. The infusion period ended when each pump had exhausted its fluid reservoir. Because the Microject PCA pump does not have a predetermined maximal reservoir volume, as do the other pumps tested, a 1000-mL bag of NS was connected to the pump that was tested for 60 h. Subsequently, the hourly infusion rate was calculated by subtracting the mass at a given hour (Mx) from that obtained after the next hour (M[x+1]). That is, the hourly infusate volume equaled (M[x+1] - Mx), and the rate equaled this volume divided by 60 min. Although the scale manufacturer reports accuracy of ±0.1 g, we were concerned about time-related drift and possible infusate evaporation over the duration of these experiments. To test for potential evaporation loss and scale drift, 100 g of NS was placed in the collection bottle with the catheter in place for 2 wk. Measurements were taken each minute, and the loss to evaporation was <0.1 g (0.1%) over the testing period. Scale drift over the 2 wk was ±0.4 g (0.4%). The ambient room air temperature was held between 20°24°C (68°75°F) during the entire study period. A temperature-monitoring device (Hobo H8; Onset Computer Corp., Bourne, MA) recorded ambient temperature every 5 min during the entire study period to ensure a uniform room temperature for the infusion pumps. Based on these data, we conclude that the apparatus was appropriate to test pump performance over the duration of at least 60 h. Each infusion pump was tested with the flow rate regulator placed in the heating unit. The temperature of the heating unit was set at the temperature that the manufacturer reported to be skin temperature (the baseline, or expected, temperature). For example, the Accufuser was calibrated by the manufacturer for a flow rate of 5.0 mL/h at 32°C, whereas the I-Flow was calibrated at 31°C. If a manufacturer-recommended temperature was not included with the infusion pump, then 31°C was used. Each test was performed twice with a new infusion pump unit. If the infusion rate during the second trial differed more than ±10% of the original trial at any point, a third trial was performed. The trials were combined to produce a mean profile for each pump at the baseline (expected) temperature. After this, all pumps (except the Microject PCA) were tested again by using the same protocol, but with the heating unit set 4°C more than the baseline temperature. The Microject PCA infusion rate is controlled electronically and is relatively temperature independent over a small-scale temperature change (e.g., 4°C). Infusion duration (measured) was considered to end when the measured flow rate decreased <50% of the set rate. Data were reported as mean ± SD. Overall comparisons were made by using analysis of variance on ranks with post hoc Tukey pairwise testing, if appropriate. P < 0.05 was considered to be statistically significant.
The infusion rate profiles for the six portable pumps tested are illustrated in Figure 3.
Consistency Of the 6 pumps tested, only the MedFlo II had an infusion rate differing >10% between the first 2 trials at each temperature, requiring a third trial. The first and second trials at each temperature differed <10% at any point during the infusion for the Accufuser and Pain Pump units. The first and second trials at each temperature differed <5% at any point during the infusion for the C-Bloc and Microject PCA pumps.
Accuracy
Flow Profile To various degrees, all three of the elastomeric pumps (Accufuser, C-Bloc, MedFlo II) infused at a rate faster than expected initially, and then returned closer to their set rates for much of the infusion (Fig. 3). This increased flow rate resulted in a decreased overall infusion duration for the Accufuser and C-Bloc pumps (Table 3). The spring-powered Sgarlato pump had a similar initial period of rapid infusion, but its rate consistently declined and fell below its expected value after approximately 12 h. This resulted in an increased infusion duration of >15%. In contrast, the infusion duration for the vacuum-powered Pain Pump was decreased by >15% as a result of its consistently fast infusion rate for the entire infusion duration. For its entire duration, the electronic Microject PCA pump infused at a slower rate than it was programmed for, and this rate progressively decreased over time (from 5% to 15% below expected).
Temperature Effects
This investigation demonstrates that the infusion rate consistency and accuracy of portable pumps often used to provide postoperative continuous regional analgesia are variable (Table 3). Factors such as pump power source and ambient temperature impact pump infusion rate (Fig. 3). Both the elastomeric- and spring-powered pumps infused at faster-than-expected rates initially, with infusion rates decreasing over the infusion duration. The vacuum pump had consistently faster-than-expected infusion rates whereas the electronic pump infused at a consistently slower-than-expected rate. Increasing flow-regulator temperature increased infusion rates by >10% in 2 of the elastomeric pumps. The faster-than-expected infusion rates led to a decreased total infusion duration (Table 3).
Implications For many of the pumps described in this investigation, temperature influenced the rate of infusion (Table 3, Fig. 3). For the C-Bloc and MedFlo II pumps, an increase of 4°C resulted in an increased flow rate of >10%, whereas the Accufuser and Pain Pump were affected to a lesser extent. Although the Sgarlato pump uses an infusion-regulating device similar in appearance and placement to these other pumps, the manufacturer states that it is temperature independent. This was confirmed in our investigation. The degree to which the temperature sensitivity of a given pump should influence a decision regarding its use is highly situation dependent. For example, our institution is located in Florida where summer temperatures often reach 39°C, increasing skin and ambient temperature, which affects the flow regulators of various infusion pumps. After this study, we instructed our patients to remain in air-conditioned environments when using a temperature-sensitive infusion pump during the summer months. This investigation only varied temperature with an increase of 4°C. A larger increase should theoretically increase the flow rate more than reported here, whereas a temperature decrease should theoretically result in a flow rate decrease. However, this speculation is based on the physics of the flow regulator technology and requires additional investigation for confirmation.
Battery Charge
Pump Choice
Study Limitation In conclusion, portable pumps often used for local anesthetic infusion during continuous regional analgesia exhibit varying degrees of delivery rate accuracy and consistency. Furthermore, increases in temperature result in an increased infusion rate of various degrees for many of the infusion pumps investigated. Healthcare providers should take these factors into consideration when choosing and using a portable infusion pump for local anesthetic administration. Controlled clinical studies are needed to investigate how local anesthetic infusion rate variability affects patient analgesia.
Funding for this project was provided by the University of Florida, Department of Anesthesiology. All infusion pumps, except the I-Flow, were donated by manufacturers. The authors thank Jenny Kline Ilfeld, MD, for her valuable editorial contributions.
An abstract of this report was presented at the annual meeting of the American Society of Regional Anesthesia, Chicago, IL, April 26, 2002.
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