Observational in-vitro study comparing the efficacy of fluid warming devices: Ranger © 24500 versus ANIMEC AM-2S

Introduction

Perioperative hypothermia, defined as a patient core temperature of <36°C, is common but potentially preventable. The National Institute for Health and Clinical Excellence has released guidelines for the management of perioperative hypothermia in adults, recommending the routine use of a fluid warming device when intravenous fluid of more than 500 ml is administered. ¹ Preventing perioperative hypothermia is important as this is associated with numerous adverse outcomes such as coagulopathy, increased risks of wound infection rates, weakening of respiratory muscles and cardiovascular morbidities.^2–4 General anaesthesia increases the risk of hypothermia due to inhibition of thermoregulatory control. ⁵ Infusion of unwarmed fluids and blood products further increases the risk of hypothermia. ¹

Currently, there are several different types of fluid warming devices that can be used for the prevention of perioperative hypothermia. One of the heating methods is via the passage of the intravenous tubing or cartridge through metal heating blocks. In our institution, two such warming fluid devices are the Ranger© 24500 (Arizant Healthcare Inc., Eden Prairie, MN, United States) and the ANIMEC AM-2S (ELLTEC Co., Ltd, Nagoya, Japan). The Ranger^© 24500 (Ranger) uses a disposable plastic cartridge inserted between conductive warming plates with a pre-determined distance between the warmer and the patient due to the proprietary set. The ANIMEC AM-2S (ANIMEC) involves no disposable sets and requires insertion of the intravenous tubing between the warming plates, with the distance between the warmer and the patient determined by the user. Although the Ranger has been shown to deliver consistent temperatures, ⁶ its use is associated with higher costs and is, therefore, not as readily available as the ANIMEC, which is cheaper.

In this study, we compared the intravenous fluid warming capabilities of the Ranger versus the ANIMEC at different flow rates. We also wanted to determine whether using two ANIMEC fluid warmers placed in series could increase its efficacy.

Methods

We conducted a prospective, observational in-vitro study comparing the temperatures of the intravenous fluid delivered by the Ranger and the ANIMEC at various flow rates. We had three comparative trial groups. In trial group 1, we evaluated the use of an ANIMEC fluid warmer. In trial group 2, we evaluated the use of two ANIMEC fluid warmers placed in series. In trial group 3, we evaluated the use of a Ranger fluid warmer. The devices in all three trial groups were evaluated with three different flow rates, that is, 1 ml/min, 5 ml/min and 10 ml/min. To simulate clinical conditions, this study was carried out in the operating theatre at room temperature, and temperature measurements were made on the operating theatre table. In all three trial groups, a bag of 500 ml 0.9% sodium chloride at room temperature was connected to a 180 cm intravenous tubing and a Terumo Terufusion TE-172 infusion pump (Terumo Medical Corporation, Somerset, NJ, United States). A calibrated, Philips - 21075A temperature probe (Philips Healthcare, Andover, MA, United States) was inserted at the distal end (where an intravenous cannula in a patient would be connected to) of the intravenous tubing to record the temperature of the infused fluid. In trial group 1, the ANIMEC was attached to the intravenous tubing in such a way that it was 10 cm away from the thermometer. In trial group 2, two ANIMEC fluid warmers were attached to the intravenous tubing in series with the distal ANIMEC placed 10 cm away from the measurement point. In trial group 3, the 180 cm intravenous tubing was connected to the Ranger standard flow disposable set/Model 24250. When each device was turned on, we ran the fluid at the pre-determined flow rate (1 ml/min or 5 ml/min or 10 ml/min) via the Terumo Terufusion TE-172 infusion pump and temperature was measured every 1 min for a consecutive 30-min period.

The study was repeated twice such that three sets of data were collected for each trial group. Descriptive statistics was used to illustrate the differences in the fluid warming capabilities of the devices. Means and standard deviations (SDs) for the recorded temperatures were calculated based on the three sets of data for each trial group. Line graphs with Temperature (°C) as the vertical axis and Time (min) as the horizontal axis based on the means were plotted. All data was recorded and analysed using Microsoft Excel 2016 (Microsoft, Redmond, WA, United States).

Results

This study was conducted at room temperature which ranged from 21.3°C to 23.4°C (mean 22.5 (1.06)°C). Baseline fluid temperature in the three groups ranged from 21.1°C to 23.7°C (mean 22.3 (1.31)°C). At a flow rate of 1 ml/min (Figure 1), utilizing two ANIMEC fluid warmers delivered higher temperatures than using one ANIMEC fluid warmer. The Ranger delivered fluid with the lowest temperatures throughout the 30-min period. At this low flow rate, the temperatures of the delivered fluid were not consistent. Fluid temperatures generally decreased until about 9–10 min and increased from 11 until 30 min regardless of device used. The mean temperatures for the devices over the 30-min periods were 24.11 (0.62)°C for the trial group using two ANIMEC fluid warmers; 21.49 (0.35)°C for the trial group using one ANIMEC fluid warmer; 21.01 (0.38)°C for the trial group using the Ranger.

OPEN IN VIEWER

Figure 1.

Comparison temperature for 1 × ANIMEC (), 2 × ANIMEC () and Ranger () at flow rate of 1 ml/min.

At a flow rate of 5 ml/min (Figure 2), using two ANIMEC fluid warmers delivered the highest temperatures whereas the Ranger delivered the lowest temperatures. The temperatures of the fluid when delivered at 5 ml/min were reasonably consistent, except for the temperatures of fluid delivered by the Ranger, which decreased until the second minute before increasing again. The mean temperatures for the devices over the 30-min periods were 29.59 (0.10)°C, 25.47 (0.08)°C and 23.87 (0.34)°C for trial groups using two ANIMEC fluid warmers, one ANIMEC fluid warmer and the Ranger respectively.

OPEN IN VIEWER

Figure 2.

Comparison temperature for 1 × ANIMEC (), 2 × ANIMEC () and Ranger () at flow rate of 5 ml/min.

At a flow rate of 10 ml/min (Figure 3), having two ANIMEC fluid warmers placed in series produced the highest temperature, whereas using only one ANIMEC fluid warmer delivered the lowest temperatures. The mean temperatures over the 30-min periods were 29.27 (0.10)°C for the trial group using two ANIMEC fluid warmers, 27.32 (0.42)°C for the trial group using the Ranger fluid warmer and 24.78 (0.12)°C for the trial group using one ANIMEC. The temperatures of the fluid delivered by either one or two ANIMEC fluid warmers were reasonably consistent throughout the 30-min period. The temperatures of the fluid delivered by the Ranger, however, increased from 1 min up until the end of the evaluation period at 30 min.

OPEN IN VIEWER

Figure 3.

Comparison temperature for 1 × ANIMEC (), 2 × ANIMEC () and Ranger () at flow rate of 10 ml/min.

Discussion

The ideal fluid warming device for intraoperative use should be capable of delivering warmed fluids across a range of flow rates and conditions. Device-specific flow rates and infusion tubing length can affect the temperature of the delivered fluid. The ability of the Ranger and the ANIMEC to deliver warmed fluid is dependent on fluid flow rate and the distance between the patient and the warming device. The time and surface area for heat exchange and heat loss along the intravenous tubing between warmer and patient will determine the eventual fluid temperature being delivered to the patient. In this study, the temperatures of the fluid were not consistent at lower flow rates. At a flow rate of 1 ml/min for all devices and 5 ml/min for the Ranger, the temperatures of the fluid decreased at the commencement of fluid infusion before increasing gradually after a few minutes. At the beginning of the infusion, fluid within the intravenous tubing distal to the warming devices cool due to heat loss to the cooler ambient temperature. Although the fluid was kept at room temperature, the draft from the air-conditioning vent above the operating table could have resulted in further heat loss, causing the drop in initial temperatures of the fluid observed. The fluid temperature, however, increased after a few minutes as warmed fluid was then delivered from the warming devices to the temperature probe at the distal end of the intravenous tubing. At higher flow rates (5 ml/min for the ANIMEC and 10 ml/min for all devices), this initial decrease in fluid temperature was not seen. This is likely because at a higher flow rate, warmed fluid was rapidly delivered to the distal end of the intravenous tubing where the temperature probe was placed.

Overall, all warming devices and set-up produced the lowest temperatures at a flow rate of 1 ml/min. At this flow rate, although the fluid spent more time in contact with the heating elements of the warming devices, the fluid lost more heat to the cooler ambient temperature due to the long transit time between the warmer and the patient. This was evidently so as the temperatures increased when the flow rates were increased to 5 ml/min across all three groups and 10 ml/min for the Ranger and for utilizing two ANIMEC warmers. When using only one ANIMEC fluid warmer, the fluid temperatures were higher at a flow rate of 5 ml/min compared with a flow rate of 10 ml/min. At a higher flow rate of 10ml/min, there was probably insufficient contact time between the heating elements of the warmer and the intravenous fluid to allow adequate warming of the cool fluid. However, this was not observed when two ANIMEC warmers were used. Using two ANIMEC warmers produced the highest temperatures as this effectively doubled the contact time between the heating elements of the warmer and the intravenous fluid.

The manufacturers recommend an operating flow rate of 1 to 12 ml/min for the ANIMEC ⁷ and keep-vein-open to 500 ml/min for the Ranger. ⁸ It was previously shown that at a high flow rate of 153 ml/min, the Ranger performed reasonably well and delivered a temperature of 35.3°C. ⁹ In this study, it appears that at significantly lower flow rates of 1 ml/min, 5 ml/min and 10 ml/min, the Ranger delivered much lower fluid temperatures. The highest temperature achieved at a flow rate of 10 ml/min was only 27.83 (1.27)°C. A possible reason for this could be the set-up of the Ranger warming device – the warming cartridge of the standard flow disposable set/Model 24250 is 150 cm away from the end of the tubing where the temperature of the fluid is measured. The fluid, after passing through the warming device, is thus excessively cooled by the ambient air before reaching the distal end of the tubing.

It was previously calculated that the infusion of 1 l of intravenous fluid at 21°C reduces core body temperature by more than two times that of the same volume of fluid infused at 33°C. ¹⁰ Although the temperatures recorded in our study were lower than the ideal core body temperature of 37°C it would be reasonable to assume that any increase in temperature of the intravenous fluid infused, no matter how minimal, would have a positive impact on preserving core body temperature.

Our study suggests that utilizing two ANIMEC fluid warmers simultaneously produced higher temperatures at flow rates of 1 ml/min, 5 ml/min and 10 ml/min. We decided to evaluate the use of placing two ANIMEC fluid warmers in series along the intravenous tubing as the ANIMEC is cheaper, easier to set up and more readily available at our institution. The cost of an ANIMEC fluid warmer is US$365 (S$500) compared with that of the Ranger at US$3645 (S$5000). Furthermore, the use of the ANIMEC AM-2S fluid warmer requires no additional infusion set as the intravenous tubing is inserted directly between the conductive warming plates of the device. The use of the Ranger, on the other hand, requires a disposable plastic cartridge which costs US$19 (S$26) that is inserted into the device. These factors would translate into lower patient costs and a greater ease of use with the ANIMEC fluid warmer.

This study has several limitations. First, we evaluated the warming devices only at low flow rates of 1 ml/min, 5 ml/min and 10 ml/min. At these flow rates, the warmed fluid may cool within the intravenous tubing before delivery to the patient, thus rendering the warming ineffective. We, however, chose to evaluate the warming devices at these flow rates as we felt that this simulates most non-resuscitative clinical conditions, when intravenous fluid infusion is required for fluid maintenance/replacement for intra-operative fluid losses. Secondly, the temperatures observed in this study were different from the heating capabilities that were reported by the respective manufacturers. The Ranger is designed to warm up to about 41°C ⁸ whereas the ANIMEC is designed to warm up to about 37°C. ⁷ The lower than expected measured temperatures could be partly explained by our experimental set-up. We performed all observations and recordings on an operating table in an operating theatre suite with air-conditioning vents above it. The measured temperature could have been affected by the air-conditioning draft from the vents above the operating table. Despite this, we elected for our experimental set-up as we felt that it simulated actual operating theatre conditions. The third limitation involves the practicality of setting up two ANIMEC warming devices, as this may be more cumbersome when compared with the standard set-up of a single ANIMEC. The advantage of using the ANIMEC warmers is its small, compact size which allows two ANIMEC warmers to be hung easily from an intravenous pole. Fourth, we did not conduct a control arm where fluids were not warmed by any warming device. We believed that it would be fair to assume that the temperature at the end of the infusion line would be similar to the recorded baseline fluid temperature. Finally, our study did not determine the optimal flow rate required to achieve the maximum heating capability for each of the warming devices. Further studies are required before such recommendations can be made.

Our data suggests that when infusing intravenous fluid into patients at low flow rates of up to 5 ml/min, it appears that the warming capability of the ANIMEC warming device is superior to the Ranger fluid warmer. Considerations can also be made to use two ANIMEC warming devices placed in series, to achieve warmer temperatures when only low flow infusion rates of up to 10 ml/min are required. Where applicable, the fluid warming device should be placed as close to the patient as possible to minimize heat loss to the cooler ambient environment.