Review of Blood and Fluid Warming Methods

where V_fluid is the volume of blood or IV fluids administered, C_p is the specific heat, and T_core and T_fluid are the core body temperature and the temperature of the blood or IV fluids, respectively. In Eq. (1), W_patient is the weight of the patient and the patient-specific heat is 0.83 kcal/L/° C [4–7].

As the difference between the temperature of the infused intravenous fluid and the core body temperature increases, the patient typically experiences a greater drop in the mean body temperature. If the fluid requirement is greater in comparison to the patient's body weight, there will also be a larger potential drop in the body temperature [6]. It is reported that even a small reduction in core temperature can significantly impact postoperative outcomes, affecting patient satisfaction and recovery [7–11].

The general guideline for a transfusion is regardless of the warming method used, the warming device must not cause any damage to the fluids that are being administered and must work in an effective, reliable, safe, and convenient manner. The goal during resuscitation is the normalization of body temperature, 37 °C. However, the temperature of blood should never exceed 42 °C. In addition, the warming method must be able to heat intravenous fluids traveling at flow rates of 90–200 mL/min in cases of gravity-dependent, less emergent situations, and at higher flow rates of 200–600 mL/min in more emergent situations [12].

This paper reviews the performance of existing blood and IV fluids warming methods in three broad categories:

• active blood and fluid warming methods

• passive blood and fluid warming methods

• improvised methods

In this section of the review paper, a comprehensive summary of active blood and fluid warming devices developed over the years is included. Active methods include devices that warm the intravenous fluid by using electric heating or battery power. These warming devices can be categorized into three groups based on the technology they employ. These groups are dry heat, use of temperature-controlled water bath, and countercurrent heat exchanger. In dry heat warming technology, the IV tubing passes through heating elements. Some of the rare associated complications are reported as current leakage and temperature control malfunction. One of the main advantages of these warming devices is that there is no risk of contamination of the fluid inside the IV tubing since the outside of the tubing is not in direct contact with a warming fluid such as water in a water bath. In the temperature-controlled water bath warming technology, thinner-walled intravenous tubing with greater heat transfer capacities is made to pass through a warm water bath. Typically, a longer IV tubing is required for adequate heat exchange, which also limits the efficiency of these systems during rapid transfusions. In addition, there is a risk of infection if the outlet port encounters contaminated water and potential overheating may cause hemolysis. Countercurrent warming technologies use a countercurrent flow of heated water around the intravenous tubing. This method allows for the rapid infusion of warmed blood or IV fluids while resuscitating victims of hemorrhagic shock but it is relatively expensive [12]. Figure 1 illustrates these groups and the examples of warming devices in each. Fluids tested in the earlier studies are saline, blood, red blood cells, Crystalloid, Lactated Ringer's solution, and Hextend.

In 1992, Uhl et al. [13] performed a comparative study investigating the warming capabilities of the Fenwal blood warmer (Fenwal Laboratories, dry heat), the DW-1000 (American Pharmaseal Co., dry heat), the Flotem IIe (Datachem Inc., dry heat), the Hemokinetitherm (Dupaco Inc., water bath), and the H250 and H500 (by Level 1 Technologies, countercurrent). Testing was performed with blood and saline at various flowrates ranging from 10 mL/min up to 500 mL/min. Among these devices, H500 was the only one that was capable of heating both fluids to the desired temperature of 35 °C at flowrates greater than 100 mL/min effectively. At lower flowrates, all warming technologies demonstrated comparable performance.

For flowrates up to 125 mL/min, Smallman and Morgan [14] examined the heating effectiveness of the Level 1 Hotline (countercurrent) compared to the Fenwal blood warmer (dry heat). Saline at 4 °C and 18 °C at the tube inlet was tested at flowrates ranging from 1 mL/min up to 70 mL/min. For flowrates lower than 16 mL/min, the research study utilized an IMED 960A pump while for higher flowrates, an American Optical roller pump was used. Smallman et al. [14] demonstrated that Level 1 Hotline outperforms Fenwal at flowrates less than 70 mL/min for both inlet fluid temperatures. A similar study was performed by Browne et al. Comparing the heating capabilities of the Level 1 Hotline blood warmer series to Fenwal. A Level 1 warming system uses water kept at 40 °C that is pumped through a countercurrent tube-in-tube heat exchanger. Level 1 500 series achieved outlet temperatures of saline close to body temperature at all flowrates and outperformed Fenwal. In 1991, Presson et al. [15] tested Level 1 500 series with packed red blood cells and reported lower outlet fluid temperatures for an inlet fluid temperature of 4 °C. Specifically, they measured an outlet temperature of 32 °C at 440 mL/min.

In their research study, Patel et al. [16] compared the fluid-warming capabilities of five devices using different heat exchange technologies: the FW537 (steel foil), H1000 (countercurrent water), Hotline (countercurrent water), BairHugger 500 (convective air), and Flotem IIe (dry heat). The H1000 and Hotline were tested with 20 cm and 84 cm extensions, while the BairHugger and Flotem used 84 cm extensions to simulate clinical conditions requiring an injection port near the patient. Experiments were conducted at 21 °C with crystalloids, specifically Lactated Ringer's solution, and packed red blood cells diluted in saline. Temperatures were measured at the outlet, distal end, and simulated patient entry point, using flow rates of 6.5–25 mL/min based on anesthetic records. For diluted red blood cells, rapid gravity and pressure flows were tested. The H1000 and Hotline maintained the warmest temperatures of 36.0–39 °C across flow rates, while the Flotem was consistently coldest (29.5–33 °C). Considerable heat loss occurred with 84-cm extensions at moderate flows, minimized by 20 cm extensions. For gravity-infused diluted red blood cells, the H1000, Hotline, and FW537 outlets measured 42 °C. The study concluded that standard warmers like Flotem IIe are ineffective at moderately rapid flowrates, while the H1000 and FW537 sufficiently heated high volumes. Flotem, Hotline, and BairHugger are not recommended for pressure-driven infusions due to low outlet temperatures (<30 °C). The Hotline and H1000 are suitable for routine moderate flowrate administration. One of the main limitations of this study is the wide confidence intervals for the flowrates.

Blood warming devices are commonly used during fluid administration in surgery to mitigate a decrease in mean body temp (MBT). Horowitz et al. [17] investigated the flow rates and warming efficacies of two warmers, Hotline and Ranger, during pressure infusion with IV bag pressures of 90 mm Hg and 300 mm Hg. In addition to pressure infusion, various IV catheter sizes were tested. Hotline fluid warmer uses active warming that is accomplished by enveloping the intravenous fluid line with a layer of circulating warm water under temperature control. It is important to note that Hotline is not recommended for use with pressures exceeding 300 mm Hg. The Ranger fluid warmer operates by using the application of dry heat in which IV tubing is surrounded by electrical heating elements. These elements generate controlled heat, warming the fluid as it passes through the tubing before entering the patient's body. The findings of the investigation suggest that, at lower transfusion rates (1–4 L/h), the Hotline warmer achieves higher temperatures for both red blood cells and saline. However, in situations that require higher transfusion rates (>4 L/h), the Ranger fluid warmer demonstrates superior performance by avoiding a rapid reduction in mean body temperature (MBT) [17].

The warming capabilities of four fluid warming devices that are surrogates for military use in forward treatment areas, were tested in a research study by Dubick et al. [18]. Two of these devices were floor models including Level 1 model 1000 and FMS 2000, while the other two were portable, namely, Thermal Angel and Ranger. The performance of each device was evaluated with three different fluids: Lactated Ringer's solution (LR), Hextend, and Packed Red Blood Cells (PRBCs). During testing, temperature measurement probes were placed in the fluid bag and at the exit from the warming device outflow catheter, which would then be attached to the patient in a typical application. Each device was tested with inlet fluid temperatures of 20 °C and 5–7 °C and at flowrates of 150 mL/min and 250 mL/min. Dubick et al.'s study compared the devices based on the setup time, the steady-state temperatures achieved, the warm-up time, the prime volume, the time for tested fluids to reach the steady-state temperature, and the temperature difference between the starting temperature and the warmed fluid temperature. The fluid warmers were tested on their ability to reach clinically minimal infusion temperatures of 32 °C and 35 °C. At both flow rates evaluated, the FMS 2000 and Ranger warmed room temperature LR to at least 37 °C at the outlet port, whereas the fluid was only warmed to 35.4 °C by Thermal Angel. At the low flowrate setting, Ranger and FMS 2000 warmed refrigerated LR, Hextend, and PRBCs to at least 36 °C. Although at this flowrate, Thermal Angel warmed cold Hextend and PRBCs to >33 °C, it could only warm cold LR to 27.3 °C. At the high flowrate, all four fluid warmers warmed cold Hextend and PRBCs to at least 35 °C, whereas Thermal Angel and Level 1 did not warm cold LR above 35 °C. The average warmed temperature generally occurred earliest with the FMS 2000, which suggests that this device was able to warm fluid to a more consistent temperature than the other systems. In addition, Dubick et al.'s research study also compared the devices based on the ease of operation, the weight and size of the equipment, and the device's bubble-suppressing ability [18].

Based on the results of Dubick et al.'s study, Level 1 and FMS 2000 were both de-emed suitable for Department of Defense forward surgical units. Although the FMS 2000 is compact and self-contained, it may be better suited for use in aircraft or more forward sites where power is available. The Ranger was superior to Thermal Angel in warming fluids at the temperatures and flow rates observed in this study and could be used for immediate first aid at the front-line medical facilities, as long as power is available. However, Thermal Angel is most likely to be adapted for far-forward combat environments because it is battery-powered [18].

Satoh et al. [19] conducted a research study to compare the warming capabilities of three fluid warming devices with flow rates ranging from 2 to 100 mL/min and initial fluid temperatures of 21–23 °C and 3–5 °C. The devices tested were the Hakko blood warmer HBW-5 (water-bath warmer), the Medi-Temp III (dry heat plate warmer), and the Hotline HL-90 (IV fluid tube warmer). During testing, a normal saline solution was used and the maximum temperature achievable for each device were 38 °C, 41 °C, and 41 °C, respectively. For temperature measurements, thermocouples were placed at the end of a 1.0 m tube connected to the outflow of the warmer. The ambient air temperatures were kept between 22 and 24 °C. Their results show that the IV fluid tubing warmer (Hotline HL-90) is the most effective at low flow rates (<40–50 mL/min) since it had the highest temperature delivered for both initial fluid temperature ranges. However, it should be noted that the tubing of this device is thick, long, and heavy and is a potential source of air bubble emboli. At high flow rates (>60 mL/min), the Medi-Temp III warming system was the most effective. Although the Hakko blood warmer HBW-5 was the least expensive compared to the others, it was also the least effective [19].

The warming capability of the Buddy Lite and enFlow fluid warmers was assessed in a research study by Bruells et al. [20]. They conducted a series of tests, measuring the temperature of the fluid as it passed through the devices, with flow rates ranging from 25 mL/min to 100 mL/min for saline at room temperature and 10 °C. Their results show that the Buddy Lite maintains a higher outlet temperature compared to the enFlow at a flowrate of 25 mL/min. However, at higher flow rates of 75 and 100 mL/min, the Buddy Lite shows a significant decrease in its heating capability, resulting in a lower outlet temperature. This trend persisted, regardless of whether the saline was at room temperature or 10 °C. Additionally, Bruells et al. observed a notable drop in temperature along the 1 m outflow tube. Overall, both the Buddy Lite and enFlow fluid warmers were effective in warming the fluid at low flow rates. Both warmers are compact and can be conveniently placed near the patient. One distinction between the two devices is that the Buddy Lite can operate using batteries that can last for several hours, while the enFlow requires an external power supply [20].

Small heating devices, that use boxed heating plates with limited warming areas, may lead to reduced warming ability. To investigate the effect of warming area, three small fluid warming systems, enFlow, Fluido compact, and Thermosens were compared in a research study performed by Zoremba et al. [21] in 2018. Experimental testing was performed by pumping saline at 24.2 ± 0.5 °C and precooled (lowest value at 5.5 ± 0.3 °C) through a section of the infusion line using a roller pump. A fluid warming system was located 10 cm down the infusion line from the roller pump, and temperature probes were placed at four locations including the inlet and exit from the fluid warming system, and 50 cm and 100 cm away from the exit. For each measurement, saline was pumped through the system at flowrates of 25, 50, 75, and 100 mL/min. When saline at 24.2 ± 0.5 °C was used, no significant difference was observed between the performance of the three devices at any of the flow rates with each trial achieving body temperature fluid up to 100 cm away from the fluid warming system. However, when precooled saline was administered, the temperature of the warmed saline at 75 mL/min and 100 mL/min flowrates for the Fluido compact test were significantly lower than the ones achieved at 25 mL/min and 50 mL/min. A degraded performance was also observed for Thermosens at 100 mL/min flowrate. The enFlow performed the best of the three fluid warming systems by achieving fluid at body temperature (38 °C) across all flowrate and saline temperature conditions [21].

Lehavi et al. [22] compared four battery-operated in-line fluid warmers: the Belmont Buddy Lite, Carefusion enFlow, Thermal Angel TA-200, and QinFlow Warrior. They tested the devices by using saline solution for a range of input temperatures and flow rates. The IV bag was positioned 100 cm above the device to generate 74 mm Hg pressure. At 10 °C and 20 °C, the devices were evaluated at 50, 100, and 200 mL/min flow rates after fully charging the batteries. The Buddy Lite warmed the solution at 20 °C and 50 mL/min but struggled at higher flow rates and lower temperatures. The enFlow battery depleted at 20 °C and 50 mL/min. Thermal Angel performed well at 20 °C but output temperatures dropped substantially at 10 °C and higher flow rates. The Warrior consistently maintained output temperatures of ∼37 °C across all conditions, warming the most fluid, but was the largest and heaviest device. The enFlow and Warrior performed the best overall, while Thermal Angel was limited by battery capacity and flowrate and the Buddy Lite required high input temperatures and low flow rates. This research study had some limitations including testing only at room temperature with fixed flow and input temperatures and using saline solution as the only test fluid. Future studies could address these limitations by testing at different temperatures, with variable flow and input temperatures, and using other fluids such as refrigerated blood.

Blakeman et al.'s [23] research study evaluates four portable fluid-warming devices: Buddy Liter, Buddy Lite AC, Thermal Angel, and °M Warmer. Buddy Liter and Buddy Lite AC use a disposable cartridge within a reusable heater unit while Thermal Angel and °M Warmer use disposable heater units. All these devices connect to intravenous tubing and heat the fluid as it passes through the tubing. The performance of these devices was observed with the understanding that blood should be warmed to normal body temperature, 37 °C, before administration to the patient. The devices were evaluated using two different fluids, normal saline (NS) and expired packed red blood cells. A nonemergent flowrate of 125 mL/hour and an emergent flowrate was achieved by using a pressure bag inflated to 300 mm Hg. For each case, the volume of fluid infused was 1 L. Since this study focused on blood warmers for military use, the testing was conducted at three simulated altitudes, ground level, 8,000 feet, and 16,000 feet, to assess the impact of changes in battlefield elevation. The environmental temperature was controlled at 23.9 ± 0.4 °C. Fluid temperature was measured before the input port and after the output port of each warmer, with data collection occurring at 1-second intervals beginning with the initiation of flow and terminating with the complete infusion of the 1 L bag. The activated fluid warmers had temperature set points as follows: Both the Buddy Liter and the Buddy Lite AC had a temperature set point of 38 ± 2 °C. Thermal Angel had a temperature set point of 38 ± 3 °C, and the °M Warmer had a temperature set point of 39 ± 3 °C. None of the devices warmed fluids to normal body temperature, 37 °C. Only the °M Warmer heated NS and PRBCs to ≥32 °C and PRBCs to ≥35 °C more than 80% of the time at the emergent flow with mild hypothermia (classified as body temperature between 32 °C and 35 °C). A higher flowrate seems to correlate with decreased performance for each warming device, but altitude seems to have little effect on device performance. No device has a distinct physical or operational advantage as there is no significant difference between the volume or weight of any of the devices.

In a research study performed by Weatherall et al. [24], the warming performance of four commercially available blood warming devices is compared over a range of clinically relevant temperatures at different flow rates within an in vitro blood circuit. These devices are Thermal Angel TA-200, Hypotherm X LG, °M Warmer, and Belmont Buddy Lite. The three flow rates tested are 50 mL/min, 100 mL/min, and 200 mL/min. The temperature of the fluid, red blood cell concentrate, in the circuit was controlled at 4 °C in the collection bag position of the circuit prior to each trial with an ambient temperature of 23 °C. Temperature measurements were taken using thermistors located distal to the collection bag, proximal to the blood warming device, and distal to the blood warming device. Control sample data were collected by excluding the blood warming device from each of the flowrate scenarios. Relative to the control, all four blood warmers heated the fluid. However, across all three trials, the heating performance deteriorated as the flowrate increased. The Buddy Lite is observed to have the most significant decrease in performance when the flowrate was increased to 100 mL/min. The authors noted that at these flow rates, the Buddy Lite appears to heat intermittently. In each scenario, the °M Warmer performs the best, but never reaches the 38 °C body temperature threshold, although closely heating the fluid to that threshold in the flowrate scenarios of 50 mL/min and 100 mL/min [24].

Vallier et al. [25] provide a comparative review of three battery-operated blood-warming devices on warming capabilities in austere environments. These devices are Buddy Lite, enFlow, and Thermal Angel. Among these devices, enFlow is listed as the ideal fluid warmer for massive transfusion since it can reach high temperatures (up to 39.5 °C with saline at an initial temperature of 5 °C) faster, and achieve higher flowrates (up to 100 mL/min) compared to Buddy Lite and Thermal Angel. (Buddy Lite achieved an output temperature of 31.8 °C when saline at 5 °C was used, while Thermal Angel achieved an output temperature of 36.4 °C). Further investigation is, however, required to validate the performance of enFlow with blood. It should be noted that one of the reported issues associated with enFlow is aluminum elution into IV fluids due to the unprotected aluminum heating element in the device.

Wilson et al. [26] performed a quasi-experimental study comparing the thermal performance of two Baragwanath Rewarming Appliances (BaRA devices) to the Hotline device to warm IV fluids. The first BaRA device has the IV tubing submerged in a warm water bath during implementation while the second one wraps the IV tubing around a forced air warmer. The IV fluids tested are dextrose water, Ringer's lactate, Voluven at room temperature, and refrigerated PRBC. Wilson et al. conducted 219 experiments with transit tubing distances of 100 cm and 200 cm. The warming capabilities of the devices are tested at flowrates of 36 mL/min, 60 mL/min, and 100 mLmin. Their results indicate that the warm water bath device kept at 43 °C with 200 cm of submerged IV tubing provides exit temperatures similar to the Hotline device at all flowrates tested.

Tongsukh et al. [27] investigated the use of a 1.5 m long heater set at 42 °C (Barkey autocontrol 3XPT; Barkey, Leopoldshoehe, Germany) over the IV line to warm the saline before it is administered to patients. Infusion flowrates tested were 100, 300, 600, 900, and 1200 mL/h. The temperature of saline was tested at the inlet and exit of the IV tubing and their study reports the following temperature rise for each flowrate: 10.9 °C ± 0.1 °C, 11.5 °C ± 0.1 °C, 10.2 °C ± 0.1 °C, 10.1 °C ± 0.7 °C, and 8.4 °C± 0.2 °C at flowrates of 100, 300, 600, 900, and 1200 mL/h, respectively. One limitation of this study is that, during testing, the ambient temperature was kept at 24 °C. There is no testing available for this method to show the effect of low ambient temperatures.

Table 1 illustrates the summary of literature research that has been performed on the performance of active blood and fluid warming methods. Most of the devices have been tested with saline. The performance of these devices may be different depending on the ambient conditions and with other fluids such as blood or colloid.

Passive methods can be described as methods that do not make use of external or internal electrical power. Examples as illustrated in Fig. 2 include the use of chemical heat such as latent heat warmers, flameless ration heaters, Meal Ready to Eat (MRE) hot packs, and Hothands warmers.

In 2020, Roxby et al. [32] investigated the safety and efficacy of fluid-warming devices powered by latent heat. The fluid warmer tested was produced and sterilized by Logikal Health Products in Morisset, New South Wales. The fluid warmer consisted of a plastic bag with two compartments, separated by a breakable plastic strip. One compartment contained 950 mL (1.7 kg) of liquid calcium nitrate tetrahydrate. The second compartment contained infusion tubing and a small amount (50 g) of solid calcium nitrate tetrahydrate. When in use, the breakable strip was bent back and forth a few times until it broke, allowing the liquid calcium nitrate tetrahydrate to flow into the infusion tube compartment. The fluid warmer was held upright, allowing the liquid calcium nitrate tetrahydrate to pass through the broken strip and solidify upon contact with the solid calcium nitrate tetrahydrate. It was shown that the latent heat warms the red blood cell units to approximately 35 °C. One of the main advantages of a latent heat warmer is that it can increase the temperature of the fluid to normal body temperature without the need for battery packs or an external electrical power source. The latent heat fluid warmer was found to be safe and did not overheat. With a mass of 2 kg and a volume of 1 L, it warmed up red blood cells in less than 1 min. The limitation of this study is that although the latent heat warmer was designed for emergency and retrieval situations with higher flow rates, this study analyzed the warmer with patients receiving outpatient transfusion support at slower flow rates [32].

A heating system that uses flameless ration heaters (FRH) and an insulated sleeve for the consistent intravenous delivery of saline solution was investigated by De-Clerk et al. in 2015 [30]. The efficacy of the heating system was tested for a transfusion flowrate of 77 mL/min when the ambient temperatures were 3 °C, 10 °C, and 20 °C. The insulated sleeve consisted of an 8 mm thick neoprene layer covered with cloth. The FRH uses an oxidation-reduction reaction of a magnesium salt with water. The results of the study show that the heating system achieves a saline solution temperature in the range 39.7 °C – 45.6 °C at the inlet of the IV flow line. However, one of the main drawbacks of this heating system is that the number of FRHs needed to achieve healthy transfusion temperatures depends on the environment temperature [30].

Hypothermic patients can be effectively treated by administering intravenous fluids warmed to between 40 °C and 42 °C. Platts-Mill et al. [31] evaluated various methods of warming intravenous fluid at 5 °C for a bolus infusion in a cold environment to an infusion temperature range of 35 °C to 42 °C. All testing was conducted in a cold room maintained at 5 °C. In this research study, four warming measures were tested: Kwik-Heat Instant Hot Pack, Wilderness IV Warmer, Meal Ready to Eat (MRE) hot packs, and backpacking stove and metal pot. The evaluation included fourteen types of application methods, incorporating insulation through a synthetic fleece jacket wrapped around the fluid bag and varying the number of heating packs applied to the bag. Each warming measure was applied to a fluid bag connected to intravenous tubing extending 100 cm from the warming area, with a thermocouple placed 3 cm from the tubing's end to measure the infusion temperature. Six of these fourteen warming methods were applied to a 1 L fluid and eight warming methods were applied to a 500 mL fluid bag. The two warming methods that produced mean infusion temperatures within the desired range were (1) using two MRE hot packs attached to a 500 mL fluid bag with a 10-minute delay and (2) placing the 500 mL fluid bag in a pot of water that is heated over a backpacking stove until the surface temperature of the fluid in the fluid bag reached 75 °C. These two methods exhibited effective warming, providing valuable insights into optimal approaches for intravenous fluid warming in cold environments. Platts-Mill et al. Reported several limitations to their study including performing testing at only one environmental temperature (5 °C) and not incorporating the effects of wind conditions, altitude, and sun exposure [31].

While the research studies performed by Roxby, Declerk, and Platts-Mill et al. [30–32] assessed the application of passive fluid warming systems directly to the IV bag, Rodriguez et al. [33] studied the application of passive warming systems to the IV tubing (instead of the bag). The heating systems used with IV tubing in the study were a HotHands hand warmer, a Meal Ready to Eat (MRE) packet, and a heating blanket. The 250 mL of dextrose bag was infused during a 9–11-min infusion time. The results of the study demonstrate that the average and the maximum temperature increase are 12.68 °C ± 3.92 °C and 16.93 °C respectively with the use of one MRE packet. The heating blanket results in a similar average temperature rise of 12.24 °C ± 2.39 °C without the use of hot water to activate it. HotHands only achieves an average temperature increase of 4.88 °C ± 2.79 °C. One of the limitations of this study is that the exit temperature appeared sensitive to the initial (starting) temperature and even small variations in the starting temperature caused large variations in the exit temperature. Nonetheless, none of the methods achieved a desirable exit temperature, i.e., higher than 35 °C.

The viability of warming red blood cells using latent heat storage has been tested by McEwan and Roxby in 2007 [34]. In latent heat storage warmer, the heat released when a liquid solidifies is used to warm intravenous fluids. One of the main advantages of such a method is not having any reliance on electrical energy or battery power. For the infusion flowrate of 50 mL/min, the fluid temperature increased from 4 °C to 35 °C [34].

Another method that has been tested to reduce heat loss is to insulate the IV tubing. Piek and Stein [35] assessed the efficacy of three insulation techniques in reducing heat dissipation while infusing warmed IV fluids at a high flowrate in a room at an ambient temperature of 5 °C. The insulation techniques examined were wrapping the tubing with a cotton bandage, utilizing a reflective emergency blanket, and combining both methods. The IV tubing was positioned vertically in the room to enable the fluid to travel from the container through the tubing. Before each trial, the Lactated Ringer's solution was heated to 45 °C. To gauge temperature changes, three thermocouples were utilized—the fluid temperature at the tubing inlet and exit and one measuring room temperature. The study consisted of four cases—one for each insulation method and a control. The control condition exhibited an average temperature loss of 5.28 °C from inlet to exit. When a cotton-conforming bandage was used to wrap the tubing, the temperature drop through the tubing decreased to 3.53 °C. Similarly, when the tubing was wrapped with a reflective emergency blanket, the temperature loss decreased to 3.53 °C. The most effective approach, however, was to wrap the tubing with both the reflective emergency blanket and the cotton-conforming bandage. This method results in a temperature loss of 3.06%. Therefore, the study recommends using this method to achieve maximum heat conservation. One limitation of this research study is that the flowrate of IV fluids was not controlled during the experiments, which may affect the accuracy of the results [35].

In their research study, Milligan et al. [29] compared temperature changes in packed red blood cells (PRBCs) using three improvised warming methods—exposure to body heat, direct sunlight, and gel heat pads—against a commercial fluid warmer, specifically the Belmont Buddy Lite, within a simulated prehospital setting. A control unit was also employed for reference. The PRBCs were placed in the axilla of an investigator for 5 min before infusion. During the tests, the thermal gel pad was activated at the beginning of each run, with the distal section of the giving set wrapped around it five times to reach an internal temperature of 54 °C. The PRBCs were exposed to direct sunlight at a temperature of 26 °C for 5 min. The Belmont Buddy Lite was integrated into the system, operating at a flowrate of 50 mL/min, as recommended for temperatures below 10 °C. The blood was initially stored at 4.5 °C. The temperature was measured at three different points and each method was tested three times with two units of blood, both of which were reused.

All warming devices demonstrated a significant increase in temperature, but the Belmont Buddy Lite consistently warmed the blood to a level close to physiological. The average temperatures achieved by each method were as follows: body heat 17.2 ° C ± 0.8 °C, exposure to sunlight 20.2 ° C ± 0.8 °C, gel heat pads 18.8 ° C ± 0.8 °C, Buddy Lite 35.2 ° C ± 0.8 °C, and the control group 14.7 ° C ± 0.6 °C. It was observed that a room air temperature of 23.4 °C had a significant effect on raising the temperature of the blood in the control group, showing the influence of the environment on the warming process. Therefore, it is important to take into account the prehospital environment and measures taken to insulate the IV tubing from that environment. The results of Milligan et al. indicate that improvised blood warming methods may lack control, potentially resulting in uneven warming. There were several limitations associated with this study. The tested flowrate of 50 ml/min is relatively slow, and mechanical pumps, commonly used in blood transfusions, were not employed in this study. Also, expired donated blood was used for testing without hemolysis analysis. The collection and cooling process for blood reuse could potentially introduce variables affecting the warming devices [29].

Convective warming of intravenous fluids by keeping the IV bags close to the defroster vents has been studied by Lyng et al. [36]. In this study, a 1 L intravenous fluid bag of 0.9% sodium chloride was placed above a vehicle windshield defroster vent for 30 min. The temperature inside the car and the external ambient temperatures were recorded as 20.1–22.3 °C and −2.9 °C, respectively. 10 independent trials were conducted for each condition driven on the same route. The initial temperature of the bag was recorded as 19.4 °C. With a probe placed through a medication port inside each fluid bag, Lyng et al. were able to record the fluid temperatures. An average temperature increase of 14.1 °C was measured with the intravenous fluid bag placed on the windshield. This method did not provide temperatures close to the “normal” body temperature of 37 °C. The mean temperature of the control intravenous fluid bag was unchanged. The mean start temperature was 20.1 °C, and the mean end temperature was 22.3 °C. The external environmental temperature was -2.9 °C and had no impact on the results.

It is important to note that improvised blood warming methods that involve direct concentrated heat application such as placing the unit near a radiator or a high-temperature heat source should be avoided. These methods can potentially lead to overheating of the blood unit, resulting in hemolysis and rendering the blood unusable [10].

The performance studies on warming methods and technologies that have been conducted thus far provide opportunities for future work. Some recommendations for future studies are as follows

• Effects of fluid type on heating performance. Most warming devices have been tested with saline. The performance of these devices may vary based on fluid properties.

• Low-temperature effects on heating performance. The inclusion of low ambient temperature conditions (at or below freezing) such as those encountered in the Arctic (or at higher altitudes) in the performance study may add more clarity to the true performance envelope of the warming methods and technologies. Additionally, this may also help identify other potential issues, especially with battery-powered active warming methods since battery performance typically deteriorates at lower temperatures.

• High-altitude effects on heating performance. Although a preliminary high-altitude study has been conducted on a few active warming devices, more in-depth studies are needed on the effects of high-altitude (low ambient pressure), especially for passive technologies and improvised warming methods.

• Comprehensive comparison of warming techniques across categories. This may suggest potential advantages or disadvantages of a technique or method over others in the same usage scenario. Further, combining and using warming techniques from different categories together may result in better heating performance in some scenarios.

Blood and IV fluid warming methods can be broadly categorized as active, passive, or improvised methods. Active methods use electrical power to heat the fluid during the transfusion or infusion process while passive methods primarily rely on the heat released during exothermic chemical reactions or phase change to do the same. Additionally, low-cost and easily accessible improvised warming methods are often employed in lieu of active and passive warming methods. Active methods generally offer a greater degree of control in their heat output compared to passive and improvised methods. The studies conducted to characterize the performance of the different warming devices and options available under each broad category indicate that the heating performance strongly depends on many (independent) factors such as the type of fluid, the initial fluid temperature, the fluid flowrate, and the pressure on the fluid (infusion) bag. The devices that achieved the desired heating performance in the studies did so only for specific sets of conditions for the independent factors. More specifically, most studies were limited in scope and lacked substantial data to suggest that one technology or method was superior to others in most usage scenarios.

There is no conflict of interest. The views and opinions expressed herein are those of the author and do not purport to state or reflect the position of the United States Government or any agency thereof, including the United States Military Academy, the Department of the Army, or the Department of Defense.

• U.S. Army Combat Capabilities Development Command Army Research Laboratory (Funder ID: 10.13039/100006754).

T_core =

core body temperature

T_fluid =

temperature of blood or intravenous (IV) fluids

C_pfluid =

specific heat of blood or IV fluids at T_fluid

C_ppatient =

patient-specific heat (0.83 kcal/L/° C)

V_fluid =

the volume of blood or IV fluids being administered

W_patient =

weight of the patient