From Meal to Medical Aid: Transforming MRE Heaters into IV Fluid Warmers
🕖 Reading Time, 9 minutes
Although the ideal fluid for resuscitation of trauma patients is blood, in most austere and tactical settings, particularly outside of military Special Operations, IV crystalloids are still used. Even with hypotensive resuscitation strategies, providing IVF only to maintain a mean arterial pressure of 50 mmHg, some IV fluid is administered.
Administration of cold IV fluid can decrease the body temperature of the casualties. Hypothermia in hypotensive trauma patients increases mortality along with coagulopathy, acidosis, and possibly hypocalcemia, therefore ideally all IV fluid should be warmed before administration.
In austere and tactical settings, IV fluid warmers are frequently unavailable.
Historically, many “improvised” techniques have been suggested to warm IV fluid before administration to a casualty. However, do they actually work to increase the temperature of the fluid, not just in the IV bag, but when it actually reaches the casualty?
Traditionally, medics on US Army Special Forces teams carried two bags of IV fluid, connected by a cravat, around their necks and against the body to try and maintain some warmth of the fluid in cooler environments.
The only study looking at this used cave (8C ambient temperature) explorers carrying 500 ml bags of Lactated Ringers at a starting temperature of 6C, in a thin fleece pouch, over underwear, and beneath insulated clothing, on either the abdomen or axillae. The IV fluid temperature increased to 15C at one hour and 19.2C at two hours of carriage. The experiment was only conducted with three carriers. The results didn’t change significantly, even if the wearer hiked for two hours. That experiment was run with four carriers. The researchers found if the IV fluid was heated to 58C and the experiment rerun, fluid decreased to 42C at 1 hour, and then 35C at 2 hours.1 The authors concluded though carrying IV fluid against the body helped offset the cold temperature of fluid, it did not raise it even remotely close to actual body temperature.
Heat loss from the IV tubing itself is significant.
Even with prewarmed IV fluid, heat is lost to a cold environment from IV tubing during administration of the fluid to the casualty. In a 5℃ lab, bags of Lactated Ringers at a starting temperature of 44℃ lost 5.3℃ of heat running through uninsulated IV tubing over a nine-minute bolus with an 18-gauge catheter.
If the IV tubing was wrapped in a “conforming cotton bandage or reflective emergency blanket material” the tubing heat loss was only 3.6℃. If both were used it was only 3.1℃. However, the authors said this wrapping was time consuming raising the question whether it is practical.
Based on this study we can assume exposed IV tubing will lose 5.3℃ of IV fluid heat before the fluid reaches the casualty and we are yet to have a good and easy IV tubing insulation option.2
What if we actually warm the IV tubing itself?
250 ml bags of D10W in a refrigerator at a mean temperature of 2.6℃ had the IV tubing coiled in five loops just proximal to the end and various heat packs applied to the IV tubing coil. One MRE heater was taped to each face of the coil, 2 used in total, a Ready Heat blanket was folded over the face of the coil, or two HotHands heat packs, one on each side of the coil were used. All heaters were activated 15 minutes before use. Five runs were conducted with each heater type.3
MRE heaters increased the delivered fluid temperature an average of 12.7℃, Ready Heat an average 12.2℃, and HotHands an average of 4.9℃. Fluid was delivered over 9 to 11 minutes.
Although none of these techniques increased refrigerated IV fluid to a physiologic level, both the MRE heaters and Ready Heat blanket did substantially increase the temperature of the IV fluid by just heating the IV tubing coil.
What if we used MRE heaters to warm the IV bag itself?
One MRE heater was able to increase 1-L of room temperature (20℃) Lactated Ringers by 16℃ in 13 to 19 minutes. The same heater was able to increase refrigerated IV fluid (10℃) by 18℃ in 14 to 20 minutes. Two MRE heaters increased refrigerated IV fluid by 31℃ in 8 to 20 minutes. They found that fluid temperature reached a plateau at ten minutes. Each experimental temperature was tested ten times.4 This showed proof of concept that two MRE heaters could increase cold (10℃) IV fluid to slightly above body temperature, but the thermometer used to measure the temperature was inside the IV bag. Based on the previous study,2 the IV fluid will still lose 5.3℃ heat during administration to the casualty.
Another MRE heater study looked at an 8 mm closed cell foam insulating pouch and 3 to 5 MRE heaters (activated with 30 ml water each, then removed after ten minutes of warming) in their ability to increase the temperature of 1 L 0.9% NS delivered through an 18-gauge catheter flowing “wide open”.5
Five MRE heaters in a 3℃ lab with 3C IV fluid, increased IV fluid temperature to 48.2℃. The final delivered fluid temperature was 42.6℃.
Four MRE heaters in a 10℃ lab with 10C IV fluid, increased IV fluid temperature to 54℃. The final delivered fluid temperature was 45.6℃.
Three MRE heaters in a 20℃ lab with 20℃ IV fluid, increased IV fluid temperature to 46.3℃. The final delivered fluid temperature was 39.7℃.
Now we are starting to see a way to provide physiological temperature IV fluid to a casualty at clinically relevant flow rates.
What if we warm the IV fluid and IV tubing?
In a 5℃ cold room, 5℃ temperature 500 ml or 1 L bags of 0.9% NS with the IV tubing coiled in seven loops, and various heat packs were placed in a fleece jacket. All final temperatures were measured at the distal end of the IV tubing with bolus flow through an 18-gauge catheter. Infusion time for 1-Liter and 500 ml were 29 minutes and 14 minutes, respectively.6
Two MRE heaters (activated with water to the fill line) attached to the 500 ml IV bag and removed after ten minutes increased the administered fluid temperature from 5℃ to 39℃. They also increased the actual temperature of the fluid in the IV bag from 5℃ to 59℃ in five minutes for 500 ml bags and 5℃ to 50℃ for 1-liter bags.
Two Kwik-Heat instant hot packs were also tried but could only raise a 500 ml bag to 21℃.
Although there are dog studies showing no vessel damage administering IV fluid up to 65℃ through central lines, it is recommended IV fluid be warmed to only 40 to 42℃ before administration.
What does all this mean?
If you need to administer bolus IV fluid in an austere or tactical prehospital environment, without a dedicated battery powered fluid warmer, then two MRE heaters, activated and removed after ten minutes, on either side of a cold 500 ml IV bag with the IV tubing looped into seven coils, all in a fleece jacket will provide body temperature (39℃) IV fluid to your casualty. If you are using 1 L IV bags you will need between 3 and 5 MRE heaters and some kind of insulation depending on ambient and IV fluid starting temperature.
No one has tried these MRE heater experiments without something insulting around the warmers and IV bag, but a jacket of some type isn’t typically difficult to come by in a cold tactical environment.
Keeping cold 500 ml bags of IV fluid against the body will transfer some heat to the IV bag but only increase the bag’s temperature about 13℃. Without MRE heaters on the IV tubing, the IV fluid will lose 5.3℃ heat during administration. In cold environments this total net heat gain will only be about 8℃. Although better than administering functionally refrigerated IV fluid, just carry MRE heaters. If you’re not aware, MRE heaters release hydrogen gas on activation and are safest in an open space, not immediately around flame.
References
1Mortimer RB, Hurtt H. Intravenous fluid warming with body contact in a wilderness setting. Wilderness Environ Med. 2008 Summer;19(2):144-5; author reply 145-6. doi: 10.1580/07-WEME-LE-166.1. PMID: 18513111.
2Piek R, Stein C. Three insulation methods to minimize intravenous fluid administration set heat loss. Prehosp Emerg Care. 2013 Jan-Mar;17(1):68-72. doi: 10.3109/10903127.2012.702190. Epub 2012 Jul 26. PMID: 22834910.
3Rodriguez A, Algaze I, Almog R, Katzer RJ. Heating Intravenous Fluid Tubing in an Experimental Setting for Prehospital Hypothermia. Air Med J. 2021 Jan-Feb;40(1):41-44. doi: 10.1016/j.amj.2020.10.009. Epub 2020 Dec 9. PMID: 33455624.
4Garcia GD, Modesto VL, Lee KT. Avoiding hypothermia in trauma: use of the flameless heater pack, meal ready to eat, as a field-expedient means of warming crystalloid fluid. Mil Med. 2000 Dec;165(12):903-4. PMID: 11149058.
5DeClerck MP, Lipman GS, Grahn DA, Cao V, Wieland M, Troxel T, Heller HC. A Chemical Heat Pack-Based Method For Consistent Heating of Intravenous Fluids. Wilderness Environ Med. 2015 Sep;26(3):412-6. doi: 10.1016/j.wem.2015.02.004. Epub 2015 Jun 19. PMID: 26094579.
6Platts-Mills TF, Stendell E, Lewin MR, Moya MN, Dhah K, Stroh G, Shalit M. An experimental study of warming intravenous fluid in a cold environment. Wilderness Environ Med. 2007 Fall;18(3):177-85. doi: 10.1580/06-WEME-OR-051R1.1. PMID: 17896849.
