The incidence of anesthetic-induced hypothermia in small animals has remained moderately consistent throughout the past 30 years. In 1973, Evans et al1 reported an incidence of 100%, and in 2012, Redondo et al2 reported an incidence of 97.4%. Numerous adverse effects related to prolonged recovery, postoperative complications, and death have been reported in veterinary and human medicine.1–7 Despite great medical advances in many aspects of veterinary and human medicine, anesthetic-induced hypothermia remains prevalent. There is a lack of veterinary information on combatting this problem.
There are 3 stages of anesthesia-induced hypothermia.8–10 The first stage occurs within the first hour of general anesthesia and is characterized by a sudden temperature decrease of 1° to 5°C because of redistribution of heat from the body core to the periphery. The second stage occurs 2 to 4 hours after induction of anesthesia and results in a much slower linear decline of temperature because of the disparity between increased heat loss and decreased metabolic heat production of the anesthetized patient. The third stage occurs after approximately 5 hours of anesthesia, and temperature often stops decreasing during this phase. This thermal plateau reflects a steady state in which heat loss equals heat production.8–10 Given that the most profound decrease in temperature occurs early, within the first stage of general anesthesia, methods to combat hypothermia should be targeted at this first stage.
Three basic types of warming methods include passive surface, active surface, and active core.11 Traditional passive surface methods include use of cotton blankets, surgical drapes, plastic sheeting, and bubble wrap. Active surface methods include use of circulating water blankets, forced-air delivery systems, and warming panels. Traditional passive surface and active surface warming devices fail to adequately restore normothermia once the first stage of hypothermia has occurred because of peripheral vasoconstriction.4,9,12–14 Studies15–17 reveal that active surface warming prior to anesthetic induction helps decrease the severity of the initial temperature decline in adult humans. Unfortunately, this is impractical in most veterinary patients. Thus, passive and active surface warming are suboptimal in maintaining normothermia in the first stage, and fluid warmers warrant further exploration.
Two main categories of fluid warming devices are distance dependent and distance independent. Most distance-dependent devices enable the fluid line to be placed in an S-shaped channel through the heating plate. Temperature sensors that are in contact with the tubing control the heating and regulate effluent temperature. Distance-independent devices consist of disposable heat exchangers that have concentric tubes in which a heated fluid (usually water) passes through the outer wall of the tubing; simultaneously, the IV infusate flows through the inner tubing. Heat is transferred from the hot outer tubing through the wall of the tubing to the infusate. These devices work most efficiently when the flow of the heated water is higher than that of the IV fluid infusate. They also overcome the problem of in-line cool down by actively warming the patient's fluids to the patient connection.18–20
Despite technological advances and the need to prevent secondary anesthetic-induced hypothermia in veterinary medicine, no product has documented superiority. No in vitro or in vivo output temperature studies have compared the efficacy of currently available veterinary distance-dependent and distance-independent devices with the simultaneous evaluation of the wide range of anesthetic fluid flow rates for small animal patients. Additionally, studies have not concurrently investigated the effect of altering the location of distance-dependent devices or whether there is a benefit to use of prewarmed fluids in conjunction with a fluid warming device.
The purpose of the study reported here was to compare the warming capabilities of commonly available IV fluid line warmers on the basis of in vitro output temperature differences at a variety of flow rates and distances and the use of room temperature (approx 22°C) fluids versus prewarmed fluids. We hypothesized that a distance-dependent blood and fluid warmera marketed for both human and veterinary use would be a superior warming device, compared with a distance-dependent fluid warmerb marketed for veterinary use only as a fluid warmer, because it is the more expensive of the 2 devices and has been used reliably in our hospital for many years. We also hypothesized that connecting the warming device closest to the catheter hub would result in warmer temperatures at all IV fluid flow rates and that the warmest output temperatures would be achieved with higher IV fluid flow rates. We also hypothesized that a distance-independent blood and fluid warmerc would be an even better warming device because of its distance-independent design properties and documented superiority in humans.20–22 We hypothesized that the addition of prewarmed fluids would result in the warmest output temperatures and that the use of a warming device would yield higher output fluid temperatures, compared with the use of room temperature or prewarmed fluids alone, because these are commonly used in clinical settings.23
Materials and Methods
A serial phase design was chosen to isolate factors such as distance and temperature of the infusate that could contribute to final output fluid temperatures. First, the effect on output temperatures was investigated by altering distance in distance-dependent IV fluid warmers at clinically relevant IV flow rates (phase 1). The warming capabilities of 2 distance-dependent IV fluid warming devices (a blood and fluid warmera marketed for human and veterinary use [product A] and a fluid warmerb marketed for veterinary use and recommended for use with fluid [product B]) at 4 distances (0, 4, 8, and 12 cm from the device to the test vein) and room temperature fluids at flow rates of 20, 60, 100, 140, 180, 220, 260, and 300 mL/h were compared. Once the superior distance-dependent device was found, it was compared with a distance-independent device to eliminate distance as a factor (phase 2). The device from phase 1 with the greatest warming capability was compared with a distance-independent IV blood and fluid warmerc with a triple-lumen tubing design (product C) with room temperature fluids at the same flow rates at a distance of 0 cm. Lastly, for the device from phase 2 with the greatest warming capability, the effect of incubated fluids (38°C), compared with room temperature fluids, was evaluated against room temperature and incubated fluids alone (phase 3).
To evaluate the temperature of IV fluids entering the vein of a patient, an in vitro test station was constructed (Figure 1). A test vein was made from an extension setd cut to a length of 6.35 cm with an injection cap placed on the proximal end. Four 18-gauge needle holes were made 3 mm apart in the center of the test vein for an egress route. A 5-mm piece of tubing was inserted into the distal end of the test vein. This centralizer tubing was used in each test vein with each trial to ensure that the thermocoupler needle would be centered in the test vein and not against the wall of the tubing. A precision fine wire K-type thermocouple needlee was then inserted distally into the test vein. The tip of the thermocoupler needle extended just beyond the centralizer tubing. A 20-gauge IV catheter was inserted into the test vein and attached to the IV fluid line exiting the fluid warming device being tested.
All experimental procedures were conducted in the same test room. At the start of each data collection day, ambient room temperature was recorded in 4 locations in the testing room with a calibrated thermometer.f Prior to starting, all 4 ambient temperature readings were within 1°C, and mean temperature was 21.7°C. All thermocouplerse were placed in a warm water bath with a thermometerf for calibration. Once the thermocouple needle temperatures were within 1°C of each other, they were deemed accurate. The needles were connected to an analog-digital input-output boardg with associated calibrationh and data transfer.i Data were recorded directly in a commercially available software program spreadsheetj during the flow periods. A commonly used IV fluid solutionk and standard fluid pumpsl were used for all trials. Fluid pumps were set to administer only the specified amount of fluid for the trial being performed. There was approximately 15 to 20 minutes between trials; test veins were not changed. Fluid administration sets were changed between trials. Each fluid line was 264 cm long, and each extension set was 76 cm long. All trials were randomly performed; each trial was performed 3 times for 30 minutes at each flow rate.
The temperature of fluids was verified with the thermometerf to be 21°C. The incubatorm temperature was set at 38°C and was checked before each trial with the thermometer.f Incubated fluids were used within 5 minutes following removal from the incubator to simulate a clinical situation.
Schematic illustration of an in vitro fluid line setup (A) and illustration of an in vitro fluid line setup at 0 cm (B) in a study to compare 2 distance-dependent IV fluid line warming devices (a blood and fluid warmer and a veterinary-specific fluid warmer) and a distance-independent blood and fluid warmer at a variety of flow rates and distances. In panel B, the veterinary-specific fluid warmer is illustrated.
Citation: Journal of the American Veterinary Medical Association 244, 12; 10.2460/javma.244.12.1423
Statistical analysis—A mixed modeln analysis was used to compare the mean temperatures among the devices while accounting for repeated measures. Pairwise comparisons among the devices at different distances and flow rates with Bonferonni adjustments were performed if the main effects in the mixed model were significant (P ≤ 0.05). Significance was defined as P ≤ 0.05.
Results
Phase 1—The distance-dependent veterinary-specific fluid warmer (product B) had significantly (P = 0.025 for 20 mL/h; P < 0.001 for all other flow rates) greater warming capability than the distance-dependent blood and fluid warmera (product A) at every flow rate evaluated (Figure 2). Fluids warmed with product B were a mean of 8.1°C (range, 2.9° to 10.1°C) warmer than fluids warmed with product A for all flow rates.
As flow rate increased for both devices, output fluid temperature increased to a certain point. The highest temperature occurred at a flow rate of 180 mL/h for product B and 140 mL/h for product A. With greater flow rates, temperature decreased slightly for both devices and then plateaued. Product B had significantly (P < 0.001) greater warming capabilities than product A for all combined flow rates for each distance tested (Figure 3).
Comparison of the warming capabilities (mean ± SD temperature) of 2 distance-dependent IV fluid warming devices (a blood and fluid warmer marketed for human and veterinary use [dashed line] and a fluid warmer marketed for veterinary use not recommended for use with blood [solid line]) used with room temperature (approx 22°C) fluids at flow rates of 20, 60, 100, 140, 180, 220, 260, and 300 mL/h.
Citation: Journal of the American Veterinary Medical Association 244, 12; 10.2460/javma.244.12.1423
For both distance-dependent warming devices, at a distance of 0 cm, fluids were significantly warmer than at a distance of 4, 8, and 12 cm for all flow rates. At a distance of 4 cm, fluids were significantly warmer than at a distance of 8 and 12 cm for all flow rates. At a distance of 8 cm, fluids were significantly warmer than at a distance of 12 cm for all flow rates except 260 and 300 mL/h.
Comparison of the warming capabilities (mean ± SD temperature) of 2 distance-dependent IV fluid warming devices at distances of 0, 4, 8, and 12 cm from the device to the test vein. See Figure 2 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 244, 12; 10.2460/javma.244.12.1423
Comparison of the warming capabilities (mean ± SD temperature) of a distance-dependent veterinary fluid warmer (solid line) and a distance-independent blood and fluid warmer (dashed line) with room temperature fluids at flow rates of 20, 60, 100, 140, 180, 220, 260, and 300 mL/h.
Citation: Journal of the American Veterinary Medical Association 244, 12; 10.2460/javma.244.12.1423
Phase 2—The distance-dependent veterinary-specific fluid warmer (product B) had significantly greater warming capability than the distance-independent blood and fluid warmer (product C) at flow rates of 60, 100, 140, and 180 mL/h (Figure 4). Trial 2 of the 3 trials of product C at 100 mL/h was eliminated from statistical analysis because the device was inadvertently not turned on. Fluids warmed with product B were a mean of 3.7°C (range, 2.9° to 4.3°C) warmer than fluids warmed with product C for flow rates of 60, 100, 140, and 180 mL/h. Fluids warmed with product C were significantly warmer than fluids warmed with product B at a flow rate of 300 mL/h only.
Phase 3—Output temperature of both room temperature and prewarmed fluids warmed with product B were > 36.4°C at all flow rates except 20 mL/h. There was no significant difference between room temperature and prewarmed fluids with product B. Output temperature of room temperature and prewarmed fluids alone were < 22°C at all flow rates. There was no significant difference between room temperature and prewarmed fluids alone. Both room temperature and prewarmed fluids warmed with product B were significantly (P < 0.001) warmer, compared with room temperature or prewarmed fluids alone alone (Figure 5).
Comparison of the output temperatures (mean ± SD) for room temperature fluids warmed with a veterinary-specific fluid warmer (solid line), incubated fluids warmed with a veterinary-specific fluid warmer (dashed line), room temperature fluids alone (dotted line), and incubated fluids alone (dashed-dotted line).
Citation: Journal of the American Veterinary Medical Association 244, 12; 10.2460/javma.244.12.1423
Discussion
Studies have not been performed to evaluate the warming capability of currently available IV fluid line warmers in the context of veterinary use. The manufacturers of the 2 distance-dependent fluid warmers (product A and product B) state that output temperatures are between 32° and 41°C. Per the manufacturers, the range of output temperature depends on many factors, including ambient temperature, flow rate, starting temperature of fluids, and distance to the patient.
By use of a distance-dependent IV fluid warmero that is no longer manufactured, Faries et al18 determined that fluids were warmest when the device was placed closest to the theoretical patient as well as when the fluids were administered at a faster rate. In confirmation with our hypothesis as well as the Faries et al18 study, we confirmed that 2 distance-dependent fluid line warmers functioned best at faster flow rates and when they were placed closest to the theoretical patient. This phenomenon is referred to as in-line cool down effect.19,20 This occurs as a result of conduction, which is the transfer of energy (fluid heat) between objects (administration tubing), and convection, which is the transfer of energy (fluid heat) to the environment, progressing to achieve equilibrium.19 As the distance from the warming device to the patient increases and as the fluid flow decreases, time for heat transfer to occur via convection and conduction increases.19 Seemingly contrary to this theory, both product A (at 140 mL/h) and product B (at 180 mL/h) had an optimal flow rate at which they reached their maximum temperature. At higher flow rates, output fluid temperatures slightly decreased and then plateaued. At higher flow rates, we can theorize that the fluids did not have an adequate amount of time to heat up in the device. This is clinically useful information because one must be conscious of the location of the fluid line warmer to maximize the warming capability of the device and additional warming modalities may be needed for lower flow rates.
Contrary to our hypotheses, product B was the superior heating device at several flow rates (60, 100, 140, and 180 mL/h), compared with product C. This was an unexpected finding because product C is a distance-independent warming device, which has recently gained extreme favor in the human field for its ability to maintain high output fluid temperatures at low and high flow rates.20–22 Consistent with our hypothesis, product C was significantly warmer at 300 mL/h. The manufacturer of product C states that the set point temperature of the recirculating fluid is 41.9°C and output temperature varies depending on flow rates, with consistent temperatures > 40.0°C for flow rates 250 to 2,000 mL/h.24 Newly published veterinary anesthetic fluid flow rates are 5 mL/kg/h for dogs and 3 mL/kg/h for cats.25 A patient weighing approximately 60 kg would require a 300 mL/h flow rate; thus, product C may actually be superior to product B at higher rates, which would be more common for an adult human.
Phase 3 revealed 3 major findings. First, use of the fluid warmer provided superior output fluid temperatures, compared with room temperature only or prewarmed fluids, which was in contrast to the study by Chiang et al.26 Second, there was no difference between prewarmed and room temperature fluids, which was in agreement with the study by Chiang et al.26 Third, the fluid warmer performed the same regardless of whether prewarmed or room temperature fluids were used. Fluid output temperatures were always > 36.4°C for all flow rates ≥ 60 mL/h with product B. Prewarmed and room temperature fluids had output temperatures < 22°C for all flow rates. Core body temperature in dogs and cats is 37.8° to 39.3°C.5 Chiang et al26 concluded that, despite significant temperature differences, the use of incubated fluids with or without their fluid warmer offered minimal benefit over room temperature fluids. The disparity between the findings of Chiang et al26 and the first major finding of the study reported here could partially be explained by the fact that they placed their fluid warmer at a distance of 9 cm. The manufacturer of that warmer states that the device should be 2.5 cm from the patient.26 On the basis of the in-line cool down effect phenomenon, we can theorize that higher temperatures may have been obtained if their device were closer to the theoretical patient.
Limitations of the present study included its in vitro design. Only 3 types of readily available IV warming devices, only 8 flow rates, and only 1 type of IV fluid solution were evaluated. Therefore, we could not extrapolate how output fluid temperature translates to patient core body temperature, the ease of use of the devices in vivo, or the warming capability of other IV warming devices. An in vivo study would be needed to confirm whether patients are warmer with IV fluids at higher output temperature. Additionally, we could only theorize output temperatures for flow rates < 20 and > 300 mL/h. The thermocouple needles and the thermometer were calibrated within 1°C of each other; however, more precise thermometers are available for greater accuracy of calibration. We used only 1 commonly available IV fluid solution. All isotonic salt solutions have the same specific heat, so the type of isotonic crystalloid used is not relevant,27 but we cannot make conclusions about other fluids such as plasma, packed RBCs, or blood.
The distance-dependent veterinary-specific fluid warmerb produced significantly warmer output temperatures, compared with the distance-dependent veterinary and human blood and fluid warmera at all flow rates and at certain flow rates for the distance-independent blood and fluid warmer.c Placing distance-dependent IV fluid line warmers as close as possible to the catheter insertion site is recommended. There was no benefit to use of prewarmed fluids with the distance-dependent veterinary-specific fluid warmer.b The output temperatures of room temperature and prewarmed fluids were less than core body temperatures of dogs and cats. Strong consideration should be given to providing a fluid warmer and additional passive or active heating to patients receiving fluids at a low rate (≤ 60 mL/h).
ANIMEC (AM-2S) fluid warmer, Elltec Co Ltd, Nagoya, Japan.
iWarm fluid warmer, Midmark Animal Health, Versailles, Ohio.
Hotline fluid warmer, Smiths Medical Inc, Norwell, Mass.
Extension set, Baxter Healthcare Corp, Deerfield, Ill.
Precision fine wire thermocouple, Omega Engineering Inc, Stamford, Conn.
National Institute of Standards and Technology–calibrated thermometer, Control Co, Friendswood, Tex.
I/OA-D conversion board, Omega Engineering Inc, Stamford, Conn.
InstaCal 5.31, Measurement Computing, Middleboro, Mass.
DAS Wizard 2.03, Measurement Computing, Middleboro, Mass.
Microsoft Excel, version 10.06501, Microsoft Corp, Redmond, Wash.
Plasmalyte, Abbott Animal Health, Abbott Park, Ill.
Baxter Fluid Pump, Baxter, Deerfield, Ill.
Boekel Incubator 132000, Boekel Scientific, Feasterville-Trevose, Pa.
SAS, version 9.2, SAS Institute Inc, Cary, NC.
TEMPCARE TC-1, Elltec Co Ltd, Nagoya, Japan.
References
1. Evans AT, Sawyer DC, Krahwinkel DJ. Effect of a warm-water blanket on development of hypothermia during small animal surgery. J Am Vet Med Assoc 1973; 163: 147–148.
2. Redondo J, Suesta P, Gil L, et al. Retrospective study of the prevalence of postanesthetic hypothermia in cats. Vet Rec 2012; 170: 1–5.
3. Pottie RG, Dart CM, Perkins NR, et al. Effect of hypothermia on recovery from general anesthesia in the dog. Aust Vet J 2007; 85: 158–162.
4. Beal MW, Brown DC, Shofer FS. The effects of perioperative hypothermia and the duration of anesthesia on postoperative wound infection rate in clean wounds: a retrospective study. Vet Surg 2000; 29: 123–127.
5. Armstrong S, Roberts B, Aronsohn M. Perioperative hypothermia. J Vet Emerg Crit Care 2005; 15: 32–37.
6. Posner L. Perioperative hypothermia in veterinary patients. NAVC Clin Brief 2007; 5: 19–23.
7. Oliver JA, Clark L, Corletto F, et al. A comparison of anesthetic complications between diabetic and nondiabetic dogs undergoing phacoemulsification cataract surgery: a retrospective study. Vet Ophthalmol 2010; 13: 244–250.
8. Sessler DI. Mild perioperative hypothermia. N Engl J Med 1997; 336: 1730–1737.
9. Sessler DI. Temperature monitoring. In: Miller RD, ed. Anesthesia. 5th ed. Philadelphia: Churchill Livingstone, 2000; 1367–1389.
10. Matsukawa T, Sessler D, Sessler A, et al. Heat flow and distribution during induction of general anesthesia. Anesthesiology 1995; 82: 662–673.
11. Oncken AK, Kirby R, Rudloff E. Hypothermia in critically ill dogs and cats. Compend Contin Educ Small Anim Pract 2001; 23: 506–521.
12. Sessler DI, Moayeri A. Skin-surface warming: heat flux and central temperature. Anesthesiology 1990; 73: 218–224.
13. Sessler DI. Consequences and treatment of perioperative hypothermia. Anesthesiol Clin North Am 1994; 12: 425–456.
14. Cabell LW, Perkowski SZ, Gregor T, et al. The effects of active peripheral skin warming on perioperative hypothermia in dogs. Vet Surg 1997; 26: 79–85.
15. Franklin MA, Rochat MC, Payton ME, et al. Comparison of three intraoperative patient warming systems. J Am Anim Hosp Assoc 2012; 48: 18–24.
16. Sessler DI. Complications and treatment of mild hypothermia. Anesthesiology 2001; 95: 531–543.
17. Just B, Trevien V, Delva E, et al. Prevention of intraoperative hypothermia by preoperative skin-surface warming. Anesthesiology 1993; 79: 214–218.
18. Faries G, Carden J, Pruitt KM, et al. Temperature relationship to distance and flow rate of warmed IV fluids. Ann Emerg Med 1991; 20: 48–50.
19. Smith C, Wagner K. Principles of fluid and blood warming in trauma. Int Trauma Care 2008; 18: 71–79.
20. Patel N, Smith C, Pinchak A, et al. Prospective, randomized comparison of the Flotem IIe and Hotline fluid warmers in anesthetized adults. J Clin Anesth 1996; 8: 307–316.
21. Horowitz PE, Delagarza MA, Pulaski JJ, et al. Flow rates and warming efficacy with Hotline and Ranger blood/fluid warmers. Anesth Analg 2004; 99: 788–792.
22. Schnoor J, Weber I, Macko S, et al. Heating capabilities of the Hotline and Autoline at low flow rates. Paediatr Anaesth 2006; 16: 410–416.
23. Dix GM, Jones A, Knowles TG, et al. Methods used in veterinary practice to maintain the temperature of intravenous fluids. Vet Rec 2006; 159: 451–455.
24. Smiths Medical Inc. Hotline blood and fluid warmer. Available at: www.internetmed.com/sites/default/files/Smiths_Fluid_Warmer_-_General_technical_manual.pdf. Accessed Feb 1, 2013.
25. Davis H, Jensen T, Johnson T, et al. 2013 AAHA/AAFP fluid therapy guidelines for dogs and cats. J Am Anim Hosp Assoc 2013; 49: 149–159.
26. Chiang V, Hopper K, Mellema M, et al. In vitro evaluation of the efficacy of a veterinary dry heat fluid warmer. J Vet Emerg Crit Care (San Antonio) 2011; 21: 639–647.
27. Gentilello LM, Moujaes S. Treatment of hypothermia in trauma victims: thermodynamic considerations. J Intensive Care Med 1995; 10: 5–14.
