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The effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia
Resuscitation 80 (2009) 766–768
Contents lists available at ScienceDirect
Resuscitation journal homepage: www.elsevier.com/locate/resuscitation
Clinical paper
The effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia夽 T.J. Mader ∗ Department of Emergency Medicine, Baystate Medical Center, 759 Chestnut Street, Springfield, MA 01199, United States
a r t i c l e
i n f o
Article history: Received 8 September 2008 Received in revised form 3 April 2009 Accepted 13 April 2009 Keywords: Induced hypothermia Resuscitation Heart arrest Post-resuscitation care
a b s t r a c t Background: This study was done to determine the effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia. The study hypothesis was that cold saline would warm rapidly during simulated infusion and that an insulating SIGG neoprene pouch would slow the process. Methods: Paired 1-l bags of normal saline [with or without an insulating SIGG neoprene pouch (NEO+ and NEO− respectively)] were refrigerated together for at least 24 h. With an ambient room temperature (RT) between 32 and 34 ◦ C, the fluid was allowed to flow unrestricted through standard tubing connected to a 20-guage angiocath while the line reservoir temperature was monitored every 30 s. The order of the bags was pre-determined and alternated for each session. During 5 sessions, ten 1-l bags were included (5 NEO+ and 5 NEO−). The data were analyzed descriptively using Stata SE v8.1 for Macintosh. Results: The average ambient RT during the experimental sessions was 32.6 ◦ C (StDev: 0.8 ◦ C). The relative humidity was a constant 16%. The average low saline temperature at the beginning of infusion was 6.2 ◦ C (StDev: 2.7 ◦ C). The average rate of infusion was 48.2 cm3 /min (StDev: 3.7 cm3 /min). The average rise in saline temperature during the first 15 min of the infusion was 2.9 ◦ C (StDev: 1.2 ◦ C). The average high saline temperature reached near the end of the infusion was 13.4 ◦ C (StDev: 4.1 ◦ C). The average temperature change during infusion was 7.2 ◦ C (StDev: 3.5 ◦ C). The baseline data for the NEO+ and NEO− samples were not statistically different. The average temperature change over the first 15 min for the NEO+ group was 2.0 ◦ C (95% CI: 1.4 ◦ C and 2.5 ◦ C) and for the NEO− group it was 3.9 ◦ C (95% CI: 2.6 ◦ C and 5.1 ◦ C). The average change over the entire infusion for the NEO+ group was 4.3 ◦ C (95% CI: 3.1 ◦ C and 5.5 ◦ C) and for the NEO− group it was 10.2 ◦ C (95% CI: 7.4 ◦ C and 12.9 ◦ C). Conclusions: During simulated infusion to induce therapeutic hypothermia, cold saline begins to warm toward ambient temperature but the rate is not rapid. An insulating SIGG neoprene pouch slows the rate of warming. © 2009 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Mild induced therapeutic hypothermia is currently recommended for survivors of out-of-hospital cardiac arrest who meet specified criteria.1,2 It is generally believed that postresuscitative hypothermia should be induced as early as possible. This notion has lead many emergency medical services to initiate this process by a combination of surface cooling and infusion of cold saline immediately upon return of spontaneous circulation in the prehospital setting. The induction of mild therapeutic hypothermia with a rapid infusion of 4 ◦ C crystalloid has recently been shown to
夽 A Spanish translated version of the abstract of this article appears as Appendix in the final online version at doi:10.1016/j.resuscitation.2009.04.022. ∗ Tel.: +1 413 794 1661; fax: +1 413 794 8070. E-mail address: [email protected]. 0300-9572/$ – see front matter © 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.resuscitation.2009.04.022
effectively lower body temperature in out-of-hospital cardiac arrest survivors prior to hospital arrival without causing any adverse consequences.3–5 This study was done to determine the effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia. The study hypothesis was that cold saline would warm rapidly during simulated infusion and that an insulating SIGG neoprene pouch would slow the process. 2. Materials and methods Five SIGG neoprene 1-l pouches were purchased online from Biome Living Brisbane City, Queensland Australia. Paired 1-l bags of normal saline [one with and one without an insulating SIGG neoprene pouch (NEO+ and NEO− respectively)] were stored together in a 1.8 ft3 commercial grade refrigerator (Danby model DAR192w; Guelph, Ontario) on the maximum cold setting [mean temperature
T.J. Mader / Resuscitation 80 (2009) 766–768
767
Table 1 Summary of main results (temperature in degrees Celsius). All (n = 10)
Ambient temperature Infusion time (min) Infusion rate (cm3 /min) Beginning (low) temperature Ending (high) temperature Temperature change at 15 min Temperature change during infusion
NEO (+) (n = 5)
NEO (−) (n = 5)
Mean
(StDev)
Mean
(95% CI)
Mean
(95% CI)
32.6 20.9 48.2 6.2 13.4 2.9 7.2
0.8 1.7 3.7 2.7 4.1 1.2 3.5
32.6 20.4 49.3 6.9 11.2 2.0 4.3
(31.4,33.9) (18.4,22.4) (44.6,54.0) (3.6,10.1) (7.3,15.0) (1.4,2.5) (3.1,5.5)
32.6 21.5 47.2 5.5 15.7 3.9 10.2
(31.6,33.5) (19.4,23.5) (42.5,51.9) (1.9,9.1) (10.7,20.7) (2.6,5.1) (7.4,12.9)
of 0.14 ◦ C (StDev: 1.98 ◦ C, range: 6.67 ◦ C)] for at least 24 h. With an ambient room temperature between 32 and 34 ◦ C each bag, in turn, was connected through standard intravenous tubing to a 20-guage angiocath and the fluid was allowed to flow unrestricted from a height of 5 ft. The ambient room temperature and relative humidity (Acu-Rite model 00613 Digital Hygrometer; Jamestown, NY) were recorded along with the line reservoir temperature (Esophageal temperature probe model 21090A; Philips Medical Systems, Andover, MA) every 30 s during infusion. The first measurement was recorded at the stable nadir upon initial connection and the last was taken on the half-minute just before the reservoir emptied. The order of the bags was pre-determined and alternated for each session. The second bag of fluid was not removed from the refrigerator until just before use. Set-up was accomplished in less than 30 s for each episode and no further manipulations were made after initiation of flow. During 5 sessions, 10 1-l bags were included (5 NEO+ and 5 NEO−). The data were entered into a spreadsheet and analyzed descriptively using Stata SE v8.1 for Macintosh. 3. Results The main results are presented in the Table 1. The average ambient room temperature during the experimental sessions was 32.6 ◦ C (StDev: 0.8 ◦ C), while the relative humidity was a constant 16%. The baseline data for the NEO+ and NEO− samples were mathematically the same. There was a statistically significant difference in the temperature change between the two groups at 15 min as well as over the total infusion time. 4. Discussion A combination of exposure and surface cooling along with a rapid infusion of cold intravenous crystalloid has been shown to
facilitate induction of therapeutic hypothermia following return of spontaneous circulation after out-of-hospital cardiac arrest. In a recent study by Kampmeyer and Callaway, a simple, effective and inexpensive method for storage of cold saline to maintain two 1l bags of normal saline near 4 ◦ C for up to 24 h is described.6 The results of this current study, as illustrated in Fig. 1, demonstrate that cold saline, when removed from the cold storage unit for simulated infusion, gradually warms toward ambient temperature. An insulating neoprene pouch has a statistically significant effect on the magnitude of warming during the infusion period with the greatest effect occurring in the last few minutes of infusion (Fig. 2). This experiment was done in a small office with a room temperature that was not artificially manipulated but by coincidence happened to be in the range recommended as the therapeutic hypothermia target temperature.1,2 Though the room has a Southeasterly exposure, there was no direct sunlight during the sessions and a negligible change in room temperature occurred. All infusions were from the same height through the same intravenous tubing with a 20-guage angiocath attached to the end. There was a wide range of infusion rates (41.7–54.1 cm3 /min) due in part to slight differences in the time to reach minimum temperature on the monitor before temperature recording began and random variability in flow rates through the tubing. There were no differences between the two experimental groups, however. Two time points were chosen a priori for comparative analysis. The first (15 min) was chosen as a realistic transport time from return of spontaneous circulation to hospital arrival and transfer of care to the emergency department staff7 and the second was at the completion of infusion to assess the overall change in temperature between the two groups. At both time points, the differences were statistically significant though the absolute temperature changes (2.9 ◦ C and 7.2 ◦ C respectively) and differences (2.0 ◦ C vs. 3.9 ◦ C and 4.3 ◦ C vs. 10.2 ◦ C, respectively) were relatively small. One could
Fig. 1. The change in saline temperature over time by group during simulated infusion to induce mild therapeutic hypothermia. The solid line error bars indicate the NEO+ and the interrupted line error bars indicate the NEO− standard deviations at each time point.
768
T.J. Mader / Resuscitation 80 (2009) 766–768
Fig. 2. The change in saline temperature over time during the last 5 min of each unit’s infusion by group. Zero indicates infusion completion.
argue that since the fluid temperatures are still well below body and target temperatures, the infusion would still be effectively cooling the cardiac arrest survivor in the clinical setting, all be it, the NEO− somewhat less efficiently. Considering the laws of thermodynamics, in light of the urgency to reach target temperature, and given concerns about volume overload in some patients, optimal efficiency in cooling becomes an important consideration for inducing mild hypothermia.8 The small differences in temperature observed between the two groups in this study over the course of the infusion, therefore, are not only statistically significant but also may be clinically significant as well. An unexpected and important finding noted during this experiment was that although all of the paired 1-l bags of normal saline were placed in the same refrigerator on the same setting for similar periods of time, the starting temperatures of the paired units varied considerably. The average difference between paired bags was 3.1 ◦ C but one pair differed by 7.9 ◦ C. The range of starting temperatures for the whole sample was between 0.8 ◦ C (in the NEO− group) and 9.8 ◦ C (in the NEO+ group). The reason for this observation is unclear. One might speculate that since the refrigerator was not medical grade and the precision of the temperature regulation within the unit, as noted above, was suboptimal, the internal temperature of the unit might have fluctuated unpredictably during each session. In addition, the placement of the 1-l bags of saline within the unit may have influenced the temperature of the solutions. Though this was considered in advance and their relative positions within the unit were rotated each time in an attempt to minimize this effect the combination of these factors may explain the differences. In any event, these findings suggest that one cannot assume that the cold saline being used to induce mild therapeutic hypothermia is of consistent starting temperature based solely on similar cooling methods. 4.1. Limitations This experiment was a simulation only and the conditions and results may not reflect actual clinical conditions. The infusions were carried out within a relatively narrow but practical range of ambient temperatures representing worst case scenario conditions—a hot summer day, out of direct sunlight, and without benefit of air conditioning. Most ambulances and emergency departments are air-conditioned, however, so the ambient temperature typically encountered in most cases would not be this warm. Depending of the season and geographic locations, the environment may differ considerably and these results should be interpreted with this in mind.
The saline temperature was measured during a set rate of infusion. Longer or shorter durations of infusion might affect the rate of warming. It was noted, for example, that toward the end of each infusion that there was an upward inflection of the temperature curve that could vary in magnitude based by the duration of the infusion. 5. Conclusions During simulated infusion to induce therapeutic hypothermia, cold saline begins to warm toward ambient temperature but the rate is not rapid. An insulating SIGG neoprene pouch slows the rate of warming. To maximize the efficiency of mild therapeutic hypothermia induction, it may be advisable to use an insulating sleeve or change to a freshly cooled liter of cold saline upon arrival in the emergency department prior to completing the field infusion. Conflict of interest statement The authors have no conflicts of interest to declare. Acknowledgements None. This was not a sponsored study. References 1. Nolan JP, Morley PT, Vanden Hoek TL, et al. Therapeutic hypothermia after cardiac arrest: an advisory statement by the advanced life support task force of the International Liaison Committee on Resuscitation. Circulation 2003;108: 118–21. 2. 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2005;112:IV84–IV88. 3. Kim F, Olsufka M, Longstreth Jr WT, et al. Pilot randomized clinical trial of prehospital induction of mild hypothermia in out-of-hospital cardiac arrest patients with a rapid infusion of 4 degrees C normal saline. Circulation 2007;115: 3064–70. 4. Kamarainen A, Virkkunen I, Tenhunen J, Yli-Hankala A, Silfvast T. Induction of therapeutic hypothermia during prehospital CPR using ice-cold intravenous fluid. Resuscitation 2008;79:205–11. 5. Bruel C, Parienti JJ, Marie W, et al. Mild hypothermia during advanced life support: a preliminary study in out-of-hospital cardiac arrest. Crit Care 2008;12:R31. 6. Kampmeyer M, Callaway C. Method of cold saline storage for prehospital induced hypothermia. Prehosp Emerg Care 2009;13:81–4. 7. Davis DP, Fisher R, Aguilar S, et al. The feasibility of a regional cardiac arrest receiving system. Resuscitation 2007;74:44–51. 8. Vanden Hoek TL, Kasza KE, Beiser DG, et al. Induced hypothermia by central venous infusion: saline ice slurry versus chilled saline. Crit Care Med 2004;32:S425–31.
Contents lists available at ScienceDirect
Resuscitation journal homepage: www.elsevier.com/locate/resuscitation
Clinical paper
The effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia夽 T.J. Mader ∗ Department of Emergency Medicine, Baystate Medical Center, 759 Chestnut Street, Springfield, MA 01199, United States
a r t i c l e
i n f o
Article history: Received 8 September 2008 Received in revised form 3 April 2009 Accepted 13 April 2009 Keywords: Induced hypothermia Resuscitation Heart arrest Post-resuscitation care
a b s t r a c t Background: This study was done to determine the effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia. The study hypothesis was that cold saline would warm rapidly during simulated infusion and that an insulating SIGG neoprene pouch would slow the process. Methods: Paired 1-l bags of normal saline [with or without an insulating SIGG neoprene pouch (NEO+ and NEO− respectively)] were refrigerated together for at least 24 h. With an ambient room temperature (RT) between 32 and 34 ◦ C, the fluid was allowed to flow unrestricted through standard tubing connected to a 20-guage angiocath while the line reservoir temperature was monitored every 30 s. The order of the bags was pre-determined and alternated for each session. During 5 sessions, ten 1-l bags were included (5 NEO+ and 5 NEO−). The data were analyzed descriptively using Stata SE v8.1 for Macintosh. Results: The average ambient RT during the experimental sessions was 32.6 ◦ C (StDev: 0.8 ◦ C). The relative humidity was a constant 16%. The average low saline temperature at the beginning of infusion was 6.2 ◦ C (StDev: 2.7 ◦ C). The average rate of infusion was 48.2 cm3 /min (StDev: 3.7 cm3 /min). The average rise in saline temperature during the first 15 min of the infusion was 2.9 ◦ C (StDev: 1.2 ◦ C). The average high saline temperature reached near the end of the infusion was 13.4 ◦ C (StDev: 4.1 ◦ C). The average temperature change during infusion was 7.2 ◦ C (StDev: 3.5 ◦ C). The baseline data for the NEO+ and NEO− samples were not statistically different. The average temperature change over the first 15 min for the NEO+ group was 2.0 ◦ C (95% CI: 1.4 ◦ C and 2.5 ◦ C) and for the NEO− group it was 3.9 ◦ C (95% CI: 2.6 ◦ C and 5.1 ◦ C). The average change over the entire infusion for the NEO+ group was 4.3 ◦ C (95% CI: 3.1 ◦ C and 5.5 ◦ C) and for the NEO− group it was 10.2 ◦ C (95% CI: 7.4 ◦ C and 12.9 ◦ C). Conclusions: During simulated infusion to induce therapeutic hypothermia, cold saline begins to warm toward ambient temperature but the rate is not rapid. An insulating SIGG neoprene pouch slows the rate of warming. © 2009 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Mild induced therapeutic hypothermia is currently recommended for survivors of out-of-hospital cardiac arrest who meet specified criteria.1,2 It is generally believed that postresuscitative hypothermia should be induced as early as possible. This notion has lead many emergency medical services to initiate this process by a combination of surface cooling and infusion of cold saline immediately upon return of spontaneous circulation in the prehospital setting. The induction of mild therapeutic hypothermia with a rapid infusion of 4 ◦ C crystalloid has recently been shown to
夽 A Spanish translated version of the abstract of this article appears as Appendix in the final online version at doi:10.1016/j.resuscitation.2009.04.022. ∗ Tel.: +1 413 794 1661; fax: +1 413 794 8070. E-mail address: [email protected]. 0300-9572/$ – see front matter © 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.resuscitation.2009.04.022
effectively lower body temperature in out-of-hospital cardiac arrest survivors prior to hospital arrival without causing any adverse consequences.3–5 This study was done to determine the effect of ambient temperature on cold saline during simulated infusion to induce therapeutic hypothermia. The study hypothesis was that cold saline would warm rapidly during simulated infusion and that an insulating SIGG neoprene pouch would slow the process. 2. Materials and methods Five SIGG neoprene 1-l pouches were purchased online from Biome Living Brisbane City, Queensland Australia. Paired 1-l bags of normal saline [one with and one without an insulating SIGG neoprene pouch (NEO+ and NEO− respectively)] were stored together in a 1.8 ft3 commercial grade refrigerator (Danby model DAR192w; Guelph, Ontario) on the maximum cold setting [mean temperature
T.J. Mader / Resuscitation 80 (2009) 766–768
767
Table 1 Summary of main results (temperature in degrees Celsius). All (n = 10)
Ambient temperature Infusion time (min) Infusion rate (cm3 /min) Beginning (low) temperature Ending (high) temperature Temperature change at 15 min Temperature change during infusion
NEO (+) (n = 5)
NEO (−) (n = 5)
Mean
(StDev)
Mean
(95% CI)
Mean
(95% CI)
32.6 20.9 48.2 6.2 13.4 2.9 7.2
0.8 1.7 3.7 2.7 4.1 1.2 3.5
32.6 20.4 49.3 6.9 11.2 2.0 4.3
(31.4,33.9) (18.4,22.4) (44.6,54.0) (3.6,10.1) (7.3,15.0) (1.4,2.5) (3.1,5.5)
32.6 21.5 47.2 5.5 15.7 3.9 10.2
(31.6,33.5) (19.4,23.5) (42.5,51.9) (1.9,9.1) (10.7,20.7) (2.6,5.1) (7.4,12.9)
of 0.14 ◦ C (StDev: 1.98 ◦ C, range: 6.67 ◦ C)] for at least 24 h. With an ambient room temperature between 32 and 34 ◦ C each bag, in turn, was connected through standard intravenous tubing to a 20-guage angiocath and the fluid was allowed to flow unrestricted from a height of 5 ft. The ambient room temperature and relative humidity (Acu-Rite model 00613 Digital Hygrometer; Jamestown, NY) were recorded along with the line reservoir temperature (Esophageal temperature probe model 21090A; Philips Medical Systems, Andover, MA) every 30 s during infusion. The first measurement was recorded at the stable nadir upon initial connection and the last was taken on the half-minute just before the reservoir emptied. The order of the bags was pre-determined and alternated for each session. The second bag of fluid was not removed from the refrigerator until just before use. Set-up was accomplished in less than 30 s for each episode and no further manipulations were made after initiation of flow. During 5 sessions, 10 1-l bags were included (5 NEO+ and 5 NEO−). The data were entered into a spreadsheet and analyzed descriptively using Stata SE v8.1 for Macintosh. 3. Results The main results are presented in the Table 1. The average ambient room temperature during the experimental sessions was 32.6 ◦ C (StDev: 0.8 ◦ C), while the relative humidity was a constant 16%. The baseline data for the NEO+ and NEO− samples were mathematically the same. There was a statistically significant difference in the temperature change between the two groups at 15 min as well as over the total infusion time. 4. Discussion A combination of exposure and surface cooling along with a rapid infusion of cold intravenous crystalloid has been shown to
facilitate induction of therapeutic hypothermia following return of spontaneous circulation after out-of-hospital cardiac arrest. In a recent study by Kampmeyer and Callaway, a simple, effective and inexpensive method for storage of cold saline to maintain two 1l bags of normal saline near 4 ◦ C for up to 24 h is described.6 The results of this current study, as illustrated in Fig. 1, demonstrate that cold saline, when removed from the cold storage unit for simulated infusion, gradually warms toward ambient temperature. An insulating neoprene pouch has a statistically significant effect on the magnitude of warming during the infusion period with the greatest effect occurring in the last few minutes of infusion (Fig. 2). This experiment was done in a small office with a room temperature that was not artificially manipulated but by coincidence happened to be in the range recommended as the therapeutic hypothermia target temperature.1,2 Though the room has a Southeasterly exposure, there was no direct sunlight during the sessions and a negligible change in room temperature occurred. All infusions were from the same height through the same intravenous tubing with a 20-guage angiocath attached to the end. There was a wide range of infusion rates (41.7–54.1 cm3 /min) due in part to slight differences in the time to reach minimum temperature on the monitor before temperature recording began and random variability in flow rates through the tubing. There were no differences between the two experimental groups, however. Two time points were chosen a priori for comparative analysis. The first (15 min) was chosen as a realistic transport time from return of spontaneous circulation to hospital arrival and transfer of care to the emergency department staff7 and the second was at the completion of infusion to assess the overall change in temperature between the two groups. At both time points, the differences were statistically significant though the absolute temperature changes (2.9 ◦ C and 7.2 ◦ C respectively) and differences (2.0 ◦ C vs. 3.9 ◦ C and 4.3 ◦ C vs. 10.2 ◦ C, respectively) were relatively small. One could
Fig. 1. The change in saline temperature over time by group during simulated infusion to induce mild therapeutic hypothermia. The solid line error bars indicate the NEO+ and the interrupted line error bars indicate the NEO− standard deviations at each time point.
768
T.J. Mader / Resuscitation 80 (2009) 766–768
Fig. 2. The change in saline temperature over time during the last 5 min of each unit’s infusion by group. Zero indicates infusion completion.
argue that since the fluid temperatures are still well below body and target temperatures, the infusion would still be effectively cooling the cardiac arrest survivor in the clinical setting, all be it, the NEO− somewhat less efficiently. Considering the laws of thermodynamics, in light of the urgency to reach target temperature, and given concerns about volume overload in some patients, optimal efficiency in cooling becomes an important consideration for inducing mild hypothermia.8 The small differences in temperature observed between the two groups in this study over the course of the infusion, therefore, are not only statistically significant but also may be clinically significant as well. An unexpected and important finding noted during this experiment was that although all of the paired 1-l bags of normal saline were placed in the same refrigerator on the same setting for similar periods of time, the starting temperatures of the paired units varied considerably. The average difference between paired bags was 3.1 ◦ C but one pair differed by 7.9 ◦ C. The range of starting temperatures for the whole sample was between 0.8 ◦ C (in the NEO− group) and 9.8 ◦ C (in the NEO+ group). The reason for this observation is unclear. One might speculate that since the refrigerator was not medical grade and the precision of the temperature regulation within the unit, as noted above, was suboptimal, the internal temperature of the unit might have fluctuated unpredictably during each session. In addition, the placement of the 1-l bags of saline within the unit may have influenced the temperature of the solutions. Though this was considered in advance and their relative positions within the unit were rotated each time in an attempt to minimize this effect the combination of these factors may explain the differences. In any event, these findings suggest that one cannot assume that the cold saline being used to induce mild therapeutic hypothermia is of consistent starting temperature based solely on similar cooling methods. 4.1. Limitations This experiment was a simulation only and the conditions and results may not reflect actual clinical conditions. The infusions were carried out within a relatively narrow but practical range of ambient temperatures representing worst case scenario conditions—a hot summer day, out of direct sunlight, and without benefit of air conditioning. Most ambulances and emergency departments are air-conditioned, however, so the ambient temperature typically encountered in most cases would not be this warm. Depending of the season and geographic locations, the environment may differ considerably and these results should be interpreted with this in mind.
The saline temperature was measured during a set rate of infusion. Longer or shorter durations of infusion might affect the rate of warming. It was noted, for example, that toward the end of each infusion that there was an upward inflection of the temperature curve that could vary in magnitude based by the duration of the infusion. 5. Conclusions During simulated infusion to induce therapeutic hypothermia, cold saline begins to warm toward ambient temperature but the rate is not rapid. An insulating SIGG neoprene pouch slows the rate of warming. To maximize the efficiency of mild therapeutic hypothermia induction, it may be advisable to use an insulating sleeve or change to a freshly cooled liter of cold saline upon arrival in the emergency department prior to completing the field infusion. Conflict of interest statement The authors have no conflicts of interest to declare. Acknowledgements None. This was not a sponsored study. References 1. Nolan JP, Morley PT, Vanden Hoek TL, et al. Therapeutic hypothermia after cardiac arrest: an advisory statement by the advanced life support task force of the International Liaison Committee on Resuscitation. Circulation 2003;108: 118–21. 2. 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2005;112:IV84–IV88. 3. Kim F, Olsufka M, Longstreth Jr WT, et al. Pilot randomized clinical trial of prehospital induction of mild hypothermia in out-of-hospital cardiac arrest patients with a rapid infusion of 4 degrees C normal saline. Circulation 2007;115: 3064–70. 4. Kamarainen A, Virkkunen I, Tenhunen J, Yli-Hankala A, Silfvast T. Induction of therapeutic hypothermia during prehospital CPR using ice-cold intravenous fluid. Resuscitation 2008;79:205–11. 5. Bruel C, Parienti JJ, Marie W, et al. Mild hypothermia during advanced life support: a preliminary study in out-of-hospital cardiac arrest. Crit Care 2008;12:R31. 6. Kampmeyer M, Callaway C. Method of cold saline storage for prehospital induced hypothermia. Prehosp Emerg Care 2009;13:81–4. 7. Davis DP, Fisher R, Aguilar S, et al. The feasibility of a regional cardiac arrest receiving system. Resuscitation 2007;74:44–51. 8. Vanden Hoek TL, Kasza KE, Beiser DG, et al. Induced hypothermia by central venous infusion: saline ice slurry versus chilled saline. Crit Care Med 2004;32:S425–31.