Hypothermia following exertional heat stroke treatment

Treating exertional heat stroke: Limited understanding of the female response to cold water immersion

According to an expansive body of research and best practice statements, whole-body cold water immersion is the gold standard treatment for exertional heat stroke. However, as this founding evidence was predominantly drawn from males, the current guidelines for treatment are being applied to women without validation. Given the recognised differences in thermal responses experienced by men and women, all-encompassing exertional heat stroke treatment advice may not effectively protect both sexes. In fact, recent evidence suggests that hyperthermic women cool faster than hyperthermic men during cold water immersion. This raises the question of whether overcooling is risked if the present guidelines are followed. The current mini-review examined the literature on women's response to cold water immersion as a treatment for exertional heat stroke and aimed to clarify whether the current guidelines have appropriately considered research investigating women. The potential implications of applying these guidelines to women were also discussed.

Consensus Statement- Prehospital Care of Exertional Heat Stroke

Exertional heat stroke (EHS) is one of the most common causes of sudden death in athletes. It also represents a unique medical challenge to the prehospital healthcare provider due to the time sensitive nature of treatment. In cases of EHS, when cooling is delayed, there is a significant increase in organ damage, morbidity, and mortality after 30 minutes, faster than the average EMS transport and ED evaluation window. The purpose of this document is to present a paradigm for prehospital healthcare systems to minimize the risk of morbidity and mortality for EHS patients. With proper planning, EHS can be managed successfully by the prehospital healthcare provider.

Cold-Water Immersion Cooling Rates in Football Linemen and Cross-Country Runners With Exercise-Induced Hyperthermia

CONTEXT

  Ideal and acceptable cooling rates in hyperthermic athletes have been established in average-sized participants. Football linemen (FBs) have a small body surface area (BSA)-to-mass ratio compared with smaller athletes, which hinders heat dissipation.

OBJECTIVE

  To determine cooling rates using cold-water immersion in hyperthermic FBs and cross-country runners (CCs).

DESIGN

  Cohort study.

SETTING

  Controlled university laboratory.

PATIENTS OR OTHER PARTICIPANTS

  Nine FBs (age = 21.7 ± 1.7 years, height = 188.7 ± 4 cm, mass = 128.1 ± 18 kg, body fat = 28.9% ± 7.1%, lean body mass [LBM] = 86.9 ± 19 kg, BSA = 2.54 ± 0.13 m2, BSA/mass = 201 ± 21.3 cm2/kg, and BSA/LBM = 276.4 ± 19.7 cm2/kg) and 7 CCs (age = 20 ± 1.8 years, height = 176 ± 4.1 cm, mass = 68.7 ± 6.5 kg, body fat = 10.2% ± 1.6%, LBM = 61.7 ± 5.3 kg, BSA = 1.84 ± 0.1 m2, BSA/mass = 268.3 ± 11.7 cm2/kg, and BSA/LBM = 298.4 ± 11.7 cm2/kg).

INTERVENTION(S)

  Participants ingested an intestinal sensor, exercised in a climatic chamber (39°C, 40% relative humidity) until either target core temperature (Tgi) was 39.5°C or volitional exhaustion was reached, and were immediately immersed in a 10°C circulated bath until Tgi declined to 37.5°C. A general linear model repeated-measures analysis of variance and independent t tests were calculated, with P < .05.

MAIN OUTCOME MEASURE(S)

  Physical characteristics, maximal Tgi, time to reach 37.5°C, and cooling rate.

RESULTS

  Physical characteristics were different between groups. No differences existed in environmental measures or maximal Tgi (FBs = 39.12°C ± 0.39°C, CCs = 39.38°C ± 0.19°C; P = .12). Cooling times required to reach 37.5°C (FBs = 11.4 ± 4 minutes, CCs = 7.7 ± 0.06 minutes; P < .002) and therefore cooling rates (FBs = 0.156°C·min-1 ± 0.06°C·min-1, CCs = .255°C·min-1 ± 0.05°C·min-1; P < .002) were different. Strong correlations were found between cooling rate and body mass (r = -0.76, P < .001), total BSA (r = -0.74, P < .001), BSA/mass (r = 0.73, P < .001), LBM/mass (r = 0.72, P < .002), and LBM (r = -0.72, P < .002).

CONCLUSIONS

  With cold-water immersion, the cooling rate in CCs (0.255°C·min-1) was greater than in FBs (0.156°C·min-1); however, both were considered ideal (≥0.155°C·min-1). Athletic trainers should realize that it likely takes considerably longer to cool large hyperthermic American-football players (>11 minutes) than smaller, leaner athletes (7.7 minutes). Cooling rates varied widely from 0.332°C·min-1 in a small runner to only 0.101°C·min-1 in a lineman, supporting the use of rectal temperature for monitoring during cooling.

Evaluation of Various Cooling Systems After Exercise-Induced Hyperthermia

CONTEXT

Rapid diagnosis and expeditious cooling of individuals with exertional heat stroke is paramount for survival.

OBJECTIVE

To evaluate the efficacy of various cooling systems after exercise-induced hyperthermia.

DESIGN

Crossover study.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twenty-two men (age = 24 ± 2 years, height = 1.76 ± 0.07 m, mass = 70.7 ± 9.5 kg) participated.

INTERVENTION(S)

Each participant completed a treadmill walk until body core temperature reached 39.50°C. The treadmill walk was performed at 5.3 km/h on an 8.5% incline for 50 minutes and then at 5.0 km/h until the end of exercise. Each participant experienced 4 cooling phases in a randomized, repeated-crossover design: (1) no cooling (CON), (2) body-cooling unit (BCU), (3) EMCOOLS Flex.Pad (EC), and (4) ThermoSuit (TS). Cooling continued for 30 minutes or until body core temperature reached 38.00°C, whichever occurred earlier.

MAIN OUTCOME MEASURE(S)

Body core temperature (obtained via an ingestible telemetric temperature sensor) and heart rate were measured continuously during the exercise and cooling phases. Rating of perceived exertion was monitored every 5 minutes during the exercise phase and thermal sensation every minute during the cooling phase.

RESULTS

The absolute cooling rate was greatest with TS (0.16°C/min ± 0.06°C/min) followed by EC (0.12°C/min ± 0.04°C/min), BCU (0.09°C/min ± 0.06°C/min), and CON (0.06°C/min ± 0.02°C/min; P < .001). The TS offered a greater cooling rate than all other cooling modalities in this study, whereas EC offered a greater cooling rate than both CON and BCU (P < .0083 for all). Effect-size calculations, however, showed that EC and BCU were not clinically different.

CONCLUSION

These findings provide objective evidence for selecting the most effective cooling system of those we evaluated for cooling individuals with exercise-induced hyperthermia. Nevertheless, factors other than cooling efficacy need to be considered when selecting an appropriate cooling system.

Rapid cooling after acute hyperthermia alters intestinal morphology and increases the systemic inflammatory response in pigs

The study objective was to determine the direct effects of rapid cooling after acute hyperthermia on intestinal morphology and inflammatory response in pigs. In four repetitions, male pigs (N = 36; 88.7 ± 1.6 kg) were exposed to thermoneutrality (TN; n = 3/rep; 19.5 ± 0.1°C) for 6 h or heat stress (HS; 36.4 ± 0.1°C) for 3 h, followed by a 3-h recovery period of rapid cooling (HSRC; n = 3/rep; rapid TN exposure and ice water dousing for 1.5 h) or gradual cooling (HSGC; n = 3/rep; gradual decrease from HS to TN). Rectal (TR) and gastrointestinal tract (TGI) temperatures were obtained every 15 min for 6 h. In repetitions 1 and 2, blood was collected at 60 and 180 min during HS and 30 and 60 min during recovery, and then pigs were euthanized at 180 min of recovery and duodenum, ileum, and colon tissue were collected to evaluate intestinal morphology. HS increased (P < 0.01) maximum TR (40.7°C) and TGI (41.5°C) compared with TN treatment (38.9 and 39.3°C, respectively). Recovery reduced TR (P < 0.01; 0.4°C) in HSRC vs. HSGC pigs, but TGI was similar (40.7°C). HSRC reduced (P < 0.01) villus height-to-crypt depth ratio in the duodenum (34%) and ileum (46%) vs. HSGC pigs. Serum LPS concentration was greater in HSRC pigs (P = 0.04; 68.5% and 52.4%, respectively) compared with TN and HSGC pigs, and TNF-α concentration tended to be greater (P = 0.06; 41.2%) compared with HSGC pigs during recovery. In summary, rapid cooling reduced TR but had no effect on TGI, and this may be linked to increased intestinal damage and a systemic inflammatory response.

Toll-like receptor 4 and high-mobility group box 1 are critical mediators of tissue injury and survival in a mouse model for heatstroke

The molecular mechanisms that initiate the inflammatory response in heatstroke and their relation with tissue injury and lethality are not fully elucidated. We examined whether endogenous ligands released by damaged/stressed cells such as high-mobility group box 1 (HMGB1) signaling through Toll-like receptor 4 (TLR4) may play a pathogenic role in heatstroke. Mutant TLR4-defective (C3H/HeJ) and wild type (C3H/HeOuJ) mice were subjected to heat stress in an environmental chamber pre-warmed at 43.5 °C until their core temperature reached 42.7°C, which was taken as the onset of heatstroke. The animals were then allowed to recover passively at ambient temperature. A sham-heated group served as a control. Mutant mice displayed more histological liver damage and higher mortality compared with wild type mice (73% vs. 27%, respectively, P<0.001). Compared to wild type mice, mutant mice exhibited earlier plasma release of markers of systemic inflammation such as HMGB1 (206 ± 105 vs. 63 ± 21 ng/ml; P = 0.0018 and 209 ± 100 vs. 46 ± 32 ng/ml; P<0.0001), IL-6 (144 ± 40 vs. 46 ± 20 pg/ml; P<0.001 and 184 ± 21 vs. 84 ± 54 pg/ml; P = 0.04), and IL-1β (27 ± 4 vs. 1.7 ± 2.3 pg/ml; P<0.0001 at 1 hour). Both strains of mice displayed early release of HMGB1 into the circulation upstream of IL-1β and IL-6 responses which remained elevated up to 24 h. Specific inhibition of HMGB1 activity with DNA-binding A Box (600 µg/mouse) protected the mutant mice against the lethal effect of heat stress (60% A Box vs. 18% GST protein, P = 0.04). These findings suggest a protective role for the TLR4 in the host response to severe heat stress. They also suggest that HMGB1 is an early mediator of inflammation, tissue injury and lethality in heatstroke in the presence of defective TLR4 signaling.