Cold-induced stress responses during a self-rescue exercise from accidental immersion in ice water in military personnel

Introduction Cold-water immersion induces autonomic stress responses including sympathetic cold shock and parasympathetic diving and trigeminovagal reflexes, potentially leading to arrhythmias or bronchospasm. Another important complication of cold-water immersion is hypothermia. This study evaluates physiological responses during ice-self-rescue training to assess safety and temperature monitoring accuracy. Methods We conducted a prospective observational cohort study of 80 healthy Mountain Infantry soldiers during a standardized ice-self-rescue training in Norway (air temperature −10 °C). Participants underwent partial immersion (0.5 °C water) resulting in transitory submersion (<5 s). They were equipped with continuous 3-lead ECG (n = 34) spirometry (n = 26); and core temperature monitoring (ingestible telemetric capsule (n = 23) versus bilateral tympanic thermometry with and without ear channel occlusion (n = 34). Primary outcomes included cardiac rhythm changes, lung function parameters and temperature measurement agreement. Results ECG analysis revealed significant post-immersion tachycardia (median increase of 17 bpm, p = 0.03) and increased RR-interval variability (+90 ms, p < 0.01), without malignant arrhythmias. Spirometry showed no clinically significant changes in FVC or FEV 1 and peak expiratory flow. Tympanic readings underestimated core temperature post-immersion (median difference −2.8 °C versus capsules, p < 0.01), with ear canal occlusion did not improve accuracy (p = 0.15). Conclusion Supervised cold-water immersion during military training exercise elicited expected autonomic stress responses without life-threatening complications in healthy soldiers. These findings suggest structured cold-water training can be safely conducted for fit individuals. Tympanic thermometry proved unreliable following immersion, even after ear channel occlusion. Ingestible capsule thermometry may be a viable approach if invasive measurement is not possible. Clinical Trial Registration This trial was registered under “ Cold induced stress reactions during cold water immersion ” in the German Clinical Trial Register (DRKS) under the registration number DRKS00032345 ( https://www.drks.de/search/de/trial/DRKS00032345/details ).