Accidental hypothermia and local cold injury: physiological and epidemiological studies on risk

PhD dissertation abstract Background and objective The objectives were to first determine incidence and contributing factors to cold‐related injuries in Northern Sweden, both of those that led to hospitalisation and those that led to fatality. A further aim was to assess post‐cooling hand‐rewarming responses and the effects of training in a cold environment, both on fingertip rewarming and on function of the autonomic nervous system, to evaluate if there was adaptation related to prolonged occupational cold exposure. Methods In a retrospective analysis, cases of accidental cold‐related injury with hospital admission in Northern Sweden in 2000–2007 were analysed. Cases of fatal hypothermia in the same region in 1992–2008 were analysed. A cohort of volunteers was studied before and after many months of occupational cold exposure. Subject hand‐rewarming response was measured after a cold hand immersion provocation, and was categorised as slow, moderate, or normal in rewarming speed. This cold provocation and rewarming assessment was performed before and after their winter training. Heart rate variability (HRV) was analysed from the same cold provocation/recovery sequences. Results For the 379 cases of hospitalisation for cold‐related injury, annual incidences for hypothermia, frostbite, and drowning were 3.4/100,000, 1.5/100,000, and 1.0/100,000 inhabitants, respectively. Male gender was more frequent for all categories. Annual frequencies for hypothermia hospitalisations increased during the study period. Hypothermia degree and distribution of cases were 20% mild (between 32°C and 35°C), 40% moderate (31.9–28°C), and 24% severe (< 28°C), while 12% had temperatures over 35.0°C. The 207 cases of fatal hypothermia showed an annual incidence of 1.35 per 100,000 inhabitants: 72% in rural areas, 93% outdoors, and 40% found within 100 m of a building. Paradoxical undressing was documented in 30%. Ethanol was detected in femoral vein blood in 43%. Contributing comorbidity was common, including heart disease, previous stroke, dementia, psychiatric disease, alcoholism, and recent trauma. Post‐training, baseline fingertip temperatures, and cold recovery variables in terms of final rewarming fingertip temperature and vasodilation time increased significantly in moderate and slow rewarmers. Cold‐related injury (frostbite) during winter training occurred disproportionately more often in slow rewarmers (four of the five injuries). At ‘pre‐winter training’, normal rewarmers had higher power for low‐frequency and high‐frequency HRV. After cold acclimatisation (post‐training), normal rewarmers showed lower resting power values for the low‐frequency and high‐frequency HRV components. Conclusions Hypothermia and cold injury continue to cause injury and hospitalisation in the northern region of Sweden. Assessment and management is not standardised across hospitals. With the identification of groups at high risk for fatal hypothermia, it should be possible to reduce the incidence, particularly for highest risk subjects: rural, living alone, alcohol imbibing, and psychiatric diagnosis‐carrying citizens. Long‐term cold‐weather training may affect hand‐rewarming patterns after a cold provocation, and a warmer baseline hand temperature with faster rewarming after a cold provocation may be associated with less general risk for frostbite. HRV results support the conclusion that cold adaptation in the autonomic nervous system occurred in both groups, although the biological significance of this is not yet clear.