Autonomic nerve system responses for normal and slow rewarmers after hand cold provocation: effects of long-term cold climate training

Abstract

Differences among individuals concerning susceptibility to local cold injury following acute cold exposure may be related to function of the autonomic nervous system. We hypothesized that there are differences in heart rate variability (HRV) between individuals with normal or more pronounced vasoconstriction following cold exposure and that there is an adaptation related to prolonged cold exposure in autonomic nervous system response to cold stimuli. Seventy-seven young men performed a cold provocation test, where HRV was recorded during cold hand immersion and recovery. Forty-three subjects were re-examined 15 months later, with many months of cold weather training between the tests. Subjects were analyzed as 'slow' and 'normal' rewarmers according to their thermographic rewarming pattern. For the 'pre-training' test, before cold climate exposure, normal rewarmers had higher power for low-frequency (PLF) and high-frequency (PHF) HRV components during the cold provocation test (ANOVA for groups: p = 0.04 and p = 0.005, respectively). There was an approximately 25 % higher PHF at the start in normal rewarmers, in the logarithmic scale. Low frequency-to-high frequency ratio (PLF/PHF) showed lower levels for normal rewarmers (ANOVA for groups: p = 0.04). During the 'post-training' cold provocation test, both groups lacked the marked increase in heart rate that occurred during cold exposure at the 'pre-training' setting. After cold acclimatization (post-training), normal rewarmers showed lower resting power values for the low-frequency and high-frequency HRV components. After winter training, the slow rewarmers showed reduced low-frequency power for some of the cold provocation measurements but not all (average total PLF, ANOVA p = 0.05), which was not present before winter training. These HRV results support the conclusion that cold adaptation occurred in both groups. We conclude that further prospective study is needed to determine whether cold adaptation provides protection to subjects at higher risk for cold injury, that is, slow rewarmers.