| Summary: | Swimming is the most popular participation sport in the UK with open water swimming seeing a rise in popularity over the last decade. However, cold water immersion is not without significant risks. Drowning represents the third leading cause of accidental death worldwide and in those aged 1 to 14 years it represents the leading cause of accidental death in some countries. Given the physical, physiological and psychological differences between adults and children, the latter are considered particularly at risk of cold-related illness and hypothermia. The 'cold shock' response on initial immersion, and the insidious onset of hypothermia and swim failure that accompanies prolonged exposure and swimming, are recognised to be potentially fatal. This has been well documented in adults, and whilst it is assumed that similar responses occur in children there is little quantitative evidence of this to date. Furthermore, adults show an habituation of their 'cold shock' response and change in their cooling rates following repeated exposure to cold water, however there are no data that show similar changes in children. This study examined the physiological and subjective responses of children to cold water on initial immersion and on prolonged immersion whilst swimming, and assessed for any adaptation in these responses following a year of repeated swim training in cold water. It was hypothesised that: children would demonstrate a 'cold shock' response on initial immersion that would habituate following a period of acclimatisation; children would demonstrate faster cooling rates than those seen in adults whilst swimming in cold water, and their rate of cooling would adapt over a year of cold water swim training. METHOD 17 children aged 10 and 11 years old were recruited from applicants to the Bristol English Channel Swim Team (an attempt by a group of children to be the youngest relay team to swim the Channel). They underwent a five-minute static immersion in 15 QC water, during which their cardiovascular, respiratory and metabolic responses were recorded. Ten of these participants went on to swim for up to 40 minutes in 15 QC water, during which their heart rates, gastrointestinal temperatures and oxygen consumption were measured. The gastrointestinal temperatures of participants during the re- warming phase (post immersion) was also monitored and recorded. Subjective thermal sensation and comfort were recorded prior to immersion, after five minutes static immersion, and at the end of the swims. Following a year of regular cold water swim training, eight participants returned to complete the five-minute static immersion and five of the original ten swimmers completed a swim of up to 40 minutes in 15 QC water. This allowed us to identify any evidence of adaptation in their 1 initial responses to immersion in cold water and cooling rates whilst swimming. The data gathered were compared to adult data collected in the same laboratory during a different experiment. RESULTS An increase in heart rate, respiratory frequency and inspiratory volume was seen in all participants in the first few minutes of immersion. However, responses were found to be smaller in children compared to adul~s (P < 0.05), and no significant attenuation was seen in these after a year of regular exposure to cold water. Children did however feel warmer (P < 0.01) and more comfortable (P < 0.05) following five minutes of static immersion after a year of cold water swim training. There was great variability in the rate of cooling between children, likely due to differences in their anthropometric profiles. Sum of skinfolds was found to hold the greatest correlation with rate of deep body cooling (R2 = 0.4157). The mean (SD) cooling rate of the children whilst swimming was 2.5 (3.06) °C.h-1. No statistical difference was found in the cooling rates of five children following a year of cold water swim training. No difference was found between children and adults in their cooling rates whilst swimming, however the trend in both groups of a slower rate of cooling following acclimatisation became significant (P = 0.026) once the child and adult data were pooled. DISCUSSION This study provides evidence that the 'cold shock' response exists III children, but is possibly smaller than that seen in adults. The lack of attenuation in this response following acclimatisation is a surprising finding that warrants further investigation, although it may be that the children were pre-acclimatised prior to their initial immersion, or due to a small sample size. This study provides quantitative data on cooling rates of children swimming in cold water. It finds that children maintain their deep body temperature as effectively as adults, which is likely partly explained by a greater percentage body fat (P < 0.05) and higher relative heat production, as measured by oxygen consumption (P < 0.05), seen in the children. The data support an habituation of children's subjective thermal awareness and goes some way to suggest that children aged 11 - 12 years old exhibit an insulative adaptation following regular swimming in cold water. It is hoped that this study provides a better understanding of children's physiological responses to accidental and non- accidental immersion, and will aid risk assessments in any projects involving paediatric immersion in cold water.
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