Hyponatremia of Exercise in Hawaiian Ironman Triathletes

Abstract

Hyponatremia of exercise is a frequent condition among Ironman Triathletes and is a common cause of collapse. Denned as a plasma sodium concentration less than 135 mmol/liter, it is most often caused by drinking hypotonic fluid in excess of sweat losses. Slower runners and females have been thought to be most at risk, presumably because they have a greater opportunity to drink excessively. PURPOSE: To examine the characteristics of athletes presenting to the medical tent with hyponatremia during the 2004 Hawaiian Ironman Triathlon in terms of their race finish time and sex. METHODS: Records of all Triathletes with plasma sodium levels less than 135 mmol/liter were reviewed and correlated with race finish times and sex. 1727 entrants were grouped into 6 classes based on their race finish times, which were labeled as class I (<10 hours; 7.1% of male entrants and .5% of female entrants), class II (<11 hours; 28% of males and 6.7% of females), class III (<13 hours; 37.6% of males and 47.2% of females), class IV (<15 hours; 12% of males and 26% of females), class V (<17 hours; 7.2% of males and 9.3% of females) and class VI (did not finish; 8.1% of males and 10.2% of females). Hyponatremia levels were denned as mild (130–134), moderate (125–129) and severe (120–124). RESULTS: 55 plasma sodium levels measured in the medical tent on race day for patients who were symptomatic, 40 in men and 15 in women. 18 were found to have plasma sodium levels <135 mmol/liter, including 14 males and 4 females. 35% of all males measured had hyponatremia, while only 26.7% of females had it. The 14 males represented 1.08% of the 1297 male entrants, while the 4 females represented .93% of the 430 female entrants. Of the 14 males with hyponatremia, 10 were mild, 2 were moderate and 2 were severe. Of the 4 females, 3 were mild and 1 was moderate. Correlating the hyponatremia with race finish times, we found that of the 10 males with mild hyponatremia, 5 were class II, 4 were class III and 1 was class IV. Of the 2 males with moderate hyponatremia, 1 was class II and 1 was class III and in the 2 males with severe hyponatremia, 1 was class III and 1 was class VI. Of the 3 females with mild hyponatremia, all were class III, while the 1 female with moderate hyponatremia was class VI. CONCLUSIONS: Our data suggest no significant difference in the incidence of hyponatremia between male and female Ironman entrants. We found the majority of hyponatremia cases (13) were mild in nature and mostly represented class II and III runners (12). Slower runners seemed to be at no greater risk as no cases seen in the slowest group (class V), and only 1 in the class IV group. These data indicates that females and slower Ironman Triathletes are at no greater risk for hyponatremia.