Accidental hypothermia and active rewarming: the metabolic changes observed above and below 32°C

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Background

In accidental hypothermia the underlying physiological mechanisms responsible for poor outcome during rewarming through 32°C remain obscure. Deleterious changes in plasma calcium (Ca²⁺), magnesium (Mg²⁺), and acid-base concentrations have been reported in other forms of tissue injury, whilst parathyroid hormone (PTH) and interleukin 6 (IL-6) levels are elevated. Accordingly this study investigated the changes which occur in Ca²⁺, Mg²⁺, pH, PTH and IL-6 during rewarming.

Methods

Eight patients, 4 males and 4 females, age 45 to 85 years, admitted with core temperatures <35°C were included in the study. Patients were rewarmed with dry warm blankets and fluid replaced by crystalloid at 40°C. Blood for Ca²⁺ and Mg²⁺, pH, PTH and IL-6 were collected at presentation, during rewarming, and at 24 h.

Results

Four patients were admitted with mild hypothermia (32–35°C) and four with moderate (28–32°C) hypothermia. All patient clinical parameters responded favourably to rewarming. Rewarming to 32°C had no significant effect on the presenting acidosis (P=0.1740), although above 32°C pH increased with temperature (P<0.0001). There was a negative correlation between pH and both Ca²⁺ (P=0.0005) and Mg²⁺(P=0.0488) below 32°C; above this temperature the relationship was significant only for Ca²⁺ (P=0.0494). PTH and Ca²⁺correlated positively (P=0.0041) and negatively (P=0.0039) in moderate and mild hypothermia respectively. IL-6 and Ca²⁺ also correlated positively (P=0.0039) and negatively (P=0.0018) when presentation temperature was below and above 32°C respectively.

Conclusions

During rewarming pH remains unchanged until patient temperature reaches 32°C. Ca²⁺ and Mg²⁺ decline is associated with this pH change. Poor outcome is associated with presentation temperature (<32°C), nonphysiological correlation between PTH and Ca²⁺, and age (≥ 84 years). IL-6 levels mirror this PTH change but further study is required to explain these findings.