Samples of blood were taken at five to 10-minute intervals during cooling, circulatory arrest, and rewarming in eight consecutive patients undergoing open cardiac surgery under profound hypothermia at nasopharyngeal and oesophageal temperatures of approximately 10°C. and the serum potassium levels were estimated. Urine samples were also collected from six of the eight patients and the total potassium excretion calculated per minute. It was found that there was a tendency for the serum potassium level to rise towards the end of cooling. A further more significant rise occurred during circulatory arrest, and on rewarming there was a pronounced fall of approximately 2·5 mEq./litre, the lowest level being reached at 27·5°C. Above 27·5°C. there was no further significant change in the serum potassium level. It is suggested that the acidosis which occurs during circulatory arrest and a depression in the function of the cell membrane at very low temperatures are at least partly responsible for the changes in serum potassium. Certainly the excretion of potassium in the urine does not account for them.
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