Why is hypothermia used during open heart surgery?

Cool the heart: Hypothermia is used to cool the heart while on a blood bypass machine. Cooling the heart results in less oxygen use by the heart muscle while the heart is receiving less blood and less oxygen during the bypass surgery itself. The cooling also helps to slow the heartbeat making surgery on the heart vessels easier.

Suspended animation: Cooling the body slows down cell metabolism, requiring less oxygen and reducing buildup of waste. The hope is that the heart muscle will "wake up" when warmed and pump effectively. Some surgeries require circulatory arrest where the body is cooled very deeply to slow down metabolism of the brain and other organs, then bypass stopped. This time is kept as short as possible, and bypass resumes.