In the 1940s and 1950s, Canadian surgeon Wilfred Bigelow demonstrated in animal models that the length of time the brain could survive stopped blood circulation could be extended from 3 minutes to 10 minutes by cooling to 30 °C before circulation was stopped. He found that this time could be extended to 15 to 24 minutes at temperatures below 20 °C. He further found that at a temperature of 5 °C, groundhogs could endure two hours of stopped blood circulation without ill effects. This research was motivated by a desire to stop the heart from beating long enough to do surgery on the heart while it remained still. Since heart-lung machines, also known as cardiopulmonary bypass (CPB), had not been invented yet, stopping the heart meant stopping blood circulation to the whole body, including the brain.