Cardiopulmonary bypass machines were essential to the development of deep hypothermic circulatory arrest (DHCA) in humans. By 1959, it was known from the animal experiments of Bigelow, Andjus and Smith, Gollan, Lewis's colleague, Niazi, and others that temperatures near 0 °C could be survived by mammals, and that colder temperature permitted the brain to survive longer circulatory arrest times, even beyond one hour. Humans had survived cooling to 9 °C, and circulatory arrest of 45 minutes, using external cooling only. However, reaching such low temperatures by external cooling was difficult and hazardous. At temperatures below 24 °C, the human heart is prone to fibrillation and stopping. This can begin circulatory arrest before the brain has reached a safe temperature. Cardiopulmonary bypass machines allow blood circulation and cooling to continue below the temperature at which the heart stops working. By cooling blood directly, cardiopulmonary bypass also cools people faster than surface cooling, even if the heart is not functioning.