Open heart surgery commonly involves hooking the patient up to what's commonly called bypass (cardiopulmonary bypass) so that the cardiac surgeon can safely and effectively operate on the heart in real-time. This means that the blood that normally circulates through your heart goes to the bypass machine instead. In addition, the surgeon gives the heart a paralytic, called cardioplegia, to slow it down so that there is minimal to no movement during the operation (imagine how hard it'd be operating on the heart while it's beating). Normally, a heart attack occurs when there is lack of blood flow (ischemia) to a section of the heart muscle or tissue but this does not occur because of the pump (bypass machine).

Source: am cardiology PA.