Before initiation of CPB, the CPB machine is "primed" with a volume of crystalloid, colloid or blood. Following anticoagulation and cannula placement, release of clamps on the venous cannula commences CPB. Blood drains from the patient into the bypass machine and the perfusionist starts the pump flow. Assuming the aortic valve is competent, following institution of CPB, the heart will be drained of blood by the venous cannula and becomes empty. At the same time, the normal arterial waveform loses pulsatility as the main driver of aortic blood flow becomes the non-pulsatile pump of the bypass machine. Consequently, full bypass flow is non-pulsatile. However, it is important to remember that the presence of a pulsatile aortic pressure wave does not preclude the patient's circulation being sustained by the CPB machine. As long as the heart has an electrical rhythm that can sustain a coordinated contraction, it will continue to beat and will eject any blood delivered to it. At times, the venous cannulae do not sufficiently drain venous blood into the bypass machine resulting in some venous return going to the heart—producing partial bypass. At other times, the surgeons will reduce venous return to the bypass machine intentionally leaving the patient on partial bypass. Patients with aortic insufficiency often fill their heart when on bypass and may develop distended hearts during CPB due to the incompetent aortic valve. Therefore in these patients, until the aorta is cross-clamped, it is important to keep the heart ejecting in order to prevent the heart from overdistention and ischemic injury. If the heart is ejecting, the patient will have both a pulsatile waveform and be supported by partial bypass. Pulsatility on the arterial pressure trace during CPB can also be observed when an intra-aortic balloon pump is triggering. When unsure whether the patient is or is not on full or partial bypass ask the perfusionist and surgeon to avoid serious management errors.