Cardiopulmonary bypass (CPB) is used in many heart surgery procedures to provide a still and bloodless heart for surgeon to work on while the blood flow to the reset of the body is continued. The brain is the most sensitive limb of the body during cardiopulmonary bypass operation, not to mention that it is mot most important organ of the body. Even slightest damages to the brain result in the loss of functionality. There are different mechanisms of brain damage following a heart surgery such as thrombosis, embolism of main arteries, micro-embolism, and ischemia caused by inadequate cerebral perfusion. Changes in cerebral autoregulation mechanism caused by CPB might cause abnormalities in cerebral blood flow (CBF). Abnormalities in CBF autoregulation during CPB might create the risk of cerebral hypoperfusion. Cerebral blood flow autoregulation ensures provision of oxygenated blood to the brain based on the cerebral oxygen demand. In addition, an accurate autoregulation protects the brain against ischemia caused by acute variation of intra-arterial blood pressure (Ono et al., 2012). In general, blood flow regulates itself in response to its demand for oxygen so that an increase/decrease in cerebral metabolic rate for oxygen (CMRO2) results in relative changes in CBF. This relationship is called flow-metabolic coupling, which implies that cerebral delivery O2 (CDO2) should supply cerebral metabolic needs. Since CDO2 is equal to CBF multiplied by arterial oxygen content, each one of the variables that control CBF or arterial oxygen content can also affect the relationship between CDO2 and CMRO2.

Open heart surgery is prescribed to treat a variety of heart diseases and problems. Heart surgery became possible and further improved by introduction of extracorporeal blood circuit or cardiopulmonary (CPB).The reasons for using CPB include oxygenating the blood, removing Co2 from the blood, controlling the patient’s body temperature, diverging blood from the surgery field toward the pumping system and creating a bloodless organ to work on while preserving other body organs. Employing CPB is a major challenge as it affects different aspects of the patients’ body physiology. Taking into account that the effects of CPB on the hemostasis mechanisms of body, monitoring a group of key parameters is essential to preserve life stability. Pump flow rate (PRF) during CPB is permanently controlled by a perfusionist who usually finds the optimum blood flow rate based on body surface area (BSA). It is highly important to make sure that an adequate blood flow is supplied during CPB operation. Low pump flow rate might lead to an attenuation of perfusion to the body limbs, ischemia and body organ damage. Excessive flow rate for a short period of time, on the other hand, might empty arterial reservoir, which risks air embolism, red globin damage, and trauma. Thereby, it is very critical to adjust PFR during CPB to a perfect level. The adjusted PFR must ensure a bloodless surgery area and adequate oxygenation, which makes finding a proper flow rate a challenging task. The accepted range of PRF during CPB is 2.2-2.4li/min per square meter of body surface. The logic of using BSA to calculate PFR during CPB is to provide the blood flow needed to supply metabolism of each square meter of body surface. However, the PRF determined by this technique might not be adequate to meet the metabolism. Differences between individuals like body weight and thyroid problems do not allow us to find a fixed PFR for all patients.