Cerebral perfusion pressure

Cerebral perfusion pressure, or CPP, is the net pressure of blood flow to the brain. It must be maintained within narrow limits because too little pressure could cause brain tissue to become ischemic (having inadequate blood flow), and too much could raise intracranial pressure (ICP).

CPP can be defined as: $$ CPP = MAP - ICP $$

CPP is regulated by two balanced, opposing forces: Mean arterial pressure, or MAP, the arithmetic mean of the body's blood pressure, is the force that pushes blood into the brain, and intracranial pressure is the force that keeps it out. Thus raising MAP raises CPP and raising ICP lowers it (this is one reason that increasing ICP in traumatic brain injury is potentially deadly). CPP, or MAP minus ICP, is normally between 70 and 90 mmHg in an adult human, and cannot go below 70 mmHg for a sustained period without causing ischemic brain damage. Children require pressures of at least 60 mmHg.

Autoregulation
The brain maintains proper CPP through a process called autoregulation. To lower pressure, blood vessels in the brain called arterioles dilate, or widen, creating more room for the blood, and to raise pressure they constrict, or narrow. Thus, changes in the body's overall blood pressure do not normally alter cerebral perfusion pressure drastically. At their most constricted, blood vessels create a pressure of 150 mmHg, and at their most dilated the pressure is about 60 mmHg. When pressures are outside the range of 50 to 150 mmHg, the blood vessels' ability to autoregulate pressure through dilation and constriction is lost, and cerebral perfusion is determined by blood pressure alone, a situation called pressure-passive flow. Thus, hypotension (inadequate blood pressure) in a can result in severe cerebral ischemia in patients with conditions like brain injury, leading to a damaging process called the ischemic cascade.

Other factors that can cause loss of autoregulation include free radical damage, nervous stimulation, and alterations in blood gas content. Amounts of carbon dioxide and oxygen in the blood affect constriction and dilation even in the absence of autoregulation: excess carbon dioxide can dilate blood vessels up to 3.5 times their normal size, lowering CPP, while high levels of oxygen constrict them. Hypoxia, or inadequate oxygen, also dilates blood vessels and increases blood flow. Blood vessels also dilate in response to low pH. Thus, when activity in a given region of the brain is heightened, the increase in CO2 and H+ concentrations causes cerebral blood vessels to dilate and deliver more blood to the area to meet the increased demand. In addition, stimulation of the sympathetic nervous system raises blood pressure and blocking it lowers pressure.