Autoregulation is the process by which the brain maintains almost constant cerebral blood flow (CBF, ml/100 gm/min) in the presence of fluctuations in cerebral perfusion pressure (CPP, mm Hg).[28] Little change in CBF is observed over a broad range of CPP (50 to 130 mm Hg) because of the ability of precapillary resistance to adjust in response.[11,20] When CPP increases above 140 mm Hg, vasoconstriction becomes maximal and CBF increases dramatically.[14] When CPP decreases below 40 mm Hg, vasodilation becomes maximal and CBF diminishes linearly. This autoregulatory vasodilation is known as Stage 1 hemodynamic compromise.[27] As CPP decreases further, the ability to vasodilate is lost and CBF decreases. The brain compensates by increasing the extraction of oxygen from the blood to maintain the cerebral metabolism of oxygen.[11,22] Stage 2 hemodynamic failure or misery perfusion is characterized by decreased CBF and increased oxygen extraction.[20,36] As CPP diminishes further, oxygen metabolism is disrupted causing cellular dysfunction and eventually cell death.
The normal range of CBF has been established by PET and varies widely (45 to 110 ml/100 g/min).[31] The range of CBF in the setting of acute stroke has been stratified in an attempt to identify potentially recoverable tissue. Typically, CBF values below 10 ml/100 g/min indicate infarction whereas values between 10 and 22 ml/100 g/min identify tissue that is ischemic but not yet infarcted. The latter range is referred to as the ischemic penumbra.[7,15] CBF values between 23 and 44 ml/100 g/min represent oligemic tissue. Autoregulatory control can result in normal CBF in the setting of decreased CPP. Moreover, when the metabolic demands of the brain are low, CBF values can decrease even though CPP is normal.
As a single hemodynamic parameter, CBF is insufficient to assess perfusion status. Other hemodynamic parameters such as cerebral blood volume (CBV, ml/100 g) and mean transit time (MTT, secs) have been investigated in an attempt to elucidate microcirculatory perfusion. The relationship of these three parameters is reflected by the central volume principle, which states that MTT=CBV/CBF.
No comments:
Post a Comment