Myogenic Regulation (Autoregulation)

Autoregulation refers to the capacity of the cerebral circulation to adjust its resistance so that it can maintain CBF constant over a wide range of mean arterial pressure (MAP) values. In normal human subjects, the limits of autoregulation occur at MAP values of approximately 70 and 150 mm Hg (see Fig. 21-4 ). The lower limit of autoregulation (LLA) has been widely quoted as an MAP of 50 mm Hg. Although this number may be correct for some animal species, the available data argue that the LLA is considerably higher in humans.^[53] Note that the units used on the x axis of "autoregulation curves" will influence the correct inflection points of the curve. When the x axis is "mean arterial pressure," the normal average LLA is not less than 70 mm Hg (with considerable interindividual variation).^{[54] [55] [56] [57] [58]} However, cerebral perfusion pressure (CPP) is the ideal independent variable, but because ICP is not usually measured in normal subjects, CPP (MAP - ICP) is rarely available. Assuming a normal ICP in a supine subject of 10 to 15 mm Hg, an LLA of 70 expressed as MAP corresponds to an LLA of 55 to 60 mm Hg expressed as CPP.

Above and below the autoregulatory plateau, CBF is pressure dependent (pressure passive) and varies linearly with CPP. Autoregulation is influenced by various pathologic processes and, in addition, by the time course over which CPP changes occur. Even within the range over which autoregulation normally occurs, a rapid change in arterial pressure will result in a transient (3 to 4 minutes) alteration in CBF.^[59]

The precise mechanism by which autoregulation is accomplished is not known. According to the myogenic hypothesis, changes in CPP lead to direct changes in the tone of vascular smooth muscle; this process appears to be passive. NO may participate in the vasodilation associated with hypotension in some species ^{[60] [61]} but not, according to a single study, in primates.^[62] Autonomic innervation of cerebral blood vessels may also contribute to autoregulation of blood flow (discussed in the next section).