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Reactivity of the Cerebral Vessels
Autoregulation

Autoregulation is the ability of the brain to maintain a relatively constant level of blood flow over a wide range of mean arterial blood pressures. This is accomplished by the vasoconstriction of the arterioles when blood pressure increases and the vasodilatation of the arterioles when the blood pressure decreases. In patients with severe atherosclerotic disease, long standing hypertension and brain injuries, this mechanism can be disturbed. If autoregulation is not fully functional, the patient becomes at increased risk for ischemia and infarction when blood pressure falls below critical limits.

The MCA's are continuously monitored bilaterally during a rapid, transient inflating and deflating of blood pressure cuffs placed around the patient's thighs. After inflation to suprasystolic values, the cuffs are then rapidly deflated, causing a volume of blood to be shunted into the legs and a transient drop in blood pressure of 12 to 20 mmHg. Middle cerebral artery blood flow velocities also decrease instantaneously with the blood pressure drop but return to baseline within a few seconds when autoregulation is intact. When the return of blood flow velocities passively follow the return of the blood pressure (which takes several minutes), autoregulation is exhausted. There are nine possible grades of autoregulation. Normal values are 5 +/- 1.

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a. Normtension- Normal vasomotor tone seen in the arterioles during normotension. b. Induced Hypotension- Vasodilation occurs following a transient, rapid drop in blood pressure, this restores blood flow and demonstrates the autoregulatory response.