Central nervous system effects

Autoregulation is highly prominent in vascular beds of the heart, kidneys, and lungs, but is most important in the cerebral circulation. In the normal state, cerebral autoregulation maintains a stable cerebral blood flow rate despite fluctuations in mean arterial pressure over a range of 60 to 150 mmHg. When the upper limit of cerebral autoregulation is exceeded for a period of time, hypertensive encephalopathy results, with cerebral edema, petechial hemorrhages, and microinfarcts. This is usually associated with very high pressures (e.g. diastolic pressure generally exceeding 140 mmHg). These patients typically present with nausea, vomiting, severe headaches, visual changes and, in extreme cases, mental obtundation or seizures ( Powers 1.9.9.3.).

Individuals with normal cerebral autoregulation can generally tolerate relatively rapid reductions in their systemic blood pressure, but this may not be true for patients with hypertensive emergencies, particularly those with hypertensive encephalopathy. Chronic elevation in systemic pressure, even without underlying cerebral vascular disease, can impair normal autoregulatory capability such that a substantial decrease in cerebral blood flow occurs at a higher mean arterial pressure than would be true in normal individuals. In individuals with chronic untreated severe hypertension, the lower limit of cerebral autoregulation can be reset to occur at a higher mean arterial pressure, such as 110 mmHg, instead of the 60 to 70 mmHg found in normotensive individuals. These changes in cerebral autoregulation have direct clinical relevance and influence the target blood pressure in patients with hypertensive emergencies. In both normotensive and hypertensive individuals, the lower limit of cerebral autoregulation is generally about 25 per cent below the baseline mean arterial pressure. However, the baseline mean arterial pressure is usually not known, and so a target range of 110 to 120 mmHg is generally recommended during treatment (Pow§.r.s...,199.3; E§n§c§k.,199.4).

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