Chapter 1 Cerebral blood flow regulation Maintenance of adequate cerebral blood flow is essential for normal brain func- tion and survival. The brain receives approximately 15% of cardiac output and is responsible for approximately 20% of total body oxygen consumption 28. Apart from oxygen delivery cerebral blood flow facilitates disposal of excess CO2 and transport of vasoactive substances. Because of the brain’s limited ability to store energy cerebral blood flow needs to be controlled effectively. To adjust cerebral blood flow to an adequate level the diameter of cerebral ves- sels is changed. The variable resistance to flow is encountered in the cerebral arteriolar bed mainly; the major cerebral conductance arteries are in principle noncompliant and act merely as a conduit for the pulsatile arterial flow from the aorta to the brain 18. Cerebral blood flow (CBF) is dynamically adjusted to changes in the perfusion pressure, the metabolic activity of the brain, humoral factors and autonomic nerve activity 9. Control of cerebral blood flow to adjust for changes in cerebral perfusion pres- sure is called cerebral autoregulation. In 1959, Lassen 15 first described cerebral autoregulation as the tendency of cerebral blood flow to remain approximately constant when cerebral perfusion pressure changes over a wide range, typically from 60 to 150 mmHg. In his review, Lassen showed that there is a lower blood pressure limit of cerebral blood flow autoregulation. When perfusion pressure decreases below this limit a reduction in cerebral blood flow occurs. Hyperten- sive forced vasodilatation had been implied in the literature for many years 24, but it was as late as the early 70’s that an upper blood pressure limit of autoregu- lation was demonstrated 26. Increase of perfusion pressure above this upper limit of autoregulation leads to increase of cerebral blood flow. However, unlike what would be expected, below the lower limit vasodilation is still not maximal. Häggendalet al 14 demonstrated that hypercapnia may increase cerebral blood flow even if the blood pressure is below the lower limit of autoregulation. Simi- larly, MacKenzieet al 16 observed, in the anesthetized cat during controlled hypotension, pial arteriolar dilation at pressures below the lower limit of auto- regulation. Thus, at pressures moderately below the lower limit of autoregula- tion, drug-induced vasodilatation might normalize cerebral blood flow. Disease states of the brain may impair or abolish cerebral autoregulation 22. Autoregulation is lost in severe head injury or acute ischemic stroke, leaving surviving brain tissue unprotected against the potentially harmful effect of blood pressure changes. Likewise, autoregulation may be lost in the surroundings of a space-occupying brain lesion, be it a tumor or a hematoma. In many such disease states, autoregulation may be regained by passive hyperventilatory hypocapnia 8
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