CEREBRAL BLOOD FLOW

Impaired cerebral blood flow disorders are extremely common and factors such as the lesion site, existing collateral’s, and the amount of tissue affected determines the actual neurological deficit that results. The impaired blood flow may have a number of causes. Things such as alterations in blood pressure, changes in the arterial walls, and occlusions of the arterial lumen are some of the more important causes.

The brain is supplied with blood by two internal carotid arteries and two vertebral arteries. These arteries form the anastomosis known as the Circle of Willis. In 1951, two researchers, McDonald and Potter demonstrated that, "the blood supply to half of the brain is provided by the internal carotid and vertebral artery of that side, and that their respective streams come together in the posterior communicating artery at a point where the pressure of the two is equal and they do not mix." (Snell, p514). This is important to keep in mind when considering just how significant the collateral circulation truly is. "If, however, the internal carotid or vertebral artery is occluded, the blood passes forward or backward across that point to compensate for the reduction in blood flow. The circulus arteriosus also permits the blood to flow across the midline, as shown when the internal carotid or vertebral artery on one side is occluded." This provides some relief for occlusions in the major vascular supply. Another important finding, was that the blood that flows from the two vertebral arteries remain on the same side of the lumen and does not mix while passing through the basilar artery. These are important items for the major vascular supply, however, once the branches penetrate the brain substance, there are no more anastomosis.

By far the most important factor influencing blood flow through the brain is the arterial blood pressure. A number of factors oppose this force such as, "a raised intracranial pressure, increased blood viscosity, and narrowing of the vascular diameter." Cerebral blood flow is relatively unchanged even with such opposing forces. This involves a special autoregulatory response. When arterial blood pressure drops, then the cerebral vascular resistance also is lowered. The opposite is also true to prevent a drastic increase in pressure within the cerebral vasculature. However, this autoregulatory system breaks down when the pressure falls very low.

The main factor influencing cerebrovascular resistance is the diameter of the cerebral blood vessels. The vessels are innervated by postganglionic sympathetic fibers. They will respond to norepinephrine, but apparently, this does not play a major part in controlling the vascular resistance. The two most important controlling substances are oxygen and carbon dioxide. Carbon dioxide is the most powerful vasodilator for cerebral blood vessels. Oxygen has strong vasoconstrictor effects on these vessels.

There are a number of things that produce cerebral ischemia, as was mentioned above. The first category includes those diseases that cause a change in blood pressure. First, an interruption of cerebral circulation. The brain is irreversibly damaged quickly following occlusion of the blood flow. "It has been estimated that neuronal function ceases after about one minute and that irreversible changes start to occur after about 4 minutes, although this time may be longer if the patient’s body has been cooled. Cardiac arrest due to coronary thrombosis is the most common cause of this condition." (Snell, p.515) Another disorder that can interrupt the circulation is postural hypotension. This may occur when blood accumulates in the venous system of the limbs or when venous return is diminished. These cause a drop in the cardiac output and a lowered arterial blood pressure. In general, the blood pressure must fall quite significantly before it affects the cerebral blood flow. Different states of shock may lead to decreased blood flow. Such things as hemorrhagic, psychogenic, and hypovolemic shock may be the underlying cause. These states lead to a decrease in the blood flow and possible ischemic conditions. Hyperventilation in anxiety episodes can lead to alkalosis causing vascular constriction and a subsequent decrease in perfusion. In diseases such as polycythemia vera, there is an increase in blood viscosity, which leads to a decrease in cerebral blood flow. Another disorder known as carotid sinus syndrome may lead to blood flow problems. The carotid sinus is very sensitive to arterial blood pressure changes and is responsible for the autoregulation of arterial pressure. An increase in impulses traveling by way of the glossopharyngeal nerve to cardio-inhibitory and vasomotor centers provided a negative feedback control system for blood pressure. In some patients, this reflex is hypersensitive and may cause a sudden drop in arterial pressure leading to an ischemic event.

A second category that is worthy of mentioning is that of diseases of the heart. Any pathological condition that leads to a decreased cardiac output can cause a decrease in cerebral blood flow. Such things as heart block, ventricular fibrillation, or infarction can be the cause of the decreased output.

A third category involves the diseases of the arterial walls. The most common finding in narrowed arteries of the brain is atheroma. This hardening of the arteries, as it is frequently called, can affect the arteries leading to the brain or those within the skull and brain itself. A number of other conditions can further complicate this condition, such as, myocardial infarction or anemia. Atherosclerosis is common in patients who are of middle or old age. It may complicate other conditions such as, diabetes and hypertension. The actual effect of the atheroma depends on the location and extent of the lesion. If a large portion of the brain undergoes ischemia, then the neuroglia in that region will proliferate and invade the area. If the lesion is in a small area supplying a limited amount of nervous tissue the damage and sequelae may be small. However, if there is generalized ischemia due to narrowing of the cerebral arteries the brain will undergo a diffuse atrophy.

A fourth category involves those diseases that result in a blockage of the arterial lumen. There are two main causes of embolism. First, a fat globule may occlude one of the cerebral arteries. This usually occurs after one of the long bones of the body have been fractured. The fat globules come from the yellow marrow and enter the circulation through the nutrient vein. These globules pass throughout the circulation and may block many small cerebral arteries. The second and most common cause of occlusion is a thrombus. The thrombus may occur anywhere from the left heart to the cerebral vasculature. Some of the common sites of thrombus formation include atherosclerotic plaques on the common carotid, the internal carotid, or the vertebral arteries. Other sites of formation include the mitral and aortic valves. The incidence of cerebral thrombosis is higher among women taking oral contraceptives, especially those which are on high-dose estrogen-progesterone combinations.

The most effective treatment of the stroke patient must be based on the underlying cause of the lesion. With MRI, CT, and other technology, clinicians are better equipped to identify the cause of the stroke. The choice of drug therapy for treating these patients varies. The indications for anticoagulant therapy are still be determined. An important concern in drug therapy is the role of intracranial pressure (ICP). Some ICP problems can be controlled with therapy involving "hyperventilation, judicious dehydration, and corticosteroid therapy." (Stern, p.33). Barbiturate therapy can also be used to lower ICP. Another option is high dose boles of mannitol. Mannitol decreases ICP, but can have adverse effects on a fetus in a gravid stroke patient. It can markedly change the fetal fluid balance. Low doses of mannitol may be effective without injury to the fetus, but the effects are still not known. The main focus of the physician in treating a stroke patient must be the individual patient and his/her underlying lesion. Cerebral blood flow deficiency is not limited in its scope of patients it can afflict. The care of stroke patients is changing and the optimal management of the patient’s condition demands the careful consultation of a well-informed team of physicians.

BIBLIOGRAPHY:

Auer, L. M. & Ladurner, G. "Alterations of the Cerebral Blood Volume," p.p. 233-38.

Snell, R. S. "The Blood Supply of the Brain," Clinical Neuroanatomv for Medical Students. Little, Brown and Co., Boston/Toronto, p.p. 507-24.

Stern, B. J. "Cerebrovascular Disease and Pregnancy’ Neurological Disorders of Pregnancy". p.p. 32-34.

Walton, John. "Disorders of the cerebral circulation," Brains Diseases of the Central Nervous System. Oxford Univ. Press, New York; p.p. 219.