Cerebral vascular resistance is one of the most important variables in the autoregulation of cerebral blood flow. It is a flow resistance with which the cerebral vessels oppose the blood flow of systemic blood pressure. Autoregulation is impaired in severe brain injury in the setting of trauma, tumors, or cerebral hemorrhage.
What is cerebral vascular resistance?
Cerebral vascular resistance is one of the most important variables in the autoregulation of cerebral blood flow. Cerebral vascular resistance is defined by medicine as a flow resistance of the cerebral vessels. The vessels of the brain oppose the blood flow of the systemic blood pressure with the cerebral vascular resistance. They narrow or widen their vessel diameter depending on the systemic blood pressure values. Thus, cerebral vascular resistance is a regulatory variable in the blood flow to the human brain. The regulatory circuit is a protective mechanism for life support in the presence of altered blood pressure values. Like all vessels, cerebral vessels are equipped with a layer of muscle fibers. This layer of muscle can contract or relax. Relaxation leads to vasodilatation with an increase in blood flow. Contraction causes vasoconstriction with decreased blood flow. Since the brain can tolerate neither too little nor too much blood flow, the vessels must respond to changing blood pressure levels with regulatory relaxation or contraction. Brain damage due to excessive and insufficient blood supply can be prevented in this way. The tissue of the human brain is also the most sensitive and specialized tissue in the human body. Nerve cells in the brain are involved in every human bodily process. Without the highly specialized brain tissue, the human being is thus not viable. Brain death, unlike cardiac death, is thus equated with actual death. Cerebral vascular resistance prevents this brain death.
Function and task
Blood serves as an important transport medium in the human body, carrying vital oxygen as well as nutrients and messengers. Thus, the condition of deficiency blood flow means oxygen and nutrients deficiency. All cells of the body are thus dependent on an adequate blood supply for survival. In the brain, inadequate blood pressure levels are particularly tragic because of the life-sustaining functions of the brain. The human body has various mechanisms for life support. This is especially true for the area of the brain, which is particularly worthy of protection and vital because of its many tasks. A protective mechanism exists, for example, for cerebral blood flow. In the presence of systolic blood pressure values of 50 to 150 mmHg as well as intracranial normal pressure values, the cerebral vessels can respond to changes in arterial mean pressure with adjustments in vascular resistance. This resistance regulation corresponds to a response to maintain cerebral blood flow constant. Autoregulation of cerebral blood flow is critical primarily for adequate blood supply to the brain. Brain damage due to lack of oxygen or nutrients is prevented in this way. Cerebral vascular resistance is directly related to blood gases. When the CO2 partial pressure within the arterial blood increases, a relaxation reaction of the cerebral vessels occurs against the background of constant blood pressure values. Blood flow in the brain area increases with cerebral vascular dilatation. The same mechanism applies in the other direction. Thus, a decreasing partial pressure of CO2 in arterial vessels causes cerebral vascular resistance to increase. As a consequence, cerebral blood flow decreases. In this way, the brain is adequately perfused even during hypoventilation and hyperventilation. Carbon dioxide is the most important influencing variable on the vascular resistance of the cerebral vessels. A somewhat smaller influencing variable is present with the partial pressure of oxygen. When the pO2 in the arterial blood decreases, the cerebral arteries may dilate. However, this only occurs in the case of a strong drop. In this case, the pO2 falls below 50 mmHg. As a result of dilatation, blood flow to the brain increases due to resistance changes within the cerebral vessels. This process is also intended to prevent brain damage due to inadequate blood flow.
Diseases and ailments
Cerebral vascular resistance mechanisms do not survive certain situations. Without these mechanisms, the brain is no longer protected from increased and decreased blood supply, and the risk of brain death increases. More severe damage to the brain can occur, for example, in the context of trauma, cerebral hemorrhage, brain tumors, and edema. These pathophysiological conditions, on the one hand, disable the blood-brain barrier. On the other hand, they affect cerebral autoregulation. The processes of autoregulation can thus be so massively disturbed in the context of the aforementioned conditions that cerebral perfusion produces an immediate change in arterial mean blood pressure. In this process, the sensitive neurons come to harm. In addition, the autoregulatory mechanism of cerebral perfusion is overloaded at systemic blood pressure levels below 50 mmHg and above 150 mmHg. In this case, the autoregulation adjusts to the vessel diameters, but it can no longer compensate for the abnormal blood flow even by maximum adjustment. Decreased blood flow leads to ischemia, resulting in a lack of oxygen and nutrients. At blood flow reductions of half, full oxygen exhaustion is initiated as an additional compensatory mechanism. At levels below 20 milliliters per 100 grams per minute, reversible changes in brain cells occur. Decreases in blood flow below 15 milliliters per 100 grams per minute cause irreversible death of brain neurons within seconds. Hyperemia is the opposite event, i.e., too high a blood flow rate. In this case, intracranial pressure rises, causing compression-related damage to brain tissue. In hypertensive crises, the upper limit of autoregulation is exceeded and cerebral edema develops. Permanent hypertension also shifts the limits of autoregulation upward.
