Cerebral blood flow (CBF) is the supply of blood to the brain over a period of time. For adults, CBF is typically
750 milliliters per minute, or 15% of
cardiac output. This corresponds to an average perfusion of 50-54
milliliters of blood per 100 grams of brain tissue per minute. CBF is tightly
regulated to meet the metabolic
requirements of the brain. Too much blood (clinical state of normal homeostatic response to hyperemia)
increases intracranial pressure (ICP), which can compress and damage delicate
brain tissue. .. Low blood flow (ischemia) occurs when blood flow to the brain
falls below 18-20 ml per 100 g / min, and tissue death occurs when blood flow
falls below 8-10 ml per 100 g / min. increase. The biochemical cascade, known
as the ischemic cascade, is triggered in brain tissue when the tissue becomes
ischemic, which can lead to brain cell damage
and death. Physicians should take steps to maintain proper CBF for
patients with conditions such as shock, stroke, cerebral edema, and traumatic
brain injury.
Cerebral blood flow is determined by many factors, including: B. Blood viscosity, vasodilators, and net pressure of blood flow to the brain. This is known as cerebral perfusion pressure, which is determined by the body's blood pressure. Cerebral perfusion pressure (CPP) is defined as mean arterial pressure (MAP) minus intracranial pressure (ICP). For the average person, it should be at least 50mmHg. Intracranial pressure should not exceed 15 mm Hg (20 mm Hg ICP is considered to be intracranial hypertension). Cerebral blood vessels can change blood flow through them by changing their diameter in a process called cerebral autoregulation. When systemic blood pressure rises, it contracts, and when it falls, it expands. Arterioles also contract and dilate in response to different chemical concentrations. For example, it expands when the carbon dioxide level in the blood is high, and contracts when the carbon dioxide level is low. Assume a person with a arterial partial pressure of 40 mmHg (normal range 38-42 mmHg) and a CBF of 50 ml per 100 g / min. If PaCO2 is reduced to 30 mmHg, this corresponds to a 10 mmHg reduction from the initial PaCO2 value. As a result, for every 1 mmHg reduction in PaCO2, the CBF is reduced by 1 mL per 100 g / min, resulting in a new CBF of 40 mL per 100 g / min of brain tissue. In fact, for every 1 mmHg increase or decrease in PaCO2 in the 20-60 mmHg range, there is a corresponding CBF change of approximately 1-2 mL / 100 g / min, or 2-5% of CBF, in the same direction. [14] Therefore, small changes in respiratory patterns can cause large changes in global CBF, especially from PaCO2 fluctuations.
No comments:
Post a Comment