Relevant Anatomy


The intracranial compartment is a closed cavity with a very limited ability to accommodate additional volume. Initially, following a hemorrhagic stroke, "circulating blood and CSF are displaced to offset the extra volume and ICP remains unchanged. However, beyond a certain threshold, if intracranial contents continue to increase in volume, then ICP will quickly increase as there is diminished compliance in the fixed intracranial vault (Open Anesthesia, 2012)."

Meninges: The brain is covered by 3 meninges: dura mater, arachnoid mater, and the pia mater.

The subarachnoid space, located between the arachnoid membrane and the pia mater, contains the cerebrospinal fluid (CSF) that cushions and supports the brain. It also contains a network of arteries and veins that support the metabolic needs of the brain. In the event of an arterial subarachnoid hemorrhage, bleeding occurs into the subarachnoid space. While bleeding continues intracranial pressure (ICP) will increase because of the arterial pressure exerted within a closed system.
Normal Intracranial Pressure (ICP) is 5-15 cm H2O). When ICP increases, veins are compressed, venous outflow is reduced and perfusion therefore decreases
.

Arterial circulation:

imageThe right and left internal carotid arteries and the basilar artery are the primary arterial blood supply for the human brain. The basilar artery joins the two internal carotid arteries and other communicating arteries to form an anastomotic ring of arteries at the base of the brain known as the circle of Willis. Three pairs of arteries arise from the circle of Willis, the anterior, middle, and posterior cerebral arteries, these vessels become progressively smaller as they extend along the surface pia mater until they penetrate the pia, eventually becoming capillaries that supply the cerebral cortex.

The brain is only about 2% of the body's total weight, yet it receives approximately 20% of the heart's resting cardiac output. Obviously, significant blood volume courses through the cerebral circulation every minute.

In a sample of 895 men and women aged 55 years and older, the mean total cerebral blood flow (tCBF) was been found to be 497.4 mL/min. (Vernooij, 2008). In the same study, tCBF was found to decrease with age and was lower for women than men but total brain perfusion was 4 mL/min per 100 mL of brain tissue greater in women.

Presence of hypertension, a higher BMI, former smoking, and presence of carotid stenosis were all associated with lower tCBF. The study also found women had a significantly lower basilar blood flow than men, whereas flow in the carotid arteries did not differ between sexes (Vernooij, 2008).

imageVenous circulation: Venous blood is drained from the brain via deep veins and dura sinuses. The veins and sinuses connect to the jugular veins or paravertebral veins.

Cerebral spinal fluid: adult CSF volume of about 140 mL. fills the subarachnoid space, ventricles and cisterns . It is produced at a rate of about .35 mL/min. by choroid plexuses and absorbed by the arachnoid villi at about 500mL/day (Raftery, 2008). The cranial CSF volume is cycled about 3.7 times per day. In the event of a subarachnoid hemorrhage, blood mixes with the CSF. Clots can then form and collect in the ventricles, cisterns and foramen. If the clots block the normal flow of CSF hydrocephalus can result. Hydrocephalus can result in decreased perfusion because of increased ICP.


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