Steps towards a fully computerized brain : the cerebral circulation.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
"Patience is power. Patience is not an absence of action; rather it is 'timing' it waits on the right time to act, for the right principles and in the right way." - Venerable Archbishop Fulton J. Sheen The brain weighs only 2% of the total body and yet receives nearly 20% of the cardiac output. Metabolism changes locally in different regions of the brain in order to interpret various stimuli. Because of this, an increase in blood flow to the area is required to replenish resources and remove the build-up of waste products without depleting the needs of the other regions. The structure responsible for directing blood around the brain is known as the cerebral arterial circle. Blood flow through the circle is governed largely by compliance and resistance of the distal vessels. Resistance is highly dynamic as it responds dramatically to small changes in the radius. Increases within local metabolic activity releases vasodilators. This enlarges the radius, decreases the resistance, and allows for more blood flow to the area. It is not fully understood how sensitive blood flow through the cerebral arterial circle is to these changes. A one-dimensional computer model was used to study the recruitment pattern within the cerebral arterial circle when the peripheral resistance was decreased. Even with a bilateral reduction of 10% in the largest efferent arteries, there was no notable decrease in flow within the remaining, non-stimulated, efferent arteries in a complete circle. Thus demonstrating the collateral capability of the circle. The peripheral resistance, represented as a lumped parameter, was manually reduced to allow the same amount of blood flow through each of the efferent arteries individually (Same Flow Tests). These were followed by reductions that allowed for the maximum amount of blood flow through each of the efferent arteries (Maximum Flow Test). These tests were performed on a complete circle and two circles containing common variations; one with a missing right proximal portion of the anterior cerebral artery (referred to as No-A1) and the other with a missing proximal portion of the posterior cerebral artery (No-P1). Results for the Same Flow Tests and Maximum Flow Tests were very similar, so the Maximum Flow Tests were used to determine which configuration had the most impact upon the collateral capabilities of the cerebral arterial circle. For the unilateral reductions, the No-A1 circle had the most potential of inhibiting the collateral capabilities of the cerebral arterial circle. In the bilateral reductions, the No-A1 variation had the most impact on a singular efferent artery. However, the No-P1 configuration had the greatest effect on the overall collateral capabilities of the cerebral arterial circle. While it was first thought that a complete circle would have the best configuration to redistribute blood flow in response to the decreases in peripheral resistance, surprisingly this was not always the case. Following these tests, the one-dimensional model was coupled to a symmetrically, bifurcating H-tree and autoregulatory CO₂ model. This made the peripheral resistance values fluctuational and pendent upon metabolic activity. Two different tests were performed on the fully coupled circle. For the first test, all of the efferent arteries expressed an equal amount of CO₂ decrease at the same time (referred to as the All-CO₂ test). The changes in flow were compared to a cerebral arterial circle with no autoregulation at normal levels of CO₂. The results were similar to those of a circle with a single coupled artery. The second test had all of the efferent arteries coupled to the autoregulatory model but only one vessel expressed changes in CO₂ levels (One-CO₂). The results were similar to those of the All-CO₂ test at normal levels of CO2. Noticeable changes were present when the CO₂ levels were low (in the allocated vessels). Most of the vessels demonstrated a decrease in blood flow in the All-CO₂ test while the opposite was true regarding the One-CO₂ test. The cerebral arterial circle plays a very important role in maintaining homeostasis in the brain. A main component that dictates the amount of flow required to be replenished by the cerebral arterial circle is the peripheral resistance. The peripheral resistance determines the amount of blood flow to the different areas of the brain. Keeping it as a constant, as it was for the first half of the tests, or allowing it to be autoregulated can have a large impact on the flow and, therefore, the collateral capability of the cerebral arterial circle.