Background and Purpose: The principle of minimum work is a parametric optimization model for the growth and adaptation of arterial trees. It establishes a balance between energy dissipation due to frictional resistance of laminar flow (shear stress) and the minimum volume of the vascular system, implying that the radius of the vessel is adjusted to the cube root of the volumetric flow. The purpose of this study is to verify whether the internal carotid artery system obeys the principle of minimum work.

No abstract

Background and Purpose: The cerebral arteries present an optimum blood flow/vessel radius relation. However, branch angles may vary widely in the cerebral arteries because the parametric optimization of branch angles is irrelevant in terms of energy cost. The position of the flow divider in extracranial arteries has been suggested to be optimum in flow orderliness. No data exist on the flow divider of cerebral arteries. Thus, we hypothesized that in the cerebral arteries the apex of the bifurcations, which is known to be the

A quantitative analysis of the contributions of the cranial and spinal compartments to the cerebrospinal fluid pressure‐volume curve was made using dogs. The curve was determined by rapid continuous injection of fluid into the cisterna magna with simultaneous measurement of the pressure. Spinal block at the C 1 level was produced by inflation of an epidural rubber balloon allowing the recording of the pressure‐volume curve for the isolated cranial system. By subtraction of the two curves obtained, the spinal pressure‐volume curve could be calculated. 70 % of the variation in volume within the system was related to the spinal section and 30 % to the cranial section. The intracranial curve represents the effects on the fluid pressure of forced alterations in the volume of the intracranial vascular bed. The spinal compartment has a quantitatively defined and probably mechanically important function as an expansion vessel for the intracranial system.

The effects of repeated subarachnoid hemorrhages have been investigated experimentally in dogs. The main objectives were to determine the tolerance to repeated hemorrhage and to study the changes occurring during the repeated bleeds, in intracranial pressure, EEG, ECG, systemic arterial pressure and respiration. The natural course of an intracranial hemorrhage was simulated by shunting blood from a femoral artery through a drop recorder into five different sites in the craniospinal system: the chiasmatic cistern, a lateral ventricle, the cisterna magna, the lumbar subarachnoid space and into the cerebral tissue of the left frontal lobe. The hemorrhage was allowed to continue until it stopped spontaneously. Each bleed resulted in a transient rise in intracranial pressure to the level of the arterial pressure, followed by a return to a steady state value. The time taken for the attainment of the steady state was increasingly prolonged. The final steady state pressure increased with each bleed. Ultimately, a stage was reached where the hemorrhage resulted in a sustained high pressure at the level of the arterial blood pressure, producing failure of vital functions and an irreversibly isoelectric electroencephalogram. The average number of bleeds necessary to produce this state in the case of hemorrhage into brain parenchyma was 3 (range 2-4), into the lateral ventricle, 4 range 3-5), and into the cisterna chiasmatica, 5 (range 2-7). After 5 hemorrhages into the cisterna magna and the spinal subarachnoid space, a local resistance at the bleeding site was built up which prevented further bleeding.