Abstract
Abstract: :
Purpose: To examine the role of the small vessels (arterioles, capillaries, venules) in the autoregulation of the retinal microcirculation (RMC) which means a compensation of the decrease of the retinal perfusion pressure. That is important as insufficiencies of the function of the autoregulation of the RMC are connected with ocular diseases. In the case of glaucoma the vascular cause may be a decrease in blood pressure or an increase in IOP resulting in a loss of perfusion pressure. Methods: The simulation is based on the measuring results of a clinical study [Nagel ‘02] where arterial and venous diameters were recorded [RVA, Imedos GmbH, Germany] before, while and after a moderate increase of the IOP (to values of about 38mmHg) of healthy subjects. The large arteries dilate (+2%) during the IOP increase and the veins constrict (-1%). The simulation tool is a knowledge-based model of the RMC programmed in VC++. It is based on the vessel network structure consisting of four quadrants parallel to each other. Each quadrant is composed of five vessel compartments (arteries, arterioles, capillaries, venules, veins) connected in series. The model parameters are geometrical parameters (vessel lengths...) as well as measurable bloodrheological values. The Casson Flow is used to calculate the nonlinear flow-pressure relation and the model parameters blood flow, blood velocity and retinal perfusion pressure in large-caliber vessels. The simulation should calculate the diameter changes of small arteries and capillaries that are necessary to compensate the increase of IOP to keep the flow constant. Results: A reduction of the retinal perfusion pressure caused by increase of IOP applied in the clinical studies means a reduction of flow to 50%. The simulation results show that the changes of the arterioles alone can't compensate flow reduction. Only the increase of the assumed capillary diameter to 100% allowed the arterioles to compensate flow loss by increasing their diameter to 20...30% . Conclusions: The results would mean that diameter changes react stronger than expected. The arterioles change more than ten times stronger than the arteries.
Keywords: blood supply • computational modeling • retinal connections, networks, circuitry