113. Occlusion of single cortical venules results in decreased blood flow in upstream capillaries
J. Nguyen1, N. Nishimura1, C. Iadecola2 and C. Schaffer1
1Biomedical Engineering, Cornell University, Ithaca; 2Neurology and Neuroscience, Weill Cornell Medical College, New York, New York, USA
Background and aims: Microvascular lesions in the brain play an important role in the development of cognitive decline.1 While recent animal models of small strokes have investigated the changes in blood flow that result from the occlusion of cortical arterioles and capillaries, the redistribution of blood flow following a venule occlusion remains poorly understood. This is largely due to the lack of an appropriate animal model. To study the consequences of single venule occlusions on cortical blood flow we use nonlinear optical techniques to induce clot formation in targeted venules and to study blood flow changes in upstream capillary beds.
Methods: Through a craniotomy in urethane-anesthetized rats, vascular topology (Figure 1A) and blood flow is measured using two-photon excited fluorescence microscopy of intravenously injected fluorescein-dextran. Blood flow velocity is determined by tracking the motion of unlabeled red blood cells. Occlusions are induced by tightly focusing high intensity, femtosecond laser pulses into a targeted venule. Nonlinear absorption of the laser energy in the focal volume injures the vessel wall, initiating the natural clotting cascade (Figure 1B).2
Results: Measurements of blood flow in brain capillaries were taken before and after the occlusion of an ascending venule, i.e. a venule that brings flow from the cortical capillary beds to a vein on the brain surface (Figure 1C). For capillaries one and two branches upstream from the clotted venule, flow decreased to 20%±3% (average±s.e.m.; 16 clots across 16 rats; P<0.001, one-way t-test) of baseline value, while for vessels three and four branches upstream, speed decreased to 51%±7% (P<0.001) of baseline. Additionly, we observed reversed blood flow in 55% of the capillaries one or two branches upstream from the occlusion (red arrows, Figure 1C).
a.Baseline two-photon image of vasculature. Surface venule and arteriole outlined in blue and red, respectively. Red circle indicates a penetrating arteriole and blue circle represents an ascending venule.
b.Post-clot two-photon image. Red ‘X’ indicates location of clot.
c.Mapped vessel network with flow speed after the clot as a percentage of baseline speed in individual vessels. Red arrows indicate vessels that have reversed flow direction after clot.
Conclusions: This reduction in blood flow speed suggests that occlusion of a single venule perturbs microvascular flow in upstream capillaries over a relatively large vascular territory. Such previously unrecognized reduction in flow can deprive nearby neurons of the nutrients required for normal function leading to cell injury and death, and may potentially play a role in the brain lesions responsible for cognitive decline.
Figure 1 Case study of an ascending venule occllusion. Case study of flow changes in capillaries after the occlusion of an ascending venule.
1. Vermeer et al. NEJM 2003;348:1215.
2. Nishimura et al. Nat Meth 2006;3:99.