The role of the cerebral capillaries in acute ischemic stroke: the extended penumbra model

The role of the microcirculation in delayed cerebral ischemia and chronic degenerative changes after subarachnoid hemorrhage

The mortality after aneurysmal subarachnoid hemorrhage (SAH) is 50%, and most survivors suffer severe functional and cognitive deficits. Half of SAH patients deteriorate 5 to 14 days after the initial bleeding, so-called delayed cerebral ischemia (DCI). Although often attributed to vasospasms, DCI may develop in the absence of angiographic vasospasms, and therapeutic reversal of angiographic vasospasms fails to improve patient outcome. The etiology of chronic neurodegenerative changes after SAH remains poorly understood. Brain oxygenation depends on both cerebral blood flow (CBF) and its microscopic distribution, the so-called capillary transit time heterogeneity (CTH). In theory, increased CTH can therefore lead to tissue hypoxia in the absence of severe CBF reductions, whereas reductions in CBF, paradoxically, improve brain oxygenation if CTH is critically elevated. We review potential sources of elevated CTH after SAH. Pericyte constrictions in relation to the initial ischemic episode and subsequent oxidative stress, nitric oxide depletion during the pericapillary clearance of oxyhemoglobin, vasogenic edema, leukocytosis, and astrocytic endfeet swelling are identified as potential sources of elevated CTH, and hence of metabolic derangement, after SAH. Irreversible changes in capillary morphology and function are predicted to contribute to long-term relative tissue hypoxia, inflammation, and neurodegeneration. We discuss diagnostic and therapeutic implications of these predictions.

Imaging of cerebral ischemia: from acute stroke to chronic disorders

Cerebral ischemia manifests widely in patient symptoms. Along with the clinical examination, imaging serves as a powerful tool throughout the course of ischemia-from acute onset to evolution. A thorough understanding of imaging modalities, their strengths and their limitations, is essential for capitalizing on the benefit of this complementary source of information for understanding the mechanism of disease, making therapeutic decisions, and monitoring patient response over time.

Cerebral Ischemia and Reperfusion Increases the Heterogeneity of Local Oxygen Supply/Consumption Balance

Background and Purpose—

After cerebral vessel blockage, local blood flow and O 2 consumption becomes lower and oxygen extraction increases. With reperfusion, blood flow is partially restored. We examined the effects of ischemia-reperfusion on the heterogeneity of local venous oxygen saturation in rats in order to determine the pattern of microregional O 2 supply/consumption balance in reperfusion.

Methods—

The middle cerebral artery was blocked for 1 hour using the internal carotid approach in 1 group (n=9) and was then reperfused for 2 hours in another group (n=9) of isoflurane-anesthetized rats. Regional cerebral blood flow was determined using a C 14 -iodoantipyrine autoradiographic technique. Regional small vessel arterial and venous oxygen saturations were determined microspectrophotometrically.

Results—

After 1 hour of ischemia, local cerebral blood flow (92±10 versus 50±10 mL/min per 100 g) and O 2 consumption (4.5±0.6 versus 2.7±0.5 mL O 2 /min per 100 g) decreased compared with the contralateral cortex. Oxygen extraction increased (4.7±0.2 versus 5.4±0.3 mL O 2 /100 mL) and the variation in small vein (20–60 μm) O 2 saturation as determined by its coefficient of variation (=100×SD/mean) increased (5.5 versus 10.5). With 2 hours of reperfusion, the blood flow decrement was reduced and O 2 consumption returned to the value in the contralateral cortex. Oxygen extraction remained elevated in the ischemic-reperfused area and the coefficient of variation of small vein O 2 saturation increased further (17.3).

Conclusions—

These data indicated continued reduction of O 2 supply/consumption balance with reperfusion. They also demonstrated many small regions of low oxygenation within the reperfused cortical region.