Three-Dimensional Imaging of Microvascular pO2 During Ischemic Stroke

Lay Summary

Using Cellular Imaging to Determine Whether Oxygen Therapy May Limit Stroke Damage

Investigators will use cellular imaging in an animal model of stroke to quantify decreases in brain oxygenation that occur during stroke, and to assess the therapeutic potential of providing oxygen following the stroke to protect nearby brain tissues from further damage.

Ischemic strokes are produced when a normal blood flow to the brain is blocked, cutting off delivery of its precious oxygen cargo. Normally, blood oxygen concentrations diffuse to cross blood vessel walls, and the oxygen is then taken up by cells to support their metabolic functions. During an ischemic stroke, the lack of oxygen sets off a cascade of damaging events around the stroke area. As oxygen-starved brain cells die, brain tissue damage ensues. Stroke therapies often target the “ischemic penumbra,” the outer region of brain tissue damage, where the tissue is potentially salvageable. One experimental therapy, called “normobaric hyperoxia” (breathing of high levels of oxygen) has been shown in animal studies to reduce the size of the infarcted brain area and increase brain tissue oxygen levels in the penumbra.

The investigators now will take the first step to determine whether normobaric hyperoxia is likely to benefit stroke patients and to identify the underlying processes involved. They will combine two imaging techniques in animal models to characterize normal oxygenation and blood flow dynamics and determine how these differ in mild and severe ischemic stroke.  Specifically, they will use phosphorescence quenching methods to map the delivery and utilization of dissolved oxygen by brain cells, and two photon fluorescence microscopy to provide a 3-deminsional quantification of the level of degenerating neurons and the size of ischemic tissue damage. By combining these techniques, they will determine the degree of oxygenation achieved through normobaric hyperoxia and determine whether high increased oxygen therapy levels correlate with lower levels of damaged brain tissue.

Significance: If results show that oxygen therapy is correlated with lower levels of post-stroke tissue damage in the animal model, the research would provide important new evidence in support of human stroke therapy studies.

Abstract

Three-Dimensional Imaging of Microvascular pO2 During Ischemic Stroke

During ischemic stroke, a cascade of events occurs throughout the ischemic territory that ultimately leads to cell death and tissue damage. Many neuroprotective strategies currently under development are designed to block components of this complex cascade of events by partially restoring blood flow and oxygenation to the ischemic tissue. Normobaric hyperoxia is one such potential stroke treatment that has been investigated recently, since it has the potential to increase oxygen delivery to the ischemic tissue and prolong the therapeutic window.

Recent studies have demonstrated that normobaric hyperoxia can lead to reduced infarct size, increased pO2 in the penumbra, and increased oxy-hemoglobin concentrations in the ischemic core, suggesting that hyperoxia may be a feasible treatment strategy. Despite these promising results however, the detailed changes in oxygenation throughout the ischemic territory remain only partially understood, primarily due to a lack of methods capable of high resolution in vivo quantification of oxygenation with three-dimensional resolution. In this proposal we will use two photon excited phosphorescence lifetime quenching measurements to quantify pO2 levels in the somatosensory cortex with micron scale resolution in three-dimensions.

The specific aims of our proposal are to (1) quantify microvascular oxygenation in single, subsurface capillaries and arterioles in non-ischemic mice under normal and hyperoxic conditions, and (2) quantify the microvascular oxygenation changes during acute stroke and subsequent normobaric hyperoxia treatment. Together, these aims will provide the first detailed study of the three-dimensional oxygenation changes within the normal and ischemic brain and have important implications for a range of cerebrovascular diseases.