Background
Anoxic depolarization (AD) and periinfarct spreading depressions (PISDs) perturb membrane ionic balances and impose additional metabolic burden on ischemic brain. We developed a multimodal optical imaging system to investigate the effects of AD and PISDs on oxygen delivery and utilization in the core and penumbra with high spatial and temporal resolution.
Methods
We performed real-time 2D multispectral reflectance imaging and laser speckle flowmetry through intact skull, to simultaneously image changes in oxyhemoglobin (oxyHb), deoxyhemoglobin (deoxyHb), total hemoglobin (rCBV), and rCBF in focal cerebral ischemia and reperfusion. Ischemia was induced by distal middle cerebral artery occlusion (dMCAO) for 60–90 minutes through a temporal craniotomy, in intubated and ventilated mice under full physiological monitoring.
Results
dMCAO reduced rCBF to 20–30% of baseline in the core, while rCBV initially remained unchanged (B, vertical line). Within 1–2 minutes after dMCAO AD developed (B, arrowhead) and caused a large reduction in rCBV, suggesting active vasoconstriction or passive vascular collapse. This was accompanied by a further drop in rCBF in the core (15–20% of baseline). DeoxyHb abruptly increased in the core upon dMCAO, and did not significantly change during AD, whereas changes in oxyHb closely followed those of rCBV and rCBF. PISDs (2–3/h) caused further reductions in rCBV, rCBF and oxyHb both in the core and the penumbra (C, *). DeoxyHb was increased by PISDs both in the core and the penumbra, suggesting that oxygen consumption continues in these ischemic territories. A ring of relative increase in rCBV invariably surrounded the infarct rim. Upon reperfusion (D, vertical line), rCBF, rCBV and oxyHb increased and deoxyHb decreased towards baseline levels; in about 30% of experiments deoxyHb decreased below pre-ischemic levels, suggesting that oxygen utilization in the severely ischemic core failed to recover completely upon reperfusion.
Conclusion
These data suggest that AD and PISDs increase oxygen consumption and reduce oxygen delivery in both the core and the penumbra. Multispectral reflectance imaging simultaneously with LSF provides better understanding of the dynamic changes in rCBF and tissue oxygenation during acute focal cerebral ischemia, and may identify irreversible metabolic injury in focal ischemia and reperfusion. (See Figure 1).
(A) Regions of interest used to quantify the changes in oxyHb, deoxyHb and total Hb. C: core, P: penumbra, NI: non-ischemic cortex. (B) Changes at the onset of dMCAO (vertical dashed line) and after AD (arrowhead). (C) Effects of PISDs (*). (D) Changes upon reperfusion (vertical dashed line). Horizontal lines indicate pre-ischemic baseline. Vertical axis shows changes in oxyHb, deoxyHb and total Hb (rCBV) in arbitrary units. Horizontal axis shows time after dMCAO. (B), (C), and (D) are representative tracings from separate experiments.