By measuring oxygen use in the brain, researchers at the School of Medicine can determine which patients with blocked carotid arteries are at high risk for a stroke.
The findings, reported in the Sept. 23-30 issue of the Journal of the American Medical Association, could have implications for reviving a discarded surgical procedure that increases blood flow to the brain.
Robert L. Grubb Jr., M.D., the Herbert Lourie Professor of Neurological Surgery and professor of radiology; William J. Powers, M.D., professor of neurology and associate professor of radiology; and colleagues report that patients with complete blockage of the carotid artery and a condition called hemodynamic failure face a stroke risk six to seven times greater than those with just a blocked carotid artery. Hemodynamic failure occurs when the brain is not receiving the normal amount of oxygenated blood.
Using positron emission tomography (PET) images, the investigators measured blood flow and oxygen metabolism in the brains of 81 study subjects. All had complete blockage of one of their carotid arteries and had suffered either a stroke or a transient ischemic attack, which resembles a stroke but resolves itself in a few hours. The PET scans suggested which patients were in hemodynamic failure by revealing the amount of blood reaching their brains and the percentage of oxygen that the brain used.
Normally, these values are closely matched. Powers said, "Using PET, we confirmed that in some patients with blocked carotid arteries, there was an imbalance between blood supply and the amount of oxygen used by the part of the brain supplied by the blocked vessel."
The brain normally uses about 30 percent of the oxygen that's carried to it in the blood, but this amount can double during hemodynamic failure. Of the 81 patients studied, 39 had PET scans that showed increased oxygen extraction, and 42 had normal scans. In the group with increased oxygen extraction, 12 patients (31 percent) suffered strokes. Only three of the patients (7 percent) with normal oxygen extraction rates had strokes. And 11 of the 12 strokes suffered by patients with increased oxygen extraction occurred on the same side of the brain as the artery blockage, near the brain area that was using more oxygen than normal.
"We were very surprised by the number of strokes in the group with hemodynamic failure," Grubb explained. "We did not anticipate the discrepancy, but we think this study clearly indicates that oxygen metabolism in the brain is important to stroke risk and that available medical treatments do little to alter risk."
But the investigators think surgical therapy might be helpful. They want to revisit a surgical technique called extracranial-intracranial (EC-IC) bypass surgery. In the operation, a neurosurgeon takes an artery outside of the brain and connects it to a vessel inside the brain. The approach is designed to improve blood flow to the brain and reduce the stroke risk.
A 1985 multicenter study showed that the EC-IC bypass was no better than medical therapy in reducing risk of stroke, but that was before the risk of hemodynamic failure had been clarified. Now, Powers and Grubb hope to test the surgery in a study of patients with known hemodynamic failure. The bypass surgery might benefit this group if it can raise blood volume and lower the percentage of oxygen extracted from the blood.
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