Direct observation of the human cerebral microcirculation during aneurysm surgery reveals increased arteriolar contractility.

BACKGROUND AND PURPOSE The effects of aneurysmal subarachnoid hemorrhage on morphology and function of the cerebral microcirculation are poorly defined, partly due to the lack of suitable techniques to visualize the microvessels in vivo. We used orthogonal polarization spectral (OPS) imaging on the brain cortex during aneurysm surgery to directly observe the small cortical blood vessels and quantify their responses to hypocapnia. METHODS In 16 patients undergoing aneurysm surgery, the diameter changes of small cortical vessels (15 to 180 microm) were observed using OPS imaging. Ten patients were operated on early (within 48 hours after bleeding) and 6 underwent late surgery. Immediately after dura opening, the response to hyperventilation of arterioles and venules was observed with OPS imaging under sevoflurane anesthesia. RESULTS In patients operated on early, layers of subarachnoid blood were clearly visible. In this group, hyperventilation resulted in a 39+/-15% decrease in arteriolar diameter with a "bead-string" constriction pattern occurring in 60% of patients. In late surgery and in controls, no subarachnoid blood was seen. The arteriolar diameter decrease with hyperventilation was 17+/-20% in patients undergoing late surgery and 7+/-7% in controls. Venules were not affected by hyperventilation in any of the groups studied. CONCLUSIONS OPS imaging allows direct in vivo observation of the cerebral microcirculation enabling us, for the first time, to visually observe and quantify microvascular reactivity in the human brain. The present study demonstrates increased contractile responses of the cerebral arterioles in the presence of subarachnoid blood, suggesting increased microvascular tonus with possibly greater susceptibility to ischemia.