Cerebral Oxygen Saturation-Time Threshold for Hypoxic-Ischemic Injury in Piglets

BACKGROUND: Detection of cerebral hypoxia-ischemia (H-I) and prevention of brain injury remains problematic in critically ill neonates. Near-infrared spectroscopy (NIRS), a noninvasive bedside technology could fill this role, although LAIRS cerebral O-2 saturation (Sc-O2) viability-time thresholds for brain injury have not been determined. We investigated the relationship between H-I duration at Sc-O2 35%, a viability threshold which causes neurophysiological impairment, to neurological outcome. METHODS: Forty-six fentanyl-midazolam anesthetized piglets were equipped with LAIRS and cerebral function monitor (CFM) to record Sc-O2 and electrocortical activity (ECA). After carotid occlusion, inspired O-2 was adjusted to produce H-I (Sc-O2 35% with decreased ECA) for 1, 2, 3, 4, 6 or 8 h in different groups, followed by survival to assess neurological outcome by behavioral and histological examination. RESULTS: For H-I lasting 1 or 2 h, ECA and Sc-O2 during reperfusion rapidly returned to normal and neurological outcomes were normal. For H-I more than 2-3 h, ECA was significantly decreased and Sc-O2 was significantly increased during reperfusion, suggesting continued depression of tissue O-2 metabolism. As H-I increased beyond 2 h, the incidence of neurological injury increased linearly, approximately 15% per h. CONCLUSION: A viability-time threshold for H-I injury is Sc-O2 of 35% for 2-3 11, heralded by abnormalities in LAIRS and CFM during reperfusion. These findings suggest that LAIRS and CFM might be used together to predict neurological outcome, and illustrate that there is a several hour window of opportunity during H-I to prevent neurological injury.