Improved Neuroprotection with Hypothermia Delayed by 6 Hours Following Cerebral Hypoxia-Ischemia in the 14-Day-Old Rat

Abstract

Since hypothermia may be a potential treatment for perinatal cerebral hypoxic-ischemic injury, we used an established neonatal model of hypoxia-ischemia to determine the time after injury at which cooling had the best protective effect. Fourteen-day-old Wistar rats were subjected to right carotid artery ligation and hypoxia (8% O 2 for 90 min). Immediately at the end of hypoxia (defined as 0h), animals were either maintained at normal body temperature until sacrifice (normothermia) or subjected to hypothermia. In a preliminary study, the effects of a reduction in temperature and the duration of such cooling were investigated; animals were cooled (until brain temperature reached 33°C or 30°C) for 2, 4, or 6 h commencing immediately after hypoxia. In a second study, animals were cooled (brain temperature 30°C) for 6 h commencing at either 0, 2, 4, or 6 h after the end of hypoxia. Sham-operated animals were used as controls. Twentyfour hours after hypoxia-ischemia, cerebral energy metabolism was measured by phosphorus magnetic resonance spectroscopy, and at 5 d cerebral infarction was measured by planimetry. In normothermic animals the ratio of phosphocreatine/inorganic phosphate (PCr/Pi) had fallen markedly 24 h following hypoxiaischemia. In contrast, animals cooled between 6 and 12 h displayed high PCr/Pi ratios similar to those in control animals. Similarly, after 5 d, infarct area was significantly reduced only in animals cooled between 6 and12 h after injury. These results indicate that cooling between 6 and 12 h after hypoxia-ischemia is more effective in reducing cerebral injury than other cooling regimes and suggest that the physiologic events during this period are critical for understanding cerebral infarction. Term infants who suffer significant hypoxia-ischemia (HI) during birth later develop cerebral injury and neurodevelopmental impairment that may in the severest cases prove fatal. Studies using phosphorus magnetic resonance spectroscopy ( 31 P MRS) have demonstrated that these babies usually possess normal cerebral energy metabolism soon after resuscitation, but impaired oxidative phosphorylation, measured as the ratio of the concentrations of phosphocreatine/inorganic phosphate (PCr/Pi), develops some 24 h later as demonstrated by a low PCr/Pi that persists for many days (1). This delayed impairment of energy metabolism was not due to continued HI, nor was it associated with intracellular acidosis (2), but its magnitude was linearly related to the severity of later impairment and reduced brain growth (3).A similar pattern of changes has been observed in neonatal animal models of HI (4 -6). Recently, we have shown that changes comparable to those observed in human term infants occur in a neonatal rat model of HI (5, 7). Thus, in the 7-day-old rat, HI produces a severe disruption of cerebral energy metabolism during the insult, which resolves on reoxygenation. However, there is a second decline in energy metabolites beginning some 10 to 12 h after the insult (5, 7). The decrease in PCr/Pi at 10 to 12...

Combination of Systemic Hypothermia and N-acetylcysteine Attenuates Hypoxic-Ischemic Brain Injury in Neonatal Rats

“…Hypothermia improves survival and neurologic outcomes in neonatal animal models of severe HI injury (4 -7), and in recently completed clinical trials of HI neonates when instituted within 6 h of birth (8 -11). Animal data shows that initiation of hypothermia between 6 and 12 h after HI reduces cerebral injury (12,13), perhaps moderating secondary injury cascades during a time when cellular recovery is still possible. However, some inflammatory mediators are merely delayed by hypothermia treatment (14), indicating that hypothermia may be most helpful in extending the therapeutic window after HI injury.…”

Time Course of Brainstem Pathophysiology during First Month in Term Infants after Perinatal Asphyxia, Revealed by MLS BAER Latencies and Intervals

“…Experiments in animals have shown that a large number of treatments would reduce brain injury if given soon after the insult (1). Recently, moderate hypothermia to protect against brain injury has undergone vigorous investigation by many researchers around the world (2,4,6,7,31). Cooling the brain to approximately 32°C is one of several interventions that can be applied after HI to modify the process of brain cell death and so reduce brain damage.…”

Long-Term Neuroprotective Effect of Postischemic Hypothermia in a Neonatal Rat Model of Severe Hypoxic Ischemic Encephalopathy: A Comparative Study on the Duration and Depth of Hypothermia

“…The 7-d-old rat with unilateral carotid artery ligation and subsequent exposure to 8% hypoxia is the most commonly used model to evaluate the efficacy of therapeutic interventions in the developing brain (12), and several studies have used this Rice-Vannucci model (RVM) to test the effects of HT (10,11,(13)(14)(15)(16)(17)(18)(19). However, an experimental design composed of a shorter duration of hypoxia, representing "mild" ischemic lesions, is not appropriate for evaluating the neuroprotective effect of HT against severe HIE.…”