Abstract 2058 Neuroprotective Strategies for Hypoxic-Ischemic Encephalopathy Platform, Sunday, 5/2

Introduction: Intra-ischemic hypothermia of only 3-6°C is accepted as a potent neuroprotective agent in both adult and newborn animal models. From a clinical perspective however, post - ischemic hypothermia would provide a far more practical and efficient method of brain protection. In previous studies we had found that brief post-ischemic hypothermia for 3 hrs. did not reduce brain damage in a model of severe perinatal HI (Ped. Res. 1993;34:525-529). Brief periods of post-ischemic hypothermia have been show, however, to delay though not completely ameliorate the brain damaging effects of a "moderate" HI insult (Brain & Dev. 1997;19:326-338, Ped Res. 1998;43:325A). The purpose of the present study was to define the duration of variable degrees of hypothermia required to effect neuropathologic outcome following a "moderate" HI insult in the immature rat. Methods: Seven-day postnatal rats were anesthetized with halothane (4% induction: 1% maintenance) during which they underwent unilateral common carotid artery ligation. Following a 2 hr period of recovery with their Dams, the animals were placed in 500 ml glass jars and exposed to hypoxia in 8% oxygen/balance nitrogen for a period of 90 min. (moderate injury), via inlet/outlet portals. Thermoregulation during HI was maintained by placing the jars in waterbaths held at 37°C. Animals were then recovered for varying periods of either 3, 6, 12, or 24 hrs in thermo-controlled waterbaths at 28, 31, 34, or 37°C or with their Dams (nesting temp - 34°C). In separate groups of animals, core (rectal) temperature was monitored continuously for up to 24 hrs in order to determine the effect of environmental temperature on true body and brain temp. Given that immature rat pups will become hypoglycemic and hyperketonemic (which has been also shown to be neuroprotective -Ped Res., 1993; 31:138-142) if away from their Dams for this length of time, the rat pups were supplemented with continuous subcutaneous infusions of 15% glucose. Subsequently the animals were returned to their Dams, and nurtured to 30 days of age, at which time brains were examined neuropathologically.

Results: Mortality rates for animals receiving HI for 90 min. ranged from 0% to only 2.29%. No animals died during hypothermic recovery. Glucose values following supplementation measured at 1, 4, 8 and 24 hrs of recovery were within normal limits for all recovering temperatures compared to those animals recovered with their Dams. Mean core temperature for animals recovered at 28, 31, 34, and 37°C were 30.4, 32.6, 35.4, and 37.7°C respectively. Hypothermia for 3 hrs post-HI at any temperature was not neuroprotective at 23 days of recovery (30 days of age). Post-HI hypothermia of 28°C for 6 hrs significantly (p<0.05) decreased brain damage compared to rats maintained at 34 or 37°C for the same duration. Post-HI hypothermia of 12 and 24 hrs provided identical results. Rat pups maintained at 28°C for 12 and 24 hrs had significantly (p<0.05) lower mean brain damage (9.81 +/- 1.61 and 7.24 +/- 1.57, respectively) compared to those maintained at environmental temperatures of 34 °C (14.95 +/- 1.89 and 10.86 +/- 1.74) and 37°C (18.04 +/- 1.95 and 14.18 +/- 1.49), and those maintained with their Dam (14.46 +/- 1.81 and 10.73 +/- 1.73), respectively for 12 and 24 hrs. Twelve and 24 hrs of post-HI hypothermia at 31°C provided significant neuroprotection compared to those recovered at 37°C (p<0.05). Conclusions: 1) Brief (3 hrs) post-HI hypothermia is ineffective in providing neuroprotection. 2) Post-HI hypothermia of 28°C for 6 hrs provides significantly and permanently decreases brain damage, and 3) Post-HI hypothermia beyond 12 hrs provides no additional benefit.

(The authors gratefully acknowledge the support of the Heart & Stroke Foundations of Saskatchewan and Canada).