Brain Temperature Reduction During Ischemia Increases with Immaturity† 1051

Brain temperature (Tb) is a critical determinant of the response and outcome to ischemia. Different Tb may exist in immature vs. mature animals during ischemia due to differences in the balance between heat loss and production, but this has not been explored. To examine this issue, Tb was measured during control and at 2 min intervals during 16 min of partial brain ischemia in 8 newborn miniswine (Nb, age 5±3d, weight 1.4±.2 Kg, x±SD) and 10 older miniswine (Old, age 34±5d, weight 6.1±1.2 Kg). Tb was measured directly with thermocouples implanted to a depth of 2 cm beneath the cortical surface, and cerebral blood flow (CBF) was measured with fluorescent microspheres during control and at 2 intervals during ischemia (6-9 and 12-15 min of the 16 min interval) to verify a constant intraischemic CBF reduction in each animal. Ischemia was induced by inflation of a neck cuff plus hypotension. At control rectal temperature (Tr) did not differ between groups (38.4±0.2 vs 38.6±0.2°C for Nb and Old, respectively), but Tb was lower in Nb compared to Old(38.4±0.2 vs. 38.8±0.3°C, p<.01). By design a broad range of CBF reductions were studied in each group during ischemia. In Nb and Old mean intraischemic CBF (% of control, 100%) was 31±27 and 33± 33% respectively, and in each group CBF at 6-9 and 12-15 min did not differ. During ischemia Tr was stable and never decreased by more than 0.2°C in either group. Changes in Tb were expressed as rectal-brain temperature difference (ΔTrb). In Nb ΔTrb was 0.0±0.1°C at control and increased to 0.9±0.7°C at 16 min of ischemia. In Old ΔTrb was -0.2±0.3 at control (p=.02 vs Nb) and increased to 0.1±0.4°C at 16 min of ischemia (p<.01 vs Nb). CBF was inversely correlated with ΔTrb for Nb when measured from 6-9 min of ischemia(r²=.69, p=.01) and 12-15 min of ischemia (r²=.64, p=.02). In contrast there was no correlation between CBF and ΔTrb for Old during ischemia. The thickness of the overlying scalp plus calvarium differed between groups; 2.69±0.55 vs 6.55±1.26 mm in Nb and Old respectively(p<.01). Ischemia results in lower Tb in Nb compared to Old, and the lower Tb could contribute to the resistance of brain to ischemia in younger compared to older mammals. The presence of less insulating tissue (scalp, calvarium) in Nb compared to Old may be responsible for greater intraischemic heat loss in Nb relative to Old.