Superficial brain is cooler in small piglets: neonatal hypothermia implications.

Objective: Hypothermia was not neuroprotective in low body weight (BW) infants on subgroup analysis in a recent clinical trial of selective head cooling (SHC) in neonatal encephalopathy (CoolCap Trial).

Methods: The BW dependence of regional cerebral temperature was investigated in 14 newborn piglets under normothermia (38.5 degrees C), whole-body cooling (WBC; 36.5, 34.5, 32.5, and 30.5 degrees C), or SHC (20, 15, and 10 degrees C).

Results: Normothermia: Lower BW led to lower superficial brain temperature (p < 0.01). Deep to superficial brain and rectal to superficial brain temperature gradients increased with decreasing BW (both p < 0.05). WBC: Lower BW led to lower superficial brain temperature and higher rectal to superficial brain temperature gradient (p < 0.05 and p < 0.01, respectively). SHC: For lower BW, superficial and deep brain temperatures decreased (p < 0.01 and p < 0.05, respectively), whereas rectal to deep, rectal to superficial, and deep to superficial brain temperature gradients increased (p < 0.05, p < 0.01, and p < 0.05, respectively). Compared with SHC alone, superimposition of WBC (34.5 degrees C) reduced all regional temperatures (all p < 0.001); gradients were unaffected.

Interpretation: Brain cooling (under normothermia, WBC, or SHC) was more efficient with lower BW due to greater head surface area-to-volume ratios. In the CoolCap Trial, low BW infants might have been excessively cooled. WBC and SHC may require BW adjustment to accomplish consistent regional temperatures and optimal neuroprotection.