Significant selective head cooling can be maintained long-term after global hypoxia ischemia in newborn piglets

Selective head cooling (SHC) combined with mild body cooling is currently being evaluated as a potentially therapeutic option in the management of neonatal hypoxic-ischemic encephalopathy. It is proposed that SHC enables local hypothermic neuroprotection while minimizing the deleterious side effects of systemic hypothermia. However, there is little evidence that it is possible to cool the brain more than the body for a prolonged period of time. The aim of this study was to examine whether the brain (T(deep brain)) could be cooled to below the rectal temperature (T(rectal)) in our piglet hypoxia ischemia (HI) model for a period of 24 hours, using a head-cooling cap.

Eight anesthetized piglets (median age: 15 hours) had subdural and intracerebral basal ganglia temperature probes inserted. After a 45-minute global HI insult (known to produce permanent brain damage), SHC using a cap perfused with cold water (5 degrees C-24 degrees C) combined with overhead body heating to maintain T(rectal) at 34 to 35 degrees C was performed for 24 hours.

The piglets were cooled to a median T(rectal) of 35.0 degrees C (interquartile range [IQR]: 34.7-35.3) for 24 hours. During this time, the median T(deep brain) was 31.4 degrees C (IQR: 30 degrees C-32.2 degrees C), with a median T(rectal) to T(deep brain) gradient of 3.4 degrees C (IQR: 2.7 degrees C-4.8 degrees C). At the end of the cooling period, this gradient was still maintained at a median of 3.3 degrees C (IQR: 2.9 degrees C-3.7 degrees C). The ability to obtain the gradient was not influenced by the size of the piglet (1300-1840 g). Cap cooling lowered scalp temperature (T(scalp)) to a median of 24.9 degrees C (IQR: 22.2 degrees C-29.2 degrees C) and subdural temperature to a median of 28.1 degrees C (IQR: 25.8 degrees C-29.5 degrees C) but did not result in either skin injury or superficial brain hemorrhage. There was no clinically useful correlation between T(scalp) and T(deep brain) or between T(scalp) and T(subdural).

This study using our piglet HI model shows that it is possible by means of a head-cooling cap to cool the brain more than the body for a 24-hour period while keeping the core temperature mildly hypothermic. However, we were unable to predict temperatures inside the brain using surface temperature probes on the head.