Differences in brain temperature and cerebral blood flow during selective head versus whole-body cooling

Objective. To compare brain temperature and cerebral blood flow (CBF) during head and body cooling, with and without systemic hypoxemia. Methods. Seventeen newborn swine were studied for either measurement of brain temperature alone (n = 9) or measurement of brain temperature and CBF (n = 8). All animals were ventilated and instrumented, and temperature probes were inserted into the rectum, into the brain at depths of 2 and 1 cm from the cortical surface, and on the dural surface. Blood flow was measured with microspheres. The protocol consisted of a control period, an interval of either head or body cooling, and cooling with 15 minutes of superimposed hypoxia. After a 1-hour recovery period, animals were exposed to the same sequence except that the alternate mode of cooling was evaluated. Results. Head cooling with a constant rectal temperature resulted in an increase in the temperature gradient across the brain from the warmer central structures to the cooler periphery (brain 2 cm - dura temperature: 1.3 ± 1.1°C at control to 7.5 ± 3.5°C during cooling). Hypoxia superimposed on head cooling decreased the temperature gradient by at least 50%. In contrast, body cooling was associated with an unchanged temperature gradient across the brain (brain 2 cm - dura temperature: 1.5 ± 1.2°C at control to 1.1 ± 0.9°C during cooling). Hypoxia superimposed on body cooling did not change brain temperature. Both modes of brain cooling resulted in similar reductions of global CBF (∼40%) and O₂ uptake. Conclusion. Brain hypothermia achieved through head or body cooling results in different brain temperature gradients. Alterations in systemic variables (ie, hypoxemia) alters brain temperature differently in these 2 modes of brain cooling. The mode of brain cooling may affect the efficacy of modest hypothermia as a neuroprotective therapy.