Technique of selective spinal cord cooling in rat: methodology and application

In a number of interventions, it is desirable to be able to produce a rapid but readily reversible change in spinal cord temperature (SCT) without altering general body temperature and to maintain this selective spinal cord hypothermia stable for an extended interval. To accomplish this, we developed a technique of subcutaneous perfusion cooling in rat. This was accomplished by constructing a copper heat exchanger which was readily implanted into subcutaneous space overlying the upper thoracic to upper sacral spinal segments. The heat exchanger was then perfused with fluid from an external temperature bath maintained at (8 degrees C) at a perfusion rate of 100 ml/min. The temperature of the heat exchanger was controlled by regulating the pump with a feed back controller driven by a thermocouple placed percutaneously into the paraspinal musculature. A series of studies were performed to demonstrate the characteristics and utility of this cooling technique. Lowering the pump set point to 24 degrees C resulted in a fall in the intrathecal temperature (ITT) to 27 +/- 0.3 degrees C within 15 min with no significant changes observed in rectal temperature (37.5- > 37.2 degrees C). Change in intrathecal temperature showed a highly significant correlation with changes in paravertebral muscle temperature (r = 0.977). The hypothermic state could be readily maintained for extended intervals up to 5 h and an underbody heating pad was used to maintain rectal temperature between 35-36.5 degrees C. Lowering the ITT from 37 degrees C-27 degrees C evoked a temperature-dependent increase in the latency of precooling spinal somatosensory evoked potentials (SSEPs) with the highest sensitivity observed in postsynaptic components. Returning the set point temperature back to 37 degrees C produced a rapid recovery of the SSEPs latencies. Consistent with previously published data, selective spinal cord hypothermia (27 degrees C) provided complete protection against otherwise injurious interval of normothermic ischemia produced by balloon occlusion of the descending aorta. This technique provides a simple, relatively non-invasive and reliable experimental tool for studying the effect of selective, acute and/or prolonged spinal cord hypothermia.