Epidural versus subdural spinal cord cooling: cerebrospinal fluid temperature and pressure changes

Background. Regional spinal cord cooling can increase the tolerable duration for spinal cord ischemia resulting from aortic clamping. We compared the efficacy of epidural and subdural cooling and the effect of the resulting cerebrospinal fluid-pressure (CSF) increases on spinal cord motor neuron function. Methods. In 8 pigs, CSF temperature and pressure were assessed in the subdural space at L4, T15, and T7. Saline was infused at 333, 666, and 999 ml/h at four consecutive locations: L4 subdural, L4 epidural, T15 subdural, and T15 epidural. First, the influence of CSF-pressure increases during normothermic infusion on transcranial motor evoked potentials (tc-MEPs) was assessed. Then, hypothermic infusion (4°C) was performed to assess CSF-temperature changes. Results. During normothermic infusion, baseline CSF pressures increased uniformly from 6 ± 4 mm Hg to 34 ± 18, 42 ± 17, and 50 ± 18 mm Hg with increasing infusion rates (p< 0.001), and did not differ between epidural or subdural infusion. Tc-MEPs indicated spinal cord ischemia in 6 animals when CSF pressures reached 65 ± 11 mm Hg. During hypothermic infusion, CSF temperatures decreased from 37° to 35 ± 1.2°, 31 ± 2.2°, and 28 ± 2.8°C, but increasing CSF-temperature gradients were observed between the infusion location and distant segments. Subdural cooling resulted in lower CSF temperatures (p< 0.001), but caused larger CSF-pressure increases (p< 0.001). Conclusions. Subdural and epidural infusion cooling produce localized spinal cord hypothermia in pigs. The concurrent pressure increases, however, are uniformly distributed and can result in tc-MEP evidence of ischemia.