Effects of head-down tilt on cerebral blood flow and somatosensory-evoked potentials in rabbits

Changes in cerebral blood flow (CBF) and somatosensory-evoked potentials (SEPs) were studied in rabbits exposed to head-down tilt (HDT) at 45° and 75°. The animals were anesthetized with alpha chloralose and the lungs were artificially ventilated. CBF was continuously measured by laser Doppler flowmetry (LDF), and SEPs were recorded as responses of the cortex to median nerve stimulation. In the 45° HDT rabbits, CBF did not change significantly in the parietal cortex during 1 h of HDT. In contrast, in the 75° HDT rabbits, CBF did not change significantly within 5 min after the onset of HDT, but decreased gradually to 79% of the pre-HDT baseline value at the end of 1 h of HDT. The latency and amplitude of SEPs did not change significantly throughout the experiment in any group. These results suggest that CBF and SEPs do not change significantly during 1 h of 45° HDT and that 75° HDT disturbs the regulation of the cerebral circulation but does not affect cortical somatosensory response, at least for 1 h.