Comparative studies of regional CNS blood flow and evoked potentials in the cat. Effects of hypotensive ischemia on somatosensory evoked potentials in cerebral cortex and spinal cord.

Functional resistance to graded hypotensive ischemia of various segments of the somatosensory pathway was determined in anesthetized cats by repeated concurrent recordings of regional blood flow measured by hydrogen clearance, and evoked potentials (EPs), of dorsal horn of lumbar spinal cord and cerebral cortex. During normal resting CNS blood flow (CBF), there were significant successive reductions of EP amplitudes, recorded from presynaptic spinal components (634, 424-949 microV; re-linearized mean and 95% confidence limits of log-transformed data) compared to postsynaptic spinal (359, 247-522 microV) and presynaptic cortical (50, 32-79 microV) and to postsynaptic cortical components (33, 22-50 microV). During ischemia amplitudes of EPs in spinal cord and cerebral cortex showed significantly different behaviors. The presynaptic spinal component was virtually independent of regional blood flow down to 12 percent of resting values, the postsynaptic cortical component exhibited strongest positive correlations (r = 0.45) with flow. In both regions postsynaptic amplitude was more sensitive to flow changes than respective presynaptic amplitudes. Despite similar regression coefficients for intermediate segments of somatosensory pathway, only postsynaptic spinal components were significantly correlated (r = 0.40) with regional flow. Presynaptic cortical amplitudes were variable and no significant flow dependence was demonstrated. Results suggested that in comparable degrees of regional ischemia of CNS functional integrity is determined by numbers of synaptic transmissions involved locally. Comparatively simple structures, e.g. the spinal cord, are less susceptible to ischemia and complex neuronal networks, e.g. the cerebral cortex, are more susceptible.