In 14 ventilated, normocapnic baboons anaesthetised with alpha-chloralose, local CBF (hydrogen clearance) and the amplitude and latency of local components of the somatosensory evoked potential (SEP, median nerve stimulation) were measured bilaterally in ventrobasal thalamus (VPL), medial lemniscus (ML), and cerebral cortex before and during progressive ischaemia, produced by occlusion of the right middle cerebral artery and subsequent controlled reductions in mean systemic blood pressure (MSBP). The first significant reduction from control of the left cortical SEP amplitude occurred in the range of 30-40 mm Hg MSBP, but those of the VPL and ML responses only below 30 mm Hg; in the range of 20-30 mm Hg, the average SEP amplitudes in cortex, VPL, and ML were 8.6, 72.6, and 90.7% of control, respectively. In terms of local CBF, the cortical SEP threshold was in the range of 15-20 ml/100 g/min (as in previous work), that of VPL in the range of 10-15 ml/100 g/min, but the ML response was only markedly reduced below 10 ml/100 g/min. Thus, the differential ischaemic sensitivity of the SEP between the three regions was clearly demonstrated. These results indicate that as one descends the neuraxis, there is an increasing resistance of electrophysiological function to systemic hypotension, together with a decreasing threshold for local ischaemia.

No abstract

A model for studying changes in local CBF and evoked potentials in selective thalamic ischaemia has been developed. The arterial supply to the posterior thalamus (mainly from the posterior choroidal arteries) was occluded in the baboon using a transorbital approach to the region of prepontine and ambient cisterns. Local CBF was measured by the hydrogen clearance method using electrodes introduced into the nucleus ventralis posterior lateralis of thalamus as well as cortex on both sides. The production of focal ischaemia was demonstrated by a significant decrease in thalamic CBF and confirmed by examination of the brain perfused with carbon particles.

Intracranial pressure (ICP) differences, change of local blood flow (CBF) using the hydrogen clearance technique, change in the somatosensory evoked potential (SEP) to median nerve stimulation and pupillary size were investigated during progressive elevation of the ICP (using an extradural balloon) in 6 anaesthetized baboons. CBF was measured in the frontal cortex, somatosensory cortex, thalamus (nucleus ventralis posterior lateralis-VPL), medial lemniscus (ML), lateral lemniscus (LL) and caudate nucleus (CN). Conduction along the somatosensory pathway between C2 at the neck and VPL was compared with conduction between VPL and primary somatosensory cortex. The amplitude of the cortical SEP was also studied. ICP gradients between hemispheres developed as the pressure was increased to in excess of 50 mm Hg. CBF was significantly reduced from control in the cortex and VPL on the side ipsilateral to the balloon at 50 mm Hg ICP. A significant decrease in ML flow occurred bilaterally at 70 mm Hg ICP. Conduction time was increased significantly between the right VPL and cortex at a pressure of 50 mm Hg. The amplitude of the cortical response was significantly reduced at 30 mm Hg on the right side and 50 mm Hg on the left. Aniscoria occurred at 50 mm Hg ICP and the pupils became dilated at 70 mm Hg. The SEP was possibly more sensitive than the pupillary reactions as an indication of tentorial herniation in these experiments.