Cerebral Blood Flow and Oxygen Consumption in Normocapnia and Hypercapnia: Modulating Influence of Paravertebral Sympathetic Blockade at the Low Thoracic Level

The objective of the present study was to explore whether the systemic consequences of sympathoadrenal activation influence the cerebral circulatory and metabolic effects of hypercapnia in the rat. To that end, a bilateral blockade of the sympathetic chain was performed at the low thoracic level by paravertebral injection of local anaesthetic. The injection was followed by a reduction in blood pressure and, in comparison to animals injected with local anaesthetic intramuscularly, those with paravertebral blockade showed lower blood and tissue concentrations of glucose and lactate. Overall ("cortical") CBF and CMRO2 were measured with a 133xenon modification of the Kety-Schmidt technique, and local CBF was estimated autoradiographically with 14C-iodoantipyrine as the diffusible tracer. Paravertebral blockade failed to modify the circulatory response to hypercapnia, nor did it prevent the increase in CMRO2d previously noted in this preparation. In animals maintained ventilated on 70% N2O, paravertebral blockade reduced overall CBF by 30% and local CBF by 30-40%, with a suggested but statistically nonsignificant reduction in CMRO2. In unparalysed, awake animals the blockade failed to affect local CBF. It is concluded, therefore, that blockade of the sympathetic chain causes a reduction of CBF only in the stressful conditions prevailing in paralysed and ventilated animals.