Cerebral Impedance Changes After Circulatory Arrest

Cerebral impedance in the rabbit was measured between two electrodes placed on the dorso-lateral aspect of a hemisphere. The real part (resistance) was by far the larger component of the impedance. The effect of circulatory arrest on the reciprocal of the resistance (conductance) was examined. After cutting the aorta the brain conductance decreases, first slowly for 1–5 minutes. Then there is a sudden large drop in conductivity which is closely correlated in time with the development of a cortical negativity (asphyxial potential). After the rapid drop, the conductivity continues to decrease at a diminishing rate until after 4–5 hours the conductivity is only 20–30% of the value before circulatory arrest. The following factors were found to contribute to the drop in conductivity. Cooling of the brain to room temperature (25°C) causes a decrease of about 20%. Draining of fluid from the cortical surface and lateral sinus and perhaps draining of intercellular fluid accounted for a drop of 15–20% of the original conductivity. The emptying of blood vessels was estimated to cause a decrease of the order of 10%. Including its onset and decline the rapid conductance drop may cause a decrease of 30–35% of the original conductivity. It was postulated that the rapid conductivity drop is due to a transport of ions from the intercellular compartment into the cells and fibers of the brain tissue caused by a sudden increase in membrane permeability which also would cause the asphyxial potential.