Our understanding of the relationship of cerbral electrical impedance (CEI) to cerebral ischemia (CI) has been primarily based on studies of focal ischemia in the cat. The gerbil is another experimental model extensively used by this laboratory in its studies of cerbral ischemia. In view of our extensive use of gerbil model, it was of interest and desirable to extend the characterization of impedance changes induced by ischemia to the gerbil. A redesign and fabrication of a new electrode array was necessary because of the small size of the gerbil brain. Otherwise the methodology was duplicative of the cat studies. The CEI changes we observed in the gerbil following bilateral carotid occlusion resembled those seen in the cat in many ways; the impedance arose after a small delay, rapidly climbed 200% or more, leveled off on a plateau and returned to the preischemic value approximately in an exponential fashion. The rate of fall was significantly slower than in the cat. Multiple occlusions similar to our model on transitory ischemic attacks (TIA) were also studied. These studies focused on BLCA occlusion ischemias of short duration, such as 3 minute occlusions repeated two or three times. Typically, we noted the slightly longer delay of rise in CEI after cessation of CBF of about 3 minutes before impedance change was evident. The most striking finding was the time of delay in recirculation, which increased markedly with successive ischemias. Not until recirculation began was there lowering of the impedance which, however, tended to remain elevated after circulation was reestablished.
