The role of electrode size on the incidence of spreading depression and on cortical cerebral blood flow as measured by H₂ clearance

Cerebral blood flow was measured by the H₂ clearance method 30 and 60 min after the implantation of 300, 250, 125, or 50 μm diameter platinum-iridium electrodes 2 mm deep into the right parietal cortex of normothermic, normocarbic halothane-anesthetized rats. Another group of animals had 50 μm electrodes inserted 1 mm. In all animals, the presence or absence of a wave of spreading depression (SD) was noted at the time of implantation, with recordings made with glass micropipettes. H₂ flow values were compared with those measured in gray matter from the same anatomical region (but from different rats), using [³H]nicotine. The incidence of SD ranged from 60% following insertion of 300 μm electrodes to 0% with 50 μm electrodes. H₂ clearance flows also varied with electrode size, from 77 ± 21 ml 100 g^-1 min^-1 (mean ± standard deviation) with 300 μm electrodes to 110 ± 31 and 111 ± 16 ml 100 g^-1 min^-1 with 125 and 50 μm electrodes, respectively (insertion depth of 2 mm). A CBF value of 155 ± 60 ml 100 g^-1 min^-1 was obtained with 50 μm electrodes inserted only 1 mm. Cortical gray matter blood flow measured with [³H]nicotine was 154 ± 35 ml 100 g^-1 min^-1. When the role of SD in subsequent flow measurements was examined, there was a gradual increase in CBF between 30 and 60 min after electrode insertion in those animals with SD, while as such change was seen in rats without SD. These results indicate that the choice of electrode size and implantation depth influences the measurement of CBF by H₂ clearance. CBF values equivalent to those obtained with isotopic techniques can be acutely obtained with small (50 μm diameter) electrodes inserted 1 mm into the cortex. While the occurrence of SD does influence CBF in the period immediately after implantation, a relationship between electrode size and measured flow is present that is independent of SD.