Regional Differences in Cerebral Edema After Traumatic Brain Injury Identified by Impedance Analysis

Objective: Cerebral edema is a common and potentially devastating sequel of traumatic brain injury. We developed and validated a system capable of tissue impedance analysis, which was found to correlate with cerebral edema. Methods: Constant sinusoidal current (50 μA), at frequencies from 500 to 5000 Hz, was applied across a bipolar electrode unit superficially placed in a rat brain after traumatic brain injury. Rats were randomized to three groups: severe controlled cortical injury (CCI), mild CCI, or sham injury. At 60 h post-CCI, cerebral voltage and phase angle were measured at each frequency at the site of injury, at the penumbral region, at the ipsilateral frontal region, and in the contralateral hemisphere. Impedance measurements were also obtained in vivo. The electrical properties of varied injuries and specified locations were compared using a repeated measures analysis of variance (RMANOVA), were correlated with regional tissue water percentage using regression analyses, and were combined to generate polar coordinates. Results: The measured voltage was significantly different at the site of injury (P < 0.0001), in the penumbra (P = 0.002), and in the contralateral hemisphere (P = 0.005) when severe, mild, and sham CCI rats were compared. Severely injured rats had statistically different voltage measurements when the various sites were compared (P = 0.002). The ex vivo measurements correlated with in vivo measurements. Further, the impedance measurements correlated with measured tissue water percentage at the site of injury (R² = 0.69; P < 0.0001). The creation of a polar coordinate graph, incorporating voltage and phase angle measurements, enabled the identification of impedance areas unique to normal, mild edema, and severe edema measurements in the rat brain. Conclusions: Electrical measurements and tissue water percentages quantified regional and severity differences in rat brain edema after CCI. Impedance was inversely proportional to the tissue water percentage. Thus, impedance measurement can be used to quantify severity of cerebral edema in real time at specific sites.