A square signal wave method for measurement of brain extra- and intracellular water content.

Brain tissue electrical impedance is a commonly used method to evaluate the dynamics of brain edema. We have found the square wave impedance method simpler and more cost-effective than the currently used sine wave impedance method. This square wave method avoids the necessity for expensive frequency control and amplitude-phase measuring devices as well as simplifying on-line data processing. In our experiments the electrical impulse was generated by a pulse generator of Macintosh data acquisition system. The signal (I = 11 muA, t = 2-20 ms) was delivered every 2-3 s external electrodes of a tetrapolar system through a specially designed isolation-calibration device. This electrode system was inserted into the cerebral cortex of experimental animals (rat). The cerebral cortex was found to have linear electrical properties in the 5-30 muA range. Our impedance measurement system was tested in calibration trials, and showed system reliability and accuracy. The system was also tested in pilot experiments, in vivo, in a rat brain osmotic edema model.