| Description |
Electrical impedance has been used to measure a variety of physiological events and to assess characteristics of tissues and organ systems. The technique requires the generation of an alternating current field within the biological system by means of surface electrodes. When current is passed through living tissue to measure its impedance, special consideration must be give to the structures between the electrodes. In order to avoid stimulation of irritable tissues, the duration and frequency of the current generated must be carefully evaluated. As the current is applied to the living tissue, the spatial manner in which it is distributed is dependent upon the resistivity characteristics of the various tissues and fluids it encounters, and upon the physical orientation of the tissues with respect to the electrodes. The biological effects over time of current density and applied charge have not been adequately investigated. Measurement of the urinary bladder volume appears to the clinical application which would require monitoring, and hence current exposure, for long periods of time. An electrical analog and simplified model of the lower abdomen predicts that insufficient energy is produced to cause a thermal injury when 75 kHz and 0.2 mA are applied via electrodes with a surface area of 3.14 cm2. In addition, the estimated applied charge with these same parameters predicts no chemical alterations physically disturbing to the exposed tissues. The study examined grossly and histologically the biological effects of 75 kHz, 0.2 mA with an electrode surface area of 0.63 cm2 on 92 immobilized rats who had been exposed to the current for controlled periods of time to simulate long term use of impedance monitoring. Also examined were 60 rats who served as controls for the effects of immobilization or the electrode materials. None of the 152 rat tissue samples showed any evidence of maceration, abrasion, erythema, gross edema, bleeding, necrosis, cellular edema, revascularization, coagulation necrosis, nuclear condensation, or gas bubble formation. There was no evidence of observable injury, thermal, chemical, or physical, when the current exposure simulated long term use of an impedance measuring device. A valuable noninvasive technique, impedance could be safely used by nurses in the clinical setting to monitor, single, intermittent, or continuous changes in physiological events. |
