An apparatus for measuring complex electrical admittance and/or complex electrical impedance in animal or human patients includes a first electrode and at least a second electrode which are adapted to be disposed in the patient. The apparatus includes a housing adapted to be disposed in the patient. The housing has disposed in it a stimulator in electrical communication with at least the first electrode to stimulate the first electrode with either current or voltage, a sensor in electrical communication with at least the second electrode to sense a response from the second electrode based on the stimulation of the first electrode, and a signal processor in electrical communication with the sensor to determine the complex electrical admittance or impedance of the patient.