INTRODUCTION
To investigate the effect of excitation frequency on transthoracic impedance measurement, an electrically-isolated impedance measurement circuit was developed and validated. Sinusoidal excitation was used, with the frequency adjustable by an external voltage input. This device was capable of monitoring both the in-phase and the out-of-phase components of the transthoracic impedance by quadrature demodulation. The amplitude of the excitation current was limited to 50 ?A.