A minute ventilation sensing device, comprising: excitation current electrodes for imposing a current field in the thoracic cavity; an exciter configured to supply excitation current between the excitation current electrodes as a bipolar excitation current waveform at a specified excitation frequency and amplitude, wherein the excitation current waveform is output as a strobe made up of a specified number of excitation current waveform cycles with each strobe repeated at a specified strobing frequency; voltage sense electrodes configured to generate a voltage sense signal corresponding to a potential difference between two points in the thoracic cavity; sampling circuitry configured to sample the voltage sense signal during the excitation waveform strobe at a specified sampling rate that corresponds to the excitation frequency; a demodulator configured to generate a weighted average of the voltage sense signal samples of each strobe with a filter coefficient for each sample that is positive or negative in accordance with the polarity of the excitation current waveform such that an impedance portion of the voltage sense signal is low-pass filtered while an external field portion of the voltage sense signal is high-pass filtered; circuitry configured to filter the demodulated voltage sense signal samples into a ventilation band to thereby generate a ventilation signal; circuitry configured to derive a signal proportional to minute ventilation from the ventilation signal; circuitry excitation current adjusting initiate a noise detection operation, sample the voltage sense signal when no excitation current is supplied and process the samples to detect a noise level; and, excitation current adjusting circuitry configured to adjust the excitation current in accordance with the detected noise level, characterized in that the excitation current adjusting circuitry configured to adjust the excitation current amplitude in response to the detected noise level in order to approximately maintain a specified signal-to-noise ratio. excitation current electrodes for imposing a current field in the thoracic cavity; an exciter configured to supply excitation current between the excitation current electrodes as a bipolar excitation current waveform at a specified excitation frequency and amplitude, wherein the excitation current waveform is output as a strobe made up of a specified number of excitation current waveform cycles with each strobe repeated at a specified strobing frequency; voltage sense electrodes configured to generate a voltage sense signal corresponding to a potential difference between two points in the thoracic cavity; sampling circuitry configured to sample the voltage sense signal during the excitation waveform strobe at a specified sampling rate that corresponds to the excitation frequency; a demodulator configured to generate a weighted average of the voltage sense signal samples of each strobe with a filter coefficient for each sample that is positive or negative in accordance with the polarity of the excitation current waveform such that an impedance portion of the voltage sense signal is low-pass filtered while an external field portion of the voltage sense signal is high-pass filtered; circuitry configured to filter the demodulated voltage sense signal samples into a ventilation band to thereby generate a ventilation signal; circuitry configured to derive a signal proportional to minute ventilation from the ventilation signal; circuitry excitation current adjusting initiate a noise detection operation, sample the voltage sense signal when no excitation current is supplied and process the samples to detect a noise level; and, excitation current adjusting circuitry configured to adjust the excitation current in accordance with the detected noise level, characterized in that the excitation current adjusting circuitry configured to adjust the excitation current amplitude in response to the detected noise level in order to approximately maintain a specified signal-to-noise ratio.