A device is presented for evaluating whether an episode of sleep apnea is occurring in a patient suffering from chronic sleep apnea disorder, for delivery of appropriate therapy. The device includes circuitry adapted to respond to a cardiac signal generated by the heart. Switching circuitry diverts passage of the heart signal through both a high impedance path and a substantially lower impedance path, and a differential amplifier processes the resulting signal pairs to ascertain the difference in magnitude between the two signals of each pair. An analyzer thereof determines changes in the patient's ventilation, from which inordinately reduced patient ventilation is detected to assess possible occurrence of an episode of sleep apnea. If the analyzer denotes change of ventilation between otherwise regular respiratory cycles, an actual episode of sleep apnea is indicated. A stimulus generator responds to such indication to generate an appropriate electrical therapy for delivery to a preselected location in the patient's body to induce ventilation so as to terminate the apnea episode.