A rate-responsive pacemaker employing a rate control parameter of respiratory minute volume, derived over a unipolar lead. The pacemaker performs the minute volume measurement by periodically applying a measuring current between the lead and a reference point on the pacemaker case. This measuring current has frequency components in a range from approximately 10 kilohertz to 1000 megahertz. Application of this measuring current allows the pacemaker to detect the voltage which arises from the applied current and, from the detected voltage, to measure the patient's spatial impedance. Spatial impedance and minute volume vary as a function of the patient's pleural pressure. The pacemaker derives minute volume and rate-responsive pacing rate from the spatial impedance measurement.