Physiological activity of a patient is detected and analyzed by means of a single sensor situated in the vascular system of the patient. Pulmonary activity is derived from analysis of changes in cardiac activity, such as sensed from blood flow, pressure or volume changes, measured in a preferred embodiment as impedance change within the right heart. Pulmonary activity is separated from cardiac activity implicit in such changes by filtering lower and higher frequency components. The separated signals are processed to derive control signals for stimulating the heartbeat in a pacemaker system, preferably by a single sensor-stimulator electrode implanted within the right heart. Thus, a single functional parameter, namely intra-cardiac impedance, varying both with the intrathoracic pressure fluctuations following respiration and with cardiac contraction is representative of the pulmonary activity and of the cardiac activity. Resulting derived sub-signals then provide information regarding cardiac and pulmonary activity and are used to monitor the patient's condition and control variably the rate of a cardiac pacemaker.