Variation in cardiac chamber volume or pressure as a controlling parameter

A rate adaptive cardiac pacer is described in which the impedance versus time information derived using impedance plethysmography or the pressure versus time information derived from a pressure transducer in a ventricular chamber is signal processed to recover a modulating envelope due to volume or pressure changes occasioned by respiratory activity. Either or both of the respiratory interval or respiratory depth may be combined in an appropriate rate control algorithm (62) with other parameters also derived from the impedance versus time signal to develop a rate control signal (64) for an implanted pacer.