An intracardiac impedance-derived parameter, Half Cycle Activity (HCA) is used in sensing and controlling cardiac activities in implantable cardiac devices such as bradycardia pacemakers and cardioverter defibrillators. This impedance-derived parameter correlates closely with physical workload and at the same time provides hemodynamic feedback information. Thus, it allows a pacemaker system to implement accurately an increase in hemodynamically driven pacing rate, as well as to limit an inappropriate decrease of driven pacing rate advised by another sensor such as an accelerometer. In addition, it determines the maximum pacing rate for the pacemaker so as to prevent hemodynamic compromise. Therefore, the HCA parameter may be used to determine a Hemodynamic Upper Rate Limit and a Hemodynamic Lower Rate Limit, and thus define a Hemodynamic Pacing Range that is compatible with the range of physiological rate. This allows a closed-loop control of the pacing rate. In an implantable cardioverter defibrillator system, the hemodynamic tolerance of arrhythmias can also be assessed by using the HCA parameter to adjust therapy provided to a heart, such as antitachycardia pacing and/or cardioverting shocks.