Ideal physiological form of pacing responds to body's needs for varying cardiac output indipendent of the requirement to synchronize with atrial activity. The atrial synchronous pacemakers are superior to the ventricular inhibited fixed rate pacing mode. Alternative methods of sensing physiological demands with single chamber ventricular pacing that produce a physiological rate response to exercise are described. System based on QT sensing and pacing system responsive to changes in respiratory rate have undergone successful clinical trials. Hemodynamics of rate responsive pacing are described. The highest cardiac performance during stress test in rate responsive pacing was achieved with the respiratory dependent pacemaker.