 Heart rate lowering is a cornerstone of the management of chronic heart failure CHF and the degree of lowering is closely related to improvements in longevity, hospitalisation rate and heart function. The investigators have shown that increasing heart rates using pacemakers does not increase exercise capacity in CHF patients. This might be because the optimal heart rate range for contraction is narrower for patients with CHF. At higher heart rates, heart contraction might be less strong. The investigators now want to examine the relationship between heart rate and heart contraction in patients with heart failure and pacemakers. Tailored pacemaker heart rate settings, individually optimised for heart contractility as assessed by cardiac ultrasound, will be investigated to examine the effects on exercise tolerance . 