In 1995, 2249 dual chamber pacemakers were implanted in the Czech Republic. These pacemakers make it possible to set an optimal AV delay between the atrial and ventricular impulse. Although the optimization of the AV interval has its well defined physiologic advantages, it does not seem to be necessary in otherwise healthy individuals with a good atrial and ventricular function. In these patients the default value, usually about 170 ms, is acceptable. However, AV interval optimization--i.e. finding the interval at which the atrial contribution to ventricular filling is maximal--should be done in all patients with left ventricular dysfunction, indicated for pacing because of bradyarrhthmia. In this subset of patients, even a small improvement in ventricular filling is believed to be clinically useful. Moreover, it has been documented, that in some types of ventricular dysfunction the so-called "primary optimization" (i.e. optimization of the AV interval in patients, in whom the pacemaker is not indicated for bradyarrhthmia but for ventricular dysfunction that might be improved by AV interval optimization) may be clinically useful. It is the case in patients with hypertrophic obstructive cardiomyopathy, dilated cardiomyopathy with presystolic regurgitation and AV interval prolongation, and perhaps even in some patients with impairment of ventricular systolic function and substantial prolongation of the AV interval. Despite all that, optimization of the AV interval is not routinely performed because even the best available optimization procedures (stroke volume measurements at different AV intervals by aortic Doppler echography) is observer dependent, time-consuming and costly.