A single-chamber pacemaker operates in VVI mode when only one lead is positioned in the ventricle; the VVI mode can also be programmed in a dual-chamber pacemaker. Programming of the base rate is essential. Indeed, in patients with sinus dysfunction, if the base rate is too high and above the sinus rate, ventricular pacing becomes permanent with

1. inversion of the physiological atrioventricular activation sequence (retrograde conduction and pacemaker syndrome) if there is no retrograde atrioventricular block; atrial contraction occurs with ventricular contraction in a quasi-synchronous manner, while the atrioventricular valves are closed, causing a retrograde flow towards the pulmonary veins and the vena cava. Pacemaker syndrome results from a complex combination of hemodynamic, neurohumoral and vascular alterations secondary to the loss of atrioventricular synchrony. Symptomatology, which is sometimes very disabling due to increased atrial pressure and venous pressure, includes dyspnea, orthopnea, pulsations in the neck and chest, palpitations, chest pain;
2. loss of atrioventricular synchrony if there is no retrograde conduction.

Conversely, when the minimum pacing rate is below the spontaneous sinus rate (e.g. 40 beats/minute), the patient is not paced when the device is in sentinel mode. This enables:

1. to reduce energy consumption and prolong battery life;
2. to avoid retrograde conduction;
3. to pace effectively only when there is occurrence of a paroxysmal atrioventricular conduction disor