Atrium and ventricle are sensed as well as paced, but there is no tracking of atrium. This is used as a fall back mode in atrial tachycardias. The upper rate and lower rate intervals are equal for both atrium and ventricle. The pacemaker will maintain a minimum atrial and ventricular rate. Pacing never occurs above the lower rate interval. If the spontaneous atrial rate goes up, the ventricular paced rate will not go up. But if the spontaneous atrial impulses get conducted to the ventricles, the ventricular rate will go up. DDI mode can be considered as a DDD – VAT mode. DDD pacemaker is supposed to have the functions of AAI, VVI and VAT. In DDI mode, VAT function is not there, but AAI and VVI functions remain.

Upper tracking rate

Upper tracking rate is the maximum rate at which P synchronous pacing is delivered. Above this rate either a second degree block pattern occurs. The type of block depends on the programmed type of conduction ratio algorithm of the pacemaker.

Upper sensing rate

Upper sensing rate is the maximum rate at which a rate responsive pacemaker is programmed to pace in the appropriate chamber depending on the input from the sensor.

Automatic mode switching

When there is a paroxysmal atrial tachycardia which is above the upper rate interval, the pacemaker switches to either DDI/DDIR or VVI/VVIR mode. It switches back to DDD/DDDR mode once the tachycardia subsides.

What are the situations where the impedance is elevated?

Any discontinuity in the lead or any loose contact at the lead – pacemaker interface can increase the lead impedance.